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1 The role of cone-beam computed tomography in the planning and placement of implants Philip Worthington, MD, DDS, BSc, FDRCS; Jeffrey Rubenstein, DMD, MS; David C. Hatcher, DDS, MSc, MRCD(c) Successfully providing dental implants to patients who have lost teeth and frequently the surrounding bone relies on the careful gathering of clinical and radiological information, on interdisciplinary communication and on detailed planning. Early in the evolution of osseointegration surgery, clinicians placed implants in regions of maximum bone volume without fully taking into consideration where the crown eventually would be placed. Biomechanics, esthetics and maintenance can be compromised if the implant location does not align properly with the crown, ultimately resulting in substandard outcomes, including failure. In worst-case scenarios, implant placement can result in implants that are placed suboptimally and ultimately remain in the bone unrestored. Implant planning should be driven by prosthodontic concerns, using a crown-to-bone approach. The crown-to-bone concept takes into account the site-specific restorative requirements, followed by finding the ideal location for the supporting implant and finally investigating the bone to determine the feasibility of implant placement. IMAGING Imaging options began with twodimensional (2-D) imaging tech- ABSTRACT Background. Three-dimensional imaging, particularly cone-beam computed tomography (CBCT), has made significant contributions to the planning and placement of implants to replace missing teeth. The accuracy of CBCT data can be used to fabricate a surgical guide that transfers the implant planning information to the surgical site to facilitate implant placement. The authors describe a method for applying CBCT data to aid in the planning and placement of implants. Methods. The authors outline clinical goals for implant planning and placement and describe the anatomical and prosthetic requirements for successful implant placement. They also present imaging solutions, including CBCT scanning and software analysis, to the clinical goals. Conclusions. Virtual implant planning using CBCT data allows the clinicians to create and visualize the end result before initiating treatment. CBCT scans are accurate and cost effective and can be used to improve communication and coordination of a multidisciplinary team to achieve the desired clinical outcome. Virtual planning allows clinicians to investigate multiple treatment scenarios until the optimum treatment plan is attained. The optimized virtual plan may be converted through modeling to create a surgical guide for clinical implementation. Clinical Implications. The precise planning and delivery of implants to replace missing teeth can avert recognized and concealed treatment problems. This process aids the clinician and benefits the patient. Key Words. Cone-beam computed tomography; implant planning; surgical guide; virtual planning; simulations. JADA 2010;141(10 suppl):19s-24s. Dr. Worthington is a professor emeritus, Department of Oral and Maxillofacial Surgery, School of Dentistry, University of Washington, Seattle. Dr. Rubenstein is a professor and the director, Maxillofacial Prosthetic Service, Department Restorative Dentistry/Division of Prosthodontics, School of Dentistry, University of Washington, Seattle. Dr. Hatcher is a clinical professor, College of Dental Medicine, University of Southern Nevada, Henderson, and an adjunct associate clinical professor, Arthur A. Dugoni School of Dentistry, University of the Pacific, San Francisco, and he maintains a private practice in oral and maxillofacial radiology. Address reprint requests to Dr. Hatcher at 99 Scripps Drive, Suite 101, Sacramento, Calif , David@ddicenters.com. JADA 141(10 suppl) October S

2 niques and now include three-dimensional (3-D) imaging techniques. Diagnostic information, treatment planning and treatment benefits levels have increased with the use of 3-D imaging techniques (Table 1 ). Using 3-D virtual planning techniques before treatment has resulted in optimal implant placement and improved clinical outcomes. The development of 3-D scanning such as cone-beam computed tomography (CBCT) 1 instead of planar films has led to improved visualization and comprehension of the anatomy in the areas in which implants are being planned for placement. 1 Computed tomographic (CT) scans and CBCT scans reproduce the anatomy with a submillimetric accuracy. 