Quantitative in vivo studies of median nerve sliding in response to wrist, elbow, shoulder and neck movements

Objective. To examine median nerve sliding in response to upper limb movements in vivo. To determine whether the median nerve can be unloaded. Design. Exploratory study in healthy subjects. Background. Impaired sliding may lead to neuropathic symptoms. In vivo results for neural dynamics in normal subjects are essential to understand changes in upper limb disorders. Methods. Ultrasound imaging of the median nerve during 40° wrist extension, 80° shoulder abduction, 90° elbow extension, and 35° contralateral neck side flexion. Frame by frame cross-correlation of image sequences to measure nerve sliding and strain. Results. Nerve excursion in the forearm and upper arm ranged from 0.3 mm for neck side flexion to 10.4 mm for elbow extension. Additional strain in the forearm for wrist extension was 1.1% (SEM, 0.2%), for shoulder abduction 1.0% (SEM, 0.2%), and for neck side flexion 0.1% (SEM, 0.1%). With the limb flexed, sliding was delayed and sometimes the nerve or the nerve fascicles had a wavy appearance. Conclusion. The median nerve is unloaded when the shoulder is adducted or elbow flexed. When the arm is extended (90° shoulder abduction, 60° wrist extension, and elbow straight) the total additional strain in the forearm will be 2.5–3.0%. Even in this position the strain is likely to be below levels that impair blood flow or conduction. Therefore, the median nerve appears well designed to cope with changes in bed length caused by limb movements.