An acromioclavicular joint separation, or AC separation or dislocation is a frequent injury among physically active people that play contact sports including AFL, soccer, rugby and martial arts.
An AC joint injury (separation or dislocation) should not be confused with a shoulder dislocation or rotator cuff tear. It involves an injury to the area between the collar bone and the front part of the shoulder blade called the acromion.
In this guide, you will learn everything you to need to know about how to treat an AC joint injury and how to recover from it.
What is the acromioclavicular joint?
The acromioclavicular joint is at the front of the shoulder. The joint is formed between the outer aspect of the collar bone (clavicle) and the scapular (shoulder blade).
A projection of this scapula bone is called the acromion and this is what articulates with the clavicle to form the acromioclavicular joint. There is one at the front of each shoulder.
The acromioclavicular joint is an inherently weak joint. It is a synovial joint (lubricated with joint fluid). It has a small disc within the gap between these bones that is surrounded by fluid. Surrounding this joint is capsule and ligaments that help to stabilise this joint.
Unfortunately, these structures are thin and can be easily damaged by direct injury to the area.
Thick ligaments help to stabilise the collarbone and offer some stability to the nearby acromioclavicular joint. These ligaments attach from the bottom of the outer aspect of the clavicle to another projection of bone from the scapula called the coracoid process.
These ligaments are called the coraco-clavicular ligaments and can also be damaged as a result of trauma to the shoulder region.
This joint compresses when the arm is brought across the chest or during overhead activities and over time can cause damage to the disc and result in pain around your shoulder.
What causes an AC joint sprain?
Injuries to the acromioclavicular joint can be either acute (new) or chronic (wear and tear).
Acute injuries to the AC joint are often as a result of direct injury to the front and top of your shoulder. Sports that involve contact aspects like AFL, rugby, basketball, soccer and martial arts are common causes of this injury.
The weakness of this joint also doesn’t help in providing resistance against a significant force.
Injury to the acromioclavicular joint can also occur by falling onto your out stretched arm. The force from such a fall can transmit up into your shoulder and result in damage to the tissues around this weak joint (joint capsule and shoulder ligaments) and cause either an AC joint sprain or AC separation / dislocation.
Chronic injuries to the acromioclavicular joint are common. Repetitive overhead or cross body maneuvers over many years can cause wear and tear of the joint and its contents resulting in injury and pain from this joint.
This kind of damage is called acromioclavicular joint arthritis. It tends to occur more commonly on your dominant upper limb side and can be accelerated with carrying heavy objects above shoulder height.
With an AC joint injury, you may experience:
- General shoulder pain and swelling
- Swelling and tenderness over the AC joint
- Loss of shoulder strength
- A visible bump above the shoulder
- Pain when lying on the involved side
- Loss of shoulder motion
- A popping sound or catching sensation with movement of the shoulder
- Discomfort with daily activities that stress the AC joint, like lifting objects overhead, reaching across your body, or carrying heavy objects at your side
How is an AC separation diagnosed?
Injury to your acromioclavicular joint is firstly diagnosed from taking a careful history of your shoulder pain which typically centres around pain at the top and front aspects of your shoulder.
Pain is often increased with movement of the arm especially with above shoulder level movement and when the arm is brought across your chest.
When examining your shoulder, your doctor will touch his joint and confirm that it is the source of your symptoms. Compressing the joint using stress testing maneuvers also guides the diagnosis.
Plain radiographs (X-ray) targeted at your AC joint confirms the diagnosis of AC joint injury or degeneration. Common findings include reduction in joint space (the gap between the acromion and clavicle), spurs or osteophytes (extra bone growth that occurs with arthritis), bone loss (osteolysis) or mis-alignment of the joint which indicates an AC separation or dislocation.
Another adjunct involves imaging the opposite shoulder to compare the changes and differences between both sides.
If further clarification about the AC joint is required, performing a CT scan or MRI scan can be useful. An MRI scan is particularly useful in highlighting problems with this joint – changes include increase signal which highlights fluid, swelling and oedema from joint disease.
The MRI can also inspect the shoulder ligaments that help stabilise the AC joint and might be injured.
