Background: To evaluate the association between age, sex, BMI, waist/hip ratio, smoking, religion, ethnicity, education and knee osteoarthritis.

Methods: Eligible subjects were randomly included from participants of Tehran COPCORD study, of whom 480 subjects with knee osteoarthritis were compared to 490 subjects without (case-control study). Using a questionnaire developed by COPCORD group (Asia & Oceania), we enquired about the risk factors of knee osteoarthritis i.e. age, sex, BMI, Waist/Hip ratio, religion, ethnicity, education and smoking. Knee osteoarthritis was defined using ACR criteria. Each knee was unit of analysis using GEE technique to evaluate these associations.

Results: Age (OR; 1.096; CI95%: 1.091-1.1; P: 0.00) and sex (OR; 2.85; CI95%: 2.49-3.28; P: 0.00) showed significant association with knee osteoarthritis. Overweight (OR; 1.81; CI95%: 1.28-2.55; P: 0.00) and obesity (OR; 3.3; CI95%: 2.34-4.66; P: 0.00) both showed higher risk for knee osteoarthritis. The association between waist/hip ratio and knee osteoarthritis showed an OR of 5.28, CI95%: 0.89-31.44; P: 0.07. However, this association was only borderline significant. People with different religion or ethnicity and smokers had no extra risks for knee osteoarthritis. Higher education is a protective factor for knee osteoarthritis as people who had university education compared to people with no/primary education showed a lower risk for knee osteoarthritis (OR; 0.54; CI95%: 0.38-0.78; P: 0.00).

Conclusions: Our study confirmed that elderly, females, overweight and obese people are at higher risk to develop knee osteoarthritis as found in western societies. Higher education is a protective factor against knee osteoarthritis. Ethnicity, religion and smoking showed no extra risk of knee osteoarthritis.