Can You Get a Ganglion Cyst on Your Knee?

Ganglion cysts absolutely can develop in and around the knee, though they are far less well known there than the classic bump on the back of the wrist. Knee ganglion cysts turn up inside the joint itself, attached to ligaments and cartilage, embedded in the fat pad behind the kneecap, or sitting just beneath the skin on the outer surfaces of the knee. They often get mistaken for other knee problems or overlooked entirely until an MRI picks them up for an unrelated complaint. The variety of locations and the overlap with other cyst types make knee ganglion cysts a more complex topic than most people expect.

Where Exactly They Show Up

The knee joint is a surprisingly hospitable environment for ganglion cysts. Inside the joint, they most frequently attach to the cruciate ligaments. In one review of 85 intra-articular ganglion cysts, 49 arose from the anterior cruciate ligament (ACL) and 16 from the posterior cruciate ligament (PCL).¹ Another imaging study of 20 cysts found half originated from the PCL, a quarter from the ACL, and a quarter within the infrapatellar fat pad.² A separate series found about 61% of intra-articular knee ganglia sitting in the intercondylar notch, the bony groove between the two rounded ends of the thighbone.³

Outside the joint, ganglion cysts can form in soft tissue around the kneecap, along the sides of the knee near the fibular head, or in the calf muscle just below the knee. One case described a large ganglion cyst that dissected into the gastrocnemius muscle behind the knee, initially mimicking a Baker’s cyst.⁴ Extra-articular ganglion cysts around the knee were surgically treated in one study, with functional knee scores improving substantially after removal and MRI at three months showing no recurrence in all but one case.⁵

What Causes Them to Form

Nobody has nailed down a single cause. The leading theory points to repetitive microtrauma that triggers connective tissue to undergo a process called mucinous degeneration: fibroblasts in and around ligaments and joint capsules get stimulated by repeated minor injuries and begin producing excessive amounts of a thick, jelly-like substance rich in hyaluronic acid.⁶ That mucin accumulates under a thin fibrous wall with no true cell lining, and the result is a ganglion cyst. Other proposed mechanisms include herniation of the synovial membrane (the tissue lining the joint capsule) and even congenital factors, though no single explanation accounts for every case.⁷

Occupational and activity-related stress on the knee seems relevant. One case involved a kindergarten teacher who spent much of her workday kneeling; she developed a large ganglion cyst in the infrapatellar fat pad that eventually prevented her from bending her knee.⁸ Another involved a runner whose cyst on the PCL became symptomatic after increasing her training mileage for a half-marathon.⁹ Despite these suggestive examples, many patients who develop knee ganglion cysts have no clear history of trauma or overuse. In one large series of 85 cases, the patients with symptomatic cysts had no histories of previous knee injury and no clinical signs of instability or cartilage damage.¹

How a Knee Ganglion Cyst Feels

Knee pain is the dominant symptom. In a series of 31 patients with cruciate ligament ganglion cysts, knee pain was the chief complaint in every single case.¹⁰ The pain tends to be vague and intermittent, which makes it easy to blame on general “knee trouble” rather than a specific mass. Because many of these cysts sit deep inside the joint, you often cannot feel them from the outside. They tend to cause a dull ache during activity, sometimes accompanied by a sensation of the knee catching or locking when the cyst grows large enough to physically obstruct normal motion. In one pediatric case, a 7-year-old boy could not fully straighten his knee because of a cyst filling the intercondylar notch.¹¹

Cysts in Hoffa’s fat pad, the wedge of tissue behind the patellar tendon, can produce visible swelling on the front and sides of the knee. Pain in this area may worsen with extension, since the fat pad gets compressed each time you straighten the leg.¹² Extra-articular cysts sitting closer to the skin are easier to notice: you may feel a firm, rubbery lump that shifts slightly under pressure.

Ganglion Cysts Versus Baker’s Cysts

This is one of the most common mix-ups around knee lumps, and the distinction matters because the two conditions respond to different treatments. A Baker’s cyst (popliteal cyst) forms in the back of the knee when joint fluid from an underlying problem like arthritis or a meniscal tear herniates into the popliteal bursa. It is essentially a balloon of synovial fluid that connects back to the joint space. A ganglion cyst, by contrast, has a fibrous wall with no epithelial lining and contains thick mucoid material rather than watery joint fluid. It does not necessarily communicate with the joint cavity at all.

