The Subacromial Complex (SAC) is a functional space within the shoulder joint, not a single anatomical structure. Situated beneath the bony arch at the top of the shoulder, this space connects the upper arm bone (humerus) to the shoulder blade. The SAC’s composition allows for the arm’s wide range of motion, contributing significantly to the shoulder’s stability and mobility, especially during overhead activities.
Structural Makeup
The boundaries of the Subacromial Complex are defined by bony elements and soft tissues. Superiorly, the space is roofed by a protective arch formed by the acromion—a bony projection extending from the shoulder blade—and the coracoacromial ligament.
Inferiorly, the complex is bordered by the head of the humerus, the ball portion of the shoulder joint. Soft tissues occupy the narrow gap between the bony structures, which typically measures about 1.0 to 1.5 centimeters in height. The most relevant soft tissue elements are the subacromial bursa and the tendons of the rotator cuff muscles.
The rotator cuff, a group of four muscles, has tendons that pass through this space, with the supraspinatus tendon being centrally located. The subacromial bursa is a fluid-filled sac layered between the acromial arch and the rotator cuff tendons. This bursa acts as a cushion, reducing friction during arm movement.
Essential Functions
The primary role of the Subacromial Complex is to facilitate the frictionless movement of the arm. The subacromial bursa provides a lubricated surface, allowing the rotator cuff tendons to glide effortlessly underneath the bony acromion during arm elevation. This mechanism prevents the soft tissues from being compressed and damaged.
The SAC also contributes to the dynamic stability of the shoulder joint. The rotator cuff tendons depress and centralize the humeral head within the glenoid socket. This downward pull prevents the humeral head from migrating upward and colliding with the acromion during overhead movements.
Common Injuries and Ailments
Dysfunction within the SAC often leads to Subacromial Impingement Syndrome (SAIS), the most frequent cause of shoulder pain. This syndrome occurs when the soft tissues within the space are compressed or “pinched” during arm movement. Compression can result from bone spurs developing on the underside of the acromion or from inflammation and swelling of the soft tissues.
Another common ailment is subacromial bursitis, characterized by inflammation and swelling of the subacromial bursa. When inflamed, the bursa occupies more of the limited space, intensifying friction and compression on surrounding tendons. This condition causes localized pain and tenderness, often noticeable when the arm is moved away from the body or when resting at night.
Impingement can progress to affect the rotator cuff tendons, leading to tendinopathy or a partial or full-thickness tear. The supraspinatus tendon is the most commonly injured structure due to its location at the narrowest point of the complex. Symptoms include pain with overhead reaching, a painful arc of motion, and weakness when attempting to lift or rotate the arm.
Identifying and Resolving Issues
The diagnostic process for SAC issues begins with a physical examination by a clinician. Specific clinical tests, such as the Neer or Hawkins-Kennedy maneuvers, are performed to reproduce compression and elicit pain, helping to pinpoint the source of discomfort. Imaging studies are then used to confirm the diagnosis and assess the extent of the damage.
Plain X-rays are often the first step, used to visualize bony structures and detect bone spurs or variations in acromion shape that narrow the space. Ultrasound or Magnetic Resonance Imaging (MRI) provides clearer detail on soft tissues, allowing for the evaluation of the bursa and identification of rotator cuff tears or inflammation. A diagnostic injection of a local anesthetic into the subacromial space can offer immediate pain relief, confirming the SAC as the origin of the symptoms.
Initial treatment for most SAC problems is conservative, focusing on reducing inflammation and restoring normal function. This non-surgical approach includes rest or activity modification to avoid painful movements, and non-steroidal anti-inflammatory drugs (NSAIDs) to manage pain. Physical therapy is a primary component, aimed at strengthening the rotator cuff and periscapular muscles to improve shoulder mechanics and increase the subacromial space dynamically.
If symptoms persist despite conservative efforts, a corticosteroid injection into the subacromial space may be considered for prolonged relief from inflammation. If non-surgical treatments fail after several months, surgical options are explored. Procedures are often performed arthroscopically and may involve subacromial decompression, which removes bone spurs and a small portion of the acromion (acromioplasty) to widen the space. If a significant rotator cuff tear is present, the surgery will also include tendon repair.