There are 21 national parks and 583 trails in South Korea, and annual visitors to these trails have increased from 40,958,773 in 2012 to 46,931,809 in 2013. To support visitors accounting for the entire population of South Korea, 1,105 national park workers patrol the trails and perform disaster prevention, safety management, visitor guidance, and rescue duties [1]. National park workers perform their work in nature itself owing to the work characteristics and are exposed to various safety and health hazards. Particularly, because they climb trails that are not level and their duties include weeding, pruning, installing or repairing equipment, and transporting rescued people, the incidence of work-related musculoskeletal disease may be high. However, previous reports are limited to the incidence of musculoskeletal symptoms of national park visitors and mountain climbers [2, 3], and there is a lack of research about the musculoskeletal diseases of national park workers who are more consistently exposed to danger than visitors or climbers. In one study about the association between musculoskeletal subjective symptoms and the type of work [4], 61.7 % of national park workers were symptomatic, a higher proportion than that of automobile industry workers (37.8 %) or aircraft maintenance mechanics (25.8 %) [5, 6]. Furthermore, considering that in a study of male professional climbers, 46 % had subjective musculoskeletal symptoms [7], the incidence of musculoskeletal diseases is high in national park workers, who climb more often than professional climbers. Considering the incidence of musculoskeletal symptoms according to body part, it is highest in legs/knees (38.3 %), followed by shoulders (32.8 %), waist (25.8 %), hands/wrists/fingers (18.0 %), arms/elbows (13.3 %), and neck (12.5 %).

National park workers are always exposed to the risk of musculoskeletal diseases owing to the nature of their work, and particularly, the incidence of musculoskeletal symptoms in the knee area is high. However, there is a lack of evaluation regarding the association between specific work factors and musculoskeletal diseases using objective tests rather than complaints of subjective symptoms. This study investigated the prevalence of menisci lesions in knees, which had a high rate of subjective musculoskeletal symptoms among body parts, in national park workers using magnetic resonance imaging (MRI). Moreover, demographic factors as well as work factors related to the menisci lesions were investigated through a survey. The menisci play a pivotal role in knee functions such as load sharing, pressure absorption, contact stress reduction, and safety maintenance [8], and evaluation of the menisci is important in knee diseases caused by chronic loads on the knees [9]. Therefore, menisci evaluation would be helpful in studying musculoskeletal disease of knees in national park workers.

This study was designed to investigate the prevalence of the menisci lesions in national park workers and work factors affecting this prevalence.

In this study investigating the menisci lesions of national park workers based on the grading system described by Crues et al., 59.7 % of the workers had the grade 2 or higher in MRI signal of the menisci. In a study of 85 people aged 40 years or more evaluating the menisci lesions according to the same grading system as our study, the incidence of grade 2 or higher lesions was 19.8 % in subjects <60 years of age and 40.6 % in those ≥60 years of age. In another study of 115 participants from a general population between 13 and 78 years old, the incidence of grade 2 or higher lesions was 28.2 %. Therefore, we found that the prevalence of the menisci lesions is higher in national park workers than in the general population, perhaps because their walking distance is greater than that of the general population owing to the nature of their work; furthermore, their work involves walking repeatedly uphill and downhill rather than on the level.

In this study, the cumulative intensity of trekking, which accounts for the greater workload, rather than demographic factors, in national park workers, statistically significantly increased the risk of the menisci lesions in multivariate regression analysis considering both demographic and work-related factors. However, it was difficult to find an article analyzing the effect of trekking of national park workers or professional climbers on the menisci lesions, but the burden of walking on the menisci could be estimated by evaluating the association between walking and the menisci lesions in nonprofessional long-distance runners. In a study of 26 nonprofessional long-distance runners who had at least 10 years of running experience with no previous history of knee trauma or knee surgery, 23.1 % had grade 2 or higher menisci lesions [18]. The reason the risk of menisci lesions was higher in national park workers was supposedly frequent walking on uneven ground and change of trekking pace owing to repeated uphill and downhill walking. Trekking long-distance uneven trails particularly stresses the knees of workers. Trekking more than 2 miles (3.2 km) increases the menisci lesions by 1.8 times [19], and particularly, downhill trekking can result in great loads on the lower limbs including knees and consequently increase the incidence of knee diseases [20].

