The effect of implant‐abutment connections on peri‐implant bone levels around single implants in the aesthetic zone: A systematic review and a meta‐analysis

Abstract Objective To systematically review literature about the effect of different implant‐abutment interface designs on peri‐implant bone level changes, implant loss and mid‐buccal mucosa changes around single implants in the anterior maxilla. Reviewing three connection configurations: Platform switched conical (PS‐conical); Platform switched parallel (PS‐parallel); Platform matched parallel (PM‐parallel). Methods A detailed search was carried out in Pubmed, EMBASE, Cochrane, Scopus, Open Gray and African journals Online (until December 1, 2020) and was restricted to clinical prospective studies of at least 1 year and with at least 10 human participants. A meta regression analysis was carried out primarily on the pooled peri‐implant bone level changes followed by implant loss and mid‐buccal mucosa level change. Risk of bias was assessed with RoB 2.0 and ROBINS‐I. The manuscript complied with the PRISMA guidelines and was registered in the PROSPERO database (ID: 225092). Results A total of 5513 hits gave 44 eligible articles for the analyses. Bone level change did not differ significantly between the two platform switched connections; their bone loss scores were significantly lower than PM‐connection. The PS‐conical connections have significantly lower implant losses than the PM connection. Mid‐buccal mucosa level change was comparable between the three connection configurations. Moderate to high risk of bias was detected in the included studies. Conclusions The performance of PS‐conical and PS‐parallel connection configurations both favored bone loss scores compared to the PM‐parallel connection configuration. All three demonstrated mid‐buccal mucosa changes that were small and did not differ significantly amongst the groups.


| INTRODUCTION
The range of implants used in restorative dentistry has become increasingly sophisticated and, simultaneously, more complex (Buser et al., 2017). The variation has become so large, that it is difficult for clinicians to choose between the various available components (Shafie & White, 2014). One of these components is the implantabutment connection configuration. A clear distinction can be made between external and internal implant-abutment connections. External connections are hardly used anymore because of the susceptibility to complications (Gracis et al., 2012;Koo et al., 2012;Steinebrunner et al., 2005). Internal connections have a number of geometric variations (Koutouzis, 2019). For example, the internal implant geometry is parallel-walled or conical/tapered. The parallel-walled connection types are often equipped with various indexing/anti-rotational features like an octagon or a hexagon, and exist with a platform switch or a platform match. A conical/tapered connection type implicates a cone-in-cone or has a conical portion in the inner cervical part, with or without indexing features in the apical part (Shafie & White, 2014).
All the previous mentioned variations were developed in an effort to reduce mechanical failure (Ceruso et al., 2017) and to minimize crestal bone resorption (Koutouzis, 2019). It is presumed that the long-term survival and success of implant treatment can be affected by peri-implant crestal bone resorption (Schwartz-Arad et al., 2005).
This bone resorption can affect the stability of the mucosa, and may therefore affect the aesthetic outcome, which is particularly interesting in the aesthetic zone (Belser et al., 2009;Fürhauser et al., 2005;Jemt, 1997). Eliminating, or at least reducing the amount of bacterial leakage, could have a positive influence on peri-implant tissue stability, and thus on the aesthetics. It has been suggested that an internal implant-abutment connection with a conical configuration is the most stable connection, with less bacterial leakage, than the other configurations (Zipprich et al., 2018).
In 2018, Caricasulo et al. (2018) performed a systematic review, with a meta-analysis, of the difference between conical, internal and external connection configurations. They concluded that significantly less bone loss occurs with conical and internal connections compared to external connections. Although they did not distinguish between platform switching and platform matching, they concluded after performing an additional analysis that a platform switch might be of more importance in preserving peri-implant bone levels than the connection configuration itself. The finding that platform switching might have a positive effect on preserving peri-implant bone levels was also mentioned by Hsu et al. (2017). However, Caricasulo et al. (2018) andHsu et al. (2017) did not focus specifically on the aesthetic region or on a possible effect on the aesthetic outcome.  et al., 2009) that the internal hexagon performs better aesthetically, but were not able to quantify their observations. Additionally, they did not distinguish between platform-switched and platform-matched connections, but this might be a noteworthy nuance when considering the Caricasulo et al. (2018) outcome.
Since the aesthetic outcome of a restoration in the aesthetic region is so important, it would be interesting to know if the implant abutment connection can contribute to achieving the best possible results. So far, to the best of our knowledge, a systematic review with a meta-analysis of the effect of solely internal connections in the aesthetic region, distinguishing between platform switching and matching, is not available. Therefore, the aim of our systematic review was to determine whether the type of implant connection configuration, that is, platform switched conical connections (PS-conical), platform switched parallel connections (PS-parallel) or platform matched parallel connections (PM-parallel), has a significant impact on periimplant bone changes, whether an implant-abutment connection influences implant survival, and whether the stability of the midbuccal mucosa level, as a factor determining the aesthetic outcome, is influenced by the connection configuration.

