Chapter 31. Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion. Carl E. Misch 684 IMPLANT SURGERY

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1 684 IMPLANT SURGERY Chapter 31 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion Carl E. Misch The primary goal of root form surgery is to insert an endosteal implant into the available bone of an edentulous site in the proper location and angulation so that it may be used as a prosthetic abutment. An endosteal implant, made of biocompatible material, will predictably obtain rigid fixation when specific surgical principles are followed. The soft tissue should cover or approximate the implant for primary healing. The bone should receive as little trauma as possible. The smaller the devital zone, which forms around an implant subsequent to surgical trauma, the more likely rigid fixation will occur. The bone should initially be within 30 to 40 mm of the implant body, 1 which should be rigidly fixated and not move during healing. Movement of 100 mm or more to the bone-implant interface has been shown to result in intervening fibrous tissue. 2 The surgical site should remain free of infection, which may result in a bacterial smear layer on the implant body and impair the development of a close bone-implant contact during healing. The health, volume, and density of the recipient bone are critical to the preparation, fixation, and healing of an endosteal implant. Figure 31-1 The most common position for root form implants in an edentulous mandible is between the mental foramina. There are more than 100 types of root form implant designs readily available on the commercial market. A consistent philosophy is the desire for clinical rigid fixation corresponding microscopically to a direct boneimplant interface, without intervening fibrous tissue on a major portion of the implant body. 3 The surgical procedures for endosteal root form placement are des cri bed in this book by region, bone volume, and bone density. This chapter addresses a generic surgical approach to the anterior and posterior mandible in adequate volume (Division A or B) and quality of bone (D2) for the placement of a screw-type root form implant. SURGICAL APPROACH TO THE ANTERIOR MANDIBLE The most common location for root form implants in the completely edentulous mandible is between the mental foramina, in the region of the mandibular symphysis (Figure 31-1). The prosthesis type and amount of support for the patient force factors and bone density are first determined. If a fixed or RP-4 prosthesis is planned, most often five root form implants are usually indicated (see Chapter 14) in the A, B, C, D, and E positions (starting from the right of the patient to the left). 4 Posterior implants may also be indicated, depending on the existing force factors and other parameters. Panographic radiographs or computed tomography (CT) scan imaging is evaluated for potential pathology, bone volume, and anatomical landmarks such as mental foramina and crest of ridge. A lateral radiographic view may be taken with a transitional prosthesis in place, which permits the evaluation of bone volume in height, the angulation of the bone in the midline, and the crown/remaining bone height ratio. A surgical template is fabricated before surgery to indicate the incisal edge position of the final restoration and help determine 684

2 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 685 Box 31-1 Anesthesia Block: Bilateral Akinosi Lidocaine 2%, 1:100,000 epinephrine Marcaine 0.5%, 1:200,000 epinephrine Higher hit rate Fewer reinjections at end of surgery Less postoperative pain Marcaine possibly at end of surgery proper implant location and angulation for the specific prosthesis (see Chapter 13). A standard surgical setup is prepared (see Figure 31-56). Patient Preparation The patient is prepared for surgery following the pharmacologic protocol typical for implant surgery (see Chapter 21). Because two carpules have a higher hit rate for block anesthesia, compared with one carpule, bilateral mandibular block anesthesia is administered with both lidocaine 2% 1:100,000 epinephrine and a long-acting anesthetic, such as bupivacaine (Marcaine) 0.5% with 1:200,000 epinephrine (Box 31-1). The use of a long-acting anesthetic is beneficial in three ways: (1) the added volume of anesthetic increases the success of the mandibular block; (2) a full arch mandibular implant insertion surgery may last 2 hours; therefore repeated injections at the end of the surgery are less indicated; and (3) in addition, long-acting anesthetics may reduce postoperative pain. In fact, it is advantageous to readminister the long-lasting block anesthetics at the conclusion of the surgery when its duration is longer than 1 hour. 5 Akinosi Technique The Akinosi technique (closed mouth) for mandibular anesthesia is preferred in the completely edentulous patient, because the occlusal plane and opposing premolar position used as references for the traditional Halsted dental injections are unavailable or inaccurate (Figure 31-2). 6 The Akinosi block procedure is administered with the mouth in an almost closed position or, because the patients often wear a denture, in an approximate occlusal vertical dimension. In addition, it is a benefit when the patient positions the jaw toward the side of the injection (i.e., slide the jaw to the right for a right mandibular block). A long, 27-gauge needle is used in the syringe. The needle is bent 30 degrees near the base, so the needle direction on a horizontal plane will be away from the midline. This is advantageous because the ramus flares lateral as it proceeds distal. The syringe and needle are placed parallel to the occlusal plane, at the height of the maxillary mucogingival junction. The needle penetrates approximately half its length (25 to 30 mm) before aspiration and injection of anesthetic. There is no bony landmark. The Akinosi block is usually less painful for the patient, because the anesthetic fluid is injected into the top of the pterygoid triangular space, which has more room for the solution. In addition, the top of this triangular space has fewer muscle fibers in the pathway of the injection compared with the penetration site of the Halsted block and therefore is associated with less discomfort (Box 31-2). Additional injections of anesthesia with the administration of local anesthetic (such as lidocaine 2% with epinephrine 1:100,000) by infiltration in the labial, lingual vestibules, and the inferior border of the Grow-Gates Akinosi Halstead Auriculotemporal n. Chorda tympani Inferior alveolar n. n. to mylohyoid Buccal n. Lingual n. A B Figure 31-2 A, The Akinosi block procedure for local anesthesia is more effective than the traditional dental injection in the edentulous patient. B, Akinosi technique: The patient s mouth is almost completely closed or at the approximate occlusal vertical dimension in a denture wearer. The cheek is retracted with the free hand to expose the posterior teeth. The syringe is aligned parallel to the occlusal plane of the maxillary molars, and the needle is positioned level with the mucogingival junction of the maxillary second and third molars. The needle is inserted into the buccal mucosa as close as possible to the medial surface of the mandibular ramus to a depth of 25 to 30 mm without contacting bone. After careful aspiration, the anesthetic solution is deposited, approximately halfway between the mandibular foramen and the neck of the condyle into the middle of the pterygomandibular space.

3 686 IMPLANT SURGERY Box 31-2 Akinosi Nerve Block Advantages Anesthesia Inferior alveolar Lingual Buccal Mylohyoid High success rate Low rate of positive aspiration Reduced incidence of trismus, paresthesia Reduced pain anterior mandible are also required, because the inferior third of the mandible is innervated by cervical nerves (C2-C3). Typically, the injections are in the facial sites in front of the mental foramina and the midline, whereas the lingual injections are lingual to each canine region. An extraoral and intraoral scrubbing and draping of the patient are then performed. The benefit of aseptic technique is well documented in general. 7 However, a controversy exists regarding sterile versus clean surgical technique for endosteal implant placement. 8,9 Both the clean and sterile techniques use instruments, implant components, and hand pieces that are sterile. An actual sterile environment cannot be achieved within the oral cavity 10,11 ; therefore the term aseptic is often used. A primary issue is the need to use sterilized gloves, gowns, patient drapes and covers for the light, hand piece cord, and evacuation tips. In addition, can the surgeon and assistant touch nonsterile items in the operating room? Scharf and Tarnow compared sterile versus clean conditions with 386 implants in 92 patients. 12 Success rates were within 0.5% for the mandible and 1.6% for the maxilla. Kraut placed 2620 implants with a clean technique with a 1.26% surgical failure at implant uncovery and 0.5% early implant failure after function. 9 Therefore these reports suggest the clean technique is adequate for implant surgery. Brånemark Surgical Approach The original Brånemark surgical approach was used in a hospital environment, with an aseptic protocol similar to ortho pedic implants. There are several advantages to this approach. Normal skin flora, including Staphylococcus epidermidis and Staphylococcus aureus, are able to adhere to implanted biomaterials A bacterial smear layer may develop, which will be difficult for the body to remove by phagocytosis or bone cell activity. Antimicrobial mouth rinses (e.g., chlorhexidine) significantly reduce bacterial count in the saliva for more than 4 hours. 16 Chlorhexidine has been shown to reduce microbial complications when used perioperatively. 17,18 Povidoneiodine alcohol solutions have antibacterial properties used in preoperative site disinfection. 19 Although a sterile technique is not possible within the mouth, scrubbing around and within the mouth with these agents reduces the risk of a bacterial smear layer by inadvertent contamination by direct contact with the implant or indirectly via a glove or instrument. The clinical decision for an aseptic versus clean environment rests with the individual clinician, because documentation exists for either approach. However, it is prudent to reduce the risk of infection preoperatively and postoperatively. Although rare, infection may occur as a postoperative complication. Intraoral surgery is assimilated to a class II or clean contaminated surgery. Incidence of infection for class II surgery is 10% to 15%, but can be reduced to about 1% if appropriate technique and prophylactic antibiotics are prescribed. 20 The risk of infection with endosteal implants when preoperative and postoperative antibiotics are used appears to be even less. 9,21,22 What Is Clean? The question remains: What is clean? In one office, clean may result in the doctor looking for instruments in a drawer, adjusting a chair, touching the mask, moving the patient s hair out of the operative site, and so on. Therefore the issue appears to be not whether sterile gloves and gowns are used, but what protocols are in place to reduce the risk of contamination reaching the implant. It is prudent to train the staff and doctors in aseptic technique to reduce these risks. Training for an aseptic protocol relative to intraoral and extraoral scrubs and touching only sterile or scrubbed regions within the operatory are prudent to decrease risk. More invasive procedures such as sinus bone grafts exhibit a range of postoperative infection from 3% to 20%. 23,24 Based on this, the author suggests that an aseptic surgical procedure be used for most sinus surgeries. For full-arch implants or bone grafts, larger regions of tissue are exposed with increased risks to patients if infection occurs, so aseptic technique should be considered. In many dental offices the staff needs constant retraining for procedures that are not often performed. The routine aseptic surgical approach ensures the quality and consistency of the asepsis when it is most indicated. Intraoral and extraoral scrubbing with iodine, povidone-iodine (Betadine), or 0.12% chlorhexidine gluconate may be performed by a scrubbed assistant. A surgical scrub and gowning is performed by the doctor during the patient preparation. On return to the operating room, the patient s final drapes are positioned. The surgical gloves are rinsed and wiped with sterile saline to remove any powder or contaminant that may be inadvertently transmitted on the implant surface during the subsequent surgery. Infiltration with 2% lidocaine with 1:100,000 epinephrine into the labial and lingual mucoperiosteum evaluates the depth of anesthesia and provides some initial hemostasis. In

