The mechanism of the effect of obesity in knee osteoarthritis - The mediating role of malalignment

Objective. Obesity is most strongly linked to osteoarthritis (OA) at the kn ee. Varus malalignment was examined as a possible local mediator that may i ncrease the impact of body weight at the knee, versus the hip or ankle, Com partment load distribution is more equitable in valgus than in varus knees, and valgus knees may better tolerate obesity. We therefore tested whether 1) body mass index (BMI) is correlated with OA severity in vans knees, 2) t he BMI-OA severity correlation is weaker in valgus than in varus knees, 3) BMI is correlated with the severity of varus malalignment, and 4)the BMI-me dial tibiofemoral OA severity relationship is reduced after controlling for vans malalignment. Methods. In 300 community-recruited patients with knee OA, 2 groups (varus and valgus) were identified based on dominant knee alignment on a full-limb radiograph, i,e,, the angle formed by the intersection of the femoral and tibial mechanical axes. Severity of knee OA was assessed by measurement of the narrowest joint space width on radiographs of knees in a fluoroscopy-co nfirmed semiflexed position. Results. Alignment direction was symmetric (or neutral in 1 limb) in 87% of patients, One hundred fifty-four patients had varus knees and 115 had valg us knees. BMI correlated with OA severity in the vanes group (r = -0.29, P = 0.0009) but not in the valgus group (r = -0.13, P = 0.17). BMI correlated with malalignment in those with varus knees (r = 0.26) but not in those wi th valgus knees (r = 0.16). The partial correlation of BMI and OA severity, controlling for sex, was reduced from 0.24 (P = 0.002) to 0.04 (P = 0.42) when varus malalignment was added to the model, Conclusion. BMI was related to OA severity in those with varus knees but no t in those with valgus knees. Much of the effect of BMI on the severity of medial tibiofemoral QA was explained by varus malalignment, after controlli ng for sex. Whether it precedes or follows the onset of disease, varus mala lignment: is one local factor that, may contribute to rendering the knee mo st vulnerable to the effects of obesity.