Bursa is a thin, lubricated fluid-filled sac located at points of friction between a bone and the surrounding soft tissue, such as skin, muscles, ligaments, and tendons like a fluid filled balloon.
About 160 bursae are present in adult body size of which depends on the location of the bursae.
A bursal sac is made up of a synovial membrane, or synovium, that produces and contains synovial fluid.
A bursa is said to be superficial when present between bones, tendons or skin, or deep if present between bone and overlapping muscles.
Subacromial and iliopsoas bursae are examples of deep bursae. Olecranon and prepatellar bursae are examples of superficial bursae.
Developmentally, there are two types of bursae – constant and adventitial
Constant bursa develops during the development of the embryo, are lined with endothelial and contain synovial cells. These are present between bones, muscles or skin.
These bursae develop in response to repeated friction and pressure. These lack endothelial and synovial cells. The bursae over bunion and osteochondroma are examples of these bursae.
Important Locations of Bursae in Body
There are approximately 160 bursae in the human body. Important Bursae are described below.
Bursae of Upper Limb
This lies between the acromion and the rotator cuff. It separates the supraspinatus tendon from the overlying coracoacromial arch and the deltoid muscle and cushions the coracoacromial ligament from the supraspinatus muscle
When the arm is resting at the side, the bursa protrudes laterally from beneath the acromion, when the arm is abducted, it rolls medially beneath the bone.
Subscapular bursae are found between the anterior surface of the scapula and the posterior chest wall.
There are two olecranon bursae. One lies between the tendon of the triceps and the posterior ligament of the elbow and the olecranon. The other is more superficial between the attachment of the triceps to the olecranon and the skin.
Bursae of Hip
It lies deep to the gluteus maximus over the ischial tuberosity.
Iliopsoas bursa is the largest bursa in the body. It lies between the iliopsoas tendon and the lesser trochanter, extending upward into the iliac fossa.
It has superficial and deep components. Superficial bursa lies between the tensor fascia latae and the skin. The deep bursa located between the greater trochanter and the tensor fasciae latae.
Medial collateral ligament
This bursa is located between the superficial and deep medial collateral ligament of the knee.
Pes anserinus bursa is located between the conjoined hamstring tendons and underlying superficial collateral ligament.
It is located anteriorly over the patella, between patella and skin
Superficial part lies between the patellar ligament and the skin. The deep component lying between the patellar ligament and the proximal anterior tibia
Also called Baker cysts, these are located in the posterior joint capsule of the knee.
Bursae Around Ankle
These are two bursae and generally considered a single entity for clinical purpose, found at the level of insertion of the Achilles tendon. The superficial one is located between the skin and the tendon, and the deep one is located between the calcaneus and the tendon.
Bursa functions to reduce frictions between two surfaces. However, repeated friction or trauma or infection may cause the bursa to get inflamed, a condition known as bursitis. Inflammation of bursa causes synovial cells to multiply and increases collagen and fibrin-rich fluid production. A granulation tissue may replace the synovial lining and fibrosis may occur
Bursitis is most common in the shoulder, elbow, knee, and hip. Other places of occurrence are the ankle, big toe, buttock, and wrist.