Using data collected from the CHARLS, a national population survey with a 4-year follow-up, our study found that the cumulative incidence of symptomatic knee OA over 4 years among Chinese adults aged ≥ 45 years was 8.5%. Our study also showed significant variations of the incidence by province. To the best of our knowledge, this is the first study to report the incidence of symptomatic knee OA among the Chinese population. The findings may provide valuable information for health-care policy makers, allowing them to better allocate health-care resources and develop evidence-informed health-care planning by province. In particular, the findings may have important implications for those provinces with higher incidence.
Few studies have examined incident symptomatic knee OA. In the Framingham Osteoarthritis Study, with a ~ 8.1-year follow-up, the incidence rate of symptomatic knee OA was 6.7% (0.8% per year) . In the current study, with a ~ 4-year follow-up, the estimated incidence of symptomatic knee OA is 8.5% (2.1% per year), higher than that in the Framingham Osteoarthritis Study.
Some studies have examined the incidence of radiographic knee OA. One study in the Japan showed that the incidence rate of K/L grade ≥ 2 knee OA was 2.9% per year . Another study in the U.K. showed that the incident rate of K/L grade ≥ 2 knee OA was 2.5% per year . However, a study in the Spain showed that the incidence rate of knee OA was identified using International Classification of Diseases (ICD)-10 codes, was 0.64% per year . However, the definitions of the cases varied among these studies.
Other longitudinal studies conducted in developed countries reported risk factors including gender, age, occupation, BMI, education, household income, diabetes mellitus, hypertension, cardiovascular disease (CVD), and physical activity [5, 10, 19–22]. Our study also identified a number of risk factors associated with the development of symptomatic knee OA, including the female gender, residing in a rural area or West China, a lower level of education, lack of physical activities, hypertension, heart disease, kidney disease, and digestive disease. However, the findings should be interpreted with caution, as the causal pathways of these factors have not yet been clearly understood. The reported results about risk factors may represent total or direct effect, depending on the causal relation among the risk factors .
The incidence of symptomatic knee OA was significantly higher in women, consistent with previous studies [10, 13, 24]. This is likely due to women doing more household work, having a higher awareness of knee OA, or having less muscle strength and low bone mineral density. Consistent with other studies, our analysis found that senior ages were associated with higher risk of symptomatic knee OA [3, 10, 13, 25]. In addition, our study found that the incidence was highest among respondents aged 60–69 years, then decreased after 70 years, possibly because elderly persons generally do not do heavy physical work after the age of 70 years; thus, have less load on their knees. Our study also found that those with a higher level of education had a lower risk of symptomatic knee OA, consistent with previous studies [9, 26]. This is likely due to those receiving less education being more likely to be employment in physical labor.
In our study, those residing in rural areas or in West part of China had a higher risk of symptomatic knee OA. Meanwhile, the provinces with the highest incidence were mainly in the West region. These findings were consistent with some previous cross-sectional results [9, 25, 27]. This may be because residents in rural areas often have less-privileged socioeconomic conditions and limited access to health-care resources, while undertaking more physical labor. The difference among the three regions is also attributable to the difference in terrain and socioeconomic imbalance.
We found that people doing some sort of physical activities (e.g. dancing, body building) often had lower risk of symptomatic knee OA, which is similar to the findings of a review about OA . As shown earlier, a light and moderate level of activity may be associated with less subsequent disabilities, such as knee OA . This finding suggested that regular physical activity is always warranted for preventing the development of this condition.
Self-reported hypertension, heart disease, kidney disease, and digestive disease are associated with symptomatic knee OA, as shown by our study. One set of meta-analysis results showed that hypertension was significantly associated with higher symptomatic knee OA . One possible explanation is that they share traditional risk factors, such as chronic inflammation. One study confirmed that CVD was a risk factor for knee OA15. A higher risk of CVD has also been observed in people with OA . For these analyses, CVD included heart disease. It is possible that heart disease and symptomatic OA have a bidirectional relationship with OA. Similarly, self-reported kidney disease and digestive disease were associated with symptomatic knee OA is possible that kidney disease and digestive disease may be caused by symptomatic knee OA. The chronic inflammation and nonsteroidal anti-inflammatory drug (NSAID) treatment in symptomatic knee OA patients are reported to increase the risk of getting kidney disease and digestive disease [31, 32]. Further studies are warranted to confirm the relationship between these diseases and knee OA. Another possible explanation might be that persons with these diseases have more contact with health care and, thus, are more prone to receive a diagnosis of arthritis.
Our study has several strengths. Firstly, the CHARLS included a nationwide representative sample of middle-aged and older adults. The findings are generalizable to the Chinese population. Secondly, the survey was conducted using a strict quality-control program, and the study participants were chosen according to a strict multistage probability sampling procedure. Finally, we have reported both the incidence and associated risk factors for symptomatic knee OA, which is helpful for healthcare policy development and clinical practice.
Our study has some limitations. Firstly, the respondents in the CHARLS did not undergo radiographic assessment, the diagnosis of symptomatic knee OA was based on self-reported knee pain and self-reported arthritis diagnosis by a physician, which differed from other studies using recommendations for diagnosis of knee OA . However, this definition has been used in published studies [9, 34]. Secondly, the data for other chronic diseases were based on self-reporting. Hence, the associations we observed might be affected by people with these diseases having more contact with health care and, thus, being more prone to receive a diagnosis of arthritis. However, our findings were generally consistent with previous studies [15, 21]. Finally, findings regarding risk factors should be interpreted with caution since different definition of knee OA have different risk factors, and our findings may differ from other studies focusing on other diagnosis of knee OA.