The shoulder girdle or pectoral girdle is the set of
bones in the
appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting sh ...
which connects to the
arm on each side. In humans it consists 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 r ...
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 eith ...
; in those
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
with three bones in the shoulder, it consists of the clavicle, scapula, and
coracoid. Some
mammal
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 ...
ian species (such as the
dog and the
horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
) have only the scapula.
The pectoral girdles are to the upper limbs as the
pelvic girdle is to the lower limbs; the girdles are the parts of the
appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting sh ...
that anchor the
appendage
An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body.
In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including ante ...
s to the axial skeleton.
In humans, the only true anatomical
joint
A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...
s between the shoulder girdle and the axial skeleton are the
sternoclavicular joints on each side. No anatomical joint exists between each scapula and the
rib cage; instead the
muscular connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compact
pelvic girdle; because the
upper limb
The upper limbs or upper extremities are the forelimbs of an upright-postured tetrapod vertebrate, extending from the scapulae and clavicles down to and including the digits, including all the musculatures and ligaments involved with the shou ...
is not usually involved in weight bearing, its stability has been sacrificed in exchange for greater mobility. In those species having only the scapula, no joint exists between the
forelimb
A forelimb or front limb is one of the paired articulated appendages (limbs) attached on the cranial ( anterior) end of a terrestrial tetrapod vertebrate's torso. With reference to quadrupeds, the term foreleg or front leg is often used inste ...
and the
thorax
The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the c ...
, the only attachment being muscular.
In humans
The shoulder girdle is the anatomical mechanism that allows for all upper arm and shoulder movement in humans. The shoulder girdle consists of five muscles that attach to 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 r ...
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 eith ...
and allow for the motion of the
sternoclavicular joint (connection between sternum and clavicle) and
acromioclavicular joint (connection between clavicle and scapula). The five muscles that comprise the function of the shoulder girdle are the
trapezius muscle (upper, middle, and lower),
levator scapulae muscle
The levator scapulae is a slender skeletal muscle situated at the back and side of the neck. As the Latin name suggests, its main function is to lift the scapula.
Anatomy
Attachments
The muscle descends diagonally from its origin to its inserti ...
,
rhomboid muscles (major and minor),
serratus anterior muscle, and
pectoralis minor muscle
Pectoralis minor muscle () is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major in the human body.
Structure
Attachments
Pectoralis minor muscle arises from the upper margins and outer surface ...
.
Joints
The shoulder girdle is a complex of 5 joints that can be divided into two groups. 3 of these joints are true
anatomical
Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...
joints, while 2 are
physiological
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...
("false") joints.
[Physiological joints are called "false" joints because they lack the characteristics of "true" anatomical joints, such as tendons and a capsule, but they are still joints because the gliding movements they provide play an important biomechanical role. ()] Within each group, the joints are mechanically linked so that both groups simultaneously contribute to the different movements of the shoulder to variable degrees.
In the first group, the scapulohumeral or
glenohumeral 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 ...
is the anatomical joint mechanically linked to the physiological subdeltoid or
suprahumeral joint (the "second shoulder joint") so that movements in the suprahumeral joint results in movements in the
glenohumeral joint.
In the second group, the scapulocostal or
scapulothoracic joint is the important physiological joint that can not function without the two anatomical joints in the group, the
acromioclavicular
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.
Structu ...
and
sternoclavicular joints, i.e. they join both ends 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 r ...
.
Glenohumeral joint
The
glenohumeral 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 ...
is the articulation between the
head of the humerus and the
glenoid cavity 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 eith ...
. It is a
ball and socket type of
synovial joint with three rotatory and three translatory degrees of freedom. The glenohumeral joint allows for
adduction,
abduction
Abduction may refer to:
Media
Film and television
* "Abduction" (''The Outer Limits''), a 2001 television episode
* " Abduction" (''Death Note'') a Japanese animation television series
* " Abductions" (''Totally Spies!''), a 2002 episode of an ...
,
medial and
lateral rotation,
flexion and
extension
Extension, extend or extended may refer to:
Mathematics
Logic or set theory
* Axiom of extensionality
* Extensible cardinal
* Extension (model theory)
* Extension (predicate logic), the set of tuples of values that satisfy the predicate
* Ext ...
of the arm.
Acromioclavicular joint
The
acromioclavicular joint is the articulation between the
acromion process of the scapula and the
lateral 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 r ...
. It is a
plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
Sternoclavicular joint
The
sternoclavicular joint is the articulation of the
manubrium
The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. S ...
of the
sternum and the first
costal cartilage with the
medial 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 r ...
. It is a
saddle
The saddle is a supportive structure for a rider of an animal, fastened to an animal's back by a girth. The most common type is equestrian. However, specialized saddles have been created for oxen, camels and other animals. It is not kno ...
type of synovial joint but functions as a plane joint. The sternoclavicular joint accommodates a wide range of scapula movements and can be raised to a 60° angle.
