Impact of implant–abutment connection and positioning of the machined collar/microgap on crestal bone level changes: a systematic review

OBJECTIVES To address the following focused question: What is the impact of implant-abutment configuration and the positioning of the machined collar/microgap on crestal bone level changes? MATERIAL AND METHODS Electronic databases of the PubMed and the Web of Knowledge were searched for animal and human studies reporting on histological/radiological crestal bone level changes (CBL) at nonsubmerged one-/two-piece implants (placed in healed ridges) exhibiting different abutment configurations, positioning of the machined collar/microgap (between 1992 and November 2012: n = 318 titles). Quality assessment of selected full-text articles was performed according to the ARRIVE and CONSORT statement guidelines. RESULTS A total of 13 publications (risk of bias: high) were eligible for the review. The weighted mean difference (WMD) (95% CI) between machined collars placed either above or below the bone crest amounted to 0.835 mm favoring an epicrestal positioning of the rough/smooth border (P < 0.001) (P-value for heterogeneity: 0.885, I2: 0.000% = no heterogeneity). WMD (95% CI) between microgaps placed either at or below the bone crest amounted to -0.479 mm favoring a subcrestal position of the implant neck (P < 0.001) (P-value for heterogeneity: 0.333, I2: 12.404% = low heterogeneity). Only two studies compared different implant-abutment configurations. Due to a high heterogeneity, a meta-analysis was not feasible. CONCLUSIONS While the positioning of the machined neck and microgap may limit crestal bone level changes at nonsubmerged implants, the impact of the implant-abutment connection lacks documentation.