Testing the "Sixth Sense" in Patients with Knee Arthritis

The ability to tell where your joints are positioned or how they are moving is called proprioception. Tiny sensors, called proprioceptors, are located in muscles, ligaments, tendons, and joint capsules. They send signals to the central nervous system to keep you in tune with your body's whereabouts.

This "sixth sense" gets a little fuzzy as we age or when we get fatigued. Scientists thank that joint problems like osteoarthritis can impair proprioception. This recent study investigated whether knee osteoarthritis disturbs sensory signals from the knee.

Researchers used a dynamometer to get precise measurements of joint proprioception. The dynamometer is a machine that can be set to move a joint at a certain speed through a preset arc of motion. It then gives a computer readout of speed, direction, and joint angle. Patients are tested to see if they can tell where their joint is positioned or which direction it is moving.

The authors tested 117 patients who were scheduled for knee replacement surgery because of knee arthritis. First, they compared the results of the patients to people of the same age without arthritis. Second, they compared signals between knees in patients who had one healthy knee and one arthritic knee. Third, they tested whether joint sense was worse in the most arthritic knees.

People with arthritis scored significantly lower than people with healthy knees, supporting the theory that osteoarthritis impairs joint sense. Unexpectedly, people with knee osteoarthritis in one knee showed impairments in the other knee, too, even when it appeared normal on X-rays. And joint sense didn't necessarily decline as osteoarthritis worsened.

These results made the authors question whether the muscle and ligament problems from osteoarthritis really are responsible for problems with proprioception. The authors speculate that impairments in joint sense might start even before the problems of knee osteoarthritis can be seen. They conclude that further studies are necessary to understand the relationship between problems with joint sense and osteoarthritis.