Hypothesis
That occupation and occupational factors are associated with an increased risk of radiographic spondylolisthesis.
Participants
Under the approval of our institutional review board, the present study, entitled the Wakayama Spine Study (WSS), was performed with a sub-cohort of the Research on Osteoarthritis/Osteoporosis Against Disability (ROAD) study. The ROAD study was initiated as a nationwide, prospective study of bone and joint diseases in population-based cohorts. A detailed profile of the ROAD cohort has been previously reported [20,21]. Therefore, in brief, the ROAD study included 3,040 inhabitants (1,061 men and 1,979 women) aged 23-95 years recruited from resident registries in three communities. ROAD included an urban community, Itabashi-ku, but the WSS, which for convenience was the sampling frame for the current study, only included participants from the two rural communities near Wakayama: Hidakagawa and Taiji. Hidakagawa-cho, is a mountainous community located in the center of Wakayama, which had a population of 11,300/330 km2 with 29% of jobs in primary industries (agriculture, forestry, fishing and mining), 24% in the secondary industries (manufacturing and construction), and 47% of jobs in the service industry. Taiji-cho, is a seacoast community located south of Wakayama, with a population of 3,500/6 km2 . In comparison with the above, 13% of the Taiji population work in the primary industries, 18% in the secondary industries and 69% work in the service industry. The ROAD study team made a second visit to Hidakagawa and Taiji between 2008 and 2010. Of the inhabitants who participated in this second visit, 1,063 volunteers were recruited for MRI. Fifty-two of these declined to attend the examination, and the remaining 1,011 were registered in the Wakayama Spine Study. All participants provided their written, informed consent for the MRI examination. Participants who had sensitive implanted devices (such as a pacemaker) or other disqualifiers were excluded. In total, 977 participants underwent lumbar spine MRI. Ten participants who had undergone a previous lumbar operation were excluded, and 29 participants who were younger than 40 years were excluded. All participants in the WSS were invited to complete an interviewer-administered questionnaire which included 400 questions about demographic factors, lifestyle factors, occupation, and occupational exposures and underwent lumbar spine radiographs and anthropometric measurements. Everybody was eligible to participate, regardless of age, gender and symptoms at baseline, providing that they could give written, informed consent and were able to complete the questionnaire and undergo spinal radiography (pregnant women were excluded). Complete radiographic and occupational data were available for 722 participants (245 males, 477 females), mean age 70.9 years, range: 53-93 years.
The study was approved by the ethics committees of the University of Tokyo and the Tokyo Metropolitan Institute of Gerontology.
Occupation and Occupational activities
A lifetime occupational history was collected alongside details of 7 types of specific work exposures: sitting on a chair for ≥2 hours/day; standing for ≥2 hours/day; kneeling for ≥1 hour/day; squatting for ≥1 hour/day; driving for ≥4 hours/day; walking ≥3 km/day; going up and down stairs ≥30 floors/day; climbing up slopes or steps for ≥1 hour/day and; lifting loads weighing ≥10 kg at least once a week. For the current study, the information on occupational title and exposures was derived from the respondent’s principal occupation (that in which the participant had worked for the longest duration). For comparison, occupations were grouped according to the nature of work as follows: Clerical/technical; agricultural / fishermen; factory/construction; clinical / housekeepers / shop workers / hairdressers / dressmakers; teachers and “other” (for all remaining types of work).
Assessment of lumbar spondylolisthesis
Lumbar spine radiographs were performed according to a standardised protocol to include the intervertebral levels from L1-L2 to L5-S1. Anteroposterior and lateral radiographs of the lumbar spine were acquired with patients in a standing position. The radiographs were all read without the knowledge of participant symptoms, occupational exposures or function by one experienced orthopaedic surgeon (YI). In line with other epidemiological studies of radiographic spondylolisthesis [22-24], the %slip was calculated as the distance of sagittal translation between adjacent vertebral endplates. A patient was defined with spondylolisthesis if they had a slip ≥5% anteriorly or posteriorly at any lumbar level on the lateral views [22-24].
Statistical analysis
Participants’ demographic and lifestyle characteristics were summarized using means (SDs) where normally distributed and medians (inter-quartile ranges, IQRs) when not and counts (%) separately for those with spondylolisthesis (cases) and those without (controls). Differences in categorical and continuous characteristics between cases and controls were assessed using chi-squared and t-tests, respectively. The effects of type of occupation (using clerical/technical experts as a baseline category), and occupational activities on spondylolisthesis were assessed using logistic regression modelling, before and after adjusting for demographic characteristics, and were summarized by odds ratios (ORs) and 95% confidence intervals (CIs).
As the main focus of this study was to explore the association between occupational factors and spondylolisthesis, and many of the older participants had stopped working as much as 20-30 years prior to their X-ray, we repeated the analyses separately for those <75 and ≥75 years of age, allowing a decade after retirement. Statistical analyses were performed using Stata V.12.1 (StataCorp, College Station, Texas, USA).