Knee bursitis typically shows up as a distinct, localized swelling on or around the kneecap, often looking like a soft, fluid-filled bulge just beneath the skin. Depending on which bursa is inflamed and whether infection is involved, the swelling can range from a painless lump to a hot, red, tender mass that makes bending the knee difficult. The exact appearance varies enough that the same condition can look dramatically different from person to person.
What a Bursa Actually Is and Why It Swells
A bursa is a small, fluid-filled sac that sits between structures that move against each other, like skin and bone, or tendon and bone. Its job is to reduce friction. The knee has several of these sacs in different locations, and they can be grouped into those around the kneecap (patella) and those elsewhere around the joint.1PubMed Central. Bursae around the knee joints When a bursa becomes irritated, whether from repetitive pressure, a direct blow, infection, or an underlying inflammatory condition, it fills with extra fluid and swells. That swelling is the hallmark visual sign of bursitis. Because different bursae sit in different spots around the knee, the location of the swelling tells you a lot about which one is involved.
Prepatellar Bursitis and the Classic “Egg on the Kneecap” Look
The most recognizable form of knee bursitis is prepatellar bursitis, sometimes called “housemaid’s knee.” The prepatellar bursa sits directly in front of the kneecap, just under the skin, so when it fills with fluid the result is hard to miss: a rounded, squishy swelling centered over the front of the knee. In mild cases it may look like a slight puffiness that you notice mostly when comparing one knee to the other. In moderate-to-severe cases, the swelling can be striking, like a soft egg or water balloon sitting on top of the kneecap.
This type tends to affect people whose work or hobbies involve prolonged kneeling. Historically that meant housemaids and clergy, but gardeners, carpet layers, plumbers, and tatami craftsmen are among those at risk today.2PubMed Central. Housemaid’s Knee (Prepatellar Septic Bursitis) Pain when kneeling is a consistent complaint, along with difficulty walking when the swelling is significant. One case report described a 70-year-old man who had a clearly defined cystic mass measuring roughly 5 by 2 centimeters in front of his kneecap after six months of symptoms, containing cloudy fluid visible on ultrasound.3International Journal of Biomedicine. Diagnostic Imaging of Chronic Prepatellar Bursitis (Housemaid’s Knee): An Elderly Patient Case Report That gives you a sense of scale: a lump roughly the size of a small lime, sitting right over the front of the knee.
Infrapatellar Bursitis and the “Clergyman’s Knee”
Just below the kneecap, a second bursa called the deep infrapatellar bursa sits behind the lower part of the patellar tendon, right above the bony bump at the top of the shinbone. When this one swells, the visual is subtler than prepatellar bursitis because the swelling is deeper and partially hidden behind the tendon. You may notice fullness or a soft lump just below the kneecap rather than directly on top of it. On imaging, the inflamed bursa appears as a fluid-filled pocket immediately behind the lower patellar tendon and in front of the tibial tuberosity.4PubMed Central. A Case of Chronic Deep Infrapatellar Bursitis Complicated by Patellar Tendinopathy and its Evaluation With Musculoskeletal Ultrasound
Because this bursa is deeper, the skin over it may not look as obviously swollen or red. Pain tends to localize to the area just below the kneecap and can worsen with activities like climbing stairs, squatting, or kneeling in an upright posture (as opposed to the more forward-leaning kneeling that aggravates prepatellar bursitis). This distinction is why infrapatellar bursitis has historically been called “clergyman’s knee,” a nod to the upright kneeling posture common during prayer.
Suprapatellar Bursitis and Swelling Above the Kneecap
A third location worth knowing is the suprapatellar bursa, which sits above the kneecap and communicates with the knee joint itself. When this bursa swells, the puffiness appears in the lower thigh, just above the kneecap. It can be tricky to identify visually because swelling in this area can look very similar to a general knee joint effusion from arthritis or a meniscal tear. In fact, distinguishing suprapatellar bursitis from an effusion caused by osteoarthritis, rheumatoid arthritis, or cartilage damage is one of the more common diagnostic challenges.5PubMed Central. Knee Bursae: A Comprehensive Review of Clinical Evaluation, Imaging Differentiation, and the Expanding Role of Biologic Therapies – Section: Differential Diagnosis If the swelling is above the kneecap and you also have significant stiffness or trouble fully straightening the leg, your doctor will want to sort out whether the problem is in the bursa, in the joint, or both.
