Crestal bone loss has been documented to be 1 of the several possible reasons for dental implant failure. Studies have shown that the crestal bone loss in the first year of functional loading was between 0.9 and 2 mm, with 0.05 to 0.13 mm loss every subsequent year.1–5 Several factors such as surgical trauma, occlusal load, periimplantitis, microgap, and so on have been held responsible for bone loss experienced during implant function. A brief description of each of these follows:
The damage caused by surgery and possible infections associated with the process may contribute to implant failure. Such implants often exhibit fibrous connective tissue around them. In addition to other factors of surgical trauma, the heat generated during drilling may cause damage, inviting inflammatory and traumatic response. Reports suggest that temperature of the bone reaches up to 47°C during drilling, which could be dependent on drilling speed, force applied, or both.6–10 Authors later questioned this on the basis that the heat generated should affect a larger area, and hence the damage should not be restricted to only the crestal bone.11–14 Another factor that does not support this hypothesis is that bone loss on implant loading does not happen in a manner similar to natural teeth. In natural teeth, osseous surgery is followed by horizontal bone resorption unlike characteristic saucerization observed in case of early loss of implants.6,15–17
Several studies have shown that occlusal overload often results in the failure of successfully osseointegrated implants.18–21 The cortical bone is known to be very sensitive to shearing force and stress at the crestal bone has been thought to promote chances of bone loss.22 The mineralization and strengthening of bone in subsequent years may prevent implant failure. However, in the initial periods of loading, this stress has been thought to contribute significantly to crestal bone loss.23 Although occlusal overload may be an important causative of crestal bone loss, such bone loss has been observed, invariably implying other important causes.
Several studies have reported correlation between plaque accumulation and implant failure.24–27 This may happen anytime during the implant life depending on other factors such as inflammation, occlusal load, etc. A study to understand the differences in the microbial flora around implants evaluated microbial flora around successful and failed implants and reported that periimplantitis was a site specific infection with microbial flora similar to chronic periodontitis.28 Successful and failed sites exhibited different microbial flora with small amounts of coccoid bacteria around healthy sites, whereas the failed sites consisted more diverse bacteria with the presence of black pigmented Bacteroides, gram-negative anaerobic rods, spirochetes, and Fusobacterium sp.
Implants usually come in 1- and 2-piece varieties. In the 1-piece implant, there is no question of microgap. Conversely, a microgap exists between the components of a 2-piece implant. This microgap may provide a place for bacterial colonization and food debris. Quirynen and van Steenberghe29 reported the presence of significant quantities of microbes in the threads of the implants with predominant numbers of coccoid cells (86.2%) and nonmotile rods (12.3%). The authors suggested that the source of these microbial species might be the microgap between the abutment and implant and that these microbes may result in periimplantitis. Hermann et al30 in a well-designed study on dogs evaluated the impact of microgap on crestal bone changes. The authors used 2-piece implants with the 2 pieces screwed together in 1 group and 2 pieces welded together in the other group. Both the groups had microgap variants with sizes between <10 and 100 μm. The study reported that the 2-piece screwed implants experienced more bone loss in comparison with the implants where the 2 pieces were welded together. However, no effect of the size of microgap on crestal bone loss was observed.
This is the layer of the connective tissue, which provides a barrier between the oral environment and the crestal bone. The seal prevents bacterial invasion and entry of the food debris in the implant area. Most of the studies till date have reported the biological width to be between 2 and 3 mm in dental implants.31 The establishment of biological width is considered to be a reason for crestal bone loss by several authors.32–36 This has been confirmed by studies placing implants at different depths with respect to the crestal bone.37–40 The placement of implant 1 mm above the bone level results in minimum bone loss, more loss was observed placing implants at the level of crestal bone, and maximum loss occurred in cases where the implant was placed 1 mm below the level of the crestal bone.41 This showed that the process of establishment of biological width was essential and perhaps the most important factor affecting crestal bone loss.
