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PH-MS01

Osteoarthritis

How the cartilage, bone and lining of a synovial joint remodel together when repair cannot keep pace with damage, and why that causes use-related pain, stiffness, crepitus and bony swelling.

By the end, you can

  • Describe the structure and function of articular cartilage in a synovial joint.
  • Explain how chondrocytes and enzymes break down cartilage matrix in osteoarthritis.
  • Describe the changes in subchondral bone, osteophytes and synovium that accompany cartilage loss.
  • Relate pain on use, short morning stiffness, crepitus and deformity to the mechanism.
  • Distinguish osteoarthritis from rheumatoid arthritis by mechanism and pattern.
  • Recognise eburnation, osteophytes and subchondral cysts, the Kellgren–Lawrence grades and the key tests of osteoarthritis, and explain how each main treatment acts on its mechanism.

What it is

Osteoarthritis is a disease of the whole synovial joint in which articular cartilage is progressively lost, the underlying bone remodels and forms osteophytes, and the synovium is mildly inflamed. It is the commonest form of arthritis and is not simply passive “wear and tear”: the joint’s cells actively respond to mechanical and biochemical stress.

2 types to tell apart
Primary (idiopathic)
No single cause; related to age, genetics, sex and body weight. Typically knees, hips, hands and spine.
Secondary
Follows a recognisable cause: previous injury, meniscal or ligament tears, malalignment, developmental hip disorders, inflammatory arthritis or crystal disease.

Normal function

A healthy synovial joint

In a synovial joint such as the knee, the bone ends (the and ) are covered by smooth articular cartilage. The encloses the joint and is lined by a thin synovium that makes lubricating synovial fluid. The spread load across the knee.

Articular cartilage is hyaline cartilage: a few cells (chondrocytes) in a large amount of matrix made of type II collagen and proteoglycans such as aggrecan. Proteoglycans hold water under pressure, so cartilage behaves like a firm, slippery cushion that spreads load onto the bone beneath.

Cartilage has no blood vessels or nerves and its cells divide little, so it repairs slowly. Chondrocytes constantly break down and replace the matrix, keeping a balance that loading, within limits, helps maintain.

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