bursa

**Definition/Structure:** A bursa (pl. bursae) is a closed, potential space lined by thin synovial membrane and containing a small amount of lubricating synovial-like fluid. Unlike joint capsules, bursae do not usually communicate with the joint (exceptions: suprapatellar bursa, subscapular bursa, and gastrocnemius–semimembranosus bursa may communicate with the joint space).

**Function:** Reduce friction and cushion pressure points where tendons, muscles, ligaments, or skin glide over bony prominences.

**Types:** - *Anatomic (true) bursae* are present at birth and are consistently located (e.g., subacromial/subdeltoid, trochanteric, prepatellar, olecranon, pes anserine, retrocalcaneal, iliopsoas). - *Adventitious (acquired) bursae* develop in response to chronic friction or pressure (e.g., over bunions, ischial tuberosity).

**Bursitis (pathophysiology):** Inflammation from repetitive trauma/overuse, acute trauma, crystal deposition (gout, pseudogout), systemic inflammatory disease (RA), or infection (septic bursitis). - *Septic bursitis:* usually superficial bursae (olecranon, prepatellar); *Staphylococcus aureus* is the most common organism (~80%). Presents with warmth, erythema, fever, and a skin break. Diagnosed by aspiration (fluid analysis, Gram stain, culture); treated with antibiotics ± drainage. - *Aseptic bursitis:* treated with rest, ice, NSAIDs, activity modification; corticosteroid injection if refractory (not for suspected infection).

**High-yield clinical associations:** - **Subacromial (subdeltoid) bursitis:** shoulder pain with overhead activity; often coexists with rotator cuff tendinopathy/impingement (supraspinatus). Painful arc 60–120°, positive Neer and Hawkins tests. - **Prepatellar bursitis ("housemaid's knee"):** prolonged kneeling. - **Infrapatellar bursitis ("clergyman's knee"):** kneeling upright. - **Olecranon bursitis ("student's/miner's/draftsman's elbow"):** leaning on elbow; also seen in gout/RA. - **Trochanteric bursitis (greater trochanteric pain syndrome):** lateral hip pain, worse lying on affected side and climbing stairs; common in middle-aged women. Often involves gluteus medius/minimus tendinopathy. - **Pes anserine bursitis:** medial knee pain below the joint line, 2–3 inches below; seen in obese women, diabetics, osteoarthritis, runners. The pes anserine comprises tendons of the **sartorius, gracilis, semitendinosus** ("Say Grace before Tea"). - **Ischial (ischiogluteal) bursitis ("weaver's bottom"):** prolonged sitting. - **Retrocalcaneal bursitis:** posterior heel pain, associated with Haglund deformity, spondyloarthropathy. - **Iliopsoas bursitis:** anterior hip/groin pain. - **Baker (popliteal) cyst:** distension of the gastrocnemius–semimembranosus bursa communicating with the knee joint, associated with joint effusion (OA, RA, meniscal tear). Can rupture and mimic DVT (calf pain/swelling); ultrasound distinguishes them.

**Differentiation from other conditions:** - Bursitis typically has pain with both active and passive ROM only at extremes (or with pressure), whereas septic arthritis causes severe pain with any joint motion and a joint effusion. - Tendinitis causes pain with resisted active motion.

**Embryology note:** Bursae develop from mesenchymal cavitation in connective tissue near joints.

**Other "bursa" usage:** *Omental bursa (lesser sac)* is a peritoneal cavity recess behind the stomach, accessed via the epiploic foramen of Winslow (posterior to the hepatoduodenal ligament containing the portal triad). The *bursa of Fabricius* in birds is the site of B-cell maturation (not in humans). Posterior gastric ulcer perforation may drain into the lesser sac.