Knee Bursitis
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Direct answer
Knee bursitis is inflammation of a synovial bursa around the knee, most often the prepatellar bursa that gives housemaid's knee its name. The decisive step is separating septic from aseptic disease, because a febrile, hot swelling needs aspiration for Gram stain and culture before any steroid is given. Staphylococcus aureus is the commonest organism in septic cases, while aseptic bursitis responds to rest, compression and NSAIDs.
What you must remember
- Prepatellar bursitis: housemaid's knee, lying superficial to the patella, and the most frequent knee bursitis, often caused by repeated kneeling on hard floors.
- Infrapatellar bursitis: clergyman's knee, a swelling below the kneecap from prolonged kneeling, with the superficial bursa lying between the skin and the patellar tendon.
- Pes anserine bursitis: tenderness below the medial joint line over the tibia, where sartorius, gracilis and semitendinosus insert, and it often coexists with medial knee osteoarthritis.
- Septic bursitis: Staphylococcus aureus is the commonest organism, so aspirate for cell count, Gram stain, culture and crystals before antibiotics are tailored.
- Aseptic bursitis: rest, NSAIDs, compression and aspiration of large effusions come first, with a corticosteroid injection reserved for persistent cases once infection is excluded.
- Baker cyst: a popliteal swelling from distension of the gastrocnemius-semimembranosus bursa, which communicates with the knee joint, so the underlying joint disorder is treated.
- Imaging: ultrasound confirms fluid and synovial thickening, while radiographs exclude a patellar fracture, a calcific deposit or a foreign body.
Working up a warm knee swelling
A 50-year-old plumber presents with a tense, tender swelling over the kneecap after months of kneeling at work. The pathway turns on one question, whether the bursa is infected.
- Locate the bursa: swelling superficial to the patella is prepatellar, swelling below the patella is infrapatellar, and medial swelling below the joint line is pes anserine.
- Test for sepsis: fever, red and warm overlying skin and a raised white cell count point towards infection and change the plan at once.
- Aspirate before treating: a sterile needle draws fluid for cell count, Gram stain and culture, and crystal analysis identifies gout or pseudogout as the cause.
- Treat aseptic cases conservatively: rest, activity change, a compression bandage and NSAIDs usually settle the swelling over several weeks.
- Inject selectively: a corticosteroid injection is an option for persistent aseptic bursitis, given only after infection has been excluded.
- Treat sepsis firmly: culture-directed antibiotics are started promptly, and bursectomy is considered if the infection fails to resolve.
Where the exam tries to catch you
Bursitis questions turn on the site of the swelling and on whether infection has been excluded.
- Trap: injecting steroid into a hot, febrile prepatellar swelling without aspirating Correct: aspirate first, since steroid into a septic bursa worsens infection and hides the organism.
- Trap: calling every anterior knee swelling prepatellar bursitis Correct: bursae are named by position, so swelling below the patella points to infrapatellar bursitis or patellar tendon disease.
- Trap: treating a popliteal swelling as simple bursitis without considering a Baker cyst Correct: a Baker cyst is a distended bursa linked to the joint, so the knee's own disorder is the real target.
Frequently asked questions
Which knee bursitis is called housemaid's knee?
Housemaid's knee is inflammation of the prepatellar bursa, which lies superficial to the patella. It results from repeated kneeling on hard surfaces and is the most frequent bursitis of the knee.
Which organism commonly causes septic prepatellar bursitis?
Staphylococcus aureus is the commonest organism causing septic prepatellar bursitis. Aspiration with Gram stain and culture confirms the organism before antibiotic therapy is tailored to the sensitivities.
Why is pes anserine bursitis mistaken for medial knee pain?
Pes anserine bursitis causes tenderness over the medial tibia below the joint line, where the sartorius, gracilis and semitendinosus tendons insert. Its pain can mimic medial meniscal or compartment pain, so careful palpation over the bursa helps separate them.
Does a Baker cyst count as knee bursitis?
A Baker cyst is a popliteal swelling caused by distension of the gastrocnemius-semimembranosus bursa, which communicates with the knee joint. It usually signals an underlying knee disorder such as osteoarthritis or a meniscal tear, so that disorder is treated alongside the cyst.