Facet Joint Cyst

A facet joint cyst is a small, fluid-filled sac that forms next to one of the facet joints — the paired joints at the back of the spine that guide and stabilise its movement. As a facet joint wears and degenerates, its capsule can weaken and balloon outward, allowing fluid to collect and form a cyst. Because these cysts sit close to the nerves, they can press on a nerve root and cause symptoms.

You may also hear these called synovial cysts, ganglion cysts, or juxtafacet cysts. The distinction between a synovial and a ganglion cyst is only a microscopic one — they look the same on a scan, behave the same way, are treated the same way, and have the same outlook — so for practical purposes the label doesn't change anything about your care.

The most important thing to understand about facet joint cysts is why they appear. They are now widely regarded as a sign of a facet joint that has become worn and slightly unstable — a marker of "micro-instability" at that segment of the spine. The excessive small movements at a degenerated joint are thought to drive fluid out through the weakened capsule, forming the cyst.

This explains some characteristic features:

The great majority of these cysts occur at the L4–L5 level — the most mobile segment of the lower spine, where load and movement are greatest.

They are frequently accompanied by facet joint arthritis and, in a substantial proportion of patients, by a slip of one vertebra on another (degenerative spondylolisthesis) at the same level.

In short, a facet cyst is rarely just a random lump. It is usually telling a story about the mechanical state of that part of the spine — and that story matters when planning treatment.

When a facet cyst grows into the spinal canal, it can compress a nerve root, and the commonest result is sciatica-type radiating leg pain (radiculopathy) in the distribution of the affected nerve. Many people also have back pain, which is often related to the underlying worn, unstable joint rather than the cyst itself. Other possible features include numbness, pins and needles, weakness, or leg pain on walking. Symptoms can be long-standing and grumbling, or occasionally come on quite suddenly. They may also fluctuate — a feature consistent with the cyst changing in size over time.

MRI is the imaging test of choice. It shows the cyst as a well-defined sac sitting next to the facet joint, and — importantly — shows its relationship to the nearby nerve and dural sac, helping confirm that the cyst is the likely cause of the symptoms. CT can add detail about the bony joint and any calcification.

Because instability is so central to these cysts, the assessment often includes standing dynamic X-rays (bending forwards and backwards). These can reveal a slip or abnormal movement that a scan taken lying still would miss — and that finding can significantly influence the treatment plan.

Conservative (non-surgical) care is usually the starting point. This includes anti-inflammatory medication, activity modification, and physiotherapy with an emphasis on core stability.

Injections are also an option. A steroid can be placed around the nerve or into the facet joint under image guidance. These can give useful pain relief, and some people get long-lasting benefit. However, it's fair to set realistic expectations: results from injections for facet cysts are variable. Studies have shown that while a meaningful minority of patients get durable relief, a substantial proportion experience only short-term improvement, and many of those eventually go on to need surgery. The cyst fluid is often thick and difficult to drain, and the cyst can refill.

Surgery is considered when symptoms are severe and persistent despite these measures, or when there is significant nerve compression. The goal is to remove the cyst and free the nerve. The key question — once again — is stability.

There are two broad surgical strategies, and the right one depends on the state of the joint and segment:

Decompression and cyst removal alone. If the spine is stable, the cyst can be removed and the nerve decompressed without fusing the segment. This relieves the radiating leg pain effectively in the great majority of patients, and most people do very well.

Decompression with fusion. If there is genuine instability or a significant slip, simply removing the cyst may not address the underlying problem — and there's a recognised risk that the cyst can recur, or that back pain persists, because the unstable joint that produced the cyst in the first place is still moving. In these situations, adding a fusion to stabilise the segment is considered. The evidence broadly supports this logic: pooled analyses have found that fusion is associated with lower rates of cyst recurrence and less ongoing back pain than decompression alone, at the cost of a bigger operation and a longer hospital stay. Reassuringly, only a small minority of patients treated with decompression alone go on to need a later fusion. The decision is therefore an individual one, weighed carefully on a case-by-case basis.

This is an area of particular interest, and where modern endoscopic techniques offer real advantages.

The traditional open operation for a facet cyst involves a degree of muscle stripping and bone removal to reach and remove the cyst. The concern with removing too much of the facet joint is that it can create or worsen the very instability that caused the cyst — sometimes tipping a patient towards needing a fusion they might otherwise have avoided.

Endoscopic cyst removal is designed to address exactly this. Working through a tiny incision, with continuous fluid irrigation and a magnified endoscopic camera placed right at the level of the pathology, the surgeon can reach the cyst and decompress the nerve while removing only a minimal amount of bone from the facet joint complex. Two routes are used depending on the cyst's size and position: a transforaminal approach (entering via the side, through the natural opening where the nerve exits) and an interlaminar approach (a small posterior muscle-splitting route, often favoured at the lowest L5–S1 level). Both allow the cyst to be carefully separated from the nerve and dural sac and removed under direct vision.

The appeal of this approach is that it preserves the supporting structures of the spine as far as possible — the aim being a thorough decompression with the lowest possible risk of destabilising the segment. In published experience, the great majority of patients undergoing endoscopic facet cyst removal achieve good or excellent outcomes, with reliable resolution of their leg pain, marked improvement in walking distance, minimal blood loss, and the convenience of a day-case or short-stay procedure.

A facet joint cyst should never be treated as an isolated lump on a scan. It is usually a signpost to a worn, slightly unstable joint, and the best treatment depends on the whole picture: how well the symptoms match the imaging, whether the leg pain or the back pain dominates, and — above all — whether the affected segment is stable. Bringing all of this together is what allows the treatment to be tailored to the individual, whether that means conservative care, an injection, an ultra-minimally invasive endoscopic decompression, or a stabilising fusion.

• —NICE — Low Back Pain and Sciatica in Over 16s (NG59)
• —NHS — Spinal Conditions
• —British Association of Spine Surgeons (BASS)

This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.