It is usually seen that the crestal bone levels are typically located approximately 1.5 to 2.0 mm below the implant-abutment junction (IAJ) at 1 year after implant restoration but are dependent on the location of the IAJ relative to the bony crest.1–6 Remodeling takes place at the bone crest and is characterized by bone resorption both horizontally and vertically, of approximately 1.5 to 2.0 mm during the first year of function.7–13 Bone remodeling occurs postfunction due to stress concentration,14 which slows down at the first thread when the force changes from shear force to compressive force induced by the thread itself.15 It has also been hypothesized that remodeling occurs to establish a proper biologic width around the dental implant.1,7,16–23 This shows a relation of soft tissue thickness and bone loss around implants. Surgical trauma,10,24 microgap,18,25–27 occlusal overload,28,29 the position of abutment inflammatory connective tissue,1 bacterial colonization of periimplant sulcus,30–33 and implant surface topography15,19,34 also control the crestal bone levels around the implant.
Platform switching refers to placing restorative abutments of narrower diameter on implants of wider diameter, rather than placing abutments of a similar diameter. It is a restorative protocol that was reported by Lazzara and Porter1 as a means of limiting circumferential bone loss around dental implants. A report of radiographic observations made over a 13-year period resulting from the placement of smaller diameter healing and prosthetic components on wider diameter implants was presented, which showed reduced vertical crestal bone loss by the inward positioning of the IAJ. The phenomenon was accidently discovered due to nonavailability of matching diameter prosthodontic components for wide diameter implants.1 Radiographic reviews after the initial 5-year period revealed that when matching diameter implants and restorative components were used, the crestal bone contacting the implant normally remodeled to approximately 1.5 to 2 mm apically to the first thread. In contrast, when smaller diameter components were placed on wider diameter platforms, the amount of crestal bone loss was reduced. The results were attributed to increase in surface area for soft tissue attachment and inward positioning of IAJ-associated inflammatory cell infiltrate away from the bone (biological advantage). Platform-switched and platform-matched implants have not shown any differences in associated microbial flora.35 Moreover, after the establishment of biologic width, there seems to be no difference in the soft tissue characteristics between both these groups.36,37
Theories have been put forward to explain the rationale of platform switching toward influencing bone loss. Biomechanical theory38 proposes that the shift of stress-concentration zone away from the bone-implant interface and directing the forces of occlusal loading along the axis of the implant leads to reduced crestal bone loss. Other theory explains that platform switching medializes the location of the biologic width and minimizes the marginal bone loss.39,40 The exact biological and biomechanical aspects of platform switching and its beneficial effects on periimplant soft and hard tissues are yet not completely deciphered. A great volume of literature is available, which establishes that platform switching limits the crestal bone loss around implants.4,41–46 However, there are studies with contradictory results leading to confusion.47–52 Hence, this systematic review was done to examine the difference in the marginal bone-level changes when the implants were restored with platform-switched prostheses and to compare the efficiency of platform switching in reducing the marginal bone loss around the implants with platform-matched prostheses. The meta-analysis was performed to get the quantified results to validate the same.
Materials and Methods
First a general PubMed search was conducted for articles published in dental journals in English. Eligible studies were identified up to October 2011 using the combined terms: “platform switching,” “platform switched implants,” “implants,” “dental implants,” “systemic review,” “meta-analysis,” “marginal bone loss,” “crestal bone loss,” “biological width,” and “microgap.” The relevant studies related to platform switching were considered.
PICO (population, intervention, comparison, and outcome) elements were described first to formulate inclusion and exclusion criteria. In patients (P) who received implant treatment (I), does the use of platform-switched implants as compared with platform-matched implants (C) result in more favorable marginal bone changes (O).
Data till October 2011 were analyzed from PubMed, EBSCO, Quintpub, Science Direct, and New York University Journal. Eligible studies were included in the meta-analysis if they met the following criteria: studies published in English, human study population, randomized controlled trials (RCT) or controlled clinical trials (CCT) with 2 treatment groups (platform-switched and platform-matched implants), more than 10 implants in platform-switched group, and mean follow-up period ≥12 months. In the presence of similar publications, only the study with most inclusive data was selected.
