Does Cephalomedullary Nail Design Influence Risk of Varus Collapse in Subtrochanteric Femur Fractures?

BACKGROUND: Cephalomedullary nails (CMNs) are frequently utilized for subtrochanteric femur fractures but a lack of cortical fit in proximal fractures may lead to varus deformity. We hypothesize that as the difference between fracture site canal width and diameter of the nail increases so will the risk of varus deformity. METHODS: A retrospective study of subtrochanteric femur fractures fixed with CMNs at an academic, level one trauma center over a 6.5-year period was performed. Patients were divided into two cohorts: CMN with sliding blade/screw (TFNA) or greater trochanteric entry recon nail (GTE). The primary outcome analyzed was the correlation between nail-canal mismatch with final neck shaft angle. Secondary outcomes analyzed included rates of nonunion and malunion. RESULTS: Seventy-one patients met the inclusion criteria (35 TFNA and 36 GTE). There was not a significant correlation between the nail-canal mismatch with final neck shaft angle for GTE nails (Pearson correlation coefficient r: -0.187, p=0.274). However, when TFNAs were subdivided into 125° and 130° cohorts, there was a stronger negative correlation for 125° (r: -0.678, p=0.001) compared to the 130° (r: -0.515, p=0.041). For 125° TFNAs, linear regression analysis demonstrated that for every 1 mm increase in nail-canal mismatch there was a decrease in final neck shaft angle by 0.757⁰. There was no difference in secondary outcomes. CONCLUSION: GTE nails and TFNAs have similar rates of malunion and nonunion when managing subtrochanteric femur fractures. However, 125° TFNAs may be associated with increased varus neck shaft angles as the nail-canal mismatch increases.