Flexural Properties of the Thermoplastic Resin for Resin Clasps

Objectives: For aesthetic reasons, a resin clasp with thermoplastic resin has been widely used for removable partial dentures instead of a metal clasp. Hence, many gingival-colored thermoplastic resins have been developed rapidly and marketed. This study assessed the flexural properties of six new thermoplastic resins.
Methods: Six types of commercially available thermoplastic resins—two polyamide resins [Bioplast (BP) and Biotone (BT); High Dental Co., Ltd., Japan], two polycarbonate resins [ReigningN (RN); Toshinyoko, Japan, and Jet Carbo-S (JCS); High Dental Co., Ltd., Japan], a polyester resin [EstheShot Bright (ESB); i-CAST, Japan], and an acrylic resin [Acrytone (ACT); High Dental Co., Ltd., Japan]—were tested in this study. As a control, polymethyl methacrylate [Acron (ACR); G-C Dental Corp., Japan] was used. The flexural properties of the elastic moduli and the breaking strengths were measured according to ISO standard 1567: 2005. Each specimen (64.0×10.0×2.5 mm) was mounted on a universal testing machine (AG-20kN; Shimazu, Japan), and a three-point bending test was carried out with a distance of 50.0 mm between the supporting points with a crosshead speed of 5.0 mm/min. The obtained data (n=6) were analyzed using a one-way ANOVA and Tukey’s multiple comparison testing (a=0.05).
Results: BP showed the lowest elastic modulus as compared with the other thermoplastic resins (p<0.05). Conversely, ACR indicated significantly elastic moduli and higher breaking strengths than did all six thermoplastic resins (p<0.05).
Conclusions: Although thermoplastic resins demonstrated lower flexural stiffness, they had greater flexural toughness to protect against clasp fracture. Theses properties would produce appropriate retention even if great undercuts form in the crown and would protect against clasp failure with repeated insertion and removal of removable dentures.