When Infection Gets Involved
The appearance of knee bursitis changes significantly when bacteria enter the picture. Septic bursitis, meaning a bursa that has become infected, tends to produce more dramatic visual signs: the skin over the swelling becomes noticeably red and feels warm or even hot to the touch. The swelling is usually more tense and painful than in non-infected bursitis. Fever and chills can accompany the local symptoms. One case involved an 83-year-old craftsman with diabetes who presented with severe pain, warmth over his right knee, and mild chills; fluid from his prepatellar bursa grew Staphylococcus aureus bacteria on culture.2PubMed Central. Housemaid’s Knee (Prepatellar Septic Bursitis)
In children, septic bursitis can look even more alarming. A case report of an 8-year-old boy described redness, swelling, and severe tenderness around the right knee following a puncture wound. The redness extended all the way up to the mid-thigh, and swelling spread to the lateral thigh and down to the ankle. He could barely move the knee, managing only about 10 to 25 degrees of total motion.6PubMed Central. Septic Bursitis in an 8-Year-Old Boy That kind of rapidly spreading redness and swelling is a medical emergency and a very different picture from the localized bump of simple, non-infected bursitis.
Diabetes and other conditions that weaken the immune system raise the risk that a bursitis will turn septic. A break in the skin near the bursa, even a small scrape or puncture, can be the entry point for bacteria. If you notice redness that is spreading, warmth that seems to be getting worse, or you develop a fever alongside a swollen knee, those are signs you should not wait on.
Acute Versus Chronic Appearance
Fresh, acute bursitis and longstanding chronic bursitis look and feel quite different. Acute bursitis comes on relatively fast, often after a specific event like a fall onto the knee or a period of unusually prolonged kneeling. The swelling is soft, the fluid inside feels thin and sloshy, and the area is typically tender to touch. If there is no infection, redness may be mild or absent.
Chronic bursitis tells a different story. When the bursa has been inflamed repeatedly or for a long time, it can thicken and develop internal scar tissue, a process that leads to a firmer, sometimes lumpy mass. Chronic bursitis may present as a painless lump in front of the knee without any redness, persisting for weeks in people who kneel repeatedly for their work.7PubMed Central. Knee Bursae: A Comprehensive Review of Clinical Evaluation, Imaging Differentiation, and the Expanding Role of Biologic Therapies – Section: Prepatellar Bursa On imaging, chronic cases can show internal divisions within the bursa (called septations), fibrotic wall thickening, and sometimes signs of old bleeding inside the sac. Because the chronic version is often painless and not red, people sometimes live with it for months before seeking care, unsure whether the lump is something to worry about.
The practical distinction matters: a painless, non-red lump that has been there for weeks is not an emergency, though it is worth having evaluated. A rapidly enlarging, hot, red, painful swelling is a different situation entirely.
What Knee Bursitis Can Be Confused With
Several other knee problems can produce swelling or anterior knee pain that mimics bursitis, and the overlap causes genuine confusion even among clinicians. Prepatellar and infrapatellar bursitis can look similar to patellar tendinopathy, which causes thickening of the patellar tendon without the obvious fluid collection. Hoffa’s fat pad syndrome produces deep pain at the front of the knee with swelling inside the fat pad, particularly noticeable when you fully straighten the leg. Suprapatellar bursitis, as mentioned earlier, can be mistaken for joint effusion from arthritis or torn cartilage. And a rupture of the quadriceps tendon, while more dramatic, can cause swelling and pain above the kneecap along with an inability to straighten the knee and sometimes a palpable gap in the tendon.5PubMed Central. Knee Bursae: A Comprehensive Review of Clinical Evaluation, Imaging Differentiation, and the Expanding Role of Biologic Therapies – Section: Differential Diagnosis
The main visual clue that points toward bursitis rather than these alternatives is the discrete, well-defined, fluid-filled swelling. If the lump is soft, localized, and you can put your finger right on it and feel fluid underneath, that is characteristic of a swollen bursa. More diffuse swelling that involves the whole joint, or pain that is deep inside the knee rather than right at the surface, points toward something else.
How Doctors Confirm the Diagnosis
In many straightforward cases, a doctor can diagnose knee bursitis by physical examination alone: the location, feel, and history of the swelling are enough. When the picture is less clear, or when infection needs to be ruled out, two main tools come into play: imaging and fluid analysis.
Ultrasound has become a particularly useful first-line imaging choice for suspected bursitis. In one study comparing ultrasound to MRI for evaluating knee bursitis, ultrasound correctly identified about 87 percent of confirmed bursitis cases, with perfect specificity (no false positives) and strong overall agreement with MRI.8PubMed Central. Knee bursitis: a sonographic evaluation That makes ultrasound a good option when a doctor wants to see exactly where the fluid is, how much there is, and whether the bursa wall looks thickened or abnormal, all without the expense and wait time of an MRI.
If infection is a concern, the gold standard is aspirating (drawing out) the fluid and sending it to a lab. The fluid gets tested for white blood cell count, bacterial culture, and sometimes crystals that might indicate gout or a similar condition. Blood tests measuring inflammation markers can support the diagnosis, though they are not specific enough on their own to confirm septic bursitis.9PubMed Central. Knee Bursae: A Comprehensive Review of Clinical Evaluation, Imaging Differentiation, and the Expanding Role of Biologic Therapies – Section: General Approach to Imaging Evaluation of Knee Bursae The appearance of the aspirated fluid itself can be telling: clear, straw-colored fluid usually indicates non-infected bursitis, while cloudy or frankly purulent fluid raises the alarm for infection.
