A synovial cyst of the spine is a fluid-filled sac that forms next to a facet joint in the spine, often in the lower back. Many cause no trouble. But when a cyst grows into the spinal canal, it can press on nerves and cause leg pain. So what is a spinal synovial cyst, what causes it, and how is it treated?

What is a synovial cyst of the spine?

Each level of the spine has a pair of small joints at the back, which guide and limit movement. Like other joints, they’re lined with a membrane called synovium. A synovial cyst is a fluid-filled sac that develops from one of these facet joints. Essentially, it’s a pouch of joint lining and fluid that bulges out next to the joint.

They’re most common in the lower back, particularly at the L4-5 level — the most mobile part of the lumbar spine. They occur in people in their 60s, with a slight predominance in women. 

You may also see the term ganglion cyst used for a similar lump near a facet joint. The technical difference is whether the sac has a true synovial lining, but in practice the two conditions behave the same and are treated identically. 

What causes a spinal synovial cyst?

Synovial cysts are linked to wear and instability at the facet joint. The main associations are:

  • Facet joint arthritis — degeneration of the joint is present in the large majority of cases

 

  • Spinal instability, including a slipped vertebra called spondylolisthesis 

 

  • Excess movement at a spinal level, which is why the mobile L4-5 level is the commonest site

Current thinking is that a worn and mobile facet joint produces extra fluid and forms a cyst. So, a synovial cyst often marks an underlying facet joint problem, not just an isolated lump.

Symptoms of a spinal synovial cyst

Many synovial cysts cause no symptoms and are found on a scan done for another reason. When they do cause trouble, it’s usually because the cyst is pressing on nearby nerves:

  • Back pain — often the first symptom, and it usually comes before any leg symptoms

 

  • Sciatica-type pain — pain radiating into the buttock and leg, from pressure on a nerve root; this is the most common symptom of a symptomatic cyst

 

  • Numbness, tingling, or weakness in the leg 

 

  • Neurogenic claudication — leg pain and heaviness brought on by walking or standing and relieved by sitting or bending forward, if the cyst contributes to narrowing of the spinal canal

The symptoms often come on gradually, but they can also fluctuate.

When a spinal synovial cyst is an emergency

Seek urgent medical care  if you develop any of these red flags:

  • numbness around the back passage, genitals, or inner thighs (a “saddle” pattern);

 

  • loss of bladder or bowel control;

 

  • rapidly worsening weakness in the legs.

These can indicate serious nerve compression (including cauda equina syndrome), which is an emergency. Rarely, a synovial cyst can bleed (haemorrhage), suddenly enlarge, and cause an abrupt, severe onset of back pain and nerve symptoms. Sudden, severe symptoms should never be “waited out.”

What else could it be? Differentials

Because a synovial cyst causes back and nerve symptoms that overlap with several more common conditions, an accurate diagnosis matters—and it’s usually made on MRI. The main differentials include:

  • Lumbar disc herniation “slipped disc”— the commonest cause of a compressed nerve root and sciatica, and the closest mimic

 

 

  • Facet joint arthritis (arthropathy) — the underlying problem the cyst arises from, which can itself cause back pain

 

  • Degenerative spondylolisthesis — a slipped vertebra, often found alongside the cyst

 

 

  • Vascular claudication — leg pain from poor circulation, which can mimic the walking-related leg pain of canal narrowing

 

 

  • Other cysts or masses near the spinal canal, and (rarely) infection

Distinguishing these is exactly why imaging is central: a synovial cyst has a characteristic appearance next to a facet joint, and the scan also reveals the underlying facet arthritis or instability.

How is a spinal synovial cyst diagnosed?

MRI is the test of choice. It shows the cyst clearly as a well-defined, fluid-filled sac next to a facet joint, along with its effect on the nerves and spinal canal, and it distinguishes a cyst from a disc problem or other cause. A cyst that has bled (haemorrhaged) looks different on MRI, which helps explain a sudden, severe presentation.

