Body mass index (BMI) is a measure of fat based on an individual’s height and weight. Based on simple physics, patients with a higher body mass index (BMI) put more physical stress on their knees. Depuy Orthopaedics, a manufacturer of knee-replacement prostheses, paid for a study that gives insight into the effect of BMI on postoperative pain associated with total knee replacement as well as knee function following the surgery.

Most recent studies show that it’s not only the mechanical impact of more weight on the knee that creates problems, but also the impact of the metabolic syndrome caused by that additional weight. Metabolic syndrome is the blood-sugar instability that leads to type 2 diabetes. This blood-sugar instability ultimately has an impact on the chemical environment of the joint, causing more bad chemicals that break it down.

A study published in the February 2015 issue of The Journal of Arthroplasty looked at 1,545 total knee-replacement patients whose pain and function scores were followed for three years after surgery. The results showed that going into surgery with a higher BMI negatively affected pain and function scores. A 10 percent increase in BMI after surgery also negatively affected the subjects’ pain and function scores. Interestingly, losing weight after surgery did not positively affect pain and function scores. Older patients were slowest to improve, and men generally improved more than women. Heavier patients or those with more disability respond less positively to knee-replacement surgery, and patients who gain weight after surgery also do more poorly.

These findings echo what already is known about high blood sugar being a risk factor for chronic pain after knee replacement, and the study provides clarity about the effects of increased BMI on knee-replacement outcome. The same negative chemical environment that breaks down tissue in the joint prior to surgery also makes healing after the surgery significantly less successful. Interestingly, data collected from patients who chose stem-cell injection instead of knee replacement show that those heavier patients with osteoarthritis of the knee seem to do as well as lighter patients.

“BMI Affects Knee Replacement Outcome” first appeared as a post on the Regenexx blog.

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