Rise of the Concept of Platform Switching
In 1991, implants with 5 and 6 mm diameter were designed for use in cases with poor bone quality. However, at that time, no matching diameter abutments were available to be fitted on these implants. Eventually, these implants were fitted with abutments of smaller diameter. The difference in diameter of the abutments and implants was 0.9 to 1.9 mm. Radiographical review of these cases after a period of 1 to 5 years revealed little bone loss in comparison with implants fitted with matching diameter components. This gave birth to the concept of platform switching. Not many studies have been made on the capability of switched platform implants to reduce crestal bone loss (Table 1). The number of studies available till date was not enough to conduct a meta-analysis to evaluate the strength of the concept. Therefore, we chose to review the available literature on human subjects, animals, or finite element analyses using dental implant models.42–51
The literature was searched in public databases: Pubmed, ScienceDirect, and Google with the following keywords: “platform switching,” “platform switching and crestal bone loss,” “dental implants and platform switching,” “the concept of platform switching,” and “platform switching and implant success.” The articles retrieved on the search were further scrutinized for their relevance to the topic.
Method of Review
The articles were reviewed in a systematic manner to extract information on following aspects:
- Extent of crestal bone loss in different implant types.
- Whether platform switching offered benefits in addition to its effects on crestal bone loss.
- Stress distribution in platform switched versus other implants.
- Whether platform switching had any drawbacks.
Literature on Platform Switching Was Scanty
Pubmed search with the above-mentioned keywords returned 142 hits (Table 1) with few relevant articles. Similarly, large number of articles retrieved from ScienceDirect and Google search were found to be irrelevant. The irrelevant articles appeared probably because of a matching keyword/phrase. The number of specific articles was counted to be 15.
Platform Switching Decreased Crestal Bone Loss
Chou et al52 undertook a clinical trial where over 1500 implants were placed on the platform switching concept (Table 2). A bone loss of 0.2 mm per year was observed in a period of 3 years. The authors concluded that bone loss was influenced by platform switching. The findings had a robust base given the large sample size and multicentric nature of the study. Li et al53 followed up their subjects radiographically for 2 to 6 years and found that platform switched implants developed less complications. Hürzeler et al54 also showed less bone resportion (−0.29 ± 0.34 mm) in platform-switched implants compared with the control group (−2.0.2 ± 0.49 mm). Cappiello et al55 also showed reduction in bone loss in platform-switched group against the standard platform. In short, almost all studies have shown platform switching to be beneficial except 1 study that reported less bone loss in test samples although the difference was found not statistically significant.57
Degidi et al58 studied the effect of platform switching on periimplant tissues. They showed that there was good connective tissue growth after 28 days of implantation with fast mineralization, which provided strength to the implant. Canullo et al56 observed that switching of 1.7 mm between abutment and the implant resulted in higher periimplant tissue stability.
Reason for Beneficial Effect of Platform Switching
Most of the authors were of the opinion that the reduced bone loss was because of development of connective tissue layer in horizontal direction as against vertical with standard platforms (Table 3). Luongo et al60 undertook histologic and histomorphometric analyses of a platform-switched implant 2 months after placement. The authors suggested that lesser bone resorption in platform switched implants may be because of alignment of the connective tissue zone from vertical to horizontal in the space provided by the narrow abutment. Degidi et al58 also showed connective tissue growth in and around the groove in platform-switched implants.
Platform Switching Offered Better Implant Load Distribution
In a finite element analysis using different crestal bone geometries, Baggi et al62 evaluated the stress distribution associated with 5 major commercially available implant types (Table 4). The authors reported that maximum stress was located at the neck of implant and that platform-switched implants demonstrated better stress performance. Deshpande et al63 undertook a study on 3D finite element model of mandible with a missing premolar. Stress value with a standard implant was higher (785 Mpa) in comparison with the platform-switched implant (465.71 Mpa). In another finite element model analyses, Maeda et al64 reported that narrow abutments offered reduced stress at the crestal bone compared with wide-diameter abutments.
The Extent of Platform Switching Required Was Unclear
Studies have used different switching extents, with the difference in diameter between implant and abutment varying from less than 1 mm to more than 2 mm, to evaluate the relation between extent of switching and effect on bone loss (Table 5). Canullo et al59 used abutments of same size (3.8 mm) with different diameters of implants (3.8 mm for control group, 4.3 mm for test group 1, 4.8 mm for test group 2, and 5.5 mm for test group 3). Histology and immunohistochemistry were done to analyze the bone structure and the cases followed up to 20 months. Bone loss in all test groups (0.896 for group 1, 0.770 mm for group 2, and 0.388 mm for group 3) was less in comparison with control group (1.548 mm). This showed inverse correlation between switching extent and the bone loss.