4 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 687 Figure 31-4 The initial incision bisects the keratinized gingiva on the crest of the ridge and extends from the first molar region to the contralateral first molar region. An accessory incision may be added off the midline to facilitate refl ection and mental foramen identification. Figure 31-3 The position of the mental foramen may be approximated by drawing an imaginary vertical line from the pupil or using one finger width lateral to the nose through the infraorbital and mental foramina. addition, when the local anesthetic is placed under the periosteum along the crest of the ridge, a hydroelevation of the soft tissue may help initial reflection of the soft tissue. Incision After anesthesia is achieved, the soft tissues and ridge height are evaluated. The lingual aspect of the mandible is palpated for undercuts and compared with the radiographic diagnostic images. Identification of the approximate position of the mental foramen facilitates the incision line design in moderately to severely atrophied mandibles and allows a more rapid reflec tion in regions around this landmark. The mandibular foramen region may be located by several methods. Palpation for the depressed foramen has been advocated, but is the most variable. 7 The overlying tissues usually prevent adequate feeling to identify the foramen with certainty. Before the patient is surgically draped for the procedure, an imaginary line may be drawn between the pupils of the eyes. A perpendicular line may then be drawn through each pupil (Figure 31-3). This vertical line passes through the infraorbital foramen and mental foramen. A surgical pen is used to mark the gingival tissue over the mental foramen region. If the patient is draped and the eyes cannot be used as a landmark, the foramen region corresponds to a vertical line drawn one fingerwidth distal to the ala of the nose and slightly distal to the corner of the mouth. Because suture line opening is the most common immediate postoperative complication, the surgical incision is designed to minimize this problem. If the crest of the ridge is above the floor of the mouth and there is greater than 3 mm of attached, keratinized gingiva on the crest of the ridge, a full-thickness incision is made, bisecting the attached tissue and scoring the bone on the crest of the ridge from the first molar region on one side to the anterior region and to the contralateral first molar region (Figure 31-4). If less than 3 mm of attached gingiva exists on the ridge, the full-thickness incision is placed so that at least 1.5 mm of the attached tissue is to the facial aspect of the incision line. This zone of attached tissue on the facial flap provides greater resistance for the sutures against tension of the mentalis muscle in the anterior region and buccinator muscle in the molar-premolar regions, which often cause the incision line opening. In addition, the blood vessels from the lingual artery may not cross over the crest of the ridge, but instead stop within the attached tissue. 25 As a result, an incision made facial to the attached tissue may cause partial ischemia to some of the crestal tissue. In addition, the incision in unkeratinized facial tissue also severs larger blood vessels, which increases bleeding and decreases vision during surgery. The incision should be done in one pass through the mucosa and periosteum and precisely score the periosteum. Additional scoring of the crestal bone after the initial incision design is usually beneficial. Areas of soft tissue invagination in prior extraction sockets are excised so as to create a clean margin for the flap, devoid of epithelium. SOFT TISSUE REFLECTION LANDMARK IDENTIFICATION A periosteal elevator (e.g., Molt 2-4, Molt A-1, Molt 7 or 9) is used to reflect a full-thickness flap. The muco periosteal flap is first reflected on the lingual aspect, which is typically less tenaciously attached to the underlying bone. The lingual reflection begins in the

5 688 IMPLANT SURGERY 20.5 mm 22.9 mm White Black Figure 31-5 A periosteal elevator is used to first refl ect the lingual full-thickness soft tissue fl ap. Two 2-0 sutures may then be used as retraction sutures, placed from each canine position to the contralateral molar site. Figure 31-6 After the lingual fl ap is tied back with retraction sutures, the facial full-thickness tissue fl ap is refl ected from the bilateral first molar region. regions of the canine and proceeds to the molar region on the same side. The contralateral canine to molar region is then reflected. This phase of the procedure most often is performed within a few seconds. The lingual flap between the canines is then reflected. This region is more difficult to reflect, because some of the tissues are embedded into the residual alveolar crest. Retraction (tie-back) sutures with 2-0 silk are then used to keep the lingual flap out of the surgical site. The right lingual canine region is tied to the left molar area, and vice versa. The sutures engage at least 5 mm of tissue, with care taken to avoid the submandibular duct. The lingual flap retraction improves visibility, negates repeated surgical assistant retraction efforts during the procedure, and reduces trauma to the thin lingual periosteal tissues (Figure 31-5). After a full-thickness periosteal flap is reflected, the periosteum must regenerate from the unreflected portion before this structure can participate in crestal bone healing. 26 Although adequate reflection to identify all bony landmarks is necessary, overzealous reflection increases soft tissue trauma and may delay bone callus formation around the implant, which may contribute to crestal bone healing. Figure 31-7 The mental foramen is approximately 22 mm from the midline; however, there is a slight variation, depending on race and sex. The facial flap is then reflected. The exact location of the mental foramina and associated nerves, arteries, and veins are identified (Figure 31-6). These are essential surgical landmarks. An anthropometric analysis of the position of the mental foramina was performed by Cutright et al. 27 The average distance from the midline was 2.2 cm, which means the usual distance between the foramina is 44 mm. Because at least 7 mm of mesiodistal bone are usually required for each 4-mmdiameter implant, the average anterior mandible has room for six implants. However, the mental foramina position is variable, depending on race, sex, and skull size. The average interforamen distance for black males was 45.8 mm; white males, 45 mm; black females, 43.8 mm; and white females, 41 mm. Therefore white females often do not have room for six implants between the foramina (Figure 31-7). When the mental foramen position is related to a tooth position, it may reside as far anterior as the canine to first premolar position, or as far distal as the second premolar to first molar root position The farther distal the foramen (and neurovascular canal), the farther distal the most distal implant (A and E sites) and the better the biomechanics for the prosthesis. When the foramen resides in its most distal position, six implants may be positioned between the foramina. When the foramen resides in its most anterior position, only four implants may be inserted to leave 3 mm of space between each implant. The most common mental foramina sites are between the first and second premolar roots, but they may be present below either the first or second premolar root. Under this condition, five implants may be positioned between the foramina. Because the primary incision was extended to the first molar region, the approach to the foramina can proceed from both anterior and posterior, which facilitates their identification. After the periosteum is reflected off the residual crest, a moist surgical sponge can be used to wipe the periosteum off the dense labial cortical plate around each foramen. This permits rapid and safe exposure of the nerve complex without trauma from instruments. Soft tissue reflection is limited in the mandibular symphysis region to avoid ptosis of

6 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion mm 4 mm A B C D E Figure 31-8 The facial full-thickness soft tissue fl ap is then refl ected. The mental foramen contents are identified, along with the crest of the anterior ridge, both anterior and posterior to the position of the foramen. The tie-back sutures may be positioned both behind and in the front of the foramen on each side. 2 mm No anterior loop 2 mm Anterior loop Figure The position of the distal implant between the foramen is affected by the presence of an anterior loop. The goal is to have the implant 2 mm in front of the mandibular nerve. When an anterior loop is present, the distal implant is placed farther mesial. Figure 31-9 An anterior loop to the mandibular canal occurs when the canal proceeds inferior to the foramen, passes under the foramen for 1 to 3 mm, and then curves superior and distal to exit the foramen. the chin, which may occur after overzealous reflection of the mentalis muscles lower attachments in moderately to severely atrophied mandibles. Tie-back sutures may be positioned to retract the facial flap during surgery. Most often these are placed in front of the mental foramen (Figure 31-8). All fibrous tissue is removed from the crest of the edentulous ridge. A bone rongeur, followed by a moist surgical sponge rubbed across the crestal surface, eliminates most fibrous adhesions. If the mandibular canal approaches the foramen at the same height, the nerve always enters the distal portion of the foramen. When this is clearly identified on the panoramic radiograph, the foramen is used as the distal landmark. The panoramic radiographs or CT images are observed to visualize a possible anterior loop of the mandibular neurovascular canal. A mandibular loop occurs when the mandibular neurovascular canal approaches the foramen from below, proceeds anterior to the foramen about 1 to 3 mm, and curves upward and distal to enter the medial aspect of the mental foramen (Figure 31-9). This has been observed almost 12% of the time for a distance of 1 to 3 mm on panoramic radiographs. 32 On the other hand, Rosenquist dissected 58 inferior alveolar neurovascular bundles and identified a 1-mm anterior loop in 43 cases. 33 A study on dry skulls noted anterior loops of 2 mm or more on 92% to 96% of the specimens. 34 Surgical dissection provided by Solar et al. 35 showed an anterior loop in 60% of mandibles ranging from 0.5 to 5 mm. Neiva et al. identified loops in 88% of cases with a range of 1 to 11 mm. 36 Dissection studies with comparison to radiographs were performed by Bavitz et al., 37 who reported 54% of loops in periapical radiographs, but only 11% where verified by dissection. Other studies concurred with the conclusion that a high percentage of false-positive or false-negative findings may occur. 38,39 For such a topography, the most distal anterior implants are placed 2 mm anterior to the anterior loop, which becomes the landmark rather than the mental foramen itself (Figure 31-10). It should be recognized that the inferior alveolar nerve complex is in the facial aspect of the bone as it exits the foramen. Therefore an implant placed in this region is most often lingual to the nerve, because the crest of the ridge is more medial than the foramen. As a result, even when a panoramic image may indicate that the implant is in the position of an anterior loop of the mandibular nerve, it most likely resides lingual to the nerve, with no paresthesia evident. Only CT scan imaging properly indicates the spatial relationship between these elements. On occasion, the nerve complex around the foramen is not clearly identified on traditional films. An anterior loop may or may not be present. A CT scan of the region may provide the information needed. Another option is to determine whether there is an anterior loop during surgery. A curved Naber s 2N probe may be gently inserted inside and along the bone on the distal half of the foramen to determine whether the nerve proceeds from the posterior region directly into the foramen. If no nerve entry is felt along the distal half of the foramen, the nerve must loop anteriorly and enter from the anterior aspect (Figure 31-11). Probing the anterior half of the foramen is not diagnostic, because the mental

