A synovial cyst is a fluid-filled sac that bulges out from a joint, usually one that is worn down or inflamed. It is not cancer. The cyst itself is harmless tissue, so any trouble comes from what it presses on, most often a nerve in the lower back. Because it usually signals a wearing or loosening joint, treatment often has to address the joint as well as the cyst. Non-surgical care sometimes helps, but it rarely gives lasting relief once a spinal cyst is clearly pressing on a nerve, and surgery brings good pain relief for about 9 in 10 people.
Where Synovial Cysts Form and What They Are Made Of
Joints are lined by a thin membrane that makes slippery synovial fluid, which keeps them moving smoothly. A synovial cyst is a pouch lined by these same cells, and it is usually still connected to the joint it came from.1PubMed Central. Cutaneous Metaplastic Synovial Cyst: A Case Report That joint is typically affected by osteoarthritis or by an inflammatory disease such as rheumatoid arthritis.2PubMed Central. Cutaneous Metaplastic Synovial Cyst: A Case Report
Why would a lining form a pouch at all? When a joint is worn, its capsule can develop weak spots, and joint fluid under pressure can bulge through one the way an inner tube bulges through a thin patch. Over time the pouch can thicken, and its wall may scar or even calcify, which is why some cysts look more solid on imaging than a simple water-filled sac. The cyst often stays connected to the joint by a narrow channel, and that connection is why fluid can sometimes be drained and then refill.
In the spine, most of these cysts form at the facet joints, the small paired joints at the back of each vertebra, usually in the lower back.3PubMed Central. Successful treatment of facet joint synovial cyst through percutaneous rupture: a case report Less often they form in the neck, mostly at the top of the spine or where the neck meets the upper back.4PubMed Central. An Updated Review on Treatment of Cervical Synovial Cyst Behind the knee, the same kind of fluid swelling is called a Baker’s cyst, one of the most common lumps around the knee.5PubMed Central. Popliteal Cyst: Etiology, Diagnosis and Management, a Current Concept Review It is a fluid-filled sac that keeps an opening into the knee joint.6PubMed Central. Ruptured Baker’s Cyst Demystified: Current Evidence, Diagnostic Strategies, and Treatment Options for an Under-Recognized Condition
A knee cyst can often be felt as a soft bulge. A spine cyst sits deep and can’t be felt from outside, and it matters because the spinal canal leaves so little room for nerves.
If you are reading an MRI report, “cyst” here means a fluid pocket, not a tumor. The report may say facet cyst, juxtafacet cyst, or synovial cyst for the same thing. The level, such as L4-L5, tells you which joint it comes from. Size matters less than whether the cyst presses on a nerve.
Ganglion cysts look similar and cause similar symptoms.7PubMed Central. Lumbar Intraforaminal Synovial Cyst in Young Adulthood: Case Report and Review of the Literature Under the microscope, though, they differ, and that difference can affect treatment.8PubMed. Ganglion cyst versus synovial cyst? Ultrasound characteristics through a review of the literature
What Causes a Synovial Cyst
In older adults, spinal synovial cysts are mainly a sign of age-related spine wear.9PubMed Central. Acute development of spinal lumbar synovial facet cyst within 1 week after lumbar decompression: illustrative case About 9 in 10 people with a spinal cyst have arthritis in the facet joint at the same level, and about half have disc degeneration there too.10PubMed. Synovial Cyst as a Marker for Lumbar Instability: A Systematic Review and Meta-Analysis As the cartilage in a facet joint thins, the joint becomes inflamed and produces extra fluid, and the capsule around it stretches. Bony overgrowth and thickened ligaments often develop at the same joint, so the cyst may be only one of several things narrowing the space around the nerve. All of this takes years, which is why these cysts appear mostly in older adults and are rare in young, healthy spines.
Doctors also see these cysts as a marker that the spinal segment is loosening.11PubMed Central. Acute development of spinal lumbar synovial facet cyst within 1 week after lumbar decompression: illustrative case Roughly 4 in 10 people with a spinal cyst have a vertebra that has slipped forward over the one below, which your report may call spondylolisthesis.12PubMed. Synovial Cyst as a Marker for Lumbar Instability: A Systematic Review and Meta-Analysis Extra movement at a loose joint is thought to help cysts form and come back.13PubMed Central. Minimally invasive surgery for lumbar synovial cysts with coexisting degenerative spondylolisthesis Repeated small strains may play a role, though doctors still debate it.14PubMed Central. Lumbar Intraforaminal Synovial Cyst in Young Adulthood: Case Report and Review of the Literature
Inflammation is another route. In rheumatoid arthritis, the joint lining becomes inflamed and makes extra fluid, and the rising pressure pushes fluid into weaker parts of the capsule, forming a cyst.15PubMed Central. Multiple extra-articular synovial cysts accompanied by rheumatoid arthritis in the bilateral elbow joints In the knee, injury, cartilage tears, overuse, osteoarthritis, or rheumatoid arthritis can all raise fluid and cause a Baker’s cyst.16Bezzy RA. Baker’s Cysts and Rheumatoid Arthritis Large cysts away from the joints are a rare complication of rheumatoid arthritis.17PubMed Central. Case Report: Unusual and Rare Presentation of Rheumatoid Arthritis as Multiple Large Synovial Cysts in Uncommon Locations
Spinal synovial cysts are most common in people in their 60s, and women are affected more often.18PubMed Central. Lumbar Intraforaminal Synovial Cyst in Young Adulthood: Case Report and Review of the Literature They are extremely rare in teens and young adults.19PubMed Central. Lumbar Intraforaminal Synovial Cyst in Young Adulthood: Case Report and Review of the Literature
So the cyst is usually a symptom of a worn joint, not a separate disease. Your report may also mention arthritis or a slipped vertebra at the same level, and those findings shape which surgery a surgeon suggests. Nothing you did suddenly caused it; it builds over years.
