Anterior instability increases superior labral strain in the late cocking phase of throwing

Superior labrum tears are common injuries in the throwing athlete. Sever contributing factors have been identified; however, the effects of instability on the superior labrum have not been studied. The goal of this cadaver study was to determine the effect of instability on superior labral strain in a model of the throwing motion. Six cadaver shoulders were mounted on an MTS machine with a strain gauge placed on the superior labrum. The shoulder was brought into the late cocking phase and strain measured before and after the introduction of a lesion to the inferior glenohumeral ligament to create anterior instability. Labral strain increased linearly in the late cocking phase of throwing in all specimens. Posterosuperior labral strain increased 160% in the unstable shoulders. This was a significant difference (P = .02). Instability increases strain in the posterosuperior labrum in the late cocking phase of throwing. This increased strain may render the labrum more prone to injury in the throwing or overhead athlete.