Effects of preload on load displacement curves of the lumbar spine.

Elastic mechanical properties of the spine are a function of the axial preload as well as the physiologic loads. The published literature does not take into account the effect of the preload. In this study we have presented a new technique for applying large preloads together with 12 physiologic loads and for measuring the resulting three dimensional motion. Some of the conclusions regarding the elastic behavior of the lumbar spine are: (1) The application of any one of the 12 physiologic loads produces a three dimensional motion consisting of three translations and three rotations. (2) The main as well as the coupled motion curve is affected by the inclusion of preloads. (3) As represented by the main motion curves, the spine becomes more flexible in the presence of preloads with the physiologic forces directed laterally or anteriorly, or moments producing lateral bending or flexion. (4) The spine becomes less flexible in the presence of preload when it is subjected to axial tension or axial torsion. (5) No appreciable change due to the preloads is noticed in the load displacement curves when axial compression, posteriorly directed force, or extension moment is applied.