The knee is a complex joint, relying on the coordinated function of three bones—the femur, tibia, and patella—along with ligaments, tendons, and cartilage for movement and stability. A bump felt near the knee can originate from various tissues, including fluid-filled sacs, bone, or soft tissue. The character of the lump—whether it is soft, hard, fixed, or movable—offers the first clue to its underlying cause. While many knee bumps are harmless, a healthcare professional must determine their source to rule out conditions requiring treatment.
Soft, Fluid-Filled Swellings
Bumps that feel soft, elastic, or fluid-filled often point to issues involving the body’s natural cushioning structures, known as bursae, or the knee’s internal lubricating fluid. Bursae are small sacs that reduce friction between bones, tendons, and muscles. When a sac becomes inflamed, bursitis develops, resulting in localized, often tender swelling.
Prepatellar bursitis, sometimes called “housemaid’s knee,” affects the bursa directly over the kneecap. This condition frequently arises from chronic irritation caused by prolonged kneeling, leading to noticeable swelling at the front of the knee. Infrapatellar bursitis involves the bursa located just below the kneecap, near the patellar tendon. Both types present with localized swelling, warmth, and pain, especially when moving the joint or applying pressure.
A Baker’s cyst, or popliteal cyst, appears as a lump at the back of the knee. This cyst is typically secondary to an underlying joint problem, such as a meniscus tear or osteoarthritis. Damage causes the joint to produce excessive synovial fluid, which is pushed into the space behind the knee, causing the sac to bulge. The cyst can cause a feeling of tightness or fullness, particularly when the leg is straightened.
Ganglion cysts can arise from a tendon sheath or joint capsule near the knee. These cysts are firm but non-bony, forming when fluid leaks from a joint or tendon sheath. Their size may fluctuate, and they usually present as a mass that is movable underneath the skin.
Hard, Fixed Growths and Bone Changes
Bumps that feel firm, unyielding, or fixed in place suggest a solid structure, such as bone, cartilage, or a benign tissue growth. The development of bony projections, known as osteophytes or bone spurs, is often associated with osteoarthritis. As joint cartilage wears down, the body grows extra bone at the margins of the joint, leading to these hard, fixed growths.
Osteophytes are typically slow-growing and can cause pain by limiting joint movement or irritating surrounding soft tissues. Their presence is a common feature used in the diagnosis of knee osteoarthritis. The bump felt may be the osteophyte itself or the surrounding inflammation and swelling it causes.
Soft tissue growths, such as a lipoma, represent another category of firm bumps. A lipoma is the most common benign tumor of soft tissue, consisting of a collection of mature fat cells. They are usually found just beneath the skin, are generally painless, and feel soft and “doughy,” though they are firmer than a fluid-filled cyst.
For younger individuals, a hard, fixed bump near the knee joint might be an osteochondroma, a benign growth composed of both bone and cartilage. This type of growth usually forms near the growth plates, most commonly around the knee, and may be discovered as a painless lump. If it presses on a nearby nerve or tendon, it can cause pain, numbness, or a snapping sensation during movement. Medical imaging is necessary to confirm the diagnosis of any fixed bony growth.
Bumps Caused by Acute Injury or Infection
A bump that appears suddenly, often following trauma or in conjunction with signs of illness, is likely related to an acute injury or an infectious process. A hematoma is a localized collection of blood that forms after a direct blow or muscle strain causes bleeding under the skin. This type of lump is often accompanied by noticeable bruising, tenderness, and swelling that develops rapidly after the injury.
Infectious causes are characterized by rapid onset and systemic symptoms, demanding immediate attention. An abscess is a localized infection that creates a pocket of pus, presenting as a raised, painful, and warm lump. Cellulitis is a more diffuse bacterial infection of the skin and soft tissues, causing generalized swelling, redness, and heat that spreads across a larger area. Both can present with fever and chills, indicating a systemic response.
Significant, immediate swelling of the knee joint following severe trauma, such as a fall or sports injury, suggests a severe internal injury, possibly a fracture or a torn ligament. This acute swelling is often due to hemarthrosis, which is bleeding into the joint space itself. Hemarthrosis is a strong indicator of damage to internal structures like the anterior cruciate ligament or a bone fracture.
Warning Signs and When to Seek Medical Care
Recognizing certain signs associated with a knee bump is important for determining the urgency of seeking professional medical evaluation. Any bump that is persistently present or slowly increasing in size should be examined by a healthcare provider, even if it is not causing significant pain. This non-urgent evaluation is needed to accurately diagnose the cause and initiate appropriate management for chronic issues like cysts or benign growths.
There are specific signs that indicate a more urgent situation, which can be remembered using the “T.R.I.P.” criteria:
- Temperature: A sudden rise, including a fever or pronounced heat radiating from the bump, suggests a possible infection.
- Redness: Spreading outward from the bump, sometimes with red streaking, points to a rapidly progressing infection like cellulitis.
- Impairment: A significant warning sign, such as a sudden inability to bear weight on the leg or a severe loss of the knee’s mobility.
- Pain: Sudden, severe, and debilitating pain, or pain intense enough to wake a person from sleep.
If the bump is accompanied by any signs of systemic infection or if the knee feels unstable or cannot support your weight, an emergency room visit is warranted. These acute symptoms suggest a serious condition like a septic joint, a rapidly spreading infection, or a significant structural injury that needs immediate diagnosis and treatment to prevent complications.