Marked Increase in Anterior Ulnar Nerve Displacement at ≥90° of Elbow Flexion in Healthy Children

Iatrogenic ulnar nerve (UN) injury is a serious complication of percutaneous medial pinning for pediatric supracondylar humeral fractures, particularly when deep elbow flexion is required for reduction. However, the flexion angle at which the nerve becomes unstable and patient characteristics predisposing to displacement remain unclear. This study aimed to determine the elbow flexion angle at which UN instability occurs in healthy children and to identify independent risk factors for anterior displacement.

A cross-sectional study of 74 healthy, asymptomatic children (144 elbows) aged 7 to 12 years was conducted. Dynamic ultrasonography was performed at 5 static flexion angles (0°, 30°, 60°, 90°, and 120°), with nerve position classified as normal, subluxated, or dislocated relative to the medial epicondyle. Multivariate logistic regression analysis identified independent predictors of instability (subluxation or dislocation) at deep flexion (≥90°), evaluating age, sex, body mass index, handedness, and generalized joint laxity.

UN instability increased progressively with elbow flexion. At 0° and 30°, the nerve remained stable in >97% of elbows. A marked increase in instability was observed at ≥90°: 60.4% demonstrated instability (50.0% subluxation, 10.4% dislocation) at 90°, rising to 94.4% (54.2% subluxation, 40.3% dislocation) at 120°. Multivariate analysis identified female sex (odds ratio 3.61; 95% confidence interval 1.50-8.67) and generalized joint laxity (odds ratio 2.61; 95% confidence interval, 1.03-6.61) as independent risk factors. Age, body mass index, and handedness were not significantly associated with displacement.

In healthy children, the UN demonstrated a pronounced angle-dependent pattern, with marked anterior displacement at ≥90° of flexion. Female sex and generalized joint laxity were associated with higher odds of sonographic instability. These findings characterize the anatomical behavior of the UN during elbow flexion and provide context for future investigation in surgical populations.

Level III. See Instructions for Authors for a complete description of levels of evidence.