Facet cyst in the lumbar spine: radiological and histopathological findings and possible pathogenesis

OBJECT: The authors define facet cyst as a cyst located beside the facet joint and exhibiting a communication with the joint, as demonstrated on arthrography and subsequent computed tomography (CT) of the joint space. The purpose of this study was to determine the pathogenesis of facet cysts based on their radiological and histological features. METHODS: Forty-six juxtafacet cysts in 45 patients (26 men and 19 women, age range 41-82 years) were surgically treated after evaluation by arthrography and subsequent CT scanning. A communication channel between the cyst and the joint was confirmed in all cases and thus the lesions were designated facet cysts. In almost all patients the involved facet joint showed moderate to severe degeneration. After a thorough preoperative radiological evaluation, these cysts were excised en bloc by medial facetectomy with the entire ligamentum flavum. The specimens were cut axially at the maximum diameter and were histologically investigated. Morphologically, the cysts exhibited three shapes, appearing as: 1) a small protrusion, 2) a semicircular cyst, or 3) a round cyst. The cyst walls consisted of elastic and collagen fibers undergoing fibrinoid degeneration, but no synovial lining cells were detected. The cystic cavities were regularly filled with fibrinoids, and myxoid degeneration was found particularly in the larger cysts. CONCLUSIONS: Facet cysts are closely related to the degeneration of the neighboring facet joint. Analysis of the findings in this histological study suggests that there are several shapes of facet cysts. The authors propose that the shape could depend on the stage of the cyst's development.