Knee Lump or Mass: Causes by Location and When to Worry

What a knee lump or mass means

In medical terms, a mass is an abnormal collection of tissue or fluid large enough to see or feel. "Mass in the knee" therefore simply means a lump. It does not imply cancer. Doctors describe a mass by its location, size, consistency (soft, rubbery, firm or bony-hard), whether it moves, whether it is tender, and how quickly it has changed. Those features point to the cause far better than the lump's presence alone.

Most knee lumps come from one of four sources:

• Fluid: cysts and bursae (small fluid sacs that cushion tendons and skin) that swell
• Soft tissue: fat (lipoma), scar or nerve tissue, and rarely sarcoma
• Bone: bony outgrowths, prominent growth plates and, rarely, bone tumors
• Skin: epidermoid cysts, warts and infected hair follicles

Knee lump causes by location

Where the lump sits narrows the list quickly.

Lump behind the knee

A Baker's cyst (popliteal cyst) is the most common lump behind the knee. Joint fluid escapes through a weak spot in the capsule and collects in a pouch between two tendons. It usually forms because something inside the knee makes extra fluid: osteoarthritis, a meniscus tear, rheumatoid arthritis or gout. It feels like a soft or tense swelling, more noticeable when standing with the knee straight, and can cause tightness when you bend fully.

A Baker's cyst can burst. Fluid then tracks down into the calf, causing sudden pain, swelling and sometimes bruising at the ankle. This looks almost identical to a deep vein thrombosis, so it needs an ultrasound the same day.

A popliteal artery aneurysm is a ballooning of the artery behind the knee. It is uncommon, occurs mainly in older men, and feels like a firm lump that pulses. It can clot and threaten the blood supply to the leg, so a pulsing lump needs prompt assessment.

Lump on the front of the knee

Prepatellar bursitis ("housemaid's knee") is a soft, often egg-sized swelling directly over the kneecap. It follows repeated kneeling (carpet layers, plumbers, gardeners) or a direct fall onto the knee. The knee usually bends normally because the fluid is outside the joint. If the skin over it becomes red, hot and very tender, bacteria may have entered through a small cut, and antibiotics and sometimes drainage are needed.

Bump below the kneecap

In children aged about 10 to 15 who play running or jumping sports, a tender bony bump on the shinbone just below the kneecap is usually Osgood-Schlatter disease: irritation of the growth plate where the patellar tendon attaches. It hurts with sport and kneeling and settles once growth finishes, though the bump itself often stays. In adults, swelling in this spot is more often infrapatellar bursitis ("clergyman's knee") or patellar tendon thickening.

Lump on the inner or outer side

• Meniscal cyst: fluid that leaks from a torn meniscus and collects at the joint line. It is often firm, tender and most prominent with the knee partly bent.
• Ganglion cyst: a jelly-filled sac attached to a tendon sheath or the joint. It can change size.
• Pes anserine bursitis: tender swelling on the inner side, about 5 cm below the joint line. It is common in people with knee osteoarthritis, overweight or diabetes.
• Osteochondroma: a bony, painless outgrowth near the growth plate, usually noticed in teenagers. It is the most common benign bone tumor.

Lumps under the skin anywhere

A lipoma is a soft, rubbery, slowly growing lump of fat that moves under the skin. An epidermoid cyst sits in the skin, often has a small dark pore and may discharge cheesy material. An abscess is red, hot and painful and may come to a head.

Could a knee lump be cancer?

Rarely. Bone and soft tissue cancers are uncommon, but the knee is one of their favored sites, especially in teenagers and young adults.

• Osteosarcoma most often starts in the lower thigh bone or upper shin bone in people aged 10 to 25. The first sign is usually deep bone pain that is worse at night, followed by a firm swelling.
• Soft tissue sarcoma can occur at any age. It is typically a painless lump that grows steadily, often deep to the muscle layer.
• Giant cell tumor of bone affects young adults around the knee and causes pain and swelling near the joint.

