A lump on the side of your knee is almost always a benign, fluid-filled structure, but the specific cause depends on exactly where the lump sits, how it feels, and what other symptoms come with it. The most common culprits are bursitis, ganglion cysts, meniscal cysts, and Baker’s cysts, though rarer possibilities like bone growths, soft-tissue tumors, and even vascular problems can produce a visible bump. Most of these lumps respond well to conservative treatment or resolve on their own, but a few features should prompt you to see a doctor sooner rather than later.
The Knee Has Many Structures That Can Swell
Your knee is surrounded by more than a dozen bursae, which are small fluid-filled sacs positioned between tendons and bone or between skin and tendon to reduce friction during movement.1PubMed Central. Bursae around the knee joints When any of these bursae become irritated or inflamed, they can swell enough to become visible and palpable, creating a soft lump on the inner or outer side of the knee. Beyond bursae, the knee joint also contains a synovial membrane, two menisci made of cartilage, several tendons passing over bony prominences, and a network of small arteries. Any of these structures can develop a cyst, growth, or swelling that feels like a lump. That wide range of possible sources is why location and associated symptoms matter more than the lump itself.
Ganglion Cysts Near the Outer Knee
If you feel a firm, rubbery lump on the outer side of your knee near the top of the fibula (the smaller bone on the outside of your lower leg), there is a reasonable chance it is a ganglion cyst. These develop at the proximal tibiofibular joint, the small joint where the shinbone meets the fibula just below the knee. A systematic review found that about seven in ten people with ganglion cysts in this location had a noticeable lateral fullness or palpable mass, roughly six in ten reported pain, and about six in ten had symptoms related to compression of the nearby peroneal nerve.2The Knee. Management and outcomes of proximal tibiofibular joint ganglion cysts: A systematic review
Peroneal nerve compression is the detail worth paying attention to. That nerve runs just behind and around the head of the fibula, and when a ganglion pushes against it, you can develop numbness or tingling along the outer calf and top of the foot, weakness when you try to lift your foot upward, or a foot drop that makes you trip. One reported case involved a 68-year-old woman who developed a lump over several months that turned out to be a large multi-loculated ganglion cyst measuring about 7 cm in length, big enough to compress the peroneal nerve significantly.3PubMed Central. Ganglion cyst at the proximal tibiofibular joint – A rare cause of compression neuropathy of the peroneal nerve If you notice any weakness or numbness along with the lump, that combination warrants imaging rather than a wait-and-see approach.
Meniscal Cysts
Meniscal cysts are another common explanation for a lump on the lateral side of the knee. These form when fluid leaks out through a tear in the meniscus, the C-shaped cartilage pad that cushions your knee joint. Lateral meniscal cysts are more frequent than medial ones. In a review of over 2,500 knee imaging studies, 50 meniscal cysts were identified, and lateral cysts outnumbered medial ones roughly two to one, with the large majority presenting as a palpable bulge.4PubMed. Clinical and arthrographic findings in meniscal cysts
A meniscal cyst typically feels like a smooth, sometimes tender bump along the joint line. It may get bigger with activity and smaller with rest, because movement forces more fluid through the underlying meniscal tear. On MRI, the cyst shows up as a well-defined fluid collection that connects to a horizontal tear in the meniscus.5PubMed. Imaging of meniscal cyst of the knee in three cases Treating the cyst alone, by draining it with a needle, tends to fail because the tear keeps feeding it. Definitive treatment usually involves addressing the meniscal tear itself, either by repair or partial removal.
Baker’s Cysts and Why They Sometimes Appear on the Side
Baker’s cysts, also called popliteal cysts, are the most talked-about knee lumps. They form at the back of the knee, in the popliteal fossa, when excess synovial fluid from the joint collects in a pouch between the calf muscles. They are typically associated with degenerative or inflammatory knee conditions like osteoarthritis or meniscal tears.6PubMed Central. Ruptured Baker’s Cyst Demystified: Current Evidence, Diagnostic Strategies, and Treatment Options for an Under-Recognized Condition When they are small, they produce a soft bulge directly behind the knee. When they grow larger, though, they can extend to the inner or outer side of the knee, which is why some people feel what seems like a lateral or medial lump that is actually a Baker’s cyst wrapping around.
The bigger concern with Baker’s cysts is rupture. If the cyst wall gives way, synovial fluid leaks into the calf muscles, causing sudden pain, swelling, and redness in the lower leg. The symptoms so closely mimic a deep vein thrombosis (DVT) that this presentation is sometimes called pseudothrombophlebitis.7PubMed. Midcalf ultrasonography for the diagnosis of ruptured Baker’s cysts In one reported case, a 54-year-old woman who had been treated conservatively for knee pain returned six weeks later with severe calf and foot swelling, and DVT was the initial working diagnosis until imaging ruled it out and revealed a ruptured Baker’s cyst instead.8PubMed Central. Ruptured Baker’s Cyst: A Diagnostic Dilemma The takeaway is that any sudden calf swelling, with or without a known knee lump, needs prompt evaluation to distinguish between a ruptured cyst and an actual blood clot.
