Evaluation of marginal misfit and stress in 3-unit implant-supported prostheses: the role of prosthetic framework and implant design

The presence of prosthetic misfit and stress in implant-supported system can compromise the longevity of treatment. Therefore, this in vitro study evaluated the influence of prosthetic framework fabrication method (CAD/CAM and overcasted) on marginal misfit and stress transmitted to implants and the effect of implant design (length and diameter) and marginal misfit on the stress levels of fixed partial denture (FPD). Three-element prosthetic frameworks were made using the CAD/CAM (n=10) and overcasted (n=10) methods. The frameworks were waxed to simulate a superior first pre-molar (pillar P) to first molar (pillar M) FPD using overcasted mini abutment cylinders. The wax patterns were overcasted (overcasted group) or scanned to obtain the frameworks (CAD/CAM group). All frameworks were fabricated in CoCr alloy. Two photoelastic models were obtained: model C with two standard Branemark implants (4.1 × 11 mm); and model S with a short implant (5 × 6 mm) and a standard Branemark implant (4.1 × 11 mm). The marginal misfit was analyzed according to the single-screw test protocol, obtaining an average value for each implant site and each framework. For implant design evaluation, different levels of marginal misfit were selected based on the misfit average of frameworks: low (< 20 µm), medium (> 20 and < 40 µm) and high (> 40 µm). The stress was measured by quantitative photoelastic analysis after tightening of frameworks to the photoelastic model C with a standardized 10-Ncm torque to evaluate the influence of framework type. The qualitative analysis in models C and S was used to compare the implant design under non-loaded condition and after 280-N load application. The results were submitted to T-test, 2-way ANOVA, and Pearson correlation test (?=.05) to evaluated the effect of framework type on the variables studied (marginal misfit and stress). The framework type and evaluation site (implant M and P) did not affect the marginal misfit values (p=.466 and p=.153, respectively) and stress (p=.602 and p=.746, respectively). Positive correlations between marginal misfit and stress was observed (CAD/CAM: r=.922 p<.0001; overcasted: r=.908 p<.0001). Under loaded condition, the short and wide implant reduced the transmitted stress to the system. It can be concluded that overcasted and CAD/CAM methods present similar marginal misfit and stress values for 3-unit implant-supported FPDs. Increasing the marginal misfit of frameworks induces greater stress in the implant-supported system. Small increments in marginal misfit increase the stress levels independently of the implant design used and the use of a posterior short and wide implant may be an approach to rehabilitate patients with reduced bone height (AU)