Ulnar nerve instability: ulnar nerve injury due to elbow flexion.

The term "ulnar nerve instability" describes the chronic conditions of subluxation and relocation of the ulnar nerve at the elbow with flexion and extension of the elbow, respectively. This condition is more common than generally thought. Recurrent subluxation of the nerve at the elbow results in a tractional and frictional neuritis. The nerve is vulnerable to trauma in its subluxed position, lying superficially on the medial humeral epicondyle. In certain cases of ulnar nerve instability associated with a tight overlying band bridging the heads of origin of the flexor carpi ulnaris, nerve injury can occur with flexion of the elbow. Thus, internal as well as external compressive factors as a cause of ulnar nerve neuropathy must be considered. Described is an elbow flexion test helpful in the diagnosis and prognosis of cases of ulnar nerve instability associated with the tight overlying band.