Maeda et al64 undertook a 3D finite element analyses to assess stress distribution in platform-switched implant connections with abutments of 3.25 mm and 4 mm diameters. The authors observed that the narrow abutment (3.25 mm) resulted in reduced stress level at cervical area of the implant. Most studies on platform switching have shown better stress distribution or lesser stress at the bone in implants switched to a higher extent. However, the question of the extent of switching remains unanswered. These studies were mostly model based. Research on animals and humans may provide further required information.
Disadvantages of Platform Switching
Almost all studies have consistently shown platform switching to be effective in reducing crestal bone loss. Only 1 study pointed out no effect of platform switching, the findings of which were presented at an International Association for Dental Research conference (2009) in Florida.57
In the clinical trial by Chou et al52 on 1500 implants placed on the platform switching concept, patients were followed up at regular intervals of 1 year up to 3 years. The authors concluded that bone loss was mainly influenced by platform switching and was independent of variables such as age, gender, bone density, and prosthesis application parameters. Becker et al61 in a study assessed the impact of platform switching on crestal bone loss in dogs, randomly fitted with abutments of matching or smaller diameter, and assigned to groups with different healing periods (1, 2, or 4 weeks). The authors measured the distance between the implant shoulder (IS) and (1) the apical extension of the junctional epithelium, (2) the most coronal level of bone in contact with the implant, and (3) the level of alveolar bone crest. The animals were killed at the designated healing period and histological examination was done. Significantly lesser crestal bone loss was observed after a 4 weeks healing period in the smaller diameter compared with matching size abutments. The authors concluded that the circumferential mismatch between 2 pieces of the prosthesis prevented vertical downgrowth of the connective tissue barrier over the said period.
Canullo et al56 undertook a prospective, controlled randomized immediate implant study on 22 patients, in which the participants were randomly distributed to 2 equal groups. After extraction, an implant of 5.5 mm was inserted at the fresh site with a titanium abutment of 3.8 mm diameter in the test group and similar abutment diameter (5.5 mm) in the control group. All implants in both groups were osseointegrated, clinically stable, and infection free with similar periodontal indices. The test group showed a deceased buccal periimplant mucosal level loss of 0.63 mm between baseline and 1-year follow-up compared with the control group Hürzeler et al54 also conducted a clinical trial to evaluate the performance of switched platforms against standard ones, such that 14 implants were based on the concept of platform switching, whereas 8 used implants and abutment of same diameter. Radiographs taken at the base time and 1 year after installation showed mean value of crestal bone height after 1 year to be significantly higher in the platform-switched cases.
Cappiello et al55 placed 131 abutments in 45 patients, 75 of which were narrower than implants by 1 mm and the remaining had similar implant-abutment diameter. The authors reported that bone loss among the platform-switched cases was lesser (0.95 ± 0.32 mm) compared with the control group (1.67 ± 0.37 mm). The strength of the study was the large sample size used for making the inference. Li et al53 evaluated 17 patients who received 26 platform-switched implants with follow-up between 24 and 74 months. X-ray studies during the follow-up revealed no complications with any of the implants and marginal bone loss of 0.13 mm after 1 year and 0.27 mm after 5 years. The study, however, lacked a control group for comparison. In a split mouth clinical trial, Telleman et al66 reported that interproximal bone level were better maintained at implants restored with platform switching.
Degidi et al58 undertook a study on a human subject to understand the course of tissue growth in an immediately loaded platform-switched implant inserted 2 mm below the alveolar crest. The patient complained of some inflammation at the implant site and was afraid of developing further complications; therefore, the implant was removed after a period of 1 month of prosthesis loading. The implant and surrounding tissues were analyzed histomorphometrically. Bone was found to be present 2 mm above the level of IS. Lamellar cortical compact bone was observed in the first 3 coronal millimeter, and infrabony pockets were completely absent. Dense connective tissue having very few inflammatory cells at the level of IS was observed. Fibrous tissue was absent at the bone-titanium interface and no downgrowth of epithelium was observed. Furthermore, osteoclasts were absent around the implant. The spaces between the implant threads were seen filled with newly formed bone tissue. The study observed that deep positioning of the platform-switched implant was suitable for growth of mineralized tissue around the implant surface. The benefit of a switched platform for providing higher surface for tissue growth to ensure more complete implant integration was an important finding of the study. The shortcoming of the study was that it presented data from only a single case.
