Associated risk factors of knee osteoarthritis: a population survey in Taiyuan, China

Knee osteoarthritis (KOA) is one of the most common disorders, and is responsible for higher morbidity, particularly in the second half of human life, during which the quality of life is particularly important. Therefore, there is a burden on health from both morbidity and cost. The International League of Associations for Rheumatology (ILAR)- China study¹ revealed that the prevalence of knee pain and KOA were significantly lower in the south than in the north of China. Furthermore, knee pain and KOA were twice more prevalent among residents in multi-storey buildings than that in single-level houses.¹ To identify the possibly associated risk factors of KOA, a population living in 6-storey buildings without elevators was surveyed from April to August 2005 in an urban district of Taiyuan, Shanxi Province, north China.

METHODS

Population characteristics
A population of 2188 adults aged 35－64 years, living in 6-story apartment buildings without elevators, was surveyed in an urban district of Taiyuan, Shanxi Province, north China.

Working team
The survey was conducted by the Department of Rheumatology, Shantou University Medical College in cooperation with the Department of Rheumatology, Second Hospital of Shanxi Medical University and the Chinese Journal of Rheumatology. The working team consisting of 10 primary health workers and 5 rheumatologists was headed by the first author. Standard training preceded the survey.

Epidemiological survey
The protocol of Asia Pacific League of Associations for Rheumatology (APLAR) Community Oriented Program for Control of Rheumatic Diseases (COPCORD) Core Questionnaire for Identification of Risk Factors for Knee Osteoarthritis was used. Printed sheet of information on the study was distributed to the target population prior to the survey. The primary health workers and rheumatologists visited and interviewed the subjects house by house in the evening. The positive respondents were examined simultaneously by rheumatologists. Lateral and anterior-posterior non-weight-bearing knee radiographs were read by 2 radiologists blindly and independently, and a consensus was reached. Suspected cases of KOA were recalled after the survey and examined by a rheumatologist to ensure international comparability of results.

Waist circumference (WC) was measured according to the World Health Organization (WHO) standard procedures.² Height and weight were also measured, and then the body mass index [BMI, body weight (kg) / height (m)²] was calculated.

Classification criteria
KOA was diagnosed according to the American College of Rheumatology (ACR) classification criteria.³ X-ray osteoarthritis was graded according to the Kellgren-Lawrence 0-IV Scoring System.⁴ Overweight was defined as BMI ≥ 24, and obesity BMI ≥ 28. Abdominal obesity was defined as WC ≥ 85 cm in men, and ≥ 80 cm in women.⁵

Statistical analysis
All data were analyzed with SPSS version 11.5 for Windows (SPSS Inc). Student's t test or one-way analysis of variance (ANOVA) was used to compare mean values, whereas the chi-square test was used to compare the prevalence. A P value < 0.05 was considered statistically significant. Variables such as sex, age, BMI, WC, education level, and smoking history, etc., were included in a binary logistic regression model for further analysis. Crude prevalence rates were adjusted to a standard Taiyuan population for age and sex constitution (nationwide consensus in 2000).⁶

RESULTS

Prevalence of knee pain
Of the 2188 subjects, 1049 were male and 1139 female. Among them, 313 (94 male and 219 female) with current and/or past knee pain made an adjusted rate of 13.6% for knee pain in total, 18.3% in women, and 8.7% in men. The prevalence rate of knee pain was significantly higher in women than in men [the chi-square test = 46.951, P < 0.001, OR = 2.418, 95% CI (1.868, 3.131)], and there was a tendency to increase with aging.

The prevalence of knee pain in the forth and second floor residents was the highest (15.2%), followed by the fifth (15.0%), first (13.3%), third (13.1%), and sixth floor (10.5%). However, there was no statistically significant difference among the different floors.

Knee radiographs
Of the 313 knee pain patients, 274 were suspected to suffer from KOA. Of the 274 patients with knee radiographs, 16 (5.8%) had grade 0, 14 (5.1%) had grade I, 232 (84.7%) had grade II, and 12 (4.4%) had grade III X-ray KOA. None reached grade IV X-ray KOA. Therefore, 244 patients were diagnosed as having KOA.

