Biomechanical Implications of Femoral Neck System (FNS) Removal following Femoral Neck Fracture (FNF) Treatment in Low Bone Mineral Density Conditions

This study aimed to investigate the biomechanical implications of Femoral Neck System (FNS) retention and removal in femoral neck fractures under varying bone mineral density (BMD) conditions, with a focus on structural stability and bone resorption risk. A finite element (FE) analysis was conducted using 3D models including intact femurs, FNS-retained (Group I), and FNS-removed (Group R) cases across three BMD levels of 0%, 20%, 40%, and 60%. Structural stability was assessed based on peak von Mises stress (PVMS) in proximal femur, while bone resorption risk was evaluated by analyzing maximum principal stress patterns and measuring strains below the critical bone-damage strain threshold (≤200 μstrain). Group I models exhibited localized stress concentrations around the FNS components, with cortical PVMS values of 30.9-34.0 MPa and cancellous PVMS 2.4-3.4 MPa, leading to a stress-shielding effect. The proportion of cancellous bone strains below the 200 μstrain threshold increased to 20.1-18.5% for Models I-0 to I-60, indicating a bone resorption risk. After FNS removal, cortical PVMS increased to 47.0–50.0 MPa, with cancellous bone PVMS returning to near-native levels (0.5-0.8 MPa). Strain analysis revealed a progressive decrease in sub-threshold strains from 3.7% to 0.1%, suggesting restored loading and reduced bone resorption risk. FNS removal can restore physiological stress distribution and reduce bone resorption risk, particularly in patients with low BMD. However, increased cortical bone stress following FNS removal may pose a risk for structural instability in osteoporotic bone. Preoperative BMD assessment and postoperative monitoring should be considered to guide clinical decisions regarding FNS removal.