Short implant in limited bone volume

Transcription

1 Periodontology 2000, Vol. 66, 2014, John Wiley & Sons A/S. Published by John Wiley & Sons Ltd Printed in Singapore. All rights reserved PERIODONTOLOGY 2000 Short implant in limited bone volume D AVID NISAND &FRANCK RENOUARD Introduction Rehabilitation of severely resorbed jaws with dental implants remains a surgical and prosthetic challenge for clinicians (25, 53). Several advanced surgical techniques have been developed over the years to restore bone volume, allowing the placement of dental implants and improving esthetic outcomes. The same surgical techniques have also been applied to improve crown-to-implant ratios, to allow the placement of longer implants and to optimize the positioning of implants for adequate load distribution. However, the latter indications remain controversial, and the increased treatment time, cost and risk of complications should be analyzed in line with the expected benefits. Sinus lift elevation, guided bone regeneration, onlay bone grafting, distraction osteogenesis and displacement of the inferior alveolar nerve were developed and applied for the management of reduced alveolar bone height. Some of these techniques, such as sinus lift elevation, are supported by a large number of publications and display excellent survival rates for dental implants (18). On the other hand, less data are available for surgical displacement of the inferior alveolar nerve, vertical augmentation or distraction osteogenesis (26, 94, 107). Moreover, long-term follow-up studies of dental implants placed in augmented bone are not available for each technique. Even for the well-documented technique of sinus lift elevation, it should be remembered that the best results, obtained with rough surface implants and biomaterial, are based only on short-term follow-up studies (87). Complex surgical techniques are often associated with complications (42). Complications may occur during surgery (such as bleeding (Fig. 1), perforation of the Schneiderian membrane (Fig. 2A D) or nerve injury) or postoperatively (including transiently or permanently altered mandibular sensation (25), graft and/or membrane exposure (Fig. 3), infections (122) and increased peri-implant bone loss (88)). Even when the risk for complications is limited, advanced surgical techniques may be contraindicated in some patients for medical or anatomic reasons. As an alternative to complex surgeries (those performed to allow the placement of longer implants or for biomechanical reasons), the use of dental implants with reduced length should be considered. Along with their simplicity, short-length implants allow for less expensive and faster treatment with reduced morbidity (43, 44). However, both survival rate and indications are still controversial. In the past, short-length implants were often associated with increased failure rates (125), which were explained by reduced implant primary stability and bone-to-implant contact, as well as by unfavorable crown-to-implant ratios. As a consequence, the use of short-length implants was mainly restricted to rescue situations. The purpose of this review was to evaluate the data available on the survival rate of short and extra-short implants and to discuss the impact of an increased crown to implant length ratio on biological and technical complications. Indications and clinical procedures for short-length implants in clinical practice are also reviewed, along with a discussion on the selection of the implant length. The paper also introduces a new concept in implant dentistry: stress-minimizing surgery. Definition There is still some controversy over the exact definition of a short-length implant. According to Striezel & Reichart (112), an implant of 11 mm is considered as short, whereas Tawil & Younan (114) stated that an implant must be 10 mm to be regarded as 72

2 Short implant in limited bone volume Fig. 1. Clinical view of the alveolar artery that may lead to bleeding complications during sinus lift procedures. short. In one recent systematic review (116) and in one recent meta-analysis (89), all implants of <10 mm were defined as short implants. For the purposes of this review, a short implant will be defined as an implant with a designed intrabony length of 8 mm (92) (Fig. 4) and an extra-short implant as a device with a designed intrabony length of 5 mm. Survival rates of short-length implants Short implants The survival rate of short-length implants has mostly been evaluated in case series with either a retrospective or a prospective design. A case series design can be defined as a collection of patients in whom dental implants of different lengths were placed and monitored for a specified period of time (95). The impact of implant length, among other parameters, was evaluated with respect to implant failure and/or bone loss (Table 1). In the first group of papers, some publications report an increased failure rate with shorter implants compared with longer implants (6, 7, 63, 65, 82, ). Of these, Winkler et al. (124) reported an overall survival rate of 74.4% for 7-mm-long implants and Herrmann et al. (63) revealed an overall survival rate of 78.2% for 7-mm-long implants. A second group of papers, although concluding that failure rates increased with short implants, still listed adequate survival rates for short-length implants (51, 64, 70, 121). For example, in the paper by van Steenberghe et al. (121), only three failures were reported among 120 placements of 7-mm-long implants, leading to an overall survival rate of 97.5% after 1 year of follow-up. Some descriptive studies have reported that the outcome with short-length implants is similar to that for longer implants (20, 21, 40, 46, 61, 67, 71, 83, 98, 99, 111, 118). A structured review explained differences, among studies, in the survival rates of short-length implants according to the following parameters: implant primary stability related to surgical bone preparation; operator s learning curve; implant surfaces; and the quality of the patient s bone (92). More recently, case series dedicated to short-length implants were A B C D Fig. 2. (A) Clinical view of a full-thickness flap elevation reaching the buccal wall of the sinus in order to perform a sinus lift procedure. (B) The trap door is created using piezosurgery to reduce the risk of membrane perforation. (C) Clinical view showing the trap door. (D) Clinical view of membrane perforation (despite the use of piezosurgery). 73

