Effect of Vacuum Versus Pressure Thermoforming and Thermocycling on Microhardness of Different Thermoplastic Retainer Materials: An In Vitro Study

Abstract To evaluate surface microhardness of three thermoplastic retainer materials (Essix PLUS Plastic, Zendura A, and Iconic Original) and to compare the effects of the thermoforming method (vacuum versus pressure) and subsequent thermocycling. This in vitro study tested 180 sheets (1.02 mm; n = 60/material). For each material, specimens were allocated to five conditions (n = 12/group): before thermoforming (BT), after vacuum thermoforming (AVT), after pressure thermoforming (APT), after vacuum thermoforming plus thermocycling (AVTT), and after pressure thermoforming plus thermocycling (APTT). Sheets were formed on a three-dimensionally printed maxillary cast using a vacuum former (Plast Vac P7) or a pressure-molding machine (Biostar), following the manufacturer's protocols. Formed sheets were cut into 9 × 13 mm specimens. For thermocycling, specimens were immersed in distilled water for 24 hours and thermocycled 1,500 cycles between 5°C and 55°C (20 s dwell per bath). Statistical Analysis Surface hardness was measured with a Vickers microhardness tester (100 g load for 15 s), and three indentations per specimen were averaged (VHN). SPSS v26 and GraphPad prism V11 were used. Normality and homogeneity were assessed (Shapiro-Wilk, Levene). Comparisons used one-way ANOVA with Tukey HSD, Dunnett's two-sided test versus BT, and independent samples t-tests. Zendura A showed the highest hardness, followed by Iconic Original and Essix Plus. Essix Plus showed no significant changes after thermoforming or thermocycling (p > 0.05). In Zendura A, pressure thermoforming increased hardness versus BT (p = 0.001); thermocycling increased hardness in the vacuum pathway (AVT→AVTT, p = 0.001). In Iconic Original, thermoforming reduced hardness versus BT (AVT and APT, p < 0.001), whereas thermocycling increased hardness within both, forming pathways (AVT→AVTT and APT→APTT, both p < 0.001). Vacuum versus pressure forming did not differ significantly before thermocycling for any material (p > 0.05). After thermocycling, differences were significant for Zendura A (AVTT > APTT, p = 0.030) and Iconic Original (APTT > AVTT, p = 0.031). Hardness is material-dependent and is influenced by thermoforming and thermal aging. Essix Plus was the most stable, while Zendura A and Iconic Original were more sensitive, with method-related differences becoming evident after thermocycling.