Retention Characteristics of Implant-Supported Milled Bar Attachments - a comparative in vitro study

Objectives: Milled bars [MB] are an alternative option for the retention of implant supported overdentures. The individual manufactured suprastructures [S] adjust precisely and rigidly to the MBs. For this purpose different materials and retention concepts [RC] were introduced. The aim of this study was to compare initial and long-term retention characteristics and wear of MBs from different materials and different RCs used to retain overdentures to dental im-plants. Methods: 7 different MB-S-combinations (20x7x2mm) with different RC were indi-vidually fabricated: gold&electro-plated-S [GG], cobalt-chromium&electro-plated-S [EG], tita-nium&electro-plated-S [TG], zirconium&electro-plated-S [ZG], cobalt-chromium&spark-erroded-S [EF], titanium&spark-erroded-S [TF], zirconium&spark-erroded-S [ZF]. 5 speci-mens of each group (n=35) were screwed to 2 implants (Xive, Friadent) and were long-term tested (5000 hysteresis cycles) in a universal test machine (v=40mm/min, s=2mm, Fmax=50N, artificial saliva=23°C). Retention forces were constantly recorded. MB-S-combinations were analysed for superficial degradation (SEM). For evaluation of the total wear the mean-retention-force [MRF] of the first and last 250 cycles were calculated and sta-tistically compared (ANOVA, Bonferroni, P0.05). Results: Initial MRF differed from 5.35N [TF] to 21.68N [EG]. After long-term cycling the resulting MRF differed from 2.41N [TF] to 18.45N [EG]. Each MB-S-combination produced a characteristic curve. MRF-change (Delta F=Fmax-final - Fmax-initial) differed from -10.13N [EF] to +2.14N [GG] (p0.001) The MRF-alteration differed from -54.95% [TF] to +17.09% [GG]. SEM-analysis revealed charac-teristic degradations of the corresponding material surfaces. Conclusion: There were differ-ences between the initial pull-off-forces of the tested MB-S-combinations. Standardized long-term-cycling exposed specific changes of the retention characteristics and resulting pull-off-forces in regard to bar material and retentive superstructure designs. Keywords: implant, milled bar, attachments, retention forces, long-term retention