2 Several methods have evolved to exploit 3-D data volumes (viewable CBCT data) to fulfill imaging goals. Specialized software allows for cross-referencing and display of multiple view angles from a specific implant site. The view angles commonly are axial, reconstructed panoramic and cross-sectional views of the jaws. The displayed reconstructed images can be measured directly on screen or on a true-sized printed copy to determine the anatomical dimensions. If clinicians are limited to a true-sized printed copy, it can be difficult for them to consider the prosthodontic management of the implant site. Clinicians can use implant radiographic guides to transfer the prosthodontic plan to the CBCT scan. A fabricated radiographic guide is positioned in the patient s mouth during the scan. Radiographic guides have evolved and can assist in a spectrum of restorative planning. The simple radiographic guide includes a radiopaque marker that indicates the desired placement position of an implant. A complex radiographic guide allows for visualization of the prosthetic tooth, occlusion with the opposing tooth, planned implant location, implant angle and thickness of the soft tissue between the bone and the tooth. CBCT visualization of the prosthodontic plan allows the clinician to evaluate the subjacent anatomy, test the feasibility of the proposed implant placement and make needed modifications to the plan to optimize the spatial and functional relationships between the planned prosthetic treatment and the anatomy. Historically, clinicians have facilitated the transfer of the crown-to-bone approach (transfer of virtual plan to surgical placement) by providing a surgical guide or template to the surgeon with the hope that the intended tooth position and implant placement would be in concert with the intended treatment goals. The radiographic guide now can be modified to become a surgical guide. A surgical guide can be designed and constructed on the basis of computer analysis of the available bone, the proximity of teeth to the proposed implant site and structures to be avoided in implant placement. This information can help clinicians comprehend better the optimal location of implant placement and ultimately lead to a better potential for a successful outcome. CBCT can accurately capture, display and provide visualization of 3-D maxillofacial anatomy. 2 For CBCT to be implemented optimally, clinicians first must consider exactly what information is essential. This step provides a clinical basis for the clinician to customize the radiographic evaluation and interpretation. CBCT can be used to assess alveolar bone height, bone width, bone quality, pathosis, interarch space, maxillomandibular relationships and temporomandibular joints. CBCT is valuable for the evaluation of the following anatomical features of the mandible, maxilla and facial bones. 3-9 Mandible. CBCT scans of the mandible can help clinicians determine the size of the lingual concavity in the symphyseal regions. Occasionally, the mandible may have an unexpected lingual concavity in the posterior region of the mandible. In the inferior alveolar canal (IAC), CBCT scans can show whether the canal is single or divided and how it is placed buccolingually. CBCT scans can help clinicians distinguish the IAC from large sinusoidal spaces in cases in which the walls of the IAC are not well defined. Studies have been published that outline the anatomical variations of the mental nerve region. 3-9 These variations can make implant placement problematic. CBCT scans can help clinicians answer the following questions: ddoes the mental nerve have an anterior loop? If so, what is its size? dis there an unusual branching pattern of the mental nerve as it leaves the IAC? How is it placed buccolingually? dis site preparation necessary? dwhat is the state of any previously placed bone graft material? ABBREVIATION KEY. CBCT: Cone-beam computed tomography. CT: Computed tomography. IAC: Inferior alveolar canal. 3-D: Three-dimensional. 2-D: Twodimensional. 20S JADA 141(10 suppl) October 2010

3 TABLE Value comparison of implant imaging modalities* commonly used to evaluate implant sites. IMAGING GOAL TWO-DIMENSIONAL IMAGING SOURCES THREE-DIMENSIONAL IMAGING SOURCES Cephalographic Periapical Panoramic Tomography Computed Tomography Cone-Beam Computed Tomography Bone Height Bone Width Long Axis or Ridge Anatomy Localized Bone Quality Pathosis Identified Jaw Boundaries Determination Virtual Planning Guide Fabrication Facilitated Communication Aid Benefit/Risk/Cost Ratio * The imaging modalities are ranked by their ability to satisfy implant planning imaging goals. Ranking scale: 0 = no value, 1 = low value, 2 = mild value, 3 = moderate value, 4 = high value. Adapted with permission of Journal of the California Dental Association from Hatcher and colleagues. 