A bulge on the side of your knee is most often a fluid-filled cyst, an inflamed bursa, or swelling tied to a meniscal tear. The specific cause depends on exactly where the bulge sits, whether it appeared suddenly or grew slowly, and whether you have pain or other symptoms alongside it. While the vast majority of these lumps turn out to be benign and treatable, a handful of rarer conditions can mimic the common ones, so understanding the possibilities helps you have a more useful conversation with a doctor.
The Most Common Culprit on the Outer Side
If the bulge is on the outer (lateral) side of your knee, near where the top of the shinbone meets the small bone on the outside of your lower leg, a ganglion cyst is one of the likeliest explanations. These cysts form at the proximal tibiofibular joint, the small joint just below and to the outside of your kneecap. They are filled with thick, jelly-like fluid and tend to grow slowly. A systematic review of 100 patients with these cysts found that about 72% had a noticeable lateral fullness as their main finding, roughly 60% reported pain, and about 57% had nerve-related symptoms like tingling, numbness, or foot drop because the cyst pressed on the common peroneal nerve running nearby.1PubMed. Management and outcomes of proximal tibiofibular joint ganglion cysts: A systematic review In one case report, a patient developed a large ganglion at this joint that directly compressed the peroneal nerve, and then a year later, a similar but smaller cyst appeared on the opposite knee.2PubMed Central. Ganglion cyst at the proximal tibiofibular joint – A rare cause of compression neuropathy of the peroneal nerve
These cysts can be soft and squishy or fairly firm, depending on how much pressure has built up inside. If yours is painless and not growing quickly, a doctor may suggest watching it. But if you notice any weakness in lifting your foot or numbness along the outer shin, that nerve compression warrants prompt evaluation. Surgical removal is sometimes recommended, and in one study, only about 8% of patients who had complete cyst excision experienced a local recurrence.3PubMed Central. Ganglion Cysts of the Proximal Tibiofibular Joint: Low Risk of Recurrence After Total Cyst Excision
Parameniscal Cysts and Meniscal Tears
Another common source of a side-of-the-knee bulge is a parameniscal cyst. These develop right next to the meniscus, the C-shaped cartilage pad that cushions the inside and outside of each knee. When a meniscal tear creates a small channel in the cartilage, joint fluid gets pumped through the tear and pools on the outer edge, forming a visible or palpable lump. These cysts can appear on either the medial (inner) or lateral (outer) side of the knee, depending on which meniscus is torn.
What makes parameniscal cysts notable is how strongly they correlate with underlying meniscal damage. In a study of patients who underwent arthroscopy or MRI, meniscal tears were found beneath the cyst in roughly 86% to 96% of cases on the medial side, and every patient with a lateral cyst overlying the body or back of the lateral meniscus had a confirmed tear.4PubMed. Association of parameniscal cysts with underlying meniscal tears as identified on MRI and arthroscopy In other words, the cyst is rarely the primary problem. It is a downstream signal that your meniscus has been damaged. There is also evidence that cyst formation is related to tear size: tears measuring about 12 mm or more along the length of the meniscus are significantly more likely to produce a cyst than smaller ones.5PubMed. Parameniscal cyst formation in the knee is associated with meniscal tear size: an MRI study
If you have a parameniscal cyst, treating it usually means treating the underlying tear. Draining the cyst alone tends to lead to recurrence because the one-way channel through the torn meniscus keeps refilling it. Surgical techniques that address both the horizontal tear and the cyst together have been described for both the medial and lateral meniscus.6PubMed Central. Submeniscal Portal for Horizontal Cleavage Tear with Parameniscal Cyst of the Lateral Meniscus
Baker’s Cyst and the Back of the Knee
Technically not the “side,” but many people describe a Baker’s cyst as being on the side because it often bulges toward the inner-back part of the knee. A Baker’s cyst (also called a popliteal cyst) sits in the space behind the knee, between two tendons. It forms when a natural valve-like opening in the joint capsule allows fluid to flow from the knee into a bursa at the back, but does not let it flow back. Cadaver studies have found that this one-way valve exists in up to 40% to 54% of healthy adult knees.7PubMed Central. Baker’s Cyst Diagnostic and Surgical Considerations
Any condition that causes the knee to produce extra fluid, such as arthritis, a meniscal tear, or a ligament injury, can feed a Baker’s cyst. The cyst acts almost like a pressure-relief valve: research suggests the volume of the cyst tracks with the size of the knee effusion, meaning the more fluid your knee is producing, the bigger the cyst grows.7PubMed Central. Baker’s Cyst Diagnostic and Surgical Considerations The valve opens during flexion and closes when the knee is straight, which is why a Baker’s cyst often feels more prominent when you bend your knee and less noticeable when it is extended.
