Keep moving to prevent osteoarthritis, say physicists

Osteoarthritis affects around half of all Americans at some point in their lives.

Osteoarthritis is a degenerative bone disease resulting from a reduction in the cartilage, leading to an increase in friction.

Cartilage is a firm, rubbery material covering the ends of the bones in the knee joint. It reduces friction in the joint and acts as a “shock absorber.”

When cartilage becomes damaged or deteriorates, it limits the knee’s normal movement and can cause significant pain, and eventually the need for knee replacement surgery.

About 80% of the volume of cartilage tissue is made up of synovial fluid. This fluid is needed to support weight and lubricate joint surfaces. The loss of synovial fluid that causes the decrease in cartilage thickness, increase in friction, bone degradation and joint pain of osteoarthritis.

Since cartilage is porous, the synovial fluid is squeezed out of the holes over time. In fact, it is constantly leaking out into the membrane-walled cavity between the upper and lower leg bones.

What stops cartilage from deflating?

Yet the symptoms associated with osteoarthritis usually take decades to develop; so, what prevents the cartilage from deflating over the course of days, months or years, in the joints?

David Burris, an assistant professor in the Mechanical Engineering Department at the University of Delaware, and colleagues believe they know how motion can cause cartilage to reabsorb liquid that leaks out.

Scientists have previously hypothesized that if movement occurs faster than the fluid can diffuse, then continuous knee movement could prevent deflation.

Using a small sphere articulated against a cartilage plug, they showed that interstitial pressure was maintained indefinitely, if the contact area moved faster than the diffusive speed of the synovial fluid.

However, it remained unclear how a knee joint does not become deflated, given the long periods of time spent sitting and standing each day, without active input.

In other words, how does cartilage reabsorb the fluid that leaks out when people are not moving?

Pressure forces fluid back into the cartilage

Burris suggested that the reabsorption process might be driven by hydrodynamic pressurization. The term hydrodynamic refers to the lubricating principle involved in the working of bearings; in this case, the knee joint.

Hydrodynamic pressurization occurs when the relative motion of two surfaces causes fluid between them to accelerate in the shape of a triangular wedge.

For example, when a normal tire travels quickly over water, pressure builds until a film forms to lubricate the interface; this is called hydroplaning. Hydroplaning leads to a complete loss of frictional control. If the tire were porous, however, the exterior fluid pressure could force fluid back into the tire.

To investigate whether hydrodynamic pressurization could refill deflated cartilage, the researchers placed larger-than-average 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 over time; 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.

Burris says:

“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.”

Medical News Today recently reported that participating in tai chi could help to prevent osteoarthritis.

Written by Yvette Brazier