After full-text analysis, studies were excluded from the statistical analysis. The principal reasons for exclusion were no control group, finite element analysis (FEA), photoelastic analysis, studies with short follow-up periods (<12 months), studies with less number of implants placed (<10 in platform-switched group), and animal studies.
Then, the electronic search was complemented by a manual search of all the bibliographies of included and excluded studies. The manual search, furthermore, included all full-text articles and other related reviews selected from the electronic search. All articles that used platform switching concept were identified (n = 176). Titles and abstracts of searches were initially screened by independent reviewers for the possible inclusion in the review. The full text of all possibly relevant studies (n = 70) was then obtained for independent assessment by the reviewers. The disagreement regarding the inclusion was solved by discussion. In all, totally 15 studies were included in the analysis. Figure 1 describes the process of identifying 15 full-text articles on the clinical studies on platform switching.
Data was extracted independently by 5 reviewers. Disagreement concerning the extracted data was resolved by discussion. Extracted data contained information on study design, number of implants placed, number of platform-switched implants, number of platform-matched implants, implant system used, implant diameter, implant length, implant abutment diameter difference on each side, implant location, level of implant placement, implant placement protocol (immediate/delayed), implant insertion torque, loading protocol (immediate/delayed), time to definitive restoration, marginal bone level changes, allocation concealment grade, Jadad score,54 follow-up period, and survival rate in both groups. Table 1 describes the characteristics of 15 included articles.
Out of the 15 included studies,35,50,51,53–66 2 studies60,62 had 3 subgroups in platform switching (test) group. Therefore, they were regarded as separate studies that furnished us 19 studies (Table 1).
Jadad quality score was used to assess the quality of the included studies (Appendix A). Cochrane65 scale for assessment of allocation was used to evaluate the validity of the included studies (Appendix B).
Meta-analysis was done by using “REVMAN 5,” COCHRANE COLLABORATION software. Funnel graph was plotted to assess the publication bias. Heterogeneity was assessed using the χ2-based Q statistic method and I2 measurement.
For marginal bone changes, the mean difference (MD) and SD was calculated at 95% confidence intervals (CIs). Following subgroups were made according to the different attributes used to describe the studies and subgroup analyses were done: study design (RCT/CCT), sample size (N > 50/N ≤ 50), implant placement protocol (immediate/delayed), implant-abutment diameter difference (≤0.4 mm/>0.4 mm), follow-up periods (12 months/>12 months), and level of implant placement (crestal/supracrestal/subcrestal).
Description of Studies
Out of all the studies, 1535,50,51,53–64 were selected. They were all published between 2007 and 2011 and had similar inclusion criteria, including the presence of sufficient alveolar bone height and width, absence of signs of local infection, adequate plaque control, type 2 or 3 bone quality, no alcohol addiction, and age 25 to 70 years. Exclusion criteria were smokers who had >10 cigarettes per day, patients with uncontrolled diabetes, pregnant ladies or lactating mothers, previous dental implant surgery, signs or symptoms of temporomandibular disorders, signs of untreated periodontitis or other mucosal and bone tissue lesions, a habit of clenching and/or bruxing, bone defects, chronic systemic disease, patients with a history of bisphosphonate therapy, presence of dehiscence or fenestration of the residual bony walls, patients with drug abuse, history of radiotherapy in the head and neck regions, psychiatric problems, and inability of the patient to provide informed consent. All patients were generally in good health. All patients were provided information about the study, and those who fulfilled the inclusion criteria and expressed interest in participating signed the informed consent and were included in the study. Patients were maintained on antibiotics during the first stage surgery and were under strict oral prophylaxis measure till the end of the studies.
