A bursa is a small, flattened sac filled with synovial fluid, which acts as a cushion and a gliding surface between bones, tendons, muscles, or skin. By reducing friction between adjacent moving structures, the bursa facilitates smooth, pain-free motion across a joint. The subacromial bursa is one of the most significant bursae in the shoulder, playing a substantial role in the joint’s extensive range of motion.
Precise Anatomical Location
The subacromial bursa is situated deep within the shoulder joint, directly beneath the prominent bony roof known as the acromion. It occupies the narrow space between the overlying bones and the underlying soft tissues. It often communicates with the subdeltoid bursa, and together they are commonly referred to as the subacromial-subdeltoid bursa.
The superior boundary is formed by the coracoacromial arch, which includes the acromion, the coracoid process, and the coracoacromial ligament. The inferior boundary consists of the rotator cuff tendons, specifically the supraspinatus tendon, and the upper part of the head of the humerus.
The bursa is located immediately beneath the large, triangular deltoid muscle. It is typically a potential space, meaning its walls are nearly touching, so any swelling quickly causes discomfort and limits movement.
The Role of the Subacromial Bursa in Shoulder Movement
The primary function of the subacromial bursa is to create a low-friction interface that enables the powerful shoulder muscles to work effectively. It permits the smooth gliding of the rotator cuff tendons as they move underneath the bony coracoacromial arch. Without this lubricating layer, repetitive friction would rapidly cause wear and tissue damage.
The bursa is particularly important during shoulder abduction, the action of lifting the arm away from the side of the body. As the arm is raised, the rotator cuff tendons slide upward toward the acromion. The bursa accommodates this translation by allowing the underlying structures to glide freely beneath the arch, preventing mechanical interference and facilitating the full range of motion required for overhead activities.
Understanding Subacromial Bursitis
The subacromial bursa is susceptible to inflammation, a condition known as subacromial bursitis. This occurs when the synovial lining becomes irritated, causing it to produce excess fluid and swell. Because the bursa is located in a confined space, even a small amount of swelling leads to severe pain and pressure on the surrounding tendons.
A frequent cause of inflammation is repetitive motion, particularly activities that involve raising the arm overhead, such as painting, swimming, or throwing. This constant friction and mechanical stress can irritate the bursa over time, leading to a chronic inflammatory response. Trauma, such as a direct fall onto the shoulder, can also cause acute bursitis.
The condition is often associated with shoulder impingement syndrome, where the space the bursa occupies narrows, compressing the bursa and the rotator cuff tendons. Anatomical factors, like bone spurs or a naturally curved acromion, contribute to this narrowing and increase the risk of developing bursitis.
The primary symptom of subacromial bursitis is an aching pain felt on the outer side of the shoulder, often radiating down the arm. This pain is worse during overhead activities, when the bursa is compressed, and can be disruptive at night, especially when lying on the affected side. The area over the bursa, just below the edge of the acromion, may feel tender to the touch. Certain health conditions, including rheumatoid arthritis, gout, and diabetes, also predispose an individual to bursa inflammation. A physical examination, including specific movements that reproduce the pain, is often used to diagnose the condition.