7 690 IMPLANT SURGERY A B Figure A, Naber s 2N probe may be used to determine the presence of an anterior loop to the mandibular canal. The probe is gently inserted along bone on the distal one-half of the foramen. B, If no nerve canal entry is palpated, the nerve must enter the foramen from the anterior aspect and an anterior loop is present. nerve complex continues anteriorly from the foramen to innervate anterior teeth and bone, regardless of whether there is an anterior loop. Therefore there is always a canal entering the anterior portion of the foramen. The incisive branch of the inferior alveolar nerve continues within the bone from the position of the foramen to the midline, where it may have some crossover from the contralateral side. This nerve complex innervates the mandibular anterior teeth, but does not provide feeling to the chin or lower lip. Therefore it does not cause paresthesia, even when violated with a drill or implant. A score mark is made with a rotary instrument facial to the crest of the ridge to delineate the most anterior position of the mandibular nerve over each mandibular foramen. This score mark may correspond to the foramen or the anterior loop of the canal, whichever is more anterior. These marks position the boundaries of the posterior surgical implant placement. The most distal portion of the implant is ideally placed 2 mm anterior to this border to allow for surgical error. The bone width at the crest is then evaluated. If it is greater than 6 mm, the crown height of the prosthesis is less than the available bone height (crown/implant ratio <1), and the direction of load to the implant body is within 30 degrees, a Division A bone volume is present. The prosthesis type is considered. An RP-4 or RP-5 prosthesis requires a minimum of 12 mm of crown height space from the occlusal plane to the crest of bone to fabricate the prosthesis and superstructure bar. If the crown height space is insufficient, an osteoplasty is indicated. An FP-1 restoration requires between 9 and 12 mm crown height space from the occlusal plane to the bony crest. If the crown-height space is greater, and the patient demands an FP-1 prosthesis, an augmentation procedure may be required before surgical placement of implants. The available bone in the anterior mandible is often Division B, with similar crown-to-bone height relationships as Division A, but the residual crest bone is 2.5 to 6 mm in width, with the ridge usually more lingual. The surgical options are osteoplasty, augmentation, or small-diameter implants, depending on the prosthesis type and the force factors of the patient. The first premolar region, usually just anterior to the mental foramen (A and E positions), is often narrower than the other anatomical sites in the mandible. As such, special attention relative to width is noted in this region. A smaller-diameter implant may be necessary in this site. As a result, a distal cantilever may need to be reduced compared with when a regular or largerdiameter implant is used. This is also the area in which the anterior loop of the mandibular canal may be present. To its benefit, the bone angulation in this area is more vertical in relationship to the occlusal plane. An osteoplasty of the crest of the ridge is often indicated, especially in Division B bone. The osteoplasty eliminates any fibrous tissue on the crest and ensures adequate width for implant placement. A bone caliper is used to measure the amount of bone to be removed in height. This caliper may also ensure there is adequate height to place implants after the osteoplasty in Division B bone. The osteoplasty may be accomplished with a bone rongeur or a high-torque hand piece with a surgical 703 bur or a crestal reduction bur under copious cooled saline irrigation, until greater than 6-mm width of bone is obtained. If several milli meters of bone must be removed to obtain the necessary width, additional resorption is probable during healing. Countersinking the crest module of the implant slightly below the remaining bone crest is considered in these instances to account for this resorption and to prevent premature loading of the implant through the soft tissue borne interim prosthesis during healing. The prosthesis must replace the vertical bone height removed during the osteoplasty. If a fixed prosthesis is the final prosthesis, care is taken to carefully evaluate the amount of bone reduced. Whenever an osteoplasty

8 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 691 is performed, the bone is harvested and stored in sterile saline in case an autogenous graft is required during a later step of the surgical procedure. When an osteoplasty is performed, the complete anterior section of the mandibular crest is reduced, not just the implant locations. The soft tissue will not follow abrupt contour changes on the crest. Instead, the gingival tissues form a thicker tissue in these regions. If the osteoplasty is done irregularly, when the abutment is placed, a greater sulcus depth occurs in the thicker tissue regions with the related complications of the increased risk of anaerobic bacteria. In addition, if a cantilever is planned for the final restoration, the osteoplasty should extend beyond the anterior ridge, under the cantilever length. Otherwise, the superstructure will need to be raised above the higher bone contour in the posterior region, which may compromise artificial tooth or attachment positions over the bar. Box 31-3 Factors Critical to Minimal Heat During Osteotomy Preparation Cooling fl uid Bone quality Minimum volume of bone removed by each drill (incremental drill diameter) Drill: sharpness, design Frequency/time of contact Depth Pressure Speed Implant Site Preparation Rotary instruments are used to prepare the implant site. Regardless of implant design or manufacturer, several surgical concepts are crucial for initial rigid fixation. A primary criterion is to limit thermal or mechanical trauma to the surrounding hard tissues (Box 31-3) Bone is very susceptible to heat. 40 Heat is generated by the drill during the implant osteotomy. A major method to reduce the bone temperature during drilling is the use of cooled irrigation. 41,42 A secondary method to reduce heat and thermal trauma is related to the amount of bone removed with each drill. 41,45 Therefore sequencing drill diameters should prepare the final osteotomy site. The drill speed and time during which the bone is prepared is one of the major factors related to bone trauma. 48,49 Additional factors such as internal versus externally cooled drills are less important. 48,55 A detailed discussion of these factors is presented in the bone density a surgical approach (see Chapter 29). Implant Site Location Most often, five implant sites are planned between the mental foramina for mandibular overdentures or fullarch fixed prosthesis, and are labeled A, B, C, D, and E, because they do not always related to tooth positions. Regardless of the number of implants (i.e., 2 to 8) being placed at the initial surgery, all five sites are determined. This permits insertion of additional implants in the future (if needed) to upgrade a prosthesis type or to solve complications. A No. 6 round bur or sharp pilot drill marks the initial implant site location in the midline (C position), unless an FP-1 prosthesis is planned. If the intended prosthesis is FP-1, the center implant is placed off the midline toward the most distal foramen by 2.5 to 3 mm. Figure After the anterior mandible is refl ected and the crest prepared for implants, five implant sites are planned. The first site (C) is usually in the midline, the sites A and E are positioned 4 mm in from of the foramen, then the B and D positions are selected. Each site should be at least 7 mm apart from center to center. After the midline implant site (C) is determined, the boundary score lines on the crest of the ridge that correspond to the anterior limit of the mandibular canal are identified. A pilot hole is made with a No. 6 round bur or pilot drill placed 4 mm medial to each distal score line (if the implants are 4 mm in diameter) to determine the center of the most distal A and E implants. This provides 2 mm of surgical error away from the boundary that identifies the mandibular nerve during bone preparation and implant placement. The distance between the midline pilot hole and distal pilot holes are then equally divided for the B and D implants. They also can be mapped with a measuring tool. The distance between each implant center site should be at least 7 mm, or 3 mm between the 4-mm-diameter implant bodies. Larger-diameter implants require more than 7 mm between each center pilot hole (i.e., 8 mm for 5-mm-diameter implants) (Figure 31-12). The initial pilot drill is used at a depth of 5 mm and evaluates the density and thickness of cortical bone on the crest of the ridge, if any remains after the osteoplasty. It can also be used for initial bone preparation in the narrow ridge. This end-cutting bur can prevent lateral perforations because it will roll away under light pressure from the harder inner cortical plate and stay within the softer trabecular bone. A No. 6 bur

9 692 IMPLANT SURGERY or a Lindemann drill may also stretch a hole in any direction to improve its final position. For example, if the initial bur hole is too lingual, the #6 or Lindemann bur can be used in the initial site to create an oblong hole more facial. As long as the initial hole is less than 2 mm from the ideal site, the final osteotomy and implant will completely obtund the initial erroneous pilot hole. In this way, the ideal location is finalized before the initial osteotomy is begun. The surgical guide template is placed in position, and the faciolingual location and ideal angulation for the implants are determined. Ideal angulation is perpendicular to the plane of occlusion and corresponds to the cingulum of the teeth for a screw-retained bar or prosthesis, or to the incisal edge for cement-retained fixed prostheses. The template is then removed so that the facial and lingual contours of bone may be observed during site preparation. The center implant site (C) is prepared first with the 2.0-mm-diameter pilot drill for the initial depth of bone preparation of 9 mm (Figure 31-13). This position should be perpendicular to the occlusal plane. If one marginal crest of bone is higher than the other (usually Figure A 2-mm-diameter pilot drill is used to prepare the center implant site for 9 mm, perpendicular to the occlusal plane. the lingual), the osteotomy depth is measured from the most inferior edge such that the implant body is positioned slightly below the lingual crest of the ridge. A direction indicator (parallel pin) is placed into the center implant site (Figure 31-14). Angulation is checked in a labiolingual and mesiodistal direction with the surgical template, which may be facilitated with guidance from an assistant (preferably standing over the midline at the foot of the chair) (Figure 31-15). The osteotomy is often not prepared while the template is in place. The implant site must have adequate bone on the labial and lingual aspects. This requires direct vision and careful observation of both cortical plates while preparing the bone. A pumping motion is used to prepare the site to the radiographically determined depth; however, this dimension is reevaluated and may be changed at this time. The bone density is noted during the initial osteotomy preparation. The procedures described in this chapter are for D2 bone density. Threaded root form implants are provided in several preestablished heights, dependent on the specific system. The ideal length of a threaded implant should be between 12 and 15 mm long in D2 bone. Because most stresses occur at the crest in favorable bone density, after an optimum implant length is obtained for each bone density, rigidly fixated implants do not appear to gain further clinical advantage when adding only length. In natural teeth, the periodontal ligament allows stress dissipation away from the crest, yet their length does not increase from the front to the back of the mouth (even though greater forces are generated in the posterior regions). Similarly, the need for longer implants or implants that engage the opposing cortical plate with an implant in a two-stage healing process is not supported in the literature. After the initial sites are prepared with the pilot drill, small-diameter (2 mm) direction indicators are used in the initial implant preparations to evaluate the need for any minor adjustments (Figure 31-16). The surgical template is inserted to evaluate implant Figure A direction indicator (parallel pin) is inserted into the center implant site. Figure The direction indicator is evaluated for angulation and proper position.

