A follower load increases the load-carrying capacity of the lumbar spine in compression

Study Design. An experimental approach was used to test human cadaveric spine specimens. Objective. To assess the response of the whole lumbar spine to a compressive follower load whose path approximates the tangent to the curve of the lumbar spine. Summary of Background Data. Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles at 80-100 N. Differences between maximum ex vivo and in vivo loads have not been satisfactorily explained. Methods. A new experimental technique was developed for applying a compressive follower load of physiologic magnitudes up to 1200 N, The experimental technique applied loads that minimized the internal shear forces and bending moments, made the resultant internal force compressive, and caused the load path to approximate the tangent to the curve of the lumbar spine. Results. A compressive vertical load applied in the neutral lordotic and forward-flexed postures caused large changes in lumbar lordosis at small load magnitudes. The specimen approached its extension or flexion limits at a vertical load of 100 N. In sharp contrast, the lumbar spine supported a load of up to 1200 N without damage or instability when the load path was tangent to the spinal curve. Conclusions. Until this study, an experimental technique for applying compressive loads of in vivo magnitudes to the whole lumbar spine was unavailable. The load-carrying capacity of the lumbar spine sharply increased under a compressive follower load, as long as the load path remained within a small range around the centers of rotation of the lumbar segments. The follower load path provides an explanation of how the whole lumbar spine can be lordotic and yet resist large compressive loads. This study may have implications for determining the role of trunk muscles in stabilizing the lumbar spine.