Body mass index and alignment and their interaction as risk factors for progression of knees with radiographic signs of osteoarthritis

Objective To investigate in which way body mass index (BMI) and alignment affect the risk for knee osteoarthritis (OA) progression. Methods Radiographs of 181 knees from 155 patients (85% female, mean age 60 years) with radiographic signs of OA were analyzed at baseline and after 6 years. Progression was defined as 1-point increase in joint space narrowing score in the medial or lateral tibiofemoral (TF) compartment or having knee prosthesis during the follow-up for knees with a Kellgren and Lawrence score ≥1 at baseline. BMI at baseline was classified as normal (<25kg/m2), overweight (25–30) and obese (>30). Knee alignment on baseline radiographs was categorized as normal (TF angle between 182° and 184°) and malalignment (<182° or >184°). We estimated the risk ratio (RR) with 95% confidence interval for knee OA progression for overweight and obese patients and for malaligned knees relative to normal using generalized estimating equations (GEE). Additionally, we estimated the added effect when BMI and malalignment were present together on progression of knee OA. Adjustments were made for age and sex. Results Seventy-six knees (42%) showed progression: 27 in lateral and 66 in medial compartment. Knees from overweight and obese patients had an increased risk for progression (RR 2.4 (1.–3.6) and 2.9 (1.7–4.1), respectively). RRs of progression for malaligned, varus and valgus knee were 2.0 (1.3–2.8), 2.3 (1.4–3.1), and 1.7 (0.97–2.6), respectively. When BMI and malalignment were included in one model, the effect of overweight, obesity and malalignment did not change. The added effect when overweight and malalignment were present was 17%. Conclusion Overweight is associated with progression of knee OA and shows a small interaction with alignment. Losing weight might be helpful in preventing the progression of knee OA.