View source for Acromion

{{short description|Bony process on the scapula (shoulder blade)}}
{{Infobox bone
| Name        = Acromion
| Latin       = acromion
| Image       = Acromion of left scapula01.png
| Caption     = Left scapula, posterior view. Acromion shown in red. 
| Image2      = Acromion of scapula06.png
| Caption2    = Skeleton, posterior view. Acromion of each scapula shown in red.
}}
In human anatomy, the '''acromion''' (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", {{plural form}}: '''acromia''') or '''summit of the shoulder''' is a bony [[Process (anatomy)|process]] on the [[scapula]] (shoulder blade). Together with the [[coracoid process]], it extends laterally over the [[Glenohumeral joint|shoulder joint]].  The acromion is a continuation of the [[Spine of scapula|scapular spine]], and hooks over anteriorly.  It articulates with the [[clavicle]] (collar bone) to form the [[acromioclavicular joint]].<ref>{{Cite journal |date=November 1958  |title=Pharmacology in Medicine. 2nd ed.|url=http://dx.doi.org/10.1002/jps.3030471148 |journal=Journal of the American Pharmaceutical Association (Scientific Ed.) |volume=47 |issue=11 |pages=838 |doi=10.1002/jps.3030471148 |issn=0095-9553|url-access=subscription }}</ref>

==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''====
{{See also|Ossification of scapula}}
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=V. J.| date=March 2000| title=Os acromiale: frequency, anatomy, and clinical implications| journal=Journal of Bone and Joint Surgery | 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| archive-date=2013-04-15| access-date=March 2, 2013| doi=10.2106/00004623-200003000-00010| s2cid=25541990| url-access=subscription |url-status=dead}}</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>{{cite journal |vauthors=Kurtz CA, Humble BJ, Rodosky MW, Sekiya JK |title=Symptomatic os acromiale |journal=Journal of the American Academy of Orthopaedic Surgeons|year=2006 |volume=14 |issue=1 |pages=12–19 |url=https://journals.lww.com/jaaos/Abstract/2006/01000/Symptomatic_Os_Acromiale.4.aspx}}</ref>{{verify inline|reason=Make sure URL is correct|date=November 2025}} 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>{{cite book |last=Arend |first=C. F. |title=Ultrasound of the Shoulder |publisher=Master Medical Books |year=2013 |chapter=2. Os acromiale: Spectrum of Ultrasound Findings |chapter-url=http://www.shoulderus.com/ultrasound-of-the-shoulder/os-acromiale-spectrum-of-ultrasound-findings/ |url-status=dead |archive-url=https://web.archive.org/web/20160323222434/http://www.shoulderus.com/ultrasound-of-the-shoulder/os-acromiale-spectrum-of-ultrasound-findings/ |archive-date=2016-03-23 |via=ShoulderUS.com}}</ref>

<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>

==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 [[bat]]s ([[Mammalia]]: [[Chiroptera]]) is particularly elongated compared to that of  [[human]]s.

[[Turtle]]s have an acromion that forms the anterior part of the triradiate [[pectoral girdle]] (together with the [[coracoid]] and [[scapula]]).  In this highly specialized [[Endoskeleton|endoskeletal]] 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 [[pareiasaur]]s and non-mammalian [[therapsid]]s the acromion is located at the dorsal tip of the scapula.
<ref>{{Harvnb|Rieppel|Reisz|1999| p=}}</ref>

In modern [[turtle]]s, the acromion projects ventrally and articulates with the [[plastron]] (the flat lower part of the shell), but it [[Evolution|evolved]] independently in [[pareiasaur]]s and turtles long before the plastron.<ref>{{Cite journal|title = The evolutionary position of turtles revised|journal = Naturwissenschaften|date = 2001-05-01|issn = 0028-1042|pages = 193–200|volume = 88|issue = 5|doi = 10.1007/s001140100228|pmid = 11482432|first1 = Rafael|last1 = Zardoya|first2 = Axel|last2 = Meyer| bibcode=2001NW.....88..193Z | s2cid=7836810 |url = http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-34962}}</ref>  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.
<ref>{{Harvnb|Lee|1996| loc=Abstract}}</ref>

<gallery>
 File:Snapping turtle shoulder girdle.JPG|The shoulder girdle of a [[snapping turtle]]. (1) Acromion, (2) scapula, and (3) coracoid
</gallery>

==Additional images==
<gallery>
 File:Acromion of left scapula - animation01.gif|Left scapula. Acromion shown in red.
 File:Acromion of scapula - animation03.gif|Animation. Acromion shown in red.
 File:Acromial angle of left scapula01.png|[[Acromial angle]] shown in red.
 File:Gray203.png|Left scapula. Posterior view. Acromion labeled at top left.
 File:Gray205 left scapula lateral view.png|Left scapula. Lateral view.
 File:Scapula post numbered.png|Left scapula. Posterior view. Acromion is "10"
 File:Gray202.png|Left scapula. Anterior view. Acromion labeled at top right.
 File:Scapula ant num.JPG|Left scapula. Anterior view. Acromion is "2"
 File:LeftScapulaLateral.jpg|Left scapula. Lateral view. Acromion is "4"
 File:Gray326.png|The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula. Anterior view.
</gallery>

==Notes==
{{Gray's}}
{{Reflist}}
* {{Cite book 
| last1     = Habermeyer | first1 = Peter
| last2     = Magosch | first2 = Petra
| last3     = Lichtenberg | first3 = Sven
| title     = Classifications and Scores of the Shoulder
| publisher = Springer | location = Heidelberg | year = 2006
| isbn      = 978-3-540-24350-2
}}
* {{Cite journal 
| last    = Lee | first = Michael S. Y.
| title   = The Homologies and Early Evolution of the Shoulder Girdle in Turtles
| journal = Proc. R. Soc. Lond. B | date = January 22, 1996 | volume = 263 | issue = 1366 | pages = 111–117 
| doi     = 10.1098/rspb.1996.0018
| s2cid = 84529868
}}
* {{Cite journal
 |last1        = Rieppel
 |first1       = Olivier
 |last2        = Reisz
 |first2       = Robert R.
 |title        = The Origin and Early Evolution of Turtles
 |journal      = Annual Review of Ecology and Systematics
 |volume       = 30
 |year         = 1999
 |issue = 1
 |pages        = 1–22
 |url          = http://www.mz.usp.br/forum/pdf/Rieppel&Reisz1999.pdf
 |doi          = 10.1146/annurev.ecolsys.30.1.1
 |bibcode = 1999AnRES..30....1R
 |access-date  = 2020-06-08
 |archive-url  = https://web.archive.org/web/20181226125040/http://www.mz.usp.br/forum/pdf/Rieppel%26Reisz1999.pdf/
 |archive-date = 2018-12-26
 |url-status   = dead
}}
* {{Cite journal 
| last1   = Warner | first1 = Jon J.P.
| last2   = Beim | first2 = Gloria M.
| last3   = Higgins | first3 = Laurence
| journal = The Journal of Bone and Joint Surgery
| date    = September 1998 | volume = 80 | issue = 9 | pages = 1320–6
| title   = The Treatment of Symptomatic Os Acromiale
| doi = 10.2106/00004623-199809000-00011 | pmid = 9759817
}}

==External links==
{{Commons category}}

{{Bones of upper extremity}}
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[[Category:Vertebrate anatomy]]
[[Category:Scapula]]