Prevalence of osteoarthritis and association between smoking patterns and osteoarthritis in China: a cross-sectional study

Osteoarthritis (OA) is the most common chronic progressive joint disease characterized by synovial tissue cartilage degeneration, inflammation, and/or thickening of the subchondral bone metabolism, leading to joint pain and stiffness and impaired physical function.1 From the epidemiological point of view, with the increasing prevalence of OA and the gradual increase in life expectancy, OA has now become one of the most important causes of disability in the world.2 In particular, the highest prevalence affecting a significant percentage is among elderly people.3 A recent study has reported that OA affects half the population aged 65 years or older, and the prevalence in postmenopausal women is double than that in men.4 Indeed, a recent meta-analysis reported a high prevalence of radiographic OA in those presenting with knee pain, with symptomatic knee OA of approximately 50%, and one-fourth of them had severe, disabling pain.5, 6

Several studies have shown that OA symptoms can seriously damage the physical and mental health and physical function of the patients, limiting daily activities and reducing quality of life.7 Some people who suffer from physical disabilities or need informal care lose their jobs, resulting in an economic burden on the family.8 Additionally, OA has been associated with an increased risk of mortality among older people.9 Since 1999, China has become an aging society; OA is a common, frequently occurring disease, and new cases are increasing every year in the elderly population.10 A large-scale survey conducted in China has shown that the prevalence of OA in the knee, hand, and cervical and lumbar spines in adults aged >40 years is as high as 46.3%.11

According to statistics, as of 2015, the current number of cases in China has risen to 120 million. At the same time, the prevalence rate in women is higher than that in men, and the age of onset is younger in women than in men12; the incidence of knee OA increases sharply in women post retirement.11,13 A French study showed that the total prevalence of OA was about 15% and that in patients >60 years old, it was 50%.14,15 Among participants aged >45 years in the Framingham study, the prevalence of knee OA was 19.2%, and the prevalence increased to 43.7% in people aged >80 years.16 According to data from the Dutch Institute for Public Health, the prevalence of knee OA in men aged 55 years or older was 15.6% and that in women was almost twice as high.17 There is a wide gap in OA prevalence rates in different regions, which may be due to differences in race, culture, lifestyle, and risk factors of OA.

A number of surveys have investigated risk factors for OA and reported factors such as age, sex, occupation, weight status, sports, and recreational activities as being important in the progression of OA.18 There is some limited evidence to suggest that mental health issues, heart disease, and hysterectomy may also be related to OA occurrence and development.6,19 However, smoking as a risk factor for OA has always been controversial. Smoking is a risk factor for a variety of diseases, including diabetes, cancer, and cardiovascular disease,6,20 and is also related to chronic pain, back pain, and rheumatoid arthritis (RA). However, a large sample size study is required to demonstrate a correlation between smoking and OA.21,22 Amin et al. conducted a study on natural history, over a period of 30 months, in patients with knee OA symptoms and found that smoking men suffered from more severe cartilage damage and more severe knee pain than nonsmokers.23 In addition, Ding et al. reported that smoking caused knee cartilage loss and defective development.24 Contrary to this, Leung et al. showed through a prospective cohort study of 63257 participants, aged between 45 and 74 years, that smoking was a protective factor for knee arthroplasty following total knee arthroplasty.25 The USA’s Health and Nutrition Examination Survey showed that knee OA was negatively correlated with the risk of smoking.26 At the same time, the Framingham population-based prospective study also concluded that smokers had a significantly lower risk of incident radiographic knee OA.27 Furthermore, smoking as a depressant in OA incidence was only observed in active smokers and not in passive smokers or in active and passive smokers. Therefore, this study explored the occurrence of OA in several different statuses of smoking. Of course, there are a couple of limitations, such as the diagnostic criteria of OA and smoking behavior classification are different by study and analyses were not conducted for associations between smoking habits (smoking amount, period, and pack-years) and OA prevalence. In addition, only a few studies have explored the precise relationship between passive smoking, active and passive smoking, and the prevalence of OA.

The main purpose of this study was to investigate the relationship between smoking patterns and the prevalence of OA in elderly people aged 50 years or older (nonsmoking, active smoking, passive smoking, and active and passive smoking) using the data of the Shanxi province.

The present study used baseline data from a cross-sectional survey. The primary purposes of this study were to evaluate the prevalence of OA in people aged 50 years or older in the Shanxi province and to identify its association with each smoking behavior. As it is well known, OA may involve any joint, usually affecting the knees, hips, hands, feet, and facet joints. In this study, knee OA was reported in 27.5% of participants, while the overall prevalence of OA was reported in 49.8% of participants (Figure 1). Knee OA was, therefore, the most common form of OA, and this is consistent with other studies.3,8,11,19,23

The prevalence of OA and knee OA was higher in the studies of the Shanxi province of China than estimates from the Framingham OA study or other studies.14,15,16,29 A knee OA study by Haq and Davatchi in the Community Oriented Program for Control of Rheumatic Disorders (COPCORD) studies showed that the prevalence of knee OA within rural areas of Iran is ten times higher than that within urban areas.30 Part of the reason for this difference may be individual differences, material conditions, and the socioeconomic environment. This research mainly concentrated on rural regions of the Shanxi province, resulting in prevalence of OA relatively higher than the global average.

