Radial nerve risk in bicortical implant trajectories for pediatric medial epicondyle fixation: A 3D MRI-based observational study

Distal humeral medial epicondyle avulsion fractures are commonly stabilized with implants directed proximally and anterolaterally. In this cohort study, we evaluated the location of the radial nerve relative to an ideal bicortical implant for pediatric medial epicondyle fractures. We aimed to (1) characterize the location of the radial nerve relative to a simulated ideal bicortical wire or screw trajectory for medial epicondyle fracture fixation and (2) recommend a safe implant trajectory. We identified 40 patients (40 arms; 19 male, 21 female) aged 7 to 16 years with 3-dimensional (3D) upper extremity magnetic resonance images done from September 2018 to 2023. Exclusion criteria were absence of 3D sequences, fused apophyses, and fractures or tumors. Multiplanar reformations were generated from 3D sequences and an ideal distal humeral implant trajectory was simulated. The optimal implant crossed centrally and perpendicular to the medial epicondyle apophysis and passed superior to the olecranon fossa. Outcome variables were (1) angle between implant trajectory to radial nerve; (2) anteroposterior distance from implant trajectory to radial nerve; and (3) “overshoot” distance from lateral humeral cortex implant exit to radial nerve. Descriptive statistics are reported. Of 208 consecutive elbow magnetic resonance imaging, 40 patients (19 male, 21 female) with a median age of 13 years (range: 8–15 years) met inclusion criteria. The angle between an optimal implant trajectory to radial nerve, when measured from the medial epicondylar entry point, was a mean 8.7 degrees ± 3.2 (range: 4.3–20.4 degrees). The mean anteroposterior distance was 0.8 cm ± 0.3 (range: 0.3–1.6 cm). The mean “overshoot” distance was 0.98 cm ± 0.44 (range: 0.32–2.37 cm). In 30 patients, the radial nerve was anterior to our ideal implant trajectory; in 10 the radial nerve was posterior. Given the proximity of the radial nerve (<1 cm from lateral humeral cortex implant exit) and the variability of its position (anterior vs posterior relative to an ideal implant trajectory), ascertaining a universal safe trajectory for bicortical implant placement for fixation of medial epicondyle avulsion fractures is not possible. Level of Evidence: IV.