The osteophytes of patellofemoral joint (OPJ) and the osteophytes of tibiofemoral joint (OTJ) were the most common radiological signs of KOA in this population (Table 1). Both of these radiographic signs presented simultaneously in 124 (50.8%) of the 244 patients. Tibiofemoral joint space narrowing (TSN) was observed in 9.8% of the patients, and patellofemoral joint space narrowing (PSN) in 0.8%. There was no significant difference in radiological characteristics between male and female patients.

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Table 1. Radiological characteristics of osteoarthritis of the knee

Prevalence of knee osteoarthritis
The adjusted prevalence rate of KOA was 10.9%; and it was significantly higher in women than in men [15.1% versus 6.3%, the chi-square test = 42.551, P < 0.001, OR = 2.578, 95% CI (1.925, 3.454)]. There was also an increased tendency in prevalence of KOA with age. The prevalence was increased more obviously after the middle age, after 40 years old in women [the chi-square test = 16.523, P < 0.001, OR = 4.826, 95% CI (2.097, 11.107)], and after 45 years old in men [the chi-square test = 14.746, P < 0.001, OR = 3.164, 95% CI (1.708, 5.862)] (Table 2).

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Table 2. Prevalence of knee osteoarthritis in different age groups

The prevalence rate of KOA was the highest in the second floor (12.9%), and the lowest (9.3%) in the sixth floor. However, no statistically significant difference was seen among the different floors, and nor significant difference between the ages of different floors (Table 3).

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Table 3. Prevalence of knee osteoarthritis of different floors

Body mass index
BMI in KOA group was significantly higher than that in non-KOA group (25.0±3.1 versus 23.7±3.1, t = 6.091, P < 0.001). Subjects with overweight and obesity were more commonly seen in KOA group than in non-KOA group (Table 4).

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Table 4. Comparison of BMI in KOA and non-KOA groups

Waist circumference
The average WC of KOA group was significantly higher than that of non-KOA group [(85.5±9.3) cm versus (84.1±9.8) cm, t = 2.043, P = 0.041]. Abdominal obesity was more commonly seen in KOA group than in non-KOA group (Table 5).

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Table 5. A comparison of WC in KOA and non-KOA groups

Other factors
The time length of occupation service in KOA group was longer than in non-KOA group [(24.3±8.4) years versus (22.6±7.7) years, t = 3.198, P <0.001]. Education level in KOA group was lower than that in non-KOA group (the chi-square test = 31.063, P < 0.001, Table 6), and the difference was more significant between under and above junior high school [the chi-square test = 17.886, P < 0.001, OR = 1.842, 95% CI (1.383, 2.453)]. Smoking history was more significantly prevalent in KOA group than in non-KOA group [the chi-square test = 4.626, P = 0.031, OR = 0.606, 95% CI (0.383, 0.958)]. There was no statistically significant difference between KOA group and non-KOA group in the distribution of occupation, communication methods for going to work place, working conditions, main working posture, and religious belief.

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Table 6. Comparison of education level between KOA and non-KOA groups

Analysis of regression
Binary logistic regression was used to analyze the following variables on the development of KOA, such as sex, age, BMI, WC, length of service, education level, and smoking history. Age, sex, and BMI were significantly associated with KOA (Table 7).

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Table 7. Association of age, sex and BMI with KOA

　DISCUSSION

Geographic factors and knee osteoarthritis
Osteoarthritis of the knee and non-specific low back pain (NSLBP) is the most popular rheumatic disorders. In the Asia-Pacific region, the prevalence of KOA was 7.50% in China,⁷ 5.78% in rural India,⁸ 22.00%－28.00% in urban and 25.00% in the rural population of north Pakistan,⁹ and 10.20% in Bangladesh.¹⁰ KOA and NSLBP are incurable with currently available therapeutic options. The only way for reduction of the burden of the two disorders is prevention. For prevention of KOA, the etiologic or risk factor for this condition should be identified in time.

