The acromion is a large, bony projection of the scapula (shoulder blade) that extends forward to form the highest point of the shoulder. This structure creates a protective arch, acting as the roof over the shoulder joint and the underlying rotator cuff tendons. Because the rotator cuff tendons must glide smoothly in the space beneath this bony roof, the specific shape of the acromion is a significant factor in shoulder health. These anatomical differences are categorized by medical professionals.
Understanding Acromial Morphology
The shape of the acromion’s undersurface is traditionally classified into three main types, a system used to predict the likelihood of shoulder problems. This classification is typically determined by taking a specialized side-view X-ray of the shoulder, often called an outlet view. The first type, Type 1, is defined by a flat undersurface, offering the most open space above the rotator cuff.
Type 2 is characterized by a curved or concave undersurface that runs parallel to the head of the upper arm bone, known as the humerus. This shape tends to narrow the space above the rotator cuff tendons slightly more than the flat type. The third variation, Type 3, features a distinct, downward-sloping or hooked shape at its most anterior point. This anatomical variation significantly reduces the available space for the tendons to move freely.
Clinical Significance of Type 1 Acromion
A Type 1 acromion is considered the most benign and structurally favorable shape for shoulder function. Because its undersurface is flat, it provides the maximum clearance for the rotator cuff tendons and the bursa sac to move without friction. This ample space means the tendons are the least likely to be mechanically irritated or compressed during arm movement.
Finding out an individual has a Type 1 acromion on an imaging report is generally not a cause for concern. This flat morphology is rarely associated with the development of chronic shoulder pain, subacromial impingement syndrome, or rotator cuff tears. When found during an imaging scan, a Type 1 acromion is often an incidental finding that does not contribute to existing symptoms.
When Acromion Shape Causes Problems
Other acromial morphologies can significantly contribute to a painful condition called subacromial impingement syndrome. This condition occurs when the soft tissues, such as the rotator cuff tendons, are pinched between the acromion and the head of the humerus during arm elevation. The Type 2 (curved) and especially the Type 3 (hooked) acromions are the shapes that reduce this subacromial space most significantly.
The Type 3 acromion, with its pronounced anterior hook, has been strongly linked to an increased risk of chronic tendon wear and full-thickness rotator cuff tears. This downward projection acts like a constant source of friction, wearing away at the underlying tendons over time. By contrast, the Type 1 acromion is the least likely shape to create this narrow outlet, which is why it is not considered serious.