Bursitis is one of those diagnoses that sounds cleaner than it is. Your knee swells, someone says "bursitis," and you picture a vague inflammation somewhere around the joint. The specifics - which sac, why it got irritated, whether it is infected - actually determine how the thing gets treated.
What bursae are and why they matter
The knee has about a dozen small fluid-filled sacs called bursae. Each one sits between a bone and the tissue moving over it (tendon, ligament, or skin) and reduces friction. Most of the time you are not aware they exist. When one swells up, it can grow to many times its normal size and become painful enough to limit walking. The location tells you which bursa is involved, and that matters.
The three most common types
Prepatellar bursitis affects the bursa directly in front of the kneecap. This is "housemaid's knee" or "carpet layer's knee" - it comes from sustained kneeling. The swelling sits at the front of the kneecap and can get quite pronounced, a rounded fluctuant mass that resembles a water balloon. Range of motion is usually preserved because the inflammation is outside the joint.
Pes anserine bursitis is on the inner side of the knee, about two to three inches below the joint line. It tends to show up in middle-aged and older adults with obesity, osteoarthritis, or diabetes, and in distance runners from repetitive stress. The pain concentrates on the inner lower knee and is often worse climbing stairs or getting out of a chair.
Infrapatellar bursitis involves two bursae below the kneecap. The superficial version develops from kneeling on hard surfaces. The deep infrapatellar bursa (between the patellar tendon and the tibia) is more common in jumping sports and is easy to confuse with patellar tendinitis without imaging.
What causes it
The prepatellar bursa gets irritated mechanically (sustained kneeling, direct blows, repetitive stress) or infected. Infection is a separate clinical problem - not just "worse bursitis" - and changes the treatment entirely. The prepatellar bursa is the most commonly infected bursa in the body because it sits right under the skin. A small skin break, sometimes invisible, lets bacteria in. Staphylococcus aureus causes most cases. Infected bursitis runs hotter, redder, and sometimes feverish - and it needs antibiotics.
Pes anserine bursitis has a different cause profile. Obesity and osteoarthritis shift how load moves through the knee, and that chronically irritates the bursa. Gout can deposit crystals in a bursa and produce inflammation that mimics infectious bursitis on examination; aspiration sorts that out.
How it is diagnosed
The diagnosis is usually clinical. The location, history of kneeling or jumping, and appearance of the swelling point clearly enough in most cases. A doctor will check range of motion: it is typically preserved in bursitis, which helps distinguish it from an internal joint problem.
When infection is suspected, the fluid gets aspirated and sent for white cell count, Gram stain, culture, and crystal analysis. A white cell count above 2,000 per microliter is suspicious; above 50,000 raises strong concern for septic bursitis. For straightforward prepatellar bursitis, imaging is rarely needed. Ultrasound is useful when the diagnosis is uncertain or aspiration is planned. MRI is reserved for cases where internal joint pathology seems possible.
Treatment
For mechanical bursitis, the first move is rest and removing whatever caused it. If kneeling is the culprit, stopping or at least using knee pads is not optional. NSAIDs help with inflammation and pain - ibuprofen or naproxen, at appropriate doses, for the acute period. Ice helps with swelling.
When the bursa is significantly swollen, aspiration drains the fluid and relieves pressure quickly. Fluid often comes back and may need to be drained again. Adding a corticosteroid injection at the same time reduces inflammation more effectively than drainage alone in randomized trials. That said, the injection carries a small risk of introducing infection and is not appropriate in every case.
Physical therapy comes in when there are contributing biomechanical issues - hip weakness, loading patterns that increase stress on the knee, poor mechanics in athletes. It matters more for pes anserine bursitis, where the underlying cause is often load distribution rather than a specific incident, than for a simple prepatellar case.
Surgery - bursectomy, removing the bursa - is reserved for chronic recurrent cases that have not responded to repeated conservative treatment. It is not common.
Septic bursitis needs antibiotics. Oral treatment covering Staphylococcus aureus (a penicillinase-resistant penicillin or a first-generation cephalosporin) works for mild cases in otherwise healthy patients. Aspiration is done both to confirm the diagnosis and to remove infected fluid, and may need repeating. Severe infection, immunocompromised patients, or cases not responding to oral antibiotics go to the hospital for IV treatment. Surgical drainage is used when antibiotics and aspiration are not enough.
Recovery
Mechanical bursitis in most people clears up in a few weeks if the aggravating activity stops. Recurrence is common when someone returns to the same activity before things have settled. Pes anserine bursitis tends to run a longer course, particularly when it is linked to osteoarthritis or obesity, because the underlying load issue does not go away.
Septic bursitis needs a full antibiotic course, typically two to three weeks, with close follow-up. It recurs more often than mechanical bursitis.
When to see a doctor
If the swelling is at the front of the kneecap, came on after kneeling, and is not particularly warm or red, rest, ice, and over-the-counter anti-inflammatories for a week or two is a reasonable start.
Go sooner if:
- The skin over the swelling is red, warm, or streaking outward
- You have fever or feel generally sick
- Swelling is not starting to improve after a couple of weeks
- You have had this before and it keeps coming back
- The pain is severe or you cannot put weight on the leg
A Baker's cyst behind the knee, a meniscal cyst, or a joint effusion from an internal problem can look similar from the outside and require a different workup.