AC Injuries are classified in three grades ranging from mild dislocation to a complete separation:
Grade I – Sprain
In this type of injury the AC joint is damaged and sprained but the ligaments that stabilise the joint are intact. There is pain from the joint but this resolves within a few weeks. There is no obvious deformity when looking at your shoulder and the X-ray will reveal a normal looking joint.
Grade II – Partial dislocation
In this grade of injury, the joint is partially damaged and dislocated. This is called a subluxation which means the joint is partially dislocated. Once again there is pain, which takes a few weeks to resolve.
This injury might cause minor external deformity – you might notice a lump over your shoulder. Imaging will reveal that joint is no longer lined up and that the end of the collar bone (clavicle) is sitting higher than the acromion.
The amount, however, should be minor and there will still be some contact between both bones which confirms a subluxation (as opposed to a dislocation).
Grade III – Complete dislocation
This is a more significant injury to the AC joint with complete dislocation between the end of the collar bone and the acromion. The ligaments stabilizing the collar bone are completely torn and as a result the collar bone lifts higher than the acromion and may include clavicle pain.
The pain is more severe in this injury, and there will be an obvious deformity over the top of the shoulder. The clavicle can also puncture through the muscles above the shoulder and tent the skin.
Very rarely, the clavicle can dislocate and move downwards and get stuck under the coracoid bone (projection of bone from the shoulder blade) – this type of injury is very uncommon.
Shoulder separation vs. shoulder dislocation
Medical terminology can be confusing and these two terms are no exception. Shoulder separation and shoulder dislocation represent two completely different entities.
Shoulder separation indicates a problem of the acromio-clavicular joint (AC joint). This joint is above the shoulder joint itself and normally an injury to this area is isolated and the shoulder joint itself is not affected.
There is visible deformity above the shoulder on observation and pain is directly over this palpable joint. Sometimes with shoulder movement, there is more pain from the AC joint and often the end of the collar bone will obviously move and be unstable.
There should be no deep pain (no shoulder joint pain) and the shoulder should move around freely. Injury to this area is commonly due to direct trauma to the front and top of the shoulder.
Shoulder dislocation is a condition where the injury affects the shoulder joint itself. The ball and socket which is made up of the humerus (upper arm bone) and glenoid (socket of the shoulder blade) dislocates in this type of injury.
Injury can force the shoulder to dislocate either forwards (anterior) or backwards (posterior). Anterior dislocations and anterior shoulder pain are far more common and result in trauma to the shoulder when the arm is above shoulder level and externally rotated.
This is particularly common in the overhead athlete. A posterior dislocated shoulder are uncommon and tend to occur with the arm across the body and with internal rotation.
Once a dislocated shoulder is sustained, it is not possible to move the shoulder without significant pain and an obvious deformity occurs.
It is very uncommon to sustain an injury to both the shoulder joint and AC joint at the same time unless very significant force is involved.
Shoulder separation vs. rotator cuff tear
Both of these conditions will cause pain and symptoms from the shoulder region.
With rotator cuff tears you will feel pain from the outer aspect of your shoulder which can radiate up into the neck region or down your upper arm. Along with shoulder pain, you might feel weakness or an inability to move your arm. You may also experience an increase in shoulder pain at night.
The rotator cuff is vital in aiding movement and affording power in the shoulder and with an acute tear (new injury) it will cause shoulder pain and dysfunction to the shoulder itself. There is no visible deformity in this condition and touching the shoulder often doesn’t cause more radiating pain.
In the case of large rotator cuff tear, you might not be able to raise your arm at all (pseudoparalysis). Most acute and traumatic rotator cuff tears will benefit from early rotator cuff surgery to re-attach the tendon to the bone. Rotator cuff surgery recovery time can take up to 6 months for full movement.
With an AC joint injury, shoulder movement and power shouldn’t be affected. Pain from the new injury to the AC joint might affect the shoulder, but once pain is controlled the function of the shoulder should be normal.
Touching the joint itself will cause pain and there will be a deformity noticeable in the higher grades of AC joint injury. Movement of the arm can cause more pain but it should be localised to the AC joint area and not from the shoulder joint.
AC joint separation treatment
All significant injuries to the AC joint warrant medical opinion and a shoulder evaluation. A medical practitioner will examine the joint for its position (and compare with the other side) and its stability.
