TRANSLATION OF THE HUMERAL HEAD ON THE GLENOID WITH PASSIVE GLENOHUMERAL MOTION

We have demonstrated that certain passive motions of the glenohumeral joint are reproducibly accompanied by translation of the head of the humerus on the glenoid. We investigated the relationship of these translations to the position of the glenohumeral joint and to applied torques and forces in seven isolated glenohumeral joints from fresh cadavera, using a six-degrees-of-freedom position sensor and a six-axis force and torque transducer. Reproducible and significant translation occurred in an anterior direction with glenohumeral flexion and in a posterior direction with extension. We also observed posterior translation with external rotation and anterior translation with cross-body movement. The translation occurring with flexion was obligate in that it could not be prevented by the application of an oppositely directed force of thirty to forty newtons. Operative tightening of the posterior portion of the capsule increased the anterior translation on flexion and cross-body movement and caused it to occur earlier in the arc of motion compared with the intact glenohumeral joint. Operative tightening of the posterior part of the capsule also resulted in significant superior translation with flexion of the glenohumeral joint. Clinical relevance: Awareness that glenohumeral translation is associated with passive motion of the shoulder is clinically important in attempts to maintain or restore normal kinematics of the shoulder. Translations occurring with passive motions represent a departure from pure ball-and-socket mechanics. The obligate translations that we identified are likely to occur during physical examination or physical therapy when the joint is passively moved to the limits of its motion, when part of the capsule becomes tight. We suggest that glenohumeral translation may also be associated with motions in sports, such as the transition between the late cocking and early acceleration phases of pitching a baseball. Posterior labral tears and calcification of the posterior part of the capsule, which are seen in pitchers, may be a result of these translations. Obligate glenohumeral translations are not the result of ligamentous insufficiency or laxity; instead, they result when the capsule is asymmetrically tight. This observation is consistent with the clinical observation that posterior translation is associated with tightness of the anterior portion of the capsule in osteoarthrosis and with excessively tight anterior repairs for glenohumeral instability.