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To investigate risk factors for low-back pain (LBP) presenting for magnetic resonance imaging (MRI), with special focus on whole-body vibration (WBV).
A case-control approach was used. The study population comprised working-aged subjects from a catchment area for radiology services. Cases were a consecutive series referred for a lumbar MRI because of LBP. Controls were age- sex-matched subjects X-rayed for other reasons. Subjects were questioned about physical factors loading the spine, psychosocial factors, driving, personal characteristics, mental health, and certain beliefs about LBP. Exposure to WBV was assessed by six measures, including weekly duration of professional driving, hours driven at a spell, and current r.m.s. A(8). Associations with WBV were examined with adjustment for age, sex, and other potential confounders.
Altogether, 252 cases and 820 controls were studied, including 185 professional drivers. Strong associations were found with poor mental health and belief in work as a causal factor for LBP, and with occupational sitting for ≥3 hours while not driving. Associations were also seen with taller stature, consulting propensity, BMI, smoking history, fear-avoidance beliefs, frequent twisting, low decision latitude and low support at work. However, associations with the six metrics of WBV were weak and not statistically significant, and no exposure-response relationships were found.
We found little evidence of a risk from professional driving or WBV. Drivers were substantially less heavily exposed to WBV than in some earlier surveys. Nonetheless, it seems that at the population level, WBV is not an important cause of LBP referred for MRI.
Occupational exposure to whole-body vibration (WBV) is very common in working populations. One British survey estimated that 7.2 million men and 1.8 million women were exposed to WBV at work in a one-week period, with an estimated vibration dose value (eVDV) exceeding a proposed British Standard action level of 15 ms−1.75 in some 374,000 men and 9,000 women.(1) Major exposures arose among lift truck drivers, mechanical truck drivers, farm workers, and drivers of other off-road vehicles, and surveys in such heavily exposed working groups have pointed to a relation between WBV and back disorders.(2-8)
However, exposures of lower vibration magnitude, such as those arising from cars, vans, buses, coaches and motorcycles, are numerically far more common in the general population (in Britain, for example, there are an estimated five million occupational users of cars and vans(1)); so evidence on risk from such sources is particularly relevant in assessing the population burden of low-back pain (LBP) attributable to WBV.
Relatively few studies have been conducted in which the predominant exposure is to common population sources of low-level exposure to WBV. However, two case-control studies by Kelsey et al suggested an elevated risk of surgically managed prolapsed intervertebral disc in truck(9) and car drivers,(10) while in a large survey of commercial travellers by Pietri et al,(11) a relation was found both between weekly hours of driving and 12 month period prevalence and cumulative incidence of LBP. An increased prevalence of LBP has also been reported among urban bus drivers when compared with maintenance workers.(12) By contrast, a community-based cross-sectional survey found little association between WBV and LBP, and estimated the overall risk and attributable burden of disease arising from WBV in British workers to be relatively small compared with lifting.(13)
In the present study we gauged the public health impact of WBV on back disorders severe enough to be referred for specialist investigation by nuclear magnetic resonance imaging (MRI) of the lumbar spine, and the extent to which such referrals were associated with WBV and professional driving as compared with other recognised risk factors for LBP.
A case-control approach was used. The study population comprised all adults aged 20-64 years resident (as defined by certain postcodes) in the catchment area served by the radiology services of the main public hospital in Southampton. Cases were a consecutive series of patients from the study population referred for MRI of the lumbar spine during 2003-2006 to the radiology department at the public hospital, or to either of two local private hospitals. Patients who had imaging because of external trauma or non-mechanical pathology (e.g. cancer, metabolic bone disease, infections, congenital disorders) were excluded, as were patients with previous back surgery. Controls were subjects who attended the emergency department of the public hospital and were X-rayed over the same period. They were identified according to a pre-defined algorithm. Eligible controls fulfilled the same residency requirement as cases and were group matched to them by sex and five-year age bands; subjects who had radiographs following a road traffic accident or who had ever had a scan or surgery to their back were excluded. Potential cases and controls were identified from the patient records of participating radiology departments.
