The single most common reason for a bump just below the kneecap is Osgood-Schlatter disease, a condition where the patellar tendon’s attachment point on the shinbone becomes inflamed and bony. It overwhelmingly affects adolescents, but the bump it leaves behind can persist for life. In adults who were never diagnosed with Osgood-Schlatter, the list of possibilities widens considerably, from inflamed fluid-filled sacs called bursae to cysts, tendon problems, and occasionally bone or soft-tissue growths. Figuring out what you’re dealing with usually comes down to your age, how active you are, and whether the bump is painful or painless.
Osgood-Schlatter Disease and the Classic Adolescent Bump
If you’re between about 10 and 16, the bump below your knee almost certainly relates to Osgood-Schlatter disease. It shows up as a hard, bony lump right on the tibial tuberosity, the small ridge of bone at the top of the shinbone where the patellar tendon anchors. The pain and swelling tend to flare with running, jumping, climbing stairs, and kneeling, then quiet down with rest.1PubMed Central. Apophysitis of the Tibial Tuberosity (Osgood-Schlatter Disease): A Review
What’s happening underneath is a tug-of-war between a growing skeleton and a powerful tendon. During growth spurts, the tibial tuberosity is made partly of cartilage that hasn’t yet hardened into bone. Repeated pulling from the patellar tendon irritates and inflames that growth plate, and the body responds by laying down extra bone. The result is a bump that can be tender to the touch and visibly raised compared to the other knee. Animal research suggests that eccentric contractions of the quadriceps, the kind that happen when your thigh muscles work hard to control downhill motion, may be a driving factor in producing Osgood-Schlatter-like changes at the tibial tuberosity.2PubMed Central. Eccentric contractions during downhill running induce Osgood‒Schlatter disease in the tibial tuberosity in rats: a focus on histological structures
The condition is far more common in kids who play sports, particularly those involving sprinting, jumping, or frequent direction changes. Boys have historically been diagnosed more often, though the gap has narrowed as participation in competitive youth sports has become more balanced. Most cases resolve on their own once the growth plate closes, usually by the late teens. The bump itself, though, often sticks around permanently as a harmless bony prominence.
When the Bump Never Goes Away
For most people, Osgood-Schlatter pain fades once the skeleton matures. But in a subset of cases, symptoms persist into adulthood. The usual culprit is a small loose fragment of bone, called an ossicle, that never fully fused with the tibial tuberosity. That unfused piece sits inside the tendon and gets irritated with activity, producing the same aching, kneeling-sensitive pain people remember from adolescence.3PubMed Central. Osgood-Schlatter Lesion Removed Arthroscopically in an Adult Patient
If you’re an adult with a bump below your knee that has been there since your teenage years and recently started hurting again, this is a likely explanation. Conservative measures like rest, anti-inflammatory medication, and physical therapy are the usual first step. When those fail, surgical removal of the ossicle reliably resolves the problem. Case reports describe complete relief of symptoms after excision, whether performed through open surgery or arthroscopically.4PubMed. Pseudoarthrosis between a patellar tendon ossicle and the tibial tuberosity in Osgood-Schlatter’s disease So if you’ve been tolerating a nagging bump for years thinking it’s just a cosmetic leftover, and it starts to hurt, it’s worth getting evaluated rather than assuming nothing can be done.
Bursitis Below and Around the Kneecap
Not every bump below the knee is bony. The knee has more than a dozen small fluid-filled sacs called bursae, and any of them can swell up when irritated. Two in particular produce visible lumps in the front of the knee.
Infrapatellar bursitis involves the bursa that sits just below the kneecap, either in front of or behind the patellar tendon. Chronic friction or direct pressure from repeated kneeling is the classic trigger, which is why this condition has historically been nicknamed “clergyman’s knee.”5PubMed Central. Infrapatellar bursitis presenting as a lump The swelling tends to be soft and squishy rather than rock-hard, and it sits right at the level of or slightly below the kneecap.
