A bursa is a small, flattened sac present throughout the human body, serving as a biological cushion near joints and bony prominences. These sacs are lined by a synovial membrane and filled with a lubricating fluid. The primary function of a bursa is to reduce friction between moving tissues, such as tendons, muscles, and bone. This allows adjacent structures to glide smoothly past one another during movement.
Precise Anatomical Placement
The subacromial bursa is situated within the shoulder joint, located just beneath the acromion, the bony roof of the shoulder blade. In most individuals, this sac communicates with the subdeltoid bursa, forming a single structure often referred to as the subacromial-subdeltoid bursa. It acts as a protective spacer deep to the large deltoid muscle that forms the outer contour of the shoulder.
The bursa lies directly on top of the rotator cuff tendons, particularly the supraspinatus tendon. It is bordered superiorly by the acromion and the coracoacromial ligament, and inferiorly by the rotator cuff. This strategic placement in the subacromial space is where the largest amount of mechanical loading and potential conflict between bone and soft tissue occurs. This close arrangement requires the bursa to function like a pillow or a ball bearing, preventing the tendons from being scraped against the overlying bone during arm movement.
Primary Role in Shoulder Movement
The main job of the subacromial bursa is to facilitate smooth, unimpeded motion in the shoulder joint. By secreting synovial fluid, the bursa ensures that the rotator cuff tendons can glide effortlessly underneath the acromion and the attached deltoid muscle. This frictionless environment is particularly important when the arm is lifted away from the body in a motion called abduction. Without the lubrication and cushioning provided by this sac, the soft tissues would rub directly against the hard, bony surfaces of the acromion, causing wear-and-tear on the tendons. The bursa essentially protects the underlying soft tissues during shoulder rotation and elevation, allowing for the wide range of motion the shoulder is known for.
Understanding Subacromial Bursitis
The most common issue affecting the bursa is subacromial bursitis, which is an inflammation of the sac. This condition results from constant friction or excessive pressure on the shoulder joint over time. Repetitive overhead activities, such as those performed by painters, swimmers, or construction workers, are causes of this irritation. Trauma, such as a direct fall onto the shoulder, can also lead to immediate swelling and inflammation. Inflammation causes the bursa to produce an excessive amount of fluid, making the sac swell and take up more of the already limited space, which can lead to a painful condition known as shoulder impingement.
The primary symptom is a dull, aching pain felt on the outside of the shoulder, which often radiates down the side of the arm. Pain is worsened by movements that involve lifting the arm, especially when reaching overhead or behind the back. Individuals may also experience tenderness when the shoulder is pressed and a limited, painful arc of motion when raising the arm. The pain can be disruptive at night, making it difficult to find a comfortable sleeping position. Other conditions like rheumatoid arthritis or diabetes can also predispose an individual to developing bursitis.