Most Baker’s cysts are painless unless they grow large enough to limit movement or, rarely, rupture. A ruptured cyst sends fluid down the calf and can mimic a deep vein thrombosis, which is why sudden calf swelling and pain after a known Baker’s cyst warrants urgent evaluation. For chronic cysts, ultrasound-guided aspiration combined with a corticosteroid injection is a well-established non-surgical option that can reduce pain and improve function, though recurrence is common if the underlying joint problem persists.8PubMed Central. Bedside ultrasound-guided aspiration and corticosteroid injection of a baker’s cyst in a patient with osteoarthritis and recurrent knee pain
Bursitis Around the Knee
The knee has over a dozen bursae, small fluid-filled sacs that reduce friction between tendons, ligaments, and bone. When one of these gets irritated, it can swell into a visible or tender bulge. The location of the bulge tells you which bursa is involved.
- Prepatellar bursitis: Swelling directly over the front of the kneecap, classically from repeated kneeling. It has been called “housemaid’s knee” for centuries.9International Journal of Biomedicine. Diagnostic Imaging of Chronic Prepatellar Bursitis (Housemaid’s Knee): An Elderly Patient Case Report The bulge is usually soft and well-defined, sitting right on top of the kneecap.
- Infrapatellar bursitis: Swelling below the kneecap. The deep infrapatellar bursa sits behind the patellar tendon and can become inflamed from overuse or direct impact, and occasionally from conditions like gout or inflammatory arthritis.10JOURNAL OF CLINICAL AND DIAGNOSTIC RESEARCH. Deep Infrapatellar Bursitis: An Unusual Cause of Chronic Anterior Knee Pain
- Pes anserine bursitis: Swelling and tenderness on the inner (medial) side of the knee, about two to three inches below the joint line. This one is easy to confuse with a meniscal tear because the most common symptom is pain along the medial joint line.11PubMed. Pes anserine bursitis: incidence in symptomatic knees and clinical presentation
Bursitis usually responds to rest, ice, and anti-inflammatory medication. If the bursa is infected (warm, red, and worsening), that is a different situation requiring antibiotics and sometimes drainage. The key distinction with bursitis is that the swelling feels superficial and moves with the skin, unlike a deeper cyst that feels anchored beneath the surface.
When the Bulge Shows Up in a Teenager
In adolescents, the most recognizable bump near the knee is the bony prominence of Osgood-Schlatter disease. It appears just below the kneecap on the tibial tuberosity, the point where the patellar tendon attaches to the shinbone. The condition is an overuse injury tied to growth: the developing bone at this attachment site cannot always withstand the pulling forces from the tendon during running, jumping, and squatting, leading to inflammation and sometimes small avulsions of the growing bone.12PubMed Central. Apophysitis of the Tibial Tuberosity (Osgood-Schlatter Disease): A Review13PubMed. Osgood-Schlatter’s disease and tibial tuberosity development
The result is a hard, bony bump that is tender to touch and gets worse with activity. It is most common during growth spurts in active kids. The bump itself is bone and does not go away entirely even after the pain resolves, which can worry parents years later. In the vast majority of cases, the condition is self-limiting and resolves once the growth plate closes, though the bony prominence usually persists as a painless lump into adulthood.
Gout and Crystal Deposits
Gout is not just a disease of the big toe. When uric acid crystals accumulate around the knee over time, they can form chalky deposits called tophi that present as firm, irregular masses. In advanced cases, these deposits can grow large enough to look like a soft-tissue tumor on imaging. One case report described bilateral knee masses caused by gouty tophi that were large enough to be visible on plain X-rays and initially prompted a biopsy to rule out cancer.14PubMed Central. Chronic Tophaceous Gout Presenting as Bilateral Knee Masses in an Adult Patient: A Case Report
Another report described a 30-year-old woman with painless swelling of the knee joint that turned out to be tophaceous gout, diagnosed after joint fluid aspiration revealed the characteristic needle-shaped urate crystals.15Dubai Medical Journal. Monoarticular Crystal Arthropathy of the Knee: Tophaceous Gout The takeaway: a firm, slowly growing bump near the knee in someone with a history of gout or elevated uric acid levels is worth investigating for crystal deposits, especially if it does not behave like a typical cyst.
Traumatic Fluid Collections
If the bulge appeared after a direct blow or a fall, a Morel-Lavallée lesion is worth considering. This is a collection of fluid, blood, and sometimes fat that forms between layers of tissue beneath the skin after blunt force or a shearing injury. The trauma separates the fat layer from the underlying fascia, creating a pocket that fills with fluid. These lesions can look and feel like a cyst but have a distinctly different origin.16PubMed Central. Diagnosis and Treatment of a Morel-Lavallée Lesion in the Lateral Knee With Point-of-Care Ultrasonography
Morel-Lavallée lesions sometimes resolve on their own but can become chronic if the body walls them off with a fibrous capsule. In those cases, aspiration or surgical drainage may be needed. The history of a specific traumatic event is the biggest clue. If you had a fall, a sports collision, or a car accident and then noticed a squishy swelling on the side of your knee days to weeks later, this diagnosis should be on the list.