Prepatellar bursitis, sometimes called “housemaid’s knee,” produces a similar fluid-filled bump but directly over the front of the kneecap rather than below it. It can develop after a direct blow or from years of occupational kneeling. One case report describes a 70-year-old man with a clearly defined cystic swelling measuring nearly five centimeters across the front of his knee, caused by chronic prepatellar bursal fluid buildup.6International Journal of Biomedicine. Diagnostic Imaging of Chronic Prepatellar Bursitis (Housemaid’s Knee): An Elderly Patient Case Report In rare instances, a traumatic event can cause the bursa to fill with blood, producing swelling that grows dramatically and persists for years if left untreated.7PubMed Central. Massive Bilateral Haemorrhagic Prepatellar Bursitis: A Case Report
The key difference between bursitis and Osgood-Schlatter is texture and location. A bursitis bump is soft, compressible, and often fluid-filled. An Osgood-Schlatter bump is hard and bony. If you press on the lump and it feels like a small water balloon rather than a marble, bursitis is the more likely culprit. Bursitis also tends to be warm when inflamed and can occasionally become infected, so redness and heat around the swelling warrant prompt medical attention.
Patellar Tendon Pain Without a Visible Lump
Sometimes what feels like a bump below the knee is actually a thickened or swollen section of the patellar tendon itself. Patellar tendinopathy, often called “jumper’s knee,” is an overuse injury caused by small tears in the tendon that connects the kneecap to the shinbone. It shows up as localized pain and tenderness right at the bottom edge of the kneecap or along the tendon below it.8International Journal of Scientific Research in Engineering and Management. Prevalence of Jumpers Knee in Basketball Players You might not see a distinct lump, but you can feel a thickened, tender area when you run your fingers along the tendon.
This condition is common among athletes in sports that demand explosive leg power, particularly basketball, volleyball, and track and field. It differs from Osgood-Schlatter in one important way: the problem is in the tendon tissue, not the bone. The treatment focus is therefore on tendon rehabilitation, usually through controlled loading exercises, rather than waiting for a growth plate to close. Persistent cases sometimes develop a nodular thickening in the tendon that can be mistaken for a mass, so imaging helps distinguish tendon disease from something more worrisome.
Sinding-Larsen-Johansson Syndrome
A related condition worth knowing about is Sinding-Larsen-Johansson syndrome, which produces pain and swelling at the bottom of the kneecap rather than at the top of the shinbone. It’s another overuse injury seen mainly in young athletes, caused by excessive pulling of the patellar tendon where it attaches to the lower pole of the patella.9AAO Journal. Acupuncture Meridian-Based Myofascial Release to Treat Knee Pain in Sinding-Larsen–Johansson Syndrome: A Case Report If your bump or pain seems to be right at the kneecap itself rather than on the shinbone below, this condition may explain it. Like Osgood-Schlatter, it usually resolves as the skeleton matures.
Cysts and Fluid-Filled Masses
The area around the knee is one of the most common locations for cysts, and they can show up in front of, beside, or behind the joint. These range from simple ganglion cysts to more complex fluid collections tied to internal joint damage. A pictorial review in a radiology journal categorizes them into true cysts (including synovial cysts, bursal cysts, ganglia, and meniscal cysts) and lesions that only look like cysts on first glance, such as blood collections, abscesses, and vascular abnormalities.10PubMed Central. Cysts and cystic-appearing lesions of the knee: A pictorial essay
Meniscal cysts deserve special attention because they produce a firm, sometimes visible bump along the joint line, usually on the outer side of the knee. They almost always accompany a tear in the meniscus, the rubbery cartilage disc that cushions the joint. As joint fluid leaks through the torn meniscus, it pools outside the joint capsule and forms a cyst. Patients often describe pain, a feeling of mechanical catching or locking, and a palpable lump at the side of the knee.11PubMed Central. Parameniscal Cyst Arthroscopic Excision by the Modified Fat Pad Approach Can Shorten Recovery Time—A New Surgical Method with Retrospective Study The cyst itself is really a secondary problem; fixing the underlying meniscal tear is what prevents recurrence.
