Effect of misfit and prosthetic abutments on straind induced in overdenture-retaining system and in loosening torque of prosthetic screw

The passive fit of prosthetic frameworks is considered essential to the longevity of rehabilitation treatment on dental implants.The aim of this study was to analyze vertical misfit of overdentures bars casted in Ti cp; measure the strain, vertical misfit and loosening torque of prosthetic screws, in passive and non-passive frameworks. Twenty bars were casted with Ti cp, retained by two implants, using two types of abutments (UCLA and MA), on which they were evaluated using an optical microscope comparison, the marginal misfit inherent of casting. The misfit quantification followed the protocol of single-screw test (32 Ncm and 10 Ncm). Twenty index were manufactured (10 for each abutment). The misfit were standardized by interposing a metal ring of 200 µm between pilar A abutment and prosthetic cylinder. In the index were evaluated the misfit level, following the same protocol used for measure after casting, and the loosening torque. The measurements of strain were made by strain gauges fixed in the replicas, after retorque of prosthetic screws. The loosening torque of prosthetic screws was evaluated using a digital torque meter with 0.1 Ncm of precision. The results were statistically analyzed (ANOVA, test t, Pearson correlation). Statistical difference was found between passive and non-passive groups: for vertical misfit (p< 0.0001 for both abutments); percentage of loosening torque (UCLA, p = 0.0087; MA, p = <0.0001); mean strain (MA, p = 0.0192) independently of passivity and between the bars desadapted (p = 0.0001) regardless the abutment used. Pearson's correlation (? = 0.05) indicated a positive correlation between: misfit x loosening torque (UCLA, r = 0.6503 e p = 0.0019; MA, r =0.7338 e p = 0.0002); loosening torque x strain for UCLA ( r = 0.5373 e p = 0.0145); misfit x strain (r = 0.5678 e p = 0.0001). It can be concluded: the prosthetic screws have a higher tendency for loosening in ill-fitting prostheses; non-passive frameworks induce higher levels of strain and vertical misfit compared with passive frameworks; UCLA abutments showed the better results in non-passive prostheses; strain, vertical misfit and loosening torque were correlated factors (AU)