Ex Vivo Biomechanical Testing to Examine the Etiology of Low Back Pain as a Result of Whole Body Vibration

An association between occupational whole body vibration (WBV) exposure and the development of lower back pain (LBP) has been established through numerous epidemiological studies. However, the etiological and biomechanical mechanisms of how WBV contributes to LBP and injury are not well understood. Using field-measured occupational WBV exposures in a controlled laboratory setting, this study attempted to expose the human lumbar spine to occupational WBV exposures to better understand the mechanisms of WBV-related low back injury. The occupational exposures selected for this study were those encountered by bus drivers, who are known to have a high prevalence of LBP. Continuous and impulsive WBV accelerations collected at the floor of a bus traversing a standardized route in the Seattle metro area were played into a six-degree of freedom hydraulic shaker platform. Twelve professional drivers sat on an air suspension seat mounted to a six-degree of freedom hydraulic shaker platform. The drivers' response to the continuous and impulsive exposures at the seat and sternum were recorded. The difference between seat- and sternum-measured accelerations were used to calculate the displacement of the spine during the vibrations encountered during continuous and impulsive exposures. Based on these two exposure patterns for the bus drivers, six human lumbar functional spine units (FSUs) were placed in a servohydraulic testing system and continuously exposed to one of the two vibration signatures. A unique staining technique was used to visually differentiate preexisting intervertebral disc damage and damage brought on by cycling the FSU. No discernible damage was found for discs exposed to the continuous WBV while exposing FSUs to the impulsive WBV resulted in endplate fractures and damage to the annulus fibrosis of the intervertebral disc. These findings indicate that, for equal drive times, a more impulsive and intermittent loading signature is more likely to cause injury to components of the low back than a relatively less impulsive and more continuous signature.