Baggi et al62 analyzed 2 Nobel Biocare Implant systems (Nobel Biocare; AB, Goteborg, Sweden), 2 ITI standard implants (Institue Straumann AG, Basel, Switzerland), and an Ankylos implant (Dentsply Friadent, Mannheim, Germany). Only the Ankylos implant was based on the concept of platform switching. A 3D finite element analysis was undertaken to assess the load distribution and capability of different implant types to bear the same. The Nobel Biocare and ITI standard implants exhibited cortical bone stress values much higher (approximately 145% in tension and 290% in compression) than those of the Anklyos system. The study reported that platform switching resulted in reduced overloading risk. To assess stress distribution in platform switching, Deshpande et al63 undertook a 3D finite element model study of the mandible with a missing premolar. The authors created 2 block types: 1 with abutment matching the size of the implant and other with an abutment of lesser diameter. Stress levels calculated using von Misses stress values were more than one and a half times higher for the standard compared with the platform-switched implant. The limitation of both the above studies was that they were not done on humans or animals.
Very few studies compared the effect on bone loss of platform switching with different extents. Maeda et al64 undertook a 3D finite element analyses to assess stress distribution in standard and platform-switched abutments and observed that the narrower abutment resulted in reduced stress level at cervical bone area of the implant. Canullo et al59 study was useful in understanding the effect of switching extent on level of bone resorption. The authors recruited 42 patients in who 80 implants of different dimensions (3.8–5.5.mm) were fixed with abutments of same size (3.8 mm). At 20 months, histology and immunochemistry showed that an inverse association existed between the extent of bone loss and difference in implant-abutment dimension. Schrotenboer et al65 in a finite element analyses study evaluated stress associated with platform switching extents of 0%, 10%, and 20%. A decrease in stress at the crestal bone level from 6.3% to 5.4% was observed in case of vertical loading and from 4.2% to 3.3% in case of oblique loading from 0% to 20% switching. Most of these studies agreed that switching to a higher extent results in better load distribution.
Enkling et al57 in a split mouth study design placed 2 implants each in 25 subjects. After a period of 3 months, the implants were fitted either with a platform-switched or matching-diameter abutment and followed up radiographically at baseline, 3, 4, and 12 months. It was observed that periimplant bone loss in the switched platforms was 0.56 (SD, 0.44) mm, whereas it was 0.61 (SD, 0.57) mm around standard platforms. The bone loss in switched platforms was marginally lesser in comparison with the control group, though not statistically significant. It was concluded that platform switching might not help in preventing bone loss. This is the only study of its kind.
Luongo et al60 removed a mandibular platform-switched implant after a period of 2 months because of rehabilitation difficulties. Histological analyses of the implant revealed the presence of inflammatory connective tissue infiltrate around the implant surface up to 0.35 mm coronal to the implant abutment junction. The authors suggested that lesser bone resorption in platform-switched implants may be because of alignment of the connective tissue zone from vertical to horizontal direction in the space provided by the narrow abutment. They felt that availability of the horizontal surface for connective tissue growth resulted in establishment of the connective tissue layer without the need for crestal bone to provide the required space. The study had limitations of sample size and no control group.
Most of the studies discussed above have been undertaken on models or inadequate number of human subjects or animals. Only 1 or 2 studies used adequate sample size.52,58,59 Therefore, the efficacy of platform switching in reducing crestal bone loss needs to be further ratified in larger sample size clinical trials.
A clear majority of the studies have shown effectiveness of platform switching in combating bone loss and seem to hold great promise in implant survival. The concept needs further exploration in terms of ideal extent (difference in implant body and abutment diameter) of switching, resultant load distribution, and possible mechanisms of action.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
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