1 Maxilla. CBCT scans can help clinicians answer the following questions about the maxilla: dwhat is the size of the labial cortical concavity in the lateral incisor region? dif the maxillary sinus has an incomplete septum, is it substantial enough to accommodate an implant tip? Or is it merely a nuisance during membrane elevation? dhow substantial is the paranasal bony buttress in the maxillary canine region? Is site preparation needed? dwhat is the state of any previously placed bone graft material? dis there useful bone in the pterygoid region? Facial bones. Clinicians can use CBCT scans to determine how substantial the body of the zygoma is and what quantity of bone is available for craniofacial implants in the orbital rims and in the immediate paranasal region. They also can use CBCT scans to determine when unorthodox implant placement (for example, oblique implants in the posterior atrophic mandible) is a possibility. 10 Underlying all of these considerations is the desire to optimize implant placement to facilitate management by the restorative dentist and to satisfy the patient. But also and equally important is the desire to do no harm. We have seen many instances in which nerve injuries could have been avoided if the clinician had conducted a preliminary CBCT examination. Similarly, unintended lingual perforation of the mandible (with possible hematoma formation in the retrosymphyseal region, damage to the submandibular gland in the molar region or both) may be avoided if 3-D imaging is used. 1 MODELING SYSTEMS Several modeling systems, such as stereolithography and dot matrix model fabrication, can be used to fabricate surgical guides. Drilling sleeves facilitate the drilling precision during surgery. Although these modeling systems can be precise and accurate with regard to fabrication and guiding capabilities, there are inherent variables that can mitigate precise implant placement. These variables include dimensional change of the polymeric materials used in the fabrication of the guides, operator error in drilling despite significant constraints created by the guide designs, compression of soft tissues during guided placement, and accuracy and stabilization of the guide during placement. Implant placement involves a sequence of steps, and the inherent error in each step must be minimized. The results of evaluations of CBCT scans and comparison of them against a known standard 2,11 and standard 2-D JADA 141(10 suppl) October S

4 Figure 1. A virtual planning environment for implant replacement of teeth nos. 9, 10, 13 and 14. The patient and the stone casts were scanned using a cone-beam computed tomography (CBCT) unit. Using anatomy imaging software, Anatomage (San Jose, Calif.) converted the scans into an interactive digital model (AnatoModel, Anatomage). The AnatoModel consists of several objects and CBCT volume, all of which are registered to the same global coordinate system (shown here with volume and exploded model). Each object (maxillary model, mandibular model, tooth objects and implant objects) has a local coordinate system that allows for object manipulation and monitoring by the global coordinate system. In this case, the virtual teeth were created by duplicating and mirroring the teeth from the opposite side. images 2 have shown CBCT scans to be spatially accurate and precise. Implant planning, surgical fabrication and implant placement have been tested to an accuracy of 0.9 millimeters at the ridge crest and 1.0 mm at the apex on an in vitro model. 12 The error range in implant placement is slightly higher in vivo, likely because of variations in surgical guide placement and stability and the clinician s inability to constrain the drill exactly along the drill guide path A recent software development has simplified the process of implant planning and placement by eliminating the requirement for fabrication of a radiographic guide (Figures 1-3). 18 This development allows the clinician to scan both the patient s maxillofacial region of interest and the stone dental casts with CBCT. The scans can be completed in less than a minute, and from that point until the implant is placed, all planning and guide fabrication can be completed by means of a virtual patient model. The virtual environment uses a multiobject viewer in which patient-specific objects (including the maxillary and mandibular dentition created from stone cast scans, library of teeth and implant objects, and the CT scan of the jaw) can be combined in the same 3-D matrix for planning (Figure 1). The dentition objects created from intraoral laser scanning of the dentition or scanning the stone casts reproduces the patient s tooth anatomy with a better fidelity than that of CBCT of the natural dentition because of a reduction in beam-hardening and scatter artifacts. The combination of CBCT with intraoral laser