| Research protocol
This systematic review was conducted following the Cochrane Handbook for systematic reviews and was reported according to the PRI- The primary outcome is peri-implant bone level change and the secondary outcome are implant loss and mid-buccal mucosa level change.

| Information sources
We conducted a literature research of the following databases: PubMed, Cochrane Library EMBASE Scopus, Open Grey and African journals Online. According to the syntax rules of each database, key words and their combinations were used to identify studies published until December 2020. No time restrictions were applied ( Table 1).
The following study criteria were handled: Inclusion criteria: • Human subjects included in the studies should be ≥18 years of age.
• Implants supporting single crowns placed in the anterior region of the maxilla (second premolar to second premolar).
• Only healed sites (at least 3 months healing time after extraction) and either or not a bone augmentation procedure has been performed in an earlier session.
• Follow-up of at least 1 year after implant placement.
• Detailed information on bone level changes measured on periapical radiographs.
• Randomized clinical trials or prospective clinical studies with a minimum sample of 10 participants per study group.
• No details of the implant type.
• Did not report bone level changes.
• Bone level measurements obtained with cone beam computer tomography (CBCT) or orthopantomograms.
• Animal studies, in vitro studies, retrospective studies, reviews. Both screw-retained and cement-retained single crowns were included. There is no evidence in the literature that one type is performing better with respect to marginal bone level changes.

| Study selection
Two reviewers (C.M.M., H.J.A.M.) independently screened the results from the electronic searches, according to the inclusion and exclusion criteria, in two rounds. Articles were first screened by title and abstract. In case of disagreement or doubt, the study was moved to the next round (full text assessment). The Cohen's κ and percentage of agreement were calculated to determine the measure of agreement between the two reviewers. Any disagreement regarding the inclusion was resolved by a consensus discussion. In case of persistent disagreement, an external independent reviewer (G.M.R.) with experience in implant dentistry could be consulted.

| Statistical analysis
Inter-observer agreement was calculated with IBM SPSS Statistics 20 (SPSS, Chicago, IL). Publication bias was assessed by plotting the log odds ratio against its standard error. Pooling of implant survival, bone loss and mid buccal mucosa changes was performed by using the software Comprehensive Meta-Analysis, Version 3.3.070 (CMA, Biostat, Englewood, NJ). A random effects model was used on the pooled outcomes. To analyze sources of heterogeneity between studies, a metaregression analysis (random effects model) was performed on the connection types, that is, PS-conical, PS-parallel and PM-parallel.