3 Nisand & Renouard Fig. 3. Exposure of a titanium-reinforced membrane used for a vertical augmentation procedure in the posterior mandible. Fig. 4. Periapical radiograph of a short (8 mm) implant after 5 years of loading. performed with either a retrospective or a prospective design (Table 2). Among this group of publications, there is great discrepancy in the definition of a short implant. Some authors only included implants with a designed intrabony length of 8 mm, whereas others included longer implants (13, 32, 33, 54, 103, 110, 114, 119). One of the first case series with a special emphasis on short-length implants was published by Texeira et al. (119). They followed 67 implants, with a mean length of 8.3 mm and placed in the posterior mandible, over a period of 5 years and reported a cumulative survival rate of 94%. The efficacy of short-length implants in the restoration of the posterior mandible was also confirmed by many other authors (1, 2, 33, 37, 38, 55 58, 75, 86, 114). Positive outcomes were also reported with severely resorbed edentate mandibles by several authors (34, 52, 60, 110). Friberg et al. (52) followed 49 edentulous patients restored with 260 short implants (6 7 mm in length) and reported cumulative survival rates of 95.5% and 92.3% after 5 and 10 years, respectively. In 1998, a multicenter study, with 1 7 years of follow-up, was performed by ten Bruggenkate et al. (117), who evaluated the survival rate of 253, 6-mm-long implants in a group of 126 patients. They reported a cumulative survival rate of 94%. Of the seven implants removed, six were located in the maxilla, and the authors recommended that short implants should be used in conjunction with longer implants in low-density bone. The possibility of using short implants for the rehabilitation of the posterior maxilla was further evaluated in a 2-year retrospective study (91) involving 96 short implants (of mm) placed in 85 patients. A cumulative survival rate of 94.6% was obtained. Several publications have also reported favorable outcomes for the use of short implants in the posterior maxilla (1, 2, 5, 28, 54 56, 58, 74, 80). Fugazzotto et al. (54), who analyzed the possibility of using short implants to restore single crowns in the posterior maxilla, reported a cumulative survival rate of 95.1% in a group of 979 implants. The usefulness of short implants to support single crowns in the posterior maxillary region was also confirmed by Lai et al. (69), with a follow-up period of 5 10 years. In severely resorbed ridges, with <5 mm of available bone height, some authors also reported favorable outcomes using short implants combined with crestal sinus lifts (23, 32, 35, 50). Whereas most publications devoted to short-length implants used a conventional loading protocol, Rossi et al. (102) performed a prospective case series study, in which 40, 6-mm-long implants were loaded with a single crown, 6 weeks after placement. The authors reported a cumulative survival rate of 95%, 2 years after loading (102). A similar loading protocol was also used by Van Assche et al. (120) in the rehabilitation of edentulous maxilla using four long implants and two distal 6-mm-long implants to support an overdenture. Similar outcomes were reported in this study for long and short implants, 2 years after loading (120). Favorable outcomes were reported in only one publication (31) that used short implants (<10 mm) with an immediate loading protocol. As a result of their descriptive nature, case series studies of short-length implants only allow proofof-principle to be to established (59). More recently, experimental research, such as randomized controlled trials, was performed to allow comparison between short and longer implants (Table 3). For ethical reasons, no direct comparison between short and longer implants in an adequate bone volume can be performed. As a consequence, randomized controlled trials were performed to compare short-length implants with both advanced surgical procedures and longer implants. Two randomized controlled trials were performed by the same team (44, 47) to compare the outcomes of short-length implants in the posterior mandible with longer implants placed in 74

4 Short implant in limited bone volume Table 1. Case series in which implant length was evaluated among other parameters Authors (ref. no.) No. of patients (no. of implants) Follow-up, in months (mean) Cumulative survival rate <10 mm, % Cumulative survival rate >10 mm, % van Steenberghe et al (121) 159 (558) (12) Jemt 1991 (64) 384 (2199) Friberg et al (51) 889 (4641) From stage 1 to connection of the prostheses Bahat 1993 (6) 213 (732) 5 70 (30.3) 92.6 (Bone type II III) 86.7 (Bone type IV) 95.9 (Bone type II III) 94.5 (Bone type IV) Jemt & Lekholm 1995 (65) 150 (801) (60) Buser et al (21) 1003 (2359) Ellegard et al (40) 68 (124) 3 84 Wyatt et al (125) 77 (230) (only 13-mm implants were included) Gunne et al (61) 23 (69) (120) (only three implants were included) Lekholm et al (70) 127 (461) (120) (only 13-mm implants were included) Winkler et al (124) (2917) (36) 74.4 (7 mm) 87 (8 mm) Bahat 2000 (7) 202 (660) (only 13-mm implants were included) Brocard et al (20) 440 (1022) ( 8 mm) 83.7 ( 12 mm) Testori et al (118) 181 (485) (52.6) Naert et al (82) 660 (1956) (66) 81.5 Stellingsma et al (110) 60 (240) (12) Weng et al (123) 493 (1179) (72) 74 (7.0 mm) 81 (8.5 mm) Romeo et al (98) 250 (759) Feldman et al (46) (4891) (10-mm machined implants were included) 97.7 (10-mm Osseotite implants were included) Nedir et al (83) 236 (528) (machined) 98.4 (Osseotite) Herrmann et al (63) 487 (487) (60) 78.2 (7 mm) 95.7 Koo et al (67) 489 (521)

