Effect of abutment finish lines on the mechanical behavior and marginal fit of screw-retained implant crowns: An in vitro study

Statement of problem. The design of the implant-abutment connection has been widely researched, but the impact of different crown-abutment geometries remains unclear. Purpose. The purpose of this in vitro study was to evaluate the effect of different crown-abutment margin geometries on the mechanical behavior and fit of screw-retained implant-supported single-crown restorations by using mechanical static and fatigue tests and mastication simulation. Material and methods. A total of 45 cobalt-chromium premolar-shaped metal frameworks were fabricated for single-unit implant-supported screw-retained restorations on stock abutments and internal hexagon empty set4.25x11-mm cylindrical implants. They were divided into 3 groups according to margin geometry: S, shoulder; C, chamfer; and F, feather-edge. Three static load until fracture and 24 dynamic load tests were performed by using the International Organization for Standardization 14801:2016 standard (ISO 14801:2016) (number of cycles limit: 5x10(6) cycles, frequency: 6 Hz). The ProFatigue software program was used to optimize the procedure (S, n=12 specimens; C, n=7 specimens; and F, n=5 specimens). Six additional specimens from each group were subjected to a mastication simulation (limit number of cycles: 1x10(6) cycles, cyclic loading from P-min=30 N to P-max=300 N, frequency: 6 Hz). Results from the fatigue tests were reported descriptively, and the Fisher exact test was used to analyze the difference in failure modes. Data from maximum misfit were evaluated by photogrammetry and statistically analyzed with the Anderson-Darling test and the Kruskal-Wallis and Dunn multiple comparison tests (alpha=.05). Results. The fatigue limit was 456 N for group S, 512 N for group C, and 514 N for group F. The mean +/- standard deviation misfit was 2.6 +/- 0.1 mu m for group S, 3.8 +/- 1.1 mu m for group C, and 3.6 +/- 0.8 mu m for group F. Differences in misfit between groups S and C and between groups S and F were statistically significant (P<.05). Conclusions. Crown-abutment connections with chamfer and feather-edge margins showed better mechanical behavior, while shoulder margin exhibited better fit. However, high levels of fit were achieved for all the evaluated geometries.