Acromioclavicular (AC) Joint Dislocations
Also known as shoulder separation, it is a dislocation of the clavicle from the acromion.
The most common cause of an AC dislocation is a fall directly onto the shoulder. As such, this injury is mostly seen in sportspeople, as well as those who have been in an accident or a fall.
What happens during an AC joint dislocation or separation?
A dislocation can occur when the ligaments around the AC joint (acromioclavicular, coracoacromial and/or coracoclavicular) are stretched or torn, resulting in the collarbone (clavicle) separating from the shoulder blade (scapula).
Common risk factors include:
- Participation in contact sports such as football, hockey, soccer, basketball, or rugby
- Accidents such as motor vehicle crashes
- Gender – males are more likely than females to experience shoulder separation.
- Being prone to falls either because of age, occupation or participation in sports where falls are common (such as cycling or gymnastics)
- Previous shoulder separation – the more this injury happens, the easier and more likely it is to reoccur.
Signs and symptoms of shoulder separation
Typical symptoms of an acromioclavicular separation or joint dislocation include:
- Pain at the top of the shoulder
- Swelling or bruises at the top of the shoulder
- The need to support the affected arm with the opposite hand
- A visible lump – this is the displaced collarbone being pulled down by gravity.
- Discomfort with daily activities that stress the AC joint, such as lifting overhead, reaching across your body, or carrying heavy objects
- Hearing a popping sound when the joint moves
- Numbness, muscle weakness, or limited movement – if these signs of nerve damage are present, seek medical attention immediately.
The shoulder is made up of a complex arrangement of bones, ligaments and other structures that work together to provide an extraordinary range of movement. Unfortunately, these structures are thin and can be easily damaged by direct injury to the area.
Where is the AC joint?
The acromioclavicular (AC) joint sits at the bony point on top of the shoulder and plays a crucial role in allowing a range of arm movements, including reaching overhead, reaching across the body, and using force when pushing or pulling. It also works as a shock absorber between the collarbone and the shoulder blade.
The AC joint is formed at the junction of three bones: the collarbone (clavicle), the shoulder blade (scapula), and the arm bone (humerus). The clavicle and scapula form a socket, and the round head at the top of the humerus sits within that space.
At the highest point of the scapula blade sits the acromion. It’s the bony structure you can feel with your fingers when you press on the top of your shoulder. The acromion is an essential muscle attachment point for the function of the shoulder joint.
The acromioclavicular joint (AC joint or ACJ) is a common site of shoulder injuries, particularly for people involved in a contact sport. An AC dislocation, also known as an AC separation, can sometimes occur after a sharp blow to the shoulder, such as during a sporting clash or a heavy fall on the tip of the shoulder.
How to identify an AC joint separation
Diagnosing an AC joint separation
Diagnosis involves evaluating both the history of the patient’s injury and the injury itself. A physical examination of the shoulder is performed to assess signs of fracture or dislocation. The AC joint is right under the skin, so this examination involves carefully feeling the bones and tissues around the AC joint, as well as comparing the arm’s position to the uninjured one on the other side.
In severe cases, the deformity of the acromion will be a visible indicator of an AC joint dislocation. For more moderate cases, other tests may be required, including:
- Testing the patient’s range of motion to isolate specific areas of pain or weakness
- X-rays to help determine whether the injury is a separation, a fracture or a total dislocation
Types of AC separation / shoulder dislocations
An AC joint dislocation is usually caused by a hard knock to the tip of the shoulder, or from a fall when the patient lands directly on their shoulder tip or outstretched arm. If the force is severe enough, multiple ligaments that surround and stabilise the AC joint are torn, which can completely separate the collarbone and shoulder blade.
The severity of an acromioclavicular joint injury depends on which supporting structures are damaged, as well as the extent of the damage caused. AC separations are classified under three main grades of ascending severity:
Type I. Partial Tear: The AC ligament is partially torn, but the collarbone looks normal.
Type II. Complete Tear: The AC ligament is completely torn, putting the collarbone partially out of alignment (partial separation).
Type III. Complete Separation: The AC and CC ligaments are completely torn, putting the AC joint noticeably out of position (complete separation).
Will I need surgery for my shoulder injury?
What does AC shoulder surgery involve?
Dr James McLean specialises in shoulder, elbow, hand and wrist surgery, and is actively participating in multiple fellowships and research initiatives that make him highly qualified for AC repair surgeries.
The surgical procedure involves using small cuts in the shoulder to gain access to the injury site. Thin instruments specifically designed for arthroscopic surgery are inserted through the cuts, as well as a tiny camera that gives Dr McLean vision of the shoulder from the inside.
Using this vision projected on a screen in the operating theatre to guide him, Dr McLean performs reconstruction of the affected ligaments, using a combination of small plates and a strong synthetic loop fixation to reduce AC space and re-position the coracoclavicular ligaments so they heal as close to the bone as possible.
Dr McLean may also shave protruding bone from the acromion if it was damaged at the time of injury, or if it causes irritation or rubbing within the shoulder AC joint.
After the procedure is complete, Dr McLean carefully removes the instruments, and closes the small incisions.
Recovery after AC separation or dislocation surgery
Recovery times can vary depending on the severity of the injury and type of surgery performed.
Generally, these guidelines apply:
- Keep your surgical dressing in place until reviewed by Dr McLean’s practice nurse in the second post-operative week
- Stitches are usually removed by Dr McLean’s practice nurse in the second post-operative week
- Elbow, wrist and hand exercises are encouraged to minimise stiffness
- A sling can be used but is not strictly necessary
- No lifting more than “a cup of tea” for 6 weeks
- 6-8 weeks of progressive strengthening and motion exercises begin after the removal of the sling
- Full recovery may take up to 4-6 months
Patient resources for shoulder conditions
Dr James McLean is an Orthopaedic surgeon based in South Australia. He practices at several locations around Adelaide, including Ashford, Elizabeth Vale, Bedford Park and Parkside. Check out Dr McLean’s articles on shoulder-related and post-surgery information.