Retentive Forces and Fatigue Resistance of Thermoplastic Resin Clasps

Objectives: The objective of this study was to evaluate the retentive force of clasps made from three thermoplastic resins and cobalt-chromium (CoCr) alloy by the insertion/removal test simulating 10 years use.

Methods: On standardized premolar metal crowns 112 clasps were fabricated, including 16 CoCr (1.0 mm thick) clasps and 32 clasps (1.0 or 1.5 mm thick) from each of the following thermoplastic resins: polyetheretherketon (PEEK), polyetherketonketon (PEKK) and polyoxymethylene (POM). Specimens were divided in subgroups with clasp undercuts of 0.25 mm and 0.5 mm, respectively. Each clasp assembly was subjected to an insertion/removal test on its abutment crown for 15,000 cycles. To analyze the retention over the course of insertion/removal test, retention was measured every 1500 cycles. Data were statistically analyzed using 3-way ANOVA (α=0.05).

Results: Resin clasps with 1.5mm thickness showed higher retention (4.9-9.1 N) than clasps with 1.0mm thickness (1.2-3.1 N; P≤0.001). Resin clasps of both dimensions had significantly lower retentive force than CoCr clasps (11.3-16.3 N; P≤0.001). Clasps with 0.25 mm undercut showed significantly less retention than clasps with 0.50 mm (P≤0.001). All clasps exhibited an increase in retentive force during the first period of cycling followed by continuous decrease till the end of the cycling but it was still significantly not different compared to the initial retentive force (P=0.970).

Significance: Thermoplastic resin clasps maintained retention over 15,000 joining and separating cycles with lower retention than CoCr clasps. However, the retention of adequately designed resin clasps might be sufficient for clinical use.