| Acromion | |
|---|---|
 Left scapula, posterior view. Acromion shown in red. | |
 Skeleton, posterior view. Acromion of each scapula shown in red. | |
| Details | |
| Identifiers | |
| Latin | acromion |
| MeSH | D000174 |
| TA98 | A02.4.01.009 |
| TA2 | 1152 |
| FMA | 23260 |
| Anatomical terms of bone | |
In human anatomy, theacromion (from Greek:akros, "highest",ōmos, "shoulder",pl.:acromia) orsummit of the shoulder is a bonyprocess on thescapula (shoulder blade). Together with thecoracoid process, it extends laterally over theshoulder joint. The acromion is a continuation of thescapular spine, and hooks over anteriorly. It articulates with theclavicle (collar bone) to form theacromioclavicular joint.[1]
The acromion forms the summit of theshoulder and is a large, somewhat triangular oroblong process, flattened from behind forward. It projects laterally at first, then curves forward and upward to overhang theglenoid fossa.[2] It starts from the base of acromion which marks its projecting point emerging from thespine of scapula.[3]
Its superior surface, directed upward, backward, and lateralward, isconvex, rough, and gives attachment to some fibers of thedeltoideus, and in the rest of its extent issubcutaneous. Its inferior surface is smooth andconcave.[2]
Its lateral border is thick and irregular, and presents three or fourtubercles for the tendinous origins of thedeltoid. Its medial border, shorter than the lateral, is concave, gives attachment to a portion of thetrapezius, and presents about its center a small oval surface for articulation with the acromial end of theclavicle.[2]
There are three morphologically distinct types of acromia[4] and a correlation between these morphologies androtator cuff tear:
| Type | Appearance | Prevalence[5] | Angle of anterior slope[5] | Rotator cuff tear[5] |
|---|---|---|---|---|
| Flat |  | 17.1% | 13.18 | 3.0% |
| Curved |  | 42.9% | 29.98 | 24.2% |
| Hooked |  | 39.3% | 26.98 | 69.8% |
The acromion has fourossification centers called (from tip to base) 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 anaccessory bone. This condition is referred to asos 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.[6][7]
Four types ofos acromiale can be distinguished:[8]
This feature was common in skeletons recovered from theMary Roseshipwreck: it is thought that in those men, mucharchery practice from childhood on with the mediaevalwar bow (which needs a pull three times as strong as the modern standardOlympicbow) pulled at the acromion so much that it prevented bony fusion of the acromion with the scapula.
Although historically regarded as an incidental finding, theos acromiale may occasionally produce symptoms from subacromial impingement or instability at the site of non-union.[9][verification needed] In people with symptoms ofos acromiale, dynamicultrasound sometimes showshypermobility in the area during shoulder movement, orgraded compression with the probe.[clarify][10]
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.
Theacromion process ofbats (Mammalia:Chiroptera) is particularly elongated compared to that ofhumans.
Turtles have an acromion that forms the anterior part of the triradiatepectoral girdle (together with thecoracoid andscapula). In this highly specializedendoskeletal structure, the scapula is a dorsal (directed upwards) process attached to the first rib; the coracoid is a posteroventral (directed backward and down) process; and the acromion is a medioventral (directed inwards and down) process (also known as the prescapular process) located at the base of the scapula. This had led to some controversy regarding the evolutionary origin of turtles, because in bothpareiasaurs and non-mammaliantherapsids the acromion is located at the dorsal tip of the scapula.[11]
In modernturtles, the acromion projects ventrally and articulates with theplastron (the flat lower part of the shell), but itevolved independently inpareiasaurs and turtles long before the plastron.[12] 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.[13]
This article incorporates text in thepublic domain frompage 203 of the 20th edition ofGray's Anatomy(1918)