5 Nisand & Renouard Table 2. Case series devoted to short-length implants Authors (Ref. no.) No. of patients (no. of implants) Follow-up, in months (mean) Implant length Cumulative survival rate, % Lai et al (69) 168 (231) (86) Intrabony length 8 mm 98.7 (5 years) 98.3 (10 years) Draenert et al (38) (47) (44) 9mm 98 De Santis et al (37) 46 (107) and 7.0 mm 98.1 Perelli et al (86) 40 (55) 60 7 and 5 mm 84 Gulje et al (60) 12 (48) 12 6 mm 96 Van Assche et al (120) 12 (72) and 6 mm (two short implants and four long implants to support an overdenture) One short-implant failure Rossi et al (102) 35 (40) 24 6 mm 95 Anitua & Orive 2010 (2) 661 (1287) (47.9) 8.5, 7.5, 7.0 and 6.5 mm 99.3 Sanchez Garces et al (103) (273) (81) 10 or <10 mm (10 mm) 92.5 (<10 mm) Grant et al (57) 124 (335) 24 8 mm 99 Corrente et al (28) 48 (48) 36 <10 mm (13 implants with crestal sinus elevation) Fugazzotto 2008 (55) 1774 (2073) <10 mm (9, 8, 7 and 6 mm) Anitua et al (1) 293 (532) 4 59 (31) 8.5, 7.5 and 7.0 mm (single crown) 99.7 (short-span fixed prostheses) Malo et al (74) 237 (408) and 7.0 mm 96.2 (7.0 mm) (5 years) 97.1 (8.5 mm) (5 years) Degidi et al (31) (133) <10 mm 97.7 (immediate loading protocol) Romeo et al (99) 129 (265) and 8 mm 97.9 (8 mm) (14 years) 97.1 (10 mm) (14 years) Arlin 2006 (5) (176) and 6 mm 94.3 (6 mm) (2 years) 99.3 (8 mm) (2 years) Misch et al (80) 273 (745) and 7 mm 98.9 Renouard & Nisand 2005 (91) 85 (96) (37.6) 8.5, 7.0 and 6.0 mm 94.6 Goene et al (56) 188 (311) and 7.0 mm 95.8 Deporter et al (35) 70 (104) (37.6) 7 mm (with crestal sinus elevation) Two short implants failed Fugazzotto et al (54) 979 (979) , 8 and 7 mm 95.1 Griffin & Cheung 2004 (58) 167 (168) (34.9) 8 mm 100 Tawil & Younan 2003 (114) 111 (269) , 8.5, 8.0, 7.0 and 6.0 mm 95.5 Deporter et al (34) 52 (156) and 7 mm

6 Short implant in limited bone volume Table 2. (Continued) Authors (Ref. no.) No. of patients (no. of implants) Follow-up, in months (mean) Implant length Cumulative survival rate, % Deporter et al (33) 24 (48) (32.6) 9 and 7 mm 100 Deporter et al (32) 16 (26) 6 36 (11.1) 9, 7 and 5 mm (with crestal sinus elevation) Friberg et al (52) 49 (260) (96) 7 and 6 mm 95.5 (5 years) 92.3 (10 years) Stellingsma et al (110) 17 (68) (77) 10, 7 and 6 mm ten Bruggenkate et al (117) 126 (253) mm 94 Texeira et al (119) 26 (67) (60) 11 and 8 mm 94 Bernard et al (13) 48 (100) (36) >10, 10, 8 and 6 mm 99 Table 3. Randomized controlled trials comparing short implants and longer implants with advanced surgical procedures Authors (ref. no.) Patients (no. of implants) Mean length of follow-up (months) Area and number of implants Test Control Cumulative survival rate Remark Esposito et al (44) Felice et al (48) Felice et al (47) 60 (121) 36 Partially edentulous mandible One to three implants 28 (178) 5 months after loading Fully edentulous maxillae. Four to eight implants 60 (121) 12 Partially edentulous mandible 6.3 mm 9.3 mm and vertically augmented bone mm 7mm Test: two short implants failed Control: three long implants failed; augmentation procedure failed in two patients 11.5 mm Test: two short implants failed Control: one long implant failed and one bilateral sinus lift procedure failed 10 mm and vertical augmentation Test: one short implant failed Control: three long implants failed, and two augmentation procedures failed Statistically significantly more complications in augmented patients. Short implants experienced statistically significantly less bone loss. Short implants could be an interesting alternative to vertical augmentation as the treatment is faster, cheaper and associated with less morbidity Significantly more complications occurred in augmented patients. This pilot study suggests that short implants may be a suitable, cheaper and faster alternative to longer implants placed in augmented bone Short implants might be preferable compared with vertical augmentation, reducing the chair time, cost and morbidity 77

