Degenerative joint disease (also known as osteoarthritis or OA) is a common problem in the aging adult. Two out of 10 people over the age of 60 develop OA. Knee arthritis is especially common. Pain and loss of motion from this condition can really limit activities and lead to increasing disability.

At the same time, surgeons are able to replace the knee joint with a perfectly functioning implant called a total knee replacement (TKR). In fact, TKRs have become so successful their number has increased over 80 per cent in the past 10 years.

Scientists collecting data on TKR patients have noticed a very important trend. Once the first knee joint is replaced, it seems there is a predictable pattern of deterioration in the opposite (nonoperated) knee. It's not uncommon for patients with one TKR to end up having a second joint replacement on the other leg.

Is this a coincidence or is there a reason for this pattern? A recent study from a Biomechanics and Sports Medicine Laboratory at the University of Tennessee was able to shed some light on this. They looked at the angle of the knee in the operated and nonoperated knees of 18 patients and compared it to with an equal number of knees in healthy adults (the control group).

A seven-camera system designed to capture motion from all angles was used to evaluate the operative group and the control group. A force plate was used to measure ground reaction forces (force up from the ground through the leg to the knee). Walking speed was also recorded. Everyone wore the same shoe while walking to reduce the effect of footwear on the results.

They found an increase in the first peak knee adduction moment of the nonoperated leg (compared to healthy controls and even compared to the operated leg). The adduction moment is the angle formed by the femur (upper leg or thigh) as it connects at the knee with the tibia (lower leg). The measurement is taken in the frontal plane. That means from the front of the patient rather than from the side or from the back. The first peak moment occurs as the person steps onto the foot (stance phase).

The increase in angle suggests there may be some change in the biomechanics of the knee and leg during the stance phase of walking when the patient puts weight on the leg. It's also possible that the patient has a new knee but still walks with the abnormal gait pattern that was present before surgery. This effect could speed up deterioration of the other knee leading to osteoarthritis and the need for a second knee replacement.

Since researchers have just started looking for reasons why, we don't know if changes present before the first knee replacement are a factor or if knee angles, uneven loads placed on the knees, walking speed, or some other factors are the cause. And it's not clear yet if surgeons can predict who will develop problems on the opposite side based on these findings. More studies are needed to sort out all the effects and factors.

In the meantime, the authors suggest the results of this study indicate a need to rehab both knees after a total knee replacement is done. The operated knee is important but the effect of the nonoperated limb can't be ignored.

Clare E. Milner, PhD, and Mary E. O'Bryan, BSEd. Bilateral Frontal Plane Mechanics After Unilateral Total Knee Arthroplasty. In Archives of Physical Medicine and Rehabilitation