To investigate the association between smoking and the risk for knee osteoarthritis (OA).
Cohort, case-control, and cross-sectional studies were obtained from the Medline, Embase, and Web of Science databases. Estimates were calculated using a random-effects model. Subgroup analyses and meta-regression models were performed to investigate potential sources of heterogeneity. We further analyzed the dose-response relationship between cigarette consumption and risk of knee OA.
Thirty-eight independent observational studies that included 481,744 participants were analyzed. Those who had ever smoked had a significantly decreased risk of developing knee OA relative to those who had never smoked (RR = 0.80; 95% CI 0.73-0.88). This was unaffected by study design, and the pooled relative risks (RRs) were 0.79 (95% CI, 0.65-0.96), 0.71 (95% CI, 0.61-0.84) and 0.83 (95% CI, 0.73-0.94) for cohort, case-control, and cross-sectional studies, respectively. Analysis of subgroups stratified by gender reduced the heterogeneity from moderate to low in both males and females. The lower risk for developing knee OA was more apparent in male smokers (RR = 0.69; 95% CI 0.58-0.80) than female smokers (RR = 0.89; 95% CI 0.77-1.02) and dose-response analysis showed a linear decrease in knee OA with increased cigarette consumption.
We found an inverse association between cigarette smoking and risk of knee OA, irrespective of study design. This association was more apparent in males. However, we have not demonstrated a causal relationship between smoking and OA, and further investigations are needed.