Bursa

For Young Learners

Structure and Location

A bursa is a small flat sac with a little fluid inside. Think of a thin water balloon with barely any water in it, placed between two things that need to slide past each other.

There are quite a few around the knee. The best known sits right in front of the kneecap, just under the skin. Another sits below it, over the top of the shin bone, and one more sits deeper down.

Function

The job is to stop rubbing. Skin slides over the kneecap every time you bend your knee. A tendon slides over bone every time a muscle pulls. Without a cushion in between, those surfaces would wear against each other.

If you kneel for a long time, the bursa at the front gets squashed repeatedly and can fill with fluid. It swells into a soft lump you can see on the front of the kneecap. The joint itself is untouched, which is why the knee usually still bends normally.

For Patients

Structure and Location

Bursae are thin sacs lined with a membrane similar to the joint lining, containing a small amount of fluid. More than a dozen have been described around the knee; a handful matter in practice.

The prepatellar bursa sits between the skin and the front of the kneecap. The superficial infrapatellar bursa lies in front of the patellar tendon, and the deep infrapatellar bursa behind it. The pes anserine bursa lies on the inner side just below the joint, under the tendons that attach there.

The large pouch above the kneecap began as a bursa and usually connects with the joint itself in adults, so it behaves as part of the joint cavity.

Function

Each one reduces friction between structures that move against each other: skin over bone, tendon over bone, or tendon over tendon. They are positioned exactly where that movement occurs.

Clinical Relevance

A bursa can become irritated by repeated pressure or by a direct blow, and then fills with fluid. The prepatellar bursa is the classic example, historically associated with trades involving kneeling.

The practical distinction worth understanding is between swelling in a bursa and swelling in the joint. Bursal swelling is localised, sits in front of the kneecap or over the tendon, and usually leaves knee movement comfortable. Joint swelling is diffuse, fills the whole front of the knee, and limits bending.

A hot, red, tender bursa can be infected, particularly after a graze or a puncture, and that needs prompt assessment.

Individual bursal conditions and their treatment are covered in the conditions section.

For Medical Students

Structure and Location

Bursae are synovium lined sacs interposed at sites of friction. Around the knee the clinically relevant ones are the prepatellar, between skin and patella; the superficial infrapatellar, anterior to the patellar tendon; the deep infrapatellar, between the tendon and the tibia; the pes anserine, deep to the conjoined insertion of sartorius, gracilis and semitendinosus; and the semimembranosus and medial gastrocnemius bursa posteromedially.

The suprapatellar bursa communicates with the joint cavity in the large majority of adults and is therefore functionally part of it.

Function

Each reduces shear between adjacent moving structures. Their lining resembles synovium and responds to irritation in the same way, with increased fluid production.

Clinical Relevance

Bursitis arises from repetitive pressure, direct trauma, infection or inflammatory arthropathy. Prepatellar bursitis is the classic occupational example.

The clinical discrimination that matters is bursal from intra-articular swelling. Bursal swelling is well circumscribed, anterior or superficial, and range of movement is typically preserved. An effusion is diffuse, obliterates the parapatellar hollows and restricts flexion.

Septic bursitis is relatively common in superficial bursae because of their subcutaneous position and proximity to skin breaches, and is distinct from septic arthritis, which involves the joint itself. The distinction changes both urgency and treatment.

A popliteal cyst arises from the semimembranosus and medial gastrocnemius bursa and commonly communicates with the joint, so it typically reflects intra-articular pathology rather than being a primary problem.

Management of each is covered in the conditions section.