Rotator cuff tear infraspinatus, teres minor, and subscapularis, may also be injured. The amount of stress needed to tear a rotator cuff tendon acutely will depend on the underlying condition of the tendon prior to the stress. In the case of a healthy tendon, the stress needed will be high, such as a fall on the outstretched arm. This stress may occur coincidentally with other injuries such as a dislocation of the shoulder, or separation of the acromioclavicular joint. In the case of a tendon with pre-existing degeneration, the force may be surprisingly modest, such as a sudden lift, particularly with the arm
Rotator cuff arm, moving it outward and away from the trunk, the rotator cuff compresses the glenohumeral joint, an action known as concavity compression, in order to allow the large deltoid muscle to further elevate the arm. In other words, without the rotator cuff, the humeral head would ride up partially out of the glenoid fossa, lessening the efficiency of the deltoid muscle. The anterior and posterior directions of the glenoid fossa are more susceptible to shear force perturbations as the glenoid fossa is not as deep relative to the superior and inferior directions. The rotator cuff's contributions to concavity compression and