A joint (‘arthron’ in ancient Greek) consists of two bones (joint partners). These joint partners are covered with cartilage tissue, forming the joint, e.g., humeral head and glenoid cavity. The joint capsule is a protective sac that surrounds the joint and ensures that no synovial fluid leaks out and that the joint partners can slide smoothly against each other. Regular exercise keeps the cartilage tissue well ‘lubricated’ and provides nutrients to the joint.
If the cartilage is not well lubricated or subjected to too much stress, cracks can develop. At some point, the cartilage is worn away, and the bones rub against each other without protection. This can lead to an inflammatory reaction in the joint.
In common parlance, terms such as joint wear and tear or joint wear and tear are also used for osteoarthritis. Osteoarthritis is a progressive breakdown of cartilage tissue. It can be compared to the transmission in a car. If all the transmission parts interact smoothly with each other, there is no problem shifting gears. However, if there is sand in the transmission or if it is worn down through constant use, in the case of the joint, it would be, for example, abraded pieces of cartilage, then smooth movement can no longer take place.
In the shoulder, osteoarthritis can occur in different places. For example, the acromioclavicular joint, the glenohumeral joint, or the sternoclavicular joint (the joint between the sternum and the clavicle) can be affected.
Osteoarthritis is a normal process that develops over the years. For example, if you look at a new tire, it has a lot of tread at the beginning. Over time it gets thinner and smaller, and eventually, the whole tire is worn down, and in the worst case, you are driving on the rims. The body counteracts cartilage wear with bone compaction (sclerosis), cyst formation (fluid accumulation in the bone), and bone accretion (osteophyte formation). These are protective mechanisms of the body. Sclerosis begins due to increased pressure on the joint partners. The edges of the bone, in particular, become compacted and deformed to compensate for this pressure. The bone tries to distribute the force over a larger area so that it feels less pain.
An extreme example of this would be the fakir walking across the nail board.
If he were to step on only one nail, he would feel intense pain. On the other hand, if he steps on a board full of nails, the weight is evenly distributed over a larger area. The weight is evenly distributed over a larger area, and the pain sensation is automatically reduced.
The osteophytes or bone attachments that have developed are always a
clear sign of arthrosis. In X-rays, a joint space is always visible in healthy joints. This joint space results from the cartilage tissue, which is not visible in the X-ray.
Once the cartilage is worn away, this joint space also disappears. It gets narrower and narrower between the socket and the head, so to speak. This eliminated joint space is a sign of osteoarthritis. As the disease progresses, cartilaginous and bony structures can wear away and wear out. This can create ‘free’ joint space and, in turn, promote wear and tear. Wear can progress to the point where the position of individual bones in space (shoulder) changes significantly. The humeral head may dislocate posteriorly and cause increased wear in the posterior portion of the socket. In addition, if the rotator cuff is severely damaged, the humeral head cannot be adequately centered and supported in the joint. The humeral head no longer moves in its natural position, which forces asymmetric overloads in the joint. The glenoid cavity wears down on one side. If there is no longer any stabilization by the rotator cuff, the humeral head tends to slide upward toward the acromion. The humeral head chronically abuts the acromion. If the arm is moved in this position for a prolonged period, the humeral head rubs into the acromion. This phenomenon is also known as acetabulization (acetabulum = socket in the hip joint). The acromion becomes a ‘socket’, which is usually very painful for the individual.