Effects of long-term simulated RPD clasp attachment/detachment on retention loss and wear for two clasp types and three abutment material surfaces.

Purpose: The purpose of this in vitro study was to measure the loss of retention and wear of two clasp types (E-circlet, back action) against three abutment materials (enamel, composite, CAD/CAM ceramic) after 16,000 simulated cycles of attachment-detachment. Materials and Methods: Forty-eight models were constructed by placing either an upper first premolar or a metal die inside a metal rectangular block. Models were divided according to the abutment teeth into three groups. Group E consisted of 16 unrestored human premolars with sound enamel. Group R had 16 premolars recontoured buccally using composite resin. Group C had 16 metal dies (duplicated from a human premolar) covered by CAD/CAM all-ceramic crowns. On the models, E-circlet (E) and back-action (B) clasps were constructed to engage the model's teeth. Withdrawal and insertion cycling of clasps was carried out for 16,000 cycles by using a chewing simulator. The retention force of each clasp was measured after cycling. An acrylic replica was made for each abutment retention surface before and after cycling. Each replica was examined by SEM, and the wear areas were measured. The data were analyzed statistically using one-way ANOVA, two-way ANOVA, and Mann-Whitney tests. Results: There was no significant retention loss after 16,000 cycles (p= 0.05) of both clasps (E, B) on the three abutment materials (E, R, C). The mean of wear areas in mm2 were 1.83 +/- 0.36, 0.85 +/- 0.66, 2.37 +/- 1.88, 1.7 +/- 1.11, 0.6 +/- 0.2, and 0.06 +/- 0 for EE, BE, ER, BR, EC, and BC, respectively. There were significant differences among the wear areas of the abutment surface of the six subgroups (p= 0.05). Conclusion: The composite resin contoured surfaces showed more wear than the enamel and ceramic surfaces. E-clasps caused more wear on the abutment materials than back-action clasps.