Baker’s cysts, located behind the knee rather than below or in front of it, are another common culprit people sometimes describe as a bump “near” the knee. They are generally caused by excess fluid production inside the joint from arthritis or other inflammatory conditions. If your bump is at the back of the knee and most noticeable when your leg is fully extended, a Baker’s cyst is the likely explanation.
Bone Growths and Soft-Tissue Tumors
A bump that feels hard and is fixed to the bone itself raises the question of a bone growth. Osteochondromas are the most common benign bone tumors, and the proximal tibia (the area just below the knee) is one of their favorite locations. These growths are essentially bony outcroppings capped with cartilage. They’re usually discovered in childhood or adolescence and often produce no symptoms at all, though they can irritate nearby structures. Case reports describe osteochondromas on the upper tibia that appeared as spur-like projections and caused bursitis in the tendons that cross over them.12PubMed Central. Pes Anserinus Bursitis due to Tibial Spurs in Children
Soft-tissue tumors are a rarer possibility. Lipomas, which are fatty lumps, are the most common soft-tissue tumor in the body and can occasionally appear around or even inside the knee joint.13PubMed Central. Intra-articular Lipoma of the Knee Joint Located in the Lateral Recess: A Case Report They tend to be soft, movable, and painless. Malignant tumors around the knee are uncommon, but any lump that is growing steadily, feels firm and fixed, or is accompanied by night pain or unexplained weight loss should be evaluated promptly with imaging.
Gout and Other Inflammatory Conditions
A lump below the knee doesn’t always originate from the bone, tendon, or bursa in the expected way. Gout, a condition caused by uric acid crystal deposits, can produce masses called tophi in surprising locations, including inside the patellar tendon. One case report describes a fit, athletic 45-year-old man who developed an enlarging mass in the patellar tendon region along with worsening anterior knee pain. Conservative treatment did nothing, and a biopsy revealed the mass was a gouty tophus embedded within the tendon itself.14PubMed Central. Intratendinous tophaceous gout imitating patellar tendonitis in an athletic man
The lesson from cases like this is that when knee tendon pain comes with a palpable mass and doesn’t respond to the usual tendinopathy treatments, the diagnosis needs to be reconsidered. Gout isn’t the only systemic condition that can mimic a local knee problem. Rheumatoid nodules, crystalline deposits from pseudogout, and even rare infections can all produce lumps in the periarticular soft tissues. A history of gout, elevated uric acid levels, or other joint involvement makes these conditions worth investigating.
Hoffa’s Fat Pad and Hidden Sources of Pain
Behind the patellar tendon sits a wedge of fatty tissue called the infrapatellar fat pad, or Hoffa’s fat pad. You can’t usually see it from the outside, but when it becomes irritated, it can swell enough to feel like a bump or fullness on either side of the patellar tendon just below the kneecap. Repetitive microtrauma, impingement between the kneecap and shinbone, and even prior knee surgery can trigger inflammation in this fat pad, leading to pain and localized swelling.15PubMed Central. Hoffa’s fat pad abnormalities, knee pain and magnetic resonance imaging in daily practice
Fat pad irritation is easy to overlook because it doesn’t produce a dramatic, discrete lump the way bursitis or an osteochondroma does. Instead, people tend to notice vague fullness in the front of the knee and pain that worsens with full extension of the leg. It’s often diagnosed by exclusion after imaging rules out other causes. If you have a diffuse, tender area of swelling below your kneecap that doesn’t fit neatly into the bursitis or tendinopathy categories, the fat pad may be the source.
Bumps on the Inner Side of the Knee
If your bump isn’t directly below the kneecap but instead on the inner (medial) side of the upper shinbone, a different set of conditions comes into play. The pes anserinus is the area where three tendons from the thigh converge and attach to the inner surface of the tibia, roughly two to three inches below the joint line. A bursa sits beneath these tendons, and when it becomes inflamed, you get pes anserinus bursitis, a common cause of medial knee pain and swelling.16PubMed. Accuracy and efficacy of ultrasound-guided pes anserinus bursa injection
This type of bursitis is especially common in middle-aged and older adults, people who are overweight, and those with osteoarthritis of the knee. It can also develop in runners and swimmers. The bump, if there is one, tends to be a soft, tender area on the inner shin rather than a well-defined lump. The distinction matters because pes anserinus bursitis is treated differently from problems at the front of the knee, and it’s frequently misdiagnosed as a meniscal tear or medial compartment arthritis if the examiner doesn’t specifically check for tenderness at the pes anserinus insertion.
