Bursae: Classifications, Locations, and Clinical Significance

What are the different types of bursa?

Bursae are small, fluid-filled sacs strategically located throughout the body to reduce friction between moving structures. They are primarily categorized into two main types: adventitious (acquired) and synovial (anatomically present), with synovial bursae further classified by their location relative to surrounding tissues.

Understanding Bursae: Nature's Friction Reducers

A bursa (plural: bursae) is a flattened, sac-like structure lined by a synovial membrane and containing a small amount of synovial fluid. Its primary function is to minimize friction and absorb shock between anatomical structures that move against each other. These structures often include tendons, muscles, bones, and skin. Without bursae, repetitive movements could lead to excessive wear and tear, inflammation, and pain in joints and surrounding tissues.

The Primary Classifications of Bursae

While all bursae serve a similar purpose, their origin, location, and specific characteristics lead to distinct classifications.

Adventitious Bursae

Adventitious bursae, also known as accidental or acquired bursae, are not present at birth. Instead, they develop in response to chronic, localized pressure or repeated friction on tissues that are not normally protected by a bursa. They form as a protective mechanism where an area of the body experiences excessive stress.

• Formation: These bursae form de novo (anew) in response to external forces. The body creates a fluid-filled sac to cushion and distribute pressure.
• Location: Commonly found over bony prominences that are subject to repeated trauma or pressure.
• Examples:
  • "Bunion" (Hallux Valgus): An adventitious bursa can develop over the medial aspect of the first metatarsophalangeal (MTP) joint of the foot due to pressure from ill-fitting shoes.
  • "Student's Elbow" or "Miner's Elbow": An adventitious bursa can form over the olecranon process of the elbow from prolonged leaning on hard surfaces.
  • "Tailor's Bottom": A bursa developing over the ischial tuberosity from prolonged sitting on hard surfaces.

Synovial Bursae

Synovial bursae are anatomically distinct structures that are present at birth. They are considered part of the normal anatomy and are strategically positioned in areas where friction is inherently high due to joint movement. These bursae are typically lined with a synovial membrane, similar to joint capsules, and contain synovial fluid. Synovial bursae can be further categorized based on their anatomical relationship to surrounding structures:

• Subcutaneous Bursae:

  • Location: Situated between the skin and an underlying bone. Their role is to protect the skin from friction and pressure against the bone during movement or direct contact.
  • Examples:
    • Prepatellar Bursa: Located between the skin and the patella (kneecap). Common site for "housemaid's knee."
    • Olecranon Bursa: Located between the skin and the olecranon process of the ulna (at the back of the elbow). Common site for "student's elbow" (though this can also be adventitious).
    • Trochanteric Bursa: Located over the greater trochanter of the femur, between the bone and the skin/iliotibial band.

• Submuscular Bursae:

  • Location: Found between muscles, or between a muscle and an underlying bone. They reduce friction between large muscle groups as they slide past each other or over bony surfaces during contraction and relaxation.
  • Examples:
    • Subscapular Bursa: Located between the subscapularis muscle and the neck of the scapula, communicating with the glenohumeral joint.
    • Iliopsoas Bursa (Iliopectineal Bursa): Located between the iliopsoas muscle/tendon and the anterior capsule of the hip joint or the iliopectineal eminence.

• Subtendinous Bursae:

  • Location: Positioned between a tendon and an underlying bone, or between two tendons. They facilitate smooth gliding of tendons over bony prominences or other tendons, preventing wear and fraying.
  • Examples:
    • Subacromial (or Subdeltoid) Bursa: Located between the deltoid muscle/acromion and the rotator cuff tendons (particularly the supraspinatus tendon) and the humeral head. This is one of the most frequently inflamed bursae.
    • Pes Anserine Bursa: Located under the conjoined tendons of the sartorius, gracilis, and semitendinosus muscles where they insert onto the medial aspect of the tibia, protecting them from friction against the bone.
    • Retrocalcaneal Bursa: Located between the Achilles tendon and the calcaneus (heel bone).

• Communicating Bursae:

  • Nature: While not a separate structural type in the same way as subcutaneous or subtendinous, "communicating bursae" refers to synovial bursae that have an opening or direct connection with the cavity of a nearby joint. This means the synovial fluid within the bursa is continuous with the synovial fluid of the joint.
  • Significance: Inflammation or fluid accumulation in a communicating bursa can sometimes be related to issues within the joint itself, and vice-versa.
  • Examples:
    • Suprapatellar Bursa: Communicates with the knee joint capsule, located above the patella and beneath the quadriceps femoris tendon.
    • Subscapular Bursa: As mentioned, this bursa typically communicates with the glenohumeral (shoulder) joint.

Key Anatomical Locations and Their Bursae

Understanding the types of bursae is enhanced by knowing their common locations:

• Shoulder: Subacromial/subdeltoid, subscapular.
• Elbow: Olecranon.
• Hip: Trochanteric, iliopsoas (iliopectineal), ischiogluteal.
• Knee: Prepatellar, infrapatellar (superficial and deep), pes anserine, suprapatellar.
• Ankle/Foot: Retrocalcaneal, subtalar, intermetatarsal, adventitious bursae associated with bunions.

Clinical Significance: Understanding Bursitis

Knowledge of the different types and locations of bursae is critical in clinical practice, particularly when diagnosing bursitis. Bursitis is the inflammation of a bursa, which can lead to pain, swelling, tenderness, and restricted movement in the affected area. The specific type and location of the inflamed bursa often provide clues about the underlying cause, whether it's repetitive strain, direct trauma, infection, or an inflammatory condition.

Conclusion

Bursae are essential anatomical structures that facilitate smooth, pain-free movement by minimizing friction and absorbing shock within the musculoskeletal system. While all bursae share a common function, their classification into adventitious (acquired from external stress) and synovial (anatomically present, further divided into subcutaneous, submuscular, and subtendinous) helps to understand their origins, specific roles, and clinical implications. A comprehensive understanding of these protective sacs is fundamental for anyone interested in human movement, injury prevention, and rehabilitation.