Forearm position-dependent variations in the carpal tunnel safe zone: an ultrasonographic anatomical rationale for a pronation-based injection concept.

OBJECTIVE: To evaluate the effect of forearm rotation on the spatial relationship between the median nerve and the ulnar artery at the carpal tunnel inlet using ultrasonography, and to characterize position-dependent anatomical variations in the transverse interval relevant to the ultrasound-defined safe zone. MATERIALS AND METHODS: This prospective observational study included 120 healthy adult volunteers (60 females, 60 males; age range 18-45 years). Ultrasonographic measurements were obtained at the carpal tunnel inlet in three standardized forearm positions: supination, neutral, and pronation. The cross-sectional area of the median nerve and its distances to adjacent anatomical landmarks, including the ulnar artery, pisiform, and scaphoid tubercle, were measured. Participants were stratified by sex and palmaris longus tendon presence. Statistical comparisons were performed to assess position-dependent changes and subgroup differences. RESULTS: Forearm pronation resulted in a significant increase in the distance between the median nerve and the ulnar artery compared with neutral and supinated positions (p < 0.05), indicating widening of the ultrasonographically defined transverse interval. The cross-sectional area of the median nerve also increased in pronation. No significant differences were observed based on palmaris longus tendon presence. Sex-based differences were identified, with larger absolute measurements in males. Effect size analysis demonstrated large position-dependent effects for median nerve displacement parameters. CONCLUSION: Forearm pronation is associated with widening of the median nerve-ulnar artery interval at the carpal tunnel inlet in healthy adults. These findings describe a position-dependent anatomical variation that may be relevant to future studies of pronation-based injection planning; however, procedural application requires clinical validation.