The subacromial space is an area located in the shoulder, situated directly beneath the acromion, which is the bony projection at the top of the shoulder blade. This narrow passage is formed by the acromion and the coracoacromial ligament above and the head of the humerus (the upper arm bone) below. It serves as a conduit for several important structures. The space is not static; its size changes with arm movement.
Anatomy of the Subacromial Space
Within this confined area reside important structures for shoulder function. The tendons of the rotator cuff muscles, a group of four muscles that stabilize the shoulder, pass directly through this space. Specifically, the supraspinatus tendon is located in this region and is frequently involved in shoulder conditions.
Another structure within the space is the tendon for the long head of the biceps muscle. To facilitate smooth movement and prevent friction between these moving parts, the space also contains the subacromial bursa. This bursa is a thin, fluid-filled sac that acts as a cushion between the rotator cuff tendons and the overlying acromion. In a healthy shoulder, these components fit comfortably within the space, allowing for unhindered motion.
Role in Shoulder Movement
The primary role of the subacromial space is to permit the smooth gliding of the rotator cuff tendons, particularly during arm elevation. As you lift your arm, the rotator cuff tendons and the subacromial bursa must slide unimpeded underneath the acromion. The rotator cuff muscles also work to keep the head of the humerus centered and depressed within the shoulder socket, which helps maintain the size of the subacromial space during movement.
This arrangement allows for a wide and fluid range of motion, from reaching behind your back to throwing a ball. The dimensions of the space naturally decrease as the arm is lifted. For example, as the shoulder moves from 30 to 120 degrees of abduction (lifting out to the side), the distance between the humerus and acromion can decrease by nearly 50%. A healthy, sufficiently large space ensures this movement occurs without pain or restriction.
Subacromial Impingement Syndrome
Subacromial Impingement Syndrome (SAIS) occurs when the subacromial space narrows, causing it to pinch the rotator cuff tendons and the bursa. This compression leads to pain and irritation as these soft tissues are rubbed against the acromion bone. One common cause is the development of bone spurs on the underside of the acromion, which physically reduces the available space.
Inflammation of the structures within the space is another major contributor. Swelling of the subacromial bursa, a condition known as bursitis, or inflammation of the rotator cuff tendons, called tendinitis, can make the space more crowded. Poor posture and muscle imbalances can also alter shoulder mechanics, causing the humeral head to ride higher in the socket and close down the space. Symptoms of SAIS include pain with overhead activities, a feeling of weakness in the arm, and discomfort when lying on the affected side.
The shape of the acromion itself can also predispose an individual to impingement. Some people have a flat acromion (Type I), while others may have a curved (Type II) or hooked (Type III) shape. A hooked acromion, in particular, can significantly reduce the clearance for the underlying tendons, increasing the risk of impingement.
Treatments for Subacromial Pain
Managing pain from the subacromial space begins with conservative, non-surgical approaches. Physical therapy is a component of treatment, focusing on strengthening the rotator cuff muscles and the muscles that stabilize the scapula (shoulder blade). Improved muscle function helps control the position of the humeral head, maximizing the subacromial space during movement. Modifying activities to avoid painful overhead motions can also help reduce irritation and allow inflammation to subside. Over-the-counter anti-inflammatory medications may be used to manage pain and swelling.
If conservative measures do not provide sufficient relief, a healthcare provider might suggest a corticosteroid injection. This involves injecting a powerful anti-inflammatory medication directly into the subacromial space to reduce the swelling of the bursa and tendons, thereby alleviating pain. This can provide an opportunity for physical therapy to be more effective.
If non-operative treatments fail, surgery may be considered. The most common procedure is an arthroscopic subacromial decompression. During this minimally invasive surgery, a surgeon inserts a small camera and instruments to remove bone spurs from the acromion and resect a portion of the inflamed bursa. This procedure physically increases the volume of the subacromial space, aiming to relieve the mechanical pinching of the tendons.