A Type 1 acromion is not considered a serious condition, as this anatomical variation is associated with the lowest risk of shoulder problems related to its shape. The acromion is the bony projection of the shoulder blade (scapula) that forms the roof over the shoulder joint and the underlying rotator cuff tendons. The shape of this bony arch varies among individuals, and this difference has been classified to understand predisposition to certain shoulder injuries. Type 1 is classified as the “flat” shape, meaning its level undersurface provides ample space for the soft tissues underneath.
Defining the Acromion Types
Shoulder specialists use a system known as the Bigliani classification to categorize the different shapes of the acromion, typically identified through lateral X-ray or MRI views. This classification divides acromial morphology into three primary types, though a fourth has been described. The different types describe the degree of curvature or hook on the undersurface of the bone, which directly affects the space available for the tendons.
Type I is the flat acromion, characterized by a smooth, straight undersurface that runs parallel to the humeral head. Type II, the curved acromion, has a concave undersurface that dips downward and is considered the most common shape. Type III is the hooked acromion, where the most anterior portion of the bone is sharply angled downward.
Type 1 Acromion and Impingement Risk
The primary concern associated with acromion shape is shoulder impingement syndrome, a condition where the acromion compresses the rotator cuff tendons, particularly the supraspinatus tendon, and the bursa underneath it. This compression often occurs when the arm is raised overhead. The flat, Type 1 acromion is the most favorable shape because its level undersurface naturally provides the maximum clearance within the subacromial space.
Because of this generous space, the Type 1 acromion is associated with the lowest risk of shoulder impingement symptoms and rotator cuff tears. The curved Type II and especially the hooked Type III acromions progressively reduce this space, increasing mechanical friction on the tendons. Type III acromions are found more often in people with rotator cuff tears compared to the Type I variant.
General Shoulder Maintenance
While a Type 1 acromion is a protective factor against shape-related impingement, it does not guarantee lifelong shoulder health, as injuries can still occur due to other factors like trauma, overuse, or tendon degeneration. Maintaining good posture is important, as a slouched or rounded-shoulder position can alter the mechanics of the scapula and functionally narrow the subacromial space, regardless of acromion shape. Focusing on keeping the shoulder blades retracted and stable helps preserve the optimal alignment.
Strengthening the muscles of the rotator cuff and the surrounding scapular stabilizers is an effective way to maintain shoulder function and prevent injury. Avoiding repetitive, painful overhead motions or heavy lifting away from the body can also reduce strain on the soft tissues. Performing a consistent shoulder conditioning program two to three days a week helps maintain strength and range of motion for the long term.