A bursa is a thin, slippery, fluid-filled sac that sits wherever tissue has to glide over bone. The knee has several. When one becomes inflamed, that is bursitis — and it is a common, treatable cause of knee pain that responds well to orthobiologic treatment.
What patients usually describe:
- Swelling, often quite localised — a soft, sometimes squashy lump rather than a diffusely puffy knee
- Warmth over the area
- Some loss of motion, particularly at the end of bending
- Pain on kneeling, on pressure, or with the activity that brought it on
Which bursa
- Prepatellar — directly in front of the kneecap. The one people kneel on, historically called housemaid’s knee, and still an occupational problem for roofers, tilers, plumbers, carpet fitters and gardeners.
- Superficial infrapatellar — just below the kneecap, over the patellar tendon. Same mechanism, slightly lower.
- Pes anserine — on the inner shin a couple of inches below the joint line. Frequently mistaken for a meniscal tear, and it commonly travels with knee arthritis.
- Suprapatellar — above the kneecap. In most adults this one communicates with the knee joint itself, so swelling there is often joint fluid rather than a separate bursal problem.
Bursitis is not an infection — but the naming is confusing
This causes a great deal of unnecessary worry, so it is worth stating plainly.
Ordinary bursitis is an inflammation, not an infection. It comes from pressure, friction, a knock, repetitive kneeling, or an underlying problem such as arthritis in the joint nearby. It is not contagious, it is not dangerous, and it is treated on its merits.
Separately, a bursa can become genuinely infected, and when that happens it is called septic bursitis. Because both conditions share the word, people searching for one constantly land on the other. They are different problems with different treatment.
What points to infection rather than ordinary bursitis:
- Marked heat, well beyond the mild warmth of an irritated bursa
- Redness spreading into the surrounding skin
- Fever or chills, or feeling unwell in yourself
- Onset over hours rather than days or weeks, and pain at rest
- A cut, scrape or puncture over the swelling — the usual route in, since the front bursae sit immediately under the skin
The way that question gets settled is by taking fluid out and testing it — cell count, Gram stain and culture. No scan substitutes for it. Septic bursitis is treated with antibiotics and drainage, not with an injection.
Orthobiologics are not a treatment for septic bursitis. Nothing is injected into a bursa that might be infected — not PRP, not bone marrow concentrate, not corticosteroid. Injecting adds material to a space the body is already fighting to clear. For ordinary bursitis, orthobiologics are used here routinely.
Not the same as a Baker’s cyst
Swelling at the back of the knee is usually a different thing: a popliteal cyst, commonly called a Baker’s cyst. It is a pocket of joint fluid that has pushed backwards out of the knee, and it matters because it is usually a symptom rather than the problem — the knee is producing excess fluid for a reason, most often arthritis or a meniscal tear.
Draining the cyst without addressing why the knee is making fluid tends to be a temporary result. The useful approach is to find and treat the source inside the joint, at which point the cyst frequently settles on its own. See knee arthritis and meniscal tears.
Treatment
Platelet-rich plasma is the typical orthobiologic treatment for knee bursitis here — prepared from your own blood on the day and placed under ultrasound guidance. It is used routinely for this problem, and the case for it is the case made throughout this site: your own tissue, no cumulative ceiling, and accurate placement into the structure that is actually inflamed.
Alongside it, and often first:
- Change the load. For prepatellar bursitis this is a large part of the answer — kneeling pads, a different working posture, time off the aggravating position. Anything injected fails if the knee goes straight back onto the same floor.
- Aspiration where a tense, distended bursa is the problem in itself. Under ultrasound guidance, so the needle reaches the fluid rather than passing beside it.
- Rehabilitation, which matters most for pes anserine bursitis, where hip and thigh strength change how the inner side of the knee is loaded.
- Treat what is underneath. Pes anserine bursitis frequently accompanies knee arthritis, and a suprapatellar swelling is often joint fluid. Treating the bursa and ignoring the joint buys weeks rather than years.
Corticosteroid will settle a stubborn non-infected bursa quickly, and it is worth being thoughtful about: it thins tissue that is already thin over the kneecap, and repeated injections into a superficial bursa are a recognised route to infection later. PRP is the better-reasoned choice for most people here.
Cost and what the visit involves are set out on PRP and orthobiologics for the knee.
Why ultrasound settles most of this in the room
Every structure in question sits within a centimetre or two of the skin. Ultrasound shows whether the swelling is in the bursa or inside the joint, whether the fluid is simple or debris-filled, whether the nearby tendon is involved, and whether a swelling at the back of the knee is a popliteal cyst. It also guides the needle, which for a small bursa is the difference between treating it and missing it.
Be seen the same day for these
- A hot, red, rapidly swelling knee, particularly with fever or chills
- Redness spreading up or down the leg from the swelling
- A knee that is agonising to move even slightly
- Any swollen, painful knee in someone diabetic, immunosuppressed, or taking steroids
- A wound or puncture over a swollen bursa
Common questions
Is knee bursitis an infection?
No. Ordinary bursitis is inflammation of the bursa from pressure, friction, a knock, repetitive kneeling or an underlying joint problem. It is not an infection. The confusion comes from the fact that a bursa can separately become infected, and that condition is called septic bursitis — so the two share a word without being the same problem.
How is bursitis treated?
For ordinary knee bursitis, platelet-rich plasma is the typical orthobiologic treatment here, placed under ultrasound guidance and prepared from your own blood on the day. It is used routinely for this. Alongside it, changing the load matters — for bursitis in front of the kneecap that is a large part of the answer — and a tense bursa is sometimes aspirated first. Where the real driver is arthritis or a tendon nearby, that is what gets treated.
Can PRP be used for septic bursitis?
No. Orthobiologics are not a treatment for septic bursitis, and nothing is injected into a bursa that might be infected. That includes PRP, bone marrow concentrate and corticosteroid. Septic bursitis is managed with drainage and antibiotics guided by culture. For ordinary bursitis, orthobiologics are used routinely.
What are the symptoms of knee bursitis?
Localised swelling — often a soft, distinct lump rather than a diffusely puffy knee — warmth over the area, some loss of motion particularly at the end of bending, and pain on kneeling or on direct pressure. Marked heat with fever, or redness spreading into the surrounding skin, is a different pattern and points to infection rather than ordinary bursitis.
Is a Baker's cyst the same as bursitis?
No. A Baker's cyst, or popliteal cyst, is a pocket of joint fluid that has pushed backwards out of the knee, so it sits at the back rather than at the front or inner side. It usually signals that the knee is producing excess fluid for a reason — most often arthritis or a meniscal tear — which is why draining it without treating the source tends to be temporary.
I have pain on the inner side of my knee. Bursa or meniscus?
Both present there and they are regularly confused. Pes anserine bursitis is tender at a specific spot on the inner shin a couple of inches below the joint line and often travels with arthritis. Meniscal pain sits at the joint line itself and more often comes with catching, locking or a specific twisting injury. Examination separates them, and ultrasound can look at the bursa directly.
