Bariatric Surgery and Total Knee Replacement for Obese Patients

An X-ray of a knee with arthritis.
An X-ray of a knee with arthritis. Science Photo Library - DR P. MARAZZI/Getty Images

The morbid obesity epidemic continues to be a significant issue facing the United States. Obesity and osteoarthritis are intimately linked issues with obese people being at an almost 14 times higher risk of developing knee osteoarthritis compared to their skinnier counterparts. A strong correlation between morbid obesity and the need for total knee replacement has also been shown in the literature.

A key issue facing orthopaedic surgeons is that total knee replacement in morbidly obese patients is significantly more difficult, and has been associated with higher complication rates. Obesity has been associated with higher rates of surgical site infections, prosthetic loosening, revision surgery, and with lower outcome scores. Orthopaedic surgeons face with these issues have traditionally resorted to one of two solutions: 1) Operate on the obese patient despite higher complication rates. 2) Refuse to perform the total knee arthroplasty until the patient returns with a lower body mass index.

A third option has been gaining attention: recommending bariatric surgery prior to total knee replacement in morbidly obese patients. After these patients lose weight the theoretical outcomes of total knee replacement will be improved closer to the level of non-obese patients. A key question in this strategy is whether the additional cost and risk of bariatric surgery outweighs the benefits gained by undergoing a total knee replacement at a lower weight.

A recent study in the Journal of Bone and Joint Surgery performed a cost-utility analysis in order to explore this topic further.

Before we delve into the results of this study, let’s briefly review cost-utility analysis. This type of analysis looks at whether the gains of a certain intervention are worth the financial cost of that intervention.

Cost-utility analyses take into account the potential outcomes of the intervention investigated and look at how much that improvement cost comparing to the scenario without the given intervention. In this case the researchers looked at a theoretical morbidly obese  50 year old patient with a body mass index equal to or greater than 35 that had end stage osteoarthritis of the knee.

The researchers then compared two possible scenarios, one in which this theoretical patient underwent bariatric surgery followed by total knee replacement and the other in which the patient went straight to undergoing a total knee replacement. Using computer models they then simulated the outcomes for each of these scenarios over the following 40 years. They included data such as the costs of surgeries, as well as the average outcomes of each surgery and the probability of needing additional surgery.

The analysis is complicated and the details of the results are beyond the scope of this general article, however they did find very interesting and important results.

The study was strongly in favor of bariatric surgery before total knee replacement for patients with a BMI greater than or equal to 35 with end stage arthritis of the knee. The cost per quality adjusted life year was about $14,000. Quality adjusted life years or QALYs are a common outcome measure in cost effectiveness analysis and are an easy/quick method to look at whether an intervention is cost-effective or not. A common threshold used to call something cost-effective is $100,000 per QALY, and this falls far below that number.

A key downside to this strategy is that undergoing bariatric surgery prior to a total knee replacement will significantly delay the joint replacement. Patients are asked to wait almost two years until they lose the excess body fat. The upside is that weight-loss has been linked to a decrease in pain from knee osteoarthritis, so it’s feasible that a small subset of patients may not require the total knee replacement after successful weight loss. The other issue to keep in mind is that this analysis is not full proof, and more research is needed to see if the predictions of this computer model are accurate. However, this may be a new direction that the total knee replacement paradigm is moving in. 

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