The effect of negative intraarticular pressure and rotator cuff force on Glenohumeral translation during simulated active elevation

Objective: The objective of this study was to determine the effect of negative pressure in the intraarticular space and subacromial bursa, as well as rotator cuff force, on glenohumeral translation during active elevation. Design: Glenohumeral translation during elevation in the scapular plane was measured under greater than, less than, and equal to physiologic rotator cuff muscle force, as well as with and without the subacromial bursa and intraarticular space vented. Background: Negative intraarticular pressure has been shown to help stabilize the glenohumeral joint in passive motion, although the effect on translation during active motion has not been investigated. Methods: Eight cadaverous human shoulder specimens were tested in a dynamic shoulder simulator. Forces in the muscles of the rotator as well as the middle deltoid muscle were simulated using servohydraulic cylinders. Joint motion was measured using an ultrasonic motion analysis system. Results: Superior translation of the humerus increased 1.2 mm (SD, 0.4) upon venting of the bursa, and 2.1 mm (SD, 1.7) upon venting of the joint capsule at 25 degrees of glenohumeral elevation in the scapular plane. At 90 degrees elevation, venting the bursa did not change superior translation but increased anterior translation 1.04 mm (SD, 1.0). Venting of the joint capsule increased superior translation by 2.8 mm (SD, 2.5). Decreasing rotator cuff force increased superior translation, while reducing it did not. Conclusion: The muscles of the rotator cuff and the negative pressure of the intraarticular space and the subacromial bursa stabilize the glenohumeral joint since they restrict translation in the superior and anterior directions.