10 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 693 Figure The pilot drill is used to prepare the initial sites, starting with the center, then A and E, then B and D, using the center parallel pin as a guide. Figure Small-diameter direction indicators are placed in the initial sites. A surgical template is inserted with the parallel guide pins in place to evaluate the future implant angulation and position. angulation and position (Figure 31-17). If the angulation is incorrect by less than 20 degrees, it can usually be corrected with the next drill in the sequence provided by the manufacturer. If the angulation is to be modified by more than 20 degrees, a side-cutting drill (e.g., Lindemann) rather than end cutting is most efficient. These side-cutting drills also can stretch the initial osteotomy laterally to improve the implant position. If the angulation is beyond 30 degrees to an axis perpendicular to the occlusal plane, or if the osteotomy is too lingual or facial in relation to the incisal edge, correction of the initial osteotomy should precede any further bone preparation. The oblong osteotomy prepared to correct the problem of poor position or angulation is obliterated and made round again with successive drills of incremental diameter. After the direction indicator confirms proper implant position, the osteotomy is made to the full depth of the planed implant size (Figure 31-18). It is usually better to drill 1 mm deeper than needed (i.e., 13 mm for a 12-mm implant) when the opposing landmark permits. If the opposing landmark is perforated before obtaining the desired length of the osteotomy, the osteotomy length before perforation is noted, and a shorter-length implant is selected. Ideally, the implant should not perforate the mandible to avoid irritation of the soft tissue, especially for a sharper apex implant design, where even slight perforation may irritate the tissues. If the implant has an open basket design, a perforation may trigger fibrous tissue ingrowth within the basket, rather than bone. On occasion, bleeding from the osteotomy site may be copious, especially in the midline site, even though no bone perforation has occurred. An artery enters the lingual plate below the genial tubercles in the midline, and an anterior branch of the mandibular neurovascular canal continues beyond the foramen. The surgeon should confirm that no perforation through the lingual plate or inferior border has occurred. The majority of Figure After the position and angulation is acceptable, the osteotomy sites may be made to the full depth of the desired implant size. time, this bleeding is of no surgical consequence. It is also not associated with any soft tissue paresthesia after surgery. If the bleeding is causing concern for vision or total blood loss, a collagen sponge may be placed into the site, along with a drill or direction indicator to help obtund the site. Elevation of the head and pulling the tongue out to reduce flood flow to the lingual artery is also of benefit until the platelet plug forms and reduces the bleeding. If the osteotomy is completed, an implant may also be introduced into the site and will crush the walls of bone and halt the bleeding process. The anterior mandible may present a significant undercut on the lingual aspect between the foramina. Life-threatening hemorrhage has been reported when a drill perforates the lingual plate of the sublingual region of the mandible and injures a sublingual or submental artery, especially in the canine region If perforation of the lingual cortical plate is associated with arterial bleeding, it is critical to identify its origin (Table 31-1). Bleeding in the floor of the anterior region of the mouth can originate from the lingual artery, facial

11 694 IMPLANT SURGERY Table 31-1 Treatment of Hemorrhage at Implant Osteotomy Site BLEEDING SITE DURING IMPLANT OSTEOTOMY ARTERIES TREATMENT Posterior mandible Mylohyoid Finger pressure at the site Middle lingual of mandible Submental Surgical ligation of facial and lingual arteries Anterior lingual of mandible Terminal branch of sublingual Compression, vasoconstriction, cautery, or or submental ligation Invading the mandibular canal Inferior alveolar artery Bone graft artery, or one of its branches. The submental artery originates from the facial artery, runs along the inferior border of the mandible, and meets at the midline. Pressure over the genial angle notch (just anterior to the masseter muscle) stops this type of bleeding. If the injured artery stems from the lingual artery, the bleeding will be reduced when the tongue is pulled out, thus placing pressure against the lingual artery where it crosses over the hyoid bone. Elevating the head is also of benefit. After the bleeding is reduced, it may be stopped by collagen sponges or hemostasis after clot formation, or rarely by surgical ties or electrocautery (see Complications). A critical step in obtaining rigid osseous fixation after initial implant placement is to maintain a rigid bone-implant interface without micromovement between the time of original placement and the second stage of surgery. The implant does not have to be submerged below the bone to achieve this result. The implant does not even need to be below the soft tissue to obtain this type of interface. 60,61 However, the clinician should recognize situations such that the higher the implant is placed above the bone, the greater the risk factors for trauma that may result in movement at the interface during initial healing. As a result, when an overlying soft tissue borne prosthesis is worn, it may be advantageous to countersink the implant below the bone. On the other hand, when a prosthesis abutment is added to the implant crest module below the bone, approximately 0.5 mm of soft tissue forms below the abutment-to-implant connection. 62 Therefore, if the crest module of the implant was below the crestal bone at the time of surgical placement, crestal bone loss will occur when the abutment is added. The implant surgeon should select the crest module position of the implant in relationship to the crestal bone on an individual basis. If the cortical bone and the overlying tissue is thick, and the patient has no parafunctional habits to apply stresses to the implant during healing, the implant and healing cover height are less critical and the platform of the crest module is often placed level with the crestal bone in more dense bone. Most often, the implant crestal module abutment connection region should be placed 0 to 0.5 mm below the bone level in D2 bone, rather than countersinking the implant several millimeters below crestal bone. As a result, for most external hex systems, the hex connection and healing cover remain slightly above the bone. If the overlying tissue is thin, the superior cortical plate was removed during an osteoplasty, or the remaining bone is of softer quality such as fine trabecular (D4), the implant and healing cover should be placed below the bone crest. As a result, the osteotomy must be 1 mm deeper before implant insertion to permit the crest module to be below the bone. As a result, the final bone osteotomy depth is variable by ± 1 mm for each patient condition. After the center site is initially prepared, the most distal implant sites are prepared next, using the direction indicator in the center osteotomy for parallelism in all planes. Direction indicators may then also be placed in the distal sites, and the implant surgeon proceeds to the intermediary implant site locations in a similar fashion. Direction indicators may also be used to tamponade profuse bleeding from an implant bone drilling site until the implant is placed. This technique is useful in maintaining adequate vision and decreasing surgical blood loss. If a bone drill becomes locked in the bone during preparation, the hand piece should not be wiggled back and forth to disengage the drill. This may increase the size of the bone preparation, cause injury and necrosis to the bone, or separate the drill above or below the bone. Instead, the rotation of the drill in the hand piece is reversed, or the drill is disengaged from the engine and rotated counterclockwise with a forceps. Drills of intermediate diameter are used next for the implant bone preparation. Gradual increases in drill diameter reduce the amount of pressure and heat transmitted to the bone, especially in presence of dense and thick cortical bone. Lavelle has shown the heat generated from a drill can be transmitted more than 3 mm away from the osteotomy site and reach temperatures higher than 50 C under copious irrigation. 50 The 2.0-mm initial drill is followed by a 2.5- mm drill, and then a 3.0- or 3.4-mm drill (Figure 31-19). Some manufacturers do not use an intermediate drill; however, a decrease in the heat and trauma generated is found with the intermediate drill. 53 The softer the bone, the fewer the number of intermediate drills

12 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 695 A Figure The final drill diameter is the most critical for the osteotomy, because the bone next to this drill will be in direct initial contact with the implants. B Figure A sequence of osteotomy drill preparation is dependent on implant diameter and design, but often uses a 2.0 mm initial drill, followed by 2.5 mm, 3.0 mm and 3.4 mm as a final dimension. needed. Direction indicators are placed in bone sites medial or lateral to implant sites being prepared. Minor corrections in angulation should be accomplished during these intermediate drilling steps. The final drill diameter is used to prepare the implant receptor site under copious irrigation (Figure 31-20). This step is the most critical for initial implant insertion and healing. The bone surrounding this drill will be in direct contact with the implant. When the final drill preparation is not precise, the implant-bone region may be incongruent and gaps may decrease initial stability, which may lead to early failure. A lesser initial contact with the host bone also decreases the percentage of new bone-implant contact formation. 54 Therefore a constant pressure and direction is used with the final drill to ensure a precise, round osteotomy is prepared along with direction indicators in adjacent sites to maintain the proper angulation. A summary of potential problems during implant osteotomy preparation and suggested solutions is presented in Table The crestal collar region of any system, regardless of dimension, should be placed in contact with crestal cortical bone. Many implants present a wider crest module design than the implant body. When the Figure A crest module drill is often used in the anterior mandible, because cortical bone is present on the crest. implant is placed below the bone, these systems may need an additional step in the bone preparation, with a crestal countersink drill to increase the osteotomy diameter only in this region (Figure 31-21). If there is no marginal cortical bone, and especially if the trabecular bone is soft, the implant is inserted without a crest countersink drill procedure. This compresses the marginal rim of bone and adds rigidity to the implant during initial healing. 63 If a threaded implant system has been selected to support the prosthesis, the clinician should decide whether or not to tap the osteotomy before implant insertion. Very dense bone (D1) requires a bone tap for the entire implant height. D2 bone may also require a bone tap, especially if the implant is made of a softer grade of titanium. With thick inferior cortical plates at the apex of the osteotomy, a bone tap is also required for the entire height. If only the superior cortical plate is dense and provides initial rigidity of the implant, the bone tap is used only in this section. Studies concentrating on threaded implants inserted with and without a bone tap have shown that implants with precut threads in dense bone achieved a higher

13 696 IMPLANT SURGERY Table 31-2 Possible Problems Encountered during Root Form Surgery and Suggested Solutions PROBLEMS SOLUTIONS Improper pilot hole/location Create a new pilot hole with round bur No. 6 or stretch the existing hole with a No. 6 drill or a side-cutting drill (e.g., Lindemann). Improper drilling angulation Start a new direction with the pilot drill and 1.5-mm drill and continue with remaining procedures. Use a side-cutting drill to redirect the osteotomy. Use the assistant/ doctor force direction indicators or template to regularly check direction during the osteotomy preparation. Elliptical/eccentric preparation Continue the surgical procedures and try to use the planned implant. If the implant is loose on seating, use a wider-diameter implant if possible. If not, pack the osteotomy with autogenous graft, compress it, and place implant again. If the angle with the adjacent implant is greater than 30 degrees, remove the implant and allow the surgical site to heal for approximately 6 months. Place the implant at a later date. D1 bone density Prepare osteotomy with a constant pumping motion to avoid overheating and use higher torque and speed. Use new drills and profuse irrigation with incremental drill sequence. Use bone tap with hand ratchet. D3-D4 bone Push the drill as it rotates at decreased speed. Do not bone tap. Insert implant with hand piece. Perforated buccal or lingual bone plates Stop drilling as soon as less than 1.0 mm of bone is lateral to the drill. Use an osteotome with a tapered end to expand the osteotomy, rather than removing the bone with a drill. Threads exposed at crest of ridge If implant threads are exposed coronally, graft autogenous bone harvested intraorally with rongeurs or a trephine bur over the region. Figure A bone tap is used at less than 30 rpm with a high torque hand piece or hand ratchet and copious irrigation in D1 or D2 bone before implant insertion. The bone tap may be used selectively only in the crestal region, when the underlying trabecular bone is soft and the implant does not engage cortical bone at the apical region of the implant. percentage of bone-implant contact during the healing phase, 64 and greater bone trauma was identified with the self-tapping implant. In the cases of a fine trabecular bone, especially in the absence of crestal cortical plate, a bone tap is contraindicated because this decreases the initial stability of the implant. 63 Whenever the bone tap is required, it should be used to prepare the bone at very slow speed (less than 30 rpm) with copious amounts of irrigant. This may be accomplished with a hand ratchet or a slow-speed, high-torque, contra-angle hand piece (40:1 to 100:1 reduction) (Figure 31-22). It is imperative that the angulation and light vertical pressure used on the ratchet or hand piece follow the same path as the final drill preparation for the entire depth. Otherwise, the tap may inadvertently stray in the trabecular bone and compromise the final implant placement. The direction indicators in adjacent sites are very beneficial in helping to maintain the correct angulation. A hand ratchet rather than a high-torque hand piece should be used in D1 bone to prevent hand piece breakage or wear. The hand position of the surgeon is important in maintaining constant force and direction on the hand ratchet or hand piece during the threading process. When using a ratchet, one hand holds the ratchet while the thumb of the other hand is placed directly over the tap, the index finger retracts the lip for improved access and vision, and the middle finger is placed under the mandible in the direct path of the osteotomy. Care must be taken to let the handle of the ratchet rotate around the insertion pin, not pull or lever the pin and connected tap as it rotates. Using a hand piece to tap the implant site in D2 bone or the apical end in a D3 site presents some advantages, such as a relative ease of constant direction, especially in softer bone. The bone tap should be cleaned with saline before each new site is prepared.