Bursitis on the Inner Side
If your lump is on the inner side of the knee rather than the outer, pes anserine bursitis is a strong possibility. The pes anserine bursa sits just below the joint line on the medial (inner) aspect of the tibia, where three tendons converge. Runners, swimmers who do breaststroke, and people with osteoarthritis or diabetes are among those most prone. The area becomes tender, warm, and visibly swollen.
Occasionally, bursitis on the inner knee has an unusual underlying trigger. In two reported cases, a bony growth called an osteochondroma on the upper tibia was irritating the overlying bursa and causing recurrent pes anserine bursitis that did not respond to typical treatments. One patient, age 25, required surgical removal of the bony growth to resolve the problem.9PubMed Central. Pes anserine bursitis as a complication of tibial osteochondroma The lesson is that bursitis that keeps coming back despite rest and anti-inflammatory measures may be worth investigating for a structural cause underneath.
When Infection Gets Involved
A knee bursa can also become infected, a condition called septic bursitis. This usually happens when bacteria enter through a break in the skin over the bursa, even a minor scrape or insect bite. The prepatellar bursa (over the front of the kneecap) is the most commonly infected, but any superficial bursa around the knee can be affected. Over 80% of cases are caused by staph and other gram-positive bacteria.10Seminars in Arthritis and Rheumatism. Septic bursitis
Septic bursitis looks different from the garden-variety swollen bursa. The skin over the lump is usually intensely red, hot, and painful, and you may feel generally unwell with fever or chills. People with weakened immune systems are at higher risk of serious complications. If the infection does not clear with antibiotics and needle drainage, surgical removal of the bursa may be necessary. Do not try to wait out a red, hot, painful knee lump, especially if you have a fever.
Bone Growths Near the Knee
Osteochondromas are among the most common benign bone tumors, and they frequently occur around the knee, especially on the upper tibia.11PubMed Central. Knee locking caused by osteochondroma of the proximal tibia adjacent to the pes anserinus: A case report These are hard, bony lumps that grow outward from the bone surface. They are usually painless unless they press on a nearby tendon, nerve, or bursa. In rare cases, an osteochondroma near the pes anserinus can even cause the knee to lock. Most are discovered in adolescence or early adulthood during a growth spurt and stop growing once you reach skeletal maturity. They do not need treatment unless they cause pain, limit movement, or compress surrounding structures.
Soft-Tissue Tumors Around the Knee
The word “tumor” sounds alarming, but most soft-tissue tumors near the knee are benign. Lipomas, which are slow-growing lumps of fatty tissue, are among the most common soft-tissue masses in the extremities.12PubMed. Fat-containing soft-tissue masses of the extremities They feel soft, move slightly under the skin, and are almost never painful. A lipoma on the side of your knee can sit there for years without changing and without needing treatment.
A less common but interesting benign tumor is the giant cell tumor of the tendon sheath, also called tenosynovial giant cell tumor. About 85% of these show up in the fingers, but roughly 12% occur in large joints like the knee.13PubMed Central. Giant Cell Tumor of Tendon Sheath in the Knee These tumors grow slowly from the synovial lining and can remain symptom-free for a long time. As they enlarge, they may cause mechanical symptoms like catching, locking, or swelling of the knee.14PubMed. Arthroscopic Excision of a Localized Tenosynovial Giant Cell Tumor of the Knee: A Case Report They are locally aggressive in some cases, meaning they can recur after removal, but they do not spread to other parts of the body.
A synovial cyst, which is a fluid-filled pouch arising from the joint lining, can also present as a lump near the knee and mimic other conditions. One case report described a 56-year-old man with persistent pain on the outer knee that was initially diagnosed as iliotibial band syndrome. Conservative treatment failed, and surgical exploration eventually revealed a synovial cyst tucked under the iliotibial band.15PubMed Central. Synovial Cyst: A Culprit for Recalcitrant Iliotibial Band Syndrome: A Case Report Cases like this highlight how lumps that are not visible from the outside can still be the hidden cause of knee symptoms that do not resolve with standard treatment.
A Dislocated Fibular Head Can Mimic a Lump
Not every “lump” on the outer knee is actually a mass. Dislocation of the proximal tibiofibular joint creates a visible bony prominence near the fibular head that feels exactly like a hard lump. This typically happens when the knee is slightly bent while the foot is rotated and pointed downward, and an anterolateral dislocation is the most common pattern.16PubMed Central. Proximal tibiofibular dislocation It can occur during sports, falls, or even yoga poses that put unusual torque on the lower leg. The prominence tends to be painful, especially with weight-bearing, and a plain X-ray confirms the diagnosis. If you feel a sudden hard bump on the outer knee after a twisting injury, this is worth considering before assuming it is a cyst or growth.