7 Nisand & Renouard vertically augmented bone. The authors reported a similar survival rate along with increased treatment time and morbidity for the graft group. Moreover, in one paper (44) a significant increase in peri-implant bone loss was reported for longer implants. One randomized controlled trial was also performed by the same team (48) to compare the outcome of shortlength implants in the edentulous atrophic maxilla with longer implants placed in augmented bone. Five months after loading, similar survival rates were reported for both techniques, with less morbidity for the short-length implant group. A large number of systematic reviews and metaanalyses (Table 4) have also been performed on short-length implants (3, 30, 62, 66, 68, 78, 79, 84, 89, 90, 92, 100, 113, 116). According to these papers, there is fair and growing evidence that short-length implants can be used successfully in atrophied jaws Table 4. Systematic review and meta-analysis on short-length implants Authors (ref. no.) Type of studies (search time) Number of papers included Definition of short implants Main results Main conclusions Annibali et al (3) Jokstad 2011 (66) Pommer et al (89) Systematic Two randomized review and controlled trials and 14 meta-analysis observational studies Systematic review ( ) (See Telleman et al (116)) One randomized controlled trial and 28 prospective cohort studies <10 mm 6193 short implants from 3848 participants. The observational period was years. Cumulative survival rate was 99.1% (95% CI: ). A higher cumulative survival rate was reported for implants with a rough surface <10 mm 2611 short implants ( mm) with a mean follow-up of 3.7 years. The estimated survival rate after 2 years ranged from 93.1% (95% CI: ) for 5-mm implants to 98.6% (95% CI: ) for 9.5- mm implants Systematic review and meta-analysis of observational studies ( ) 54 publications included <10 mm 19,083 implants included. In the mandible, no impact of reduced implant length on failure was observed within the first year of prosthetic loading. A significant impact of implant length for short machined implants was observed in the anterior (odds ratio = 5.4) and posterior (odds ratio = 3.4) maxilla. Short rough-surface implants demonstrated increased failure rates in the anterior maxillary sites (1.4% vs. 0.0%) The provision of short implant-supported prostheses in patients with atrophic alveolar ridges appears to be a successful treatment option in the short term; however, more scientific evidence is needed for the long term There is growing evidence that placement of short (<10 mm) implants can be successful in the partially edentulous patient In areas of reduced alveolar bone height the use of short dental implants may reduce the need for invasive bone augmentation procedures 78

8 Short implant in limited bone volume Table 4. (Continued) Authors (ref. no.) Type of studies (search time) Number of papers included Definition of short implants Main results Main conclusions Telleman et al (116) Sun et al (113) Systematic review ( ) Systematic review ( ) 29 studies <10 mm 2611 short implants ( mm) An increase in implant length was associated with an increase in implant survival (from 93.1% to 98.6%). The estimated survival rate after 2 years for the different implant lengths was 93.1% (95% CI: ) for 5-mm implants, 97.4% (95% CI: ) for 6-mm implants, 97.6% (95% CI: ) for 7-mm implants, 98.4% (95% CI: ) for 8-mm implants, 98.8% (95% CI: ) for 8.5- mm implants, 98.0% (95% CI: ) for 9-mm implants and 98.6% (95% CI: ) for 9.5-mm implants 35 studies 10 mm 14,722 implants included, of which 659 failed (failure rate = 4.5%). The failure rates of implants with lengths of 6.0, 7.0, 7.5, 8.0, 8.5, 9.0 and 10.0 mm were 4.1, 5.9, 0, 2.5, 3.2, 0.6 and 6.5%, respectively. There was no statistically significant difference between the failure rates of short dental implants and standard implants or between those placed in a single stage and those placed in two stages (multivariate analysis). There was a tendency toward higher failure rates for the maxilla and for dental implants with a machined surface. The heterogeneity and low quality of the included studies made meta-analysis impossible There is fair evidence that short (<10 mm) implants can be placed successfully in the partially edentulous patient, although with a tendency toward an increasing survival rate per implant length, and the prognosis may be better in the mandible of nonsmoking patients Among the risk factors examined, most failures of short implants can be attributed to poor bone quality in the maxilla and a machined surface. Although short implants in atrophied jaws can achieve similar long-term prognoses as standard dental implants with a reasonable prosthetic design according to this review, stronger evidence is essential to confirm this finding 79

9 Nisand & Renouard Table 4. (Continued) Authors (ref. no.) Type of studies (search time) Number of papers included Definition of short implants Main results Main conclusions Menchero- Cantalejo et al (78) Neldam & Pinholt 2012 (84) Raviv et al (90) Romeo et al (100) Kotsovilis et al (68) Systematic review and meta-analysis ( ) Systematic review ( ) Literature review Literature review ( ) 10 mm The majority of the studies obtain a cumulative success rate similar to that of longer implants (92.5% and 98.42% for machined and roughsurface implants, respectively). The studies that record lower cumulative success rates are later studies that analyze implants with a machined surface 15 prospective nonrandomized studies, 11 retrospective nonrandomizedstudies and one review 8 mm Data on 6-mm implants were few (Straumann implants representing 441 out of 549 implants). Branemark implants, 7 mm in length, comprised 1607 implants out of Straumann implants, 8 mm in length, comprised 2040 out of 2352 implants. Failures varied between 0 and 14.5%, 0 and 37.5% and 0 and 22.9% for 6-, 7-, and 8-mm-long implants, respectively 13 studies The recent literature has demonstrated a similar survival rate for short and standard implants. Older articles have demonstrated a lower survival rate for short implants Systematic 37 studies reporting on review and 22 patient cohorts meta-analysis ( ) <10 mm or 8 mm There is no significant difference in survival between short ( 8 or < 10 mm) and conventional ( 10 mm) rough-surface implants in totally or partially edentulous patients In view of the results analyzed, rehabilitations with short implants are a reliable treatment; however, the lack of consistency in the study designs as well as the presence of bias in all of the studies reviewed make it difficult to analyze the data Short implant length was not related to observation time, installment region, failures, and dropouts were not specified; subsequently, it was not possible to perform a meta-analysis The treatment planning is a key factor for success in the use of short implants. It can be assumed that a careful treatment planning can lead the clinician to obtain a successful rehabilitation Within the limitations of this systematic review, the placement of short roughsurface implants is not a less efficacious treatment modality compared with the placement of conventional rough-surface implants for the replacement of missing teeth in either totally or partially edentulous patients 80