Guidance from NICE and sarcoma specialists suggests an urgent ultrasound or MRI for any soft tissue lump that is larger than 5 cm, growing, deep, painful or recurring after removal, and an urgent X-ray for unexplained bone pain or swelling.

Symptoms that come with a knee lump

How doctors diagnose a knee mass

A doctor will ask when you first noticed the lump, whether it has grown, whether it followed an injury or kneeling, and whether you have pain, knee swelling, locking, fever or weight loss. They will feel the lump, compare it with the other knee, check its relationship to bone and tendon, shine a light through it (fluid-filled cysts often glow), and examine the joint and the circulation.

Tests depend on what they find:

• Ultrasound: quick and painless. It shows whether a lump is fluid or solid and checks for a blood clot or aneurysm.
• X-ray: shows bone lumps, arthritis and calcification.
• MRI: the best test for meniscal tears, deep masses and suspected tumors.
• Aspiration: drawing fluid with a needle relieves pressure and lets the lab check for infection, gout crystals or blood.
• Biopsy: for any solid mass with worrying features, done at a sarcoma center before surgery so the tumor is not disturbed.

Treatment and home care

Treatment depends entirely on the cause.

• Baker's cyst: treat the knee problem behind it. Many shrink when arthritis or a meniscus tear is managed. Options include ice, a compression sleeve, physical therapy, draining the cyst and a steroid injection into the joint. Surgery to remove the cyst alone often fails because the fluid source remains.
• Prepatellar bursitis: stop kneeling, use knee pads, apply ice for 15 to 20 minutes several times a day and take an anti-inflammatory if safe for you (follow the label dose; avoid NSAIDs with kidney disease, stomach ulcers or some heart conditions). Infected bursitis needs antibiotics.
• Osgood-Schlatter: reduce jumping and running until pain settles, ice after sport, and quadriceps and hamstring stretches.
• Ganglion or meniscal cyst: observation, aspiration, or arthroscopic repair of the underlying tear.
• Lipoma or skin cyst: removal only if it is growing, painful or bothersome.
• Tumors: managed by a specialist sarcoma team with surgery and sometimes chemotherapy or radiotherapy.

Do not try to squeeze or drain a knee lump yourself. Puncturing a bursa or cyst can introduce infection into the joint.

Frequently asked questions

What is the definition of a mass in the knee?

A mass in the knee is any abnormal lump of tissue or fluid in or around the knee joint that can be seen, felt or found on a scan. The term describes a finding only. Most knee masses turn out to be cysts, swollen bursae or fatty lumps, not cancer.

Is a lump behind the knee serious?

Usually not. The most common cause is a Baker's cyst, which reflects a problem inside the knee such as arthritis or a meniscus tear. Get same-day care if the calf suddenly swells and hurts, because a burst cyst and a blood clot look alike and need an ultrasound to tell apart.

Why do I have a squishy lump on my kneecap?

A soft, fluid-filled swelling directly over the kneecap is usually prepatellar bursitis, often caused by kneeling or a fall. It tends to settle with rest, ice and knee pads. See a doctor if it turns red, hot or very painful, or if you develop a fever.

What does a cancerous lump on the knee feel like?

Sarcomas are typically firm, fixed and steadily growing, and may be deep within the muscle. Bone tumors usually cause deep aching pain, often worse at night, before a lump appears. Any lump larger than 5 cm (2 in) or growing over weeks should be scanned.

Can a knee lump go away on its own?

Many do. Bursitis from kneeling often settles within a few weeks of avoiding pressure. Baker's cysts can shrink when the underlying knee problem calms down. Ganglion cysts sometimes disappear spontaneously. Bony lumps and lipomas do not go away, but they are usually harmless.

What kind of doctor should I see for a knee lump?

Start with your primary care doctor, who can examine it and order an ultrasound or X-ray. You may then be referred to an orthopedic or sports medicine specialist. A suspected tumor should go to a specialist sarcoma center before any biopsy or surgery.