ANATOMY

Understanding a few basic points about the anatomy of this nerve is key to making the diagnosis of ulnar neuropathy, or one of its mimics. The ulnar nerve is derived from the spinal nerve roots C8 and T1. These fibres pass through the lower trunk and medial cord of the brachial plexus. The ulnar nerve itself arises from the plexus in the proximal axilla, then lies on the medial aspect of the upper arm (fig 1). The anatomy of the nerve at the elbow is particularly important. Here the nerve lies in the bony ulnar (condylar or retroepicondylar) groove behind the medial epicondyle of the distal humerus (fig 2). As it emerges from this groove, it passes under the aponeurotic arch of the flexor carpi ulnaris muscle (also called the humeroulnar arcade). This is formed from the attachment of the muscle to the medial epicondyle and the olecranon (fig 2). Its edge usually lies about 1 cm distal to a line joining those two points.