Does Implant Staging Choice Affect Crestal Bone Loss?

Does Implant Staging Choice Affect Crestal Bone Loss? J Oral Maxillofac Surg 70:307-313, 2012 Hakimeh Siadat, DDS, MS,* Mehrdad Panjnoosh, DDS, MS, Marzieh Alikhasi, DDS, MS, Masoud Alihoseini, DDS, Seyed Hossein Bassir, DDS, and Amir Reza Rokn, DDS, MS Purpose: The purpose of the present study was to compare the crestal bone loss around implants placed according to either a 1-stage or 2-stage implant installation procedure using a digital subtraction radiography technique. Materials and Methods: In the present randomized clinical trial, screw-shaped tapered implants were inserted in the posterior mandible of patients needing fixed partial dentures. In each edentulous area, according to the randomization table, 1 implant was inserted using a 1-stage procedure (group 1) and 1 was placed using a 2-stage approach (group 2). The implants were temporized with the relined denture after 2 weeks. All implants were functionally loaded with fixed partial dentures after 3 months. Crestal bone loss (primary outcome variable) was measured using a digital subtraction radiography technique. Standardized radiovisiographs were taken after implant insertion, after fixed partial denture installation (3 months after surgery), and after 6 and 12 months of functional loading. The data were analyzed using the Wilcoxon signed ranks test ( 0.05). Results: Eleven patients (mean age 46.9 years, 3 women and 8 men) were included in the study. A total of 34 implants were inserted, 17 using a 1-stage protocol and 17 using a 2-stage protocol. Three months after implant placement, the 2-stage implants showed significantly more crestal bone loss (0.65 0.71 mm) than the 1-stage implants (0.41 0.53 mm; P.02). However, after 6 and 12 months of functional loading, both groups showed comparable changes in bone level (P.05). Conclusions: No differences were found between 1-stage and 2-stage implant placement in crestal bone loss after 1 year of functional loading. 2012 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 70:307-313, 2012 There are 2 surgical approaches for dental implant placement: 1-stage (nonsubmerged) and 2-stage (submerged) placement techniques. In the former approach, the implant is placed so the soft tissue flap is placed around the coronal portion of the implant body or healing abutment. In the latter surgical approach, the implant is placed at or below the alveolar crest and the soft tissue is closed over the implant. Then, after an appropriate period, a second procedure is performed to expose the implant platform and attach the healing abutment. 1,2 During the initial phase of healing, most implant systems recommend that the fixture should be submerged. There are several reasons for such a recom- *Associate Professor, Implant Research Center and Department of Prosthodontics, Tehran University of Medical Sciences School of Dentistry, Tehran, Iran. Assistant Professor, Department of Oral and Maxillofacial Radiology, Tehran University of Medical Sciences School of Dentistry, Tehran, Iran. Assistant Professor, Dental Research Center and Department of Prosthodontics, Tehran University of Medical Sciences School of Dentistry, Tehran, Iran. General Dentist, Tehran University of Medical Sciences School of Dentistry, Tehran, Iran. General Dentist, Dental Student Research Center, Tehran University of Medical Sciences School of Dentistry, Tehran, Iran. Associate Professor, Implant Research Center and Department of Periodontics, Tehran University of Medical Sciences School of Dentistry, Tehran, Iran. This project was supported by grant 3422 from Tehran University of Medical Sciences. Address correspondence and reprint requests to Dr Alikhasi: Dental Research Center and Department of Prosthodontics, Tehran University of Medical Sciences School of Dentistry, North Amirabad, Tehran, Iran; e-mail: m_alikhasi@yahoo.com 2012 American Association of Oral and Maxillofacial Surgeons 0278-2391/12/7002-0$36.00/0 doi:10.1016/j.joms.2011.09.006 307