The doctor or shoulder specialist will also check that the shoulder itself isn’t affected by the injury – to ensure that a dislocated shoulder, rotator cuff tear or shoulder injury hasn’t occurred.
Once an AC joint injury is diagnosed clinically, the doctor or shoulder surgeon might investigate with plain X-rays. It is often useful to get bilateral views (x-rays of both your shoulders) to compare and diagnose the degree of injury that has occurred.
Getting a stress x-ray view of the AC joint can also be very useful in deciding if the joint is stable or unstable – in this technique you will be asked to hold a heavy object in the injured arm and then x-rays are taken whilst the joint is stressed.
These films can be compared with the films taken without stress to check for shoulder instability.
Depending on the degree of injury to the AC joint, your doctor will advise you on the best shoulder treatment. A lower grade AC joint sprain can be treated conservatively and without an arthroscopic shoulder operation.
Pain relief, rest and eventually exercises and physiotherapy can alleviate these lower grade injuries.
If there is a higher-grade injury or significant shoulder instability, your orthopaedic surgeon may recommend surgery to stabilize the AC joint and reduce it back into its normal position.
AC joint injury treatment & surgery
Surgery to reduce and stabilize the AC joint is recommended in high grade injury / dislocations of the AC joint and in instances where nonsurgical measures have failed and there is ongoing pain, dysfunction or instability.
In the new or acute injury setting, the rationale with surgery is reduce the AC joint (bring it back into alignment). Once the joint is back into position, the CC ligaments (coraco-clavicular ligaments) can heal in the proper position and length, and provide long term stability and position back to this joint.
There are numerous surgical techniques to achieve this. Surgery can involve arthroscopic (key hole) or open surgical methods. Options can include a device that connects the clavicle to the coracoid (eg. dog-bone device) or application of hook plate that relies on pushing the collar bone down with a hook under the outer bone (acromion).
Irrespective of the surgical technique or device, the aim is the reduce the joint so that the CC ligaments can heal and afford long term stability of this joint.
If a hook plate is used, this will need to be removed once the ligament is healed – often this is performed after the 4-6 month mark from the initial surgery.
The surgical treatment of chronic AC joint injuries is different to acute AC joint surgical management. Chronic injuries are ones that have been sustained for more than 6-8 weeks. In this setting, the CC ligaments have healed in an elongated position and simply reducing the joint will not afford long term stability to the AC joint.
The surgery is this setting is a ligament reconstruction. The aim is to replace the native CC ligaments with a new ligament to stabilise the AC joint. Options include using hamstring tendons (from the knee) or other tendons from elsewhere in the body.
Synthetic ligaments are also an option. Using a device to secure the AC joint (as is used in the acute surgical setting – CC fixation or a hook plate) is also often employed during this surgery to give this joint supplementary stability in the initial period to allow the ligament reconstruction to heal and afford long term AC joint stability.
If a ligament reconstruction is not performed in the setting of a chronic injury, the surgery will fail eventually as the native CC ligaments are lengthened and is what is needed for long term function and stability of your AC joint.
AC joint sprain and recovery time
First 4 weeks after AC joint surgery
After surgery, your arm will be placed into a sling for the first 4 weeks. During this phase, your physiotherapist will show exercises that prevent your shoulder from getting stiff but allowing the AC joint to heal. It is safe to lift your arm to shoulder level during this phase and work on shoulder blade control.
Between 4-8 weeks after AC joint surgery
During the intermediate phase, you can discard your sling and progress your shoulder movement and commense rotator cuff strengthening exercises. Shoulder blade (scapular) control is also very important in this phase.
Full recovery after AC joint surgery takes 6 to 12 months.
The late phase aims are to restore full shoulder movement, optimize rotator cuff strength and scapular control and enhance shoulder power, strength and endurance.
Overall achievable targets include driving from 6-8 weeks, light lifting from 6 weeks (avoid significant lifting until 3 months at a minimum), breast stroke from 6 weeks, freestyle from 3 months and no contact sports for 6 months.
Can an AC joint injury be avoided?
Avoiding these injuries are difficult given the inherent weakness of this joint. However, limiting contact or direct force to the area can avoid a sprained AC joint or dislocation.