Acromioclavicular or AC Joint

The AC joint is located at the top of the shoulder where the acromion (part of the shoulder blade or scapula) and collarbone (clavicle) form a joint (Figure 1).

Fig. 1 - Acromioclavicular Joint (AC Joint)

Fig. 2 - Capsule and intra-articular disc

The joint is enclosed in a capsule or sac and there is a disc of tissue (meniscus) which lies between the 2 bones (Fig. 2).

Fig. 3 - Coracoclavicular and acromioclavicular ligaments

The collar bone is held down against the acromion by the capsule and the coracoclavicular ligaments. The acromioclavicular ligament passes from the acromion to the coracoid bone (Fig. 3).

The AC joint is a common site of pathology and of injuries to the shoulder.


1.    Injuries

2.    Overuse and osteolysis

3.    Osteoarthritis (wear and tear)


These occur as either a direct blow e.g. direct injury when rugby tackler takes the impact on the top of the shoulder. Most common mechanism of injury is falling onto the point of the shoulder. These injuries are particularly common in rugby players, cyclist and motor cyclists. Patients will feel pain over the top of the shoulder and often will have an abrasion just behind the AC joint.

There are various grades of injury;

Grade   I    –  minor tear or strain of the capsule

Grade  II   –  complete rupture of the capsule but the coracoclavicular ligaments

are intact. The end of  collar bone rises up a small amount (Fig. 4)

Grade III  –  complete rupture of the capsule and the coracoclavicular ligaments; the end

of the collar bone rises right up (Fig. 5)

Grade IV   –  Grade III with the end of the collar bone tearing through the muscles at the

back of the AC Joint. The end of the collar bone comes to lie just under the skin.

Grade V and VI – more extreme types.

Clinical Features

The patient points to the area of the AC joint as the site of their pain and there is tenderness over the AC joint often with bruising and there may be an abrasion over the top of the shoulder just behind the AC joint or over the shoulder blade. With Type III, IV and V there is an obvious deformity.

One must carefully examine the shoulder itself as there may be an injury to other structures e.g. the rotator cuff tendons. It may be difficult to ascertain this initially as the examination is hampered because of the patient’s pain.

Fig 4 – Grade II ACJ dislocation

Initially the patient may not be able to lift their arm at all. In Grade I, the x-rays are normal whereas in Grade II, the clavicle is slightly elevated (Fig. 4) .

An x-ray of the opposite side may be necessary for comparison to see subtle differences.

Fig 5 – Grade III ACJ dislocation

In Grade III onwards, the diagnosis is very clear on the x-ray (Fig. 5).


For Types I and II, the treatment is non operative with rest, icing and anti-inflammatories. If the pain does not settle within a few weeks then a cortisone injection can be given into the joint.

In Types IV, V and VI, surgery is indicated.

The treatment for Type III is controversial with some surgeons preferring to fix it and others leaving it. In most cases (90 – 95%) with a type III, when treated without an operation, the pain will settle and patients will regain full function of their arm but will have a ‘bump’ on the top of the shoulder. It essentially ends up being a cosmetic deformity.

In a person who works continually with arms above shoulder height, then surgery may be advised.

Recent literature has shown that there is a higher failure rate with early surgery as compared with late reconstruction.

Approximately 5 to 10% of patients with a type III dislocation will continue to be troubled by it. There may be persistent pain over the end of the collar bone or a nagging pain over the muscles of the shoulder blade. In these patients a late reconstruction may be done and the results of this are generally very good.


There are many different operations described and devices available for stabilizing the end of the collar bone.

The operations are aimed at reconstructing the torn coracoclavicular ligaments Fig. 6. This can be done using synthetic material, donor tendon or transfer of the patient’s own ligament. One of the more well recognised procedures is the modified Weaver Dunn operation (Fig. 6 and Fig. 7). In this operation, the coracoacromial ligament is detached form the acromion and is transferred into the cut end of the collar bone or clavicle.

Fig. 6 - Torn coracoclavicular ligamenst

Fig. 7 – Modified Weaver Dunn Procedure


The AC joint may become painful because of overuse that may occur in overhead sports and in weight training.

Clinical Features

The onset is usually gradual and patients feel the pain right over the top of the shoulder and can point exactly to the spot. The pain may radiate into the neck and is exacerbated when the patient lies on the shoulder. Overhead activities, putting hand behind their back and taking the arm across the body will bring on or exaccerabate the pain. The features on examination are classical with point tenderness over the AC joint, pain on cross body adduction, pain on resisted adduction and pain on distraction test.


Fig. 5 - Osteolysis

These may be completely normal. In weight lifters there may be Osteolysis of the end of the clavicle where on x-ray the end of the bone becomes fuzzy, eaten away or disappears

(Fig. 5).


Initially is rest, icing and anti-inflammatories. If this treatment is unsuccessful then a cortisone injection is given. These are very effective (a course of injections may be necessary) and in one study where patients were followed-up 5 years after injection, 80% still had no recurrence of pain. If the injections do not resolve the condition then an arthroscopy and excision of the end of the clavicle may be performed.


Osteoarthritis or ‘wear and tear’ of the clavicle is very common. Many people will have this but not have pain from it. In one study where people without shoulder pain had an MRI of their shoulder; there were wear and tear changes in 60% of people at 40 years of age and changes in 90% of people at 60 years of age.

Clinical Features

Some people may just notice a bump on the top of their shoulder. Some people however do develop pain. The clinical features are as above.


Fig. 6 - Osteoarthritis

X-rays may show narrowing of the joint space between the acromion and clavicle, cysts in the end of the clavicle and/or osteophytes (spurs; extra bone formation) at the edges of the joint (fig. 6).


Is the same as above, where initially anti-inflammatories are tried, then injection and finally arthroscopy and removal of the end of the clavicle.


The surgery is done arthroscopically (‘key hole’) and involves removing a small amount of bone from the end of the clavicle so as to produce a gap of approximately 1 cm between the clavicle and acromion. Thus the two bones no longer rub together causing pain, the meniscus is also removed which is often damaged. This 1 cm gap fills with scar tissue and the bones continue to move together as a joint but without any pain and there is no loss of strength. The results of this procedure are excellent (>95% success rate). It may however take patients up to 6 months to be functioning completely normally.

Leave a Reply