Abstract and Introduction
Abstract
Objectives To determine whether smoking is protective against the development of osteoarthritis (OA).
Methods 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.
Results 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.
Conclusions 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.
Introduction
It is accepted that smoking is one of the major risk factors for many conditions such as cancer, diabetes and cardiovascular diseases. In contrast, smoking is reported to have a negative association with risk of developing certain conditions such as ulcerative colitis, Alzheimer's disease and Parkinson's disease. However, unlike the convincing evidence for the harm of smoking in many conditions, the evidence for the benefit of smoking is contentious. A recent study in Alzheimer's disease, for example, found that the negative association of this disease with smoking is biased by tobacco industry-funded/affiliated studies.
Smoking is associated with the elevated risk of back pain, chronic widespread pain and rheumatoid arthritis. Controversially, smoking has been reported to have a protective association with osteoarthritis (OA). However, not all studies consistently report this finding. It is unclear what mechanisms account for this phenomenon or, indeed, if any exist. Proposed reasons for a reduced risk of OA in smokers are the decreased physical sporting activity undertaken by smokers and that smokers tend to have lower body weight. However, the results are not supported after adjustment for physical activity and body weight. In contrast, some studies report that smoking may be associated with a greater risk both of cartilage loss and knee pain in OA.
The objective of this study was to assess whether there is a true negative association through a meta-analysis of observational studies. We hypothesised that a negative association between smoking and OA may result from selecting controls from hospital settings where patients from other departments (eg, cardiovascular) may have higher exposure to smoking than the general population.