LAUSR

OBJECTIVE Symptomatic distal interlocking screws in retrograde femoral nailing are common due the difficulties of imaging the trapezoidal femur. Screws appearing to have appropriate length on imaging may possibly be prominent, creating symptoms. Screw trajectory may influence the degree of this radiographic error. We hypothesize that external rotation of screw trajectory will increase measurement error of screw length. DESIGN Retrospective. SETTING Urban Level I Tertiary Trauma Center. PARTICIPANTS 283 patients with Computer Tomography (CT) scans of the native knee were retrospectively identified. Simulation was done of the trajectory of an interlock at 20 mm and 40 mm proximal to the nail entry point, which represent common screw positions associated/not associated respectively, with removal. The distance between the radiographic medial cortex and the tip of the transverse screw was calculated (D). The angle (Ψ) between the transverse trajectory and a modified trajectory aimed at the most medial cortex to avoid radiographic measurement error was calculated. Geometric modeling was utilized to calculate the measurement error (D) in the event of accidental external rotation. The angle of the medial slope was also measured (Θ). INTERVENTION Review of CT imaging of normal distal femora. MAIN OUTCOME MEASUREMENTS CT measurements of distal femora. RESULTS The mean distance (D) at 20/40 mm was 4.21 [95%CI 4.02-4.402] and 2.03 mm [95%CI 1.78-2.83], respectively (p < 0.0001). The mean angle (Ψ) between the transverse and modified trajectory at 20/40 mm was 12° [95%CI 11.5-12.5] and 9.60° [95%CI 9-10.2], respectively (p < 0.0001). External rotation by a similar amount nearly triples the measured difference (D). The measured medial slope was significantly increased as screws were placed more proximal (Θ20 mm 46.5 vs Θ40 mm: 48.7 °, p < 0.00001). CONCLUSION The distance between the perceived medial cortex and the tip of the most transverse screw is 4.21 mm and could account for painfully prominent screws. In more proximal screws this distance is decreased. Internal rotation of the screw trajectory 12° can reduce this distance (D), which has implications in nail design. External rotation, amplifies this difference nearly three-fold. Surgeons should avoid external rotation of the aiming arm to prevent prominent screws.