14 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 697 Figure The high-torque hand piece is used to thread the implant into the anterior mandible at less than 30 rpm. Figure Five anterior implants have been threaded into position between the mental foramina, and two implants are inserted over the top of the foramen. Figure The hand piece insertion of the implant occurs after the osteotomy has been suctioned, to remove any debris or fl uid, which may create hydrostatic pressure or friction during the insertion process. Figure The first-stage cover screw is inserted into the implant body. This prevents soft or hard tissue growth over the implant platform during healing and protects the antirotational hex for abutment connection. Implant Placement The implant sites are flushed with saline to remove any debris and suctioned to reduce hydrostatic pressure during implant insertion. The implants are now ready to be threaded into position at a speed similar to the bone-tapping procedure (Figures 31-23, 31-24). The direction-indicator pins may still be needed in adjacent sites to guide the implants in the correct angulation in soft trabecular bone. The implant surface has been prepared by the manufacturer to be clean and uncontaminated. Handling the implant body is poor protocol and may alter the surface chemistry. Instead, the implant is placed directly into the implant site by means of a preattached insertion mount. Some manufacturers require the surgical team to attach the implant mounts before placement, but this procedure complicates the process with no benefit. After the first threads of the implant body engage the bone, the same speed used to tap the bone is used to insert the implant. The surgeon s hand position is identical to that maintained during the bone tapping procedure. Sterile saline may be used after the implant has begun its descent into the implant osteotomy, but is not suggested. The patient may attempt to swallow as a result of the irrigant and during this process may close and change the angulation of the implant. The hand ratchet is used at the end of implant insertion to check the final stability of the implant body. The surgeon should not overtighten the implant in place. Excessive tightening of the implant in its final position may cause microfractures and compromise the entire implant thread-to-bone profile and interface development. When in doubt, the implant should be slightly unthreaded so as to decrease pressure at the devel oping interface. The marginal cortical bone should seal the implant periphery without voids or cracks. If any defects are present, the bone chips harvested from the drills or from an osteoplasty are used to fill any deficit (Figures 31-25, 31-26). On rare occasions, the implant may not be rigid after implant placement. A mobile implant may not heal predictably with a direct bone interface. The implant may be fixated by inserting it deeper in the osteotomy or by the placement of a larger-diameter implant. In addition, the lateral cortical plates may be compressed with a mallet and blunt instrument to compress the bone against the implant. The surgical decision to

15 698 IMPLANT SURGERY obtain rigid fixation is based on the width and height of bone available. The internal aspects of the implant bodies are thoroughly irrigated to remove any blood or debris before the placement of the cover screw. The first-stage cover screw is then inserted into the implant body. An antibiotic paste may be placed on the threads of the cover screws before their insertion. In theory, this paste helps seal the cover screw implant body connection and decrease the risk of bacteria growth within the implant body during healing. However, no clinical study is available to confirm this hypothesis. The cover screw is threaded to final position, then loosened and tightened again. The implant should not rotate during this process. When the cover screw is threaded into position, care is taken not to overtighten it. The stress transmitted may rotate the implant body and also complicate the cover screw removal at the second stage of surgery. Soft Tissue Adaptation The crestal bone is evaluated for any sharp edges, and a rongeur or bone file is used as indicated. The surrounding bone is also evaluated, because final soft tissue contour is directly dependent upon its topography. The surgical site is thoroughly irrigated with saline to remove any debris or bone fragments. The lingual retraction sutures are cut and removed. The tissues are approximated without tension, and a 3-0, resorbable, horizontal mattress interrupted suture is placed at the midline. An interrupted horizontal mattress or a continuous suture is placed on the distal aspect of the midline suture. It is important that the labial periosteum be joined to the lingual periosteum. Surface epithelium should not contact the periosteum along the suture line, because this may result in delayed healing and incision line opening. The sutures should be placed approximately 3 to 5 mm from the incision line, and 3 to 5 mm apart. A firm pressure is applied to the reflected soft tissue for 4 minutes after the suturing is completed. This helps prevent bleeding under the soft tissue flaps and helps the tissue adhere to the bone, because fibrin released from the blood is natural tissue glue. A postoperative panoramic radiograph is taken at the conclusion of the procedure. Because the anterior implants and primary healing covers are often placed after an osteoplasty, the labial flange of a removable prosthesis is overextended and needs to be shortened. The internal surface of the prosthesis is aggressively relieved 3 to 5 mm. A soft tissue conditioner replaces the acrylic. The tissue conditioner is also relieved over the implant sites so that the prosthesis is not in contact with the soft tissue directly over the implant sites. The patient may wear the denture during the healing process; however, the less it is worn, the less likely premature loading of the implant will occur. Incision line opening is a possible consequence of wearing the denture during the first 2 postoperative weeks; however, this is rarely a problem if the denture is properly modified and the patient wears it primarily for social encounters. The patient should not use denture adhesives or other products that may enter the incision line. The denture should not be worn at night, and the patient is instructed to coat it with petroleum jelly or soak it in water to maintain the pliable nature of the conditioner. SURGERY IN THE POSTERIOR MANDIBLE DIVISION A There are 20 million Americans missing all of their mandibular teeth, and 40 million partially edentulous people are missing all their posterior teeth in one or more quadrants (Figure 31-27). Therefore 60 million people are candidates for implants in the posterior mandible. A primary indication for root form implants is the Division A posterior mandible. Dental implants are rarely inserted in the mandibular second or third molar location, and the first premolar is anterior to the mental foramen. Therefore the posterior mandible is primarily concerned with the second premolar and first molar region. On occasion, surgery in the posterior region of the mouth is complicated by a small crown height space, especially when opposing teeth are present. The implant drills and hand piece require a space of at least 20 mm to the bony crest, so the osteotomies may be prepared with proper angulation. In addition, the implant and insertion tools often require at least 20 to 25 mm of space. This may be most difficult in the second molar region because the amount of space decreases posteriorly. If opposing teeth have extruded, the occlusal plane should be corrected before implant placement to increase the crown height space for surgery. On Figure The posterior mandible is often missing multiple adjacent teeth.

16 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 699 occasion, a shorter-length implant may be indicated to reduce the crown height space needs of the surgery. The posterior mandible has greater initial width than the anterior mandible. As such, Division A bone is more commonly found early after tooth loss. The Division A bone has a height measurement that is determined from the crest of the edentulous ridge to the opposing landmark of the mental foramen or mandibular inferior alveolar nerve, artery, and vein. The ideal bone height includes 2 mm of surgical safe zone above the landmarks to decrease the risk of paresthesia by inadvertent violation of the structures. Therefore a 12-mm-long implant requires 14 mm of available bone height and a 9-mm-long implant requires 11 mm of bone height. The ideal length of an implant in the posterior regions is 12 mm or more; however, 9 mm has proven predictable when implant width, number, or design is also considered (Figure 31-28). 65 After the resorption of the edentulous ridge has evolved into Division B, it may last for more than a decade, so this condition is also often observed. Unlike the anterior mandible, the posterior mandibular Division B bone is less likely to be modified to Division A by osteoplasty, because the mandibular canal is on average 12 mm above the inferior border of the mandible and rarely permits enough crestal bone reduction to increase the width and still have enough height of bone for the Division A criteria. Therefore bone augmentation in Division B posterior mandibles is often warranted. Many aspects of the surgical approach are similar to root form surgery in the anterior mandibular region. However, because the mandibular canal is the opposing landmark, rather than a thick cortical plate such as in the anterior regions, attention is specifically brought to the neurovascular complex. The study models of the patient are duplicated, and a diagnostic wax-up of the final prosthesis is made. This step is important in determining the position of the Figure A panoramic image of an edentulous posterior mandible, missing both premolars and molar. There is abundant bone height above the mandibular canal. Most often, the position of the mandibular canal may be observed in the first molar site. abutment posts for the prosthesis (including the plane of occlusion, the height of the abutment for cement, and whether it is adequate for long-term cement retention), the need for pontics, the angulation of the post in relation to the abutment teeth, and the mesiodistal position of the implant. The study models, diagnostic wax-up, panoramic radiograph, and other diagnostic images are all available at the time of surgery. The patient is premedicated, as suggested for any implant surgery, and anesthetized using an Akinosi block with both lidocaine 2% with 1:100,000 epinephrine and a long-acting anesthetic. Postoperative discomfort is reduced with long-acting anesthetic agents. The patient is prepared for surgery with a preoperative intraoral and extraoral Betadine or chlorhexidine scrub. The surgical team also scrubs and gowns in the standard aseptic method. SOFT TISSUE REFLECTION The surgical region is checked for regional anesthesia with local infiltration of 2% lidocaine with 1:100,000 epinephrine. Local infiltration also provides hemostasis and improved visibility during the soft tissue incision. In addition, when the local anesthetic is injected on the crest of the ridge and under the periosteum, a hydroelevation of the tissue may help the initial tissue reflection. The primary incision is made on the crest of the ridge from the retromolar papilla (at the base of the pad) to the anterior tooth abutment. If there is 3 mm or more of attached gingiva on the crest of the edentulous ridge, the incision bisects this tissue. This places half of the attached gingiva width on each side of the cover screws or low-profile permucosal extensions. If there is less than 3 mm of attached keratinized tissue on the crest, the incision is made more lingually so at least 1.5 mm of the attached tissue is placed to the facial aspect of the implant abutments. In this way, the buccinator muscle is separated from the incision line and does apply tension and cause opening of the incision line during the initial healing, and gingival complications are reduced over the long term. The crestal incision is often continued in the sulcus and interproximal of the posterior teeth to the ipsilateral canine (Figure 31-29). A secondary incision is made lateral to the retromolar pad and up the ascending ramus, to the height of the occlusal plane, and just below the two attachments of the temporalis muscle (Figure 31-30). On rare occasion, the buccal artery crosses over the ramus in this region and may be severed during the vertical release incision. This artery is a branch of the maxillary artery. Therefore a hemostat is readily available and clamps the soft tissue on the lingual aspect of the flap if excessive bleeding is noted. After 10 minutes, the hemostat may

