The AC joint is a synovial diarthrodial articulation between the acromion and distal clavicle, separated by a fibrocartilaginous disk reinforced on all sides by AC ligaments. These AC ligaments provide both horizontal and vertical stability to the AC joint. Additionally, the coracoclavicular (CC) ligaments, composed of the conoid ligament medially and trapezoid ligament laterally, provide additionally vertical stability to the AC joint.
Classic history and presentation: A patient presenting with an AC joint separation will typically be a young, active male endorsing pain in this area at the superior aspect of the shoulder, often resulting from a fall or direct blow to the side of the shoulder. There is often a prominence and obvious deformity over the shoulder at the lateral aspect of the clavicle, with tenderness to palpation in this area.
Prevalence: It is most common in individuals in their 20s and is 5 times more likely to occur in men than in women, mostly occurring during sporting activities, especially contact sports.
Risk factors: The main risk factor for this condition is participation in sporting activities, particularly contact sports, either organized or recreationally.
Pathophysiology: AC joint separations are traumatic injuries that can range from mild sprains of the AC ligament to complete dislocations of the joint, resulting from disruption of the AC and/or CC ligaments, accompanied by damage to the surrounding musculature and fascia. The injury results from a direct trauma to the lateral shoulder causing depression of the shoulder girdle, leading to disruption of the ligamentous support of the AC joint, starting usually with the AC capsular ligaments and advancing to the CC ligaments, depending on the degree of trauma. This results in instability / displacement of the AC joint, depending on the structures damaged. If only the AC ligament is disrupted, horizontal AC instability will be appreciated. If both the AC and CC ligaments are disrupted, there will be both horizontal and vertical instability of the AC joint. The deformity may be irreducible if there is interposed fascia, muscle, or cartilage.
Grade / classification system: The Rockwood Classification is a recognized classification system that can be useful in dictating treatment of these injuries.
- Type 1 – AC ligament sprain, CC ligaments intact, radiographs appear normal, there is no displacement at the AC joint.
- Type 2 – AC ligament torn, CC ligaments sprain, AC joint disruption and normal CC distance, reducible AC joint, less than 25% displacement.
- Type 3 – AC ligament torn, CC ligaments torn, radiographs show AC joint disruption and increased CC distance, superior clavicle displacement, reducible AC joint, 25%-100% displacement.
- Type 4 – AC and CC ligaments torn with posterior displacement of clavicle through deltotrapezial fascia, not reducible.
- Type 5 – AC and CC ligament disruption with greater than 100% superior displacement of clavicle, increased CC distance, not reducible.
- Type 6 – AC and CC ligament disruption with inferior clavicular displacement, rare high energy injury, not reducible.
