Pressure-based finite element analysis of setup distance in clear aligner canine retraction comparing bodily and tipping movements

In clear aligner therapy, setup distance is a critical design parameter governing orthodontic force magnitude, yet it is commonly determined empirically without explicit biomechanical justification. This study proposes a pressure-based biomechanical framework for evaluating setup distance during maxillary canine retraction. Three-dimensional finite element models were constructed with setup distances ranging from 0.05 to 0.35 mm, and both bodily and tipping movement designs were analyzed. Hydrostatic pressure distribution within the periodontal ligament (PDL) was calculated, and the area fraction within the physiological pressure range was quantified as a spatial metric associated with mechanically favorable conditions for bone remodeling. As setup distance increased, the proportion of PDL within the physiological pressure range initially increased and subsequently decreased as suprasystolic pressure regions expanded. Bodily movement exhibited a broader effective setup-distance range (0.10-0.25 mm) characterized by relatively uniform pressure distribution, whereas tipping movement showed a narrower effective range centered around 0.25 mm due to pronounced cervico-apical pressure gradients limiting expansion of the optimal pressure zone. These findings demonstrate that PDL pressure distribution provides a biomechanical basis for interpreting setup-distance selection in clear aligner design.