We tried to assess the value of posterior epidural fat for distinguishing isthmic spondylolisthesis from degenerative lumbar spondylolisthesis on midline sagittal MR images.

The midline sagittal MR images of the lumbar spines were retrospectively studied for 50 patients without spondylolisthesis, for 78 patients with isthmic spondylolisthesis and for 43 patients with degenerative spondylolisthesis. The anteroposterior diameter of the posterior epidural fat (ADEF) was measured at each intervertebral disc level by two radiologists and these values were then compared between each group. To normalize for difference of body size, the posterior epidural fat ratio (PEFR) at each level of spondylolisthesis and at L1-2 were also determined for each level of spondylolisthesis, and the PERF was compared between each group. Statistical analysis was performed by the chi-square method.

For the patients with isthmic spondylolisthesis, the ADEFs at the spinal levels with spondylolisthesis were significantly greater than those ADEFs in the control group that were measured at the corresponding disc levels (p<0.05). For the patients with degenerative spondylolisthesis, the ADEFs at the spinal level with spondylolisthesis were significantly less than the ADEFs in the control group that were measured at the corresponding disc levels (p<0.05). The PEFRs obtained at L4-5 were 1.37±0.12 for the control group, 2.61±1.31 for the patients with isthmic spondylolisthesis, and 0.60±0.05 for the patients with degenerative spondylolisthesis. The PEFRs obtained at L5-S1 were 2.25±1.32 for the control group, 3.47±1.69 for the patients with isthmic spondylolisthesis and 1.65±0.18 for the patients with degenerative spondylolisthesis. At both levels, the PEFRs were greatest for the isthmic spondylolisthesis group and smallest for the degenerative spondylolisthesis group, and all the differences were statistically significant.

The posterior epidural fat, which is easily seen structure on the midline sagittal MR image, is significantly increased in isthmic spondylolisthesis, but it is decreased in degenerative spondylolisthesis, and this could be useful in distinguishing isthmic spondylolisthesis from degenerative spondylolisthesis.