Subjects were mailed a questionnaire and a single reminder as necessary after four weeks. Cases were asked about their history of LBP and sciatica, and their recent disability was assessed using the Roland-Morris questionnaire.(14) - Cases and controls also completed a common question set on: current or more recent job and other occupations held for more than 12 months; physical and psychosocial risk factors at work; professional driving and exposure to WBV (vehicle types, duration and intensity); personal characteristics (e.g. height, weight, age, sex, smoking habits); mental health (low mood, somatising tendency); fear-avoidance beliefs; beliefs regarding work as a cause or aggravation of LBP; and propensity to consult over LBP.
Various standard instruments of inquiry were used. Low mood was assessed using the Mental Health Section of the SF-36 (SF-36 MH),(15) with subjects categorised into bands (best, intermediate, worst) according to approximate thirds of the distribution of scores across all subjects. Somatising tendency was assessed using elements of the Brief Symptom Inventory,(16) a validated self-reported measure of distress, comprising items on bothersome nausea, faintness or dizziness, chest pain and breathing difficulties during the past 7 days. The number of symptoms reported as ‘extremely’, ‘quite a bit’ or ‘moderately’ distressing were summed, with the data analysed in three bands (0, 1, ≥2 distressing symptoms). Fear-avoidance beliefs were assessed using elements of the validated Fear-Avoidance Beliefs scale of Waddell et al,(17) a sum being made of the number of statements with which the respondent agreed (0, 1-2, 3-4). Three questions were also asked concerning beliefs about work as a cause or aggravation of LBP and two on attitudes to consulting (whether it was important to see the doctor straightaway at the first sign of trouble, whether neglecting problems of this kind could lead to permanent health problems);(18) in each case a sum was made of the number of items of agreement. Questions on occupational psychosocial risk factors were based on the Karasek model,(19) with subjects classified according to decision latitude (three bands) and support (three bands), as well as self-reported job satisfaction (two bands). Subjects were categorised by age in three bands and by height in rough thirds of the overall distribution by sex; and a count was made of the number of anatomical sites outside the back (knees, hips, shoulders, neck, wrist/hand, elbows) with pain lasting ≥1 day in the past four weeks (coded as 0, 1-2, 3-6). A series of questions were also asked about exposure in the current or most recent job to digging, lifting ≥10 kg (times/day), bending the trunk (times/day), twisting (times/day), standing (hours/day), sitting while not driving (hours/day) and unloading a vehicle by hand.
Finally, occupational exposure to WBV in the current or most recent job was assessed according to six metrics: (1) professional driving for ≥1 hour/day; (2) professional driving ≥3 hours at a spell; (3) average weekly hours driven for the most common source of exposure (in three bands); (4) average weekly hours driven for all exposure sources (in five bands); (5) maximum r.m.s. of any machine (three bands: 0, − 0.5, ≥0.6 ms−2) and (6) current A(8) r.m.s. (0, >0 - <0.5, 0.5 - <1.15, ≥1.15 ms−2, where 0.5 ms−2 represents the daily exposure action level and 1.15 ms−2 the daily exposure limit value in the European Directive 2002/44/EC on mechanical vibration(20)). To establish these last two metrics, questions were asked about the time driven in a typical week for each of a pre-defined list of vehicles, as well as for an open category. Externally acquired estimates of vibration magnitude (the vertical, z-axis, frequency-weighted acceleration on the seat in accord with BS 6841 (1987) or ISO 2631 (1997)) for the various commonly reported exposure sources(1) were then applied, and dose measures estimated according to a standard methodology agreed by the VIBRISKS European Research Consortium.(21)
Although not a feature of this report, all cases had images of the lumbosacral spine taken according to routine departmental practice. Ethical approval was given by the NHS Southampton and South West Hampshire Local Research Ethics Committee.
Analysis was restricted to cases whose present episode of LBP came on in their current or most recent job and to controls who gave a current or most recent job history. It focussed on two main outcome groups – a) all cases and b) cases with a Roland Morris Score >10 (the median value for all cases) versus controls. Associations with each outcome were explored by logistic regression and expressed as adjusted odds ratios (OR) with associated 95% confidence intervals (95% CI). Separate models were constructed in relation to personal risk factors, occupational risk factors other than WBV, and professional driving and WBV exposures. All models were adjusted for age and sex (as factors of group matching and recruitment). Subsequently, for each outcome-control comparison, a stepwise forward selection regression model was fitted with age and sex constrained to be included in the model and the significance level for inclusion of other variables (personal, occupational and WBV-related) set at P<0.20.
