A cohort study of sciatic pain and measures of internal spinal load in professional drivers

Introduction Current epidemiological data are insufficient to outline an exposure-response relationship between whole body vibration (WBV) and disorders of the spine. The EU Directive on mechanical vibration has established daily action values and exposure limit values for WBV expressed in terms of A(8) or VDV calculated from the highest value of the vibration accelerations determined in the x-, y-, and z-axes (A(8)max in ms-2 r.m.s.; VDVmax in ms-1.75). These latter are measures of “external” dose and only partially reflect the spinal load acting on the disc and/or endplates of the vertebrae. In a working draft of ISO/TC 108/SC 4/WG 15, a method is proposed to estimate internal spinal forces for occupational exposures to vibration containing multiple shocks [1]. The prediction of the lumbar spinal response to vibration is estimated by means of Finite Element (FE) models anatomically adapted to the anthropometry and posture of the exposed workers. The assessment of health risk is based on the estimation of the daily compressive dose Sed (MPa) and the risk factor R (non-dimensional units) calculated from the internal static and dynamic spinal forces and other individual parameters such as the age of the subject at which exposure started and the daily and lifetime duration of exposures to WBV. The aim of this cohort study of professional drivers was to investigate the relation of sciatic pain, a severe symptom of low back disorders often associated with herniated lumbar disc, to measures of daily WBV exposure and internal lumbar load derived from the biodynamic and epidemiological databases implemented by the German and Italian arms of the EU VIBRISKS project. Methods The cohort included 537 male professional drivers of earth-moving machines (n=124), fork-lift trucks (n=169), and public utility vehicles (n=244). They were investigated over a two-year follow up period (2003-2006). Personal, occupational, and health histories were collected by means of the VIBRISKS questionnaire. Sciatic pain was defined as LBP radiating to one or both legs below the knee. The question on sciatic pain concerned the last 7 days and the previous 12 months. Vibration measurements were made on representative samples of the industrial machines and vehicles used by the professional drivers, according to ISO 2631-1 (1997). Daily vibration exposure was expressed as A(8)max (ms-2 r.m.s.) or VDVmax (ms-1.75), according to the EU Directive. The daily compressive dose Sed (MPa) and the risk factor R (units) were calculated at six lumbar spine levels (T12-L1 to L5-S1), according to ISO/WD 2631-5. The risk assessment was carried out on the basis of the lumbar spine level with the highest values of Sed and R factor. Physical work load other than when driving was evaluated from 11 questions and categorised as mild, moderate, hard, and very hard load grade. A measure of psychosocial work environment was derived from 5 questions concerning job decision, job support, and job satisfaction, and divided into categories of increasing psychosocial load (good, reasonable, a little poor, poor work environment). The relations of 7-day or 12-month sciatic pain to individual characteristics, work-related risk factors, measures of daily vibration exposure (A(8)max, VDVmax) or measures of internal load (Sed, R factor) were expressed as odds ratios estimated by the generalised estimating equations method for repeated measures over time. Results At the cross-sectional survey, the point prevalences of 7-day and 12-month sciatic pain in the professional drivers were 8.4% (n=45) and 23.1% (n=124), respectively. Herniated lumbar disc, diagnosed by CAT scan or MRI, was reported by 10.1% of the drivers (n=54). Over the follow up period, the cumulative incidence was 8.3% for 7-day sciatic pain, 21.8% for 12-month sciatic pain, and 5.2% for herniated lumbar disc. A(8)max and VDVmax were significantly correlated with Sed (r=0.72 and 0.60, respectively), and to a lesser extent with R factor (r=0.44 and 0.32, respectively). After adjustment for potential confounders, Sed and R factor showed stronger associations with 7-day and 12-month sciatic pain than A(8)max and VDVmax (Table 1). There was a significant trend of increasing occurrence of both 7-day and 12-month sciatic pain with the increase of Sed and R factor treated as continuous variables; no significant trend was observed for A(8)max and VDVmax. Herniated lumbar disc, previous lumbar trauma, and physical work load other than when driving were significantly associated with 12-month sciatic pain, while poor psychosocial work environment showed some associations with 7-day sciatic pain. Height and smoking were not related to either 7-day or 12-month sciatic pain. Conclusions The findings of this prospective cohort study showed that measures of internal lumbar load, such as Sed and R factor, were better predictors of the occurrence of sciatic pain in a population of professional drivers than measures of external dose (A(8)max, VDVmax). However, our findings suggest that the proposed ISO limit values for R factor (<0.8 for a low probability of an adverse health effect, >1.2 for high probability of an adverse health effect) tend to underestimate the health risk to the lumbar spine. References [1] ISO/WD 2631-5. Mechanical vibration and shock – Evaluation of human exposure to vibration – Part 5: Methods for evaluation of vibration containing multiple shocks. DIN, 2011. ISO/TC 108/SC 4/WG 15.