Our previous study has suggested that latitude might be one of the risk factors of KOA.¹¹ In the present study, the prevalence rate of knee pain in Taiyuan, was lower than that in Beijing (13.6% versus 38.7%),¹² but still higher than that in Shantou, south of China (13.6% versus 10.1%).¹¹ The prevalence rate of KOA was significantly higher than that in Shantou (10.9% versus 1.3%－3.2%),¹³ and was quite similar to that in Beijing (10.9% versus 9.6%).¹² Beijing, Taiyuan, and Shantou are situated at 40˚, 37.9˚ and 23˚ north latitude respectively, suggest- ing that latitude might be one of the risk factors of KOA.¹¹ Further studies are recommended to confirm whether the discrepancy of osteoarthritis prevalence due to the latitude may be only accused for the distinct climate. Epidemiological studies of geographical distribution of the adult osteoarthritis prevalence in 19 provinces of China showed that osteoarthritis was more common in epidemic area and circumjacent area of Kaschin-Beck's disease (KBD), but less common in southeast China.¹⁴ The results of our investigation were coincided to these findings. Further study should be done for elucidating the effect of geographic factors on the development of osteoarthritis.

Age, sex and knee osteoarthritis
In this study, the prevalence of KOA was increased with age (the Chi-square test = 83.114, P < 0.001). It was increased more significantly after 40 years old in women, and after 45 years old in men, suggesting that age is another risk factor of KOA.

The prevalence of KOA was reported to be significantly higher in women than that in men.^15,16 Our study showed that the prevalence of KOA in women was higher than that in men (15.1% versus 6.3%, OR = 2.578), indicating that sex is an important risk factor of KOA.

Body mass index and knee osteoarthritis
A strong correlation was found between high BMI and KOA,¹⁷ weight reductions could obviously decrease the incidence of KOA by 25%－50%. Our report showed that BMI in KOA group was significantly higher than that in non-KOA group (t = 6.091, P < 0.001). The prevalence rate of KOA was significantly higher in overweight cases than in normal-weight ones (OR = 2.141); this finding was quite similar to Soeroso's report.¹⁸

Ergonomic factors and knee osteoarthritis
In the ILAR-China study, the prevalence of both knee pain and KOA had a two-fold increase in residents in multi-story buildings without elevators compared with those living in single-level houses. Thus ergonomic factors might be one of the risk factors of KOA.¹

The National Health and Nutrition Examination Surveys (NHANES) showed that prevalence of KOA was less 0.1% in 25－34 years old people, and was 10%－20% in 65－74 years old people.¹⁹ For diminishing the interference of age, a general population sample of 35－64 years old living in 6-level apartment buildings for over 10 years without elevators was selected into this study. During this period they would have been presumed at low risk of KOA, and consequently would not have altered their accommodation because of knee problems. The results showed that there was no statistically significant difference among the different floors in prevalence of KOA (the Chi-square test = 5.178, P = 0.395).

Because no population sample living in single-level houses had been surveyed, we can not compare the prevalence of KOA between multi-story buildings and single-level houses in this study. However, the above-mentioned results implied that climbing stairs might not be an independent risk factor associated with KOA. A research indicated that low-impact physical activities, such as running and jumping, did not seem to be a risk factor of KOA, if there was no sustained trauma or injury.²⁰ Therefore, the repetitive movement of knee joint also did not seem to promote the development of KOA, but high- intensity occupation activities did.^21,22 Accordingly, climbing stairs may have the same effect on KOA, but it needs to be confirmed by more extensive studies.

Other risk factors
In the present research, there were more or less differences between KOA group and non-KOA group in WC, time length of occupation service, education level, and smoking history, but such factors showed no statistically significant correlation with KOA by multinomial logistic regression analyses. Further study is needed to explore the role of these factors.

This study showed that geography, aging (aged >40 years in woman and >45 years in man, respectively), female, and overweight (BMI ≥24) might be the associated risk factors of KOA, suggesting that the prevention of KOA should be initiated before the middle-age, particularly in the female. Controlling body weight and avoiding obesity are important in preventing KOA. Other factors such as climbing stairs, WC, time length of occupation service, education level, and smoking history might also be the factors contributing to the development of KOA, but further study is required to elucidate the role of these factors.

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