Finally, as some 113 cases were recruited from the private hospitals (targeting full case ascertainment) but no controls came from these centres (since accidents and emergencies in the study population were only treated at the public hospital), we explored possible selection bias by re-running the analyses with exclusion of private cases.
All analyses were performed using Stata 10.0 software.
Altogether, 758 cases and 2,306 controls were approached. Usable replies were received from 393 (52%) of the cases and 980 (43%) of the controls. The major reason for non-response was failure to return a questionnaire, but other reasons included moving away (7 cases and 38 controls), postal errors (1 case and two controls), serious concomitant illness (1 case) and mental handicap (5 controls). Among the remaining 393 cases, four were excluded because they did not confirm LBP on questionnaire and seven were ineligible because of previous surgery to the back; while among the 980 controls, 97 were excluded because they had previously had either a scan or surgery to the back. A further 108 cases were excluded because their LBP began before their current/most recent job, 1 case and 18 controls had never held a job, 13 cases and 45 controls gave an incomplete occupational history, and 8 cases gave incomplete information concerning the timing of LBP onset and current/most recent job start/stop dates. Thus, a total of 252 cases and 820 controls were finally included in the occupational analyses.
Table 1 summarises the characteristics of subjects included in the analyses of personal and occupational risk factors. Controls had most commonly attended for X-rays to the wrist-hand and ankle-foot. They were well matched to the cases by age and gender. They had also started their current or most recent job at a similar interval before scan or X-ray (median interquartile range (IQR): cases, 8.7 (4.3 to 16.3) years; controls, 6.9 (2.9 to 16.5) years).
Among the cases, the median duration of the current episode of LBP (defined as the interval since last free of pain for as much as a month) was 1.0 (IQR 0.5-2.2) years and 79% reported sciatica (pain spreading down the leg to below the knee or causing distal neurological symptoms). The median Roland Morris score for the past four weeks was 10 (IQR 5-16). Altogether, 68% of cases reported taking at least two weeks off work in the last year because of symptoms.
Table 2 compares the distribution of demographic and personal characteristics in cases and controls. Associations were found with tall stature, somatising tendency, poor SF-36 MH score and propensity to consult over LBP. Additionally, among cases with a Roland Morris Score >10, there were associations with BMI, current smoking status, and fear-avoidance beliefs, while associations with poor SF-36 MH and somatising tendency were strengthened. Thus, for example, the OR vs. controls was raised in the worst vs. best band of SF36 MH score, by 1.7-fold among cases as whole and by 4.8-fold in severe cases.
Table 3 records the associations with occupational risk factors other than WBV, with adjustment for age and sex. Associations were found with twisting, sitting while not driving, job support, belief in work as a cause of LBP and lack of decision latitude. For the most part associations were strengthened when analysis focussed on severe cases. Thus, the OR for twisting (>20 times/day vs. not at all) was 1.4 in the all cases vs. controls comparison but 2.2 in Roland Morris Score >10 vs. controls.
The study included 185 professional drivers (42 cases and 143 controls), and of these 164 reported driving a single vehicle occupationally. The predominant exposure was to cars (115 reports), there being also 25 lorry drivers, eight bus drivers, seven drivers of forklift trucks, six ambulance drivers, two drivers of loaders, and one tractor driver. The median weekly exposure time for drivers was 16 hours (IQR 10-30 hours), and the estimated median A(8) was 0.79 (IQR 0.56-1.24) ms−2 r.m.s..
Few positive associations were seen between the six metrics of whole-body vibration and the two case outcomes. Table 4 presents effect estimates for each exposure definition with adjustment for age and sex. In the comparison of severe cases vs. controls (but not in all cases vs. controls), professional driving for ≥3 hours at a time was associated with a higher odds of LBP (1.3); and increases in ORs were found in the bands with A(8) ≥0.5–1.15 vs. 0 ms−2 (OR 1.3) and regular driving for <15 hours per week. But no finding was significant at the 5% level and no exposure metric showed an exposure-response pattern.
