Distal Femoral Interlocking Screw Length Can Be Estimated Using “Anatomic Zones” During Femoral Intramedullary Nailing

Purpose: The aim of this study is to evaluate whether “anatomic zones” along the distal femur can be used to estimate distal interlocking screw length during femoral intramedullary nailing. Methods: A retrospective “anatomical control” cohort was used establish anatomic zones along the length of the distal femur and estimated screw length within each zone. Estimated screw lengths for each zone were based on mean cortex-to-cortex length at each zone as measured on computed tomography (CT). A prospective cohort was enrolled to evaluate agreement between these estimated screw lengths and depth-gauge measurements for each zone. Agreement was evaluated using mean differences between estimated screw lengths and depth-gauge measurements, an intraclass correlation coefficient (ICC), and Bland-Altman analysis. Results: The retrospective cohort of 56 patients was used to establish eight anatomic zones. Screw length estimates ranged between 42.1mm in the most proximal zone to 80.2mm in the most distal zone. The prospective cohort included 74 patients (126 screws), was 63.51% male (n=47), with mean age 44.2±24.7 years, mean height 173.2±12.7cm, and mean BMI 28.5±10kg/m ² . The mean absolute value difference between zone-estimated screw lengths and depth-gauge measurements was 3.9mm±2.9mm, ranging 2.7mm-5.6mm depending on zone. The ICC was 0.94 (95%CI [0.91,0.96], p<0.01). Bland-Altman analysis revealed a bias of -0.86mm with limits of agreement at +8.6mm and -10.3mm. Conclusion: Anatomic zone-based screw length estimates derived from CT data show strong agreement with depth-gauge measurements and are a potential strategy to reduce operative time and errors in screw length selection.