The small bony lump on the top of your shoulder is the acromio clavicular joint (ACJ). It’s where the collar bone (clavicle) meets the top part of the shoulder blade (acromion). Several ligaments hold the joint in place.
Over time, the bones of the joint can wear out, become larger, and develop around the joint. This can then result in a degenerative disease known as osteoarthritis. The diseased bone can then cause damage to the joint ligaments. The cushioning membrane within the joint (the bursa) can also become inflamed and thickened, causing pressure and inhibiting movement of the joint.
People who use their arms for extended periods of time (especially constant overhead lifting) are susceptible to ACJ arthritis. Another cause is an old injury, such as . Any activity that can put pressure on the joint (throwing sports, racquet sports, swimming etc) may eventually cause arthritis, too.
You may notice a weakness in the upper arm and an aching pain on performing overhead activities. There may also be swelling in the joint as it is used. It can disrupt sleep, especially if you turn to lie on the affected shoulder.
If you’re over 40, the symptoms above are good indicators of ACJ osteoarthritis. X-rays will show the degeneration of the bone. An MRI scan may be used to rule out or highlight other problems such as tendon or ligament tears. will clearly show whether you have degeneration of the bone.
If there is osteoarthritis in your acromio clavicular joint, it may be necessary to shave the end of the clavicle – this is called a resection arthroplasty.
During the the thickened cushioning tissue (bursa) is removed, and any badly damaged ligaments will also be removed. The last 1cm of the collar bone will be cut off, and any bone spurs or diseased bone will be trimmed and smoothed out.
After the operation, you will have to wear a shoulder sling for two days, and will be given mobilisation exercises, including stretching, straight away. After a few weeks, you will start strengthening exercises. Six weeks after surgery, you should be able to return to all your normal activities.