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What is osteoarthritis?

Osteoarthritis (OA) is also known as 'degenerative' arthritis and 'wear and tear' arthritis. It is the most common joint disease, and knee osteoarthritis is the most common cause of chronic disability in the elderly.

Primary and secondary arthritis

There are two types of OA: primary and secondary. Primary OA is where no underlying predisposition is present and OA has developed as a wear and tear process, either due to weakened joints, or abnormal stresses on weight-bearing joints, or just as the normal aging process.

In secondary OA, an underlying cause can be identified such as trauma due to occupation or sport, congenital or development abnormalities, or metabolic disorders. The progression of the disease is similar for both primary and secondary causes of osteoarthritis

The healthy joint

A normal, healthy joint allows smooth, free movement, and provides stability during weight bearing. The hyaline cartilage that surrounds all articulating surfaces is an excellent shock absorber, and with lubrication of synovial fluid within the joint capsule, provides an almost frictionless surface for the bone ends to glide over each other.


In the early stages of OA, the cartilage actually thickens as it tries to repair itself in compensation. But the new repair cartilage does not cope as well with mechanical stress as the original hyaline cartilage, and becomes soft and pitted and flakes away.

Deep ulcers form and may reach the bony surface underneath. The bone itself becomes the new articular surface, and becomes worn smooth like ivory (eburnation). Pockets of fluid form in the bone marrow (subchondral cysts), and bony spurs (osteophytes) grow at the margins of the joint which changes it's shape and may restrict movement. Inflammation of the joint space (synovial cavity) and thickening of the joint capsule may also restrict movement.

Risk factors contributing to primary osteoarthritis

  1. Increasing age - is the biggest risk factor for osteoarthritis. In a radiographic survey, 2% of women under 45 had OA, 30% between the ages of 45 to 64 years, and 68% 65yrs and over had evidence of OA. Articular cartilage is highly resistant to wear and tear, but over the course of a lifetime repetitive impact will eventually cause damage. Also, as the body ages, the ability of the cartilage to repair itself is reduced. Osteoarthritis is therefore a common feature of aging.
  2. Obesity - increases the risk of knee OA by 1.5 in men over 36, and doubles the risk for women over 36. Obesity especially plays a role in the more severe cases of knee OA. The risk of severe knee OA is doubled in obese men over 36yrs, and trebled in obese women over 36yrs. Furthermore, obese people who have not yet developed OA, can reduce their risk of knee OA by 50% with a weight loss of only 5kg.

Due to the nature of primary OA, the joints most greatly affected tend to be the weight bearing joints as this is the location of the greatest mechanical stress. This means primary OA is more common in the hip, knee and spine.

Causes of secondary osteoarthritis

  1. Trauma - acute injury to a joint from sport or accident may lead to osteoarthritis in later life. Anterior cruciate and meniscus damage lead to knee OA, and a fracture of the ankle may cause ankle OA. Chronic trauma from repetitive stress on the joint can also be a cause of OA. Occupations such as construction or assembly line work, or any other jobs that require a lot of lifting and bending show an increased risk in developing back and knee osteoarthritis. Sport also appears to be a risk factor. Osteoarthritis affects the ankles of ballet dancers, the fingers of boxers, and the hips and knees of elite athletes. However, excluding major trauma, there is no data to support an association between s

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