The destruction of the cartilage causes an abnormal reaction in the adjoining bone and an inflammatory reaction in the joint. The term "arthrosis" (or “degenerative arthritis”) is used to describe degenerative joint disease characterized by joint wear. With increasing age a general "wearing down" of the joints can be observed in a large number of individuals. As a consequence of age – but also of stress and strain on the joint - the smooth cartilage tissue layer is increasingly worn off; as a result, the sliding joint function is impaired.

Degenerative arthritis is a disease that develops over many years and even decades.

The symptoms therefore never occur without warning.

Every instance of overloading or improper loading can cause injury to the cartilage on the joint surfaces. As a result, a number of cartilage cells may die; grooves and tears occur. One of the reasons this degenerative process (during which the joint is literally worn down) can take place is that joint cartilage is not replaced in adults. The cartilage soon becomes rough and breaks down into fibers. Small pieces of cartilage detach from the surface and irritate the membranes inside the joint. This result is a reaction of the internal joint membrane culminating in inflammation and joint effusion. This, in turn, results in a disturbance of movement sequences. For the affected individual, these processes are associated with pain.

Basically, all joints of the body can be involved. However, degenerative arthritis occurs most frequently in the joints of the lower extremities, which already have to carry the load of the person's body weight.

Degenerative arthritis causes pain; at first the pain only occurs during exercise (the distances the person can walk become shorter and shorter) but eventually the pain does not stop even at rest. A typical syndrome is the "start-up pain" experienced by arthritis sufferers when they get up in the morning or, later in the day, when they take their first steps after sitting for a long time; the affected joint is "rusted". At the same time, joint mobility decreases. The disadvantage of the inevitable tendency to "protect" the joint is that the lack of exercise results in weakening of the muscles and causes a further deterioration of the nutritional situation of the cartilage.

To stay strong, the muscles need movement and loading. The cartilage needs movement and a certain loading so that it is sufficiently nourished. In patients with chronic polyarthritis (rheumatoid arthritis), numerous joints are affected and destroyed. In this form of arthritis, the inflammatory process takes a different course than in degenerative arthritis; however, the final stage of both diseases is the same.

During the defense reaction mounted against the patient's own body, special substances are produced which lead to inflammatory swelling of the joint mucosa. As a result of this process, the path normally taken by nutrients en route to the cartilage is substantially damaged and a destructive process takes place on the cartilage surface and at the bone-cartilage junction.

The result is increasing deformation and inability to use the joints. The course can be slow, insidious or rapidly progressive.

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How does a knee joint arthrosis develop?

According to conservative estimates, arthritic joint changes can be demonstrated in nearly all individuals at age 75.

Additional causes for the development of an arthrosis are: overweight, deformities of the legs (e.g. knock knees or bowlegs), insufficient exercise or poor circulation in the joints. During each step a person takes, the knee is loaded with four times his or her body weight. This loading increases proportionally in overweight individuals. Other factors which can trigger the development of an arthrosis are overloading and injury of the knee joint during work, recreational activities or sports, e.g. during professional soccer. Stress loading of the joint may also trigger the development of arthrosis.

Any disease affecting the cartilage, bone, joint mucosa or synovial fluid can lead to arthrosis.

In a complex and heavily used joint, such as the knee joint, both the axes of movement and the anatomical shape must be coordinated if the joint is to function without any notable wear for an entire lifetime. Similarly, the muscles and ligaments which both move and support the knee joint must work flawlessly.