Osteoarthritis is a disease of hyaline cartilage, the gristle that caps the ends of long bones in joints. Cartilage is composed of cells, collagen, proteoglycans, and water. Aside from the cells, the major component of cartilage is collagen.

Collagen is oriented horizontally at the surface and vertically deep where it anchors to bone.

Cartilage is both pliable and stiff. This unique property is due to the fact that the proteoglycans attract water from the inside of the joint and this increases the stiffness of cartilage.

The cells inside cartilage are called chondrocytes. The chondrocytes are surrounded by what is called the extra cellular matrix. Think of grapes inside a jello mold.

Chondrocytes require a certain amount of load bearing in order to remodel and maintain a normal environment for cartilage. There is an intimate relationship between cartilage and the underlying bone and load bearing is also responsible for normal bone metabolism to occur.

The ability of cartilage to withstand impact and loading is due to its ability to rapidly exchange water with the surrounding synovial (joint) fluid.

That being said, there are several factors that can cause cartilage damage.
For example, immobilization reduces the synthesis of proteoglycans and this leads to cartilage softening. Too much loading or too much "shearing" of cartilage can damage chondrocytes.

The ability of chondrocytes to repair injury is poor. Because cartilage has no blood vessels, the ability to start an inflammatory response that leads to healing is not present. Damage to cartilage leads to the death of chondrocytes in the immediate area. These chondrocytes are not replaced.

The development of osteoarthritis is a complicated process involving genetic factors, abnormal loading, and inability to repair damage to cartilage.

There have been various attempts to stimulate cartilage repair. For example, the theory behind microfracture surgery is that by supplying mesenchymal stem cells through drilling into the bone, a type of inflammatory process with tissue repair might be possible. Unfortunately, the type of cartilage that is generated is fibrocartilage, not the stronger hyaline cartilage that is seen in a normal joint.

Mosaicplasty is a procedure where cartilage plugs are placed in an area where there is cartilage damage. While decent results are seen, this technique is reserved for small areas of cartilage damage. It does not appear to be a suitable technique to treat osteoarthritis.

Autologous chondrocytes implantation is a another type of procedure where cartilage cells are harvested from a non-weight bearing portion of the joint, digested and cultured in the laboratory, and then re-implanted in the host. Results have been mixed.

Techniques involving the use of autologous mesenchymal stem cells which are harvested from the bone marrow and fat of the host and introduced into the joint after irritation of the joint, in order to produce inflammation, look promising in preliminary studies.

However, a number of factors need to be considered. The biomechanics of the joint, the age of the patient, and the presence of aggravating factors such as excessive weight indicate a poor prognosis with this procedure.