Larger nail-shaft gap as a potential risk factor of femoral head varus in patients treated with proximal femoral nail antirotation fixation: a retrospective, observational, single-center study with corresponding numerical simulation.

Background: Deterioration of postoperative local biomechanical environment is a primary contributor to varus deformity of the femoral head in patients with intertrochanteric fractures treated with proximal femoral nail antirotation (PFNA) fixation. Theoretically, increasing the number of threaded anchors can optimize the local biomechanical environment and enhance the fixation stability. According to this principle, a larger gap between the intramedullary nail and the surrounding femoral shaft at the distal locking screw may compromise fixation stability by reducing the number of thread anchorages. However, whether this issue can lead to a higher incidence of biomechanical varus deformity of the femoral head remains to be determined. The main objective of this study was to investigate the clinical and biomechanical effect of nail-shaft gap on the incidence of femoral head varus in PFNA-treated patients. Methods: This study conducted a comprehensive investigation that included both a retrospective clinical analysis and numerical biomechanical simulations. In the retrospective component, data from patients with PFNA-fixed intertrochanteric fracture were collected. The nail-shaft gap was measured in the distal locking screw location. The varus alignment of the femoral head was evaluated based on measurements of the neck-trunk angle, which were obtained from anterior-posterior radiographs taken immediately postoperatively and at 6 months postoperatively. Additionally, patient demographics-including age, sex, body mass index, bone density (represented by T-score), and fracture types (stable or unstable)-were recorded. Regression analysis was performed to identify the effect of changes in gap size on femoral head varus. Furthermore, biomechanical alterations resulting from variations in nail-shaft gap size were investigated to elucidate the mechanisms underlying clinically observed phenomena. Results: A larger nail-shaft gap was not significantly correlated with the progression of femoral head varus (correlation coefficient =0.169; P=0.145), nor was it identified as an independent risk factor for this complication (P=0.145). In contrast, a lower T-score (correlation coefficient =-0.269; P=0.019) and unstable fracture type (correlation coefficient =0.26; P=0.023) were significantly correlated with femoral head varus. Moreover, the results of the multivariable regression analysis indicated that a lower T-score (P=0.005) and unstable fracture (P=0.034) were determined to be independent risk factors for this complication. Furthermore, while displacement values of the femoral head increased under both physiological and compressive loads with larger gap sizes, stress values in both the femoral head and antirotation blade remained comparable across different models. Conclusions: A larger nail-shaft gap does not lead to more severe varus deformity of the femoral head from a biomechanical perspective. This phenomenon may be attributed to the optimal anchorage capability of the cortical shaft, even in elderly patients with osteoporosis.