17 700 IMPLANT SURGERY Figure The posterior incision line is made on the crest of the edentulous ridge and often extends to the distal of the canine. Figure The distal release incision extends lateral to the retromolar pad, along the ascending ramus. A B Figure A, The lingual fl ap is first refl ected off the crest. B, The facial fl ap is refl ected and, when a computed tomography scan is not available, usually exposes the superior aspect of the mental foramen. be released and the bleeding will be arrested. Usually a secondary vertical release incision is also made just distal to the mandibular canine (which includes the distal papilla), which is always anterior to the mental foramen. The lingual flap is first reflected only a few millimeters beyond the lingual crest of the ridge. Almost all concavities in bone of the posterior mandible occur on the lingual aspect in conjunction with the submandibular fossa. Therefore the region should be palpated and evaluated with a CT scan when available before osteo tomy preparation. A full-thickness periosteal flap is then reflected on the facial, exposing the underlying crest and lateral surface of the edentulous mandible in the region of the foramen (Figure 31-31). When a CT scan of the region is not available and intraoral radiography is not easily present, the facial reflection should expose several millimeters of the lateral aspect of the edentulous ridge and the superior portion of the mental foramen. By exposing this area, the implant surgeon is less likely to inadvertently perforate the bone, which would result in fibrous tissue formation next to the implant in the region. In addition, the mental foramen is almost always identified in mandibular posterior surgery to determine the zone of safety above the mandibular canal when a CT scan is not available before surgery. 32 The retromolar papilla should be elevated to the lingual to expose the underlying bone when implants may be placed in the mesial aspect of the second molar. The bone under the papilla in this area is usually more than 5 mm wide and 5 mm in length. The region is palpated just inferior to the retromolar pad to identify the submandibular anatomic landmarks. Severe undercuts inferior to this region are typical. As a result, root form implants are rarely placed under the papilla, but its reflection improves vision and access in the mesial half of the second molar area. MANDIBULAR NERVE LOCATION: CLINICAL ZONE OF SAFETY The risk of mandibular canal penetration during endosteal implant placement in the posterior mandible is a serious concern. At the time of surgery, intrusion into the mandibular canal results in an increased risk of hemorrhage, visibility impairment, and increased

18 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 701 potential of fibrous tissue formation at the surface of the implant. More important, the patient experiences altered nerve sensation in the form of anesthesia, paresthesia, or hyperesthesia. This may affect the patient s lifestyle during eating, kissing, and applying lipstick and is a common cause for litigious action against the implant dentist. Consequently the surgical boundary is often set conservatively 2 mm above the mandibular canal to establish a surgical zone of error Procedures to Identify the Mental Foramen A common approach to identify the existing available bone above the inferior alveolar canal is to start at the foramen, because this is the location in the posterior mandible where the reduction of the vertical height of bone begins. Compared with the anterior mandible, where the opposing landmark is the inferior border of the mandible, the posterior mandible is limited by the foramen and inferior alveolar complex. The anteroposterior position of the mental foramen is variable and may correlate as far forward as the apex of the first premolar to as far distal as below the mesial root of the first molar. In a study by Cutright et al., the mental foramen location variance was different for white and black populations (Figure 31-32). 27 Mandibles for whites had the foramen farther forward, with 5 of 76 mandibles being under the first premolar (6.6%), 24 between the premolars (31.6%), 39 below the second premolars (51.1%), and 8 between the second premolar and first molar root (10.5%). Mandibles of black people had the foramen between the premolar 11 of 78 times (14.1%), below the second premolar 41 times (52.6%), between the second premolar and first molar 25 times (32%), and below the mesial root of the molar 1 time (1.3%). Therefore the average foramen of white people was below or in front of the second premolar (89.3%) and the average foramen in blacks was below or distal to the second premolar (85.9%). The mental foramen vertical position is usually found more coronal and facial than the mandibular canal (Figure 31-33). Agthong et al. stated the foramen was 14 to 15 mm from the inferior border. 66 Neiva stated the position was 12 mm (range, 9 to 15 mm) from the lower cortex of the mandible. 36 Fishel et al. noted in a white population, the foramen was coronal to the apex of the first premolars 38.6% of the time, 15.4% at the apex, and 46% apical to the apex position. The position of the foramen in relation to the second premolars was 24.5% coronal, 13.9% at the apex and 61.6% apical. 29 Therefore the vertical position of the foramen is variable. On occasion, more than one mental foramen may be present. However, this appeared to occur less than 2% of the time in whites and 5.7% in blacks. 67 Jacobs et al. evaluated panoramic radiographs and could observe the foramen 94% of the time, but only 49% were clearly detectable. 68 Fishel et al. found that Zone White Black 1 6.6% 0% % 51.1% 10.5% 0% 14.1% 52.6% 32% Figure The mandibular foramen may extend as far forward as the first premolar to as far distal as between the second premolar and first molar roots. It is often correlated to sex and race. Figure The mandibular foramen is usually coronal and facial to the position of the mandibular canal. The foramen is cone shaped, with the outer rim larger than the inner dimension of the foramen. even the mental foramen was only visible on approximately 50% of periapical radiographs. 29 On the other hand, Yosue and Brooks 69 showed that the mental foramen could be identified on a panoramic radiograph 87.5% of the time. However, only 64% of the radiographs evaluated clearly showed the accurate position of the foramen. These same authors evaluated a panoramic x-ray and the actual skull in four subjects. The panoramic radiographs illustrated the mental foramen in the correct position less than 50% of the time, and often it was significantly closer to the inferior border of the mandible than its actual position. In addition, the appearance of the foramen varied with the positioning %

19 702 IMPLANT SURGERY of the mandible in relationship to the focal length used while obtaining the radiograph. 69 Conventional tomography and CT are usually more diagnostic as to the position of the mental foramen. 28,70,71,72 Procedures to Identify the Mandibular Canal The available bone height over the mandibular canal is critical to determine whether adequate bone is present to place root form implants. The most common methods to assess this dimension are periapical radiographs, panoramic radiography and CT images with or without reformatted images. Several clinical approaches to the posterior mandible have been proposed using radiographic techniques. Usually the inferior alveolar nerve proceeds anteriorly, inferiorly, then horizontally and laterally below the apices of posterior teeth and curves superiorly to exit at the mental foramen. It is reported to never transverse lower than 6 mm below the height of the foramen. 73 In the region of the first mandibular molar, the mandibular canal is often difficult to identify on a periapical or panoramic radiograph. A cortical lining of the canal is not always present in the molar region. The angulation of the bone in the posterior mandible progressively evolves from almost vertical in the premolar region to 15 degrees in the first molar and 25 degrees in the third molar region. A periapical radiograph cannot be parallel to all three of these planes. In addition, the floor of the mouth prevents the periapical x-ray from being ideally positioned in relation to the edentulous ridge. As such, foreshortening or elongation of the image is expected. Because the x-ray beam of a panoramic radiograph originates from below the patient s mandible, the location of the canal in reference to the crest of the ridge is dependent on the buccolingual position of the mandibular canal. If the nerve canal proceeds along the lingual aspect of the mandibular body, the canal will be projected more superior toward the crest. When the nerve canal proceeds toward the foramen on the buccal of the mandibular body, it will be projected more inferior in relationship to the crest. In other words, the distance from the crest to the mandibular canal may be the same in two different patients, but appear on the panoramic films as two different positions (Figure 31-34). Despite obvious limitations, a panoramic radiograph is often used as the initial method to determine the height of the bone available over the mandibular canal. Most often, a panoramic radiograph can identify the entry of the mandibular inferior alveolar nerve into the ramus, because the lingula is cortical bone and the canal has a cortical lining, surrounded by softer trabecular bone. The mental foramen is also able to be identified on most films. Although a bifid canal has been observed, its occurrence is less than 1%. 74 L B A When present, the canal that exits the mental foramen is the opposing landmark, because sensory nerves that do not innovate soft tissue are of little concern for paresthesia. After the panoramic radiograph is observed and the bone height over the radiographic canal directly measured on the radiograph, the patient may be placed in one of three categories: (1) there is obviously enough radiographic bone height over the canal and foramen (>15 mm), (2) there is obviously not enough height over the landmarks (<9 mm), or (3) the adequate bone height is questionable. The magnification of the radiograph is initially ignored to determine in which of the three categories the available bone height is placed. A 25% manufacturer-supplied magnified image of the implant body may be placed over the radiograph to help select the desired size of implant to be inserted in the abutment positions. This magnified image is not accurate, but may help to determine whether the patient obviously has enough bone. These magnified images often prove imprecise in marginal cases because panoramic radiographs do not have a uniform magnification rate. Additional variables such as the patient s head position may significantly alter magnification by as much as 55%. In addition, at the time of surgery, the crest of the edentulous ridge is often modified by osteoplasty to increase the width of crestal bone. The premeasured bone height is reduced by a dimension that may be difficult to assess. A study by Sonick et al. compared the location of the mandibular canal with panoramic and CT radiographs of a cadaver mandible. 72 The average distortion of the panoramic image was 3.0 mm (range, 0.5 to 7.5 mm) and the CT average was 0.2 mm (range, 0 to 0.5 mm). Therefore the panoramic method of mandibular nerve location would be limited to obvious extremes of abundant or inadequate bone. The cortical lining of the mandibular canal is often absent in the first molar region and even when present may be confused with a vascular region surrounded by B B A X-ray film Figure Although the distance from the crest of the ridge is similar in both of these nerve locations, when the canal resides more buccal in the mandibular body, it appears more apical on the x-ray film.

