Study population
The study subjects were 698 workers whose tenure was longer than 7 years among 1,105 workers who worked in 20 Korean national parks in 2014 (excluding Hallasan National Park managed by Jeju Special Autonomous Province). An orthopedist visited each national park and performed history taking and physical examinations. The subjects with knee symptoms and positive signs in McMurray test or Apley test were taken knee MRI scan. The knee MRI was performed in 124 national park workers.
All study subjects were informed of the purpose of this study. The subjects were consented to participate under the 'Ethics, consent & permissions' heading and to publish to report individual patient data. This study was approved by the Ethics Committee of Wonjin Institute for Occupational and Environmental Health. The reference number is IRB-2016-001.
Study methods
Questionnaire survey
Demographic information such as sex, age, body mass index (BMI), smoking, alcohol drinking, regular exercise, and leisure activity as well as work-related factors such as tenure, overtime labor, holiday use, and the trekking times were investigated through a survey. Subjects were also asked whether they performed stair flight and kneeling movements within the past month and about previous knee injuries, previous knee pain, and past medical history.
High-signal grading of the menisci on knee MRI
Knee MRI was performed on 124 workers from November 2014 to February 2015. The MRI was taken to the workers with knee symptoms and positive physical examination. This study used 1.5 Tesla MRI (Philips MRI system, Achieva, Netherlands) and a knee joint coil, and measurement was conducted in the supine position with knee joint extension. The workers were examined using a standard six-sequence MRI protocol (axial, coronal, and sagittal T2-weighted Turbo Spin Echo (TSE) sequence; coronal proton density-weighted spectral presaturation with inversion recovery sequence; and coronal and sagittal proton density-weighted TSE sequence).
An orthopedist and a radiologist respectively read these images of the menisci of the knee joint using a grading system (grade 0–3) based on the MRI signals described by Crues et al. [10]: grade 0, no high signal in the menisci; grade 1, irregular type of high signal in the menisci; grade 2, linear type of high signal in the menisci; and grade 3, extension of high signal to the margin of the menisci. If the MRI scan area contained both knees, we used the result of the menisci with higher MRI signal grade as the dependent variable in the readings on the knees of the subjects.
Cohen’s kappa coefficient was calculated to evaluate consistency between readers. According to Viera AJ et al., a kappa coefficient of 0.61–0.80 means substantial agreement, indicating high consistency between two readers [11]. The kappa coefficient of the results read by two readers in this study was 0.622, corresponding to substantial agreement. If some knee MRI signal grade by the two readers did not match each other, the final judgment for the grade was determined under the agreement of them.
The grade 0 is no MRI high signal. The grade 1 is irregular MRI signal change in the menisci, not linear transformation [12]. In the case of grade 1, even if patients have symptoms on their knees, some surgery isn’t required. The diffuse degenerative process for this grade 1 is mainly observed in the arthroscopy as well. The study of Crues et al. classified the grade 1-2 in MRI signal of menisci as no torn, and the grade 3 was only as torn. Of course most radiologists interpret grade 3 as a tear of menisci, and more than 90 % of grade 3 signal require a procedure or surgery [13]. To mention of grade 2, however, the grade with knee symptoms can be observed the intrasubstance tear in the arthroscopy may require some surgery of menisci [14]. Moreover, in approximately 10 % of grade 2 signal, some tear of menisci can be seen on arthroscopy, which tends to be underestimated as it is usually on the posterior horn of the medial menisci [15, 16]. Accordingly, we judged that the classification between the grade 1 and grade 2 should be separated.
Finally, the grade 0 without MRI high signal and the grade 1 with irregular MRI signal change which may have diffuse degenerative process through arthroscopy were categorized to the same group. Meanwhile, the grade 2-3 with ‘the menisci tear’ in arthroscopy was classified as the other group, which may require surgery.
Classification according to location, site, and type of menisci tear
We investigated the classification of menisci tear according to location (medial and lateral), site (anterior horn, body, and posterior horn), type (longitudinal, horizontal, radial, and complex), and other factors (discoid menisci and root tear) of the tear among the subjects. If the location and the site of menisci tear were multiple regions, we only described and classified for the region that had more severe damage in the menisci. It may need detail descriptions for the menisci tear in order to find out the cause of occurrence of the tear.
Calculation of the cumulative intensity of trekking
To calculate the cumulative intensity of trekking of national park workers, the mean trail distance (km), the difficulty of the trail, the tenure at each national park (months), and the number of treks per month for each worker from the start of work until the present were investigated. The Korea National Park Service (KNPS) investigated the degree of the slope, width, distance, and road surface conditions for 1700 km of trails in national parks from 2011 to 2013 using Global Positioning System (GPS) measurements [17]. Based on this information, this study estimated the mean trail distance and the difficulty of the trails of the affiliated national parks. Also, information about the workplace and tenure (years) of workers was provided by the KNPS, and the mean trekking times per month in the corresponding parks were investigated through a self-administered survey.
Based on the above data, the intensity of trekking was defined in three ways as shown in Fig. 1. The first method defined the intensity of trekking as the worker’s tenure at the affiliated national park, whereas the other two methods also took into consideration other factors such as distance and difficulty of trails. The intensity of trekking was calculated using the three methods for each national park, and the intensity of trekking of national parks workers from the beginning of their employment until 2014 was summed to calculate the cumulative intensity of trekking.
The cumulative intensity of trekking 1 was divided into two groups based on the tenure 17 years. 17 years was the average tenure of the workers in KNPS. Each cumulative intensity values of trekking 2 and 3 were divided into three parts by the two points on the basis of 33 % and 66 % of the distribution and they were classified into three groups: low, medium, high.
Statistical analysis
First, we performed Chi-square tests to see if there were differences in the menisci lesions grade according to general characteristics (sex, age, BMI, smoking, alcohol drinking, regular exercise, previous knee injury, previous knee pain, and past medical history), occupational characteristics (overtime labor, holiday use, physical burden of work, and stair flight and kneeling movements), the classification of menisci tears, and the three ways of calculating the cumulative intensity of trekking.
Second, we evaluated the variables used in the Chi-square test using simple logistic regression analysis, and the crude odds ratio (OR) as well as 95 % confidence interval (CI) of the menisci lesions were calculated.
Finally, adjusted ORs and 95 % CIs were calculated using multivariate logistic regression analysis after establishing three different models according to the adjusted variables: model 1, adjusted for general and occupational characteristics and tenure; model 2, adjusted for general and occupational characteristics and the cumulative intensity of trekking 2; and model 3, adjusted for general and occupational characteristics and the cumulative intensity of trekking 3.
The significance level was set to <0.05.