Effect of Cephalic Screw Positioning in Intramedullary Nailing of Unstable Trochanteric Fractures (AO/OTA 31A2): A Biomechanical Study

Trochanteric fractures are common fragility injuries with high one-year mortality. In unstable patterns (AO/OTA 31A2–A3), intramedullary nails are widely used, yet fixation failure persists. Cephalic screw positioning is a key modifiable factor, but its biomechanical impact for intramedullary constructs in unstable fractures is not fully quantified. The aim was to evaluate how cephalic screw positioning influences implant/bone mechanics in unstable AO/OTA 31A2 fractures fixed with a proximal femoral nail using finite element analysis. A computed tomography based FE model of an AO/OTA 31A2.2 fracture in a healthy adult femur was instrumented with a short proximal femoral nail. Five cephalic screw configurations were analysed under single-leg stance (2.5 × body weight plus abductor/iliopsoas forces) over ten gait cycles. Outcomes were Von Mises stresses in bone/implant and femoral-head sliding along the screw axis. Model validity against experimental/validated datasets achieved 10% agreement. All models converged. Stress concentrations consistently occurred at the superior screw aperture of the nail. Malpositioned screws produced local implant stresses up to 696 MPa and cancellous overload up to 87 MPa, while central-subchondral placement yielded more uniform cancellous loading (69 MPa). Femoral head sliding was lowest with central-subchondral placement (Δ = 1.9 mm) and more than doubled with eccentric/short trajectories (Δ = 4.3 mm). Cephalic screw positioning strongly determines construct mechanics. Inferior-central subchondral placement minimises stress concentrations and collapse, providing a plausible mechanism for reduced cut-out risk. Intraoperative control of tip-apex distance/calcar-referenced TAD and restoration of anteromedial cortical support are recommended.