20 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 703 Figure A computed tomography reformatted image of a mandible allows the identification of the mandibular canal in several planes. Therefore the width of available bone above the nerve, the height of available bone, and the angulation of the bone may be determined. trabecular bone. As such, tomograms or an individual CT image may not easily determine the position of the mandibular canal. A computer-generated tomogram series of reformatted images represents an improved diagnostic tool for identifying bone width, height, and mandibular canal location (Figure 31-35). Therefore when the diagnosis is unclear, the use of a CT scan and the reformatted images into a computer software package (i.e., Materialise, Implant Logics) is a considerable advancement in diagnostics for the location of the mandibular canal. Using a CT scan is most indicated when the panoramic radiograph gives an uncertain answer relative to the available bone in the region. When questionable bone height is present, the CT scan is indicated to determine a more exact canal position and whether an implant may be inserted above the mandibular canal. A surgical approach without block anesthesia has been suggested to allow implants placement above the canal in questionable ridges. 75 In this technique, local anesthesia is achieved by infiltration into the surrounding soft tissues. The bone is then prepared without anesthesia, because few sensory nerve fibers exist in this region, except those of the mandibular canal. The patient is instructed to notify the surgeon if sensation or discomfort is experienced, which theoretically correlates to the rotary instrument approaching the mandibular nerve. However, the infiltration surgical approach presents several disadvantages. With this approach, sedation must be minimal to enable the patient to communicate intelligently during the procedure. The misinterpretation of a pressure phenomenon or bur vibration raises the patient s and doctor s anxiety regarding nerve encroachment. Additional sensory mandibular nerve pathways are often present within the bone. These nerves innervate teeth but little or no soft tissue of the face. Apprehension or misinterpretation by the doctor or patient with this technique can result in the selection of a short endosteal implant of lesser bone-implant surface area. This condition may lead to greater bone loss and failure and is not recommended. The placement of endosteal implants medial to the mandibular canal has been reported. 76 However, the position of the canal in a transverse plane is not reliable. The inferior alveolar nerve enters the medial aspect of the ramus above the lingula and exits on the lateral aspect of the mandibular body at the mental foramen. The crossover from medial to lateral aspects usually occurs in the third to second molar region, but this is highly variable. Attempting to place an implant medial or lateral to the canal as a result of an average anatomic location or, more recently, with information from the CT image has been suggested This technique requires precise control of the angulation of penetration at the crest of the edentulous ridge. Surgical error of a few degrees may result in perforation of the canal. Visualization of a few degrees is most difficult in the posterior regions of the mouth when observing the mandible from the lateral aspect of the patient. As a result, the general rule is to place the implant above the mandibular canal. ZONE OF SAFETY The zone of safety is defined as an area within the bone that can safely support implants without fear of impingement on the mandibular neurovascular bundle. A zone of safety for the placement of posterior mandibular endosteal implants was established by Misch in 1980 by the evaluation of 530 panoramic radiographs of partially edentulous patients. In 1989 this evaluation was confirmed by Misch and Crawford with an additional 324 consecutive panoramic radiographs. 32 The zone of safety is a clinical method to assess a safe zone to insert endosteal implants. It is used when a CT image does not confirm the presence of adequate available bone height. Therefore when periapical or panoramic images are the only diagnostic tool to assess available height and abundant height seems present (>15 mm), two treatment methods may be used: 1. A 2-mm pilot drill prepares an implant site 9 mm deep. A 9-mm-long force direction indicator is positioned in the osteotomy, and a periapical radiograph is made. The indicator is evaluated in light of the opposing landmark, and a decision is made to remain at 9 mm or reprepare the site to 12 mm deep. 2. The mental foramen is reflected and the safe zone is established before drilling the initial osteotomy depth. The zone of safety is determined either on panoramic radiographs or clinically during surgery as follows. Line A is drawn parallel to the posterior plane of occlusion, at the level of the residual crestal ridge and at the sites of implant placement. Line B is drawn at the most superior aspect of the mental foramen parallel to line

21 704 IMPLANT SURGERY Inferior alveolar nerve 43% 97.5% 5.5% 100% Line A Zone of safety Line B inferior and anterior to the position of the foramen, then loops superior and distal to reach the foramen. An anterior loop was observed in 12% of the panoramic radiographs and up to 3 mm in length anterior to the foramen. 32 Therefore when this condition exists, first premolar implants should usually be placed above the mental foramen position. Mandibular neurovascular canal Mental foramen Figure Mandibular zone of safety. Draw an imaginary line parallel to the plane of occlusion at crest of the posterior ridge (A) and another parallel line at the top of the foramen (B). The height of bone between these lines is the zone of safety to the mid first molar region. A. Lines A and B are joined with a perpendicular line C. The length of line C is the safe zone measurement to the mesial half of the first molar (Figure 31-36). The results of studies by Misch and Crawford 32 indicated the prevalence of the mandibular canal below the second line (B) on radio graphs, using the landmark of the mental foramen. A zone of safety was observed 100% of the time mesial to the middle of the mandibular first molar. The most common position of the mandibular canal was 2 mm or more inferior to line B (drawn from the top of the foramen). Therefore the area within the zone of safety is above the mandibular canal and allows a surgical safety zone that most often approximates 2 mm. In the region of the distal half of the first molar, the mandibular canal was below line B (within the safe zone) in 97.5% of radiographs. In the region of the mesial half of the second molar, the mandibular canal was below line B in 43% of the radiographs; in the distal half of the second molar, it was only 5.5% (Table 31-3). Because implants are not often placed in the second molar area, the zone of safety has proven a useful clinical tool. The position of the foramen is also an anatomical landmark for mandibular implant placement in the premolar region. Because the foramen is most often found distal to the first premolar, implants are often considered safe to place in the first premolar region, just anterior to the foramen. An anterior loop of the mandibular canal occurs when the mandibular nerve proceeds Safe Zone Procedure The zone-of-safety technique is used before and during surgery for endosteal implant placement in the posterior mandible, when a CT scan was not used to deter mine the height of available bone and when a panoramic radiograph clearly indicates enough bone is present above the canal (>15 mm). First, radiographic estimates of vertical bone height are made by measuring the distance between lines A and B in the safe zone. Correction of this measurement (from magnification) may be accomplished by using calibrated balls or wires and using ratios of magnification or an arbitrary 25% magnification. The approximation of the available bone height is used to develop the treatment plan. An implant height may be initially determined, with the understanding that the safe zone is actually measured at the time of surgery and may be different because of osteoplasty or incorrect magnification allowance. If the final estimates of available bone height are less than 15 mm to the canal, it is suggested a CT scan be obtained before surgery to determine precise mandibular nerve position in this region. Bone Preparation If the edentulous ridge is narrower than 6 mm, an osteoplasty is performed under copious irrigation, as long as the remaining height of bone is greater than 12 mm above the foramen position. As a result of the osteoplasty, the crest width is increased, but the anatomic landmark of the mandibular canal is now closer to the remaining crest of bone. The ideal implant position is level with both cortical plates. Bone remodels to the height of the lowest cortical plate. The height of the available bone is then determined, forward of the mesial half of the first molar. An imaginary line A is drawn parallel to the Table 31-3 Mandibular Canal Location: Posterior Mandible* MESIAL FIRST DISTAL FIRST MESIAL SECOND LOCATION MOLAR MOLAR MOLAR DISTAL SECOND MOLAR (100%) 316 (97.5%) 149 (43%) 18 (5.5%) (1.8%) 166 (51%) 34 (41%) 0 2 (<1%) 8 (5.5%) 172 (53%) +, Within zone of safety; 0, below zone of safety but not touching canal;, touching or below top of canal. * Data from 324 consecutive panographic radiograph measurements.

22 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 705 occlusal plane on the crest of the residual ridge after osteoplasty. Another parallel imaginary line B is established at the superior aspect of the mental foramen. The height of the available bone in the second premolar and mid-first molar region corresponds to the distance between these lines and represents the maximum height of the endosteal implant in this region (Figure 31-37). The radiograph is then evaluated to confirm and extend the zone of safety beyond the mid-first molar region. If the canal appears to remain at the same distance as the zone of safety in the distal of the first molar region (97.5% of the time), the safe zone measurement is expanded to the distal root region. If the canal appears to rise 2 mm or more in the mesial of the second molar region (about 60%), the safe zone measurement is reduced accordingly in this region. The distal half of the second molar region is almost always reduced from the safe zone measurement (94% of the time) and usually is at least 4 mm less than the safe zone height. The osteotomy preparation may be started if adequate bone height is present. It is recommended to err on the side of safety in the region past the mesial half of the first molar, using the panoramic radiograph as a guide. This clinical approach has been used successfully over two decades in thousands of surgical procedures without paresthesia. The premolar site implant osteotomy may proceed directly over the foramen to the same safe zone measurement without the implant osteotomy impinging on the nerve, based on three anatomical considerations. Although the crossover position of the mandibular canal from the medial aspect of the ramus to the lateral area of the body of the mandible is variable and may occur anterior to the second molar region, the mandibular nerve always exits the mandible facially at the foramen. Therefore the implant osteotomy is lingual to the canal and its contents in the foramen region. In addition, the mandibular foramen is cone shaped, with the base of the cone oriented facially and its summit lingually (see Figure 31-33). Therefore, when the height measurement is made from the top of the external portion of the foramen, this represents the more superior portion of the cone-shaped foramen. In addition, the nerve exits from a path inferior to the foramen, so the nerve approaches below the height of the external measurement. Thus the nerve is apical and lateral to the area of the implant osteotomy, when it is directly over the foramen and at the height of the mental foramen. After the measurement is made for available bone height, the implant osteotomy locations may be established every 7 mm (for 4-mm-diameter implants), starting 3.5 to 4 mm distal from the last abutment tooth (Figure 31-38). An abutment location point is made on the midcrestal bone of the ridge with a round No. 6 or pilot bur (Figure 31-39). A measuring tool may be used to measure the interimplant distance on one side and the tooth-to-implant position on the other side. If the pilot holes for the implant osteotomies are positioned too close to each other, it may be separated by an Figure A measuring tool is placed on the distal of the tooth, so that the center of the implant site is 3.5 to 4 mm distal to the tooth. In this way, the 4-mm diameter implant will be 1.5 to 2 mm distal to the tooth. Figure A probe is positioned over the mental foramen and measures the vertical distance to the crest of the edentulous ridge. This measurement is compared with the preoperative x-ray and determines the zone of safety for posterior implant length. Figure The implant sites are at least 7 mm apart in the posterior mandible so that the implants will have 3 mm of bone between them.

