Geometric Analysis of Dural Compression in Non-traumatic Spinal Instability

ABSTRACT

We studied the extent of dural sac compression and the contributory factors involved during lumbar flexionextension motion in non-traumatic spinal instability patients. In a total of 79 patients, spinal instability at L4-L5 or L5-S1 had been clinically and radiologically diagnosed. Their lateral flexion-extension lumbar myelograms were reviewed and we selected 48 cases which had adequate lumbar mobility, and clear and unequivocal measuring points. Those patients were divived into four groups of 12, as follows Group I transient low back pain without radiologic abnormalities(Control); Group II spinal instability without displacement(SIWD) had been clinically and radiologically diagnosed; Group III symptomatic degenerative spondylolisthesis(DSL); Group IV symptomatic isthmic spondylolisthesis(ISL). Using the OPTIMAS 5.2 image analyzing program, we measured dural AP distance(DD) at intervertebral disc level and dural area(DA) at each functional motion segment, as seen on flexion-extension lumbar myelograms. The results were as follows (1) In the control group and SIWD group, the degree of dural sac compression was not significantly different. (2) In the DSL group, DD and DA at the L4-L5 level were markedly lower than in the control group(p<0.05). In the ISL group, DD was significantly higher(p<0.05), but DA was slightly lower than in the control group. In addition, DD and DA in the ISL group were significantly higher than in the DSL group. (3) At the L5-S1 level, these changes were variable and there were no significant trends. In conclusion, we discovered that in determining the extent of dural sac compression in spondylolisthesis, measuring both dural distance and dural area is more valuable than measuring either of these alone. This is because in this condition, anterior displacement of the vertebral body or posterior displacement of the mobile lamina plays an important role. We also found that the different mechanisms of dural sac compression seen in the DSL and ISL group, as well as the variable changes seen at the L5-S1 level, are due to anatomical characteristics.