The role of the elbow musculature, forearm rotation, and elbow flexion in elbow stability: an in vitro study.

The goal of this study was to define the relative passive contributions of the major muscle groups about the elbow to varus-valgus stability and to determine whether these contributions vary with forearm rotation and elbow flexion. Fourteen cadaveric upper extremities were tested with a custom elbow testing device. The biceps, brachialis, and triceps muscles were loaded to simulate passive tension. The origins and insertions of the remaining muscles that cross the elbow were left intact to assess the contributions of their passive tension to elbow stability. For each specimen, varus-valgus laxity was measured at 30 degrees , 50 degrees , and 70 degrees of elbow flexion with the forearm in full supination, pronation, and neutral rotation, yielding 9 total positions of assessment. Six specimens (series 1) were tested for varus-valgus laxity after the following sequence of conditions: (1) unloaded biceps, brachialis, and triceps; (2) loaded biceps, brachialis, and triceps; (3) release of lateral elbow muscle tension; (4) release of medial elbow muscle tension; and (5) transection of the anterior bundle of the ulnar collateral ligament (UCL). Eight specimens (series 2) were assessed under the same conditions, only with the order of the last 2 conditions reversed for further comparison. Release of the lateral muscles alone increased varus-valgus laxity by a mean of 0.6 degrees to 1.4 degrees , but this was statistically significant only at positions of forearm pronation in series 1 (P < .012) and only at 2 of 9 positions in series 2 (30 degrees of flexion in pronation and 50 degrees of flexion in neutral rotation, P < .049). Release of the medial muscles alone caused a further increase in varus-valgus laxity by a mean of 0.5 degrees to 1.2 degrees , but this was only statistically significant at 30 degrees , 50 degrees , and 70 degrees of flexion in supination (P < .014) and 70 degrees of flexion in pronation (P = .044) in series 1 and only at 30 degrees , 50 degrees , and 70 degrees of flexion in supination in series 2 (P < .046). Release of the anterior bundle of the UCL resulted in a statistically significant increase in elbow varus-valgus laxity at all elbow and forearm positions by a mean of 1.8 degrees to 3.2 degrees (P < .001). Unloading the biceps, triceps, and brachialis caused significant increases in varus-valgus laxity at most elbow testing positions, independent of the position of forearm rotation (P < .046). Thus, the medial elbow musculature and lateral elbow musculature affect total elbow varus-valgus stability to roughly equal magnitudes, and the anterior bundle of the UCL affects stability to over twice the magnitude of either muscle group. The medial elbow musculature mostly affects elbow stability with the arm in supination and the lateral musculature in pronation, where the passive tension in the respective muscles is increased. Furthermore, the medial elbow musculature provided stability to the elbow when the forearm was supinated even with a deficient anterior bundle of the UCL, emphasizing its role as a secondary stabilizer.