Keep moving to prevent osteoarthritis, says a team of physicists

Exercise could help prevent osteoarthritis according to findings presented by physicists at a US conference in California.
Osteoarthritis is a degenerative bone disease that results from a reduction in cartilage that leads to an increase in friction.
Cartilage is a rubbery material that covers the ends of the bones in the knee joint, which reduces friction in the joint and acts as a shock absorber. So when cartilage deteriorates or becomes damaged, it limits the knee’s normal movement and causes pain.
The vast majority of the volume of cartilage tissue is made up of synovial fluid. This fluid supports weight and lubricates joint surfaces. Loss of synovial fluid causes the decrease in cartilage thickness, increases friction, bone degradation and joint pain.

Cartilage is porous so the synovial fluid is squeezed out of the holes over time. It is constantly leaking into the membrane-walled cavity between the upper and lower leg bones.

New findings

Now a team from the University of Delaware claims that motion can cause cartilage to reabsorb liquid that leaks out… and that hydrodynamic pressurisation is key. This happens when the motion of two surfaces causes fluid between them to accelerate in the shape of a triangular wedge.

To investigate if hydrodynamic pressurisation could refill deflated cartilage, the researchers placed cartilage samples against a glass flat to ensure that there would be a wedge. They found that at slow sliding speeds (less than would occur in a joint at typical walking speeds) cartilage thinning and an increase in friction occurred, but as the sliding speed increased toward typical walking speeds, the effect was reversed.

They concluded that hydrodynamic pressures, which force fluid flow into the cartilage, must have counteracted the fluid that had been exuded. David Burris, who led the work, comments:

“We observed a dynamic competition between input and output [of synovial fluid]. We know that cartilage thickness is maintained over decades in the joint, and this is the first direct insight into why. It is activity itself that combats the natural deflation process associated with interstitial lubrication.”

Image credit: Stefan Powell

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