Synovial cyst of the lumbar spine

In one minute

A synovial cyst is a small sac of joint fluid that has bulged out of a worn facet joint into the spinal canal, where it presses on a nerve root. It appears mostly in the sixties, at L4-L5, in people whose facet joints have been wearing for years. It feels like sciatica: pain down one leg, sometimes with numbness, and in some people the walking-limited pattern of stenosis. MRI (magnetic resonance imaging) shows it as a round, fluid-bright pouch next to the joint. Rest, medication and an injection help some people; needle rupture helps fewer than surgery; surgical removal, through a 7 mm endoscopic incision when the segment is stable, is the definitive treatment when the leg pain will not settle.

What it is

The facet joints at the back of each segment are true joints with a lining that produces fluid and a capsule that contains it. When the joint wears, the capsule weakens, and a pouch of it can push through the ligament that lines the canal. That pouch is the cyst. Radiologists call it a synovial cyst, a facet joint cyst, a juxtafacet cyst (next to the facet) or, when it has no lining, a ganglion cyst. For the patient the distinction changes nothing.

In the systematic review of 966 reported patients, 75.4% of cysts were at L4-L5; L5-S1 and L3-L4 come next. Patients present in their mid-sixties. Cervical and thoracic cysts exist but are rare, about 4% of the total between them.

Causes and risk factors

The cyst is a product of facet joint wear, so its risk factors are those of facet arthropathy: age, disc degeneration at the same level, and a segment that moves more than it should. The strongest single association is degenerative spondylolisthesis: the 2004 review found a slip at the cyst level in up to 40% of patients. A cyst can also form at a level that was decompressed by an earlier operation; in one endoscopic series, 9 of 25 cysts had grown at a level with a previous laminectomy.

How it develops

Abnormal movement irritates the joint. The lining inflames and makes more fluid, the capsule stretches and thins, and a pouch of it is pushed through the ligamentum flavum into the canal, on the side nearest the joint. Once inside, the cyst behaves like a small mass: it presses the nerve root against the disc or the bone. Because it sits at the back and side of the canal, it usually catches the root that is about to leave at the next level down, which is why an L4-L5 cyst produces L5 sciatica.

Symptoms

Leg pain is the main complaint. In the pooled review, 69.6% of patients presented with radicular pain (pain along the course of one nerve root) and 48.3% with back pain. The leg pain follows the root the cyst is pressing: down the outside of the leg to the top of the foot for L5, down the back of the leg to the sole for S1. Numbness or pins and needles in the same territory are common. When the cyst is large or there is stenosis alongside it, walking brings on heaviness in the legs that eases with sitting (neurogenic claudication).

A cyst cannot be told from a disc herniation by symptoms alone, though the patients are older and, in our experience, the back pain is more often the mechanical ache of a worn joint, worse on arching backward.

Diagnosis

The examination looks for the root involved: which part of the leg has changed sensation, which muscle is weak, which reflex is reduced, and whether raising the straight leg reproduces the pain.

MRI is the test. On the T2-weighted images (the MRI sequence on which fluid looks bright) the cyst is a round, bright, fluid-filled pouch immediately next to the facet joint, at the back and side of the canal. CT (computed tomography) shows the joint itself and the bone around the cyst. Standing and bending X-rays are needed as well, because the choice of operation depends on whether the segment is stable: the AO Spine recommendations of 2026 make radiographic stability the deciding factor.

If you send images for an opinion, include the sagittal and axial T2 slices at the level of the cyst, the report, and any standing X-ray. Say which leg hurts and how far down.

Treatment without surgery

A short period of reduced activity, a course of anti-inflammatory medication chosen by the doctor, and physiotherapy once the acute pain settles are reasonable first steps, and for some people the leg pain settles for a period. An epidural or root injection of local anaesthetic and steroid can calm the inflamed root.

Needle treatment is the next option: under CT or X-ray guidance the joint is injected until the cyst ruptures, or the cyst is aspirated. The evidence is consistent that this works less often than surgery. The pooled analysis in World Neurosurgery found cyst resolution in 58% of percutaneous procedures against 90% for surgery, with repeat procedures needed in 29% of percutaneous cases and under 1% after decompression. The 2022 meta-analysis of 2,226 patients found satisfactory improvement in 66.2% after rupture or aspiration against over 80% after surgical removal. The AO Spine appraisal notes that more than half of patients treated by rupture need surgery within a year. It remains a fair choice for someone who cannot or will not have an operation.

