Definition
A bursa (bursa synovialis or simply bursa) is a small bag filled with synovial fluid, which occurs in many parts of the human body, especially in the area of the musculoskeletal system, to reduce stress caused by pressure and friction. On average, there are about 150 bursa sacs in the human body, which are one to a few centimeters wide and long, depending on their location. Unless they are inflamed, they have a flat tissue structure.
All bursa sacs are located between a hard base (usually bone) and a type of soft tissue. Depending on the structure of this second tissue, different types of bursa are distinguished: There is another criterion by which bursae can be divided into two groups:
- Skin bursa (bursa subcutanea): Located under the skin in places where it would otherwise border directly on bone.
- Tendon bursa (bursa subtendinea): Located below tendons above a bony structure.
- Bursa subligamentosa: Lying between ligaments and bones.
- Muscle bursa (submuscularis): Separates the muscle from its bony support.
- On the one hand, there are the so-called constant (congenital) bursae, which exist in every human being from birth on and are always in the same place.
- On the other hand, there are acquired or reactive bursae in the body, which only develop after birth as a reaction to a certain stimulus. They are therefore found in different locations and not in all people. Many skin bursae belong to this group.
Structure of a bursa
However, all bursae are built up in the same way, very similar to joint capsules and tendon sheaths. They consist of two layers:
- On the outside there is a connective tissue layer, the Membrana fibrosa or Stratum fibrosum.
- The synovial layer, the membrana synovialis or stratum synoviale is located below. This is responsible for producing and reabsorbing the fluid present in the bursa (synovial fluid).
