The present study is the first study to our knowledge to describe trends in the prevalence and incidence of osteoarthritis in China. Using data from the health-insurance claims of nearly 18 million people, we found that the 10-year average age-standardized prevalence and incidence rates of osteoarthritis in Beijing were, respectively, 5.21% and 28.54 per 1000 person-years. Furthermore, prevalence increased significantly within a decade. In addition, we also found that the AAPC in the prevalence and incidence during the decade was the largest in the low-age group (18–34 years), indicating that osteoarthritis was diagnosed at younger ages.
Our results showed that the age-standardized prevalence of osteoarthritis in people over 18 years-old was 5.21%, which is close to the findings of the Australian scholar Minaur (5.5%) . The highest prevalence of osteoarthritis was 20.7%  and the lowest was 1.6%  in relevant and comparable studies. Compared with related studies in other countries, the prevalence of osteoarthritis in Beijing is at a low-medium level. We attribute the difference in prevalence among studies to differences in race, life and work styles, and other variables. Furthermore, the prevalence observed in the present study increased significantly within a decade, which is similar to the change in prevalence observed in other countries, such as the United States and the United Kingdom [6, 21]. The reason for this may be the aging of the population and increased risk factors for osteoarthritis (such as low physical activity and high body mass index) in recent years [22–24]. In addition, the present study found the AAPC in the prevalence of osteoarthritis was greatest in the age group under 35 years-old. We should be alert to the phenomenon that the rate of osteoarthritis is increasing among young people, not only among older adults. More efforts are needed in the future to prevent osteoarthritis in young people.
Previous studies have shown that the prevalence and incidence of osteoarthritis are higher in women than in men [25–27], which is consistent with the results of the present study. This phenomenon may be attributed to differences in hormone levels, muscle strength, and health-seeking behavior between the genders [28–30]. Estrogen levels in postmenopausal women are significantly lower than premenopausal and male levels, which may affect cartilage metabolism and change the mechanical environment of joints . Moreover, men and women have different sensitivity and tolerance to disease, and women are more likely to seek timely medical treatment than men are . This suggests that men may be less likely to be diagnosed with osteoarthritis, which is reflected in the lower prevalence and incidence among men in this study.
It is known that the prevalence and incidence of osteoarthritis increases with age. Our study showed that the prevalence and incidence of osteoarthritis increased significantly after 55 years-old, and that the average crude prevalence from 2008 to 2017 of people over 55 years-old was 14.86%: 10.65% in males and 20.36% in females. These results are consistent with the results of other studies [33–35]. The reasons may include the following [30, 36, 37]: (i) the cell functions and properties of articular cartilage change with age and it responds differently to cytokines and growth factors; (ii) the articular cartilage secretes less synovial fluid with age, which reduces lubrication of the joints; (iii) muscle strength is reduced with age, so it is difficult to support the surrounding articular cartilage, thereby accelerating cartilage wear; and (iv) after menopause, changes in hormone levels can cause bone hyperplasia and accelerate the onset and progression of osteoarthritis. However, in our study, the prevalence and incidence of osteoarthritis in people over 85 years-old were lower than they were in the 55–64 and 65–74 age groups. The explanation for this may be related to an increased comorbidity rate and a decreased rate in medical visits due to osteoarthritis in this group of people. Another possible explanation for this phenomenon is survivor bias, which allows relatively healthy people to survive to the oldest age group.
This study has certain strengths. Currently, there is a lack of epidemiological studies on large samples of people with osteoarthritis in China. We used data from the health-insurance claims of nearly 18 million people to estimate the prevalence and incidence of osteoarthritis in Beijing in the past 10 years, and determined the prevalence trend for osteoarthritis. These findings about osteoarthritis could provide valuable evidence for other developing cities in China, and even in cities in other countries in the future. Moreover, the estimates of prevalence and incidence were based on a dynamic population, which is closer to real-world population changes.
The limitations of this study include the following. Since the data were based on hospital visits, we could not obtain information about people who suffered from osteoarthritis but did not see a doctor. This may result in an underestimation of prevalence and incidence rates. In addition, although the BMCDE covers more than 80% of the resident population of Beijing, we could not obtain information about some immigrants in this city, which may cause selection bias. Finally, because the BMCDE includes only claims data, information on socioeconomic status and health behaviors was not available. Therefore, we could not do a more detailed analysis, such as the identification of risk factors.