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.
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].
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).