The rotator cuff (often times mispronounced as the rotator cup) consists of four muscles which hold the ball of the shoulder (a.k.a. humeral head) into the socket (a.k.a. glenoid). The four muscles can be remembered by the acronym “SITS.” They stand for Supraspinatus, Infraspinatus, Teres Minor and Subscapularis. Each muscle is responsible for a select motion. They are as follows:
- Supraspinatus- abduction (a.k.a. elevation of the arm)
- Infraspinatus- external rotation (a.k.a. rolling the arm outward)
- Teres Minor- external rotation (a.k.a. rolling the arm outward)
- Subscapularis- internal rotation (a.k.a. rolling though arm inward)
Collectively, these muscles and their tendons work together by surrounding the ball (head of the humerus) and, with tension, help seat the ball into the socket (glenoid).
Since your ball is three times the size of your socket, the rotator cuff requires a lot of help in holding the joint together. There are two other structures which are instrumental in holding the ball into the socket (a.k.a. glenohumeral joint). The first structure is called your glenoid labrum. The glenoid labrum attaches the larger ball to the smaller socket and acts like a suction cup (like the end of a toy dart) in order to hold the two together. In fact, the labrum is of similar makeup as the aforementioned suction cup. It
has a rubbery-like texture. The second structure is the joint capsule. The joint capsule is a stiff fibrous membrane composed of avascular tissue (no blood flow) which envelops the shoulder joint (glenohumeral joint) and assists the rotator cuff and the labrum in holding the ball into the socket. Since the shoulder joint (glenohumeral joint) is classified as a synovial joint (a joint that holds fluid), the joint capsule helps hold the synovium (a viscous fluid) in and around the joint for lubrication. Any one of the four rotator cuff muscles, the labrum or the joint capsule can be acutely injured (traumatic) or chronically injured (happens over time, normal wear and tear). Regardless of the mechanism of injury (MOI), acute or chronic, they are classified as a Grade I (1/3 tear), Grade II (2/3 tear) or a Grade III (full-thickness tear). The treatment, whether conservative (physical therapy) or aggressive (surgery), depends upon the Grade, location and number of structures torn.