How is a spinal synovial cyst treated?

Treatment depends on symptoms and ranges from simple measures to surgery.

Conservative (non-surgical) treatment

For milder symptoms, non-surgical treatment is tried first:

  • Activity modification, physiotherapy, and pain relief

 

  • A facet joint or epidural steroid injection — an image-guided injection of corticosteroid into or around the affected facet joint can relieve symptoms, and sometimes the cyst is aspirated (drained) at the same time. It’s a reasonable step, but recurrence is common. Some cysts return after aspiration or injection, and relief is often short- to medium-term rather than permanent. It can still provide genuine relief and delay or avoid surgery for some people.

A trial of conservative treatment over several months is reasonable for symptoms that aren’t severe or rapidly progressing.

Surgery

Where symptoms are severe, surgery is the definitive treatment. This involves removing the cyst and decompressing the nerves. It generally gives good results. Where there’s underlying instability or a slipped vertebra, the surgeon may also recommend fusing the segment to stabilise it and reduce the chance of the cyst returning. The choice of procedure is individualised and made with a spinal surgeon. Emergency surgery is occasionally needed for the acute presentations described above.

Where does a sports medicine doctor fit in?

A sports and exercise medicine doctor assesses back and leg pain, recognises red flags, and arranges the right imaging (MRI). Your doctor can guide treatment — including image-guided injections where appropriate — and refer on to a spinal surgeon when surgery is the better option.

Frequently asked questions about spinal synovial cysts

Are spinal synovial cysts dangerous?

Most are not — many cause no symptoms at all and are found incidentally. When they do cause symptoms, it’s usually back and leg pain that can be managed. Urgent attention is needed for sudden, severe symptoms; rapidly worsening leg weakness; or any loss of bladder or bowel control, or saddle numbness. Rarely, a cyst can bleed and enlarge suddenly, causing acute symptoms.

What’s the difference between a synovial cyst and a ganglion cyst in the spine?

The difference is whether the sac has a true synovial (joint) lining — a synovial cyst does, but a ganglion cyst doesn’t. In practice, though, this distinction doesn’t change anything: the two look similar, cause the same symptoms, and are treated the same way. 

Can a spinal synovial cyst go away on its own or without surgery?

Sometimes. Some cysts resolve with conservative treatment. Reports describe cysts shrinking or disappearing on follow-up scans. Facet or epidural steroid injections can relieve symptoms, though recurrence is common and the benefit may be temporary. So a trial of non-surgical treatment is reasonable for milder cases. But cysts that don’t respond, or that cause significant nerve compression, are best treated surgically.

Why do spinal synovial cysts keep coming back?

Because the cyst arises from an underlying problem—a worn, unstable, or excessively mobile facet joint — aspirating or injecting the cyst treats the sac but not the cause, so the cyst can reform. This is also why, when surgery is needed and there’s significant instability or a slipped vertebra, the surgeon may add a fusion to stabilise the segment. 

What causes a spinal synovial cyst 

They’re strongly linked to facet joint arthritis and spinal instability, especially a slipped vertebra (degenerative spondylolisthesis). The most mobile level of the lower back, L4-5, is the most common site, which supports the idea that excess movement and wear at a facet joint drive cyst formation. 

Final word from Sport Doctor London about spinal synovial cysts

A synovial cyst of the spine is an uncommon cause of back and leg pain, arising from a worn or unstable facet joint, most often at L4-5 in the lower back. Many are silent and need no treatment. Physically painful cases are diagnosed on MRI and can be managed with physiotherapy, injections, or surgery. The key is to recognise the red flags that need urgent care, get the right imaging, and treat the underlying facet joint problem, not just the cyst.

If you have persistent back or leg pain, Dr Masci can assess you in London, arrange the right imaging, and guide treatment or refer you where needed. Contact the team here or call +44 (0) 203 488 0350.  

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