23 706 IMPLANT SURGERY Figure A 2-mm end-cutting drill used with copious amounts of saline prepares the osteotomies to 9 mm (when more than this height is available). Figure A periapical radiograph is taken with the direction indicator in place to confirm the implant position in relation to the adjacent roots and mandibular canal. Figure The force direction indicator is positioned into the osteotomy and evaluates implant position. additional 1 mm by stretching the hole away from the problematic implant or tooth with a side-cutting Lindemann drill. This same technique is used if the osteotomy is too facial or lingual. A 2-mm end-cutting drill with copious amounts of saline then prepares the initial osteotomy at 9 mm deep (when more than this height is available) (Figure 31-40). After the implant osteotomy is 2 mm in diameter, a force direction indicator (parallel pin) is inserted (Figure 31-41). The patient gently closes into occlusion, and the direction of force is noted. The indicator should be positioned between the central fossa and palatal cusp of the maxillary teeth. This permits the occlusal loads to be ideally directed onto the implant body. The surgical template is fitted over the remaining teeth and compared with the pilot hole positions. A periapical radiograph may be made at this time to verify the position away from the adjacent roots and confirm the available bone height (Figure 31-42). If the preselected implant length needs to be modified from that planned on the basis of discrepancy between the initial radiographic estimations and present clinical measurements, a different-length implant is selected at this time. The available bone and implant should be at least 9 to 10 mm in height from the crest of the ridge to the inferior portion of the implant body. Largerdiameter implants (or two regular size implants) are used in the molar regions when possible to account for the greater force magnitude in the posterior regions of the mouth. The depth of the millimeter gauge lines inscribed on the shank of the drills most often do not include the cutting edge of the drill and are not to the actual depth of the drill. Most drills have a sharp V-shaped apical portion to improve their cutting efficiency. The V-shaped apical portion of the drill (called a Y dimension in engineering) is often not included in the depth measurements of the commercial drills and may measure as much as 1.5 mm on larger drill diameters. As a result, implant companies often provide a caution statement included in the implant company surgical manual (drills prepare sites deeper than the depth gauge lines indicate) (Figure 31-43) In addition, some companies include the healing cap height along with the implant depth gauge lines. Some systems make the drill depth gauge 1 mm longer than the implant (a 13-mm implant has a corresponding line at 14 mm on the drill). Other systems make the actual implant 1 mm shorter than the corresponding drill gauge (a 13-mm implant is really 12 mm, and the drill line is 13 mm). Some manufacturers make the depth line of the drill the same as the implant length. This variance in corresponding depths becomes critical when preparing bone sites over the mandibular canal or when exact measurements are required. Therefore it is necessary to evaluate the manufacture s drills and compare them

24 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 707 Y 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.4 mm Drill diameter 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.2 mm 3.4 mm 3.7 mm 4.0 mm 4.2 mm 4.4 mm 4.7 mm Y dimension 0.43 mm 0.58 mm 0.74 mm 0.86 mm 0.94 mm 0.99 mm 1.07 mm 1.17 mm 1.22 mm 1.27 mm 1.35 mm Figure The Y dimension or cutting edge of a drill is often not included on the depth lines of a drill. Therefore the 9-mm depth line on a 4.7-mm-wide drill may actually represent mm of cutting depth. 14 mm 12 mm 10 mm 15 mm 13 mm 11 mm 9 mm 7 mm Figure The depth lines on these drills include the Y dimension. The larger-diameter drills have no V-shaped end and therefore no Y dimension (BioHorizons Dental Implants, Birmingham, Ala.). with the length of the implant before preparing a posterior osteotomy above the mandibular canal. The apex drill design was modified by Strong et al. 82 to eliminate the Y dimension of a drill (Figure 31-44). The flat end in the larger-diameter drills also acts as a stop, because it does not proceed deeper than the osteotomy depth of the pilot drill. This drill design is of most benefit in the posterior mandible, when the implant approaches the mandibular nerve location. Osteotomy preparation proceeds following the surgical protocol adapted to the bone density (Figure 31-45). A 2.5-mm end-cutting drill is followed by a 3-mm drill and the final drill dimension for the system employed. A countersink drill and a bone tap are only used for D1 bone. In the posterior mandible, D2 bone most often uses a countersink drill for a larger crest module, but does not use a bone tap. D3 bone does not use a crest module drill or a bone tap. A posterior mandible that has been edentulous for several years may present a fine trabecular pattern under a thin crestal cortical plate. The less dense trabecular bone does not require the entire osteotomy depth to be prepared before implant placement. When the available bone height is marginal and the trabecular bone is less dense, the drill may be stopped 2 mm shorter than the full osteotomy depth. In this way, the risk of nerve injury is significantly reduced. A countersink drill is not indicated under these conditions. The implant is inserted to the complete depth by compression of the trabecular bone around the apical 2 mm of the implant. The crest module of the implant compresses the thin cortical plate and increases initial fixation. When the trabecular bone is fine, countersinking the implant may place it below the crestal cortical bone, causing decreased stability during healing. The posterior mandible may exhibit greater flexure and torsion compared with the anterior Y Pilot ø2.5 ø3.0 ø3.4 Figure The drill sequence protocol is related to the bone density. The 2.0-mm pilot drill is most often followed by the 2.5- and 3.0-mm intermediate drills. The 3.4-mm final drill may be followed by implant insertion in D3 bone. mandible as a result of parafunction, and the crestal bone may be required to stabilize the implant during healing. On occasion, the lingual cortical plate at the end of the osteotomy may prevent complete seating of the implant, as a result of the submandibular fossa in the region. In these cases, the depth of the osteotomy must be completely prepared, as in the anterior mandible. The surgical sites are thoroughly rinsed to remove any debris or contaminants before inserting the implant. Otherwise, the implant may compress the contents of the site beyond the apex and place pressure against the mandibular nerve. This may cause a transitional paresthesia. The bone tap (in D1 bone) and implant insertion is performed at a reduced speed of less than 30 rpm with a high-torque, slow-speed hand piece (Figure 31-46). Implant insertion most often occurs without irrigation. The crest module is often placed D3

25 708 IMPLANT SURGERY Figure A low-speed hand piece threads the implant into position, usually without irrigation. Figure A one-stage implant surgical approach is often used in good-quality bone and when the patient does not wear a soft tissue borne transitional prosthesis. Figure A, For external hex designs, the implant platform may be placed even with the crest of bone as suggested for D2 bone and when the implant is less at risk of movement during healing. B, In D4 bone, the implant is more at risk of movement during healing. As a result, it is countersunk below the crest of bone, so the implant cover screw is level with the crestal bone. The location of the implant platform determines the depth of implant osteotomy. level with the crest of the ridge in soft bone and slightly above the crest in good-quality bone (Figure 31-47). The first-stage cover screw is inserted for a two-stage surgical approach, or a permucosal healing extension (PME) is inserted for a one-stage surgical approach. A one-stage approach is used when the patient does not wear a soft tissue borne partial denture. This approach eliminates the second-stage surgery and allows the tissue to be mature at the time of prosthetic fabrication (Figures and 31-49). The tissues are approximated and sutured with 3-0 or 4-0 polyglycolic acid PGA or Vicryl suture. The accessory anterior incisions are approximated first to ensure proper tissue placement after reflection. Most often, the suture is positioned within the mid-papilla in the distal of the canine from the facial. The suture then picks up the lingual aspect of the papilla and proceeds to the medial of the canine. After it is looped around the canine, it is tied to the suture end and the distal papilla is pulled medial up against the distal of the canine. An interrupted suture is placed below in the vertical release incision to allow primary closure of the soft tissue in the correct position. A continuous nonlocking suture or interrupted sutures may be placed Figure The cover screw or permucosal extensions are exposed in the one-stage surgical approach. The tissues are approximated with a resorbable suture (PGA or Vicryl). along the crest of the ridge. Pressure is then applied to the soft tissue over the surgical site for 4 minutes to arrest any bleeding and help adapt the periosteum to the bone. IMMEDIATE POSTOPERATIVE PROCEDURES AND INSTRUCTIONS After surgery for the anterior mandible, the patient s mouth is rinsed with saline, and moist gauge sponges are rolled and inserted over the surgical site. Several other 4 µ 4-inch gauze sponges are rolled together and placed intraorally between the arches. The patient bites on the gauze packs for approximately 1 hour. A panoramic radiograph is taken at the conclusion of surgery while the transitional prosthesis is modified (Figure 31-50). The radiograph is closely evaluated to ensure proper implant placement in relationship to the opposing landmarks or surrounding structures. If inadequate placement is observed or infringement on the mandibular canal is evident, the implant

26 Root Form Surgery in the Edentulous Anterior and Posterior Mandible: Implant Insertion 709 Figure A panoramic radiograph is taken at the conclusion of the surgery to confirm the implant position away from the mandibular nerve and adjacent teeth. placement should be corrected at this surgical visit. On rare occa sions it has been necessary to remove the sutures and reposition an implant after analysis of the panoramic radiograph. The threaded implant may be unthreaded one to two revolutions to lift an implant away from a neurovascular canal when necessary. Decompression of the nerve complex decreases the risk of paresthesia. It is far better to perform this correction surgery while the patient is still anesthetized and the soft tissue is not healed than to wait for a future appointment. In the posterior partially edentulous patient, the posterior region most often is out of the esthetic zone and does not require a transitional prosthesis during hard and soft tissue healing. The usual course of events after the surgical procedure is uneventful. If the patient needs to wear a removable partial denture with a metal framework, it is recommended to make holes through the metal mesh in the saddle area corresponding to implant placement before relining with a soft tissue conditioner. Removing only the acrylic under the metal framework on the saddle area does not provide adequate relief for the surgical site, because the acrylic is usually 0.8 mm thick. Occasionally edema develops in the floor of the mouth, accompanied by minor hematoma formation. However, the edema may extend extraorally and include the chin, especially if the inferior cortical plates were perforated. An occasional short-term paresthesia of the lip is possible because of the edema. Analgesics and corticosteroids are recommended the first few days following the surgery, and the protocol outlined in Chapter 21 is implemented. The patient is provided with written postoperative instructions highlighting areas such as rest, application of ice and pressure with gauze packs, and steps to follow in case of bleeding, suture line opening, and pain. Prescription of chlorhexidine gluconate rinses and the use of salt water rinses are also recommended. Usual postoperative instructions similar to those after oral surgery procedures are implemented. The patient returns in 10 to 14 days for suture removal and observation, even though resorbable sutures are used. Polyglycolic resorbable sutures require 6 to 8 weeks to resorb, and on rare occasion small fistulae form in this time frame. Any fistula, regardless of cause, affects the ph in the area and can cause bone loss; therefore the sutures are removed at the postoperative visit. Tissue conditioners are selected for the immediate postoperative period because they are able to change dimension during the first 24 to 36 hours under pressure. As a result, excess force from swelling or occlusion on the tissue in any region will cause modification of the material. However, tissue conditioners become more rigid faster than the soft tissue liners. In addition, sili cone-based products harbor more bacteria and promote yeast formation; therefore the tissue conditioner is removed at the suture removal appointment. The inter nal surface of the denture is then relieved, and a soft tissue liner is placed, which is also relieved over the implants in the surgical region. The patient is scheduled for an additional soft tissue checkup in 3 weeks to confirm full maturation of the soft tissue. COMPLICATIONS Nerve Injury One of the most troublesome complications short and long term of bone harvesting (i.e., ramus block graft), third molar extractions, and implant surgery in the posterior mandible is nerve injury (Figure 31-51). The two most common nerves of concern are the inferior alveolar nerve (IAN) and the lingual nerve. The trauma from surgery or administration of local anesthetic to branches of the fifth cranial nerve may lead to loss or altered sensation (paresthesia) or, worse, to painful symptoms (dysesthesia). A thorough presentation of this topic is beyond the scope of this text; however, the clinician should know how to evaluate and refer when appropriate when associated symptoms Figure Implant surgery in the posterior mandible is a primary cause of mandibular nerve injury.