Precision of fit and retention force of cast non-precious-crowns on standard titanium implant-abutment with different design and height.

Objective The cost-effectiveness of cast nonprecious frameworks has increased their prevalence in cemented implant crowns. The purpose of this study was to assess the effect of the design and height of the retentive component of a standard titanium implant abutment on the fit, possible horizontal rotation and retention forces of cast nonprecious alloy crowns prior to cementation. Materials and Methods Two abutment designs were examined: Type A with a 6° taper and 8 antirotation planes ( Straumann Tissue- Level RN) and Type B with a 7.5° taper and 1 antirotation plane ( SICace implant). Both types were analyzed using 60 crowns: 20 with a full abutment height (6 mm), 20 with a medium abutment height (4 mm), and 20 with a minimal (2.5 mm) abutment height. The marginal and internal fit and the degree of possible rotation were evaluated by using polyvinylsiloxane impressions under a light microscope (magnification of ×50). To measure the retention force, a custom force-measuring device was employed. Statistical analysis: one-sided Wilcoxon rank-sum tests with Bonferroni- Holm corrections, Fisher′s exact tests, and Spearman's rank correlation coefficient. Results Type A exhibited increased marginal gaps (primary end-point: 55 ± 20 μm vs. 138 ± 59 μm, P < 0.001) but less rotation ( P < 0.001) than Type B. The internal fit was also better for Type A than for Type B ( P < 0.001). The retention force of Type A (2.49 ± 3.2 N) was higher ( P = 0.019) than that of Type B (1.27 ± 0.84 N). Reduction in abutment height did not affect the variables observed. Conclusion Less-tapered abutments with more antirotation planes provide an increase in the retention force, which confines the horizontal rotation but widens the marginal gaps of the crowns. Thus, casting of nonprecious crowns with Type A abutments may result in clinically unfavorable marginal gaps. [ABSTRACT FROM AUTHOR]