Objective: Complete implant integration may not occur with medically compromised patients such as those with osteoporosis, those on steroids, and post menopausal patients. Questions arise as to the stresses exerted at the implant-bone interface during securing abutment cylinders to implants. The objective of this photoelastic study was to evaluate the stress generation during torque application to abutment screws on implants with different levels of bone integration. Methods: Composite models of overall dimensions 4 x 4 x 1cm were fabricated using individual photoelastic plastics to simulate trabecular and 1.0mm thick cortical bone. Threaded 10mm long implants (3i), of diameters 3.25mm, 4.0mm, and 5mm were embedded into individual models. Models were made with implants representing the following degrees of simulated integration: 100%, 50%, 25%. Partial integration was represented by blocking portions of the threads. Appropriate abutment cylinders and screws were installed on the implants. A calibrated torque wrench (ITI) was used to tighten the screws to 20N/cm. The stresses developed in the bone simulants were recorded in the field of a circular polariscope and the resultant isochromatic fringes digitally analyzed. Results: The lowest and least stress concentration occurred with the completely integrated 5mm diameter implant. The highest stresses were noted at the crestal bone of the smallest diameter implant at 25% integration, as well as at the junction of non-integration areas. Intermediate torquing stresses were generated with the mid-sized implant regardless of degree of integration. Conclusions: Results demonstrated increasing torque-induced stress associated with decreasing implant diameter and degree of integration. Exceptional care should be exerted when installing abutment cylinders of small diameter in clinically incomplete integration situations.

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