A bursa sac in the knee is a small, fluid-filled pouch that sits between bones, tendons, muscles, and skin to reduce friction and cushion pressure points as the joint moves. Your knee contains several of these sacs, and most people never think about them until one becomes irritated or inflamed, a condition called bursitis. Understanding where these sacs are and what they do helps explain why certain knee pain and swelling patterns develop.
What a Bursa Does
Every time you bend, straighten, or twist your knee, dozens of structures slide past each other. Bones press against tendons, tendons glide over other tendons, and skin stretches across bony surfaces. Without something to ease that contact, the repeated friction would quickly damage soft tissue. Bursae solve this problem. Each sac is lined with a thin membrane that produces a small amount of lubricating fluid, similar to the fluid inside the joint itself. This creates a nearly frictionless cushion that allows structures to move smoothly against one another.
Think of a bursa like a tiny water balloon wedged between two surfaces. When pressure is applied, the fluid distributes the force evenly instead of concentrating it on one spot. This is especially important in the knee, which absorbs tremendous loads during walking, running, squatting, and kneeling.
Where the Knee’s Bursae Are Located
The knee has roughly a dozen bursae, but four tend to cause the most problems clinically.
- Prepatellar bursa: Sits directly over the kneecap, between the bone and the skin. This is the one most commonly irritated by kneeling.
- Suprapatellar bursa: Located above the kneecap, between the large thigh muscle tendon and the thighbone. It helps the tendon glide smoothly when you extend your leg.
- Infrapatellar bursae: There are actually two of these, one superficial and one deep. The superficial sac lies between the bony bump below the kneecap and the skin. The deep one sits behind the kneecap tendon, cushioning it against the shinbone.
- Pes anserine bursa: Found on the inner side of the knee, just below the joint line. It separates a group of three tendons from the shinbone and the inner knee ligament.
Other bursae exist around the outer knee, behind the knee, and near the ligaments on both sides of the joint. They’re less commonly involved in bursitis but can still become inflamed under the right circumstances.
How Bursitis Develops
When a bursa is subjected to repeated pressure, direct trauma, or infection, it responds by producing excess fluid and becoming inflamed. The sac swells, the surrounding tissue gets tender, and the knee may feel stiff or warm to the touch.
The most common trigger is repetitive kneeling or pressure on the front of the knee. Plumbers, roofers, carpet layers, coal miners, and gardeners all face a higher risk of prepatellar bursitis, which has historically been called “housemaid’s knee.” Athletes who play sports involving frequent falls onto the knees, like wrestling or volleyball, are also prone to it. The pes anserine bursa on the inner knee tends to flare up more often in runners or people with osteoarthritis.
Less commonly, a bursa can become infected. Bacteria typically enter through a cut, scrape, or puncture wound over the kneecap. Infected (septic) bursitis is more serious and requires different treatment than the standard inflammatory type.
What Bursitis Feels Like
The hallmark symptom is localized swelling. With prepatellar bursitis, you’ll often see a distinct, egg-shaped puffiness right over the kneecap that isn’t present on the other knee. The area is usually tender when pressed and may feel warm. Bending the knee fully can be uncomfortable because the swollen sac gets compressed.
Pes anserine bursitis produces tenderness on the inner side of the knee, about two to three inches below the joint line. People often notice it when climbing stairs or getting out of a car. Deeper bursae, like the suprapatellar, can cause a more diffuse aching and stiffness that’s harder to pinpoint.
If the bursa is infected, the pain tends to be more intense. The skin over the area may become red and hot, and you might develop a fever or feel generally unwell. These signs distinguish septic bursitis from ordinary inflammation, though distinguishing the two definitively requires fluid analysis. Standard blood tests aren’t always reliable here, since white blood cell counts in the bloodstream can look normal even with an active infection. The definitive test involves drawing fluid from the bursa with a needle and checking it for bacteria and inflammatory cells.
How Bursitis Is Treated
Most cases of knee bursitis resolve with straightforward home care. Rest, ice, compression, and elevation are the standard first steps. Over-the-counter pain relievers like ibuprofen or acetaminophen help manage discomfort and, in the case of anti-inflammatory medications, reduce swelling. The key is removing whatever caused the irritation in the first place. If kneeling triggered it, stop kneeling or use proper knee pads.
Swelling and pain typically improve within a couple of weeks with consistent rest. If symptoms haven’t improved after two to three weeks, it’s worth getting a professional evaluation. Left untreated, bursitis can become chronic and persist for months or even years.
For deeper bursae that don’t respond to conservative care, corticosteroid injections (sometimes combined with a local numbing agent) can provide relief. However, these injections aren’t recommended for superficial bursae like the prepatellar bursa because of a higher risk of complications, including infection, skin thinning, and damage to nearby tendons.
When a bursa is suspected to be infected, antibiotics targeting common skin bacteria are the standard treatment. In some cases, a doctor will drain the bursa with a needle both to relieve pressure and to confirm the diagnosis by analyzing the fluid.
When Surgery Becomes Necessary
Surgery is rare and reserved for cases that don’t respond to any other treatment. A bursectomy, the removal of the bursa entirely, is typically only considered for chronic prepatellar bursitis that has resisted months of conservative management. The procedure involves excising the thickened bursa sac, sometimes along with a small area of overlying skin if it has become damaged. Recovery involves wound care and gradual return to activity. Over time, the body forms scar tissue in the area that functions as a replacement cushion, though it’s not as effective as the original bursa.
Preventing Bursa Problems
If your job or hobbies put repetitive stress on your knees, cushioned knee pads are the single most practical investment you can make. Look for pads that distribute pressure across a wide area rather than concentrating it on the kneecap. For workers who spend long stretches kneeling, alternating positions regularly and taking breaks to straighten the legs makes a meaningful difference.
Runners and athletes can reduce pes anserine bursa irritation by strengthening the muscles around the inner knee and hip, maintaining flexibility in the hamstrings, and avoiding sudden jumps in training volume. If you notice mild swelling or tenderness developing, addressing it early with ice and rest prevents a minor irritation from becoming a weeks-long problem.