A tendon sheath is a tubular sleeve that wraps around a tendon, while a bursa is a small, flat sac that sits between structures like bone and tendon. Both reduce friction and contain lubricating fluid, but they differ in shape, location, and the type of movement they support. Understanding how they’re built and where they sit in the body helps explain why injuries to each one feel and behave differently.
Structure: Sleeve vs. Cushion
A tendon sheath is essentially a double-walled tube. The outer layer (called the parietal layer) lines the inside of the channel the tendon runs through. The inner layer (the visceral layer) wraps directly around the tendon’s surface. A thin bridge of tissue connects these two layers, and between them sits a small amount of synovial fluid, the same slippery liquid found inside joints. The sheath is sealed at both ends, creating a closed, lubricated tunnel. Folds of tissue at the top of the sheath give the tendon enough slack to glide back and forth.
A bursa, by contrast, is a flat, pillow-like sac. Think of it as a tiny water balloon that’s been mostly deflated. It doesn’t wrap around anything. Instead, it sits between two structures that would otherwise grind against each other, like a tendon pressing over a bony bump, or skin sliding over a kneecap. Bursae contain a thin film of fluid inside a single collapsed cavity, which lets the surfaces on either side of the bursa glide smoothly.
Where Each One Shows Up in the Body
Tendon sheaths appear wherever a tendon needs to make sharp turns or slide through tight spaces. The fingers and wrists are the classic example: long tendons travel from the forearm muscles all the way to the fingertips, passing through narrow channels along the way. Without a sheath, these tendons would fray against bone every time you curled a finger. True synovial sheaths are only found in spots that demand highly efficient lubrication because of sudden direction changes and high friction.
Bursae are found in broader, flatter areas where compression is the main problem rather than sliding through a channel. The shoulder, elbow, hip, and knee are common sites. The bursa over your kneecap, for instance, cushions the skin and patella when you kneel. The one beneath the shoulder’s bony roof protects the rotator cuff tendons from being pinched overhead. Bursae can also sit between bone and ligament, or between bone and skin at any prominent bump that takes repeated pressure.
How They Reduce Friction Differently
Both structures exist to prevent wear and tear, but they solve different mechanical problems. A tendon sheath manages linear, back-and-forth gliding. When a muscle contracts, the tendon pulls through its sheath the way a cable slides inside a bike brake housing. The sealed fluid layer keeps the tendon from catching or sticking. The fibrous outer portion of the sheath also acts as a retinaculum, a kind of strap that holds the tendon in place and prevents it from bowstringing away from the bone during strong contractions.
A bursa handles compression and shear. Rather than guiding a tendon along a path, it absorbs pressure where two surfaces press together and shift slightly. When you raise your arm overhead, the rotator cuff tendon compresses against bone, and the bursa between them spreads that load and lets the surfaces slide past one another without grinding.
What Happens When Each One Gets Inflamed
Inflammation of a tendon sheath is called tenosynovitis. Inflammation of a bursa is called bursitis. The symptoms overlap quite a bit: both cause pain and stiffness that typically worsen with movement and can feel worse at night. But because of the structural differences, the two conditions play out differently in practice.
Tenosynovitis tends to produce pain that tracks along the path of the tendon, often with a sensation of catching or locking. De Quervain’s tenosynovitis, for example, causes pain along the thumb side of the wrist that flares when you grip or twist. Trigger finger is another form, where the inflamed sheath traps the tendon and the finger gets stuck in a bent position before snapping straight. Because the sheath is a closed tube, swelling inside it quickly increases pressure on the tendon, which is why the catching sensation is so characteristic.
Bursitis, on the other hand, tends to produce more localized swelling over a joint or bony prominence. Prepatellar bursitis (sometimes called “housemaid’s knee”) creates a visible, squishy pocket of fluid right over the kneecap. Olecranon bursitis does the same thing at the tip of the elbow. Because a bursa is an open sac rather than a tight tube, it can swell outward more freely, which is why a swollen bursa often looks puffy from the outside while a swollen tendon sheath may not.
Diagnostic Differences
When a bursa is inflamed, fluid can sometimes be drawn out with a needle and tested. This is especially useful when infection is a concern. If the fluid contains fewer than 500 white blood cells per cubic millimeter, the cause is likely mechanical irritation rather than infection or crystal deposits like gout. Tendon sheaths are rarely aspirated in the same way because they hold so little fluid and sit in tighter anatomical spaces.
Recovery and Management
For both conditions, the initial approach is similar: rest, ice, and reducing the activity that triggered the problem. But the details diverge because of how each structure behaves.
With bursitis, complete immobilization can backfire. Shoulder bursitis is a good example. Keeping the shoulder still for too long can lead to muscle weakening, tissue retraction, and eventually a frozen shoulder. Physical therapy and range-of-motion exercises are important early on to maintain strength in the muscles surrounding the inflamed bursa.
Tenosynovitis sometimes requires a short period of true immobilization, especially in the hands and wrists. A splint that prevents the tendon from sliding through the irritated sheath gives the inner lining time to calm down. This makes sense given the mechanics: a bursa can tolerate gentle movement across its surface, but every time a tendon glides through an inflamed sheath, it aggravates the lining all over again.
Why They’re Often Confused
Tendon sheaths and bursae are built from the same type of tissue, produce the same lubricating fluid, and serve the same broad purpose of reducing friction. In some locations they even sit right next to each other; the shoulder, for instance, has both a subacromial bursa and several tendon sheaths within inches of one another. The key distinction is geometry: a sheath is a tube that guides a moving cable, and a bursa is a pad that cushions two surfaces pressing together. That difference in shape dictates where each one appears, how it fails, and what recovery looks like.