Herniated nucleus pulposus in isthmic spondylolisthesis: higher incidence of foraminal and extraforaminal types.

BACKGROUND: The purpose of this study was to evaluate the pattern of disc herniation and to investigate the associated symptoms in cases of isthmic spondylolisthesis. It is well known that the pathogenesis of degenerative spondylolisthesis associates with disc degeneration, followed by facet laxity and ligamentum flavum hypertrophy, which result in severe spinal canal stenosis. But isthmic spondylolisthesis is known to have a different pathogenesis. In isthmic spondylolisthesis, pseudodisc bulging is easily identified, and canal stenosis is comparatively rare. Therefore, we propose that isthmic spondylolisthesis has a different pattern of disc herniation from degenerative spondylolisthesis. We studied the type, incidence of disc herniation and clinical symptoms related to isthmic spondylolisthesis.

METHOD: This study included 132 patients with isthmic spondylolisthesis who had undergone an operation in the last 4 years. Among them, 120 patients were retrospectively reviewed. The mean age was 49.6 years, and the sex ratio was 1 to 1.93 (male to female). The disc herniation type was identified according to the classification of Bonneville.

FINDINGS: There were 78 patients (65%) with L4 to L5 spondylolisthesis and 42 patients (35%) with L5 to S1 spondylolisthesis. Eighty-two patients (68.3%) had only pseudodisc bulging without disc herniation, and 38 patients (31.7%) had pseudodisc bulging with disc herniation. Among the 38 patients with disc herniation, there were: medial type: 1 patient (2.6%), medial + posterolateral type: 1 patient (2.6%), posterolateral + foraminal type: 2 patients (5.3%), foraminal type: 7 patients (18.4%), foraminal + extraforaminal type: 25 patients (65.8%) and extraforaminal type: 2 patients (5.8%). Overall extreme lateral disc herniation (foraminal, extraforaminal) was seen in 36 patients, which was 30% of the total isthmic spondylolisthesis cases. In 26 out of 36 patients (72.2%), the dermatome was matched to the exact location of the extreme lateral disc herniation. In all cases, the extreme lateral disc herniation migrated upward slightly, about one slice (2 mm) according to thin-cut CT.

CONCLUSIONS: As expected, pseudodisc bulging without disc herniation was the most common type in isthmic spondylolisthesis. However, in cases of disc herniation, extreme lateral disc herniation occasionally occurs; therefore, every isthmic spondylolisthesis patient should be examined carefully for extreme lateral disc herniation with thin-cut axial CT or MRI, especially when the patients complain of lateralizing symptom.