308 IMPLANT STAGING AND CRESTAL BONE LOSS mendation, including minimizing the risk of infection, reducing vertical bone loss, decreasing the risk of undue early loading, and preventing apical downgrowth of mucosal epithelium. 3 However, the 1-stage approach provides a number of advantages such as avoidance of a second procedure, which results in a simplified prosthetic procedure, less chair time, and reduced treatment costs. 4-7 Longitudinal studies have shown that implants placed using both 1-stage and 2-stage techniques can be clinically successful in a large percentage of cases for extended periods. 7-9 In addition, promising results have been reported by placement of a 2-stage implant system using a 1-stage surgical procedure. 6,10 However, some studies have shown that significant differences in bone level changes can be detected between the 1-stage and 2-stage implant installation techniques during the first and second year of function. 3,10-12 There are studies reporting that micromovement during the bone healing phase, to some extent, could be favorable to osseointegration. For example, adjusted and relined denture on the implants during the healing phase could cause an optimal loading condition. However, the effects of such loading on the implants during the healing phase should be investigated further. 13 One of the assessment criteria for successful endosseous implantation is radiographic peri-implant bone changes. This criterion is a mean crestal bone loss of less than 1.5 mm in the first year after abutment connection and less than 0.2 mm in subsequent years. 14-17 Different methods have been used to measure crestal bone resorption around dental implants, from counting the number of threads on the implants to measurements using a computerized and interactive image analysis system. Nevertheless, the computerized methods are considered to provide greater accuracy than the conventional methods. 18,19 It has been shown that image analysis routines, such as digital contrast enhancement, noise suppression, and filtering, increase the diagnostic accuracy of crestal bone loss measurement. 20 However, only a few clinical studies have used computerized methods to measure the crestal bone resorption around dental implants. Moreover, most studies have not evaluated 2 approaches (1-stage and 2-stage) in the same patient. 1,3,10-12,21 Although it has been shown that proper osseointegration and a good survival rate can be obtained with both 1-stage and 2-stage implant installation techniques, 11 data are still limited regarding the risk of crestal bone loss using 1-stage and 2-stage implant placement techniques. Therefore, the research question for the present study was whether, among patients receiving dental implants, implants placed using a single-stage protocol, compared with those using a 2-stage protocol, have an increased risk of crestal bone loss. The null hypothesis was that no difference would be found in crestal bone resorption between the 1-stage and 2-stage implants. The specific aim of the present randomized clinical trial was to compare the crestal bone loss between 1-stage and 2-stage implants using a digital subtraction radiography technique. Edentulous areas were temporized with relined dentures 2 weeks after surgery. They were then functionally loaded with fixed partial dentures 3 months after surgery. Materials and Methods STUDY DESIGN AND POPULATION To address the research purpose, we designed and implemented a randomized clinical trial. The study population was composed of all patients presenting for evaluation and rehabilitation of partially edentulous areas of the posterior mandible from March 2007 to October 2009, at the Department of Implantology, Tehran University of Medical Sciences Faculty of Dentistry (Tehran, Iran). The inclusion criteria were age older than 21 years, good oral hygiene, 3 adjacent teeth missing in the posterior mandible, a sufficient amount of bone ( 10 mm) in the recipient sites, occlusal loading with the antagonistic natural teeth, and type II or III bone, as determined by the surgeon s tactile sense. The patients were excluded if they had any health-related contraindication for elective oral surgery intervention; the use any medication or drug that could jeopardize the treatment outcome; had signs of untreated periodontal disease or other mucosal and bone tissue lesions; had any signs of parafunctional habits; and if they were heavy smokers ( 10 cigarettes/day). All subjects signed an informed consent form after being informed about the treatment protocol. The study was performed according to the principles outlined in the Declaration of Helsinki on experimentation involving human subjects. The university s clinical research ethics board approved the research protocol, including the inclusion and exclusion criteria, treatment procedures, and evaluation approach. STUDY VARIABLES The primary predictor variable in the present study was the implant staging technique used. In each edentulous area, according to the randomization table, 1 implant was inserted using the 1-stage procedure (group 1) and 1 was placed using the 2-stage approach (group 2). Implant placement in the anterior and posterior sites of each edentulous area was also randomized to ensure that the implant position in the arch could not influence the results.

