The Korean National Health and Nutrition Examination Survey (KNHNES) is a national population-based study conducted by the Korean Centers for Disease Control and Prevention annually from 1998. The subjects are noninstitutionalized civilians who were randomly selected through stratified, multistage probability samples, which were based on age, sex, and residence area. This study includes questionnaires regarding health behavior and nutrition intake and health examinations such as body weight, height, blood pressure measurements, and blood tests. Specific health examinations are included according to the demand of national healthcare policies. Knee osteoarthritis examination and survey were performed in 2011.
A total of 10,589 population-based subjects were invited to participate in the 2011 survey, and 8518 agreed to participate, with a response rate of 80.4%. Of these, 1956 women aged 50 years and older were included. Knee osteoarthritis was defined radiographically as Kellgren-Lawrence grade ≥2[7, 29, 30], which was evaluated by two skeletal radiologists. A total of 830 women with knee osteoarthritis were selected, and exclusion criteria were use of osteoarthritis medication, presence of malignancies, and incomplete dataset. Finally, 564 women with knee osteoarthritis were analyzed after implementing the inclusion and exclusion criteria (Figure 1).
Written informed consent was obtained by the Korean Centers for Disease Control and Prevention from all participants. Approval from the ethical committee was exempted by the institutional review board at our hospital because this study utilized a publicly available database and did not have any potential violation of patient rights.
Data collection from the fifth KNHNES database
Demographic data including age, sex, body mass index (BMI), weight change in the past year (gain >10 kg, gain of 6–10 kg, gain of 3–6 kg, gain <3 kg, no change, loss <3 kg, loss of 3–6 kg, loss of 6–10 kg, and loss >10 kg) was collected. The parameters of height and weight were measured using standardized instruments, and BMI was calculated using the height and weight measurements. The percentage of weight change was calculated by dividing the weight change with body weight. Presence of malignant diseases and use of osteoarthritis medication were recorded using a health information questionnaire. The short form of the International Physical Activity Questionnaire was used to evaluate the subjects’ activity level, and weekly hours of vigorous-intensity, moderate-intensity, and walking activities were recorded.
Health-related quality of life was measured using the EuroQOL five-dimension (EQ-5D) index. The system comprises five dimensions including mobility, self-care, usual activities, pain/discomfort, and anxiety/depression[32, 33].
The knee osteoarthritis survey included severity of knee pain and radiographic examination. The severity of knee pain was evaluated using a 10-point numerical rating scale (NRS) (0=no pain, 10=severe pain). Knee X-rays were taken bilaterally with the subjects’ weight-bearing using an SD 3000 Synchro Stand (Accele Ray, SYFM Co., Seoul, South Korea), and the radiographic images were digitally stored. Severity of radiographic knee osteoarthritis was evaluated using the Kellgren-Lawrence grading system[7, 29, 30](grade 0, no features of osteoarthritis; grade 1, small osteophytes of uncertain significance; grade 2, definite osteophytes without impairment of joint space; grade 3, definite osteophytes with moderate joint space reduction; grade 4, definite osteophytes with substantial joint space narrowing and subchondral bone sclerosis). Presence of knee osteoarthritis was defined as Kellgren-Lawrence grades 2, 3, and 4[7, 29, 30]. Radiographic evaluation was performed by two radiologists, and the agreement between the two was 85.2% for 81 randomly selected radiographic images with an intraclass correlation coefficient of 0.767 (95% confidence interval, 0.659 to 0.844). If a disagreement in radiographic findings between the two radiologists occurred, the higher grade was adopted.
Data analysis and statistics
Women with knee osteoarthritis were categorized into three groups according to BMI measurements: those with BMI≤22.5 kg/m2, those with 22.5 kg/m2<BMI≤27.5 kg/m2, and those with BMI>27.5 kg/m2. Descriptive statistics included mean and standard deviation (SD) for continuous variables and proportion for categorical variables. Normal distribution of the data was tested using the Kolmogorov-Smirnov test. Means and frequency were compared using analysis of variance (ANOVA) test and chi-square test among the three groups, respectively. Correlation between the variables was analyzed using Pearson’s correlation coefficient or Spearman’s correlation coefficient according to data normality.
Multiple regression analysis was performed to identify the variables that significantly contributed to the level of knee pain (NRS) and health-related quality of life (EQ-5D), which were dependent variables. Candidate independent variables were selected and included in the regression model when the variable showed p<0.1 in the correlation tests. All statistical analyses were performed using SPSS version 20.0 (IBM Corp., Armonk, NY, USA), with statistical significance set at p<0.05.