When surgery is considered

Surgery is considered when leg pain persists after a fair trial of the measures above, when the root is losing strength or sensation, or, urgently, when bladder or bowel control is affected. The 2023 meta-analysis describes surgery as the standard of care for exactly these situations: failed conservative therapy, neurological deficit and evidence of instability.

There is no trial comparing early with delayed surgery for synovial cysts. What the literature shows is that a cyst large enough to cause persistent sciatica rarely resolves on its own and that the needle alternative fails in around half of patients, so a long wait mostly postpones the same operation.

Surgical options

Open removal through a laminotomy (a window in the bone at the back of the canal) is the traditional operation. In the 2023 meta-analysis of 1,251 patients it had a same-level recurrence rate of 6.36% and a revision rate of 11.3%. Tubular removal, through a narrow retractor with a microscope, had a recurrence rate of 3.01% in the same analysis.

Endoscopic removal takes the cyst off the nerve through a 7 mm incision, with the joint and the midline ligaments preserved. In the 2023 meta-analysis its recurrence and revision rates were not significantly different from open removal; in the 2022 meta-analysis satisfactory outcomes after full-endoscopic removal were about 90%, with adverse events under 2%. In the awake transforaminal series of 25 patients, leg pain fell from 7.6 to 2.3 on a 10-point scale and disability from 39.7% to 13.0% at two years, with no recurrence. The AO Spine group rates the evidence for endoscopic removal as low quality and surgeon-dependent, and considers it appropriate for carefully selected stable segments.

Removal with fusion adds a cage and screws to stop the segment moving. It has the lowest recurrence: none in the 2010 review and 0.0% in the 2023 meta-analysis, with revision in 6.67%. AO Spine recommends it when the segment is unstable or would become unstable after the decompression, and the 2022 meta-analysis found recurrence and revision almost twice as common when a degenerative slip is present at the level. Fusion is the more invasive operation, as the 2023 meta-analysis notes, and it leaves a permanently stiff level, which is why it is reserved for that group.

How we treat it at spine.uz

Send the MRI, the standing X-ray if you have one, and the report; a doctor from the team replies by voice within 48 hours, free of charge. The questions an opinion has to answer are whether the cyst explains the leg pain, whether the segment is stable, and whether it is reasonable to wait, try a needle procedure or operate.

When the segment is stable and surgery is indicated, the cyst is removed endoscopically through a 7 mm incision, with no screws or implants. Most endoscopic decompressions are done without general anaesthesia, under local anaesthesia with sedation; the anaesthetist and surgeon decide per patient. Most patients walk on the day of surgery and go home within 24 hours. When the segment is unstable and needs fusion, the team performs it fully endoscopically, with an interbody cage and percutaneous screws. Every patient receives a written conclusion from the team. Across 27,600+ endoscopic and spine operations, the team's complication rate is well below 1%.

Recovery and outcomes

Leg pain usually eases quickly once the cyst is off the root. In the 2010 review, 91.1% of patients had complete relief of leg pain and 92.5% complete relief of back pain after surgery. By a mean follow-up of about two years, back pain had returned in 21.9% and leg pain in 12.7%, and 6.2% needed a second operation, most of them a fusion for instability and mechanical back pain. Same-level cyst recurrence after decompression alone was 1.8%.

Two things improve the odds: choosing the operation by the stability of the segment rather than by the size of the incision, and preserving the joint during removal. The 2004 review notes that patients decompressed without fusion can develop a new or progressing slip afterwards, and the endoscope, with its magnified view, lets the surgeon take the cyst and leave as much of the joint as possible. Walking begins on the day of surgery, and the awake endoscopic series above was done as outpatient surgery. Desk work commonly resumes within a few weeks; heavy work waits longer.

Living with it and prevention

A synovial cyst is a sign that the facet joint underneath it has worn and that the segment moves more than it should. Keeping the trunk muscles strong and the weight down is the sensible response; no diet, supplement or brace shrinks a cyst. After removal, new back pain or a return of leg pain deserves a standing X-ray and an MRI rather than a wait, because the two things worth catching early are a mobile slip and a new cyst.