Acromion

In human anatomy, the acromion or summit of the shoulder is a bony process on the scapula. Together with the coracoid process, it extends laterally over the shoulder joint. The acromion is a continuation of the scapular spine, and hooks over anteriorly. It articulates with the clavicle to form the acromioclavicular joint.

Structure

The acromion forms the summit of the shoulder and is a large, somewhat triangular or oblong process, flattened from behind forward. It projects laterally at first, then curves forward and upward to overhang the glenoid fossa. It starts from the base of acromion which marks its projecting point emerging from the spine of scapula.

Surfaces

Its superior surface, directed upward, backward, and lateralward, is convex, rough, and gives attachment to some fibers of the deltoideus, and in the rest of its extent is subcutaneous. Its inferior surface is smooth and concave.

Borders

Its lateral border is thick and irregular, and presents three or four tubercles for the tendinous origins of the deltoid. Its medial border, shorter than the lateral, is concave, gives attachment to a portion of the trapezius, and presents about its center a small oval surface for articulation with the acromial end of the clavicle.

Variation

There are three morphologically distinct types of acromia and a correlation between these morphologies and rotator cuff tear:

Type	Appearance	Prevalence	Angle of anterior slope	Rotator cuff tear
Flat		17.1%	13.18	3.0%
Curved		42.9%	29.98	24.2%
Hooked		39.3%	26.98	69.8%

''Os acromiale''

The acromion has four ossification centers called pre-acromion, meso-acromion, meta-acromion, and basi-acromion. In most cases, the first three fuse at 15–18 years, whereas the base part fuses to the scapular spine at 12 years. However, in between 1% and 15% of cases, this osseous union fails and the acromion remains separate as an accessory bone. This condition is referred to as os acromiale, but rarely causes pain. Earlier estimates of its prevalence were as low as 1.4%, and this higher estimate was made by Sammarco in the year 2000, based on radiographic and anatomical studies.
Four types of os acromiale can be distinguished:

A non-union between the meso- and meta-acromia, the most common or typical os acromiale
A non-union between the pre- and meso-acromia
A non-union between the pre- and meso-acromia; and between the meso- and meta-acromia, atypical
A non-union between the pre- and meso-acromia; between the pre- and meso-acromia; and between the meta- and basi-acromia

This feature was common in skeletons recovered from the Mary Rose shipwreck: it is thought that in those men, much archery practice from childhood on with the mediaeval war bow pulled at the acromion so much that it prevented bony fusion of the acromion with the scapula.
Although historically regarded as an incidental finding, the os acromiale may occasionally produce symptoms from subacromial impingement or instability at the site of non-union. In people with symptoms of os acromiale, dynamic ultrasound sometimes shows hypermobility in the area during shoulder movement, or

Changes from age

Anterior acromial spur formation is an age-dependent process, with a significantly increased likelihood after the age of 50. However, variation in acromial morphological condition are not acquired from the age-related changes such as spur formation and thus contribute to impingement disease independently.

In other animals

The acromion process of bats is particularly elongated compared to that of humans.
Turtles have an acromion that forms the anterior part of the triradiate pectoral girdle. In this highly specialized endoskeletal structure, the scapula is a dorsal process attached to the first rib; the coracoid is a posteroventral process; and the acromion is a medioventral process located at the base of the scapula. This had led to some controversy regarding the evolutionary origin of turtles, because in both pareiasaurs and non-mammalian therapsids the acromion is located at the dorsal tip of the scapula.
In modern turtles, the acromion projects ventrally and articulates with the plastron, but it evolved independently in pareiasaurs and turtles long before the plastron. In these primitive ancestors, the acromion projected forward to form a strong and flexible articulation between the shoulder girdle and the clavicle. Notwithstanding these changes, the acromion of turtles retains its original function, to support the shoulder girdle and increase stride length.