Femoral Neck Stress Fracture: Can MRI Grade Help Predict Return-to-Running Time?

Background: Limited research is available regarding return-to-running (RTR) time after femoral neck stress fractures. While studies have shown the prognostic value of image-based grading scales for stress fractures at other sites, few have included femoral neck stress fractures. Purpose: To determine if the grade of femoral neck stress fractures based on magnetic resonance imaging (MRI) characteristics correlates with RTR time. Study Design: Cohort study; Level of evidence, 3. Methods: This study included 24 patients (mean age, 32.9 years; range, 18-51 years) who were diagnosed with 27 femoral neck stress fractures by MRI from 2009 to 2015 at a single sports medicine clinic. All fractures were compression sided and were treated nonoperatively. Charts were reviewed for patient demographics and RTR time. Images were graded from 1 to 4 using the Arendt stress fracture severity scale. Statistical analysis was performed using survival analysis and Cox proportional hazard model to compare the RTR time between grades. Cox regression was repeated, adjusted for age, bone mineral density (BMD), and body mass index (BMI). Results: The mean (standard error of the mean) RTR time in weeks for patients with fractures graded 1 to 4 was 7.4 +/- 2.7 (range, 4-11), 13.8 +/- 3.8 (range, 6-21), 14.7 +/- 3.5 (range, 8.5-24), and 17.5 +/- 3.4 (range, 10-32), respectively. Survival analysis indicated that there was a statistically significant effect of fracture grade on RTR time (P = .0065). The Cox model indicated a statistically significant difference in RTR time between grades 1 and 2 (P = .036), 1 and 3 (P = .014), and 1 and 4 (P = .002). The unadjusted hazard ratio was significant (P = .037). There were no statistically significant differences between the remaining grades (P = .82 for grades 2 and 3, P = .37 for grades 2 and 4, and P = .31 for grades 3 and 4). Age (P = .71) and BMD (P = .81) did not have an effect on RTR time. The hazard ratio remained significant (P = .05) after adjusting for age and BMD. BMI tended to have an effect on RTR time (P = .09). After adding BMI to the adjustment, the hazard ratio decreased in significance (P = .13), although sample size also decreased. Conclusion: Grade 2 to 4 femoral neck stress fractures require longer RTR time than do grade 1 injuries. Patients with lower BMI tend to require a longer RTR time.