Cigarette Smoking Increases the Prevalence of Hip Joint Involvement in Ankylosing Spondylitis: A Real-World Case-Control Study

Abstract

ObjectiveTo investigate the association between cigarette smoking and hip joint involvement in ankylosing spondylitis (AS).MethodsThis case-control study compared patients with AS with and without hip involvement, as defined by the Bath Ankylosing Spondylitis Radiology Hip Index. Logistic regression analysis, subgroup analysis, and sensitivity analysis were conducted to estimate the association between smoking and hip involvement in AS.ResultsThis study included 103 patients with hip involvement (cases) and 89 patients without hip involvement (controls). In univariate analysis, patients who had juvenile-onset AS (JAS), were younger, were male, had peripheral arthritis history, or had cigarette exposure were prone to hip involvement. After adjusting for confounding factors, JAS (adjusted odds ratio [aOR] 2.52, 95% CI 1.26-5.06), male sex (aOR 2.89, 95% CI 1.14-7.33), and cigarette smoking (aOR 7.23, 95% CI 2.27-23.05) were regarded to be independently associated with hip involvement in AS. Moreover, patients who smoked with exposure of less than 10 pack-years were 2.2 times more likely to have hip involvement than those without (aOR 2.21, 95% CI 1.09-4.47). This association was reproduced in subgroup analyses of males and propensity score–matched subjects, and it withstood sensitivity analysis.ConclusionSmoking is a novel independent risk factor for hip involvement in AS; even exposure of less than 10 pack-years could contribute to increased prevalence of hip involvement in AS, which underlines the significance of smoking cessation in patients with AS, especially for JAS.