Why Weight Matters: Obesity and Knee Replacement

By: Kathleen A. Hogan, MD

Obesity and knee replacement are closely linked. It is well established that people who are overweight have an increased incidence of knee arthritis at a younger age and are more likely to require knee replacement. Joint pain can limit mobility and diminish exercise tolerance which may contribute to weight gain.

What is BMI?

The relationship between height and weight is determined by a mathematical calculation to give a body mass index (BMI). A BMI of 20-25 is considered to be ideal. A person with a BMI above 30 is considered to be obese and over 40 is morbidly obese. For example, a 5’7’’ 145 lb woman would have a BMI of 23 (normal body weight). If her weight increased to 200 her BMI would be 31 (obese). At a weight of 250, her BMI would be 40 (morbidly obese). BMI helps physicians standardize our discussion of weight. However it does not take into account the distribution of that weight – the apple or pear shaped body shapes for example. Nor does it determine if weight is primarily muscle or fat.

Risks of Obesity and Knee Replacement

Why does weight matter when discussing joint replacement? Patients who are excessively overweight are at much higher risk of compilations following hip or knee replacement surgery. The risk of infection, for example rises exponentially as BMI increases over 40. The risk of a major infection after knee replacement is less than 1% in patients with a BMI of 20-25. This risk rises to 1-2% in obese patients (BMI >30), 3-4% in morbidly obese patients (BMI >40) and in a patient with a BMI of 50 the risk is over 8% This increased risk is independent of other medical problems such as diabetes, hypertension, and cardiac disease. Infection after a joint replacement can be a devastating problem. Often, the metal implants must be temporarily removed in order to eradicate the infection. Prolonged treatment with IV antibiotics is required. Only after the infection has been successfully treated can a new joint be put back in place. Elevated body weight also increases the risk of medial complications, blood clots, and even death following surgery. Diabetes, smoking, and poor nutrition are also risk factors for complications following surgery.

What Criteria Are Used?

Every surgeon has his or her criteria for determining if a patient is “too big” for a joint replacement. Body weight and BMI are important; however other heath problems such as diabetes, smoking, malnutrition, peripheral vascular disease, and the condition of the legs’ skin are also important considerations in determining if surgery can be safely performed. It is very difficult to explain to someone that their disabling arthritic pain can not be safely treated with a joint replacement because of their body weight. Joint replacement surgery can be a life altering procedure when successful, with improvement in mobility and elimination of pain. However, the complications of joint replacement can be devastating as well. If a surgeon tells you that weight loss is necessary prior to surgery, it is only because the risks of surgery outweigh the potential benefits. Loosing weight prior to knee replacement will not only improve the outcome of your surgery and lower the risk of major complications, but will also have life-long health benefits.