10 Short implant in limited bone volume Table 4. (Continued) Authors (ref. no.) Type of studies (search time) Number of papers included Definition of short implants Main results Main conclusions Renouard & Nisand 2006 (92) das Neves et al (30) Misch 2005 (79) Structured review ( ) Structured review ( ) Literature review 53 studies 8 mm A relatively high number of published studies (12) indicated an increased failure rate with short implants which was associated with operators learning curves, a routine surgical preparation (independent of the bone density), the use of machined-surfaced implants and implant placement in sites with poor bone density. Recent publications (22) reporting an adapted surgical preparation and the use of textured-surfaced implants have indicated that survival rates of short implants are comparable with those obtained with longer implants 33 studies 10 mm 16,344 implants were included, of which 786 failed (failure rate = 4.8%). Implants 3.75-mm wide and 7-mm long failed at a rate of 9.7%, compared with 6.3% for implants 3.75-mm wide and 10-mm long. Finally, 66.7% of all failures were attributed to poor bone quality, 45.4% to the location (maxilla or mandible), 27.2% to occlusal overload, 24.2% to location within the jaw and 15.1% to infections (an implant could be associated with multiple risk factors) <10 mm The use of a short implant may be considered in sites thought to be unfavorable for implant success, such as those associated with bone resorption or previous injury and trauma. Whilst in these situations implant-failure rates may be increased, outcomes should be compared with those associated with advanced surgical procedures such as bone grafting, sinus lifting and the transposition of the alveolar nerve Short implants should be considered as an alternative to advanced boneaugmentation surgeries, as surgery can involve higher morbidity, require extended clinical periods and involve higher costs to the patient 81

11 Nisand & Renouard Table 4. (Continued) Authors (ref. no.) Type of studies (search time) Number of papers included Definition of short implants Main results Main conclusions Hagi et al (62) Structured review ( ) 12 7 mm Machined surface implants experienced greater failure rates than did textured surface implants. With the exception of sintered poroussurface implants, 7- mm-long dental implants appear to have higher failure rates than those >7 mm in length Dental implant surface geometry is a major determinant in how well short dental implants performed (3, 66, 68, 78, 90, 100, 116), reducing both the need for invasive and complex surgery (30, 89, 92, 113) and treatment morbidity (30, 92). However, there is a tendency for increased failure rates with machined surface implants (89, 92, 113), placement in smokers (116) and placement in specific locations, such as the severely resorbed posterior maxilla (113, 116) and the anterior maxilla (89). Longer follow-up times of up to 10 years are also needed to confirm these findings and to evaluate the impact of annual marginal bone loss on survival rate (3, 92, 113). Extra-short implants Three case series (36, 86, 108) and one randomized controlled trial (43) were recently performed to evaluate the survival rate of extra-short implants supporting fixed partial dentures in severely resorbed posterior jaws. In the paper by Slotte et al. (108), three to four 4-mm implants were inserted in the posterior mandibles of 24 patients (87 implants in total) to support fixed partial dentures. Two years after loading, a survival rate of 92.3% was reported. Using a split-mouth design, Esposito et al. (43) compared 5-mm implants with 10-mm implants in augmented bone (with either interpositional bone blocks in the mandible or sinus lift in the maxilla) to restore either the posterior mandible (15 patients) or the posterior maxilla (15 patients). They report similar outcomes for both techniques. The use of extra-short implants allows patients to be treated with lower cost and less morbidity. However, so far only sparse short-term data are available. Further studies are needed to evaluate the long-term prognosis. Stress repartition and crown-toimplant length ratio A dogma (4) states that the prognosis of abutment teeth and prosthetic rehabilitation is related to the crown-to-root ratio. According to this statement, it is assumed that for successful prosthetic rehabilitation the crown-to-root ratio should always be 1. However, this statement was not supported in a recent systematic review (72) which reported similar outcomes for abutment teeth with or without a history of periodontal bone loss. Nevertheless, this empirically based crown-to-root ratio guideline is commonly applied for dental implant-supported restorations, frequently resulting in the placement of the longest implant possible. In areas of reduced bone volume, in which short implants must be placed, bone resorption is often accompanied by increased maxillomandibular space, with the prosthetic consequence of excessive crown height (Fig. 5). In such sites, clinicians tend to perform advanced surgical procedures to allow the placement of longer implants, thus lowering the crown-to-implant ratio. According to the definition provided by Blanes et al. (16), two types of crown-to-implant ratio can be established: the anatomical crown-to-implant ratio, in which the transition line is located at the level of the implant shoulder; and the clinical crown-to-implant ratio, in which the transition line is located at the level of the bone crest. 82