Table 5 shows the final models selected by stepwise regression. In the all cases vs. controls comparison, positive associations with being tall, in poor mental health, and having a propensity to consult over BP were confirmed, while associations with occupational risk factors (twisting, sitting but not driving, belief about work as a cause of BP) tended to strengthen. Among severe cases, the association with somatising tendency was somewhat weaker than among cases as a whole, while those with sitting but not driving and beliefs about work as a cause of BP were stronger. Associations with sitting but not driving were noteworthy in all comparisons (OR 3.4 to 3.7). Relative to Table 4, stronger but non-significant associations were found with professional driving for >3 hours at a spell (OR 1.8 to 2.3), but no dose-response pattern was found in relation to hours driven per week, and the A(8) exposure metric was not retained in either model.
When we repeated the analysis after excluding cases from the private hospitals (n= 113), the final stepwise regression models for all cases vs. controls and severe cases vs. controls showed positive associations with frequent lifting (ORs 1.5 and 1.8 respectively, for >10 vs. 0 lifts/day), frequent twisting (ORs 2.9 and 2.8, for >20 vs. 0 twists/day), sitting but not driving (ORs 2.4 and 2.4, for >3 vs. <1 hrs/day), beliefs about work as a cause of LBP (ORs 2.2 and 3.9, for 3 vs. 0 beliefs), and propensity to consult over LBP (ORs 2.6 and 2.0, for 2 vs. 0 statements agreed with). In addition there was an association with somatising tendency for all cases vs. controls (OR 1.7 for a score of ≥2 vs. 0) and with SF-36 MH score for severe cases vs. controls (OR 3.9 for worst vs. best category of score). However, no vibration or driving metrics were retained in the two finally selected stepwise models.
As judged by these findings there are positive associations between LBP referred for imaging of the lumbar spine and low mood, somatising tendency, certain beliefs about LBP and consulting propensity, as well as with being tall, smoking, and work involving frequent or prolonged twisting, sitting while not driving, and low decision latitude. Beyond these findings, which are supported by a broader research literature, we found only very limited evidence of a risk from exposure to professional driving and WBV, and none for exposures estimated to exceed the daily exposure limit value in the Physical Agents Directive of the European Union.
In weighing the findings a number of limitations need to be considered. Response was incomplete. However, this would be a source of bias in relation to questions about WBV only if non-responders had different associations with professional driving from responders, and we have no reason to expect this.
A more important limitation is that, among cases, the exposure history came after the occurrence of LBP. The relevant exposures are those that precede onset of symptoms, but the most reliable and complete information came from the most recent or currently held job. Bias could arise if workers with LBP developed symptoms in driving jobs but then moved to work with lesser exposure because of symptoms (unhealthy worker selection bias). Assessing this bias is challenging in practice, as LBP often begins early on in adulthood,(22) sometimes before employment begins,(23) and then runs a relapsing and recurrent course. Defining an exposure that predates symptoms may seem arbitrary, while the distinction between WBV as an initiating factor as compared with a factor of aggravation is also not straightforward. A censoring of recent exposure experience for cases would need to be mirrored by a censoring for controls. In practice, we focussed on the current episode of LBP and when this began, and limited analysis to cases whose symptoms began in the current or most recent job, comparing their exposure to controls reporting a current/recent job. While it remains possible that some drivers had reduced their exposure but remained within the same job, we consider the scope for this to be more limited than for a change of occupation; and no such selection was evident in relation, say, to occupational twisting.
Assessing exposures after the event has the potential also to inflate some risk estimates through reverse causation. Thus, low mood could arise as a consequence of severe LBP rather than causing it. However, it seems implausible that some exposures with positive associations could be influenced in this way – e.g. twisting, height and less likely for others – e.g. tendency to somatise.
Recruitment through secondary medical care raises the possibility that professional drivers might have sought care, or been referred, less readily for MRI investigation than other occupational groups, such as manual labourers. This could lead to under-representation of professional drivers among the cases, while not altering the general conclusion that WBV does not appear to be an important cause of back-pain cases seen by MRI services.
