Bursae are small, fluid-filled sacs that cushion and lubricate the spaces between bones, tendons, muscles, and skin. They reduce friction so these tissues can glide smoothly over each other every time you move. The human body contains over 150 of them, concentrated around the major joints like shoulders, elbows, hips, and knees.
How Bursae Work
Each bursa is lined with a thin membrane that secretes a lubricating fluid with a consistency similar to raw egg whites. This fluid forms a thin film on the inner surfaces of the sac, creating a nearly frictionless gliding surface between tissues that would otherwise grind against each other. Think of a bursa like a tiny water balloon wedged between a bone and a tendon: it lets those structures slide past one another without catching or wearing down.
The membrane surrounding a bursa is semi-permeable, meaning certain substances can pass in and out. This allows the sac to regulate its own fluid levels and respond to changing mechanical demands. The lubricating fluid inside contains compounds that provide both low-friction and low-wear properties, protecting the surrounding cartilage and soft tissues from damage during repetitive motion.
Where Bursae Are Located
Most bursae sit near large joints in the arms and legs, positioned between layers of tissue that need to move independently. Some of the most clinically relevant ones include:
- Shoulder (subacromial bursa): sits between the rotator cuff tendons and the bony roof of the shoulder, allowing your arm to lift overhead without pinching
- Elbow (olecranon bursa): covers the bony tip of the elbow, letting the skin glide freely during bending and straightening
- Hip (trochanteric bursa): cushions the outer point of the hip where large tendons cross over bone
- Knee (prepatellar bursa): protects the front of the kneecap from direct pressure, particularly during kneeling
Bursae that sit close to the skin surface, like those at the elbow and kneecap, are especially vulnerable to injury because there’s so little tissue protecting them from outside impact.
Cushioning and Protection
Beyond lubrication, bursae act as shock absorbers. Wherever a bony prominence could dig into soft tissue during movement or pressure, a bursa sits between them to distribute the force. When you lean on your elbows at a desk, the olecranon bursa absorbs that compression so the skin and tendons aren’t crushed against the bone underneath. When you kneel on a hard floor, the prepatellar bursa does the same for your kneecap.
This protective role is passive but critical. Without bursae, repetitive motions like throwing a ball, climbing stairs, or even walking would cause far more wear on your tendons and ligaments over time.
What Happens When Bursae Become Inflamed
When a bursa gets irritated or damaged, it swells with excess fluid, a condition called bursitis. This accounts for roughly 0.4% of all primary care visits, making it one of the more common musculoskeletal complaints. It typically causes localized swelling, tenderness, and pain with movement, though the joint itself usually retains its full range of motion.
Bursitis falls into two broad categories. Non-infectious bursitis is the more common type, usually triggered by repetitive pressure, overuse, or a direct blow. “Housemaid’s knee” from prolonged kneeling and “student’s elbow” from leaning on hard surfaces are classic examples. Inflammatory conditions like gout or rheumatoid arthritis can also trigger it.
Infectious (septic) bursitis occurs when bacteria enter the bursa, most often through small breaks in the skin. The limited blood supply to bursal membranes makes them especially susceptible to infection through the skin rather than from the bloodstream. Staphylococcus aureus is the most common culprit. Septic bursitis tends to produce more dramatic symptoms: fever is present in up to 77% of cases, and significant redness appears in 63% to 100%. People on immunosuppressant medications face a higher risk.
How Bursitis Is Diagnosed
Many cases of bursitis can be identified through a physical exam alone, based on the location of swelling and tenderness. When infection is a concern, a doctor may use a needle to draw fluid from the bursa and send it for testing. A white blood cell count below 500 per cubic millimeter in the fluid points toward a non-infectious cause, while much higher counts suggest infection.
Imaging comes into play when the diagnosis is unclear or the bursa sits deep beneath muscle. Ultrasound is particularly useful because it shows real-time changes during movement and can distinguish bursitis from skin infections by revealing a characteristic pattern in the fat layer over the bursa. MRI is reserved for evaluating deeper bursae that ultrasound can’t reach as easily.
Recovery and Treatment
Most bursitis is short-lived. The majority of people heal within a few weeks with rest, ice, and avoiding the activity that triggered it. The key is giving the bursa enough time to fully recover before returning to the aggravating motion or pressure. Resuming activity too soon significantly raises the chance of reinjury.
For cases that don’t resolve with rest, corticosteroid injections are the standard next step. A systematic review of trochanteric (hip) bursitis found that symptom resolution rates ranged from 49% to 100% with steroid injections. Most patients needed only a single injection, though up to a third required a second round, and a small number needed as many as five. Some studies reported a relapse rate of about 25% within ten months, so recurrence is possible even after successful treatment.
Septic bursitis requires a different approach, with antibiotics targeting the specific bacteria involved. In stubborn or recurrent cases, surgical removal of the bursa is an option, though the body can often compensate over time by forming scar tissue that mimics some of the bursa’s original cushioning function.