Bursa sacs are small, fluid-filled pouches that sit between bones, tendons, muscles, and skin near your joints. They act as cushions, reducing friction so your tissues can glide smoothly over each other when you move. The average adult body contains about 160 of these sacs, spread across nearly every major joint.
Think of a bursa like a tiny water balloon wedged between two surfaces that would otherwise grind together. Without them, the simple act of bending your knee or raising your arm would cause painful wear on the surrounding tissues.
How Bursae Work
Each bursa is lined with a thin membrane called the synovium, which produces a slippery fluid with a consistency similar to raw egg whites. This synovial fluid forms a thin film on the inner surfaces of the sac, creating a nearly frictionless barrier. The membrane’s inner lining actively secretes this lubricant, keeping the sac from drying out or losing its cushioning ability over time.
Bursae don’t just reduce friction. They also absorb impact and distribute pressure across a wider area, protecting tendons and muscles from being pinched or worn down against bone. Every time you walk, reach overhead, or kneel, dozens of bursae are quietly doing their job.
Where Bursae Are Located
Most bursae cluster around the large joints of the arms and legs, where the greatest forces and most repetitive movements occur. The shoulder, hip, knee, and elbow each contain several bursae positioned at key friction points. Your knee alone has multiple bursae: some sit between the kneecap and the skin, others cushion the tendons running along the sides and back of the joint.
Bursae also appear in smaller, less obvious locations. You have them near your heel, at the base of your big toe, and alongside the bony prominences of your wrist and ankle. Not all 160 bursae are present at birth. Some develop later in life as the body encounters new sources of friction with age and activity.
Types of Bursae
Most bursae fall into the category of synovial bursae. These are the ones you’re born with (or that develop naturally in childhood). They have a true synovial lining, produce their own lubricating fluid, and sit in predictable locations between tissues that need protection.
There’s also a less common type called an adventitious bursa. These are not part of your original anatomy. Instead, they form in response to unusual or repeated pressure on soft tissue over a bony area. A bunion is a familiar example: the swollen bump at the base of the big toe develops partly because an adventitious bursa forms there in response to friction from tight shoes or structural changes in the foot. Unlike synovial bursae, adventitious bursae lack the normal lubricating lining and are essentially the body’s improvised attempt at cushioning.
What Happens When a Bursa Gets Inflamed
When a bursa becomes irritated or swollen, the condition is called bursitis. The most common cause is prolonged or repetitive pressure, where the bursa gets compressed between a hard surface and bone. Kneeling on hard floors for extended periods, resting your elbows on a desk for hours, or performing the same overhead motion repeatedly can all trigger it. Direct trauma, like a fall onto your knee or elbow, is the second most common cause.
Bursitis can be acute or chronic. Acute cases typically come from a sudden injury, an infection, or crystal deposits (as seen in gout). Chronic bursitis develops more gradually from repetitive stress or inflammatory conditions like rheumatoid arthritis. The chronic form is essentially the result of accumulated “micro traumas” over weeks or months.
Infected vs. Non-Infected Bursitis
Not all bursitis is the same, and the distinction between an infected (septic) and non-infected (aseptic) bursa matters for treatment. Aseptic bursitis, the more common type, usually presents as mild swelling with little pain. It’s an irritation issue, not an infection.
Septic bursitis looks and feels noticeably different. In studies comparing the two, tenderness was present in 88% of septic cases versus just 36% of aseptic ones. Redness or surrounding skin changes appeared in 83% of infected cases compared to 27% of non-infected ones. Warmth around the joint showed up in 84% versus 56%. Fever occurred in about 38% of septic cases but was essentially absent in aseptic bursitis. If a swollen bursa is also hot, red, very tender, and accompanied by fever, that pattern points strongly toward infection and needs prompt medical attention.
How Bursitis Is Treated
Most non-infected bursitis improves on its own with basic self-care. Rest, ice for the first 48 hours, and over-the-counter anti-inflammatory pain relievers are typically enough to bring the swelling down. Switching to heat (a warm bath or heating pad) after the initial swelling phase can also help. Simple adjustments matter too: if you sleep on your side and have hip bursitis, placing a pillow between your knees reduces pressure on the affected area.
When these measures aren’t enough, treatment escalates gradually. Physical therapy can strengthen the muscles around the joint, reducing the load on the bursa and helping prevent recurrence. For persistent shoulder or hip bursitis, a corticosteroid injection into the bursa often provides rapid relief, and many people need only a single injection. A walking cane or other assistive device can temporarily take pressure off a hip or knee bursa while it heals.
Infected bursitis requires antibiotics. In some cases, the bursa needs to be drained with a needle. Surgical removal of a bursa is rare and reserved for cases that don’t respond to anything else. When surgery is needed, the body can typically compensate, though recovery takes longer than conservative treatment.
Preventing Bursa Problems
Since most bursitis comes from repetitive pressure, prevention is largely about reducing that pressure. If your work or hobbies involve kneeling, use knee pads. If you lean on your elbows at a desk, add cushioning or change your position regularly. Warming up before exercise and building strength in the muscles around vulnerable joints both help distribute forces away from the bursae.
Properly fitting shoes matter more than most people realize, especially for preventing adventitious bursae at the feet. Shoes that crowd the toes or create pressure points over bony areas can trigger new bursa formation and inflammation at the same time. Varying your activities rather than performing the same repetitive motion day after day also gives overworked bursae time to recover before irritation sets in.