Downhill trekking can cause loading on the knee joint, and in a previous study, downhill trekking of an 11° slope increased loading on the knee more than level walking [21]. Also, Andriacchi et al. compared the peak flexion moments of the lower limbs in various activities of daily life and reported that walking down a staircase caused four times more moment on the knee joint than other parts of the body [22]. Winter et al. reported that the peak knee joint moment increased by five times in downhill compared with level walking [23]. In the present study also, trail difficulty considering the slope and surface condition of the ground was analyzed in addition to other factors. In model 2, which did not take trail difficulty into consideration, a high cumulative intensity of trekking increased the risk of menisci lesions, but it increased even further by 5.84 times in model 3, which considered trail difficulty. This indicates that not only long-distance trekking but also the trail difficulty, which includes repeated uphill and downhill trekking, caused loading on the knees in national park workers and is a risk factor for the menisci lesions.

The study of Coggon D et al. showed that kneeling could increase the risk of knee disease. In their study for 518 patients who were listed for surgical treatment of knee, repetitive kneeling increased knee joint disease by 1.8 times [24]. Also, in a study for 625 Swedish who were engaged in works requiring frequent kneeling positions such as farming, forestry, construction industry, the risk of knee joint disease increased by 2.1 times compared with the general population of the same age group [25]. However, in the present study, kneeling did not statistically significantly increase the risk of menisci lesions. It was considered that the increase of knee loading due to specific positions such as kneeling was not great because national park workers do not often perform their work in this kind of position and trekking for facility management as well as trekking information service accounted for greater proportion of their workload.

In previous studies, demographic factors such as age and body weight in addition to work-related factors were shown to affect the menisci lesions. Anderson and Felson reported that the risk of knee osteoarthritis was increased 1.76-fold in males and 3.29-fold in females every decade [26]. Furthermore, Jerosch et al. reported that degeneration increased with age according to an evaluation of the degree of degeneration for menisci using the knee MRI signal grading system (grades 0–4) in 82 people who had no knee symptoms in the age range of 8–62 years [27]. In univariate analysis in the present study, the menisci lesions were statistically significantly increased in workers aged ≥45 years, but in multivariate analysis, it was not statistically significant. Moreover, age was not a significant factor to increase the menisci lesions when age was classified by decade (30s, 40s, and 50s or above). Unlike other studies, the effect of age on the menisci lesions of national park workers is relatively low because chronic injuries occur mainly in those ≥65 years of age in the general population and degenerative changes become greater as age increases [28], but the proportion of older workers is low as the mean age of national park workers is 45.5 years and most of them retired at the age of 57.

The American Medical Association(AMA) has reported that overweight or obesity is also a strong predictor of the menisci lesions [29]. In the study of Baker et al., in subjects aged 25–59 with tears of menisci found on arthroscopy, the OR was 2.3 (95 % CI 1.5–3.4) for a BMI of 24.1–27.0 kg/m² and 1.7 (95 % CI 1.2–2.6) for a BMI of ≥27 kg/m² [30]. In a study of subjects who had previous surgery of menisci, there was a statistically significant positive correlation between BMI and menisci lesions [31]. In the present study, there was no significant difference in the incidence of the menisci lesions according to BMI because the number of subjects with a BMI of ≥25 kg/m² was small. Furthermore, we considered that obese workers did not participate as much in trekking owing to potential knee pain or attempted to control weight by themselves to reduce knee pain.

The menisci tears may be described by the factors such as location, site, and type of the tear. In this study, the medial menisci tear was the most part according to the location of menisci. In previous study as well, the medial menisci tear were more frequent than the lateral. Because the medial meniscus is ‘C’ shaped while the lateral is a shorter incomplete circle with closer spaced ‘horns’, but also since its more secure attachments to the tibia plateau make it susceptible to shear injury [32, 33].

