Orientation of the articular processes at L4, L5, and S1. Possible role in pathology of the intervertebral disc.

In the synergistic complex formed by the intervertebral disc and posterior articular processes, the latter play a significant role to protect the disc and block forward movement of the spine. This role is of special importance at the level of the lumbosacral interface whose inclination contributes to increase shearing forces acting on the disc. The orientation of the lumbosacral articular processes modifies the distribution of the mechanical stress acting at their level. The relationship between the orientation of the articular processes and the stress transmitted to the disc was studied by computerized tomography (31 subjects without disc prolapse, 35 subjects with disc prolapse, 110 operative reports). Sagittal orientation of the facet joints, which is consistently more pronounced on the right side, seems to promote the occurrence of disc prolapse at the lumbosacral level.