Bursae: Types, Functions, and Common Conditions

What are the 4 types of bursae?

Bursae are small, fluid-filled sacs that act as cushions between bones, tendons, muscles, and skin, reducing friction and facilitating smooth movement. While numerous bursae exist throughout the body, they can be broadly categorized into four primary types based on their anatomical location and developmental origin: subcutaneous, subtendinous, submuscular, and adventitious.

Understanding Bursae: Nature's Friction Reducers

In the intricate machinery of the human body, movement is paramount. Yet, with movement comes the potential for friction between tissues. This is where bursae play a crucial role. Derived from the Latin word "bursa," meaning a pouch or bag, these synovial fluid-filled sacs are strategically located at points where tissues rub against each other. Their inner lining, similar to that of joint capsules, secretes synovial fluid, which lubricates and nourishes the bursa, enabling its shock-absorbing and friction-reducing functions. Without healthy bursae, repetitive movements could lead to significant irritation, inflammation, and pain, hindering mobility and function.

The Essential Functions of Bursae

The primary functions of bursae are multifaceted and critical for optimal musculoskeletal health:

• Friction Reduction: Bursae provide a smooth, gliding surface that minimizes friction between moving structures, such as tendons sliding over bone, or skin moving over underlying bony prominences.
• Cushioning and Shock Absorption: They act as natural shock absorbers, protecting bones and soft tissues from impact and pressure, especially in areas subjected to repetitive stress.
• Facilitating Movement: By reducing friction, bursae allow for more efficient and less painful movement of joints, tendons, and muscles.

The Four Types of Bursae

While the body contains over 150 bursae, they are classified into distinct types based on their location and, in one specific case, their developmental origin.

1. Subcutaneous Bursae

Location: These bursae are found directly under the skin, typically overlying bony prominences that are subject to external pressure or friction. Function: They protect the skin and underlying bone from rubbing and pressure, especially during movements that involve sliding or leaning. Examples:

• Olecranon bursa: Located over the bony tip of the elbow (olecranon process). Inflammation here is often called "student's elbow" or "miner's elbow."
• Prepatellar bursa: Situated in front of the kneecap (patella). Known for "housemaid's knee" due to kneeling.
• Infrapatellar bursa: Found just below the kneecap.
• Trochanteric bursa: Over the greater trochanter of the femur (hip).

2. Subtendinous Bursae

Location: These bursae are positioned directly beneath a tendon, where the tendon crosses over a bone or another tendon. Function: They reduce friction between the tendon and the underlying bone or other structures, allowing the tendon to glide smoothly during muscle contraction and joint movement. Examples:

• Subacromial bursa: Located under the acromion of the scapula and the deltoid muscle, superior to the rotator cuff tendons of the shoulder. Critical for smooth arm elevation.
• Anserine bursa: Found on the medial (inner) side of the knee, beneath the tendons of the sartorius, gracilis, and semitendinosus muscles (the "pes anserinus" or "goose's foot").
• Achilles bursa (retrocalcaneal bursa): Located between the Achilles tendon and the calcaneus (heel bone).

3. Submuscular Bursae

Location: These bursae are situated beneath a muscle or between two muscles. Function: They facilitate the smooth gliding of muscles over bones or other muscles, minimizing friction during muscle contraction and joint movement. Examples:

• Iliopectineal bursa (iliopsoas bursa): Located anterior to the hip joint, beneath the iliopsoas muscle.
• Gastrocnemius bursa: Found behind the knee joint, between the medial head of the gastrocnemius muscle and the femur.
• Subscapular bursa: Located between the subscapularis muscle and the anterior aspect of the scapula.

4. Adventitious (Acquired) Bursae

Location: Unlike the other three types, which are typically present from birth (though they can vary in size), adventitious bursae are not anatomically inherent. They develop de novo (anew) in response to abnormal or chronic friction, pressure, or trauma in areas not typically served by a bursa. Function: They form as the body's protective response to localized, excessive mechanical stress, attempting to reduce friction and cushion the affected area. Examples:

• Bunionette (Tailor's Bunion): An adventitious bursa that can form on the outside of the fifth metatarsophalangeal joint of the foot due to pressure from tight footwear.
• "Biker's Nipple": Can form over the pubic symphysis in cyclists due to prolonged pressure from the bicycle seat.
• "Student's Elbow": While the olecranon bursa is subcutaneous, chronic, excessive pressure (e.g., leaning on elbows) can lead to its significant enlargement or the formation of an adventitious bursa adjacent to it if the original bursa is overwhelmed or damaged.

Common Bursitis Locations

When a bursa becomes inflamed, it's known as bursitis. This painful condition often results from repetitive motion, prolonged pressure, trauma, or underlying inflammatory conditions. Common sites for bursitis include the shoulder (subacromial), elbow (olecranon), hip (trochanteric, iliopectineal), and knee (prepatellar, anserine). Recognizing the type of bursa involved can help in diagnosis and targeted treatment strategies.

Maintaining Bursal Health

To minimize the risk of bursitis and promote healthy bursal function:

• Practice Proper Biomechanics: Ensure correct form during exercise and daily activities to avoid undue stress on joints and surrounding tissues.
• Gradual Progression: Increase intensity, duration, or load of activities gradually to allow tissues, including bursae, to adapt.
• Use Protective Gear: Employ padding or cushioning in activities that involve prolonged pressure or impact on bony prominences (e.g., knee pads for kneeling).
• Listen to Your Body: Heed early signs of discomfort or pain, allowing for rest and recovery before inflammation escalates.
• Maintain Flexibility and Strength: Balanced muscle strength and flexibility around joints help ensure proper joint mechanics and reduce stress on bursae.

Understanding the different types of bursae and their specific roles provides a deeper appreciation for the body's intricate design and reinforces the importance of protective measures in our physical activities.