Quantifying Strain on Posterior Shoulder Tissues During 5 Simulated Clinical Tests: a Cadaver Study

Study Design: Controlled laboratory study using a repeated-measures approach.

Objective: To quantify the amount of strain on cadaver posterior shoulder tissues during simulated clinical tests across different tissue conditions.

Background: Several clinical tests are used to quantify posterior glenohumeral joint (GHJ) tissue tightness; however, the ability of these tests to directly assess the flexibility or tightness of the posterior capsule has not been evaluated.

Methods: The middle and lower regions of the posterior shoulder tissues were instrumented with strain gauges on 8 cadaver shoulder specimens. Strain was quantified on the posterior shoulder muscles, on the native posterior GHJ capsule (baseline condition), and on the posterior GHJ capsule after it was experimentally contracted using thermal energy. Five simulated clinical tests were compared across each of the 3 conditions: humerus cross-body adduction, and GHJ internal rotation with the humerus positioned in 4 combinations of plane and elevation angle. Repeated-measures analyses of variance were used to compare strain measured during the 5 simulated clinical tests across the 3 conditions, and to evaluate the change in strain after contracting the posterior capsule.

Results: There was a statistically significant interaction between tests and conditions for the middle region of the posterior shoulder. In the experimentally contracted condition, strain was greater when GHJ internal rotation was added to humerus flexion than when GHJ internal rotation was added to humerus abduction. There was a statistically significant main effect of tests at the lower region of the posterior shoulder, with internal rotation in abduction and internal rotation in the GHJ resting position demonstrating greater strain than cross-body adduction. The percent change in strain between the baseline and contracted capsule conditions did not reach statistical significance at either region.

Conclusion: Strain on an experimentally contracted posterior GHJ capsule is highest when tested with a combination of GHJ internal rotation and humerus flexion.