To establish baseline biomechanical data for six nickel-titanium (NiTi) reverse curve of Spee (RCOS) archwires in a standardized mandibular dentition model and to compare the effects of archwire cross-section and size on force systems, tooth displacement, and root/periodontal ligament (PDL) stress distribution.

A three-dimensional finite element model of the mandible, dentition, PDL, MBT brackets, and six NiTi RCOS archwires, including three round archwires (0.014, 0.016, and 0.018 inch) and three rectangular archwires (0.016 × 0.022, 0.017 × 0.025, and 0.018 × 0.025 inch), was constructed. Archwire activation was standardized using a nodal displacement-based loading approach, in which predefined vertical displacements were applied to reproduce archwire engagement into the bracket slots. Tooth-level forces and moments, total and directional displacement, and root/PDL stress distributions were analyzed under standardized loading conditions.

Increasing archwire size consistently increased force magnitudes, tooth displacement, and root/PDL stress. Round RCOS archwires generated force systems dominated by vertical components, resulting in more coordinated anterior intrusive displacement and relatively limited buccolingual and mesiodistal effects. In contrast, rectangular RCOS archwires produced more complex three-dimensional responses, including greater buccolingual and mesiodistal displacement, more pronounced moment expression, and higher root/PDL stress, particularly in the mandibular incisors.

In this standardized finite element model, round RCOS archwires favor vertically controlled intrusive displacement with lower stress levels, whereas rectangular RCOS archwires produce greater three-dimensional effects and higher root/PDL stress levels. Clinically, rectangular RCOS archwires may be considered in deep-bite patients with retroclined mandibular incisors when simultaneous vertical leveling and torque expression are desired. However, they should be used cautiously, or avoided as an initial leveling archwire, in patients with proclined mandibular incisors, reduced periodontal support, short roots, or increased susceptibility to root resorption.