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Photoelastic analysis of stress patterns from different implant-abutment interfaces.

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.