What is This?

On the second installment of my blog series “What Is This?” I wanted to look at something that you hopefully will not see in a young athlete, but is becoming increasingly common in middle-aged to older individuals. Many athletes, and for that matter, non-athletes, require surgery to repair worn out joints or repair areas of bone or muscle that have worn to a point that now cause pain. One joint that many individuals have trouble with later in life is the knee joint. It is estimated that over 600,000 knee replacement or arthroplasty surgeries are done a year in the United States alone (WebMD, 2019). There are five basic types of knee replacement surgeries: total knee replacement, partial knee replacement, patellofemoral replacement, complex/revision knee replacement, and cartilage restoration. The most common knee replacement surgery is the total knee replacement. For a total knee surgery, an orthopedic surgeon removes the damaged part of the knee and replaces it with an artificial joint made of metal, ceramic and plastic. The artificial joint is often attached to the femur and tibia with a special adhesive material such as acrylic cement. 

The images below belong to a former collegiate hurdler who underwent knee replacement. The surgery took place when the individual was 55 years of age, so no longer a competing athlete, but still very active. This individual was experiencing a considerable amount of pain in the knee joint when performing activities of daily living, as well as working out. The two images below belong to this former collegiate hurdler before and after he underwent knee replacement surgery.

In the image below (Image A), you see the pre-surgical DXA total skeletal scan as well as the isolated left knee DXA scan. In the pre-surgical isolated knee view, you can see the accumulated damage that has occurred in the joint area over the years (highlighted by the red box). It should be noted that a DXA scan would not be used to diagnose whether a replacement is necessary or not. In most cases, an X-ray and/or magnetic resonance image (MRI) would be used by an orthopedic surgeon to diagnose the need for a knee replacement as well as the type of knee replacement required.

A few weeks after a successful surgery, I was able to repeat the two DXA scans in this individual (Image B).  I repeated both the total body scan as well as the isolated left knee taken after knee replacement surgery. On the total skeletal view, you can easily see the knee replacement on the left knee as the blue object (highlighted by a red box).  If you look at the isolated scan of the left knee again, you get an excellent view of the knee replacement. Looking at the femur you can clearly see the cap that is placed around the tibia and you can see the base of the knee replacement that is placed into the tibia. While not appropriate to diagnose whether a knee replacement surgery is necessary, DXA may actually be valuable when examining the knee replacement over time. Most knee replacements used today are thought to have a lifespan of 20-30 years (WebMD, 2019). Therefore, most individuals will not have to worry about having to have a second replacement if they have had their knee replacement done late in their life. The DXA scan may be useful in these individuals to check on the bone around the actual replacement itself. Since some knee replacement surgeries use an acrylic cement to bond the replacement parts to the bone of the knee joint there is a possibility of bone loss at these contact points. If you look at the isolated post-surgery image, you can see those areas of contact between the knee replacement and the bone where the acrylic cement has been used to bond the bone and the knee replacement.  By examining these areas and looking at the bone mineral content (BMC) as well as the bone mineral density (BMD) it is possible to monitor these areas for increased bone loss due to the acrylic cement.

Take Home Message
Two things stick out for me regarding the DXA scans below. The first is that, even though it is not used to diagnose whether a knee replacement is appropriate, the DXA scan was, in fact, able to pick up the damage that made a knee replacement necessary. The second is the potential to use the DXA to monitor the health of the bone surrounding the knee replacement. Over time, subsequent scans of this individual and others like him will be useful in determining the benefits of DXA technology in monitoring long-term recovery as it relates to bone health.

DXA Images













WebMD https://www.webmd.com/osteoarthritis/guide/knee-replacement-surgery#1
Accessed April 23, 2019


About the Author: Donald Dengel, Ph.D., is a Professor in the School of Kinesiology at the University of Minnesota and is a co-founder of Dexalytics. He serves as the Director of the Laboratory of Integrative Human Physiology, which provides clinical vascular, metabolic, exercise and body composition testing for researchers across the University of Minnesota.

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