Similar to other studies, our results showed that the prevalence of OA in women was greater than in men across all age groups between the ages of 50 and 90 years (Figure 2). In the study by Srikanth et al., it is clear that the incidence of knee, hip, and hand OA in women was higher than that in men, and OA in women increased significantly after menopause.16,31 Meanwhile, prevalence data from the Framingham study showed that both symptom-related data and radiographic data were higher in women than in men aged between 63 and 75 years.32,33 In view of this phenomenon, it has been suggested that hormones may play an important role in the progression of OA; however, this has yet to be confirmed through clinical and epidemiological studies.34

Although it is well known that smoking has been associated with an increased risk of several diseases, such as diabetes, cancer, cardiovascular diseases, respiratory diseases, stroke, RA, and musculoskeletal diseases, so far reports of the consequences of smoking on OA are conflicting.21,35 This study showed that there was no significant association between active smoking and nonsmoking in both knee OA and all types of OA, and the prevalence of OA and smoking behavior were not significantly different between sexes (Table 2). Active smokers were leaner, and residual confounding from the higher BMI among the nonsmokers could lead to no association observed in the study. However, the study of the effects of smoking on knee OA in the Framingham study showed that even adjusting for weight or BMI did not change the conclusion that smoking was harmless in terms of OA.23 Several cross-sectional and longitudinal studies have confirmed that smoking seemed to decrease the risk of OA or showed no correlation.25,27,36,37,38 A study showed that when nicotine concentration is 25–100 ng/ml, the mRNA and protein expression of type II collagen in chondrocytes of the articular cartilage increase significantly and the proliferation of cells increases.39 Some researchers have suggested that this may be related to smoking and reduced physical activity. Lastly, the effects of smoking may differ for incident and prevalent knee OA. When cartilage damage is caused by other mechanisms, the further study of the harmful effects of smoking on articular cartilage may result in false-positive results. According to a study by Schouten et al., the risk of knee OA in smokers did not increase during 12 years of follow-up.40 The latest research on Chinese people aged 45–74 years in Singapore has shown no difference in total joint replacement between active smokers and nonsmokers.25

Our analysis of smoking and OA showed sex differences that were not significant (Table 2). It is well known that OA is the most common cause of disability in the elderly, while women usually suffer from a higher prevalence of cartilage dysplasia, menopausal cartilage damage, and cartilage loss.41 A recent meta-analysis showed that differences in smoking patterns between sexes were not related to the occurrence of knee OA.42 The present study showed that smoking differences between men and women were not a risk factor for OA considering four different smoking patterns, which was consistent with the results of recent studies.

The smoke inhaled by indirect smokers contains more toxic and harmful substances because they inhale smoke not filtered by cigarette filters.43 According to relevant reports, nonsmokers exposed often to secondhand smoke will be at an increased risk of bronchitis, asthma, pneumonia, otitis media, macular degeneration, neurological disorders, and other related diseases.43,44 At present, reports of the relationship between passive smoking and the prevalence of OA are highly rare. In the present study, multivariate logistic regression analysis performed after adjustment for a series of factors, including age, BMI, waist and hip circumference, sex, education background, occupation, ventilation, heating, and alcohol consumption, revealed a nonsignificant association between passive smoking and OA (Table 2). However, active and passive smoking has been shown to be a risk factor for OA (Table 2). Although in the study by Kang et al., the unadjusted results partly showed that passive smoking in males was negatively correlated with OA compared to nonsmoking.6 This finding is consistent with the results regarding passive smoking and OA after adjusting for all covariates.6 For all types of OA or knee OA, with status of active and passive smoking, the values were OR>1 and P<0.05. There was no statistical significance when studying active smokers and passive smokers separately. However, when studying active and passive smokers, this combined situation was confirmed as a risk factor for OA. We can further speculate that this may be related to the duration of exposure, the total amount of smoking, and other factors. Since our findings may be due to the small sample size in this cross-sectional observation, these results need cautious explanation. Further studies on the relationship between smoking and OA using larger samples of different smoking statuses are required to draw more definitive conclusions.

Our study has several strengths. The wide range of people in this study can represents middle-aged and elderly people in the northwest of China. The Shanxi COPCORD study represents the middle-aged and elderly people in the northwest of China. We determined the final participants through rigorous multistage sampling procedures and conducted strict quality control of the survey. Therefore, this study can objectively describe the prevalence of OA in northwest China according to social, economic, and geographical location factors. In addition, considering the confounding effects of smoking and OA prevalence, we made targeted corrections to improve the accuracy of the statistical analysis. Our findings will contribute to the development of disease prevention and health promotion strategies and help to elucidate the potential etiology of radiographic OA in Chinese patients.

This study has some limitations. A major limitation is that only associations between smoking and OA prevalence could be drawn due to the cross-sectional design. Another limitation is that the participants were aged 50 years or older and were often accompanied by other diseases with intake of a large number of drugs, including those for treatment of many other diseases; this would have affected the prevalence of OA and analysis of the influencing factors. Furthermore, there were no data concerning activities of participants, and correlation results were limited.

Using data from the COPCORD study, we observed that the prevalence of radiographic knee OA in the Shanxi province of China was high. The prevalence of OA increased with age and was higher in women of all ages than in men. Larger cohort studies involving different smoking statuses are needed to explore the relationship between smoking and OA development.