Breaking Strength of Acrylic Overdenture on Retainer

Objective: Overdentures tend to break into thin portions because the retainers act as fulcrums during service. This study investigated the effect of the reinforcement wires and differences in artificial teeth on the breaking strengths of acrylic overdentures. Method: After the basal layer of two kinds of artificial premolar teeth [composite denture teeth (DURACROSS Physio, NISSIN, Japan) and acrylic denture teeth (Bio-ACE, SHOFU, Japan)] was removed using a carbide bur, the teeth were arranged with inadequate space on the retainer. A reinforcement wire (San-Cobalt wire, Dentsply Sankin, Japan) and palatal bar (San-Cobalt palatal bar, Dentsply Sankin) were primed with Metal Primer II (GC), and embedded near the occlusal surface. Heat-cured resin (ACRON, GC) was polymerized following manufacturer instructions, and all specimens were finished to 6.5 × 8 × 70 mm. Specimens without a reinforcement wire were fabricated as a control. The breaking strengths (n=8) of two-week-aged (37 °C) specimens were measured using bending test. The strengths obtained at a crosshead speed of 2.0mm/min were analyzed by ANOVA/Tukey¢s test (a=0.01). Results: The bending strengths (N) of the composite denture teeth and acrylic denture teeth [Mean(SD)], respectively, were: control [163.6(15.5), 162.0(16.6)], reinforcement wire [251.1(23.5), 272.5(34.2)], palatal bar [394.6(49.6), 378.5 (44.6)]. There were no statistical differences between the composite denture teeth and acrylic denture teeth (p>0.01). The breaking strength of the palatal bar was the significantly highest among the specimens tested (p<0.01). Conclusion: The reinforcement wire and palatal bar increased the breaking strengths by approximately 1.7 times and 2.4 times, respectively, of the acrylic overdenture on the retainer.