Determination of vibration-related spinal loads by numerical simulation

Objective: Quantitative documentation of primary mechanical overload injury to lumbar vertebrae and discs caused by long-term exposure to heavy physical exertions or whole-body vibration at the workplace. Study design: A cross sectional study measuring vertebral height, sagittal plane displacement and disc height from lateral radiographic views of exposed cohorts and comparing the results with a normative database compiled from radiographs of unexposed, healthy subjects. Background: Current guidelines on health and safety during manual handling and lifting as well as under conditions of whole-body vibration are aimed at minimising potential risks specifically to the lumbar spine. Such regulations affect a large percentage of the labour force, and their enforcement constitutes a major economic factor. Up to now, however, the prevalence of work-related primary mechanical overload damage to lumbar vertebrae and discs has not been quantified and the effectiveness of the guidelines in preventing overload damage has not been proven. Methods: Overload damage to the lumbar spine is expected to result potentially in (i) fractures effecting a decrease in vertebral height or a wedge shape of vertebral bodies, (ii) a derangement of the sagittal plane alignment of lumbar vertebrae, resulting in dorso-ventral displacement, or (iii) primary injury to intervertebral discs or fracture of vertebral endplates, resulting in a decrease in disc height. To assess the occurrence and magnitude of such damage, archive lateral radiographic views and work histories of 355 subjects with long-term exposure to heavy physical exertions or whole-body vibration at the workplace were collected from 8 cohorts in the steel, mining and oil industries as well as from public services. Vertebral height, sagittal plane displacement and disc height were measured, employing advanced methods of image analysis compensating for distortion in central projection as well as for variation in radiographic technique, patient posture and stature. The measured data were compared with age-appropriate data of normative databases, previously compiled from radiographs of healthy, male, unexposed subjects (n=737) in the age range between 17 and 57 years. Results: Comparison with normative databases demonstrates that in the cohorts under study (save for a cohort performing a specialised task in a forward bent posture and a cohort of miners with mean age close to 60 y) neither exposure to very heavy physical exertions in manual labour nor exposure to whole-body vibration or shock loading resulted in a height decrease or wedge-shape deformation of lumbar vertebrae. Heavy spinal loading or whole-body vibration did not lead to increased sagittal plane (dorso-ventral) displacement of lumbar vertebrae. Lifting and handling very heavy objects, specifically when working in confined spaces or on uneven ground, lead to a noticeable and significant decrease in the height of lumbar discs. While exposure to vibration on damped machine operators' seats did not lead to a reduction in disc height, vibration and shock loading transmitted from unsprung seats on (in some cases) unsprung machines resulted in a noticeable and significant decrease in lumbar disc height. Conclusions: This study demonstrates, for the first time, objectively and quantitatively that spinal loading in certain industrial workplaces can result in damage to lumbar discs. Heights of vertebral bodies and sagittal plane displacement were generally unaffected. Any workplaces with characteristics similar to those identified here as detrimental are in urgent need of ergonomic redesign. That ergonomie redesign can be effective in reducing spinal overload damage is starkly demonstrated by comparing the results from vibration-exposed cohorts of machine operators using damped as opposed to unsprung seats. The question of whether heavy work is related to the prevalence of symptoms and/or resultant disability remains to be determined, but the results here suggest that existing ergonomic guidelines are justified to reduce the risk of irreversible spinal damage.