Osteoarthritis
Introduction
Osteoarthritis is a condition that results from the loss of cartilage at synovial joints, and is often accompanied by degeneration of the underlying bone.
Radiologically there are:
- osteophytes
- Joint space narrowing
Epidemiology
- most common type of arthritis
- 80% people >60 will have some radiographic features
- Only ¼ of these (20% of total population >65) is symptomatic
- rare to present below the age of 55
- OA of the hip is less common in black Africans and Chinese populations than in Caucasians
- radiologic signs are more common than symptoms
- More commonly affects women (M:F - 1:3)
- The leading cause of disability in those over 55
Aetiology
- there is a genetic component. Most commonly disease with interphalangeal involvement (nodal OA), and primary generalised OA are the inherited types
- Genes that encode for ‘collagen type II’ are thought to be involved. Several have been identified, but it is thought that many genes are involved.
- Previous trauma of a joint increases the risk of having OA in this joint.
- Obesity
- Hypermobility of joints predisposes you to OA
- Osteoporosis REDUCES the risk of OA
- Occupation – e.g. involving manual labour. Miners get OA of the hip, knee and shoulder, cotton workers – in the hand, and farmers – in the hip, sports people – often get OA in the lower limb.
Pathology
- focal destruction of articular cartilage. Inflammation is usually not involved.
- There is massive variability in the bone changes. In some individuals there may be no underlying bone change, but in others there can be massive new bone formation at the chondral margins.
- Bone underneath cartilage is referred to as subchondral bone.
What is cartilage
It is 75% water, and the rest is mainly collagen (type II), mixed in with chondrocytes, fibroblasts and proteoglycans. Cartilage is smooth and can absorb impact. It is constantly damaged in normal use, but this is repaired by chondrocytes. This damage and repair is a balanced process in normal function. Chondrocytes are found throughout the cartilage itself – and along with fibroblasts, are the only cells found in cartilage.
- Cartilage itself has no blood supply, and no nervous supply.Nutrients diffuse into the cartilage from the synovial fluid, and from the nearest blood supply – in the bone marrow.
- So where does the pain come from in OA?! The reasons are pretty complex, but it is probably mainly due to irritation of the bone surface after the cartilage has worn away.
- The precursor cells of chondrocytes also differentiate into osteoblasts.
- The structure of collagen means water molecules are held in place by electrostatic forces between sidechains of the collagen. This gives cartilage its ability to absorb impact. Throughout the day, the collagen becomes dehydrated, and we lose height, due to the intervertebral discs becoming thinner. During the night when we are laid flat, the collagen rehydrates.
In OA, this balance between damage and repair is lost. There is chondrocytes proliferation, and excess cartilage is produced, however, it is oedematous. This leads to focal erosions forming. This leads to rapid destruction of the cartilage, for which, the chondrocytes cannot compensate, and later in the progression of the disease the chondrocytes die, through apoptosis.
Adjacent areas of cartilage try to take over the role of repair, but this is inadequate.
Eventually, the synthesis of new matrix stops completely. The surface becomes fistulated and fibrillated.
- Fibrillations are small ridges in the articular cartilage surface.
Now the bone has lost its protection, it is exposed to extra stressors. The two bones of a joint may rub directly against eachother. There may be microfractures, and cyst formation.
The bone attempts to re-grow itself, but this process can be too great, and it results in the formation of osteophytes.
The disease is progressive. This can be gradual, or stepwise. In some very rare cases, radiological improvement has been seen. This could be the basis for future treatments.
Clinical features
Can affect most joints, and there are lots of different patterns. Disability results when knee and hip joints are affected.
- joint pain and stiffness
- Initially, the pain is intermittent, and often described as an ache. Typically it is provoked by movement of the affected joint, and relieved by resting the joint, but in later disease, there may also be pain at rest. It may be severe enough to wake the patient at night.
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