In human anatomy, the acromion (from Greek: akros, "highest", ōmos, "shoulder", : acromia) or summit of the shoulder is a bony process on the scapula (shoulder blade). 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 (collar bone) to form the acromioclavicular joint.
Structure
The acromion forms the summit of the
shoulder and is a large, somewhat triangular or
rectangle process, flattened from behind forward. It projects laterally at first, then curves forward and upward to overhang the
glenoid fossa.
[ Gray's Anatomy 1918, see infobox] 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 , rough, and gives attachment to some fibers of the
Deltoid muscle, and in the rest of its extent is subcutaneous. Its inferior surface is smooth and
Concave function.
Borders
Its lateral border is thick and irregular, and presents three or four tubercles for the tendinous origins of the
Deltoid muscle. 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
anterior slope[ !! Rotator]
cuff tear[
] |
| 3.0% |
| 24.2% |
| 69.8% |
Os acromiale
The acromion has four ossification 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 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 longbow (which needs a pull three times as strong as the modern standard Olympic Games 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.[Kurtz CA, Humble BJ, Rodosky MW, Sekiya JK. Symptomatic os acromiale. J Am Acad Orthop Surg 2006; 14:12-9.] In people with symptoms of os acromiale, dynamic ultrasound sometimes shows hypermobility in the area during shoulder movement, or .[Arend CF. Ultrasound of the Shoulder. Master Medical Books, 2013. Chapter on os acromiale available at ShoulderUS.com]
. Posterior side. Acromion visible at upper left, in blue.]]
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
(
Mammalia:
Chiroptera) is particularly elongated compared to that of
.
have an acromion that forms the anterior part of the triradiate pectoral girdle (together with the coracoid and scapula). In this highly specialized Endoskeleton 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 both and non-mammalian the acromion is located at the dorsal tip of the scapula.
In modern , the acromion projects ventrally and articulates with the plastron (the flat lower part of the shell), but it Evolution independently in and turtles long before the plastron.
. (1) Acromion, (2) scapula, and (3) coracoid]]
==Additional images==
Notes
External links