scanning or CBCT of stone casts provides optimal representation of surface and subsurface anatomy. This virtual environment allows clinicians to articulate the maxillary and mandibular dentition and to replace missing teeth in their ideal functional positions virtually. The quality and volume assessment of the subjacent bone and virtual planning of the implants position and axial orientation can be performed on the virtual model (Figure 2). The modeling environment allows for simultaneous iteration of all of variables (for example, tooth position, jaw articulation, bone quality, bone volume and implant placement) until the optimum treatment plan is achieved. Once the optimum implant plan has been determined, the virtual model can be submitted for fabrication of a surgical guide. CBCT scans and the associated software have led to an increase in implant placement. The next development in implant technology may be robotic or computer assistance for the surgical and restorative phases of implant therapy. This process is already in development, 19,20 albeit in its infancy. CONE-BEAM COMPUTED TOMOGRAPHY IMPLANT PLANNING PROTOCOL The following CBCT scanning protocol can help clinicians meet the imaging goals we defined previously in this article: dlimit the scanning field of view to the dental 22S JADA 141(10 suppl) October 2010

5 A B C Figure 2. The virtual placement of an implant (A) and interactive visualization of the volume and implant (B). The software computes and displays the attenuation value (bone density) of the bone adjacent to the virtual implant (B). Volume rendering of left posterior maxilla showing virtual 3-D placement of implants (C). The local coordinate system for tooth site no. 13 was activated. The bone density can be color mapped to approximate Hounsfield units (HUs). HUs express the relative attenuation value of various anatomical tissues or spaces by means of a scale that has been calibrated to give the radiodensity of distilled water an HU of zero. arch being evaluated and the crowns of the opposing arch; dorient the occlusal plane to be parallel to the horizon; dseparate the maxila B lary and mandibular teeth with a thin sheet of wax; dselect a relatively small volume element (voxel) size (0.20 mm or 0.25 mm) unless you are limited by the planc D ning software (a relatively small voxel size creates the best opportunity to visualize anatomical structures, such as the mandibular canal, particularly in patients with a low attenuation value [osteopenia]); E F duse patient immobifigure 3. Demonstration of the virtual replacement and articulation of missing teeth with the opposing lization techniques arch model (A, B and D). Note that the stone cast registration to the cone-beam computed tomography volume eliminates the beam-hardening and scatter artifacts that often occur with teeth and their metallic during scanning; (C). The model also shows gingival soft-tissue contours (C and F). During the virtual implant dscan stone models or restorations placement, the clinician can consider the location of the replacement crown and the subjacent anatomy impressions or scan (B). The three-dimensional positioning of the implants can be visualized without the model, and virtual teeth can be visualized (D). Teeth nos. 9, 10, 13 and 14 were duplicated from corresponding teeth on the teeth using an intraopposite side of the arch (E). oral laser. Creating an image portfolio a specific and communicate imaging goals and findings. The folprecise collection of image views and anatomical lowing are steps clinicians can take to create an renderings is an effective way to visualize and image portfolio. JADA 141(10 suppl) October S

6 ddisplay a reconstructed panoramic projection at a slab thickness that is sufficient to show alveolar process and teeth (approximately 15.0 mm thick). ddisplay a reconstructed panoramic projection at a slab thickness of 1.0 mm or less with nerves marked on mandible scans. ddisplay cross-sectional slices of each of the proposed implant sites (cross-sectional slices are generated automatically by the software to be perpendicular to the arch curve created on the axial section mapped for the panoramic reconstruction; these slices are displayed by selecting a 1.0-mm slice thickness and a spacing of 1.0 or 2.0 mm). dfabrication of a multiobject 3-D model of the planning sites. CLINICAL VALUE PROPOSITION A clinical value proposition is a subjective determination that considers the ratio of benefit to cost, risk and time efficiency. The variables that define a clinical value proposition include desired diagnostic information, the risks and costs to the patient, and time efficiency. The implant planning and placement exercise begins with developing clinical