|The shoulder is one of the most mobile structures in the body; consequently its joint and associated soft tissues are highly vulnerable to injury. There are two main joints in the shoulder: the acromioclavicular joint, which connects the upper part of the shoulder blade (acromion) to the collar bone (clavicle); and the glenohumeral joint, which connects the socket of the shoulder blade (glenoid) to the upper end of the long bone of the arm (humerus). Ligaments (fibrous bands of tissue that connect the bones to bones) function to stabilize the joint. A shoulder sprain results in damage to the ligaments, usually from forces strong enough to stretch and/or tear the ligaments without causing a shoulder fracture or dislocation. The ligaments most frequently affected by a shoulder sprain include the acromioclavicular, coracoclavicular, and coracohumeral ligaments, each one named according to its origin at the scapula and insertion in the clavicle or humerus. |
In addition to ligament damage, tendons (fibrous connective tissues that connect muscles to bones) also may be involved. A sprain is damage to tendons and the muscle(s) they are attached to. A shoulder strain may include damage to the muscles and/or tendons of the rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis) as well as to the biceps or triceps muscles. When the proximal biceps tendon is severely strained, the cartilage of the shoulder socket (glenoid labrum) may become detached, resulting in a superior labrum anterior posterior (SLAP) lesion. On occasion, the muscles that attach the upper arm and shoulder to the chest (pectoral muscles), the back (latissimus dorsi, teres major), or those that stabilize the scapula (trapezius, rhomboids) may also become injured. Muscle strains most commonly occur in the dominant arm and are frequently the result of a forceful eccentric muscle contraction.
Ligament sprains are graded according to the severity of ligament damage and the resulting amount of joint separation: grade I or first-degree sprains involve stretching of the ligament fibers without joint separation. Grade II or second-degree sprains reflect stretching and tearing of some ligament fibers with minimal joint displacement, and grade III or third-degree sprains involve full thickness tears (rupture) of ligament fibers that result in joint dislocation (acromioclavicular dislocation, glenohumeral dislocation). More severe shoulder sprains are less common but may be graded from IV to VI, reflecting increased displacement of the bony anatomy (described in more detail in Dislocation, Acromioclavicular Joint).
Muscle and tendon strains are similarly graded according to the severity of muscle or tendon fiber damage. A grade I strain is a mild strain in which muscles or tendons become stretched, with few torn fibers and no loss of muscle strength. Grade II strains involve a greater number of injured muscle or tendon fibers with noticeable loss of strength. A grade III strain is a severe strain in which the muscle or tendon is ruptured, resulting in complete functional loss of the affected muscle or tendon. The grading of shoulder strains or sprains is not an exact science but is based on clinical assessment and experience related to signs, symptoms, or imaging studies.
Upper arm and shoulder sprains or strains typically occur as the result of direct trauma, falling onto an outstretched arm, overuse, or repeated attempts to lift or lower a heavy weight. However, with chronic degeneration of the muscle and tendon fibers, as occurs with impingement syndrome, older individuals may report a history of gradual onset of shoulder pain without known trauma. Sitting or working with poor posture also may predispose certain individuals to chronic muscle strain. SLAP lesions may be present in up to 20% of individuals with no known history of trauma (Snyder).
Risk: Risk of a sprain or strain of the upper arm and shoulder increases with participation in overhead reaching and throwing activities, repeatedly lifting heavy weights, direct trauma to the shoulder, falling onto an outstretched arm, participation in contact sports, and poor physical condition. Men are affected more frequently than women due to the increased likelihood of occupational physical labor and participation in contact sports.
Incidence and Prevalence: It is estimated that 3.7 million individuals in the US visit their physicians each year for upper arm and shoulder sprains and strains ("Common Shoulder Injuries").
Acromioclavicular joint sprains are the most common athletic shoulder injury, accounting for 40% to 50% of the total cases requiring medical intervention (Seade).
Incidence of rotator cuff tears, especially of the supraspinatus tendon, increases with age; 32% to 37% of individuals over 40 years of age will have a partial thickness rotator cuff tear, although many are asymptomatic (Roy).
Studies of shoulder surgery report an underlying SLAP lesion discovered in conjunction with the primary shoulder disorder in 3.9% to 26% of cases, depending on the study (Kim; Tischer; Hasan).