Purpose: To investigate the peri-implant stress fields generated from four different implant-abutment interfaces under axial loading applied at the center of the implant and several millimeters away from the implant center via photoelastic analysis.
Materials and methods: Similar unthreaded and cylindric implants and abutments were fabricated and embedded in photoelastic resin with four different implant-abutment interfaces: external hex, internal hex, internal taper (11.5 degrees), and solid connection to the abutment (one piece). The samples were submitted to vertical compressive loads; one was applied at the implant center (1.5 kg; centered load), and the other was applied 6.5 mm away from the center, 4.4 mm from the outside of the outer aspect of the implant (0.75 kg; off-centered load). The maximum shear stresses were determined and observed at 46 points around the implants under the centered load and at 61 points under the off-center load in the photoelastic models. Graphics describing the maximum shear stress (y-axis) and the analyzed points (x-axis) were obtained, and areas under the curves were calculated.
Results: The centered loading (all points) resulted in small differences. The lowest amounts of stress were observed for the internal-taper implants, and values were minimally greater (0.4% to 3.3%) for the other implants. No statistically significant differences were found between groups for the centered load in any area. Under an off-center load, the internal-hex implants presented the least stress (all points). For off-center loading, the internal-hexagon implants differed significantly from the external-hex and one-piece implants and displayed the lowest stress levels.
Conclusion: Under an off-center load, the internal-hex interfaces presented the lowest stress concentrations, internal-taper interfaces presented intermediate results, and one-piece and external-hex implants resulted in high stress levels. Centralized axial loads produced similar results.