Does smoking protect against osteoarthritis? Meta-analysis of observational studies

To determine whether smoking is protective against the development of osteoarthritis (OA).Observational studies for the association between smoking and OA were systematically searched through Medline (1950-), Embase (1980-), Web of Science (1960-), PubMed, Google and relevant references. ORs and 95% CIs were directly retrieved or calculated. Current standards for reporting using MOOSE were followed. Quality-related aspects such as study design, setting, sample selection and confounding bias were recorded. Stratified and meta-regression analyses were undertaken to examine the covariates.Of 48 studies (537 730 participants) identified from the systematic literature search, 8 were cohort, 21 cross-sectional and 19 case-control. There was an overall negative association between smoking and OA (OR=0.87; 95% CI 0.80 to 0.94) and subgroup analysis confirmed this in case-control studies (OR=0.82; 95% CI 0.70 to 0.95), but not in cohort (OR=0.92; 95% CI 0.81 to 1.06) or cross-sectional studies (OR=0.89; 95% CI 0.78 to 1.01). Within case-control studies a negative association occurred only in hospital settings (OR=0.65; 95% CI 0.52 to 0.81), not in community settings (OR=0.90; 95% CI 0.75 to 1.08). The association was also seen in knee OA, radiographic OA and smoking as a secondary exposure (covariate or confounding factor). Meta-regression analysis demonstrated that a hospital setting and smoking as a secondary exposure were the major source of the negative association.The protective effect of smoking in OA observed in some epidemiological studies is likely to be false. It may be caused by selection bias, often in a hospital setting where control subjects have smoking-related conditions and studies that are not primarily designed to investigate smoking. Critical appraisal of such studies is needed.