Why this matters practically: Baker’s cysts often shrink or disappear when you fix the underlying joint pathology, such as repairing a torn meniscus or managing arthritis. Ganglion cysts do not tend to respond to those strategies. One surgical team emphasized that because ganglion cysts around the knee commonly need direct excision and do not resolve with the treatments used for Baker’s cysts, it is important for clinicians to tell them apart.⁴ If you have been told you have a Baker’s cyst behind your knee but it hasn’t responded to treatment of whatever joint problem was blamed, a ganglion cyst is worth considering.

Parameniscal Cysts and Their Relationship to Meniscal Tears

A related but somewhat different category: parameniscal cysts form at the outer edge of the meniscus, the C-shaped cartilage cushion inside the knee. These cysts are strongly associated with horizontal tears in the meniscus. In one imaging study, about 91% of meniscal cysts sat immediately adjacent to a meniscal tear, and nearly all of those tears were of the horizontal cleavage type, with the tear leading directly into the cyst.¹³ Joint fluid essentially gets pumped through the tear with each step and accumulates along the outer margin of the cartilage.

Bigger tears produce more cysts. One study found that meniscal tears measuring 12 millimeters or more along the cartilage’s circumference were associated with parameniscal cyst formation, and patients with cysts had significantly larger tears than those without.¹⁴ These cysts sometimes get lumped under the “ganglion cyst” umbrella in casual conversation, but they are pathologically distinct: parameniscal cysts contain synovial fluid pushed through a tear, while true ganglion cysts contain mucoid material produced by degenerating connective tissue. The treatment approach differs accordingly, since parameniscal cysts often resolve when the underlying meniscal tear is repaired.

When a Cyst Presses on a Nerve

One of the more concerning complications of a ganglion cyst near the knee is compression of the common peroneal nerve, which wraps around the head of the fibula just below the outer side of the knee. This nerve controls muscles that lift the foot and toes. When a cyst pushes on it, the result can be foot drop, a sudden inability to lift the front of the foot while walking. It is rare but well documented. One case report described an extraneural ganglion cyst sitting above the knee that compressed the common peroneal nerve enough to cause foot drop.¹⁵

This is where differential diagnosis becomes especially important. Both intraneural ganglion cysts and synovial sarcomas, a type of soft tissue cancer, tend to arise near the fibular head and can look similar on MRI, particularly when the mass is smaller than about 5 centimeters. Researchers have emphasized that a biopsy should be performed before surgery in ambiguous cases to avoid misdiagnosis.¹⁶ If you develop numbness, tingling, or weakness in the lower leg or foot along with a lump near the outer knee, imaging and possibly biopsy are warranted rather than a wait-and-see approach.

How Knee Ganglion Cysts Are Diagnosed

MRI is the gold standard. Standard X-rays do not show ganglion cysts because the cysts are made of soft tissue and fluid, not bone. On MRI, a ganglion cyst typically appears as a well-defined, often lobulated mass that looks dark on one type of image sequence and bright on another. That characteristic appearance, combined with the cyst’s location, usually allows radiologists to make the diagnosis without surgery.¹⁷ In the Hoffa’s fat pad case mentioned earlier, MRI showed a tubulocystic lesion with low signal on T1-weighted images and high signal on T2-STIR images, a pattern radiologists recognize as fluid-filled.¹²

Many knee ganglion cysts are incidental findings: a person gets an MRI for a different reason, perhaps suspected ligament damage after a sports injury, and the cyst turns up unexpectedly. The real diagnostic challenge is determining whether the cyst is the source of the patient’s symptoms or an innocent bystander. In the series of 85 intra-articular cases, only nine cysts were judged to be solely responsible for the patient’s knee discomfort.¹ The rest coexisted with other knee pathology, making it hard to pin down exactly how much of the pain came from the cyst.

Treatment Options

The right approach depends on whether the cyst is causing problems and where it is located. For cysts found incidentally that are not producing symptoms, observation alone is reasonable, since some have been reported to regress on their own.¹⁸ That said, the evidence for complete spontaneous resolution is thin. A systematic review of intraneural ganglion cysts found no confirmed cases of complete regression without surgery that met its quality criteria; the handful of reported regressions all had residual cysts on follow-up imaging.¹⁹ Watching and waiting makes sense for asymptomatic cysts, but it probably will not make a symptomatic one disappear.

Ultrasound-Guided Aspiration

For accessible cysts, a needle guided by ultrasound imaging can drain the thick contents. One study of ultrasound-guided therapy for knee and foot ganglion cysts found that about 90% of patients experienced immediate pain relief, and roughly three-quarters had no recurrence of symptoms at an average follow-up of about 14 months.²⁰ Some clinicians add a saline injection after aspiration to help the cyst collapse, with case reports showing good functional recovery using this approach.²¹ The downside is that recurrence is always possible, since aspiration drains the contents but leaves the cyst wall intact.