objectives, which sets into motion a sequence of steps designed to fulfill those objectives. The best implant placement plans are achieved when consideration is given to all of the key information. CBCT, unlike traditional 2-D imaging methods, provides anatomical accuracy. The use of virtual modeling provides the opportunity for clinicians to solicit and combine input from a multidisciplinary team to create a single optimal treatment plan. Using the virtual environment could be the most efficient and cost effective method for planning. Restricting the CBCT scanning field of view to only the areas of interest minimizes the radiation dose. Careful planning to avoid nerve injury, penetrations of jaw boundaries and implant proximity to adjacent teeth, as well as to facilitate implant alignment with the prosthetic elements, improves the potential for achieving successful outcomes. CONCLUSION CBCT provides the anatomical data that can generate a collaborative treatment plan and achieve optimal outcomes for the restorative dentist, radiologist, surgeon and patient. The evolution in CBCT hardware and software has been responsible, in part, for the increase in the precise planning and placement of implants while minimizing the associated risks. Disclosure. None of the authors reported any disclosures. 1. Hatcher DC, Dial C, Mayorga C. Cone beam CT for pre-surgical assessment of implant sites. J Calif Dent Assoc 2003;31(11): Stratemann SA, Huang JC, Maki K, Miller AJ, Hatcher DC. Comparison of cone beam computed tomography imaging with physical measures. Dentomaxillofac Radiol 2008;37(2): Arzouman MJ., Otis L, Kipnis V, Levine D. Observations of the anterior loop of the inferior alveolar canal. Int J Oral Maxillofac Implants 1993;8(3): Kieser J, Kuzmanovic D, Payne A, Dennison J, Herbison P. Patterns of emergence of the human mental nerve. Arch Oral Biol 2002;47(10): Kuzmanovic DV, Payne AG, Kieser JA, Dias GJ. Anterior loop of the mental nerve: a morphological and radiographic study. Clin Oral Implants Res 2003;14(4): Mardinger O, Chauschu G, Arensburg B, Taicher S, Kaffe I. Anterior loop of the mental canal: an anatomical-radiologic study. Implant Dent 2000;9(2): Rosenquist B. Is there an anterior loop of the inferior alveolar canal? Int J Periodontics Restorative Dent 1996;16(1): Solar P, Ulm C, Frey G, Metjka M. A classification of the intraosseous paths of the mental nerve. Int J Oral Maxillofac Implants 1994;9(3): Worthington P. Injury to the inferior alveolar nerve during implant placement: a formula for protection of the patient and clinician. Int J Oral Maxillofac Implants 2004;19(5): Stella JP, Abolenen H. Restoration of the atrophied posterior mandible with transverse alveolar maxillary/mandibular implants: technical note and case report. Int J Oral Maxillofac Implants 2002;17(6): Peck JL, Sameshima GT, Miller A, Worth P, Hatcher DC. Mesiodistal root angulation using panoramic and cone beam CT. Angle Orthod 2007;77(2): Sarment DP, Sukovic P, Clinthorne N. Accuracy of implant placement with a sterolithographic surgical guide. Int J Oral Maxillofac Implants 2003;18(4): Arisan V, Karabuda ZC, Ozdemir T. Accuracy of two stereolithographic guide systems for computer-aided implant placement: a computed tomography-based clinical comparative study. J Periodontol 2010;81(1): Katsoulis J, Pazera P, Mericske-Stern R. Prosthetically driven, computer-guided implant planning for the edentulous maxilla: a model study. Clin Implant Dent Relat Res 2009;11(3): Nickenig HJ, Eitner S. Reliability of implant placement after virtual planning of implant positions using cone beam CT data and surgical (guide) templates. J Craniomaxillofac Surg 2007;35(4-5): Nickenig H-S, Wichmann M, Hamel J, Schlegel A, Eitner S. Evaluation of the difference in accuracy between implant placement by virtual planning data and surgical guide templates versus the conventional free-hand method: a combined in vivo-in vitro technique using cone-beam CT part II. J Craniomaxillofac Surg (in press). 17. Moreira CR, Sales MA, Lopes PM, Cavelcanti JP. Assessment of linear and angular measurements on three-dimensional cone-beam computed tomographic images. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009;108(3): Curley A, Hatcher DC. Cone beam CT: anatomic assessment and legal issues the new standards of care. J Calif Dent Assoc 2009;37(9): Eggers G, Mühling J, Hofele C. Clinical use of navigation based on cone-beam computer tomography in maxillofacial surgery. Br J Oral Maxillofac Surg 2009;47(5): Elian N, Jalbout ZN, Classi AJ, Wexler A, Sarment D, Tarnow DP. Precision of flapless implant placement using real-time surgical navigation: a case series. Int J Oral Maxillofac Implants 2008;23(6): S JADA 141(10 suppl) October 2010