In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", plural: acromia) is a bony
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
on the
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
(shoulder blade). Together with the
coracoid process
The coracoid process (from Greek κόραξ, raven) is a small hook-like structure on the lateral edge of the superior anterior portion of the scapula (hence: coracoid, or "like a raven's beak"). Pointing laterally forward, it, together with the ...
it extends laterally over the
shoulder joint
The shoulder joint (or glenohumeral joint from Greek ''glene'', eyeball, + -''oid'', 'form of', + Latin ''humerus'', shoulder) is structurally classified as a synovial ball-and-socket joint and functionally as a diarthrosis and multiaxial joint. ...
. The acromion is a continuation of the
scapular spine
The spine of the scapula or scapular spine is a prominent plate of bone, which crosses obliquely the medial four-fifths of the scapula at its upper part, and separates the supra- from the infraspinatous fossa.
Structure
It begins at the vertical ...
, and hooks over anteriorly. It articulates with the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
(collar bone) to form the
acromioclavicular joint
The acromioclavicular joint, or AC joint, is a joint at the top of the shoulder. It is the junction between the acromion (part of the scapula that forms the highest point of the shoulder) and the clavicle. It is a plane synovial joint.
Structure ...
.
Structure
The acromion forms the summit of the
shoulder
The human shoulder is made up of three bones: the clavicle (collarbone), the scapula (shoulder blade), and the humerus (upper arm bone) as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder mak ...
, and is a large, somewhat triangular or
oblong
An oblong is a non-square rectangle.
Oblong may also refer to:
Places
* Oblong, Illinois, a village in the United States
* Oblong Township, Crawford County, Illinois, United States
* A strip of land on the New York-Connecticut border in the Unit ...
process, flattened from behind forward, projecting at first lateralward, and then curving forward and upward, so as to overhang the
glenoid fossa
The glenoid fossa of the scapula or the glenoid cavity is a bone part of the shoulder. The word ''glenoid'' is pronounced or (both are common) and is from el, gléne, "socket", reflecting the shoulder joint's ball-and-socket form. It is a sha ...
.
['']Gray's Anatomy
''Gray's Anatomy'' is a reference book of human anatomy written by Henry Gray, illustrated by Henry Vandyke Carter, and first published in London in 1858. It has gone through multiple revised editions and the current edition, the 42nd (Octob ...
'' 1918, see infobox It starts from the base of acromion which marks its projecting point emerging from the
spine of scapula
The spine of the scapula or scapular spine is a prominent plate of bone, which crosses obliquely the medial four-fifths of the scapula at its upper part, and separates the Supraspinatous fossa, supra- from the infraspinatous fossa.
Structure
It b ...
.
Surfaces
Its superior surface, directed upward, backward, and lateralward, is
convex
Convex or convexity may refer to:
Science and technology
* Convex lens, in optics
Mathematics
* Convex set, containing the whole line segment that joins points
** Convex polygon, a polygon which encloses a convex set of points
** Convex polytope ...
, rough, and gives attachment to some fibers of the
deltoideus
The deltoid muscle is the muscle forming the rounded contour of the human shoulder. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat. Anatomically, the deltoid muscle appears to be made up o ...
, 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
In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal.
In plants
A tubercle is generally a wart-like projection ...
for the tendinous origins of the deltoid. Its medial border, shorter than the lateral, is concave, gives attachment to a portion of the
trapezius
The trapezius is a large paired trapezoid-shaped surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. It moves the scapula and supports the ...
, and presents about its center a small oval surface for articulation with the acromial end of the
clavicle
The clavicle, or collarbone, is a slender, S-shaped long bone approximately 6 inches (15 cm) long that serves as a strut between the shoulder blade and the sternum (breastbone). There are two clavicles, one on the left and one on the rig ...
.
Variation
There are three morphologically distinct types of acromia and a correlation between these morphologies and
rotator cuff tear
A rotator cuff tear is an injury where one or more of the tendons or muscles of the rotator cuff of the shoulder get torn. Symptoms may include shoulder pain, which is often worse with movement, limited range of motion, or weakness. This may limi ...
:
''Os acromiale''
The acromion has four
ossification center
An ossification center is a point where ossification of the cartilage begins. The first step in ossification is that the cartilage cells at this point enlarge and arrange themselves in rows.Gray and Spitzka (1910), page 44.
The matrix in which t ...
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
An accessory bone or supernumerary bone is a bone that is not normally present in the body, but can be found as a variant in a significant number of people. It poses a risk of being misdiagnosed as bone fractures on radiography.
Wrist and hand
...
. 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
The ''Mary Rose'' (launched 1511) is a carrack-type warship of the English Tudor navy of King Henry VIII. She served for 33 years in several wars against France, Scotland, and Brittany. After being substantially rebuilt in 1536, she saw her l ...
shipwreck
A shipwreck is the wreckage of a ship that is located either beached on land or sunken to the bottom of a body of water. Shipwrecking may be intentional or unintentional. Angela Croome reported in January 1999 that there were approximately ...
: it is thought that in those men, much
archery
Archery is the sport, practice, or skill of using a bow to shoot arrows.Paterson ''Encyclopaedia of Archery'' p. 17 The word comes from the Latin ''arcus'', meaning bow. Historically, archery has been used for hunting and combat. In m ...
practice from childhood on with the mediaeval
war bow (which needs a pull three times as strong as the modern standard
Olympic
Olympic or Olympics may refer to
Sports
Competitions
* Olympic Games, international multi-sport event held since 1896
** Summer Olympic Games
** Winter Olympic Games
* Ancient Olympic Games, ancient multi-sport event held in Olympia, Greece b ...
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
Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...
sometimes shows
hypermobility in the area during shoulder movement, or .
File:Gray206.png, Plan of ossification of the scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
. Posterior side. Acromion visible at upper left, in blue.
File:X-ray of os acromiale.jpg, Radiograph of the shoulder showing an os acromiale
In other animals
The acromion process of
bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s (
Mammalia
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
:
Chiroptera
Bats are mammals of the Order (biology), order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in fli ...
) is particularly elongated compared to that of
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
s.
Turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...
s have an acromion that forms the anterior part of the triradiate
pectoral girdle
The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of t ...
(together with the
coracoid
A coracoid (from Greek κόραξ, ''koraks'', raven) is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is prese ...
and
scapula
The scapula (plural scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on eithe ...
). In this highly specialized
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
Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with scutes which covered large areas of the body. They first appeared in southern Pangea during the Middle Permian, ...
s and non-mammalian
therapsid
Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...
s the acromion is located at the dorsal tip of the scapula.
In modern
turtle
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked tu ...
s, the acromion projects ventrally and articulates with the
plastron (the flat lower part of the shell), but it
evolved
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...
in a
common ancestor
Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
of
pareiasaur
Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with scutes which covered large areas of the body. They first appeared in southern Pangea during the Middle Permian, ...
s 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.
File:Snapping turtle shoulder girdle.JPG, The shoulder girdle of a snapping turtle
The Chelydridae is a family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and '' Macrochelys''. Both are endemic to the Western Hemisphere. The extinct genera are '' Acherontem ...
. (1) Acromion, (2) scapula, and (3) coracoid
Additional images
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:Gray203.png, Left scapula. Posterior view. Acromion labeled at top left.
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.
Notes
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External links
{{Authority control
Vertebrate anatomy
Scapula