| Study identification and selection
The study selection procedure is summarized in Figure 1. A total of 5513 publications was identified after the electronic and hand search, up to the July 1, 2020. After discarding the duplicates from the output, a total of 2395 publications underwent title abstract selection whereupon 2071 did not meet the inclusion criteria. A total of 324 articles remained for full-text analysis and, of these, 281 did not meet the F I G U R E 1 Study identification and selection process. A total of 43 studies were included following the main screening process (up to July 1, 2020). The updated search resulted on December 1, 2020 resulted in one additional study in-and exclusion criteria, or they were a follow-up of an earlier study

| Risk of bias
ROBINS-1 was used on the prospective non-randomized trials and the domain with a high risk of bias was "bias due to confounding." Low risk of bias was detected in "bias due to deviations from intended interventions" and in "bias in selection of the reported result." Five studies had a high risk of bias on at least one domain (20.8%), 15 studies had a moderate risk of bias on at least one domain (62.5%), and 4 studies had a low risk of bias (16.7%) (see Figure S1).
RoB-2.0 was applied to the RCTs and a high risk of bias was seen in the domain "bias due to deviations from intended interventions." Low risk of bias was detected in the domain "bias due to missing outcome data." Eighteen studies had a high risk of bias on at least 1 domain (90%), and the remaining two studies had low risk of bias (10%; see Figure S2).

| Publication bias
The funnel plot showed no studies on the lower right part of the plot, indicating a possibility of publication bias (see Figure S3).
A meta-regression analysis revealed that the differences in bone change per year were statistically significant between the two plat-  Cosyn et al., 2015De Bruyckere et al., 2018Eghbali et al., 2018Esposito et al., 2015Friberg et al., 2019Gotfredsen, 2012Grandi et al., 2013Heydecke et al., 2019Hosseini et al., 2019Hsu et al., 2016(a) Jonker et al., 2018Kemppainen et al., 1997Meijndert et al., 2019Norton, 2004Palmer et al., 2000Raes et al., 2015Raes et al., 2018Slagter et al., 2016Vanlioglu et al., 2012Vanlioglu et al., 2014Wittneben et al., 2020Yildiz et al., 2016Zuiderveld et al., 2018   platform-switching with platform-matching, but they did not find a significant difference in implant survival rate. Vetromilla et al. (2019) concluded that a conical connection resulted in a higher implant survival rate than a platform-matched configuration, which is in line with the meta-analysis in our study.
According to our study, in contrast to bone level change platform switching seems to have little influence on the mid-buccal mucosa level. When searching for studies that compared implant abutment connections in the aesthetic region, only one RCT (Cooper et al., 2019) was set up in the anterior maxilla and reported no statistically significant differences between the three connection types concerning mid-buccal mucosa changes. The overall mid-buccal retractions in both the PS-conical, PS-parallel and PM-parallel groups were small and the differences were negligible.

| STRENGTH AND LIMITATIONS
The strength of this meta-analysis is the broad and detailed literature search in multiple databases. A limitation to this study is that the quality of the reporting in the included studies was weak and the median follow-up time was short (1 [1;5] year). Also, the meta-analysis was done for variables that can be measured in many ways (in particular bone level change and mucosa level change) and are subject to confounding factors (such as surgical and restauration protocol and implant geometry) and was thus subject to heterogeneity, which means that the outcome must be viewed with caution. Another limita-

Mean and 95% CI
F I G U R E 4 Forest plots for random effects meta-analysis of studies evaluating of bone level change in the PM-parallel group comparability purposes, resembles a linear relation that assumes that the same quantity of bone, implants or mucosa is lost every year. Yet, in real life, most remodeling takes place in the first year, and only a few changes in the years thereafter. We accepted this limitation in order to perform a meta-analysis and this approach is commonly accepted in the dental implant literature, but one should still interpret the results with caution.

| RECOMMENDATIONS FOR FUTURE RESEARCH
Due to a lack of well-designed RCTs and high quality studies, additional well designed studies are needed to be able to truly rate the effect of different implant-abutment connections in the aesthetic zone. We therefore encourage efforts to come to a consensus on how to measure and report clinical and radiographic variables, as well as aesthetic ratings, accurately and homogenously.

| CONCLUSION
The performance of conical and parallel connection configurations with a platform switch is comparable regarding peri-implant bone loss and implant loss when applied to solitary implant restorations in the aesthetic zone. Parallel walled platform matched connections showed the most bone level change and implant loss. None of the connection configurations was significantly better at preserving the mid-buccal mucosa levels.