The acromioclavicular joint (AC joint) is the joint at the top of the shoulder and is formed from the acromion, a hook-shaped projection of bone from the shoulder blade (scapula) and the collar bone (clavicle). As there are no muscles or tendons that bridge the AC joint it is made stable by the capsule and the strong ligaments that cross the joint or attach to the nearby coracoid process.
The AC joint has an important role in the movement of the shoulder and it allows the shoulder blade to achieve additional movement, specifically upward rotation on the thorax. It also allows the collar bone to rotate as the arm is elevated. It allows for the shoulder blade to tip and rotate inwards and outwards to allow for changes in the thorax as the shoulder and arm moves.
One more important role is force transference and transmission along the arm and shoulder to the collar bone. This is important in tasks and activities that involve pushing or bearing weight through the arms.
Dislocation (or separation) of the AC joint is a common injury but is not to be confused with shoulder dislocation, which is a dislocation of the glenohumeral joint. Dislocations are usually due to a traumatic event where a strong force is applied to the shoulder, such as in contact sports (rugby, hockey, mountain biking, etc.) or by a direct fall onto the top of the shoulder. This force needs to be strong enough to disrupt the AC joint ligaments by tearing or significantly stretching them, which then allows for the tip of the collar bone to come up and be raised above the acromion.
Acromioclavicular joint dislocations can be graded according to according to the degree of separation between the acromion and the collar bone. This is closely associated with the extent of injury to the AC joint ligaments. The grades will range from I to VI, and the management and recovery time will vary depending on this. Grade IV-VI injuries are rare and will often happen following extremely high impact trauma such as motor vehicle accidents. These higher-grade injuries will require surgical management.
X-rays and ultrasound scans can be useful in grading the injury and excluding any complications, such as a fracture of the collar bone.
Symptoms will depend on the grade of the injury. Pain is often the most common symptom, but with higher grade dislocations there will often be a loss of movement, deformity or lump at the top of the shoulder (as the end of the clavicle is elevated), and instability at the AC joint. Instability or increased movement at the AC joint is common following dislocation, especially with higher grade injuries, but will often improve over time following treatment.
Most grade I-III AC joint dislocations are treated successfully without surgery and will heal fully within 2-3 months. The conservative management of AC joint dislocations might require a short period of immobilisation or reduced activity, along with adequate pain control such as ice and pain medication. Once pain has started to improve over the first few days to weeks, it is important to begin a progressive rehabilitation program with a physiotherapist. This will help to regain movement, strength and confidence in using the joint and allow a full return to function or sport. Returning to contact sport following an AC joint dislocation shoulder should only be considered when full movement in regained and there is no pain on palpating or stress testing the AC joint.
In some cases, patients do not respond to conservative management and if there is continued pain and instability to the AC joint after two to three months of rehabilitation then surgical management may be discussed. This situation is more common in grade III injuries and upwards and is very uncommon in grade I-II dislocations. In specific cases, those with a grade III injury may opt to have surgery prior to attempting conservative management depending on specific occupational or sporting requirements.
A variety of surgical options are available but the exact nature of any surgery will depend on surgical preference, the grade of injury, and any associated injuries or complications. The most common surgical option is a reconstruction on the AC joint ligaments and reattachment to the end of the collar bone. As with any surgery, there may be some complications and these could include persistent instability and pain, or failure of the surgical hardware (screws and wires) due to the mobility and rotation of the collar bone with shoulder movement.