Effect of trimming line design and edge extension of orthodontic aligners on force transmission: An in vitro study

•The trimming design has a significant effect on force transmission by aligners.•Force transmission by aligners occur through specific stress concentration areas.•The thickness of aligner sheets is drastically reduced by thermoforming.•The experimental biomechanical studies of clear aligners have several limitations.•Pressure sensitive films can be used for understanding the biomechanics of aligners. To investigate how the stress distribution and forces transmitted from orthodontic aligners to the tooth surface are affected by the geometry and extension of the trimming line. Thirty-six aligners were thermoformed from Zendura FLX sheets (0.75 mm thick) and divided into four groups based on the design of the trimming line: Scalloped, Scalloped extended, Straight and Straight extended. Fuji pressure-sensitive films were used for pressure measurement. The pressurized films were scanned and evaluated. Pressures and forces were measured over the entire facial surface of an upper right central incisor (Tooth 11) and at 7 different locations [cervical, middle, incisal, mesio-incisal, mesio-cervical, disto-incisal, and disto-cervical]. In addition, the thickness of the aligners at these 7 sites was measured with a digital caliper. The active force ranged from (2.2 to 6.9) N, and the average pressure was (1.6–2.7) MPa. The highest values were recorded for the (straight extended) design, while the lowest values were recorded for the scalloped design. The forces and stresses were not uniformly distributed over the surface. When the values in each area were compared separately, significant differences were found between the different designs in the cervical area, with the scalloped design transmitting the lowest cervical forces. Aligner thickness was drastically reduced (60–75% thinning) over the entire tooth surface after thermoforming. The straight extended design of aligner's trimming line exhibited more uniform force transfer and stress distribution across the surface than the other designs. The trimming line design could have a significant impact on the clinical outcome of orthodontic aligner treatment.