Acromion

A bony ridge runs horizontally across the upper back of the scapula. This is called the scapular spine. The acromion is the broad, flat portion where the outer end of this spine bends forward. If you press your fingers against the very top of your shoulder, you can feel a flat, firm plate—that is the acromion. It is easy to feel even through clothing, so it is sometimes used as a landmark when measuring shoulder width or height.

The acromion has three main roles.

• It forms a joint — Its front end meets the outer end of the clavicle to form the acromioclavicular joint. Although this joint has a small range of motion, it is an important link that helps the scapula and clavicle move together as you raise your arm.
• It forms a roof — Together with the coracoacromial ligament in front of it and the coracoid process, the acromion forms an arch over the shoulder joint. The gap between this arch and the head of the humerus is called the subacromial space. The supraspinatus tendon and the subacromial bursa pass through this space.
• It provides muscle attachments — The middle portion of the deltoid muscle, which gives the shoulder its rounded shape, originates from the acromion, and part of the trapezius also attaches here.

When you raise your arm out to the side, the humerus is not the only bone that moves upward—the scapula also rotates. The acromion rotates upward along with the scapula, making more room for the head of the humerus to pass. As the arm rises overhead, the subacromial space narrows, then gains room as the scapula follows with the appropriate rotation. This coordinated movement of the arm and scapula is called scapulohumeral rhythm.

Viewed from the side, the undersurface of the acromion differs in shape from person to person. A classification dividing these shapes into three types was introduced in the 1980s: type I has a flat undersurface, type II is gently curved, and type III is hooked. If you have seen a description of “acromial morphology” on an imaging report, it may refer to this classification.

More recently, researchers have also been studying methods based on angle measurements rather than visual classification of shape. These include measures such as the critical shoulder angle, which expresses the inclination of the scapular glenoid and the extent of the acromion's lateral projection as an angle, and the acromion index. Acromial shape varies between individuals, and curved or hooked shapes are common even in shoulders without pain (asymptomatic imaging findings). So even if a particular shape stands out on imaging, it cannot tell you the condition of the shoulder on its own. Current shoulder care is shifting toward considering pain patterns and movement together. Pain beneath the acromion has long been called shoulder impingement syndrome, and the term “subacromial pain syndrome,” which does not narrow the cause down to a single factor, is also used today.

The acromion is also where a bag strap or seat belt may lie. Because the bone sits just beneath the skin, carrying a heavy bag on one shoulder for a long time can put pressure on this area and leave it feeling sore and tingly. Also, if you feel an uncomfortable catching sensation at the top of your shoulder when raising your arm overhead, the space beneath the acromion is involved in that movement.

• Try alternating which shoulder you carry your bag on, or adjust the strap so it does not press directly on the edge of the scapula.
• Before raising your arm, gently roll your shoulders to prepare the scapula to move along with it.
• Slowly repeating a movement of shrugging your shoulders up, then lowering them back, helps the area around the scapula move smoothly.
• If a sharp, tingling catching sensation when you raise your arm continues for more than a few days, it can help to show the movement to a professional who assesses shoulders.