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Analyzing Knee Motion in Normal Knees and Knee Implants

Many doctors are busy in the operating room fitting patients with brand new knee joints. At the same time, doctors at the Rocky Mountain Musculoskeletal Research Laboratory in Denver, Colorado, are studying the motion of these new knees.

Doctors and engineers are working together to compare normal knee movement with the motion of a total knee joint replacement. They also compare joint implants to one another. The goal is to find ways to mimic normal knee motion in the implants. Younger patients with early knee replacements need the implant to last 20 years or more. Uneven wear, knee laxity (looseness), and loss of full motion can cause implant failure.

In this study, two types of total knee replacements were studied. Motion during walking and deep knee bends were measured using high-frequency video fluoroscopy. This technique produces video X-rays that can be viewed on a TV screen. Normal knee motion is recorded during actual movement using fluoroscopy and a computer program. This study is the first to show contact points between the femur and tibia in three dimensions.

The researchers looked at contact points between the the tibia and femur bones of the knee. They analyzed the sliding and gliding motions of the knee implants and noticed when the bones separated without touching. Notably, the implant sometimes moved in an opposite pattern from a normal knee. This is called paradoxical movement. The authors think this may be a possible cause of implant failure and should be studied further.

References: Douglas A. Dennis, MD, et al. In Vivo Fluoroscopic Analysis of Fixed-Bearing Total Knee Replacements. In Clinical Orthopaedics and Related Research. May 2003. Vol. 410. Pp. 114-130.

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