The shoulder complex is made from several bones and joints, held together with several groups of muscles. The joints include:
- Glenohumeral joint
- Acromioclavicular joint
- Sternoclavicular joint
The shoulder blade (scapular) sits on the thorax as a floating bone and at it’s far end provides the socket for the shoulder (glenohumeral joint) as well as a projection of bone known as the acromion above this joint. The acromion articulates with the end of the collar bone (clavicle) to form the acromioclavicular joint.
Full shoulder movement can only be achieved due to several areas of movement being available. The majority of movement occurs in the shoulder joint helped with the rotator cuff muscles. This joint provides almost all of the rotation and approximately two thirds of the available movement when we lift our arm above head height.
The shoulder blade is able to provide almost one third of the movement when we raise our arm by tilting and rotating on the thorax. The small remaining movement is achieved by the acromioclavicular joint and the sternoclavicular joint allowing the collar bone to rotate.
Most of this movement occurs in the last few degrees of shoulder movement so tasks that require reaching and large ranges of motion are most often affected by acromioclavicular joint pathology.
The second important function of the acromioclavicular joint is to provide an initial, strong support for weight to be transferred from the upper limb to the thorax during tasks such as pushing and when bodyweight is being supported by the arms.
Thirdly, the acromioclavicular joint is part of a wider structure known as the coracoacromial arch which provides a roof above the shoulder joint. This ensures the shoulder joint cannot dislocate upwards but also allows room and a protective space for the tendons of the rotator cuff to pass through to attach to the upper arm bone.
Traumatic acromioclavicular joint pain
The acromioclavicular joint is held stable with many strong ligaments and in some cases these ligaments can be injured. To injure the acromioclavicular joint a strong force needs to be applied to the top of the shoulder so common mechanisms include shoulder tackling in contact sports or direct falls on to the point of the shoulder.
Ongoing pain can sometimes result due to the remaining instability to the joint as the ligaments will not stabilise fully as they heal.
Acromioclavicular joint osteoarthritis
Due to the acromioclavicular joint’s role in transferring the load from the upper limb to the thorax, it is particularly susceptible to developing degenerative changes known as osteoarthritis. Although there may be a genetic component to this, individuals who place repeated heavy loads through the shoulder due to sporting or occupational reasons have a higher incidence of this problem.
Activity-related acromioclavicular joint pain
In some cases, the acromioclavicular joint may become painful with no obvious cause found on imaging and clinicians will diagnose the symptoms based on a clinical examination. Often these bouts of pain may have coincided with a period of unaccustomed activity with the shoulder overhead.
Other causes of acromioclavicular joint pain
Rarely, the acromioclavicular joint may become inflamed and painful as a result of an immune system disorder. Some disorders which can cause acromioclavicular joint pain include:
- Rheumatoid arthritis
- Psoriatic arthritis
Under the vast majority of circumstances acromioclavicular joint pain can be treated with excellent results and allow the patient to function fully following treatment.
As the acromioclavicular joint has no muscles which attach directly, physiotherapy can be of limited benefit under certain circumstances. There can be some benefit to strengthening the muscles which move the scapular and following trauma to grade appropriate range of motion and strengthening exercises following trauma or surgery.
For flares of osteoarthritis corticosteroid injections may be suggested by clinicians and often have excellent results. Under most circumstances the injections will work optimally if administered under ultrasound guidance to ensure correct needle placement.
In higher grade acromioclavicular joint injuries which are unstable, stabilisation surgery may be considered. Stabilisation surgery involves placing a synthetic, replacement ligament structure looped around the end of the collar bone and bridged across the acromioclavicular joint. When tightened, this structure can replace the injured ligaments and stabilise the joint.
For cases of severe acromioclavicular joint osteoarthritis or when there is considerable enlargement of the joint which have caused congestion at the top of the shoulder joint surgery may also be considered. A keyhole (arthroscopic) technique is able to resect bony spurs (osteophytes) if these are invading the subacromial space and causing an impingement syndrome. Resecting a portion of the end of the collar bone is also considered. Following surgery, the gap is then filled with scar tissue to allow normal function but significant reduction of pain.