A meniscal cyst is a fluid-filled sac that forms next to, or within, one of the crescent-shaped cartilage pads in your knee. It develops when joint fluid gets pushed through a tear in the meniscus and becomes trapped on the other side, ballooning into a cyst. Although the name sounds alarming, these cysts are closely tied to the underlying meniscal tear that feeds them, and treating the tear usually resolves the cyst as well.
How a Meniscal Cyst Forms
The meniscus sits between your thighbone and shinbone, acting as a shock absorber and stabilizer. When the meniscus tears, especially with a particular type of horizontal split that runs along its length, the torn edges can create a passage that lets synovial fluid (the slippery liquid that lubricates the joint) leak out. As you bend and straighten your knee, pressure pumps fluid through the tear in one direction, but the flap-like edges don’t let it flow back. This one-way valve effect causes fluid to accumulate in or around the meniscus, gradually inflating into a cyst.1The Open Orthopaedics Journal. Not Always a Baker’s Cyst – An Unusual Presentation of a Central Voluminous Postero-Medial Meniscal Cyst
One study of surgically treated meniscal cysts found a horizontal cleavage tear in every single case, confirming how tightly the two problems are linked.2PubMed. Arthroscopic treatment of meniscal cysts On imaging, roughly 98% of meniscal cysts sit immediately adjacent to a meniscal tear, with a visible tract connecting the two.3PubMed. MRI appearance of meniscal cysts That direct connection is one of the features that separates a meniscal cyst from other fluid collections around the knee.
How Common They Are and Where They Appear
Meniscal cysts show up in about 4% of knee MRI scans. In a large review of over 2,500 MRI examinations, roughly 8% of all meniscal tears had an accompanying cyst, with the rate almost identical for medial and lateral tears.4PubMed. MR imaging of meniscal cysts: incidence, location, and clinical significance That means most meniscal tears do not produce a cyst, but when they do, the cyst can appear on either side of the knee.
There is an older clinical impression that lateral meniscal cysts are more common, partly because lateral cysts tend to bulge outward and are easier to feel with your hand. Imaging studies tell a more balanced story. Multiple MRI-based series have found that medial cysts actually outnumber lateral ones, accounting for roughly 56 to 66% of cases.5PubMed. New observations on meniscal cysts6PubMed. Relative incidence and morphology of lateral and medial meniscal cysts detected by magnetic resonance imaging The discrepancy with older clinical literature probably reflects a detection bias: lateral cysts sit closer to the skin surface and are more obvious on physical examination, while medial cysts are more likely to stay hidden between layers of soft tissue until imaging picks them up.
In terms of position along the meniscus, medial cysts most often form near the back (posterior) third, whereas lateral cysts tend to cluster around the middle third.7PubMed. Relative incidence and morphology of lateral and medial meniscal cysts detected by magnetic resonance imaging The anterolateral corner of the knee is another common spot, particularly for cysts that become large enough to palpate.
Symptoms You Might Notice
Small meniscal cysts can be completely silent. One MRI study of asymptomatic knees found small medial meniscal cysts in several subjects who had no complaints at all, with the cysts averaging less than a centimeter in their largest dimension.8AJR Am J Roentgenol. Prevalence and size of meniscal cysts, ganglionic cysts, synovial cysts of the popliteal space, fluid-filled bursae, and other fluid collections in asymptomatic knees on MR imaging When a cyst does cause trouble, the symptoms typically overlap with those of the underlying meniscal tear: pain along the joint line, occasional swelling, and a feeling of tightness or fullness.
Larger cysts can produce a visible or palpable bump on the side of the knee, and in some cases that bump is what drives a person to seek medical attention. One reported case described a lateral meniscal cyst that grew large enough over several years to resemble a soft-tissue tumor, eventually causing locking episodes where the knee would catch and refuse to straighten fully.9PubMed. Tumor-like meniscal cyst That degree of growth is unusual, but it illustrates why a persistent, firm lump at the knee joint line should not be brushed off.
Many people find that their symptoms fluctuate. The cyst may feel larger and more painful after activity and shrink somewhat with rest, tracking the amount of fluid being pumped through the tear. This waxing and waning can make the problem easy to dismiss until it gets big enough to interfere with daily movement.
How Meniscal Cysts Are Diagnosed
MRI is the go-to tool. It shows both the cyst itself and the meniscal tear feeding it, which matters because treating the cyst without addressing the tear usually leads to recurrence. On MRI, meniscal cysts appear as well-defined, fluid-bright collections sitting right at the edge of the meniscus, with a visible connection to the underlying tear.10PubMed. MRI appearance of meniscal cysts MRI can also distinguish meniscal cysts from lookalike fluid collections that happen to sit near the joint line but have no link to a meniscal tear.
