Being overweight can speed the progression of knee osteoarthritis, but the effect may be limited to people who are moderately “bowlegged” or “knock-kneed,” a new study suggests.

Among more than 200 adults with knee osteoarthritis who were followed for 30 months, higher body mass index, or BMI, was associated with a greater risk of worsening arthritis.

However, further analysis showed this was true only of patients who were either moderately bowlegged - meaning their legs curved outward in the area of the knee - or moderately knock-kneed - meaning their knees curved inward toward each other.

In contrast, high BMI did not speed arthritis progression in patients whose legs had a “neutral” alignment or those with a severe degree of misalignment.

Weight loss has been seen as one way overweight people might stall the deterioration in their arthritis-affected knees. The new findings suggest that any benefit of weight loss may be limited to patients with moderate malalignment in the legs, the study’s lead author, Dr. David T. Felson of Boston University, told Reuters Health.

Felson and his colleagues report the findings in the journal Arthritis & Rheumatism.

Being overweight is already considered a risk factor for developing knee osteoarthritis, in which the cartilage of the joint breaks down, leading to pain, stiffness and immobility. But the effect of body weight on the condition’s progression has been less clear, according to Felson.

To see whether limb alignment affects the relationship between BMI and arthritis progression, Felson and his colleagues followed 227 older men and women with knee arthritis for 30 months, using X-rays to estimate cartilage loss in the joint over time.

Overall, the researchers found, arthritis worsened in 23 percent of the affected knees, with the risk climbing in tandem with BMI - but only in those with a moderate degree of limb misalignment.

As for why BMI did not affect arthritis progression in people with neutral alignment, Felson said it is likely due to the fact that in these individuals, the heavy “joint loading” caused by excess pounds is fairly evenly distributed across the knee.

At the other end of the spectrum, severe misalignment in the legs may be enough in itself to worsen knee arthritis, so that no other risk factor - including a high BMI - much of an effect, according to the researchers.

Indeed, they found that limb alignment alone was a strong predictor of arthritis progression - with nearly half of the severely misaligned limbs showing progression, versus 22 percent and 9 percent of moderately misaligned and neutrally aligned limbs, respectively.

SOURCE: Arthritis & Rheumatism, December 2004.