Poster 185: Flexor Pronator Mass Contribution to Elbow Valgus Stability: A Biomechanical Project

OBJECTIVES: Identify the main muscle in the flexor pronator mass (the pronator teres (PT), flexor carpi radialis (FCR), flexor digitorum superficialis (FDS), and flexor carpi ulnaris (FCU)) that contributes most to valgus stability at a greater range of elbow motion (60-150 degrees) than in previous studies. METHODS: 8 cadaveric elbows in an in vitro biomechanical study were investigated. The specimens were mounted on a custom apparatus from the distal humerus to the fingers. Each muscle group of the flexor pronator mass was relatively loaded based on physiologic cross-sectional area. There were 6 loaded conditions: all loaded, unloaded PT, unloaded FCR, unloaded FDS, unloaded FCU, and all unloaded. A 2 N-m valgus torque was applied at a rate of 1 mm/sec. Valgus angle was measured for each loaded condition, especially between all loaded and one unloaded muscle. This allowed an increase in valgus angle for one unloaded muscle. The higher increase in valgus angle indicated the higher the contribution of that released muscle to valgus stability. This was performed at 60, 90, 120, and 150 degrees of elbow flexion to cover the range at which the UCL tears during the late cocking and early acceleration phases of throwing. The measurements were made in two conditions: first with the UCL intact and second with the UCL resected at the anterior bundle. RESULTS: There was a statistical significance in the mean change in valgus angle at all flexion angles and load conditions when the UCL was intact vs when it was resected (3.76 ± 2.78 SD vs 4.82 ± 2.99 SD degrees). The valgus angle was statistically mostly increased in the “all unloaded” condition when compared to all the other loaded conditions at all flexion angles involving both when the UCL was intact and resected. Unloading one individual flexor pronator muscle did not significantly affect the valgus stability of the elbow. There was a trend towards FDS as being the primary flexor pronator stabilizer when the elbow was taken from an all-loaded condition to one muscle that was unloaded (P value = 0.169). At 150 degrees, the elbow was statistically significantly most stable compared to all the other flexion angles. At 90 degrees, the elbow was most unstable with an increase in valgus angle of 1.2 degrees between intact and resected UCL conditions. At 60 degrees with the UCL resected, there was not a statistical significant difference in valgus angle between all unloaded and unloaded FDS conditions signifying that the FDS contributed most to valgus stability. CONCLUSIONS: Physical examination of the UCL is best performed with the elbow at 90 degrees with the forearm in neutral (most unstable out of the flexion angles we tested). At 150 degrees, the elbow is most stable to valgus stress. When the UCL is injured, the FDS appeared to contribute most to valgus stability at 60 degrees out of the four flexor pronator muscles. There was a trend towards FDS as being the primary flexor pronator stabilizer. Rehabilitation of the flexor-pronator mass should focus on the FDS.