A joint, known as ‘arthron’ in ancient Greek, is a complex structure composed of two bones, referred to as joint partners. These joint partners are encased in cartilage tissue, forming essential components of the joint, such as the humeral head and the glenoid cavity. Ensuring the smooth functioning of this joint, a protective capsule envelops it, preventing the leakage of synovial fluid and enabling frictionless movement between the joint partners. Regular physical activity plays a vital role in maintaining well-lubricated cartilage and ensuring the joint receives necessary nutrients.
In cases where cartilage is inadequately lubricated or subjected to excessive stress, it can develop cracks. As cartilage deteriorates over time, the bones can grind against each other without the cushioning protection it provides, often leading to an inflammatory response within the joint.
In everyday language, terms like joint wear and tear or joint degeneration are frequently used to describe osteoarthritis. Osteoarthritis is characterized by the progressive degeneration of cartilage tissue. This process can be likened to a car’s transmission system. If all the transmission components function seamlessly, gear-shifting proceeds without issues. However, when there is abrasion, such as worn-down cartilage, akin to pieces of cartilage breaking off, it disrupts smooth joint movement.
Osteoarthritis in the shoulder can manifest in various locations, including the acromioclavicular joint, the glenohumeral joint, or the sternoclavicular joint, which connects the sternum and clavicle.
Osteoarthritis is a natural progression that unfolds over years. To illustrate, consider a new tire with substantial tread at the outset. Over time, the tread wears down, becoming thinner and smaller. Ultimately, in the worst-case scenario, the tire is worn to the rims.
The body employs various protective mechanisms to counteract cartilage degeneration, including bone compaction, cyst formation (accumulation of fluid in the bone), and the development of bone outgrowths known as osteophytes. Sclerosis, which begins due to increased pressure on joint partners, causes the bone edges to compact and deform, redistributing the force over a broader area, thereby minimizing pain.
An apt analogy is a fakir walking across a bed of nails. If they stepped on a single nail, they would experience intense pain. However, when walking on a board covered with numerous nails, the weight distribution is even across a larger surface, leading to an automatic reduction in pain sensation.
The presence of osteophytes, or bone outgrowths, serves as a telltale sign of arthrosis. Healthy joints typically exhibit a visible joint space in X-ray images, attributed to cartilage tissue, which remains unseen in X-rays.
As cartilage deteriorates, this joint space diminishes, creating a narrowing gap between the socket and the head. This reduced joint space is a hallmark of osteoarthritis. As the condition progresses, both cartilaginous and bony structures can wear down, resulting in the formation of ‘free’ joint space and promoting further degeneration. The extent of wear can lead to significant shifts in the position of individual bones in the shoulder joint. The humeral head may dislocate posteriorly, causing increased wear in the rear portion of the socket. Furthermore, severe damage to the rotator cuff can disrupt the centration and support of the humeral head within the joint. This leads to an altered, unnatural position of the humeral head, imposing uneven loads on the joint. Over time, the glenoid cavity may experience asymmetric wear. When the rotator cuff can no longer provide stabilization, the humeral head tends to shift upwards towards the acromion, resulting in chronic contact between the humeral head and acromion. If the arm remains in this position for an extended period, the humeral head grinds against the acromion, a phenomenon known as acetabulization (akin to the hip socket), which often leads to significant discomfort for the individual.