The AC (Acromio-Clavicular) joint is a small joint at the top of the shoulder formed between the outer end of the collar bone (clavicle) and the acromion (shoulder tip) which is part of the shoulder blade that ‘leans’ forward over the top of the shoulder. There is much less movement in the AC joint compared to the main shoulder joint (glenohumeral joint) with movement only really occuring when the arm is lifted above shoulder height or across the chest. Within the joint there is a pad of cartilage called the meniscus (like in the knee joint) which helps to cushion and move the joint. By the time the patient is in their 40s this cartilage pad is often not functional and has degenerated. The capsule of the joint helps to keep it stable and the surrounding ligaments, particularly the superior acromioclavicular ligament, help to reinforce this stability to gether with the coracoclavicular ligaments.
AC Joint Dislocations
The stability of the AC joint is maintained by the acromioclavicular ligaments (running from the acromion to the clavicle) and the very strong coracoclavicular ligaments (running from the coracoid bone on the front scapula/shoulder blade to the base of the outer part of the clavicle (collar bone). These strong coracoclavicular ligaments provide vertical stability to the AC joint.
AC joint dislocations are much more common in males and are usually the result of direct impact onto the top of the shoulder tip, for example from a rugby tackle or an awkward fall such as falling over bicycle handle bars.
Dislocations to the AC joint were originally classified by Allman and Tossy into types 1-3 but the American surgeon Rockwood expanded the classification to include types 4-6 in 1998.
The types of AC joint are as follows:
Type 1 – Sprain of the AC joint ligaments and no displacement of the joint.
Type 2 – The joint is displaced vertically by 50% secondary to tearing of the AC joint ligaments and capsule. The coracoclavicular ligaments are still intact.
Type 3 – The joint is displaced vertically 100% and there is no contact between the end of the clavicle (collarbone) and the acromion (shoulder tip). The collar bone will appear higher than the acromion on the AP x-ray view. Both the AC joint ligaments and the coracoclavicular ligaments are torn.
Type 4 – As for type 3 but the clavicle (collar bone) is also displaced posteriorly (backwards) into or through the trapezius muscle. This is best seen on the axillary view x-ray.
Type 5 – This is similar to grade 3 but the vertical displacement is much more severe. The ligaments are torn and also the end of the clavicle bone detaches from the deltoid and trapezius muscles.
Type 6 – In this type the end of the clavicle displaces inferiorly and lodges beneath the coracoid process of the scapula (shoulder blade). This is a very rare injury.
Treatment of AC Joint dislocation
The majority of patients with type 1 and 2 injuries will recovery satisfactorily without surgical treatment. Occasionally some patients can suffer persistent pain which requires an injection to settle the discomfort. In a small minority of cases the discomfort persists and the AC joint may need to be excised with an operation.
The management of type 3 injuries is somewhat controversial as there is evidence that patients can do very well either with therapy alone or by having a surgical reconstruction. The majority of patients who suffer a type 3 dislocation will recover well without the need for surgery. However, all patients with non reconstructed injuries will have a cosmetic bump over their shoulder from the prominent end of the clavicle (collar bone) as opposed to only 1 in 4 patients who undergo a reconstruction. There is some evidence that patients will return to sporting activity and work quicker without surgery and just having physiotherapy. However, some patients will continue to be symptomatic from the dislocation with pain and weakness in the shoulder and require a surgical reconstruction at a later stage (See AC Joint Reconstruction operation). Some patients therefore feel that they wish for the joint to be reconstructed at an early stage so that they can start rehabilitating from the surgery sooner rather than later and also try to avoid the cosmetic deformity.
Type 4, 5 and 6 dislocations require acute surgical reconstruction by a specialist shoulder surgeon. There a numerous different surgical techniques used to reconstruct the AC joint. One technique is to use an artificial ligament wrapped around the coracoid bone at the front of the scapula and attach it to the clavicle with a screw. The artificial ligament is extremely strong and fibrous tissue also grows around it to further strengthen the repair. This replicates the function of the torn coracoclavicular ligaments and holds the end of the clavicle down in place at the level of the AC joint thereby reducing the dislocation. (See AC Joint reconstruction operation).