Effects of slip severity and loading directions on the stability of isthmic spondylolisthesis: A finite element model study

Study Design. Using a validated finite element model, the biomechanical effects of pars defect in a lumbar segment with and without different degrees of slip (up to 50% slip) were studied. Objectives. To study the effects of slip severity and loading parameters on the stability of the lytic and adjacent motion segments. Better knowledge of the biomechanics of spondylolisthesis may help formulate treatment strategies such as bracing or spinal implants. Summary of the Background Data. Clinically, spondylolisthesis exists in varying grades of anterior slip, and the biomechanical stability of the motion segments at the lytic defect and adjacent level probably varies as well. In vitro studies of L4-L5 and L5-S1 isthmic spondylolisthesis slips have concluded that an L4-L5 pars defect is more unstable than an L5-S1 pars defect. Comparing the stability of lytic motion segments with different grades of spondylolisthesis is difficult to do experimentally and therefore has not been done. Further assessing the stresses in the bone and intervertebral discs at or adjacent to a lytic defect is also difficult to study experimentally, so no data are available. Methods. A finite element model of L4-S1 was validated with and without a pars defect at L5. The model was then revised to represent different degrees of slip at L5, and six different moment loadings were applied. Results. The current study showed larger decrease in stiffness with increasing percent slip. The decrease in disc stiffness and increase in disc stresses with increasing percent slip were larger at the level of spondylolisthesis as compared to the changes in the adjacent segment. Lateral bending moment and torsion load showed the largest decrease in stiffness due to slip. At 50% slip, the maximum increase in motion ( as compared to motion in an intact segment) was seen under lateral bending moment load (about 55% at L4/L5 and 250% at L5/S1). Lateral bending also produced the largest increase in stresses due to 50% slip in the anulus and endplates (300% increase in anular stress and 190% increase in endplate stress) at L5/S1. Conclusions. The stiffness of a spondylolisthetic motion segment decreases as the slip increases. Lateral bending and torsion are moment directions causing the greatest resulting motions.