Distal Femoral Interlocking Screw Length Can be Estimated Using "Anatomic Zones" During Femoral Intramedullary Nailing.

OBJECTIVE: Depth-gauge measurements can be time-consuming and prone to technical error. The aim of this study is to evaluate whether "anatomic zones" along the distal femur can be used to help estimate distal interlocking screw length during femoral intramedullary nailing. METHODS: A retrospective "anatomical control" cohort was used to 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 validation cohort was prospectively 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 derived from CT-based measurements ranged between 42.1 mm in the most proximal zone to 80.2 mm in the most distal zone. The validation cohort included 74 patients (126 screws), was 63.51% male (n = 47), with mean age 44.2 ± 24.7 years, mean height 173.2 ± 12.7 cm, and mean BMI 28.5 ± 10 kg/m2. The mean absolute value difference between zone-estimated screw lengths and depth-gauge measurements was 3.9 mm ±2.9 mm, ranging 2.7 mm-5.6 mm depending on zone. The ICC was 0.94 (95% CI [0.91, 0.96], p < 0.01). Bland-Altman analysis revealed a bias of -0.86 mm with limits of agreement at +8.6 mm and -10.3 mm. CONCLUSION: Anatomic zone-based screw length estimates derived from CT data show strong agreement with depth-gauge measurements and are a potential adjunctive strategy to reduce operative time and errors in screw length selection without the need for additional equipment or technology.