Dynamic stabilizing function of the deltoid muscle in shoulders with anterior instability

Background: The stabilizing role of the deltoid muscle has not been extensively studied. Purpose: To study the contribution of the deltoid muscle to anterior stability of the shoulder. Study Design: Controlled laboratory study. Methods: We used nine fresh cadaveric shoulders with the arm at 90· of abduction and 90· of external rotation. The position of the humeral head was monitored by an electromagnetic tracking device with 0 and 1.5 kg of anterior translation force; with 0, 1, 3, and 5 kg of force applied to each of the anterior, middle, and posterior portions of the deltoid muscle; and with the capsule intact, vented, and with a simulated Bankart lesion. Results: With the capsule intact, anterior displacement was significantly reduced by application of load to the middle deltoid muscle. After the capsule was vented, load application to the anterior, middle, or posterior deltoid muscle significantly reduced anterior displacement. With a simulated Bankart lesion, effects of muscle loading were most apparent: anterior displacement was significantly reduced with loading of each muscle portion. Conclusion: The deltoid muscle is an anterior stabilizer of the glenohumeral joint with the arm in abduction and external rotation. Clinical Relevance: The stabilizing function of the deltoid muscle takes on more importance as the shoulder becomes unstable.