Aim: To evaluate the effect of combined thermocycling and artificial saliva pH on flexural strength, surface microhardness, as well as clasp retention and deformation of two different thermoplastic polymers.
Materials and methods: Three groups were created, heat-cured polymethyl methacrylate, acetal, and poly-ether-ether ketone (PEEK) resins. Specimens were wrapped in plastic bags containing artificial saliva with three pH values (acidic 5.8, neutral 7.2, and alkaline 8.3). Two Aker clasps materials (acetal and PEEK), for premolar and molar, were stored in neutral salivary pH. Specimens were subjected to 2,000 thermocycles (5-55°C). Surface microhardness, flexural strength, and clasp retention and deformation were evaluated before and after aging. Data were analyzed by ANOVA, Tukey's test, Student's t-test, and paired t-tests (p < 0.05).
Results: Thermal cycling at acidic and alkaline pH significantly decreased flexural strength and surface microhardness of acetal. It had no significant effect on PEEK properties. Poly-ether-ether ketone showed statistically significant higher mechanical properties in all groups. Acetal clasps exhibited a statistically significant deformation and a corresponding decrease in retention after thermocycling at neutral pH.
Conclusion: Mechanical properties of acetal, as well as its clasp retention and deformation, significantly decreased after combining thermal and pH aging and thermal cycling in neutral pH, respectively. Meanwhile, PEEK clasps were not significantly affected.
Clinical significance: Different intraoral variables may significantly affect mechanical performance, retention, and deformation of TMs used for denture base and clasp construction. Some of these TMs may behave better than the other types with recommended improvement of the design like increasing clasp cross-section area.
Keywords: Artificial saliva pH; Clasp retention; Deformation; Thermal cycling Thermoplastic resins..