Arthroscopic Resection

For intra-articular cysts, particularly those attached to the cruciate ligaments, arthroscopic surgery is the preferred treatment. A camera and instruments are inserted through small incisions, allowing the surgeon to remove the entire cyst including its walls while visualizing the interior of the joint. In one study, all patients who underwent arthroscopic resection of cruciate ligament ganglion cysts had their symptoms resolve completely, with normal joint motion restored at an average follow-up of two years.²² Another case achieved complete symptom relief after arthroscopic removal of an ACL ganglion cyst with no recurrence.²³ The emphasis across the literature is on complete removal of both the cyst contents and the fibrous wall to minimize the chance of recurrence.

Open Surgical Excision

Larger cysts or those in locations difficult to reach arthroscopically may require open surgery. This is more common with extra-articular cysts and giant cysts in Hoffa’s fat pad. The fat pad case mentioned earlier required open excision, and histopathology of the specimen confirmed the diagnosis.¹² In the extra-articular cyst study, open excision yielded strong improvements in functional knee scores and no recurrence in all but one patient, who developed very small asymptomatic cysts on follow-up MRI.⁵

Children Can Get Them Too

Knee ganglion cysts are uncommon in children, but they do occur and may behave differently from adult versions. A 7-year-old boy presented with three months of worsening knee pain and an inability to fully straighten his knee. MRI revealed a ganglion cyst within the ACL, and arthroscopic removal followed by a rehabilitation program led to complete recovery, including a return to sports.¹¹ In another case involving a child of the same age, histological examination of the removed cyst showed no mucoid degeneration, which is almost always present in adult ganglion cysts. The authors suggested that the way these cysts develop in children may differ from the degenerative process that drives them in adults.²⁴ The advent of MRI has increased the detection rate in pediatric patients, since these cysts would previously have been missed entirely on standard imaging.

Ganglion Cysts After Knee Surgery

A somewhat ironic twist: ganglion cysts can form as a complication of prior knee surgery, particularly ACL reconstruction. The tibial tunnel drilled during reconstruction provides a channel along which a cyst can develop, sometimes surfacing as a visible lump on the front of the shinbone months or even years later. In one case, a symptomatic ganglion cyst appeared along the tibial tunnel four years after an ACL hamstring autograft reconstruction.²⁵ Another report described recurrent cyst formation after ACL reconstruction, underscoring that this complication, while rare, can be persistent.²⁶ Four cases of subcutaneous pretibial ganglion cysts with direct communication to the tibial tunnel have also been documented after ACL reconstruction using hamstring or iliotibial band grafts.²⁷

Post-surgical ganglion cysts can also develop at the arthroscopic portal sites themselves, the small incisions used during knee arthroscopy. Two such cases were reported at the lateral portal region of the knee, with one resolving after surgical removal and the other doing poorly with conservative management alone.²⁸ If you develop a new lump or renewed pain near your surgical scars after knee surgery, a ganglion cyst belongs on the list of possibilities.

Mucoid Degeneration and the ACL

Ganglion cysts of the ACL are sometimes found alongside a related but distinct condition called mucoid degeneration, in which the ligament itself swells with mucoid substance and takes on an abnormal appearance on MRI. The two can coexist but are considered separate entities. Both are non-traumatic and relatively uncommon, and their exact cause remains unclear.²⁹ Mucoid degeneration can enlarge the ACL enough to mimic the symptoms of a cyst, including restricted knee motion and vague pain. When an MRI shows an abnormal ACL with surrounding or embedded cystic areas, distinguishing between a focal ganglion cyst, diffuse mucoid degeneration, and a combination of both affects how the surgeon plans treatment. A focal cyst may be resectable on its own, while diffuse degeneration throughout the ligament raises questions about whether the ligament is structurally compromised.

Why These Cysts Are Probably Underrecognized

Before MRI became routine for evaluating knee problems, intra-articular ganglion cysts were considered rare curiosities found mostly during surgery for other conditions. Their detection rate has climbed steadily with the broader use of knee MRI, suggesting that many go undiagnosed in people who never get advanced imaging. Because the symptoms overlap heavily with more common knee problems, including nonspecific pain, occasional catching, and mild swelling, a ganglion cyst can masquerade as runner’s knee, a minor meniscal issue, or early arthritis for months or years. The Hoffa’s fat pad variant in particular is described as rare in the literature, yet case reports keep accumulating, hinting that “rare” may partly reflect underdiagnosis rather than true scarcity.³⁰ If you have persistent knee pain that hasn’t responded to the usual treatments and you haven’t had an MRI, the imaging may well reveal a cyst nobody expected.

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