The average appearance of our patient with an anterior spondylolisthesis is that there is also a narrowing of the disc spacing between L5 and S1. This narrowing is attributed to the perpendicular component of the forces that are acting over the L5-S1 interbody articulation. The component vector, that is parallel to the joint surface, will pull the vertebrae toward posterior, or toward correction, while the component that is perpendicular to the surface, will enlarge the disc space. This simple vector analysis for the temporary reduction of spondylolytic anterolisthesis is verified with radiographic studies mentioned below. Before proceeding, however, an understanding of the principles of viscoelastic creep is necessary.
Viscoelastic creep is a characteristic of biological tissue.
An example of a typical creep curve from White and Panjabi is noted in Fig. 4. In a creep curve, deformation of the tissue is plotted as a function of time. The principle of creep is that the tissues will continue to deform over time, even though the load on the tissues will take place, creating a lasting alteration in the tissue mechanics, giving a lasting benefit to the forces that were applied to the tissues. Over the past ten years, we have done a radiological study of 30 patients with spondylolytic anterolisthesis. Our study consisted of exposing radiographs in the upright and inverted, gravity traction positions.