Biomechanical Thresholds for Failure: Why Cleveland Zone and Reduction Quality Matter More Than TAD in PFN

Purpose Cut-out remains a major mechanical complication following proximal femoral nailing. This study aimed to evaluate radiographic predictors of cut-out, focusing on reduction quality, screw position within the femoral head, and tip–apex distance (TAD). Methods Seventy-one patients treated with proximal femoral nailing were retrospectively analyzed. Cut-out occurred in 10 patients (14.1%). Reduction quality (Fogagnolo classification), Cleveland zone of screw placement, collodiaphyseal angle (CDA), and TAD were assessed. Univariate analyses were performed, followed by restricted multivariable logistic regression due to limited event number. A trend analysis was conducted for reduction quality. Absolute risk differences were calculated for screw position categories. Receiver operating characteristic (ROC) analysis evaluated the discriminative performance of continuous TAD. Results Cut-out was significantly associated with Cleveland zone placement (p = 0.011), with no failures occurring in favorable zones (5 or 8). Screw placement outside these zones was associated with an absolute 21.7% increase in cut-out risk. A significant linear trend was observed between worsening reduction quality and increasing cut-out risk (OR per category increase 2.56; 95% CI 1.08–6.05; p = 0.032). CDA ≤ 120° was associated with higher failure rates (p = 0.022). In multivariable analysis (poor reduction and TAD > 25 mm), no independent predictor reached statistical significance, likely due to the limited number of events. Continuous TAD demonstrated limited discriminative ability (AUC = 0.63). Inter-observer reliability was high for both TAD measurement ( r  = 0.985). Interobserver agreement for Cleveland zone classification was moderate to substantial ( κ range 0.58–0.61). Clinically, mechanical failure resulted in poor functional outcomes, with mean Harris Hip Scores dropping significantly from 72.6 in successful cases to 30.0 in the failure group. Conclusion Cut-out appears to be more strongly influenced by construct-related biomechanical factors—particularly reduction quality and screw position within the femoral head—than by TAD threshold alone. These findings suggest that precise screw placement and restoration of alignment may act as biomechanical thresholds for preventing mechanical failure.