The effect of axial load on the sagittal plane curvature of the upright human spine in vivo.

Determining the effect of load carriage on the human spine in vivo is important for determining spinal forces and establishing potential mechanisms of back injury. Previous studies have suggested that the natural curvature of the spine straightens under load, but are based on modelling and external measurements from the surface of the back. In the current study, an upright positional MRI scanner was used to acquire sagittal images of the lumbar and lower thoracic spine of 24 subjects. The subjects were imaged in standing whilst supporting 0, 8 and 16 kg of load which was applied axially across the shoulders using an apron. An active shape model of the vertebral bodies from T10 to S1 was created and used to characterise the effect of load. The results from the shape model showed that the behaviour of the average-shaped spine was to straighten slightly. However, the shape model also showed that the effect of load exhibited systematic variation between individuals. Those who had a smaller than average curvature before loading straightened under load, whereas those who had a greater than average curvature before loading showed an increase in curvature under load. The variation in behaviour of differently shaped spines may have further implications for the effects of load in lifting manoeuvres and in understanding the aetiology of back pain.