Arthritis Care Research News Alerts. 2009;61(3):329-335.

Obesity is known to be a strong risk factor for the onset of knee osteoarthritis (OA), but studies on the relationship between obesity and progression of the disease have shown mixed results and have lacked large numbers of patients. It’s important to clarify the relationship between weight loss and the risk of OA progression, since patients with knee OA might be motivated to lose weight if it could be shown that it would prevent advancement of the disease. A new study examined the relationship between body mass index (BMI) and the risk of new and progressive knee OA and found no overall relationship between obesity and the progression of knee OA. The study was published in the March issue of Arthritis Care & Research (http://www3.interscience.wiley.com/journal/77005015/home).

Led by D.T. Felson of Boston University School of Medicine, the study involved more than 2,600 participants who either had OA or were at high risk for developing it due to the fact that they were overweight or obese, had knee pain, aching or stiffness or had a history of knee injury that made walking difficult or had previous knee surgery. Participants underwent an exam, a hip bone mineral density test and X-rays of both legs at the start of the study and knee X-rays after 30 months.

The results showed that obesity was associated with an increased risk of onset of knee OA, but there was no overall effect of BMI on the risk of progression of the disease. However, there was an effect depending on how the knees were aligned: those with high BMI and neutral alignment had an increased risk of progressive knee OA, while those with valgus (knockknee’ed) alignment had a small risk, and those with varus (bowlegged) alignment showed no increased risk.

The authors suggest that obesity has the greatest effect in neutrally aligned knees because the excess load it generates acts without the influence of stress on the knee due to malalignment. In knees inwardly aligned (bowlegged), the stress generated by the malalignment accelerates progression of the disease. “Our findings suggest that this stress is sufficient by itself to produce progression, and that the excess load conferred by obesity may not be necessary as an additional factor,” the authors state.

Dr. Felson pointed out that their study did not speak to the effect of weight loss on knee pain but only on structural effects of obesity. Other studies have suggested that obesity worsens knee pain and that weight loss may alleviate this pain.

Among those that did not have knee OA, obesity increased the risk of developing the disease, regardless of the malalignment status. This may be because malalignment was less severe in subjects who did not have OA, which suggests that it occurs with the development of the disease, or because malalignment in knees with OA may not be the same as in knees without it.

“The failure to demonstrate that obesity increases the overall risk of OA progression in our study and others does not eliminate opportunities for weight loss trials aimed at slowing disease progression, especially among knees in extremities with neutral or valgus alignment,” the authors note. They point out that the study may help explain the modest effects of weight loss on OA symptoms shown in previous studies and conclude that weight loss has many positive health effects, even if it may not delay the progression of structural damage in knee OA.

Item is available via Wiley InterScience at http://www.interscience.wiley.com/journal/arthritiscare.

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