The role of load magnitude as a modifier of the cumulative load tolerance of porcine cervical spinal units: progress towards a force weighting approach

Abstract

Most research to date relating cumulative compression exposure in the low back to the reporting of pain has employed a linear summation of force (load) magnitudes to obtain estimates of total exposure. More recently, force magnitudes have been weighted (square or tetra power) based on the idea that higher load magnitudes will result in an increased risk of injury. However, no empirical evidence currently exists for the employed weighting factors. Therefore, the role of load magnitude in modifying the cumulative load tolerance of porcine cervical spinal units was assessed. Specimens were assigned to one of four loading groups, corresponding to peak loads of 40, 50, 70 and 90% of the estimated compressive strength of the joint. Cyclic loading was performed until failure or a maximum of 21,600 cycles. Lower loading magnitudes resulted in fewer vertebral fractures, with only two of ten specimens tested at 40% failing. Cumulative load tolerance at failure was found to be significantly higher (p<0.0001) in the 40% loading group than all others. The cumulative load tolerance in the 50% loading group was found to be significantly higher than either the 70 or 90% groups, which were not significantly different from each other. Height loss at failure was not significantly different between the groups (p = 0.1204). A non-linear relationship was found between cumulative load tolerance at failure and loading magnitude. This relationship was used to develop weighting factors to adjust loading magnitudes for their impact on injury development when assessing cumulative loading. An equation is provided to allow application of the weighting factors to current methods of cumulative load estimation.

Keywords:

• Cumulative loading
• Load magnitude
• Spine
• Weighting factor

Acknowledgements

This work was funded by the Natural Sciences and Engineering Research Council Canada and the AUTO21 Network Centers of Excellence, whose funding is provided by the Canadian federal government. Jack Callaghan is supported by a Canada Research Chair in Spine Biomechanics and Injury Prevention.