Ultrasound has become a useful second option, especially when you want a quick, low-cost assessment. High-resolution knee ultrasound correctly identified meniscal cysts in 31 of 32 confirmed cases in one study, with an overall accuracy of 94%.11PubMed. Meniscal cysts: detection with high-resolution sonography It is also practical for guiding a needle into the cyst if aspiration is planned.12PubMed Central. Diagnosis and treatment of a lateral meniscal cyst with musculoskeletal ultrasound The limitation of ultrasound is that it does not reliably show the meniscal tear itself, so MRI is still needed to plan treatment.
What Else It Could Be
Several other fluid collections around the knee can look and feel similar, and sorting them out matters because the treatment differs. Baker’s cysts (popliteal cysts) are by far the most common cystic finding on knee MRI, with an incidence around 38%.13PubMed Central. MRI characteristics of cysts and “cyst-like” lesions in and around the knee: what the radiologist needs to know They form at the back of the knee, communicate with the joint through a specific anatomical channel, and are usually related to arthritis or general joint irritation rather than a discrete meniscal tear.
Other possibilities include ganglion cysts that arise from ligaments or the joint capsule, inflamed bursae (the small fluid pockets that cushion tendons), and rarer entities such as hematomas, abscesses, or even tumors that happen to look cystic on imaging.14PubMed Central. Cysts and cystic-appearing lesions of the knee: A pictorial essay The hallmark that pins down a meniscal cyst is the visible trail of fluid connecting the cyst back to a meniscal tear. If that trail isn’t there on MRI, the lump is almost certainly something else.
Non-Surgical Treatment
Because many meniscal cysts are small and painless, the first approach for a mildly bothersome cyst is often watchful waiting combined with activity modification and anti-inflammatory medication. In rare cases, a cyst may resolve on its own. At least one documented case showed spontaneous resorption of an intra-articular ganglionic cyst on follow-up MRI three years later.15PubMed Central. Intra-articular Ganglionic Cysts of Knee: A Cause for Anterior Knee Pain Spontaneous resolution is the exception rather than the rule, but it does mean that immediate intervention is not always necessary.
Ultrasound-guided aspiration, where a needle is inserted into the cyst to drain the fluid, can provide quick relief. In one series of 18 patients, more than half had complete and lasting resolution of symptoms after aspiration and were able to return to high-level sport. A third, however, saw their symptoms return within a few weeks.16PubMed. Ultrasound-guided percutaneous drainage of meniscal cysts: preliminary clinical experience The main drawback is that aspiration does not fix the meniscal tear, so the one-way valve keeps pumping and the cyst can refill. Aspiration works best as a bridge, either buying time or confirming that the cyst is the pain source before deciding on surgery.
Surgical Options and What to Expect Afterward
When a meniscal cyst keeps causing trouble, the standard approach is arthroscopic surgery that addresses both the cyst and the tear feeding it. Three broad techniques are in use: arthroscopic cyst decompression, where the wall between the cyst and the joint is opened so fluid drains back inward; arthroscopic cyst excision, where the cyst is removed entirely through the scope; and percutaneous decompression under arthroscopic visualization, where a needle is advanced into the cyst from outside while the surgeon watches through a camera inside the joint.
Across all approaches, a systematic review found that roughly 88% of patients rated their outcome as “excellent” or “good,” about 86% had their symptoms resolve or were left with only minor complaints, and close to 80% returned to sport at their previous level.17PubMed Central. Arthroscopic Management of Meniscal Cysts: A Systematic Review Cyst recurrence across all arthroscopic techniques was under 10%, though the specific rate varied by technique.
That variability is worth noting. A separate systematic review reported a recurrence rate of about 10% after decompression compared with roughly 3.4% after open cystectomy.18PubMed Central. Do meniscal repairs with meniscus cyst do better than meniscectomy?—a systematic review of meniscal cyst treatment A head-to-head comparison study put the difference more starkly: the decompression group had a sixfold higher recurrence risk than the excision group over a follow-up period averaging about two years (21% versus 4%).19PubMed. Comparison of the recurrence risk of parameniscal cysts between patients treated with arthroscopic excision and arthroscopic decompression techniques Excision is more thorough, but the best technique for a given patient depends on the cyst’s size, location, and how much meniscal tissue can be preserved.
