Optimizing orthodontic mini-screw design is critical for primary stability. This study investigated the effects of macroscopic parameters—including length, diameter, taper, and thread depth—on stress distribution in surrounding alveolar bone using three-dimensional finite element analysis (FEA) immediately after orthodontic loading.

We designed a standard mini-screw (8mm length, 1.6mm diameter, 0.25mm thread depth, 1° taper with V-shaped threads) and systematically altered each parameter to create eight additional models. Each mini-screw embedded in bone was subjected to a 2N static shear load. The stress distribution in the bone and mini-screw, as well as mini-screw displacement under shear loading, were calculated to evaluate primary stability for each design.

Reducing diameter by 0.2mm increased maximum von Mises stress by 47% in bone and 33% in the mini-screw, whereas increasing diameter by 0.2mm decreased stress by 10% in both. Other design parameters had smaller effects under shear loading. Maximum bone stress consistently occurred at the mini-screw entry site and around the first thread.

Mini-screw diameter is the most influential factor affecting primary stability, particularly regarding maximum von Mises stress. Our findings highlight the importance of optimizing diameter and structural design at the bone entry site, providing practical guidance for clinical selection and mini-screw design improvements.