Effect of carbamide peroxide on biomechanical properties of vacuum-formed retainers: A split-mouth randomized controlled trial

Objective: To investigate the effect of tooth whitening on biomechanical properties of vacuum-formed retainers (VFRs). Methods: Using a split-mouth, randomised controlled trial design, thirty participants were randomly allocated to receive whitening on either the upper or the lower arch, using 10 % carbamide peroxide for two weeks. Biomechanical properties such as hardness, tensile strength, and surface roughness were assessed two weeks after whitening was completed. Results: Tensile strength of the whitening arch (mean +/- SD: 40.93 +/- 3.96 MPa) was significantly lower than that of the control (47.40 +/- 5.03 MPa) (difference 6.47 MPa, 95 % CI 4.51 - 8.42, p < 0.001). Hardness and internal roughness of the whitening arch (VHN = 14.63 +/- 2.29 N/mm2 and Ra = 1.33 +/- 0.35 <mu>m, respectively) were significantly greater than those of the control (12.22 +/- 1.86 N/mm2 and 0.96 +/- 0.29 mu m, respectively) (differences 2.41 N/mm2, 95 % CI 1.56 - 3.25, p < 0.001 and 0.37 <mu>m, 95 % CI 0.23 - 0.51, p < 0.001, respectively). The whitening arch showed greater tooth colour change (Delta E = 6.00 +/- 3.32) than the control (Delta E = 2.50 +/- 1.70) (difference = 3.50, 95 % CI 2.43 - 4.56, p < 0.001). Conclusions: Based on this short-term study, marked tooth colour change was achieved by whitening with VFRs as the whitening trays, but this changed the VFRs' biomechanical properties, including a decrease in tensile strength and an increase in hardness and internal roughness. Clinical significance: The application of carbamide peroxide in VFRs may compromise their mechanical properties.