Correlation between back shape and spinal loads

The purpose of this study was to determine the correlation between the back shape of the lumbar region and the spinal loads during activities performed in the sagittal plane. Measurements were performed in four subjects who had suffered from a compression fracture of a lumbar vertebral body which was treated with a telemeterized vertebral body replacement that is able to measure six load components in vivo. An Epionics SPINE measurement system was used to determine the lumbar lordosis angle. The relationship between the lordosis angle and the corresponding loads was quantified with the Spearman's rank correlation coefficient method. Measurements were performed during thirteen exercises in lying, standing or sitting. During upper body flexion, the force increased on average by approximately 285N and the lordosis angle decreased by 15°. The change of the force for elevating 30N in one hand was on average approximately 190N and for the lordosis angle 2°. Correlation coefficients greater than 0.6 were found for exercises that involved both large back shape and load changes, such as upper body flexion. A strong increase in spinal load can be associated with an increase or a decrease of the lordosis angle. Only for considerable changes of the lordosis angle in an upright body position was a strong correlation between lordosis angle and implant force found.