Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 48

The ulnar nerve is formed from terminal branches of the medial cord of the brachial plexus and contains nerve processes that originate from spinal cord roots located at cervical 8 and thoracic 1 levels. The ulnar nerve descends distally down the arm toward the elbow just anterior to a medially located intermuscular septum (a connective tissue sheath that both divides and provides attachment for the triceps brachial muscles located posteriorly, and brachialis muscle located anteriorly). The ulnar nerve pierces this septum and passes through a fibro‐osseous space behind the medial epicondyle of the humus that is known as the cubital tunnel. Thereafter, the ulnar nerve passes between the two heads of the flexor carpi ulnaris muscle of the forearm before proceeding distally to the medial wrist and hand. The ulnar nerve innervates several medially located forearm muscles and most of the intrinsic muscles of the hand (such as those acting on the little and ring fingers, the medial two lumbricals, interosseous muscles, and two muscles of the thumb including the adductor pollicis).

This anatomical arrangement places the ulnar nerve posterior to the elbow’s axis of motion, so that during elbow flexion, the nerve is required to stretch up to 5 mm longer than its length at rest as well as slide through the cubital tunnel (Cutts, 2007; Fadel et al., 2017). Alterations in the fibro‐osseous space and increase in intraneural pressure are believed to be key to the pathogenesis of cubital tunnel syndrome. Flexion of the elbow changes the cubital tunnel’s shape from ovoid to ellipse, decreases the cross‐sectional area of the space by 55%, and increases intraneural pressure up to 20 times higher than resting pressure (Apfelberg & Larson, 1973; Novak, Lee, Mackinnon, & Lay, 1994). The floor of the tunnel is made up of the elbow joint capsule and a supporting ligament; bones act as walls on either side (the medial epicondyle of the humerus and the olecranon process of the ulna), while the roof is made of elastic connective tissue (a retinaculum). This retinaculum has a variable structure (and may even be missing) with variations in its tightness perhaps also contributing to cubital tunnel syndrome (O’Driscoll, Horii, Carmichael, & Morrey, 1991).