Risk factors for nonunion in femoral neck fracture patients with internal fixation: A multicenter (TRON group) retrospective study.

Introduction: Femoral neck fractures (FNFs) are common in elderly individuals. When minimally displaced, they are typically treated with internal fixation. Nonunion is a complication of internal fixation of FNF, that sometimes necessitates reoperation. Radiographic parameters, including the Pauwels angle and posterior tilt angle, are risk factors for nonunion. However, these parameters are assessed solely in a two-dimensional context. We developed a novel radiographic parameter named the "Kindex." This multicenter investigation aimed to identify risk factors for nonunion following FNF fixation and evaluate the validity of this index.

Methods: This retrospective multicenter study collected data from 939 FNF patients who underwent internal fixation between 2016 and 2020 at 11 facilities (TRON group). The following exclusion criteria were applied: age <65, insufficient data, and Garden Stage III or IV fracture. Patient data, including age, sex, BMI, ASA classification, alcohol and smoking history, and comorbidities, were recorded. Radiographic measurements of the Pauwels angle and posterior tilt angle at the time of injury were used to calculate the Kindex. Correlations between the Pauwels angle, posterior tilt angle, Kindex, and nonunion were evaluated. A multivariate logistic regression analysis was performed to investigate independent risk factors for nonunion. A receiver operating characteristic (ROC) analysis was performed to determine the cutoff value of the Kindex, and the area under the curve (AUC) was calculated.

Results: The study included 594 patients (males, n = 151; females, n = 443; average age, 80.9 years). Nonunion was observed in 13 cases (2.2 %). While the Pauwels angle and posterior tilt angle did not show significant correlations, the Kindex exhibited strong correlations with both parameters. In the multivariate analysis, the Kindex and renal impairment were independent risk factors for nonunion (Kindex: OR 1.06, p = 0.015; renal impairment: OR 1.48, p = 0.021). In the ROC analysis, a Kindex of 50 was identified as the optimal cutoff value (AUC 0.72).

Conclusion: The nonunion rate after internal fixation of FNF was 2.2 %. Renal impairment and the Kindex were identified as independent risk factors. The Kindex at the time of injury may serve as a novel radiographic parameter to consider when evaluating the need for internal fixation in FNF cases.