What it is
Synovial cysts arise from the synovial lining of a degenerated lumbar facet joint. As the facet joint breaks down, joint fluid can accumulate and herniate through a weakened area of the joint capsule, forming a cyst in the spinal canal. These cysts most commonly occur at L4-5 and can grow large enough to compress a nerve root against the back of the disc or the lateral wall of the spinal canal.
On MRI, synovial cysts typically appear as smooth, well-defined structures adjacent to the facet joint, sometimes with internal fluid or hemorrhage.
Non-surgical management first
Many synovial cysts respond to targeted facet joint injection with corticosteroid — the injection can deflate the cyst or reduce the inflammatory response around it, relieving nerve root irritation. This is generally attempted before surgery for patients with tolerable symptoms and no significant neurological deficit.
Fluoroscopic or CT-guided rupture of the cyst — puncturing it through the facet joint during injection — has variable success rates and is an option in some centers before proceeding to surgical excision.
Surgical excision
When injections have failed or the cyst is causing progressive neurological deficit, microsurgical excision is effective and straightforward. Through a small posterior incision, the lamina and medial facet are partially removed to expose the cyst, which is carefully dissected free and excised along with its stalk at the facet joint.
The question of whether to add fusion is important: synovial cysts arise because the facet joint is unstable. If there is significant instability or spondylolisthesis at the level, cyst resection alone may have a higher recurrence risk and fusion may be added. For patients without instability, decompression alone is generally appropriate and avoids the recovery burden of fusion.