The painful and debilitating effects of osteoarthritis at the knee can be remedied by replacing the damaged articulating surfaces of the knee with a total knee replacement. Modern knee replacements consist of a metal alloy component on the femoral side (top of the knee) and a special plastic called ultra-high molecular weight polyethylene on the tibial side (bottom of the knee). Through recent improvements in materials, these implants have seen increased longevity, often lasting 15 years or more in the body before requiring a second surgery. As patients opt for knee replacements earlier in life, remain more active after surgery and live longer, the demands on the design and longevity of knee replacements increases.
One of the leading causes of revision surgery is periprosthetic bone loss, which means that bone near the implant has thinned. As the knee articulates during walking, the top metal component slides, rolls and rotates on the polyethylene. This relative motion causes the polyethylene to wear. Microscopic debris leaves the surface and enters the surrounding tissue, where it can set off a cascade of cellular reactions that lead to bone resorption. Currently, knee replacement designs and materials are evaluated before being implanted by machines that run accelerated tests by simulating the motion of walking at the knee.
However, walking is not the only activity of daily living. Sitting in a chair, rising from a chair, stepping up a stair and stepping down a stair are the most common activities for total knee replacement patients after walking. In a motion analysis lab, we measured the movements and dynamics of these activities using retroreflective markers, infrared cameras and force plates. Using a computational method known as inverse dynamics and a parametric numerical model, we were able to show that these activities generally have larger motions and higher forces. In particular, the secondary motions of the knee, such as internal-external rotation and medial-lateral (side-to-side) translation, are often larger in non-walking activities.
Ultra-high molecular weight polyethylene, the plastic most commonly used in knee replacements, is made of long chains of molecules. The material is highly wear resistant because the chains are entangled. As thousands of cycles of walking accumulate, the molecular chains align in the primary direction of motion. Once aligned, the material is more susceptible to wear in secondary directions. The larger secondary motions of other activities could lead higher wear. Because pre-implantation tests only include walking, the tests may be insufficient to accurately predict the wear of the implant in the body.
Currently, testing is underway to evaluate if the motions of non-walking activities do increase the wear of knee replacements. In addition, knee replacements that had been implanted and were retrieved from patients after revision surgery or death are being evaluated for wear. The results of these retrieved specimen will help determine whether the inclusion of non-walking activities in testing routines is necessary to evaluate the wear of total knee replacements.
My dance to explain my in-progress PhD thesis at the University of Illinois at Chicago (UIC) in Bioengineering was originally choreographed for live performance and adapted for the camera.
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