Meniscal preservation has become a priority in orthopedic surgery. Research since the 1970s has made clear that removing large portions of the meniscus raises the long-term risk of osteoarthritis in that compartment of the knee.20PubMed Central. The evolution of the meniscus: Where surgical advancements meet translational research – Section: Meniscectomy So surgeons try to repair the tear and preserve as much meniscal tissue as possible, removing only what is necessary. This is especially true in younger patients, where losing the meniscus early translates to decades of altered joint mechanics.
Cyst Formation After Meniscal Repair
Here is an irony that catches some patients off guard: a meniscal cyst can sometimes develop after surgery to repair a meniscal tear. One study following 102 meniscal repairs found that cysts appeared in about 14% of cases overall, but the rate varied dramatically with the repair technique used. Repairs performed with all-inside suture anchors had a 40% cyst formation rate, while inside-out sutures saw only about 2%. Having a medial meniscus tear also raised the risk substantially.21PubMed. Incidence and Risk Factors for Meniscal Cyst After Meniscal Repair
The likely mechanism is that the repair devices themselves create small channels through the meniscus, and if those channels don’t seal completely, they can mimic the same one-way valve that produces a cyst from a natural tear. This finding is relevant if you are told you need a meniscal repair, because the choice of technique may influence whether a cyst develops later. It does not mean all-inside repairs should be avoided; they have other advantages including smaller incisions and faster operating times. But the possibility of a post-repair cyst is something your surgeon should factor in, especially for medial tears.
When a Cyst Compresses a Nerve
A rare but significant complication occurs when a lateral meniscal cyst grows large enough to press on the common peroneal nerve, which wraps around the outside of the knee just below the joint line. This nerve controls your ability to lift the front of your foot, so compression can cause foot drop, numbness along the outer shin, or both. Case reports have documented lateral meniscal cysts extending around the back of the fibula and squeezing the nerve against the bone.22PubMed. Compression of the common peroneal nerve by a cyst of the lateral meniscus. A case report More recent cases have confirmed the pattern, with patients presenting with sudden-onset weakness in foot and ankle motion that turned out to be caused by a meniscal cyst rather than a spinal or neurological problem.23PubMed Central. Lateral Meniscal Cyst Diagnosed After Acute Onset of Common Peroneal Nerve Palsy
The clinical takeaway is that an unexplained foot drop or new numbness on the outer part of the lower leg, especially paired with a lump near the lateral knee, warrants imaging not just of the spine but of the knee. Peroneal nerve palsy from a meniscal cyst is treatable: draining or removing the cyst relieves the pressure, and nerve function typically recovers, though the speed of recovery depends on how long the nerve was compressed.
Meniscal Cysts in Children and Adolescents
Meniscal cysts are less commonly discussed in pediatric patients, but they do occur. Children and teenagers who develop meniscal tears, whether from sports injuries or from congenital variations in meniscal shape like a discoid lateral meniscus, can develop the same kind of cyst-forming mechanism seen in adults. A discoid meniscus is an unusually thick, disc-shaped meniscus that covers more of the tibial surface than a normal crescent-shaped one. It is more prone to tearing, and those tears can produce cysts through the same valve mechanism.
Meniscal preservation is even more important in younger patients. The long-term consequences of removing meniscal tissue are magnified when the patient has 50 or 60 years of activity ahead of them. For that reason, surgeons lean heavily toward repair rather than removal in the pediatric population, accepting a modestly higher rate of reoperation in exchange for protecting the joint surface over the long haul.
Watching and Waiting Versus Acting Early
If you have been told you have a meniscal cyst on an MRI and you are not in much pain, the natural question is whether you need to do anything about it right now. For small, minimally symptomatic cysts, many orthopedic surgeons advise a period of observation. Activity modification, physical therapy to strengthen the muscles around the knee, and occasional use of anti-inflammatory medication can manage symptoms while you and your doctor track how the cyst behaves over time.
There are a few situations where waiting is less advisable. A cyst that is growing steadily, one that interferes with your range of motion or ability to work and exercise, or one that is causing neurological symptoms like numbness or weakness around the lower leg should push the conversation toward intervention sooner. Because the cyst is almost always a downstream consequence of a meniscal tear, the tear itself also factors into the decision. A repairable tear in a young, active patient is worth fixing early to preserve meniscal tissue; a degenerative tear in an older patient with a small, tolerable cyst may reasonably be watched.
The evidence on recurrence after aspiration versus surgery underscores a general principle: treatments that address only the cyst (draining it, injecting corticosteroid around it) are palliative, while treatments that also address the tear (arthroscopic repair, partial meniscectomy with cyst decompression) are more durable. The decision is not one-size-fits-all, but understanding that the cyst and the tear are a package deal is the key to making a good one.