Studies evaluated the bone loss by comparing standardized periapical x-rays taken by long cone technique.50,51,53 Some studies fabricated custom templates for each patient and compared the periapical radiographs taken by long cone technique by image analyzing software.35,54,55,57,60,61,63 In multicenter studies,51 all centers used different radiographic sensors and films and performed digital evaluation of radiographs. Vigolo and Givani58 and Prosper et al56 used long cone technique but analyzed the radiographs by magnifying lens. Enkling et al64 in their study used panoramic radiographs by customizing it for each patient by making bite splints. The radiographs were then magnified and recordings were made. Veis et al59 took orthopantomogram (OPG) or periapical x-rays and standardized the analysis.
Ten studies51,53,55–57,60–64 out of 15 were RCTs and the rest were CCTs.35,50,54,58,59
Six out of 10 RCT51,53,55,56,60,62 studies mentioned that the randomization was done according to predetermined tables and assignment was performed using sealed opaque envelopes. One study57 did randomization by using randomization card to be scratched off before the implant surgery. They then placed implants in such a way that each patient served his/her own control.57 Two studies63,64 used a computer-generated list to randomize the patients but did not mention about the method of assignment of the procedure. A study by Pieri61 used a software program for randomization and again sealed envelopes were used at the time of the surgery for assignment of the procedure.
The range of marginal bone-level changes in test and control groups was 0.055 to 0.99 mm and 0.19 to 1.67 mm, respectively. Out of the 19 study units, 14 studies showed statistically significant difference between the test and control groups. However, 2 studies50,51 favored platform matching, 2 studies63,64 showed insignificant difference favoring platform switching, and 1 study was neutral61 (MD = 0 mm) (Table 2).
Analysis of 19 study units showed that the crestal bone experiences lesser resorption when the implants are restored with platform-switched prostheses than platform-matched prostheses with an MD of −0.34 mm (95% CI: – 0.37 to −0.30; P < 0.00001; Fig. 2). The χ2 of heterogeneity was 232.25 (P < 0.00001; I2 = 92%). Test for overall effect Z = 17.57 (P < 0.00001; Fig. 2).
In general, the subgroup analysis favored platform switching (Table 2). Analysis of RCT revealed MD = −0.29 (95% CI: −0.34 to −0.24; P < 0.00001). Likewise, CCTs demonstrated MD = −0.48 (95% CI: −0.53 to −0.42; P < 0.00001). Analysis of a smaller sample size also showed MD = −0.30 (95% CI: −0.38 to −0.22; P < 0.00001), and with a larger sample size of >50, it was MD = −0.35 (95% CI: −0.39 to −0.30; P < 0.00001).
Subgroup with implant placed immediately after extraction showed MD = −0.13 (95% CI: −0.22 to −0.04; P < 0.00001). After delayed placement of implants, MD = −0.38 (95% CI: −0.42 to −0.34; P < 0.00001).
Where implant-abutment diameter difference was ≤0.04 mm, MD = −0.35 (95% CI: −0.43 to −0.28; P < 0.00001) and where abutment difference was ≥0.04 mm, MD = −0.39 (95% CI: −0.44 to −0.34; P < 0.00001).
For follow-up period of >12 months, MD = −0.36 (95% CI: −0.41 to −0.31; P < 0.00001) and for follow-up period <12 months, MD = −0.14 (95% CI: −0.17 to −0.10; P < 0.00001).
Crestal placement showed MD = −0.40 (95% CI: −0.45 to −0.36; P < 0.00001), subcrestal placement showed MD = −0.52 (95% CI: −0.61 to −0.43; P < 0.00001), and supracrestal placement showed MD = −0.02 (95% CI: −0.06 to 0.02; P < 0.00001).
The implant tissue interface is an extremely dynamic region of interaction throughout the entire phase of implant therapy.66,67 The biomechanical stresses that develop at the implant bone interface are responsible for the remodeling of the crestal bone. The cortical bone is more susceptible to shear forces than compressive forces and the surrounding bone.68
Crestal bone loss has been a matter of concern.8,69–75 Platform switching is believed to minimize crestal bone loss around implants.1,42 The main aim of this meta-analysis was to study the influence of platform switching on crestal bone loss. Therefore, all clinical human studies, with substantial number of implants placed and significant amount of follow-up period were included in the analysis. Presence of 2 groups (test and control) helped in drawing a comparison between platform-switched and platform-matched implants.