SIADAT ET AL 309 The primary outcome variable in the present study was crestal bone loss over time. It was measured radiographically using a digital subtraction radiography technique after implant insertion, fixed partial denture (FPD) installation (3 months after surgery), and after 6 and 12 months of functional loading. SURGICAL AND PROSTHETIC PROCEDURES All patients received oral hygiene instructions for self-performed plaque control measures and had a full-mouth plaque score of less than 20%. The surgical treatment was performed with the patient under local anesthesia. A crestal incision was made, and the mucoperosteal flap was reflected on the buccal and lingual sides. To obtain tension-free adaptation of the wound margins and for close adaptation of the gingiva, if needed, periosteal relieving and contouring incisions were performed. In the 1-stage procedure, the implant was placed, and a healing abutment was immediately connected. The soft tissue was subsequently adapted to the implant and sutured. Screwshaped tapered implants (Replace Select, Nobel Biocare, Gothenburg, Sweden), 10 mm long and 4.3 mm in diameter (regular platform) were used. In the 2-stage procedure, the implant was inserted, and a cover screw was then placed. Wound closure over the 2-stage implant was achieved using horizontal mattress and interrupted sutures. Ten weeks after implant placement, second-stage surgery was performed, and a healing abutment was connected to the 2-stage implant. All implants were inserted at the crestal bone level, with acceptable primary stability (30 to 40 Ncm insertion torque). An experienced surgeon performed all surgeries. Patients received antibiotic and anti-inflammatory medication 1 hour after implant installation. Seven days after each surgical procedure, the sutures were removed. During the first 2 weeks after surgery, plaque control was maintained by rinsing the oral cavities daily with 0.12% chlorhexidine gluconate (Hexodine, Donyaye Behdasht CO, Tehran, Iran). In both groups, impressions for the removable partial denture were taken before surgery, and a removable partial denture with a 2-mm spacer was made for each patient. The relined removable partial denture with a soft liner (UFI Gel P, Voco, Cuxhaven, Germany) was used 2 weeks after surgery. Three months after surgery, the implants were functionally loaded with FPDs. All FPDs were made of gold metal and ceramic and were cemented using temporary cement (TempBond, Kerr Dental, Orange, CA). DATA COLLECTION The marginal bone level changes were measured using a digital subtraction radiography technique. The radiovisiograph (RVG) imaging system (Trophy, Marne-La Vallée, France), which uses charged coupled device sensors, was used. Radiographs were taken after implant insertion, after FPD installation (3 mo after surgery), and after 6 and 12 months of functional loading (Figs 1, 2). Standardized digital images were taken using extension cone paralleling and sensor holder-using paralleling technique. A bite index was recorded for each patient, using silicon material (Speedex Putty, Coltène/Whaledent, Cuyahoga Falls, OH). The bite index was saved and reused to provide a geometrically reproducible alignment. All digital images were obtained with a dental X-ray generator (Plan Mecca Intra, Helsinki, Finland), with a 0.7-mm 2 focal spot, a total filtration of 2 mm Al, operated at 70 kvp and 8 ma. Contrast differences were corrected before the images to be subtracted. The Photoshop CS2 (Adobe System, Tokyo, Japan) subtraction program was used to subtract the images and evaluate the bony changes after implantation. Marginal bone changes were recorded with 0.1-mm accuracy. Crestal bone loss for each implant was measured at the mesial and distal aspects and the average recorded (Fig 3). The distance between 2 threads of the implant was used for measurement calibration. All radiographic analyses were performed by an experienced oral and maxillofacial radiologist who was unaware of the bone loss measurements at the previous visits but was not kept unaware of the placement approach (1 stage vs 2 stage). STATISTICAL ANALYSIS A power calculation was performed to determine the sample size. The implant was considered as the statistical unit. It was calculated that 15 implants per group (total of 30 implants) would provide 80% power to recognize a significant difference of 1.5 mm between the test and control, using crestal bone loss as the primary outcome variable. To protect from possible withdrawals, the sample size was increased by 10%, resulting in 17 implants per group. Therefore, a sample size of 17 test and 17 control implants was recruited. The Kolmogorov-Smirnov test was used to evaluate the normality of the data. When the data did not reach normality, nonparametric methods were used. Although 6 patients had implants in both quadrants of the mandible, because both case and control implants were inserted bilaterally, clustering was not considered in the statistics. Statistical analysis for the determination of differences in the marginal bone levels were performed using the Wilcoxon signed ranks test. Statistical significance was set at 0.05.

310 IMPLANT STAGING AND CRESTAL BONE LOSS 1-stage protocol and 17 using the 2-stage protocol. Of the 11 patients, 6 had bilateral and 5 unilateral freeend edentulous regions (Table 1). A total of 132 RVGs were made to determine the alveolar bone levels around the implants. No fenestra- FIGURE 1. Radiograph of 2-stage implant A, at surgery and B, after 1 year of loading. Results The study sample included 11 subjects (3 women and 8 men) aged 21 to 57 years (mean 46.9). A total of 34 implants were inserted, with 17 placed using the FIGURE 2. Radiograph of 1-stage implant A, at surgery and B, after 1 year of loading.

