Evaluation on locking plate fixation with and without oblique screw in thumb carpometacarpal joint arthrodesis with finite element analysis.

Arthrodesis with a locking plate is widely used for thumb carpometacarpal osteoarthritis. However, complications such as nonunion and plate fracture remain a concern. Although clinical studies suggest that an oblique screw reduces the complication rate, its mechanical effect and an optimal insertion method remain unclear. We aimed to quantify the oblique screw's biomechanical effect with finite element (FE) analysis. A FE model of thumb arthrodesis was constructed using a CT-based bone model and three plate designs. For each plate, models with and without an oblique screw were tested under three loading modes to evaluate the displacement of the first metacarpal head and the maximum equivalent stress in the plate. With a T-shaped plate under bending, displacement and stress dropped from 4.10 to 0.12 mm and 621.6-72.0 MPa, respectively; other conditions showed similar trends. A simplified model showed that longer screw insertion paths enhanced fixation, reducing displacement and stress by 86% and 79%, respectively. These findings quantify the mechanical benefit of oblique screw insertion and suggest that increasing the insertion path length enhances the screw effect. Our recommendation may reduce complication risk, improve long-term outcomes, and help surgeons choose more reliable fixation strategies.