Joint lubricating fluid plays key role in osteoarthritic pain

A team at the University of Cambridge has shown how, in osteoarthritis patients, the viscous lubricant that ordinarily allows our joints to move smoothly triggers a pain response from nerve cells similar to that caused by chilli peppers.

Joint lubricating fluid plays key role in osteoarthritic pain. Photo: Jamelah e.

Osteoarthritis is the most common form of arthritis. It causes joint pain and stiffness, and in some people swelling and tenderness of the joints. The condition affects an individual’s quality of life and costs millions to the global economy, both directly in terms of healthcare costs and indirectly due to impact on the individual’s working life.

Osteoarthritis tends to occur later in life and has been largely considered as a degenerative disorder in which pain is produced by damage and wear and tear to bone and cartilage. However, in recent years it has become clear that osteoarthritis is not restricted to cartilage damage, but is a failure of the entire joint, with inflammation – the body’s response to stress and injury – being a major contributor to the pain experienced by patients.

A recent collaboration between the two pharmaceutical companies Pfizer and Eli Lilly has found that their anti-inflammatory drug, tanezumab, produced pain relief for osteoarthritic patients in a phase 3 clinical trial.

When inflammation occurs during osteoarthritis, the body produces an increased number of cells within and around the joint. These cells release inflammatory substances into the synovial fluid, the lubricant that allows joints to move smoothly. During osteoarthritis, synovial fluid becomes less viscous and these inflammatory substances come into direct contact with sensory nerve cells in the joint, producing the sensation of pain.

Researchers at the University of Cambridge and Addenbrooke’s Hospital, part of Cambridge University Hospitals, examined whether synovial fluid produced during osteoarthritis is capable of directly exciting sensory nerves supplying knee joints – those nerves responsible for transmitting pain signals.

The researchers obtained synovial fluid from consenting osteoarthritis patients at Addenbrooke’s Hospital and from post-mortem donors with no known joint disease. They then incubated knee sensory nerves isolated from mice in either healthy or osteoarthritis synovial fluid and recorded the activity of these nerves.

The team found that when incubated with osteoarthritic synovial fluid, the knee nerves were more excitable. The nerves also showed an increase in the function of TRPV1, a molecule that detects the hotness of chilli peppers (TRPV1 is also activated by heat, which is why chillis tastes hot). Although the presence of inflammatory chemicals in osteoarthritis synovial fluid has been known since 1959, this is the first evidence that synovial fluid can directly excite sensory nerves and hence is an important contributor to an individual’s experience of pain.

“In the future, this set up can be used to identify the specific components of synovial fluid that cause pain and then to test if and how a drug will be useful in arthritic pain. Since synovial fluid is regularly collected from arthritic patients as part of their treatment regime, our technique can be easily set up in laboratories throughout the world to understand and help to identify a cure for arthritic pain.” (University of Cambridge)

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