Synovial cyst

Findings

Figure 1: An axial T2 image demonstrates a high signal round structure, adjacent to the right facet and postero-lateral to the thecal sac with a low signal rim.
Figure 2 and Figure 3: Axial and sagittal T2-weighted images demonstrate a cystic structure with a low signal rim adjacent to the right facet with mass effect on the right lateral recess at the L4-L5 level.


Diagnosis: Synovial cyst


Synovial cysts are formed by degeneration of the facet joint. Ninety percent of synovial cysts occur in the lumbar spine with 70-80% occurring at L4-L5. The cysts themselves are usually 1-2 cm in size and appear round or lobulated with sharp margins. Synovial cysts are postero-lateral extradural cystic masses and are adjacent to the facet joint. Direct communication with the facet joint confirms the diagnosis but is not always visualized.

Differential diagnosis of synovial cysts include:
- Extruded disk fragments
- Ganglion cysts
- Nerve sheath tumors
- Septic facet arthritis
- Asymmetric ligamentum flavum hypertrophy.

Characteristic imaging findings of synovial cysts include a low T1 signal round lesion adjacent to the postero-lateral aspect of the thecal sac with a rim that is iso-signal on T1 and low signal on T2. Synovial cysts are bright on T2-weighted images and may demonstrate communication with the facet joint. If proteinaceous fluid or hemorrhage is present within the cyst, the signal may be more heterogeneous. The wall of the cyst may enhance with contrast.

Synovial cysts may be incidental findings or may cause varying degrees of central canal, subarticular or lateral recess narrowing. Synovial cysts are usually seen in patients greater than 60 years of age and are more common in females. Clincal presentation may result secondary to chronic low back pain, acute pain from hemorrhage into the cyst or radicular symptoms. Synovial cysts may spontaneously regress with conservative management. Conservative management includes bed rest and analgesia. Additional minimally invasive treatment includes facet injection with steroids and or percutaneous aspiration of the cyst material under CT guidance. Surgical treatment includes laminectomy with cyst excision or hemilaminectomy and flavectomy of the affected side.