SIADAT ET AL 311 FIGURE 3. Example of digital subtraction image with regions of interest. Lighter shades of gray reflect bone gain and darker shades of gray reflect bone loss. tion was found over the 2-stage implants during the healing phase. The mean standard deviation of the peri-implant bone levels for the 1-stage and 2-stage groups at different intervals are listed in Table 2. Three months after implant insertion, the 2-stage approach caused significantly more crestal bone loss than the 1-stage approach (0.65 0.71 mm versus 0.41 0.53 mm; P.02). After 6 months of functional loading, the 1-stage (0.29 0.49 mm) and 2-stage (0.21 0.40 mm) implants both showed closely similar bone levels (P.17). Also, at the end of the study, after 1 year of loading, 1-stage (0.30 0.51 mm) and 2-stage (0.16 0.30 mm) implants both had comparable bone levels (P.38). The results also showed that most of the implants exhibited crestal bone loss within the range of 0 to 0.5 mm in both groups after 1 year of loading. However, 1 implant in the 2-stage group had crestal bone loss of 1.0 mm, and 3 implants in the 1-stage group exhibited a bone loss of 1.2 mm. Discussion The present randomized clinical trial aimed to compare the crestal bone changes around implants placed according to 1-stage and 2-stage procedures in edentulous posterior mandible. Both types of implant placement were implemented in each patient for maximal comparability. The implants were temporized with relined dentures 2 weeks after surgery and FPDs were delivered 3 months after surgery. The key finding of the present study was that no significant difference was found between the 1-stage and 2-stage procedures with regard to the bone loss after 6 and 12 months of functional loading. The findings of the present study showed that irrespective of the surgical installation protocol, the implants exhibited only a small amount of radiographic crestal bone loss during the first year of function. The average crestal bone loss in the 1-stage and 2-stage groups was 0.29 mm and 0.21 mm after 6 months and 0.30 mm and 0.16 mm 1 year after insertion of FPDs, respectively. According to the generally accepted success criteria for osseointegration implants formulated by Albrektsson et al, 14 based on an average marginal bone loss of less than 1.5 mm during the first year after insertion of the FPDs, implants placed using either 1-stage or 2-stage procedures met the success criteria. Although no statistically significant difference was seen in between the 2 groups after 6 and 12 months of functional loading, the crestal bone loss was significantly greater in the 2-stage group than in the 1-stage group during the first 3 months after surgery (before FPD insertion). This was probably because when the implants are temporized by relined dentures during the healing period, 2-stage implants are loaded indirectly by way of the mucosa, and 1-stage implants can be loaded directly even if the dentures were properly adjusted and relined. 12 Because it has been shown that early loading of implants could stimulate remodeling of marginal bone, 22 lower marginal bone loss in the implants placed using the 1-stage protocol during Table 1. PATIENT POPULATION AND CLINICAL DATA Parameter Value Patients (n) 11 Age (yr) Mean 46.9 Range 21-57 Gender (n) Male 8 Unilateral implants 4 Bilateral implants 4 Female 3 Unilateral implants 1 Bilateral implants 2 FMPS (%) 16 7.2 Smokers (%) 27.3 Abbreviation: FMPS, full mouth plaque score.

