INTRODUCTION: It is known that synovial cysts arise from the facet joints after decompressive lumbar surgery. However, it is not well known about the incidence or the cause of postoperative synovial facet cysts (PSFC). The purpose of this study is to clarify the incidence and radiographic findings of PSFC.
METHODS: There were 72 patients who underwent partial facetectomy and flavectomy of the involved level for lumbar spinal canal stenosis. All patients underwent pre and post‐operative MRI and CT study. There were 40 male and 32 female and their age ranged from 42 to 85 years (mean 70 years). Follow up period was 6 to 107 months (mean 29.4 months). Cystic lesions demonstrating low signal intensity in T1 weighted and high signal intensity in T2 weighted MRI axial images, which connected with the facet joints were defined as PSFC. Degree of the facet joint degeneration was classified into four categories according to the CT findings as follows; normal, mild, moderate and severe. We also measured %slip, sagittal rotation, intervertebral disc height, Cobb's angle and facet angle on plain radiogram and CT. These measurements were compared between PSFC group (patients with PSFC) and non‐PSFC group (patients without PSFC).
RESULTS: Fifteen of 72 patients (20.8 %) had PSFC at the operated levels. Degeneration of the facet joints progressed in both groups. There was no statistically significant difference of measurements between PSFC group and non‐PSFC group.
DISCUSSION: Interestingly, the high incidence of PSFC (20.8%) was observed. Ventral side of the facet joint is covered by the flavum without capsule. Resection of the hypertrophic flavum has to be performed in decompressive surgery. As a result, the facet joint directly connects with spinal canal. As progress of the facet joint degeneration, synovitis deteriorates and hypertrophied synovium may protrude from the facet joint.