Editing Acromion (section)

==Structure==
The acromion forms the summit of the [[shoulder]] and is a large, somewhat triangular or [[rectangle|oblong]] process, flattened from behind forward. It projects laterally at first, then curves forward and upward to overhang the [[glenoid fossa]].<ref name="Gray's">''[[Gray's Anatomy]]'' 1918, see infobox</ref> It starts from the base of acromion which marks its projecting point emerging from the [[spine of scapula]].<ref name="Al-Redouan and Kacklik 2022">{{cite journal |last1=Al-Redouan |first1=Azzat |last2=Kachlik |first2=David |title=Scapula revisited: new features identified and denoted by terms using consensus method of Delphi and taxonomy panel to be implemented in radiologic and surgical practice |journal=J Shoulder Elbow Surg |date=2022 |volume=31 |issue=2 |pages=e68–e81 |doi=10.1016/j.jse.2021.07.020 |pmid=34454038 |s2cid=237348158 |url=https://www.jshoulderelbow.org/article/S1058-2746(21)00636-4/fulltext|url-access=subscription }}</ref>

===Surfaces===
Its superior surface, directed upward, backward, and lateralward, is [[wikt:convex|convex]], rough, and gives attachment to some fibers of the [[Deltoid muscle|deltoideus]], and in the rest of its extent is [[Subcutaneous tissue|subcutaneous]].  Its inferior surface is smooth and [[Concave function|concave]].<ref name="Gray's" />

===Borders===
Its lateral border is thick and irregular, and presents three or four [[Tubercle (anatomy)|tubercles]] for the tendinous origins of the [[Deltoid muscle|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]].<ref name="Gray's" />

===Variation===
There are three morphologically distinct types of acromia<ref>{{Cite journal |last1=Baechler |first1=Martin Frederick |last2=Kim |first2=David H. |date=October 2006  |title=Uncoverage" of the Humeral Head by the Anterolateral Acromion and Its Relationship to Full-Thickness Rotator Cuff Tears |url=http://dx.doi.org/10.7205/milmed.171.10.1035 |journal=Military Medicine |volume=171 |issue=10 |pages=1035–1038 |doi=10.7205/milmed.171.10.1035 |pmid=17076461 |issn=0026-4075|url-access=subscription }}</ref> and a correlation between these morphologies and [[rotator cuff tear]]:
{| class="wikitable"
 ! Type !! Appearance !! Prevalence<ref name=Habermeyer>{{Harvnb|Habermeyer|Magosch|Lichtenberg|2006| pp=1–3}}</ref> !! Angle of<br />anterior slope<ref name=Habermeyer/> !! Rotator<br />cuff tear<ref name=Habermeyer/>
|-
| Flat || [[File:Flat acromion.jpg|90px]] || 17.1% || 13.18 || 3.0%
|-
| Curved || [[File:Curved acromion.jpg|90px]] ||42.9% || 29.98 || 24.2%
|-
| Hooked || [[File:Hooked acromion.jpg|90px]] || 39.3% || 26.98 || 69.8%
|}

====''Os acromiale''====
The acromion has four [[ossification center]]s 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.<ref>{{Harvnb|Warner|Beim|Higgins|1998| loc=Introduction}}</ref><ref name="Sammarco">{{cite journal| last=Sammarco| first=VJ| date=March 2000| title=Os acromiale: frequency, anatomy, and clinical implications| journal=J Bone Joint Surg Am| volume=82| issue=3| pages=394–400| pmid=10724231| url=http://jbjs.org/article.aspx?articleid=24552| archive-url=https://archive.today/20130415031906/http://jbjs.org/article.aspx?articleid=24552| url-status=dead| archive-date=2013-04-15| access-date=March 2, 2013| doi=10.2106/00004623-200003000-00010| s2cid=25541990| url-access=subscription}}</ref>

Four types of ''os acromiale'' can be distinguished:<ref>{{Harvnb|Habermeyer|Magosch|Lichtenberg|2006| p=4}}</ref>
* 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|war bow]] (which needs a pull three times as strong as the modern standard [[Olympic Games|Olympic]] [[bow (weapon)|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.<ref>Kurtz CA, Humble BJ, Rodosky MW, Sekiya JK. Symptomatic os acromiale. J Am Acad Orthop Surg 2006; 14:12-9.</ref>  In people with symptoms of  ''os acromiale'', dynamic [[Medical ultrasound|ultrasound]] sometimes shows [[Hypermobility (joints)|hypermobility]] in the area during shoulder movement, or {{clarify span|text=graded compression with the probe|explain=too technical for the general reader|date=January 2016}}.<ref>Arend CF. Ultrasound of the Shoulder. Master Medical Books, 2013. Chapter on os acromiale available at [http://www.shoulderus.com/ultrasound-of-the-shoulder/os-acromiale-spectrum-of-ultrasound-findings/ ShoulderUS.com]</ref>

{{See also|Ossification of scapula}}
<gallery>
 File:Gray206.png|Plan of ossification of the [[scapula]]. Posterior side. Acromion visible at upper left, in blue.
 File:X-ray of os acromiale.jpg|Radiograph of the shoulder showing an os acromiale
</gallery>