Characteristics of the Extent and Onset of Osteonecrosis of the Femoral Head After Femoral Neck System (FNS) Fixation: A Minimum Two-Year Follow-Up Comparative Study with Cannulated Screws

Background/Objectives: This study aimed to compare the incidence, extent, and time to diagnosis of osteonecrosis of the femoral head (ONFH) between Femoral Neck System (FNS) and multiple cannulated screw (CS) fixation in patients with femoral neck fractures (FNFs) at a minimum follow-up of 24 months, as well as to identify factors associated with ONFH after FNS fixation. Methods: A retrospective cohort study was conducted at a single university hospital involving patients aged ≥18 years with isolated FNFs treated by closed reduction and internal fixation using either FNS or multiple CS. Reduction quality, fracture union, incidence and extent of ONFH (measured by Kerboul angle), and revision surgery due to ONFH or other causes were assessed as outcome measures. Logistic regression analyses were conducted to identify factors associated with ONFH after FNS fixation. Results: Eighty-seven patients were included, comprising 48 in the FNS group and 39 in the CS group. No significant differences were found between the groups in reduction quality, union rate, or time to union (p > 0.05 for all). The incidence of ONFH did not differ significantly between the FNS and CS groups (p = 0.314). However, the FNS group showed a significantly greater extent of ONFH (p = 0.031) and longer mean time to ONFH diagnosis (p = 0.001). No significant differences were observed in revision surgery rates due to ONFH (p = 1.000) or other causes (p = 0.624). Multivariate analysis identified fracture displacement (Garden classification stages III and IV) and poor reduction quality (“Broken S” by Lowell’s criteria) as significant predictors of ONFH after FNS fixation. Conclusions: ONFH following FNS fixation tended to exhibit a larger necrotic area and a delayed radiographic onset compared to CS fixation; however, the overall incidence and revision rates were comparable between implants. These differences may be due to initial fracture displacement and baseline severity, rather than the FNS implant itself. In the FNS group, the mean time to radiographic diagnosis of ONFH was 13.1 months, and initial fracture displacement and reduction quality were identified as risk factors for the development of ONFH following FNS fixation. Therefore, long-term and vigilant follow-up is essential after FNS fixation, particularly in patients with displaced fractures. Achieving optimal reduction quality and anatomical femoral neck alignment remains paramount to mitigating the risk of ONFH after FNS fixation.