12 Indications and clinical procedures Short implants Short implant in limited bone volume Fig. 5. Periapical radiograph showing a 7-mm-long implant with a crown-to-implant ratio of >1.5 after 7 years of loading. To the best of our knowledge, there are only a few descriptive studies (Table 5) that have evaluated the impact of the crown-to-implant ratio on peri-implant bone loss (15, 16, 97, 104, 115), implant survival rate (16, 104, 106) or the occurrence of biological and technical complications (104). Among this group of publications, three studies involved only single-tooth implant-supported restorations (15, 104, 106), thus avoiding the bias of better occlusal force distribution in studies involving mainly splinted implant restorations (16, 97, 115). These three studies (15, 104, 115) demonstrated that marginal bone loss was not related to the crown-to-implant ratio, whereas two studies (16, 97) indicated that implant restorations with higher crown-to-implant ratios displayed statistically lower marginal bone loss than did implant restorations with lower crown-to-implant ratios. According to Blanes et al. (16), the latter might be explained by the stimulatory nature of bone stress. Similar survival rates have been reported for implant restorations with high and low crown to implant ratios (16, 104, 106). The crown-to-implant ratio has also been found to have no statistically significant influence on the occurrence of biological and technical complications (104). These results are in accordance with the outcomes of a systematic review performed by Blanes (17) on the impact of the crownto-implant ratio on the survival and complication rates of implant-supported reconstructions. However, it should be remembered that the crown-to-implant ratios of most of the implant-supported restorations included were between 1.0 and 2.0, and very few data are available on crown-to-implant ratios of >2.0. Therefore, further studies should investigate the impact of crown-to-implant ratios of >2.0 on marginal bone loss, the implant survival rate and the occurrence of biological and technical complications. Short-length implants may be indicated without any dogma in areas of reduced bone height (such as the posterior maxilla and the posterior mandible) following tooth extraction. Bone height in the premolar and molar regions of the maxilla may be reduced by sinus expansion. A remaining bone height of 7 mm may indicate that short implants should be used (Fig. 6A D). With 5 6 mm of available bone, the decision to use short implants should be based on bone quality and existing risk factors for marginal bone loss over time (e.g. a history of periodontitis and smoking), as well as the patient s age. With <5 mm of available bone below the floor of the sinus, a sinus bone-grafting procedure is recommended (Table 6) (Fig. 7A E). Bone height in premolar and molar regions of the mandible may be limited by the inferior alveolar nerve. As a distance of at least 2 mm should be maintained between the implant and the inferior alveolar nerve, short-length implants may be considered only when the available bone above the nerve is 8mm (Fig. 8A D). With <8 mm of bone, advanced surgical procedures may be indicated to allow the placement of dental implants (Table 7). Short-length implants may be indicated to support single- and multiple-fixed reconstructions in the posterior jaws. For multiple tooth replacement, no strong recommendation can be made with respect to splinting or with regard to the optimal number of implants per prosthetic unit. Short-length implants may also be recommended in the treatment of a severely resorbed edentulous mandible, with four short-length implants used to support an overdenture or six short implants used to support a fixed reconstruction. In the edentulous maxilla, two short-length implants additionally placed in the distal area, together with longer implants in the premaxilla, may be indicated to support a maxillary overdenture or a fixed reconstruction. From a clinical point of view, high short-implant survival rates may be obtained using a surgical bone preparation adapted to the patient s bone quality and the implant design in order to reach sufficient initial primary stability. Moreover, a micro-rough implant surface should be selected to improve periimplant bone growth, bone-to-implant contact and bone anchorage, thus reducing the time between mechanical primary stability and biological secondary stability. 83

13 Nisand & Renouard Table 5. Studies on the impact of crown-to-implant ratio Authors (ref. no.) No. of patients (no. of implants) Follow-up (months) Type of prosthesis Mean crownto-implant ratio Crown-toimplant ratio range Survival rate (%) Bone loss (mm) Rokni et al (97) Tawil et al (115) Blanes et al (16) Schulte et al (106) Birdi et al (15) Schneider et al (104) 74 (199) % single crowns 38.2% splinted restorations 109 (262) % single crowns 87.4% splinted restorations 83 (192) % single crowns 86.5% splinted restorations 294 (889) 27.6 Single crowns Success implant: crown-toimplant ratio = 1.3 Failed implant: crown-toimplant ratio = to < % with a crown-toimplant ratio of and > % with a crown-toimplant ratio between 1 and 2; 3.4% with a crown-toimplant ratio of > % with a crown-toimplant ratio of 2 4.2% with a crown-toimplant ratio of > to < (309) 20.9 Single crowns Crown-to-implant ratio 1: Crown-to-implant ratio > 1to2: Crown-to-implant ratio > 2: Crown-to-implant ratio < 1: Crown-toimplant ratio : Crown-to-implant ratio : Crown-to-implant ratio : Crown-to-implant ratio : Crown-to-implant ratio > 2: Crown-to-implant ratio < 1: Crown-toimplant ratio 1to< 2: Crown-to-implant ratio 2: (100) 60 Single crowns Clinical crown-toimplant ratio: Anatomical crown-toimplant ratio: Clinical crown-toimplant ratio: Anatomical crown-toimplant ratio:

14 Short implant in limited bone volume Table 6. New classification for treatment of the resorbed maxilla Alveolar ridge height* Therapeutic options Bone type I, II, III <5 mm Sinus lift Sinus lift 5to 6 mm Short implants Sinus lift Bone type IV, history of periodontitis, smokers, patient age 6 mm Short implants Short implants *This classification is suitable for a residual alveolar ridge width of at least 5 mm. The long-term prognosis of short-length implants may be altered by marginal bone loss over time. Until now, available data are too scarce to draw any definitive conclusions with regard to the impact of platform-switching, micro-thread and types of connections on the peri-implant bone level of short implants. Therefore, it is strongly recommended that patients should be included in supportive therapy (96) to improve both the survival rate and the maintenance of the marginal bone level. Long-term prognosis of short-length implants may also be affected by peri-implantitis. However, it should be remembered that the removal of a short implant is a relatively simple procedure with minimal bone destruction compared with the removal of a long implant, which may jeopardize adjacent teeth or the replacement of the implant. Implant length selection For years, clinicians have tended to place the longest implants possible to improve bone-to-implant contact, implant primary stability and the crown-toimplant ratio. However, recent knowledge in implant dentistry has shown that bone-to-implant contact may also be improved by the use of micro-rough surfaces, and adequate implant primary stability can be achieved through the use of an adapted surgical preparation and new implant designs. Similarly, recent publications have shown that marginal bone loss, the implant survival rate and the incidence of complications are not related to the crown-to-implant ratio. The placement of the longest implant possible may have some drawbacks. It increases bone preparation time, exacerbating the risk of bone overheating and inappropriate bone preparation (oversized bone preparation), which ultimately could reduce implant primary stability (8). Using the longest implant possible also increases the risk for nerve injury or sinus perforation. Lastly, in the esthetic A C B D Fig. 6. (A) Preoperative cone beam computed tomography scan of a missing right secondary molar showing 8 mm of available bone below the floor of the sinus. (B) Soft tissue healing 2 months after the placement of a short-length implant (8 mm in length and 4 mm in diameter). (C) Periapical radiograph 2 years after loading. (D) Clinical view of the prosthetic restoration after 2 years of loading. 85

15 Nisand & Renouard A B C E D Fig. 7. (A) Preoperative cone beam computed tomography scan of a missing right maxillary second premolar and molars showing 1 3 mm of available bone below the floor of the sinus. (B) Preoperative cone beam computed tomography scan showing maxillary septa that may complicate Schneiderian membrane elevation. (C) The trap door is created and the Schneiderian membrane is elevated without perforation, despite the presence of an incomplete septum. (D) Postoperative cone beam computed tomography scan 6 months after the sinus lift procedure. (E) Periapical radiograph of the implant-supported fixed partial denture after 4 years of loading. anterior area (Fig. 9A E), the use of the entire available bone may lead to overly angulated implants, thus increasing the risk for gingival retraction and the need for a cemented restoration. In the posterior area, the use of the longest implant possible may be associated with incorrect implant angulation or position, with the occlusal consequence of inadequate load repartition. Therefore, there are clinical situations in which the entire available bone should not be used, to allow the surgeon to focus his limited resources on optimal three-dimensional implant positioning. 86

16 Short implant in limited bone volume Table 7. New classification for treatment of the resorbed mandible Alveolar ridge height* Therapeutic options Bone type I, II, III and IV <8 mm Advanced surgical techniques 8mm Short implants *This classification is suitable for a residual alveolar ridge width of at least 5 mm. A new concept in implant dentistry: the stress-minimizing surgery In 2011, the European Association of Dental Implantologists concluded its consensus conference on short implants with the following recommendation to avoid complications: the implant surgeon and restorative dentist should have adequate clinical experience (12). From this recommendation, one may legitimately wonder whether this conclusion encourages practitioners to avoid complications by focusing on complex bone-reconstruction solutions in order to place longer implants, or to promote alternative treatments such as partial dentures or long-span dental bridges. The statement is symptomatic of the decision-making process in implant dentistry, which rarely considers the complexity of procedures and instead tends to be based solely on success or survival rates. Thus, a few percentage points higher or lower in survival rate may be deemed sufficient to guide therapeutic decisions. It seems that, to promote the best overall patient outcomes, other factors, such as feasibility and morbidity, should be considered when making a therapeutic choice (Fig. 10A Q). Feasibility Experience The actual feasibility of a large number of more complex surgical protocols is never touched upon in many discussions surrounding therapeutic options. It seems that much effort is expended to omit the reality of clinical life for the vast majority of practitioners. Worldwide, the average number of implants placed annually by most practitioners is estimated to be fewer than 50. This figure seems to be quite minimal in terms of gaining the surgical experience required for the implementation of complex protocols. Studies of neurocognitive activity show that the part of the brain that manages both complex and novel procedures lies in the prefrontal cortex, the most anterior region of the brain (39, 73). Tasks utilizing the prefrontal cortex require conscious effort and, importantly, consume vast cognitive resources (105). Complex tasks, such as surgical procedures, as well as tasks that are unfamiliar, require the prefrontal cortex to remain active and the brain s full resources to remain accessible. However, under some conditions, specifically stress, fatigue and burnout, this access becomes impaired. Advanced surgeries represent particularly high-stress activities for less-experienced A B C D Fig. 8. (A) Preoperative cone beam computed tomography scan of a missing left first molar showing 9.5 mm of available bone above the inferior alveolar nerve. (B) Soft tissue healing 2 months after the placement of a short-length implant (8 mm in length and 5 mm in diameter). (C) Periapical radiograph 3 years after loading. (D) Clinical view of the prosthetic restoration after 3 years of loading. 87