Most menisci tear that occur in the young athletes is longitudinal. This tear is associated with ACL injury, and the medial menisci is more commonly affected for this tear [34]. However, in our study, the workers with longitudinal tears were not found. Radial tear is known to be more frequent in the lateral meniscus and can appear to be caused by athletic trauma [35]. While, in other study, radial tears occurring in the posterior horn of the medial meniscus were common [36]. The workers who had frequently trekked in our study could be damaged on the menisci by some trauma during the trekking, and the radial tear may occur to them. Horizontal tear is known as degenerative type and is more common in middle-aged people [37]. The posterior horn of the medial menisci is relatively no movable compared with other parts, and it’s weak to tear. Consequently, the horizontal tear of the posterior horn in the medial meniscus is the most emblematic of degenerative menisci tear [38]. They can occur in all age groups but increase with advancing years. On the other hand, the horizontal tear in the lateral menisci can occur to some runners [39]. Despite the average age of the subjects in our study was 44.3 years old and they retired from the KNPS in 55-59 years old, the ratio of horizontal menisci tear for them was the highest. We thought the menisci lesion by the long-period hikes similar to runners caused the tears.

This study has the following limitations. First of all, there was limitation to occur the bias in determining of the ambiguous MRI signal grade since the orthopedic surgeon directly carried out physical examinations to the subjects. Although the knee MRI was taken to the subjects with knee symptoms and positive signs in McMurray test or Apley test, the occurrence of bias in the readings of the reader who contacted to the workers directly was inevitable. In a further study, if the reader who does not contact to the subjects will read for the MRI signal grade, we think the study can increase the reliability of the readings.

Because this study was performed as a cross-sectional study, it was challenging to infer a causal association, and other results could be obtained given a different study period. Moreover, a prevalence-incidence bias could occur [40]. Additionally, because demographic and work-related factors were investigated using a self-administered survey, the intention of researchers and responders could be reflected. There was also a limitation of analytical results because 124 workers who underwent knee MRI were study subjects. A previous study evaluated not only the menisci lesions but also various diseases such as osteoarthritis of the knee, but our study did not evaluate factors related to knee diseases other than the menisci lesions. Another limitation could be that the statistically significant risk factor for the menisci lesions, the cumulative intensity of trekking, was estimated indirectly through trail distance as well as trail difficulty.

However, this was a unique study that was designed to objectively approach and evaluate the menisci lesions of workers in a specific environment, the national parks of Korea. Because various factors including demographic and work-related factors of national park workers were considered and analyzed, we could identify the strongest factor for increasing the risk of menisci lesions. Furthermore, in performing this study, reliability could be increased as an orthopedist visited workers individually, performed the physical examinations, and systematically classified the menisci lesions through the MRI high signals of the menisci. Regarding the evaluation of the cumulative intensity of trekking, not only trail distance but also trail difficulty such as slope angle and surface conditions were included and suggested that trail difficulty could affect the loading of the menisci.

Occupational musculoskeletal diseases occur owing to various factors in various types of work. Therefore, it is important to find and prevent risk factors at an early stage. The work-related factor that increased the menisci lesions in national park workers was the cumulative intensity of trekking in this study. Thus, to prevent the menisci lesions in national park workers, it is important to find a threshold of the cumulative intensity of trekking that causes injuries through a cohort study in the future. If workers who currently have no menisci lesions but a cumulative intensity of trekking that is greater than the threshold can be identified and advised to avoid trekking, the prevalence of menisci lesions of national park workers can be lowered.

The factor that most affected the menisci lesions among the workers in Korean national park was a high cumulative intensity of trekking. It is necessary to establish a threshold of the cumulative intensity of trekking for knee disease and notify the workers who exceed this threshold and transfer them to other places with less trekking.