Bursitis happens when small, fluid-filled sacs called bursae become inflamed, usually from repetitive motion, prolonged pressure, a direct injury, or an infection. Your body has over 150 of these sacs positioned at key friction points, and when one gets irritated, it swells with excess fluid and produces pain that can range from a dull ache to intense, sharp discomfort.
What Bursae Do in a Healthy Joint
A bursa is a thin, flattened sac lined with the same type of tissue that lines your joints. Inside, a thin film of synovial fluid acts as a lubricant. The sac sits between structures that would otherwise grind against each other: a tendon sliding over bone, skin stretching across a kneecap, or a muscle pressing against a bony ridge. Think of it as a tiny hydraulic cushion that absorbs compressive and shear forces so those tissues can move freely without friction.
Bursae don’t wrap around tendons the way a sleeve would. They sit at the interface where pressure concentrates most, creating a buffer zone that allows independent movement over a short distance. When this system works, you never notice it. Problems start when something disrupts the sac’s lining or overwhelms its capacity to absorb stress.
Repetitive Motion and Prolonged Pressure
The most common path to bursitis is cumulative irritation. Repeating the same movement, whether it’s throwing a ball, kneeling on a hard floor, or resting your elbows on a desk for hours, creates friction that gradually damages the bursa’s lining. The synovial tissue responds by thickening and producing excessive fluid. Blood vessels in the area become more permeable, allowing protein-rich fluid to leak in. The sac swells, and the lubricating fluid that was once thin and slippery becomes thick and sticky.
This is why bursitis so often tracks with specific activities or occupations. Carpet layers and gardeners develop it in the knee (“housemaid’s knee”). Painters and carpenters get it in the shoulder. Runners frequently develop it at the hip. The pattern is consistent: repeated stress at a single point, sustained over days or weeks, tips the bursa from normal function into inflammation.
Prolonged direct pressure works through the same mechanism, just without the motion component. Sitting on a hard surface for long stretches can irritate the bursae near the sit bones. Leaning on your elbows compresses the olecranon bursa at the elbow tip. The tissue doesn’t need friction to become inflamed; steady compression alone is enough.
What Happens Inside the Inflamed Bursa
During the acute phase, the body launches a local inflammatory response. Chemical signals, including proteins that drive inflammation and enzymes that break down tissue, flood the area. The bursa fills with fluid that can be cloudy, thick, or even tinged with blood. Swelling presses on surrounding nerves and tissues, making movement painful.
If the irritation stops, this process usually resolves on its own within a few days to a few weeks with rest and basic care like ice and avoiding the aggravating activity. But if the cause persists, the inflammation becomes chronic. The bursa’s normal lining gets replaced first by granulation tissue (the body’s early repair material) and eventually by scar-like fibrous tissue. At that stage, the bursa loses its ability to function as a cushion, and the chronic inflammation can weaken overlying ligaments and tendons. In severe, long-standing cases, those tendons can eventually rupture.
Sudden Injury or Trauma
You don’t always need weeks of repetitive stress. A single hard blow can trigger bursitis immediately. Falling onto your knee, getting hit on the elbow, or landing on your hip after a fall can damage the bursa’s lining in one event, causing rapid swelling and pain. This type tends to come on quickly and is often accompanied by visible bruising or redness. The fluid inside can become hemorrhagic, meaning it contains blood from ruptured small vessels within the sac.
Traumatic bursitis follows the same inflammatory pathway as the repetitive kind, but compressed into a shorter timeline. The key difference is that the damage is sudden rather than cumulative, and recovery often begins sooner because the source of irritation is a one-time event rather than an ongoing habit.
Infection: Septic Bursitis
Some bursitis isn’t caused by mechanical stress at all. Bacteria can enter a bursa and trigger a more serious form called septic bursitis. The most common route is through a break in the skin, such as a cut, scrape, or puncture wound near a superficial bursa. Bacteria can also spread from a nearby skin infection (cellulitis) into the sac. In rare cases, an infection elsewhere in the body reaches the bursa through the bloodstream.
Septic bursitis tends to look different from the mechanical kind. The area is typically hot to the touch, noticeably red, and more swollen. You may feel generally unwell or develop a fever. This form requires medical treatment, because antibiotics are needed to clear the infection and prevent it from spreading to deeper tissues or the joint itself. Superficial bursae, particularly at the elbow and the front of the knee, are the most vulnerable because they sit just beneath the skin with minimal soft tissue protecting them.
Underlying Conditions That Raise Your Risk
Certain health conditions make bursitis more likely to develop or more likely to recur. Rheumatoid arthritis and other inflammatory types of arthritis create a baseline of joint inflammation that can easily extend to nearby bursae. Gout, which causes uric acid crystals to deposit in and around joints, can trigger an intense inflammatory reaction inside a bursa that mimics or accompanies a gout flare.
Diabetes increases susceptibility as well, though the mechanism is less direct. Impaired circulation and changes in connective tissue health make the bursae more vulnerable to both mechanical irritation and infection. Carrying excess weight puts additional pressure on the bursae of the hips and knees, which is why higher body weight is a recognized risk factor for bursitis in those joints specifically.
Where Bursitis Develops Most Often
Bursitis can technically affect any bursa, but it clusters in predictable locations. The shoulder is one of the most common sites, particularly in people who do overhead work or sports. The elbow is another frequent location, especially in people who lean on hard surfaces or who have jobs that involve repetitive elbow bending. The hip, particularly the outer side, is a common spot for runners, older adults, and people who sleep on one side. The knee develops bursitis both at the front of the kneecap (from kneeling) and along the inner side (from activities like running or climbing stairs).
Each location has its own typical aggravating activity, but the underlying mechanism is the same everywhere: the bursa absorbs more stress than it can handle, and the lining responds with inflammation and fluid overproduction.
How Bursitis Feels Different From Tendonitis
Bursitis and tendonitis often occur in the same areas and sometimes at the same time, which makes them easy to confuse. Tendonitis pain tends to be sharpest when you actively engage the muscle attached to the affected tendon, especially against resistance. The tenderness is usually concentrated along the tendon itself or at a specific point where it attaches to bone.
Bursitis pain, by contrast, can be intense both during movement and at rest. It often produces a broader area of swelling rather than pinpoint tenderness. When infection is involved, you’ll notice warmth, redness, and swelling that tendonitis alone doesn’t typically produce. In practice, both conditions involve inflammation near a joint, and the distinction matters mainly because treatment approaches differ. Bursitis that doesn’t respond to rest and activity modification within a few weeks, or that shows signs of infection, warrants a closer look from a clinician.