Biomechanical aging behaviour of clear aligners

Introduction: The biomechanical behaviour of clear aligners can affect the clinical efficacy of tooth movement. Objectives: To investigate the aging effects of clear aligners to suggest an optimal wear time from the biomechanical perspective. Methods: Brand new "as-received" (0-day) clear aligners (Invisalign, Align Co., USA) were immersed in distilled water and placed in an incubator at a constant temperature of 37(degrees)C for 21 days to simulate the aging process encountered in the oral cavity. The biomechanical properties (flexural strength, translucency, surface roughness, hardness, and tensile strength) of the clear aligners were assessed each day. The flexural strength was measured using a digital tension gauge; translucency was assessed using the Vita Easyshade digital spectrophotometer; surface roughness was measured using a profilometer; hardness was measured by the indentation test; and tensile strength was measured using a universal testing machine. Results: The flexural strength decreased significantly after 7 days (0.22 +/- 0.01 N at 3 mm compression; 0.35 +/- 0.01 N at 5 mm compression) compared with the baseline (0.26 +/- 0.01 N, 0.41 +/- 0.01 N, respectively) (difference = 0.15 N, p < 0.05 for both). The hardness decreased significantly after 10 days (4.37 +/- 0.48 kgf/mm(2)) compared with the baseline (6.60 +/- 0.43 kgf/mm(2)) (difference = 2.23 kgf/mm(2), p < 0.05). No significant change was found in translucency (p = 0.38), internal surface roughness (p = 0.99), external surface roughness (p = 0.57), and tensile strength (p = 0.84) during the 21 days of simulated aging. Conclusions: The flexural strength and hardness of the clear aligners decreased significantly after 7 days and 10 days, respectively. Surface roughness, translucency, and tensile strength varied little during the 21 days of artificial aging.