The shoulder is a complex joint, enabling a wide range of arm movements and bearing significant loads. The acromion, a bony structure, is essential for its mobility and strength. It forms part of the shoulder’s framework, contributing to its movement capabilities and overall mechanics.
Anatomy and Exact Location
The acromion is a bony projection extending from the scapula, or shoulder blade. It forms the highest point of the shoulder, projecting laterally and curving forward to overhang the shoulder joint. This projection is a continuation of the scapular spine, a prominent ridge on the back of the shoulder blade.
You can locate your acromion by following the ridge of your shoulder blade outwards to the bony point at the top of your shoulder. Another method involves placing two fingers directly down from the top of your shoulder, where the acromion will feel flat and level. The acromion articulates with the lateral end of the clavicle, or collarbone, forming the acromioclavicular (AC) joint. This joint provides stability to the shoulder. The acromion varies in shape among individuals, such as flat, curved, or hooked, which can influence shoulder mechanics.
Function and Purpose
The acromion has multiple functions that contribute to shoulder mechanics. It acts as a protective roof over the glenohumeral joint, the main ball-and-socket joint of the shoulder. This shields the underlying humeral head and rotator cuff tendons from external forces. This bony arch also prevents upward displacement of the upper arm bone, maintaining joint stability.
Beyond protection, the acromion provides attachment points for several muscles. The deltoid muscle, responsible for lifting the arm away from the body, originates in part from the acromion. The trapezius muscle, involved in scapular movement like elevation and rotation, also inserts onto the acromion. These muscle attachments enable the shoulder to achieve a broad spectrum of motion while maintaining stability.
Common Issues Involving the Acromion
Given its role in shoulder mechanics, the acromion can be involved in several common conditions. Acromioclavicular (AC) joint sprains, also known as shoulder separations, occur when the ligaments connecting the acromion and clavicle are damaged, often due to direct trauma or a fall onto an outstretched hand. Symptoms include pain and tenderness over the joint, and in severe cases, a visible bump where the joint has been disrupted.
Another frequent issue is shoulder impingement syndrome, where the rotator cuff tendons and the bursa beneath the acromion become pinched in the narrow space, especially during overhead arm movements. This pinching can be exacerbated by certain acromion shapes, such as a hooked acromion, or by bone spurs on its underside. Acromion fractures, though less common than other shoulder fractures, result from high-energy trauma, such as a direct blow to the shoulder or a severe fall. These fractures can cause pain, swelling, and limited shoulder movement.