RCT, without doubt, is the most powerful research design to establish causality, including the effectiveness of interventions.76 It also decreases the chances of errors. Randomization and double/triple blinding decreases the chances of bias.77 However, RCT comes with its own limitations; notably cost, time, difficulty in studying rare events.78,79 Nevertheless, more RCT data make the analysis stronger.77
This systematic review and meta-analysis was conducted on 15 studies in which totally 1683 implants were placed. The forest plots revealed that the results favored the experimental side (platform switching). This is in concurrence to the clinical studies, case reports, histomorphometric studies, and biomechanical studies that platform switching had an inverse relation with bone loss.40–42,52,80–90
The line in the middle is called “the line of no effect,” which has the value of 0. There is no difference between the platform switching and matched group on this vertical line. The boxes are situated in line with the outcome value of the individual studies, also called the effect estimates. The value axis is at the bottom of the graph. The size of the green box is directly related to the “weighting” of the study in the meta-analysis. The horizontal lines through the boxes depict the length of the CI. The longer the lines and wider the CI, the less precise the study results. Arrows indicate that the CI is wider than there is space in the graph.
The weight (in %) indicates the weighting or influence of the study on the overall results of the meta-analysis of all included studies. The higher the percentage weight and bigger the box, the more influence the study has on the overall results. One column gives the numerical results for each study that are identical to the graphical display last column.
The diamond in the last row of the graph illustrates the overall result of the meta-analysis. The middle of the diamond sits on the value for the overall effect estimate and the width of the diamond depicts the width of the overall CI. The total number of participants in the platform-switched (column 2) and platform-matched group (column 3) is also summarized in the same row. The result is regarded as statistically significant if P < 0.05.
MD with its CI is shown in the last row, along with an estimate of homogeneity (the χ2 and I2 test), degrees of freedom, and the P value. Z-test has been used as the test for overall effect. Random-effects model was applied as the heterogeneity among the studies was significant. Subgroup analyses were conducted to test the heterogeneity because the test of heterogeneity had a low power, and a nonsignificant result may not be reliable to identify heterogeneity.91
The parameters used for subgroup analyses were study design, sample size, implant placement protocol, implant-abutment diameter difference, follow-up period, and level of implant placement. The result shows that the studies that were conducted as CCT35,50,54,58,59 were more in favor of platform switching than RCT51,53,55–57,60–63,66 studies. Sample size of ≥5035,50,51,56–58,60,62,63,66 showed lesser resorption than with sample size <50.35,53,55,57,59,61 A great difference was noted in the results. The implants that followed delayed51,53,54,56–60,63–65 placement protocol showed lesser crestal bone resorption than the implants that were placed immediately50,55,61 after extraction. More be the mismatch between the implant and abutment diameters, more preservation of crestal bone occurs.41,60,92,93 More stable crestal bone levels are seen when the studies were followed up for >12 months50,56–60,62,66 rather than <12 months.51,53,54,61,63–65 Subcrestal50,54,59 placement of implants with platform switching would lead to lesser crestal bone loss than crestal placement of implants.55–59,62,65 Supracrestal placement48,56,59,61 of implants reported maximum resorption of bone and it should be avoided. The funnel plot (Fig. 3) showed a slight asymmetry showing slight publication bias, which is almost negligible.93,94
It is theorized that the concept of platform switching should be applied as soon as the implant is exposed to the oral environment beginning with the healing abutment if a horizontally repositioned biologic width is to be established.1 The concept, however, has its faults. Platform switching can only be used with components that have similar designs, that is, the screw access hole must be uniform. In addition, sufficient space is needed to develop a proper emergence profile. Moreover, the stresses are transferred from the abutment to implant body rather than surrounding bone leading to increased risk of abutment/implant fracture.88,92,95