Useful questions for your doctor: Is there a slipped vertebra at the cyst’s level? Does my spine move abnormally on bending X-rays? Could inflammatory arthritis be involved, since I have other swollen joints?
Symptoms of a Synovial Cyst
Spine cysts can cause significant back pain and sciatica-type pain, which runs down the leg from a pinched nerve root.20PubMed Central. Successful treatment of facet joint synovial cyst through percutaneous rupture: a case report The most common pattern is pain down one leg from a single pinched nerve low in the back.21PubMed. Lumbar nerve root compression by intraspinal synovial cysts. Report of 8 cases People describe it as burning, electric, or aching, traveling from the buttock or hip down the leg, sometimes with tingling or numbness in part of the foot. Which part of the leg and foot is affected depends on which nerve root is squeezed, so the pattern can point to the level of the cyst.
The back pain itself may be mild or may come from the worn joint, so the leg symptoms are often what bring people in. The leg pain often hurts more than the back, follows a line down the leg, often to the foot, and tends to worsen with standing and walking. Large cysts can squeeze the bundle of nerves at the bottom of the spine and cause leg pain or heaviness when walking that eases with rest.22PubMed. Lumbar nerve root compression by intraspinal synovial cysts. Report of 8 cases
Symptoms can also worsen suddenly. Long-term inflammation can grow fragile blood vessels in the cyst wall, and a bleed into the cyst can make it swell quickly and press on nerves.23PubMed Central. Hemorrhagic synovial cyst: An unexpected cause of acute cervical spinal cord compression. Case report Pain can suddenly worsen, sometimes with weakness or numbness.24PubMed Central. Hemorrhagic Lumbar Synovial Cyst Such bleeds have followed injuries or blood-thinner use, and can happen with no trigger.25PubMed. Spontaneous hemorrhage in an upper lumbar synovial cyst causing subacute cauda equina syndrome If your pain flared suddenly, mention any blood thinners you take.
Neck cysts most often press on the spinal cord, causing clumsy hands or unsteady walking.26PubMed Central. An Updated Review on Treatment of Cervical Synovial Cyst A knee cyst usually causes swelling or tightness behind the knee.
Some signs need same-day care: new numbness in the groin or inner thighs, trouble controlling your bladder or bowels, or rapidly worsening leg weakness or foot drop. These suggest severe nerve compression that needs urgent treatment.
How Doctors Diagnose a Synovial Cyst
Symptoms from these cysts are often the same as from a herniated disc.27Radsource. Lumbar Synovial Cyst MRI, however, shows the cysts with a recognizable look, so it usually makes the diagnosis.28Radsource. Lumbar Synovial Cyst They usually appear bright on fluid-sensitive MRI images, though appearance varies.29PubMed Central. Lumbar facet joint synovial cysts: does T2 signal intensity predict outcomes after percutaneous rupture?
The workup typically starts with an exam testing strength, reflexes, and sensation in your legs. Then comes the MRI to see the cyst and the nerves. Standing or bending X-rays may be added to check for a slip or loose joint. For a knee lump, ultrasound is often enough.
The radiologist usually names the level, the side, and how much the canal or nerve exit is narrowed, and these details matter more than the cyst’s size. The same report may list disc bulges, thickened ligaments, or arthritis, so not every finding explains your pain. Matching what the scan shows to where you feel symptoms is the doctor’s main job, since scans in people without pain often show changes too.
The right label matters because a disc and a cyst can need different operations, and knowing joint instability is present changes surgical planning. Now and then, a lesion thought to be a disc or tumor on MRI turns out to be a synovial cyst at surgery.30PubMed Central. Case of the Week: Preoperative MR/CT Diagnosis of Left L2-L3 Disc Surgically Documented As Massive Synovial Cyst So your surgeon may confirm the diagnosis during the operation.
Can a Synovial Cyst Go Away Without Surgery
Rarely for lasting relief. In one group of patients with lower back cysts treated without surgery, only 1 had lasting benefit, and 8 went on to surgery with good results.31PubMed. Lumbar intraspinal synovial cysts: conservative management and review of the world’s literature Occasionally a cyst shrinks away on its own; in one case pain eased and the cyst disappeared on MRI after 18 months of non-surgical care.32PubMed Central. The diagnosis and management of synovial cysts: Efficacy of surgery versus cyst aspiration Waiting can be reasonable when symptoms are mild, but most people with clear nerve pain end up needing more.