312 IMPLANT STAGING AND CRESTAL BONE LOSS Table 2. CRESTAL BONE LOSS CHANGES AT IMPLANTS IN BOTH GROUPS Evaluation Point Implants (n) Crestal Bone Loss (mm) P Value 3 mo After surgery 2-Stage 17 0.65 0.71.02 1-Stage 17 0.41 0.53 6 mo After functional loading 2-Stage 16 0.21 0.40.17 1-Stage 16 0.29 0.49 12 mo After functional loading 2-Stage 16 0.16 0.30.38 1-Stage 16 0.30 0.51 Data presented as mean standard deviation, unless otherwise noted. first 3 months of healing could be explainable. However, judgment as to whether this is actually the case requires more investigation. An additional finding of the present study was that, in both groups, the largest amount of bone loss occurred during the first 3 months after implant installation. This finding is in line with data from previous studies, which have shown that the crestal bone level after rehabilitation seemed to remain stable in implants placed using to either the 1-stage or 2-stage protocol. 11,23 The results of the present study are supported by those from several studies that have shown comparable bone loss in both 1-stage and 2-stage implants after functional loading. 3,10,11,21,22 Cecchinato et al 11,21 compared the marginal bone changes around Astra Tech implants placed using either the 1-stage or 2-stage protocol during a 5-year period. They showed that marginal bone loss is independent of the implant installation protocol. They reported a mean marginal bone loss of 0.02 0.38 mm and 0.17 0.51 mm around 1-stage and 2-stage implants in the first year, respectively. During the second year, the amount of crestal bone loss in the 2 groups was near 0. In addition, the difference in marginal bone loss between the 2 installation protocols was not significant during 5 years of function. The difference between the present study and that of Cecchinato et al 11,21 is that conventional radiography was used in that study. Moreover, 1-stage and 2-stage implants were placed in different patients in their study, which could limit the comparability of the results. Ericsson et al 3 conducted a study with a split-mouth design in the mandible to evaluate 1-stage and 2-stage implant placement procedures. The analysis of standardized radiographs at 12 and 18 months showed that the crestal bone loss was similar in both treatment groups. The mean crestal bone loss around the 1-stage and 2-stage implants was 1.1 0.5 mm and 0.8 0.3 mm during the first year, respectively. Petersson et al 22 also reported similar results comparing the marginal bone loss of 1-stage and 2-stage placement protocols using Brånemark dental implants after 18 months (0.2 vs 0.3 mm, respectively) and 5 years (1 mm in both groups). In these cited studies, standardized radiographs were obtained and the images digitized; however, in the present study, RVGs were taken. Measurement accuracy of the marginal bone level is influenced by the precision of the radiographic technique and the measurement technique used. 22,24 In the present study, radiographs were taken using extension cone paralleling and sensor holder by paralleling technique. RVGs provide greater accuracy than conventional methods. 18,19 Therefore, the differences in the crestal bone loss between these studies could have stemmed from the different radiographic techniques used. One limitation of the present study was that the 2 implant installation procedures were only compared radiographically. Conventional radiographs show only 2-dimensional information. Therefore, crestal bone loss and soft tissue recession occurring at the buccal or lingual aspects might be missed. Thus, additional studies are recommended to combine the radiographic examination with clinical measurement to assess the differences between the 1-stage and 2-stage implant installation procedures. Another limitation of the present study was that the data were analyzed at the level of the implant, which does not take the clustering of the implants within a patient into account. However, because each patient, whether they received 2 implants or 4, had both 1-stage and 2-stage implants in each side, the clustering would have the same effect on the results of both groups. 25 In the present study, in the 1-stage group, an originally submerged implant design was used with the 1-stage procedure, and the fixtures were placed at the crestal bone level. Although this approach offers 1-stage implantation, a micro-gap between implant

SIADAT ET AL 313 and abutments remains at the bone crest level (just as in the 2-stage procedure). Because the crestal bone loss is influenced by the micro-gap position, more crestal bone loss will occur when the micro-gap is located at or below the alveolar crest. 