Clinical studies in this analysis have clearly revealed that platform switching helps in maintaining the periimplant bone cuff. Conversely, many biomechanical studies still do not comply with the same and show either no relation/little but nonsignificant relation in preserving bone loss.96–98 However the biomechanical studies using FEA have been excluded in this analysis; hence, any conclusion regarding biomechanical advantages cannot be drawn.48,93,96–104
This analysis took into consideration single-tooth implant cases. Its use in implant-supported overdenture was studied and no relation was found, negating its application in overdenture cases.105
Limitations of the Systematic Review and Meta-analysis
This systematic review analyzed all the articles that were published in English, whereas literature from the other languages were excluded from the evaluation. Heterogeneity existed among studies due to different observation periods, implant types, study populations, randomization protocols, surgical protocols, and radiographic analysis methods. Enkling et al63 observed the bone loss by taking panoramic radiographs. The measurements might not be accurate and increases the source of errors. Certain studies that had favorable results and good number of implants placed had to be excluded from the study because there was no control group.40,85,106
The radiographic analysis of the bone loss measured was not standardized, which could have added to the heterogeneity. One more cause of heterogeneity could be the different implant systems used and different torques applied to all implants. The oral hygiene maintenance of the patients also varies among patients that leads to increase or decrease in the failure or success rates. The distance of the implant collar to the bone was also not standardized, which might lead to aberrant measurements regarding bone loss. Distance between implants, which also has a potential impact on crestal bone loss, could not be standardized.107 There might be confounding factors other than platform diameter that might influence the bone resorption pattern.
Non randomized CCTs have limitations such as small sample size. This leads to lack of enough raw data to draw a firm conclusion leading to introduce a bias. However, it was postulated that CCT can complement the evidence provided by RCTs. In addition, subgroup analysis comparing RCT and CCT showed that CCT did not underestimate or overestimate the treatment effect.108,109 However, it must be mentioned that standardization of implant system, implant insertion torque, the distance of the implant collar to bone, antibiotic regimen, and oral hygiene protocols followed cannot be achieved.
Taking into account all the possible errors in collection and analysis of the data and within the limitation of the available data, the results reveal that platform switching holds promise as a simple, functional, and predictable technique for preserving periimplant crestal bone. Subgroup analyses concluded that studies that were CCTs, with sample size ≥50, placed after healing of the extraction sites showed lesser marginal bone loss. A greater mismatch between the diameters of implant and abutment leads to better bone preservation. It also validates the subcrestal placement of the implants for decreasing the crestal bone loss. It could be used in situations where a larger implant is needed but prosthetic space is limited and in the anterior region where crestal bone preservation may enhance the esthetics. However, long-term, clinical, microbiologic, pathological randomized studies on the proper diameter of abutment without deformation, as well as the effective degree of platform switching in terms of bone resorption, are still awaited.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
The authors are particularly grateful to Dr. J. Chandrashekhar, Department of Public Health Dentistry, Amrita School of Dentistry, Cochin, India, for his kind efforts in the statistical analysis performed.
Presented at 39th INDIAN PROSTHODONTIC SOCIETY Conference, December 02–05, 2011, Dubai, United Arab Emirates.
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- Was the study described as random? 1/0
- Was the randomization scheme described and appropriate? 1/0
- Was the study described as double-blind? 1/0
- Was the method of double blinding appropriate? (Were both the patient and the assessor appropriately blinded?) 1/0
- Was there a description of dropouts and withdrawals? 1/0.
Jadad Score Quality Assessment Based on Jadad Score
Range of score quality: 0–2, Low; 3–5, High.
Allocation Concealment Grade
- Grade A—Allocation concealment was adequately reported (centralized randomization either by a central office; sequential administration of coded containers to enrolled participants; on-site computer; serially numbered sealed opaque envelopes)
- Grade B—Allocation concealment is not described, but it is mentioned in the text that the study is randomized
- Grade C—Allocation concealment was inadequate.