Incidence and risk factors of symptomatic knee osteoarthritis among the Chinese population: analysis from a nationwide longitudinal study

China has the world’s largest elderly population, and the whole nation is aging fast – the proportion of aging population (≥60 years) was estimated to be 14.9% in 2013, 25.3% in 2030, and over 30% in 2050 [1]. Rapid aging has led to an increasing healthcare burden attributable to age-related diseases [2]. Knee osteoarthritis (OA) is a common disease condition associated with old age and is one of the leading causes of disabilities, which primarily affect elderly adults [3, 4]. The disease can substantially reduce quality of life [5]; severe cases may even lead to knee-joint replacement. The costs for knee OA management are usually very high, placing a heavy burden upon families and society [6‐8].

Epidemiological studies to understand the burden of knee OA are currently of great importance for health-care policy makers and clinicians. A previous study using a nationwide survey found that the prevalence of symptomatic knee OA among Chinese adults aged ≥45 years was 8.1% and increased with age [9], and concluded that future studies are needed to identify the risk factors for incident symptomatic knee OA. To date, however, no data are available on the incidence of symptomatic OA among the middle-aged and older Chinese population. Additionally, due to significant imbalances in socioeconomic development, environmental conditions, lifestyle patterns, and health-care utilization among different geographic regions in China, large variations in the incidence of symptomatic knee OA may be present among these populations. The lack of information may hinder the effective planning and execution of health-care strategies, as well as efficient use of health care sources.

In addition, understanding the risk factors for knee OA is important for managing health among adults, particularly for elderly populations. Several longitudinal studies conducted in the U.S.A., the U.K., Japan, and Europe have investigated risk factors for incident symptomatic knee OA [10‐13]. However, these studies were conducted in developed countries. The findings from them have limited implications for the Chinese population, because socioeconomic status, environmental factors, and lifestyle patterns differ substantially between the developing countries and the developed world. Although a few cross-sectional studies investigated factors associated with knee OA [9, 14‐17], to date, no longitudinal studies have been conducted to examine risk factors for knee OA among the elderly Chinese population.

In order to bridge this important evidence gap, we conducted a cohort study analysis, using data from the China Health and Retirement Longitudinal Study (CHARLS), to investigate risk factors for incident symptomatic knee OA in Chinese adults aged 45 years or older. In addition, we determined the incidence of symptomatic knee OA for this population.

In the CHARLS, 15,910 respondents were free from the symptomatic knee OA at the 2011 national baseline survey, among whom 2833 were lost to follow-up in 2013 and 2015. As a result, 13,077 remained in the cohort until 2015 and were finally used in our study (Fig. 1).

Among the 13,077 respondents included in this analysis (Table 1): 6726 (51.5%) were women; the mean age was 64.2 years (SE 0.09); 8140 (62.3%) lived in rural areas. The respondents were generally equally distributed across geographic regions (36.4, 32.4, and 31.2% from East, Central, and West of China, respectively). Among all respondents, 40.0% completed elementary school only, and 12.0% received high school or higher education; 62.9% had normal weight (BMI: 18.5–24.9 kg/m²), 6.4% were underweight (BMI: < 18.5 kg/m²), and 30.7% were overweight (BMI: ≥25.0 kg/m²). Most respondents did not have pre-specified disease conditions at the baseline survey, as per physician diagnosis.

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) [12]. 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 [10]. Another study in the U.K. showed that the incident rate of K/L grade ≥ 2 knee OA was 2.5% per year [11]. 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 [13]. However, the definitions of the cases varied among these studies.

The incidence of symptomatic knee OA was significantly higher in women, consistent with previous studies [10, 13, 19, 20]. This is likely due to women predisposed with higher bone mineral density [21]. Consistent with other studies, our analysis found that senior ages were associated with higher risk of symptomatic knee OA [3, 10, 13, 22]. 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 less heavy physical activities after the age of 70 years; thus, less heavy physical activities may reduce knee symptoms, which make the incidence of symptomatic knee OA decreased. 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, 23]. This is likely due to those receiving less education being more likely to be employment in physical labor [24]. Overweight/obesity was associated with an increased risk of incident symptomatic knee OA in our study, although it was not significant. Previous studies have shown that overweight/obesity was associated with an increased risk of both radiographic and symptomatic knee OA [15, 22, 25, 26].

In our study, those residing in rural areas or in West part of China had a higher risk of incident symptomatic knee OA. Meanwhile, the provinces with the highest incidence were mainly in the West region. These findings were consistent with previous cross-sectional results [9, 20, 27]. This may be explained by that 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 certain physical activities (e.g. dancing, body building) often had lower risk of incident symptomatic knee OA, which is similar to the findings of a review about OA [28]. As shown earlier, a light and moderate level of activity may be associated with less subsequent disabilities, such as knee OA [29]. 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 increased risk of incident symptomatic knee OA, as shown by our study. One set of meta-analyses results showed that hypertension was significantly associated with higher incidence of symptomatic knee OA [30]. 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 OA [15]. A higher risk of CVD has also been observed in people with OA [31]. 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 incidence of symptomatic knee OA is possible that kidney disease and digestive disease may be caused by medication use due to 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 [32, 33]. 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 reported both the incidence and associated risk factors for symptomatic knee OA, which is helpful for healthcare policy development and clinical practice.

Our study also has limitations. Firstly, the respondents in the CHARLS did not undergo radiographic assessment, and hence the diagnosis of symptomatic knee OA was based on self-reported knee pain and self-reported arthritis diagnosis by a physician, which differed from the diagnostic criteria used in other studies [34]. However, this definition provided the best available diagnosis for symptomatic knee OA in CHARLS and has been used in several published studies [9, 35]. Secondly, data for other chronic diseases were also collected on the basis of self-reporting. Hence, the associations, which we observed between chronic diseases and symptomatic knee OA, might be confounded by the increased contact with health care professionals in patients with long-term disease, which in turn leads to increased reception of a diagnosis of arthritis. However, our findings were generally consistent with previous studies [15, 30]. Thirdly, the question about OA diagnosis did not distinguish between OA and other arthritis (e.g. rheumatoid arthritis, gout et al.), which would lead to overestimating the incidence of OA. Nevertheless, in economically backward regions, the residents may less likely to visit physicians, which may result in underestimating the incidence of OA. Fourthly, there is 4 years of longitudinal data in our study, the results would be more reliable for a longer cohort. 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.

Using data from the CHARLS, we observed that the cumulative incidence of symptomatic knee OA among middle-aged or older Chinese adults was high, was even more common among females, and varied by province and region. Those adults not undertaking physical activities, or presenting with heart disease, kidney disease, or digestive diseases had a higher likelihood of developing incident symptomatic knee OA. Further research with more reliable diagnosis need to confirm our findings. The findings may be intriguing for health care, and may have important implications for practitioners and policy makers, particularly those from developing countries.