Synovial Cyst

Findings:

A large well circumscribed focus of fluid attenuation surrounds the widened left L5-S1 facet joint. Adjacent smooth bone scalloping is present. The structure causes mild mass effect on the thecal sac. The left L5-S1 facet joint is also hypoplastic and there is a chronic left L5 pars defect.

Discussion/Differential Diagnosis:

The relation of this structure to the facet joint, attenuation, and morphology make anything other than a synovial cyst unlikely. THe smooth bone remodeling indicates a long standing process, which is less typical. Lumbar synovial cysts (LSS) are uncommon and may be asymptomatic until they expand and cause compressive symptoms. LSS are most common at L4-5 which is the most mobile segment, followed by L5-S1 and L3-4. They are true cysts with an epithlial lining. Etiology is poorly understood but felt to arise from instability, facet degeneration, and/or spondylolisthesis. LSS may have variable signal and attenution depending on the presence of internal hemorrhage, calcification, gas, and fibrosis. An enhancing synovial cyst wall is typical on MR.

Surgery is indicated for cases refractory to conservative treatment. In addition to typical rest, PT, braces, and analgesics, conservative therapy may include CT guided aspiration and/or steroid injection. Most LSS recur within 6-12 months after aspiration, therefore surgery is more definitive.

Reference: Khan et al. Eur Spine J. 2006 Aug;15(8):1176-1182.

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