The appearance of a bump or swelling near the knee is often the result of common mechanical issues or degenerative changes. The knee joint is a complex structure involving bone, cartilage, ligaments, tendons, and fluid-filled sacs, all susceptible to stress and injury. Bumps vary widely in texture, size, and location, originating from soft, fluid-filled structures or from the hard surfaces of the bone itself. Understanding the different categories of causes helps determine the nature of the bump and appropriate action.
Soft Tissue Swellings
Soft, mobile bumps around the knee often arise from the inflammation or herniation of fluid-containing sacs, such as bursae or cysts. Bursae are small, thin sacs that function as cushions, reducing friction between bones, tendons, and muscles. When a bursa becomes irritated or sustains a blow, it swells significantly, a condition known as bursitis.
Prepatellar bursitis, sometimes called “housemaid’s knee,” occurs directly over the kneecap (patella) due to prolonged or repetitive kneeling. The prepatellar bursa, located just beneath the skin, becomes distended with excess fluid, creating a distinct, localized, and often tender swelling. Infrapatellar bursitis involves the bursa located just below the kneecap and also produces a prominent soft bump. Both types of bursitis result in localized swelling that is warm to the touch and painful when kneeling or applying direct pressure.
Another common soft tissue swelling is a Baker’s cyst, or popliteal cyst, which forms as a fluid-filled sac behind the knee. This cyst is usually a secondary issue, developing when excess synovial fluid is pushed out of the knee joint. Conditions like arthritis or a meniscus tear cause the knee to produce too much fluid, which collects in the bursa at the back of the knee. The resulting lump can cause tightness or fullness, especially when the knee is fully extended.
Tendon and Ligament Related Bumps
Some prominent knee bumps relate directly to physical stress placed on the tendons, particularly in adolescents and athletes. A common cause of a fixed, painful bump just below the kneecap is Osgood-Schlatter disease. This condition affects growing adolescents involved in sports requiring frequent running or jumping. The bump forms at the tibial tuberosity, a bony elevation on the upper shinbone (tibia) where the patellar tendon attaches.
During rapid growth spurts, the repetitive pulling action of the quadriceps muscles, transmitted through the patellar tendon, causes traction and irritation at the attachment point. Because this area is still a soft growth plate, the body responds to the stress by laying down new bone. This results in a permanent enlargement of the tibial tuberosity. Although the pain associated with Osgood-Schlatter disease resolves once the growth plate matures and fuses, the bony prominence often remains visible into adulthood.
Bumps can also develop within the tendons themselves due to chronic overuse. Patellar tendinopathy, often called “jumper’s knee,” results from repetitive loading stress that causes micro-tears and degeneration within the patellar tendon. In long-standing cases, failed healing leads to localized thickening or palpable nodules within the tendon structure. This thickening represents a change in the tendon’s tissue composition and is felt as a firmer, less mobile bump compared to a soft tissue cyst.
Bony Growths and Cartilage Issues
Bumps that are hard, fixed, and non-mobile often signal a change in the underlying skeletal structure, frequently associated with long-term wear. Osteophytes, or bone spurs, are bony growths that develop at the edges of joints affected by degenerative conditions like osteoarthritis. As the protective cartilage breaks down, the underlying bone attempts to stabilize the joint by creating these projections. Bone spurs can form on the ends of the thigh bone, shinbone, or kneecap, and when large enough, they are felt as hard, unyielding lumps near the joint line.
The prominence of certain bony landmarks, such as the tibial tuberosity, can also be a simple anatomical variation unrelated to disease. Some individuals naturally possess a more noticeable tibial tuberosity or other bony contours. These natural variations are fixed, hard, and non-tender, representing a normal part of the bone structure rather than a pathological growth.
A different type of hard bump is a loose body within the joint. These are fragments of bone or cartilage that have broken off due to injury or degenerative disease and float freely in the joint fluid. While not always fixed, they can migrate and become lodged, felt as a small, hard, and mobile lump. Loose bodies cause mechanical symptoms like a catching or locking sensation in the knee, especially if they interfere with smooth joint movement.
When a Knee Bump Needs Evaluation
While many knee bumps are harmless or resolve with conservative care, certain accompanying signs suggest the need for medical evaluation. A rapidly growing bump, especially one that appears suddenly without apparent injury, should be assessed by a healthcare professional. Any bump accompanied by a visible deformity of the knee joint or an inability to bear weight also warrants timely attention.
Symptoms of potential infection require immediate evaluation, including a bump that is intensely red, hot to the touch, and accompanied by a fever or chills. These signs may indicate septic bursitis, a bacterial infection of the bursa requiring prompt medical treatment. Mechanical symptoms suggest internal joint damage, such as a loose body or a torn meniscus. These symptoms include:
- The knee locking up.
- The knee giving way.
- A persistent, painful catching sensation.
- Limitation of the joint’s range of motion.
Seek professional diagnosis if a bump causes persistent, severe pain that does not improve with rest, or if the pain wakes you up at night.