L-4-5 ISTHMIC SPONDYLOLISTHESIS - A BIOMECHANICAL ANALYSIS COMPARING STABILITY IN L4-5 AND L5-S1 ISTHMIC SPONDYLOLISTHESIS

The authors have previously reported that the L4-5 isthmic spondylolis thesis lesion often progresses more than the L5-S1 lesion in adult pat ients. This biomechanical study compares the in vitro stability of the L4-5 isthmic spondylolisthesis lesion compared with the L5-S1 isthmic lesion. The authors also analyzed the role of the L5 iliolumbar ligam ent as a contributing factor to stability. Six fresh frozen human cada veric specimens (L4 to the sacrum including the iliolumbar ligamentous complex) were tested by applying 10 Nm flexion-extension moments. Sag ittal plane motion was measured with the specimens intact and after se quential transection of the pars interarticulares at L4 and L5 and fin ally with the iliolumbar ligaments cut at L5-S1. L4-5 and L5-S1 both s howed significant increases in rotation with the pars defect compared with normal (L4-5 = +2.0, L5-S1 = +3.2 degrees). Decreased translation of L5-S1 occurred with pars defect at this level. There were no signi ficant differences at the L5-Sl level after sectioning of the iliolumb ar ligament. Calculating the percentage difference from normal, L4-5 w ith a pars defect exhibited significantly greater relative motion comp ared with L5-S1 with the same defect; 12% more rotation, 33% more shea r, and 43% more axial translation. The iliolumbar ligament did not app ear to contribute to these differences because there was no significan t change in the L5-S1 kinematics after its transection. These results support the hypothesis that L4-5 pars defects are more unstable than L 5-S1 lesions. The iliolumbar ligament could not be implicated as the m ajor contributing factor in these differences.