Kinematics of the rotator cuff

The bony anatomy of the shoulder girdle functions to provide the foundation for the kinematics of the rotator cuff. The complexity of the scapula provides the shoulder girdle muscle origins to direct in-line vectors to maximize pull in physiologic directions with their insertion on the humerus. The orientation and shallow depth of the glenoid, with the corresponding large articular surface area of the humerus, allow a range of motion that is unlike any other joint or joint complex. This motion is based on multiple articulations that occur and a nonlinear coupling between the glenohumeral and scapulothoracic joints. Forces at the glenohumeral joint are dependent on multifactorial influences such as condition of the muscle, size of the muscle, orientation of the muscle, position of the arm, and any pathology in the muscle-tendon unit. These forces produce three broad areas of function of the rotator cuff: Rotation of the humerus about the scapula, compression of the humeral head into the glenoid, and providing muscle balance to the glenohumeral joint. Tears in the rotator cuff tendon are often symptomatic but can also affect the biomechanics of the shoulder, leading to abnormal kinematics and worsening symptoms. These tears can occur due to trauma or by degenerative etiologies. Overall, this joint complex relies on unique anatomy and intricate coupled physiology to produce a range of motion that is unmatched throughout the body.