The hospitals from which the cases were recruited were the only institutions in the study area that provided MRI scans for LBP, and the identification of eligible cases from the study population should therefore have been near complete. Similarly, the hospital from which the controls were recruited was the only provider of accident and emergency services in the study area, and this should have helped to ensure that the control group was representative in terms of relevant exposures. Bias could have occurred, however, if workers with LBP who were in white-collar jobs had more ready access to private medical care, and were therefore more likely to be investigated by MRI scanning than those from manual occupations. To explore this possibility, we conducted a sensitivity analysis, in which we examined the impact of excluding cases who had been identified from the private hospitals, but we found no major impact on the findings relating to driving and WBV.
A further challenge lies in the assessment of exposures to WBV. Estimates of dose relied on self-assessed exposure times and imputed values of vibration magnitude from external field observations. Non-differential errors could bias associations towards the null. However, there is evidence, that professional drivers make a reasonably accurate assessment of their exposure times.(24) Moreover, it seems unlikely that there would be much misclassification of an exposure metric such as professional driving for ≥ 3 hours at a time. We focussed exposure assessment on the current or most recent job, where reporting is likely to be more complete and reliable, and in doing so did not consider total lifetime hours or total lifetime dose of vibration, which (while difficult to measure reliably) may bear a different relation to LBP risk.
Finally, in assessing the null finding in relation to WBV the possibility of uncontrolled confounding should be borne in mind. Although we have no direct evidence to this effect, a factor that protected against BP in the controls, or a greater exposure to WBV from non-occupational sources among controls, might serve to obscure associations with occupational exposure to WBV.
Our finding of a lack of clear relation between LBP and WBV contrasts with several other research reports and reviews.(2-12) One explanation, given the relatively low prevalence of professional driving in our study population (17 %), is that an effect was missed by chance. The upper confidence intervals for risk estimates did not exclude a doubling of risk from professional driving for ≥ 3 hours at a time at the 5% level and the absence of any exposure-response effect by estimated A(8), although limited by the numbers with high exposure, tends to argue against this explanation.
A second possibility is that the drivers in our study - representing a population-based sample - were less heavily exposed to WBV than in surveys of occupational cohorts. Most were drivers of cars, with relatively few other sources of exposure reported. Associations with car driving have been reported in several earlier surveys,(9-11) but there are also some contrary observations in general population-based samples.(13,25) It was estimated that in only 1 in 6 to 7 of our study subjects was the A(8) ≥ 0.5 m/s−2 and in only 1 in 20 was it ≥ 1.15 m/s−2. In comparison, in positive studies from occupational settings average exposure levels were around 0.5 m/s−2 in crane drivers(26,27) and helicopter pilots,(28) 0.8 m/s−2 in lift truck drivers,(4) and 0.7-1.0 m/s−2 in tractor drivers,(6) and drivers of wheel loaders and freight containers.(29) Our findings should not be construed as arguing against WBV as a cause of LBP symptoms in more highly exposed working populations.
The finding of a clear consistent and strong association with sitting but not driving raises a third possibility – that previously reported associations with WBV were confounded by constrained sitting, a characteristic ingredient of professional driving. The data in hand provide some support for this idea, in that an association was found in the final models between continuous driving in a spell (with sitting for more than 3 hours at time) but not with hours driven in aggregate or other measures of WBV dose. A number of positive associations with sitting while not driving have been reported also in the wider literature,(30-33) but increases in risk have been modest and not wholly consistent and the balance of evidence is against this explanation.(31,34, 35)
A fourth possibility is that WBV is generally associated with mild to moderate LBP, but not with severe LBP that leads to investigation by MRI.
Whichever the explanation, our findings suggest that in the population studied, WBV seems not to be an important cause of LBP severe enough to be referred for MRI imaging of the lumbar spine. Certain aspects of mental health (low mood, somatising tendency) and health beliefs (beliefs regarding work as a cause or aggravation of back pain) may make a more important contribution.
This research was supported by the European Commission under the Quality of Life and Management of Living Resources programme, project no. QLK4-2002-02650 (VIBRISKS).