A joint (from the Ancient Greek *arthron*) consists of two bones, known as the joint partners. The surfaces of these bones that form the joint are covered with cartilage, such as the humeral head and the glenoid. The joint capsule is a protective sac that surrounds the joint, helps retain the synovial fluid, and allows the joint surfaces to glide smoothly against each other. Regular movement helps lubricate the cartilage and provides the joint with essential nutrients.

If the cartilage is not adequately lubricated or is subjected to excessive stress, cracks may develop. Over time, the cartilage can become worn away, leaving the bones to rub directly against each other without the protective layer of cartilage. This can trigger an inflammatory response within the joint.

Osteoarthritis is also commonly referred to as joint wear or joint degeneration. It is characterised by the progressive breakdown of cartilage tissue. This can be compared to the gearbox of a car. When all components of the gearbox interact smoothly, shifting gears is effortless. However, if sand gets into the gearbox or the components become worn through constant use, the mechanism can no longer operate smoothly. In the case of a joint, for example, worn cartilage fragments can interfere with smooth movement.

Osteoarthritis can affect different joints in the shoulder region. It may involve the acromioclavicular joint, the main shoulder joint, or the sternoclavicular joint, which connects the breastbone (sternum) and the clavicle.

Osteoarthritis is a natural process that develops gradually over many years. If you think of a new tyre, for example, it initially has plenty of tread. Over time, the tread gradually becomes thinner and more worn, until eventually the entire tread has been worn away and, in the worst-case scenario, you are driving on the rim.

The body responds to cartilage wear by developing bone sclerosis, cyst formation (fluid-filled cavities within the bone), and bony outgrowths (osteophyte formation). These are protective mechanisms of the body. Increased pressure on the joint surfaces leads to sclerosis. In particular, the edges of the bones become denser and may deform in an attempt to compensate for this pressure. The bone attempts to distribute the pressure over a larger surface area, thereby reducing pain.

An extreme example of this would be a fakir walking across a bed of nails. If he were to step on just one nail, he would experience severe pain. However, when he steps onto a board covered with many nails, his weight is distributed evenly over a larger surface area, automatically reducing the sensation of pain.

The presence of osteophytes or bony outgrowths is always a clear sign of osteoarthritis. On an X-ray, a healthy joint always shows a visible joint space. This joint space is created by the cartilage, which is not visible on X-rays. 

Once the cartilage has worn away, the joint space also becomes narrower and may eventually disappear. The space between the glenoid and humeral head becomes increasingly reduced. This loss of joint space is a sign of osteoarthritis. As the condition progresses, both cartilage and bony structures can become damaged and worn down.

This can lead to the formation of “loose bodies” within the joint, which can further contribute to joint wear. The degeneration can progress to the point where the position of individual bones within the shoulder changes significantly.

The humeral head may dislocate posteriorly, resulting in increased wear of the posterior part of the glenoid. If the rotator cuff is also severely damaged, the humeral head can no longer be properly centred or guided within the joint. It no longer moves in its natural position, which in turn promotes asymmetric loading of the joint. The glenoid then becomes worn predominantly on one side.

If the rotator cuff can no longer provide sufficient stabilization, the humeral head tends to migrate upward towards the acromion. It may remain in chronic contact with the acromion. If the arm is moved in this position over a prolonged period, the humeral head gradually wears against the acromion. This phenomenon is also known as acetabulization (acetabulum = the socket of the hip joint).

The acromion effectively becomes a “joint socket”, which is usually very painful for those affected.

Osteoarthritis primarily manifests as pain. Patients often report pain and a pulling sensation in the shoulder, particularly during movement.

Typical signs of osteoarthritis also include pain at night that occurs at rest.

After prolonged periods of rest, symptoms such as stiffness and “start-up pain” may occur. Pain may develop when the joint is put under stress and tends to become more intense as the load on the joint increases. Sensitivity to changes in weather and cold may also indicate the presence of osteoarthritis.

Swelling, thickening of the joint, inflammatory reactions, and noises such as rubbing or grinding during movement may also occur. Bony outgrowths can lead to deformity of the joint, which may impair mobility. As a result, the joint may gradually lose its normal range of motion.

Shoulder strength may be impaired or reduced depending on the extent to which the rotator cuff is affected.

X-rays are essential for assessing the bony structures. This diagnostic procedure allows the narrowing of the joint space and the extent of bone sclerosis along the joint margins to be assessed. X-rays provide the physician with information about whether cysts or typical bony outgrowths (osteophytes) are present and how the joint surfaces are positioned in relation to each other.

They also show whether the humeral head is centred within the joint, whether it has shifted upward or posteriorly, and whether sufficient space remains between the acromion and the humeral head.

An ultrasound or MRI scan can be performed to assess the condition of the rotator cuff. The findings from the ultrasound or MRI examination influence the choice of treatment. If, for example, the supraspinatus or subscapularis tendons are severely damaged, an anatomical shoulder replacement is not possible. In such cases, a reverse shoulder replacement is used.

A CT scan is performed as part of preoperative planning. This imaging technique provides a precise three-dimensional image of the shoulder joint and clearly shows the position of the glenoid and humeral head. This allows, for example, the alignment of the glenoid and humeral head to be assessed.

In addition, contrast-enhanced imaging can be used to determine whether the rotator cuff is intact. The examination can also provide information about the amount of fatty infiltration within the rotator cuff muscles.

Osteoarthritis cannot be cured. However, if it is diagnosed at an early stage, its progression can be slowed through various measures. The affected joint requires regular movement to support the nutrition of the remaining cartilage. Well-trained muscles can help stabilize and guide the joint more effectively, thereby reducing pain. The choice of treatment always depends on the individual stage of osteoarthritis.

Improved function and pain relief can be achieved through physiotherapy and physical treatments such as cold and heat therapy, as well as ultrasound therapy. In the early stages, muscle strengthening and joint-protection measures can often help relieve the symptoms of osteoarthritis.

In some cases, pain-relieving and anti-inflammatory medications may also be prescribed. The treating physician will determine whether corticosteroid injections into the joint are appropriate. Alternative injection treatments are also available, such as hyaluronic acid injections, which are intended to supplement the joint fluid, or PRP (Platelet-Rich Plasma) therapy, in which autologous blood plasma is injected into the joint.

These measures can help relieve symptoms. However, it is important to note that they do not address the underlying cause. Worn or damaged cartilage cannot regenerate naturally or be restored through medication. Research has made progress in cultivating cartilage cells and, in selected cases, partially replacing damaged cartilage through cartilage transplantation, particularly in the knee.

Joint replacement (arthroplasty) is the surgical treatment for advanced osteoarthritis. During the procedure, the affected joint is replaced with an artificial joint that recreates the surfaces of the glenoid and humeral head.

There are two types of shoulder joint replacements: the anatomical or conventional shoulder prosthesis and the reverse shoulder prosthesis. The choice depends on various factors and the individual circumstances of each patient.