Knee bursitis happens when one of the small, fluid-filled sacs around your knee joint becomes inflamed. These sacs, called bursae, act as cushions between bones, tendons, and skin. When they’re irritated by repeated pressure, a sudden injury, an infection, or an underlying health condition, they swell with excess fluid and become painful. The knee has several bursae, and the specific cause often depends on which one is affected.
Which Bursae Are Most Commonly Affected
The knee contains roughly a dozen bursae, but three are responsible for the vast majority of problems. The prepatellar bursa sits directly in front of the kneecap, just beneath the skin. It’s the most commonly inflamed bursa in the knee, and prepatellar bursitis has long been called “housemaid’s knee” because of its link to prolonged kneeling. The infrapatellar bursa sits just below the kneecap, near the top of the shinbone, and tends to flare up from similar kneeling or impact-related causes.
The pes anserine bursa is located on the inner side of the knee, a few inches below the joint line, where three tendons from the thigh attach to the shinbone. Pes anserine bursitis has a different profile. It’s more closely tied to body weight, hamstring tightness, and overuse from activities like running or cycling rather than direct kneeling pressure. Knowing which bursa is involved helps explain why the bursitis developed in the first place.
Repetitive Pressure and Overuse
The single most common cause of knee bursitis is sustained or repeated pressure on a bursa. Any activity that keeps you on your knees for extended periods compresses the prepatellar or infrapatellar bursa against the hard surface of the kneecap and the bone beneath it. Carpet installers, plumbers, gardeners, roofers, and tile setters are especially prone. Even regular household tasks like scrubbing floors can trigger it if you kneel frequently without padding.
The mechanism is straightforward: repeated compression damages the thin lining of the bursa, which responds by producing extra fluid and becoming inflamed. Over time, this can become a cycle where the swollen bursa is even more vulnerable to the next round of pressure. The swelling typically develops gradually over days or weeks, unlike the rapid onset from a single injury. You may first notice a soft, spongy lump over the kneecap that’s tender to touch and worsens when you kneel or bend the knee fully.
Direct Impact and Acute Injury
A single direct blow to the knee can cause bursitis symptoms to come on fast. Falls onto a hard surface, contact during sports like football or wrestling, or any impact that strikes the front of the knee can damage the bursa in one moment. The bursa responds by rapidly filling with fluid, sometimes within hours. Unlike the gradual swelling from repetitive kneeling, traumatic bursitis often produces noticeable swelling and pain the same day.
This type is common in athletes and in anyone who takes a hard fall. The initial injury may also cause minor bleeding into the bursa, which intensifies the inflammatory response and can make the swelling more dramatic than you’d expect from the force of the impact.
Bacterial Infection
Infectious bursitis, also called septic bursitis, is the most serious cause and requires prompt treatment. It develops when bacteria enter the bursa through a break in the skin. A scratch, insect bite, puncture wound, or even a small cut on or near the knee can be the entry point. Once bacteria reach the bursa, the sac fills with infected fluid.
The prepatellar bursa is particularly vulnerable to infection because it sits so close to the skin’s surface, with very little tissue separating it from the outside world. Signs that distinguish septic bursitis from other types include warmth and redness over the swollen area, fever, and pain that’s more intense than typical overuse bursitis. A healthcare provider will often draw fluid from the bursa with a needle (aspiration) to test it for bacteria. If an infection is confirmed, antibiotics are necessary, and in some cases the bursa needs to be drained surgically. Keeping any cuts or bites on your knee clean is one of the simplest ways to prevent this form.
Underlying Health Conditions
Several systemic conditions increase your risk of developing knee bursitis or make existing bursitis worse. Gout and pseudogout cause crystals to form inside joints and the surrounding soft tissue, and those crystals can irritate bursae directly. Rheumatoid arthritis creates widespread joint inflammation that can extend to nearby bursae. In these cases, bursitis tends to flare alongside the underlying disease and may affect multiple joints, not just the knee.
Osteoarthritis of the knee changes how forces are distributed across the joint. Altered movement patterns put abnormal stress on bursae that wouldn’t normally be overloaded, particularly the pes anserine bursa on the inner side of the knee. People with knee osteoarthritis are significantly more likely to develop pes anserine bursitis than those without it.
Body Weight and Muscle Tightness
Carrying extra body weight is a well-established risk factor for pes anserine bursitis specifically. The pes anserine bursa cushions the point where three hamstring-area tendons meet the shinbone, and additional load on the knee increases friction and compression at that spot. People with overweight or obesity are disproportionately affected by this form of bursitis.
Tight hamstrings compound the problem. When the muscles along the back of your thigh are chronically short or stiff, the tendons that cross the pes anserine bursa pull more tightly against it with every step, squat, or stair climb. This is why pes anserine bursitis is common in runners who neglect stretching, in people who sit for long hours and then exercise, and in anyone whose hamstring flexibility is limited. Addressing both weight and flexibility can meaningfully reduce the mechanical stress on this bursa.
Sports and Exercise Patterns
Athletes develop knee bursitis through a combination of the factors above. Runners, cyclists, and swimmers who use a breaststroke kick are prone to pes anserine bursitis because of the repetitive bending and rotating forces at the inner knee. Contact sports like football, hockey, and wrestling carry the added risk of direct trauma to the prepatellar bursa.
Sudden increases in training volume are a common trigger. A bursa that handles your usual mileage without complaint can become inflamed when you ramp up distance, intensity, or frequency too quickly. The tissues don’t have time to adapt, and the bursa absorbs more friction and pressure than it can manage. This is one reason bursitis often appears during a training buildup for a race or at the start of a new sport season.
How Knee Bursitis Is Identified
Diagnosis is usually straightforward and based on a physical exam. A provider will compare both knees, press on specific areas to locate the source of pain and swelling, check the overlying skin for redness or warmth that might suggest infection, and move your knee through its full range of motion to see which positions reproduce your symptoms. The location of the tenderness tells them which bursa is involved: directly over the kneecap points to prepatellar bursitis, while pain a few inches below the joint on the inner side suggests pes anserine bursitis.
Imaging isn’t always necessary. When the diagnosis is clear from the exam, no X-ray or MRI is needed. Imaging may be used when the provider suspects a more complex problem underneath the bursitis, such as a meniscus tear, ligament damage, or advanced arthritis, or when symptoms don’t improve as expected. If infection is a concern, fluid aspiration is the key test, since it can confirm or rule out bacteria and also distinguish gout crystals from a simple inflammatory flare.
What Recovery Typically Looks Like
Most non-infectious knee bursitis improves within a few weeks once the underlying cause is addressed. That means avoiding the activity or position that triggered the inflammation: using knee pads if you kneel for work, reducing training volume if overuse was the trigger, or stretching tight hamstrings if pes anserine bursitis is the problem. Ice, compression, and over-the-counter anti-inflammatory medication help manage pain and swelling during recovery.
Septic bursitis follows a different timeline. It typically requires a course of antibiotics and sometimes repeated drainage before the infection clears. Recovery can take several weeks, and the bursa may remain swollen longer than expected even after the bacteria are gone. Chronic bursitis that doesn’t respond to conservative measures may be treated with a corticosteroid injection to reduce inflammation, or in rare cases, surgical removal of the bursa itself. A removed bursa generally regrows over several months, though the new one may be less prone to recurrence than the original.