Ultrasonographic and Electrophysiological Evaluation of Ulnar Nerve Instability and Snapping of the Triceps Medial Head in Healthy Subjects

Objective To evaluate the relationship between ulnar nerve instability and snapping of the triceps medial head during elbow flexion. Designs Twenty-six healthy individuals were recruited. The primary outcome measures were ultrasonographic and electrophysiological parameters of the ulnar nerve. Ulnar nerve instability was classified into three types based on the degree of ulnar nerve movement: no dislocation (Type N), subluxation (Type S), and dislocation (Type D). Results In the elbow 90-degree position, the incidences of Type N, S, and D were 41 (78.8%), 8 (15.4%), and 3 (5.8%) elbows, respectively; in the full flexion position, the incidences of Types N, S, and D were 24 (46.2%), 19 (36.5%), and 9 (17.3%) elbows, respectively. Spearman's correlation coefficients between ulnar nerve instability and snapping of the triceps medial head in the elbow 90-degree and full flexion positions were 0.808 and 0.889 (P < 0.001), respectively. The ulnar sensory response in Type S was of prolonged latency and decreased amplitude compared with that in Type N or D in the elbow full flexion position. Conclusions Ulnar nerve instability increased with elbow flexion and correlated with snapping of the triceps medial head. Ultrasonography of the ulnar nerve is an important tool in ulnar nerve instability assessment.