Gout Deposits That Look Like Tumors
Chronic gout can produce deposits of uric acid crystals called tophi that accumulate in and around joints, and the knee is a common site. Tophi can grow large enough to form visible masses that look strikingly similar to tumors on imaging. In one reported case, a patient with bilateral knee masses underwent MRI that showed heterogeneous tissue consistent with tophi, but the appearance was concerning enough that a biopsy was needed to rule out cancer before the diagnosis of gouty tophi was confirmed.17PubMed Central. Chronic Tophaceous Gout Presenting as Bilateral Knee Masses in an Adult Patient: A Case Report If you have a history of gout and develop a hard lump near the knee, tophi should be on the list of possibilities before jumping to more worrisome diagnoses.
Vascular Lumps After Knee Surgery
A pulsatile lump on the side of the knee, especially one that develops in the weeks after arthroscopic surgery, could be a pseudoaneurysm. This happens when a small artery near the knee is damaged during surgery and a pocket of blood forms that stays connected to the damaged vessel, creating a mass that pulses with your heartbeat. The genicular arteries, which are small branches that supply the knee region, are the usual culprits.
In one case, a young patient developed a painless pulsatile mass on the outer side of his knee 19 days after ACL reconstruction. Doppler ultrasound and CT angiography confirmed a pseudoaneurysm fed by the superolateral genicular artery.18PubMed Central. Pseudaneurysm of the superolateral genicular artery following an anterior cruciate ligament reconstruction Similar cases have been described on the medial side, where a pseudoaneurysm of the descending genicular artery presented as pulsatile swelling just two days after surgery.19PubMed. Pseudoaneurysm of the articular branch of the descending genicular artery following double-bundle anterior cruciate ligament reconstruction The key distinguishing feature is the pulse. If you can feel rhythmic throbbing in the lump, particularly after a recent knee procedure, do not ignore it, as pseudoaneurysms can rupture if left untreated. Most are treated with guided compression or a minimally invasive embolization procedure.
When to Worry About Cancer
Malignant soft-tissue tumors (sarcomas) around the knee are uncommon, but they do occur, and catching them early matters. Clinicians use a set of red-flag features to decide which lumps need urgent investigation. These include a size greater than 5 cm, a deep location beneath the fascia, rapid growth, and pain.20Orthopaedics and Trauma. Benign and intermediate soft tissue tumours for trauma and orthopaedic exam candidates In a study of 258 sarcoma patients, about seven in ten had been referred based on these alarm symptoms, and among soft-tissue tumors specifically, size over 5 cm had the highest sensitivity and positive predictive value for detecting sarcomas.21PubMed Central. Alarm symptoms of soft-tissue and bone sarcoma in patients referred to a specialist center
On imaging, features that raise suspicion include a diffuse solid mass rather than a well-defined cyst, and evidence of bone erosion adjacent to the mass.22PubMed. Characterisation of intra-articular soft tissue tumours and tumour-like lesions To put this in perspective: the vast majority of lumps around the knee are cysts, inflamed bursae, or benign growths. But if your lump is bigger than a golf ball, feels hard and fixed in place, is growing noticeably over weeks, or hurts at rest, get it checked promptly rather than assuming it will resolve.
How Doctors Figure Out What the Lump Is
Physical examination narrows the list considerably. A soft, fluctuant lump along the joint line that changes size with activity points toward a meniscal cyst. A rubbery mass near the fibular head with tingling suggests a ganglion. A hard, immobile bump attached to bone suggests an osteochondroma. A warm, red, tender swelling with fever suggests infection.
When imaging is needed, ultrasound is usually the first step. It is inexpensive, widely available, and excellent at evaluating fluid collections, soft-tissue masses, and tendons around the knee. It can also dynamically assess whether a lump changes size with movement, which helps distinguish cysts from solid tumors.23PubMed. Ultrasound of the knee MRI is reserved for cases where ultrasound is inconclusive, the lump appears to be solid, or the doctor needs to see internal structures like menisci or ligaments that ultrasound handles poorly.24PubMed Central. Evaluation of the knee joint with ultrasound and magnetic resonance imaging For a suspected pseudoaneurysm, Doppler ultrasound or CT angiography confirms the vascular connection. And if any imaging raises concern for malignancy, biopsy is the definitive next step.
Knee Lumps in Children and Teenagers
In younger patients, the diagnostic landscape shifts. The most recognizable cause of a bony bump near the knee in an adolescent is Osgood-Schlatter disease, which produces a tender, swollen prominence just below the kneecap where the patellar tendon attaches to the shinbone. A related condition, Sinding-Larsen-Johansson syndrome, causes similar symptoms at the bottom of the kneecap itself.25Paediatrics and Child Health. Knee pain in children Both are overuse injuries common in active kids going through growth spurts, and both resolve with skeletal maturity, though the bony bump from Osgood-Schlatter can persist permanently as a painless knob.
Osteochondromas also tend to appear in children and adolescents, as the skeleton is still growing. These bony lumps can show up on the side of the knee and may be found incidentally during an X-ray taken for another reason. Because a small number of osteochondromas can interfere with growth plate function or joint mechanics, monitoring is standard practice in children even when the growth is painless. Ganglion cysts and meniscal cysts are less common in pediatric patients but are not unheard of, and the evaluation approach is similar to that in adults.