Benefits of weight loss may depend on alignment of affected leg, study suggests

A painful and sometimes crippling disease characterized by progressive cartilage loss, osteoarthritis (OA) of the knee affects an estimated 6 percent of adults over age 30.

AT present, no treatments are available that have been shown to impede the destructive course of this disease, apart from knee replacement surgery. Numerous studies have shown that being overweight increases the risk of developing knee OA, whose sufferers, on average, tend to be heavy. While doctors routinely advise patients to lose weight, researchers have yet to affirm the benefits of weight loss to prevent ongoing joint deterioration.

To better understand the effect of body weight on the course of knee OA, researchers at Boston University focused on an important predictor of disease progression: limb malalignment, defined by joint space loss at the point where the thigh and shin bones connect to the knee. Featured in the December 2004 issue of Arthritis & Rheumatism, their findings suggest that the benefits of weight loss for knee OA patients depend on the degree of alignment in the affected leg.

The researchers recruited their subjects from two studies on quality of life conducted by the Veterans Administration of the Boston Health Care System. 228 individuals with knee OA were selected; all but one completed a 30-month period of follow-up. 41 percent were women and the mean age was 66 years. Among the subjects, the diagnosis of OA was confirmed by radiographs in 394 knees. At the first follow-up examination, each subject was assessed for degree of alignment in the affected leg, which was then categorized as moderate, severe, or neutral. Malaligned limbs could be either varus (bowlegged) or valgus (knock-kneed). The body mass index (BMI) of each subject was also computed.

Of the total 394 knees studied, 90 showed disease progression. Weight gain did have a significant impact. For each 2-unit increase in BMI, researchers found an 8 percent increase in the risk of disease progression. However, this effect was limited to knees in the moderately malaligned legs. In neutrally aligned legs on the one end of the spectrum and severely maligned legs on the other, body weight had no measurable effect on the risk of OA progression. "The effect of BMI on progression was different at different levels of alignment, with the risk being much greater for limbs with moderate malalignment," affirms the study's author, David T. Felson, M.D.

Why would the impact of body weight on knee OA be restricted to legs where malalignment was moderate? Dr. Felson offers possible reasons. In patients with severe malalignment, the extreme stress already placed on local cartilage may be the only risk factor required for continued structural deterioration. In patients with neutrally aligned limbs, the increased joint loading that accompanies increased body weight would be distributed across much of the joint surface, thus protecting against further damage. Since moderate alignment increases the stress on cartilage, the addition of excess weight effectively works to aggravate the cartilage damage. For these patients, losing weight might relieve the stress and considerably slow disease progression.

"Our findings, which need to be confirmed in other studies, suggest that prevention and treatment efforts for obesity and knee OA could be efficiently targeted to those subjects with moderate malalignment," Dr. Felson concludes. "These findings may have broad implications not just for the effect of body weight on OA, but for other risk factors that affect joint loading."