Excess bodyweight, expressed as increased body mass index, is associated with osteoarthritis risk, especially in weight bearing joints. However, the strength of the association was inconsistent. The study was conducted to quantitatively assess the association between body mass index and the risk of knee osteoarthritis and investigate the difference of the strength stratified by sex, study type and osteoarthritis definition.

We used published guidelines of the Meta-analysis of Observational Studies in Epidemiology Group (MOOSE) to perform the meta-analysis. The search strategy employed included computerized bibliographic searches of MEDLINE, PubMed, EMBASE, The Cochran Library and references of published manuscripts. Study-specific incremental estimates were standardized to determine the risk of knee osteoarthritis associated with a 5kg/m² increase in BMI.

Twenty-one studies were included in the study. The results showed that body mass index was significantly positive associated with osteoarthritis risk in knee site. A 5-unit increase in body mass index was associated with an 35% increased risk of knee osteoarthritis (RR: 1.35; 95%CI: 1.21, 1.51). Magnitude of the association was significantly stronger in women than that in men with significant difference (men, RR: 1.22; 95%CI: 1.19, 1.25; women, RR: 1.38; 95%CI: 1.23, 1.54; p=0.04). The summary effect size was 1.25(95%CI: 1.18, 1.32) in case-control studies and 1.37 (95%CI: 1.19, 1.56) in cohort studies (p=0.28). Body mass index was positively associated with knee osteoarthritis defined by radiography and/or clinical symptom (RR: 1.25, 95%CI: 1.17, 1.35) and clinical surgery (RR: 1.54, 95%CI: 1.29, 1.83). The latter tended to be stronger than the former (p<0.01).

Increased body mass index contribute to a substantially increased risk of knee OA. The magnitude of the association varies by sex and OA definition.