A typical trial starts with anti-inflammatory pain relievers and activity changes, plus physical therapy to strengthen the core. Over-the-counter anti-inflammatories can calm joint irritation and ease pain, but they do not shrink a cyst that is already pressing on a nerve. Check with your doctor before taking them regularly, especially if you take blood thinners or have kidney, stomach, or heart problems. Avoiding positions that worsen your leg pain, such as long standing, can help while you wait, but complete bed rest generally does not.
Non-surgical procedures include steroid injections into the joint and draining the cyst with a needle.33PubMed Central. Successful treatment of facet joint synovial cyst through percutaneous rupture: a case report A doctor can also burst the cyst with needles under X-ray guidance; in one case relief was immediate and lasted 2 years.34PubMed Central. Successful treatment of facet joint synovial cyst through percutaneous rupture: a case report These are done as an outpatient, and if pain persists you re-evaluate after several weeks.
If your symptoms are mild and steady and you have no weakness, a period of non-surgical care carries little risk. The cyst is not cancer and does not spread, so there is no urgency from the cyst itself. The reasonable plan is to set a time to recheck with your doctor rather than waiting open-endedly.
Surgery becomes the better choice when leg weakness or numbness is getting worse, when pain limits your walking despite treatment, or when the cyst returns after needle drainage. Any bladder or bowel changes call for immediate attention.
Surgery for a Synovial Cyst
Surgery removes the cyst and frees the pinched nerve. About 9 in 10 people report good pain relief afterward, and about 8 in 10 regain some of the leg strength they had lost.35PubMed. Surgical evaluation and management of lumbar synovial cysts: the Mayo Clinic experience On a 10-point pain scale, average pain fell by about 3 points and stayed improved for over a year.36PubMed Central. Patient Outcomes After Minimally Invasive Excision of Lumbar Synovial Cysts, With and Without a Spondylolisthesis, in an Ambulatory Care Center Setting Leg pain often improves soon after the operation.
Removing bone through a small incision works well as a first operation for most people and avoids fusion.37PubMed. Minimally Invasive Surgical Decompression without Fusion for the Treatment of Lumbar Synovial Cysts: Feasibility and Long-Term Outcomes The surgeon uses a tube to reach the spine, then removes a small piece of bone and ligament to expose the cyst. The cyst is separated from the nerve covering and taken out, which gives the nerve room. Because muscle is spread rather than cut widely, there is typically less pain afterward than with open surgery. It can be done at an outpatient surgery center,38PubMed Central. Patient Outcomes After Minimally Invasive Excision of Lumbar Synovial Cysts, With and Without a Spondylolisthesis, in an Ambulatory Care Center Setting so many people go home the same day. In one study, people with a slipped vertebra improved at least as much as those without one, though a loose segment can still raise the chance of the cyst returning.39PubMed Central. Patient Outcomes After Minimally Invasive Excision of Lumbar Synovial Cysts, With and Without a Spondylolisthesis, in an Ambulatory Care Center Setting
Fusion joins two vertebrae so they no longer move. It uses screws and rods plus bone graft to hold the bones still while they grow together. A loose segment helps cysts return, so fusing the vertebrae is often recommended when a vertebra has slipped.40PubMed Central. Minimally invasive surgery for lumbar synovial cysts with coexisting degenerative spondylolisthesis Without it, cysts came back more often and lingering back pain was more common.41PubMed Central. Decompression with or without Fusion for Lumbar Synovial Cysts—A Systematic Review and Meta-Analysis In one group of people with a cyst plus spinal narrowing or a slip, only about 6 in 10 had good or excellent results with bone removal alone.42PubMed. Lumbar laminectomy for the resection of synovial cysts and coexisting lumbar spinal stenosis or degenerative spondylolisthesis: an outcome study
The choice comes down to trade-offs. Removing the cyst alone is a smaller operation with faster recovery. Fusing the vertebrae lowers the chance of the cyst returning, because it removes the abnormal motion that helped the cyst form, but it adds operating time, a larger incision or additional small ones, and a longer recovery while the bones knit together. Spine stability on X-ray is the main deciding factor, and it is worth asking your surgeon how likely recurrence is in your case.
Treating a Baker’s Cyst Behind the Knee
Because extra knee fluid drives a Baker’s cyst, treating the arthritis or tear causing that fluid is the main approach.43Bezzy RA. Baker’s Cysts and Rheumatoid Arthritis Rest and ice help during flares, and treating knee arthritis often shrinks the cyst. Fluid drainage or an injection can help stubborn swelling, and surgery is rarely needed.
A Baker’s cyst can also rupture, and this is often missed.44PubMed Central. Ruptured Baker’s Cyst Demystified: Current Evidence, Diagnostic Strategies, and Treatment Options for an Under-Recognized Condition A rupture causes sudden calf pain and swelling, which a blood clot can mimic closely. Get urgent care to rule out a clot; an ultrasound can tell the two apart.