17 Nisand & Renouard A B D C E Fig. 9. (A) Preoperative cone beam computed tomography scan showing internal resorption of a left central incisor. (B) Gingival retraction and inflammation around the left central incisor. (C) Six weeks after a flapless tooth extraction, the implant is placed together with a guided bone-regeneration procedure. The implant is inserted in the correct three-dimensional position to allow insertion of a screw-retained prosthesis and to avoid gingival retraction. (D) Periapical radiograph 6 months after the placement of a provisional crown. A shorter implant (11.5 mm in length and 3.5 mm in diameter) was used to allow the placement of a screw-retained prosthesis. (E) Soft tissue healing around the provisional crown after 6 months. practitioners, and the prefrontal cortex becomes inaccessible in precisely the complex situations where its use is a priority. Over time and with repetition, complex tasks become more routine and the advanced processing abilities of the prefrontal cortex are needed less and less to perform them. By repeatedly practicing a single type of intervention, practitioners described as experts have gradually transferred the new gesture, which was initially managed by the prefrontal cortex, to the limbic brain. The limbic brain is a group of brain structures consisting of multiple subcortical entities such as the hippocampus, the amygdala and the hypothalamus. It is located in the middle part of the brain. This part of the brain is involved in emotions and manages routine actions. This automatic mental mode requires far fewer resources and is thought to be used for as much as 80% of all activities performed. It takes time and large amounts of repetition before a complex act can be made partially routine while still being accomplished with a high success rate. Moreover, experience is necessarily obtained by 88

18 Short implant in limited bone volume the occurrence of errors and failures, which are then analyzed, primarily unconsciously, by the anterior cingulate cortex (22), allowing the brain gradually to adjust and routinize the procedure. This phenomenon is made possible as a result of brain plasticity, allowing the brain to restructure itself continuously upon exposure to new stimuli (29). Learning leads to a cerebral reprogramming that is based on successes but mainly on failures. This long learning process is necessary to transfer portions of the work of the prefrontal cortex to the limbic brain. The aim is that the majority of gestures are accomplished without having to think about them. This can occur for an entire, or for only part of, a procedure. Unconsciously, the operator uses both parts of the brain simultaneously. The act of transferring some of the workload to the limbic brain allows an individual both to conserve limited mental resources (93) and to make room in the prefrontal brain to handle new parameters from sensory input in complex situations, such as the observations that this dissection is difficult, the patient is becoming stressed, the patient is bleeding more than usual or the patient has a voluminous tongue that is interfering with surgery. These additional parameters add to the complexity of the situation, placing new demands on the prefrontal cortex. The novice practitioner who performs this type of surgery only a few times a year will not have enough experience to routinize the procedure. Therefore, every act will require significant cognitive effort, with the risk of quickly overloading the prefrontal brain. The immediate consequence of this is to induce stress, with the corollary being a significant decline in operating efficiency, which, in turn, further increases the stress level. A vicious cycle is created, eventually leading to an uncontrolled intervention that is more likely to generate complications. Whatever the scope is, expertise is created over a long learning period, estimated at about 10 years. The more a protocol is simplified and perfectly codified, the easier it is for large numbers of people to learn. The use of short implants fits perfectly into this cognitive analysis of surgical difficulty. While the ideal would be to rebuild all patients ad integrum, we must accept that in reality relatively few surgeons can acquire the necessary level of experience and expertise required to perform this type of intervention with a high and consistent success rate. Nontechnical human factors Many nontechnical parameters, such as stress, fatigue, overconfidence (14) and the lack of preparation or organization (81), can influence the outcome of a procedure. In this vein, a study of 9,830 surgical procedures in a London hospital demonstrates the importance of nontechnical factors in the occurrence of surgical complications (45). Stress is probably one of the complicating factors shared most widely by dental and maxillofacial surgeons. It is difficult for most practitioners to manage both the technical and emotional aspects of a patient who is usually under local anesthesia. The emotional impact of the relationship with the patient is a layer of complexity on top of the purely technical aspect of the procedure, making the whole treatment even more complex and demanding even more in terms of cognitive resources. Nevertheless, the stress and commensurate increased risk of complication generated by implementing complex procedures is rarely mentioned in discussions about treatment decision-making. Stress occurs when there is a mismatch between an individual s perception of the constraints imposed by the environment and that individual s perception of his or her own resources to cope with it (10, 11, 85). We may also explain stress as a conflict of resource mobilization and accessibility: when knowledge exists but it is not immediately available when needed, stress occurs. We now understand that it is not the situation that is stressful per se, but the individual s reaction to the situation. Stress in humans is 90% endogenous. Accordingly, a surgeon with less experience and little practice will be more stressed and will tend to approach complex surgeries in a more negative cognitive state. Hence, the level of alertness of the less-experienced practitioner is more likely to be affected by cognitive overload during these unusual events. Under heavy stress, practitioners or therapeutic team members may see their cognitive abilities diminish until they become incapable of making rational decisions (76, 77, 109). This state is termed mental tunneling (27, 93). Overwhelmed by stress, the practitioner is unable to analyze the situation and the surroundings. The practitioner may even be subject to regression, namely the implementation of solutions learned previously and now managed by the limbic brain, forgetting recent knowledge gains. The overstressed practitioner will then react in one of two ways: with anxiety; or simply with the urge to avoid the situation. This is an instance of the fight-or-flight response (19, 24). Physically unable to leave the operating theater, the surgeon may have the impulse to get the surgery over with, regardless of the consequences of mistakes, oversights or surgical shortcuts. Instead of prioritizing the use of their prefrontal 89

19 Nisand & Renouard A B C D E F G H I J K L M N O P Q 90