How Doctors Figure Out What It Is
The first step is almost always a physical exam. Where exactly is the bump? Is it hard or soft? Does it move? Does it hurt when you press on it, when you straighten your leg, or when you squat? These simple questions narrow the possibilities dramatically before any imaging is ordered.
When imaging is needed, ultrasound and MRI are the main tools. Ultrasound is inexpensive, widely available, and excellent at identifying superficial structures: bursitis, fluid collections, cysts, and tendon problems all show up clearly. One comparative study found that ultrasound achieved almost perfect agreement with MRI for detecting meniscal cysts, Baker’s cysts, and bony spurs, making it a reliable first-line option for many below-the-knee lumps.17PARIPEX INDIAN JOURNAL OF RESEARCH. COMPARATIVE EVALUATION OF ULTRASOUND AND MRI IN THE DIAGNOSIS OF KNEE JOINT PATHOLOGIES MRI adds value when the concern involves deeper structures inside the joint, such as meniscal tears, fat pad abnormalities, or bone lesions that need detailed characterization.18PubMed Central. Evaluation of the knee joint with ultrasound and magnetic resonance imaging
X-rays are still useful in specific scenarios, particularly when the bump feels bony. An X-ray can quickly identify an osteochondroma, an unfused Osgood-Schlatter ossicle, or early arthritis with bone spur formation. For most soft-tissue bumps, though, X-rays won’t tell you much, and ultrasound or MRI will be more informative.
After Knee Surgery
People who have had anterior cruciate ligament (ACL) reconstruction sometimes notice a new bump below the kneecap that wasn’t there before. When a bone-patellar tendon-bone graft is used for the reconstruction, a small block of bone is harvested from the tibial tuberosity. The donor site heals with new bone formation, and the result can be a slightly raised area that feels different from the surrounding surface. This bump is a normal part of the healing process and is usually painless.19PubMed Central. Femoral press-fit fixation in ACL reconstruction using bone-patellar tendon-bone autograft: results at 15 years follow-up
Post-surgical scar tissue can also create firm lumps in the soft tissue around the patellar tendon. These are sometimes called fibrotic nodules and tend to develop in the weeks to months following surgery. They can be tender initially but usually soften over time. If a post-surgical bump is growing, becoming more painful, or associated with redness and warmth, those are reasons to contact your surgeon, because infection or hardware problems need to be ruled out. Otherwise, a mild bump at the graft harvest site is something most ACL reconstruction patients learn to live with and eventually ignore.
When to Worry and When to Wait
Most bumps below the knee turn out to be benign, especially in younger people where Osgood-Schlatter accounts for a huge share of cases. But a few features push a lump into the “get this checked soon” category:
- Rapid growth: A lump that doubles in size over weeks or months needs imaging to rule out a tumor.
- Night pain: Pain that wakes you from sleep and isn’t related to how you positioned your knee can be a red flag for bone pathology.
- Redness and warmth: These suggest infection, particularly in the setting of bursitis, and may need antibiotics or drainage.
- Locking or catching: Mechanical symptoms suggest internal derangement, such as a meniscal tear with a cyst, rather than a simple surface bump.
- Numbness or tingling: A lump that compresses a nerve can cause sensory changes in the lower leg. This is uncommon but warrants evaluation.
A bump that has been present for years, doesn’t change in size, and only hurts with direct pressure or prolonged kneeling is overwhelmingly likely to be harmless. For adolescents dealing with the classic Osgood-Schlatter scenario, activity modification and time are usually all that’s needed. For adults with a new or changing lump, a visit to a sports medicine physician or orthopedic specialist can quickly sort through the possibilities with a focused exam and, if needed, a straightforward ultrasound.