Treatment and What to Expect Visually During Recovery
For non-infected bursitis, the initial approach is usually conservative: avoiding the activity that triggered the swelling, icing, compression, and anti-inflammatory medications. The swelling typically goes down gradually over days to weeks. During this time you will see the lump flatten and soften, though a small amount of residual puffiness can linger for a while, especially if the bursa has been inflamed before.
When conservative measures do not resolve things, aspiration (draining the fluid with a needle) provides immediate visible relief. The lump deflates noticeably, sometimes almost completely, right after the procedure. A corticosteroid injection is sometimes given at the same time to reduce inflammation and lower the chance of the fluid coming back. For chronic or recurrent cases, ultrasound-guided aspiration and injection improves accuracy and reduces complications compared to doing it blind.7PubMed Central. Knee Bursae: A Comprehensive Review of Clinical Evaluation, Imaging Differentiation, and the Expanding Role of Biologic Therapies – Section: Prepatellar Bursa
Corticosteroid injections are effective but come with their own set of cosmetic side effects worth knowing about. In rare cases, the injection site can develop localized skin lightening (hypopigmentation) and loss of the underlying fat layer, creating a visible dip or discolored patch on the skin. One reported case involved a 32-year-old woman who developed a well-defined area of lighter skin and fat atrophy over her right knee two months after receiving a triamcinolone injection.10PubMed Central. Hypopigmentation after intra-articular corticosteroid injection These changes are uncommon and often improve over time, but they are worth being aware of, particularly if you have darker skin where the color change would be more noticeable.
Bursitis in Children
Knee bursitis in kids is far less common than in adults, and when it does occur, infection is a bigger concern. Children are more likely to develop septic bursitis after a skin break, especially around the kneecap where falls and scrapes are common. The visual presentation can be more dramatic in children, with redness and swelling spreading well beyond the knee itself. In the pediatric case mentioned earlier, an 8-year-old boy’s infection caused erythema extending from the knee up to the mid-thigh and edema reaching down to the ankle, all from what started as a small puncture wound over the front of the knee.6PubMed Central. Septic Bursitis in an 8-Year-Old Boy
If a child develops a warm, red, swollen area around the knee after a scrape or cut, the threshold for seeking medical attention should be lower than it would be for an adult with a similar-looking swelling. Children’s immune systems react differently, and infections can escalate quickly. The usual “wait and ice it” advice that works for a construction worker’s mildly swollen knee does not safely apply to a child with spreading redness.
Red Flags That Change the Urgency
Most knee bursitis is a nuisance rather than a danger. But certain signs shift the situation from “schedule an appointment when convenient” to “get seen today.” Those signs include:
- Spreading redness: If the red area is expanding outward from the swelling over hours, that suggests the infection is moving beyond the bursa into surrounding tissue.
- Fever or chills: Systemic symptoms alongside a swollen knee strongly suggest infection.
- Rapid worsening: A swelling that was mild yesterday and is tense, hot, and very painful today is behaving differently from typical non-infected bursitis.
- Inability to bear weight: While some stiffness and discomfort with walking is normal in bursitis, being unable to put any weight on the leg suggests either a severe infection or a different problem entirely.
- A wound near the swelling: Any break in the skin over or near a swollen bursa is a potential entry point for bacteria and warrants closer evaluation.
People with diabetes, those on immunosuppressive medications, and anyone with a weakened immune system for any reason should treat any new, warm, swollen area around the knee with extra caution. The combination of compromised immunity and a superficial bursa close to the skin surface makes the prepatellar bursa particularly vulnerable to infection in these groups.
Pes Anserine Bursitis and Other Less Visible Types
Not all knee bursae produce obvious visible swelling when they become inflamed. The pes anserine bursa, located on the inner (medial) side of the knee just below the joint line, is a common source of knee pain that rarely produces a visible lump. Instead, pes anserine bursitis shows up as tenderness when you press on the inner part of the upper shin, pain that worsens with stair climbing, and sometimes mild swelling that blends into the overall contour of the inner knee. Because there is no dramatic bump to point to, pes anserine bursitis is more often felt than seen.
Similarly, bursae on the back of the knee, including those associated with Baker’s cysts, can produce swelling in the popliteal fossa (the soft spot behind the knee) that is easier to feel with your hand than to see in a mirror. These posterior swellings tend to cause a sense of tightness or fullness behind the knee, especially when fully bending or straightening the leg. The visual picture of knee bursitis, in other words, depends heavily on which of the knee’s many bursae is involved. The front-of-the-knee varieties are the most visually obvious; the ones on the sides and back are sneakier.