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.

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

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 ...

shipwreck: 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

. 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 ...

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 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 bir ...

) 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, ...

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 ...

(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

). 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 ...

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 ...

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

* * * *

External links

{{Authority control Vertebrate anatomy Scapula