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Acromioclavicular dislocation (more commonly called AC separation) refers to separation of the collarbone (clavicle) from the highest point (acromion) of the shoulder blade (scapula). This injury is often confused with a dislocation of the shoulder (glenohumeral dislocation), which is actually a separation of the bone of the upper arm (humerus) from the socket in the shoulder blade (glenoid cavity of the scapula). The acromioclavicular joint capsule is made up of several groups of ligaments that stabilize the joint and allow motion.
Dislocation of the acromioclavicular joint is usually the result of a downward force applied to the acromion from a fall directly onto the shoulder or on to an outstretched arm. In this type of injury, the clavicle remains in place while the scapula is driven down, resulting in a tear to one or more ligament groups in the acromioclavicular joint capsule. Injuries to the AC joint are graded from I to VI depending on the degree of ligament damage and the resulting separation of the joint. Grades I and II are more commonly called sprains of the AC joint, incomplete dislocation, or subluxation. Grade I injuries typically involve stretched or partially torn AC ligaments and joint capsule. Grade II injuries are caused by more significant forces, and the AC ligament and joint capsule may rupture. Grades III, IV, V, and VI constitute a true dislocation. Grade III injuries involve tearing of all ligaments, resulting in complete AC joint dislocation. Grades IV through VI also involve a complete dislocation, and may additionally include injury to the surrounding muscles (muscle strains or muscle tears). The clavicle can pierce the muscle around the shoulder (trapezius) when dislocated. An AC dislocation disrupts shoulder function, limiting arm motion. Instability of the joint may cause deformity when the lateral end of the clavicle rises higher than the acromion.
Other injuries sustained from a fall or blow directly on the shoulder may include tears in deltoid and trapezius muscles attached to the clavicle; fracture of the acromion, clavicle, or scapula; and disruption of the articular cartilage of the AC joint. Shoulder injury from falling onto an outstretched arm is more likely to result in a humeral neck fracture or glenohumeral joint dislocation, and a heavy object falling on the shoulder is more likely to result in a fracture than dislocation.
Risk: Individuals who participate in contact sports (football, soccer, rugby, hockey), cycling, or who do heavy, one-armed lifting are most at risk for this injury. Men are at greater risk than women for AC dislocations, which occur with a male-to-female ratio of about 5:1 (Prybyla). Incidence and Prevalence: In the general population, AC dislocations are most common during the third decade of life. Among athletes, AC separations occur most often between ages 10 and 20 (Malone). Incomplete injuries to the AC joint are 2 to 3 times more common than complete dislocations. AC dislocations account for approximately 12% of all shoulder dislocations (Malone). |
Source: Medical Disability Advisor