26,27 Therefore, more investigations are needed to address the correlation of crestal bone loss with the micro-gap position and considering the implant placement technique. The hypothesis regarding the marginal bone loss of 1-stage and 2-stage implant installation approaches was verified because the results of our study indicated that both groups had similar radiographic bone levels after 1 year of functional loading. Overall, the present results suggest that the crestal bone loss around implants does not depend on whether the implant is placed using a 1-stage or 2-stage approach. Within the limitations of the present study, it was concluded that no differences result between implants placed using either the 1-stage or 2-stage procedure regarding crestal bone loss after 1 year of functional loading. References 1. Collaert B, De Bruyn H: Comparison of Brånemark fixture integration and short-term survival using one-stage or two-stage surgery in completely and partially edentulous mandibles. Clin Oral Implants Res 9:131, 1998 2. Fiorellini JP, Buser D, Paquette DW, et al: A radiographic evaluation of bone healing around submerged and non-submerged dental implants in beagle dogs. J Periodontol 70:248, 1999 3. Ericsson I, Randow K, Glantz PO, et al: Clinical and radiographical features of submerged and nonsubmerged titanium implants. Clin Oral Implants Res 5:185, 1994 4. Zechner W, Kneissel M, Kim S, et al: Histomorphometrical and clinical comparison of submerged and nonsubmerged implants subjected to experimental peri-implantitis in dogs. Clin Oral Implants Res 15:23, 2004 5. Garcia RV, Kraehenmann MA, Bezerra FJ, et al: Clinical analysis of the soft tissue integration of non-submerged (ITI) and submerged (3i) implants: A prospective-controlled cohort study. Clin Oral Implants Res 19:991, 2008 6. Engquist B, Astrand P, Anzén B, et al: Simplified methods of implant treatment in the edentulous lower jaw: A controlled prospective study. Part I: One-stage versus two-stage surgery. Clin Implant Dent Relat Res 4:93, 2002 7. Kim DM, Badovinac RL, Lorenz RL, et al: A 10-year prospective clinical and radiographic study of one-stage dental implants. Clin Oral Implants Res 19:254, 2008 8. Lambrecht JT, Filippi A, Künzel AR, et al: Long-term evaluation of submerged and nonsubmerged ITI solid-screw titanium implants: A 10-year life table analysis of 468 implants. Int J Oral Maxillofac Implants 18:826, 2003 9. Simonis P, Dufour T, Tenenbaum H: Long-term implant survival and success: A 10-16-year follow-up of non-submerged dental implants. Clin Oral Implants Res 21:772, 2010 10. Becktor JP, Isaksson S, Billström C: A prospective multicenter study using two different surgical approaches in the mandible with turned Brånemark implants: Conventional loading using fixed prostheses. Clin Implant Dent Relat Res 9:179, 2007 11. Cecchinato D, Olsson C, Lindhe J: Submerged or non-submerged healing of endosseous implants to be used in the rehabilitation of partially dentate patients. J Clin Periodontol 31:299, 2004 12. Ericsson I, Randow K, Nilner K, et al: Some clinical and radiographical features of submerged and non-submerged titanium implants: A 5-year follow-up study. Clin Oral Implants Res 8:422, 1997 13. Friberg B, Sennerby L, Linden B, et al: Stability measurements of one-stage Brånemark implants during healing in mandibles: A clinical resonance frequency analysis study. Int J Oral Maxillofac Surg 28:266, 1999 14. Albrektsson T, Zarb G, Worthington P, et al: The long-term efficacy of currently used dental implants: A review and proposed criteria of success. Int J Oral Maxillofac Implants 1:11, 1986 15. Smith DE, Zarb GA: Criteria for success of osseointegrated endosseous implants. J Prosthet Dent 62:567, 1989 16. Misch CE: Dental Implant Prosthodontics (ed 1). St. Louis, CV Elsevier, 2005, pp 18-31 17. Piao CM, Lee JE, Koak JY, et al: Marginal bone loss around three different implant systems: Radiographic evaluation after 1 year. J Oral Rehabil 36:748, 2009 18. Mörner-Svalling AC, Tronje G, Andersson LG, et al: Comparison of the diagnostic potential of direct digital and conventional intraoral radiography in the evaluation of peri-implant conditions. Clin Oral Implants Res 14:714, 2003 19. De Smet E, Jacobs R, Gijbels F, et al: The accuracy and reliability of radiographic methods for the assessment of marginal bone level around oral implants. Dentomaxillofac Radiol 31: 176, 2002 20. Wenzel A: Computer-aided image manipulation of intraoral radiographs to enhance diagnosis in dental practice: A review. Int Dent J 43:99, 1993 21. Cecchinato D, Bengazi F, Blasi G, et al: Bone level alterations at implants placed in the posterior segments of the dentition: Outcome of submerged/non-submerged healing. A 5-year multicenter, randomized, controlled clinical trial. Clin Oral Implants Res 19:429, 2008 22. Petersson A, Rangert B, Randow K, et al: Marginal bone resorption at different treatment concepts using Brånemark dental implants in anterior mandibles. Clin Implant Dent Relat Res 3:142, 2001 23. Astrand P, Engquist B, Anzén B, et al: Nonsubmerged and submerged implants in the treatment of the partially edentulous maxilla. Clin Implant Dent Relat Res 4:115, 2002 24. Li G, Engström PE, Welander U: Measurement accuracy of marginal bone level in digital radiographs with and without color coding. Acta Odontol Scand 65:254, 2007 25. Weyant RJ, Burt BA: An assessment of survival rates and withinpatient clustering of failures for endosseous oral implants. J Dent Res 72:2, 1993 26. Piattelli A, Vrespa G, Petrone G, et al: Role of the microgap between implant and abutment: A retrospective histologic evaluation in monkeys. J Periodontol 74:346, 2003 27. Hermann JS, Buser D, Schenk RK, et al: Crestal bone changes around titanium implants. A histometric evaluation of unloaded non-submerged and submerged implants in the canine mandible. J Periodontol 71:1412, 2000