Effects of mechanical properties of thermoplastic materials on the initial force of thermoplastic appliances

Abstract

Objective: To measure the forces delivered by thermoplastic appliances made from three materials and investigate effects of mechanical properties, material thickness, and amount of activation on orthodontic forces.

Materials and methods: Three thermoplastic materials, Duran (Scheu Dental), Erkodur (Erkodent Erich Kopp GmbH), and Hardcast (Scheu Dental), with two different thicknesses were selected. Values of elastic modulus and hardness were obtained from nanoindentation measurements at 28°C. A custom-fabricated system with a force sensor was employed to obtain measurements of in vitro force delivered by the thermoplastic appliances for 0.5-mm and 1.0-mm activation for bodily tooth movement. Experimental results were subjected to several statistical analyses.

Results: Hardcast had significantly lower elastic modulus and hardness than Duran and Erkodur, whose properties were not significantly different. Appliances fabricated from thicker material (0.75 mm or 0.8 mm) always produced significantly greater force than those fabricated from thinner material (0.4 mm or 0.5 mm). Appliances with 1.0-mm activation produced significantly lower force than those with 0.5-mm activation, except for 0.4-mm thick Hardcast appliances. A strong correlation was found between mechanical properties of the thermoplastic materials and force produced by the appliances.

Conclusions: Orthodontic forces delivered by thermoplastic appliances depend on the material, thickness, and amount of activation. Mechanical properties of the polymers obtained by nanoindentation testing are predictive of force delivery by these appliances.

Figure 1.

Custom-made epoxy model with small force sensor at left maxillary central incisor: (a) frontal view; (b) occlusal view.

Figure 2.

Fabrication of setup model and thermoplastic appliance for measuring orthodontic force. (a) Impression of maxillary teeth by silicone rubber impression material. (b) Trimmed cast made from high-quality stone. (c) Left maxillary central incisor dissected with diamond disk. (d and f) Adjusted activation (0.5 mm or 1.0 mm) using silicone rubber impression material and fixed with high-quality stone. (g and h) Thermoplastic appliance made with pressure-molding machine. (i) Placement of thermoplastic appliance.

Figure 3.

Representative loading-unloading curves for three thermoplastic materials: Duran, continuous line; Erkodur, dashed line; and Hardcast, dashed-dotted line.

Figure 4.

Mean hardness and elastic modulus for thermoplastic appliances with standard deviations. Comparisons are presented for each material. Left side, Duran; middle, Erkodur; and right side, Hardcast. * P < .05 by Tukey test. Hardcast showed significantly lower hardness and elastic modulus than the other two thermoplastic materials.

Figure 5.

Representative results for measuring orthodontic force obtained from Duran thermoplastic appliances: 0.5-mm thickness and 0.5-mm activation, continuous line; 0.75-mm thickness and 0.5-mm activation, dashed line; 0.5-mm thickness and 1.0-mm activation, dashed-dotted line; and 0.75-mm thickness and 1.0-mm activation, double-dotted line.

Figure 6.

Mean forces from thermoplastic appliances with standard deviations. Comparisons are presented for each material at two different thicknesses. Left side shows results for activation of 0.5 mm; right side shows results for activation of 1.0 mm. Left bar, Duran; middle bar, Erkodur; and right bar, Hardcast. * P < .05 by Tukey test. Hardcast showed significantly lower orthodontic force than the other two thermoplastic materials.

Figure 7.

Mean forces from thermoplastic appliances with standard deviations. Comparisons are presented for each material at two different thicknesses. Upper row shows results for 0.5-mm activation; lower row shows results for 1.0-mm activation (Table 1). * P < .05 by Student's t-test. Appliances fabricated from thicker materials delivered greater forces.

Figure 8.

Mean forces from thermoplastic appliances with standard deviations. Comparisons are presented for each material at two different activations. Upper row shows results for thinner thermoplastic materials; lower row shows results for thicker thermoplastic materials (Table 1). Left side, Duran; middle, Erkodur; and right side, Hardcast. * P < .05 by Student's t-test. Appliances receiving greater activation generally delivered lower forces.