Vascular Mimics Behind the Knee
A popliteal artery aneurysm is rare, especially in women, but it can masquerade as a Baker’s cyst or a soft-tissue mass. The popliteal artery runs behind the knee, and when it develops an aneurysm (a ballooning of the artery wall), the resulting lump can feel similar to a cyst. The key danger is that once the aneurysm thromboses (fills with clot), it loses its pulsatile quality and becomes even harder to distinguish from a benign lump by feel alone.17PubMed Central. A Thrombosed Popliteal Artery Aneurysm Masquerading as a Benign Soft-Tissue Mass: A Case Report Emphasizing the Role of Multimodal Imaging
One case involved an 81-year-old woman with tingling in her leg caused by a 10-cm popliteal artery aneurysm, large enough to compress surrounding nerves.18PubMed Central. Open Repair of a Giant Popliteal Artery Aneurysm Presenting with Nerve Compression Symptoms Popliteal aneurysms are most common in older men with a history of atherosclerosis or other aneurysms elsewhere. If your behind-the-knee bulge pulses, or if ultrasound shows blood flow within it, that changes the diagnosis and the urgency entirely.
When a Lump Is Not What It Seems
The uncomfortable reality is that a small number of knee-area lumps are not cysts, bursitis, or benign growths. Soft tissue sarcomas are rare cancers that can arise in muscle, fat, or connective tissue near joints. Because they grow slowly and are often painless at first, they frequently get mistaken for benign conditions like a Baker’s cyst or an inflamed bursa. A study examining cases of soft tissue sarcomas around joints found that the initial presentation radiologically resembled benign synovial pathology, which led to delayed diagnosis and treatment.19PubMed Central. Soft Tissue Sarcomas Mimicking Benign Inflammatory Processes: A Diagnostic Dilemma
This is not meant to frighten you. Soft tissue sarcomas account for a very small fraction of all lumps around the knee. But there are red flags that should prompt you to get imaging sooner rather than later: a lump that is growing steadily over weeks, a mass that is deep and feels fixed to underlying structures rather than movable, a lump larger than about 5 cm (roughly the size of a golf ball), and any mass that is firm or hard rather than squishy. Night pain and unexplained weight loss are also warning signs, though they tend to occur later.
How Doctors Figure Out What It Is
The diagnostic process for a knee bulge usually starts with a physical exam. Where the lump sits, how it feels, whether it transilluminates (glows when a light is pressed against it, suggesting clear fluid), and whether it changes size with knee movement all offer clues. But physical exam alone is often not enough to distinguish between possibilities.
Ultrasound is typically the first imaging step because it is quick, inexpensive, and can show whether a mass is fluid-filled or solid, and whether there is blood flow inside it. MRI is the next level and gives a much more detailed picture of what the mass is, where it originates, and how it relates to surrounding structures like menisci, ligaments, and nerves. Both modalities are considered highly useful for evaluating the range of knee conditions described above, each with different strengths depending on the suspected diagnosis.20PubMed Central. Evaluation of the knee joint with ultrasound and magnetic resonance imaging
If imaging raises any concern about a solid mass or an unusual appearance, a biopsy may follow. For suspected gout, joint fluid aspiration can confirm the diagnosis by identifying urate crystals under a microscope. For a suspected vascular cause, Doppler ultrasound or CT angiography can show blood flow patterns that distinguish an aneurysm from a cyst.
Practical Steps if You Find a Bulge
Most side-of-the-knee bulges do not require emergency care, but a few situations call for urgency. Sudden swelling with severe pain and an inability to bend or straighten the knee could indicate a ruptured Baker’s cyst, a joint infection, or a large meniscal tear with a locked knee. Numbness, tingling, or foot weakness associated with a lateral knee lump suggests nerve compression that should not wait weeks for evaluation. And any lump that is growing quickly, feels rock-hard, or appeared without an obvious injury or overuse history deserves imaging to rule out something more unusual.
For the garden-variety cysts and bursitis that make up the majority of cases, the first line of treatment is usually conservative: rest, ice, compression, and over-the-counter anti-inflammatories. If a Baker’s cyst or bursa is large or symptomatic enough to interfere with daily life, aspiration with or without a corticosteroid injection is a reasonable next step. Addressing the root cause matters most with conditions like parameniscal cysts, where the meniscal tear feeding the cyst needs to be dealt with to prevent recurrence, and Baker’s cysts, where treating the underlying arthritis or cartilage damage reduces the fluid production that keeps the cyst refilling.