We examined 186 military helicopter pilots who had undergone annual pilot physical examinations at a military hospital and 94 military clerical workers who had also received annual general physical examinations. The helicopter pilots were exposed to WBV, and 0.76 m/s² was the mean overall level of exposure to WBV per pilot according to the ISO 2631-1 method of International Organization for Standardization [24].

The helicopter pilots examined in this study regularly wore standard flight helmets of Korea Air Force during the day. During night operations they may also have had night vision goggles attached to their helmets. Pilots fly 12 hours per week during the daytime and three hours per week during the nighttime.

Unlike helicopter pilots, clerical workers are not exposed to WBV. The military clerical workers examined in this study otherwise worked in the same workplaces as the exposed group and there were few differences in the office working conditions between the two groups. Considering the similarity of the working conditions in both groups, except for exposure to WBV, and in order to reduce the bias due to the healthy worker effect, we decided to use military clerical workers as a control group. The clerical workers each worked eight hours per day typing and correcting official documents using computers.

Basic demographic variables and past disease histories of the subjects were surveyed using written questionnaires and interviews that were given during the medical examinations. Among the survey subjects, all who commuted long distances by car, were exposed to WBV while enjoying hobbies like motorcycle riding, mountain terrain bicycle riding, off road driving or had suffered any injury or disease that could trigger musculoskeletal diseases like ligament injury of knee and ankle joint, spondylolisthesis or scoliosis, were excluded. Questionnaires and interviews were completed for 164 of the 186 helicopter pilots (response rate, 88.2%) and 88 of the 94 clerical workers (response rate, 93.6%).

The questionnaire consisted of items relating to general and work-related characteristics. Information was obtained regarding age, height, weight, smoking, alcohol consumption (any alcoholic beverages more than once a week or less than once a week), and exercise (exercise more than three times per week for more than 1 hour each time, or not). Participant body mass indexes (BMI) were categorized as normal (<23 kg/m²), overweight (≥23, <25 kg/m²), or obese (≥25 kg/m²) according to the Korean Society for the Study of Obesity standards for Koreans [25]. Questions concerning work-related characteristics included length of service, accumulated flight hours, and average flight hours per day.

Since radiographic examinations of the cervical and the lumbar spines were not included in the annual health examinations of either the helicopter pilots or the clerical workers, radiographic examinations of the cervical and the lumber spines were performed after obtaining informed consent from the study subjects. Four views of the cervical and lumbar spines (anteroposterior, lateral, and both obliques) of each subject were obtained. All radiographs were independently reviewed by two radiologists. To reduce bias, the radiologists were given no information relating to the subjects' occupations. Degenerative changes of the lumbar spine were defined when there were severe disc height changes (<25% of adjacent discs), any osteophyte formation or any endplate sclerosis [26]. Degenerative changes of the cervical spines were defined when there were any abnormalities such as irregular odontoid processes, suggestions of narrowed foramina, hypertrophic or subluxated facets, lipping and/or narrowing of disk spaces, or any other osteophytic or ankylosis findings of the cervical spine [27]. Severe vertebral problems such as spondylolysis, spondylolisthesis, kyphosis, and scoliosis were excluded from the degenerative changes considered in this study.

To determine the significance of any differences between the exposed and control groups in general and work-related characteristics and radiologic findings, the chi-square test was used for discrete variables and Student t-test was used for continuous variables. To identify factors affecting degenerative changes in the cervical and lumbar spines of the two groups, a logistic regression analysis was used to obtain crude odds ratios. Flight hours, flight frequency, service length, and age were set as each variable because they are correlated with each other. Adjusted BMI, drinking, smoking, and exercise were set as common variables. Multivariate logistic regression analyses were also used to correct for the effects of age, length of service (<10 years, ≥10 years), flight hours, and flight frequency.

Accumulated flight hours were used for the statistical analysis because it was impossible to directly measure accumulated WBV. The accumulated flight hours and flight frequency were analyzed and expressed in units of 100 hours and 100 times, respectively. Statistical analyses were based on a significance level of 0.05.