The acromion is a bony projection extending from the scapula, forming the roof of the shoulder joint. This structure protects underlying soft tissues, including the rotator cuff tendons and the bursa. Variations in the acromion’s contour can significantly affect shoulder function and health. Specific shapes can predispose an individual to shoulder problems, and this article focuses on the Type 2 acromion and its associated risk of shoulder impingement.
Understanding Acromial Classification
The acromion shape is categorized using the Bigliani classification, which identifies three primary types based on the contour of the bone’s underside. Type 1 is characterized by a flat undersurface, offering the most spacious subacromial area for tendons to move freely, and is least associated with shoulder pathology.
The Type 2 acromion is defined by a gentle, convex curve or down-sloping character along its inferior surface. This curved shape is a common anatomical variation, present in approximately 43% of the population. Although it is a normal variation, this subtle curvature begins to slightly encroach on the space beneath it.
Type 3 acromion has a hooked shape, featuring a prominent anterior projection that severely limits the available space. This morphology is the most restrictive and is strongly correlated with chronic tendon issues. Structurally and clinically, the Type 2 acromion sits between the flat Type 1 and the hooked Type 3.
Type 2 Acromion and Impingement Risk
The Type 2 acromion is clinically significant due to its relationship with Subacromial Impingement Syndrome (SIS), a common cause of shoulder pain. Impingement occurs when the rotator cuff tendons and the fluid-filled bursa are compressed between the arm bone and the acromion. The curved undersurface of the Type 2 acromion naturally reduces the volume of this subacromial space.
This anatomical narrowing makes soft tissues vulnerable to friction and compression, especially when the arm is lifted overhead. Repeated contact between the curved bone and tendons leads to irritation, inflammation, and pain. The bursa, a sac designed to provide smooth gliding, can become inflamed (bursitis), further reducing the space.
The risk of developing impingement syndrome is progressively higher across the classification types, increasing from Type 1 to Type 2, and then to Type 3. Research indicates that the prevalence of shoulder impingement increases significantly in those with a Type 2 shape, highlighting the direct impact of the curved contour on mechanical irritation. This Type 2 shape is considered a predisposing factor, especially in people who perform repetitive overhead activities.
Identifying and Treating Symptoms
Individuals with Type 2 acromion who develop impingement commonly report pain that intensifies when lifting the arm, reaching overhead, or reaching behind the back. Night pain is also frequent, especially when lying on the affected shoulder, as body weight compresses the irritated structures. The pain often occurs within a specific range of motion, typically between 60 and 120 degrees of arm elevation, known as the painful arc.
Diagnosis begins with a physical examination, where a physician performs specific tests, such as the Neer and Hawkins signs, to reproduce the impingement pain. Imaging is then used to confirm the diagnosis and assess the pathology. A standard X-ray, particularly a supraspinatus outlet view, can visualize the acromial shape and confirm the Type 2 curve.
Diagnosis
A magnetic resonance imaging (MRI) scan is often ordered to provide a detailed view of the soft tissues, revealing inflammation in the bursa or any partial tearing within the rotator cuff tendons.
Treatment Options
Initial management is conservative, focusing on reducing inflammation with nonsteroidal anti-inflammatory drugs (NSAIDs) and physical therapy to improve shoulder mechanics and strengthen the rotator cuff muscles. If pain persists despite several weeks of conservative care, a corticosteroid injection into the subacromial space may be used to deliver potent anti-inflammatory medication directly to the site of irritation. If conservative treatments fail, a surgical procedure called acromioplasty, or subacromial decompression, may be recommended to shave the curved undersurface of the acromion and create more space for the tendons.