Scapulocostal joint
The scapulocostal joint (also known as the scapulothoracic joint) is a
physiological
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemica ...
joint formed by an articulation of the
anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
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 eith ...
and the
posterior thoracic rib cage. It is musculotendinous in nature and is formed predominantly by 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 th ...
,
rhomboids and
serratus anterior muscles. The
pectoralis minor also plays a role in its movements. The gliding movements at the scapulocostal joint are
elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § ...
,
depression,
retraction,
protraction and superior and inferior rotation of the scapula. Disorders of the scapulocostal joint are not very common and usually restricted to
snapping scapula.
Suprahumeral joint
The suprahumeral joint (also known as the subacromial joint) is a physiological joint formed by an articulation of the
coracoacromial ligament
The coracoacromial ligament is a strong triangular ligament between the coracoid process and the acromion. It protects the head of the humerus. Its acromial attachment may be repositioned to the clavicle during reconstructive surgery of the acro ...
and the
head of the humerus. It is formed by the gap between the humerus and the acromion process of the scapula. This space is filled mostly by the
subacromial bursa and the
tendon
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...
of
supraspinatus. This joint plays a role during complex movements while the arm is fully flexed at the glenohumeral joint, such as changing a lightbulb, or painting a ceiling.
Movements
From its neutral position, the shoulder girdle can be rotated about an imaginary vertical axis at the medial end of the clavicle (the sternoclavicular joint). Throughout this movement the scapula is rotated around the chest wall so that it moves laterally and the glenoid cavity is rotated 40–45° in the horizontal plane. When the scapula is moved medially it lies in a frontal plane with the glenoid cavity facing directly laterally. At this position, the lateral end of the clavicle is rotated posteriorly so that the angle at the acromioclavicular joint opens up slightly. When the scapula is moved laterally it lies in a sagittal plane with the glenoid cavity facing anteriorly. At this position, the lateral end of the clavicle is rotated anteriorly so that the clavicle lies in a frontal plane. While this slightly closes the angle between the clavicle and the scapula, it also widens the shoulder.
The scapula can be elevated and depressed from the neutral position to a total range of ; at its most elevated position the scapula is always tilted so that the glenoid cavity is facing superiorly. During this tilting, the scapula rotates to a maximum angle of 60° about an axis passing perpendicularly through the bone slightly below the spine; this causes the inferior angle to move and the lateral angle .
Injury
Shoulders are a common place for tissue injuries, especially if the person plays overhead sports such as tennis,
volleyball
Volleyball is a team sport in which two teams of six players are separated by a net. Each team tries to score points by grounding a ball on the other team's court under organized rules. It has been a part of the official program of the Sum ...
, baseball, swimming, etc. According to Bahr's major injury related statistics, shoulder dislocations or subluxations account for 4% of injuries in adults ages 20–30 and 20% of shoulder injuries are fractures. Damage to the shoulder and adjacent features can fluctuate in severity depending on the person's age, sport, position, recurring shoulder dysfunction, and many other factors. Some other common shoulder injuries are fractures to any shoulder girdle bones i.e.
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 r ...
, ligamentous sprains such as
AC 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 ...
or GH ligaments, rotator cuff injuries, different labral tears, and other acute or chronic conditions related to shoulder anatomy.
Shoulder girdle pain can be
acute
Acute may refer to:
Science and technology
* Acute angle
** Acute triangle
** Acute, a leaf shape in the glossary of leaf morphology
* Acute (medicine), a disease that it is of short duration and of recent onset.
** Acute toxicity, the adverse ef ...
or
chronic and be due to a number of causes.
Inflammation
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
or injury of associated
tendons
A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its abilit ...
,
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
,
muscles,
nerves,
ligaments
A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the:
* Peritoneal ...
, and
cartilage
Cartilage is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck ...
can all cause pain. Also, past injury compensation, and stress can result in complicated shoulder pain.
Disorders
In humans,
winged scapula is a condition in which the shoulder blade protrudes from a person's back in an abnormal position.
Other animals
The pectoral girdle demonstrates an enormous variation in
amniote
Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are dis ...
s (vertebrates adapted to a full terrestrial life), both among extant species and along evolutionary lines, and determining
homologies for individual pectoral elements is difficult. Except for the sternum, these elements (along with the
pelvic girdle) were, however, present in early
bony fishes
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...
before there were even limbs, arising from their ancestral external armor plates. In digitless
choanates the
cleithrum, clavicle, and
interclavicle are
dermal and linked to the caudal part of the head while the humerus articulates with a small
scapulocoracoid bone. As the first digits appeared, the pectoral structure lost its direct connection to the head skeleton while the scapulocoracoid grew more prominent and started to face laterally. In true tetrapods the dermal part of the girdle was gradually reduced and the scapulocoracoid split into a dorsal scapula and a ventral coracoid.
[Fig. 1]
/ref> As for the sternum, it also came from the fusion of the inner ends of the pectoral girdles in tetrapods
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct therapsi ...
, subsequently growing between the ribs.
In dinosaurs
In dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is the ...
the main bones of the pectoral girdle were 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 eith ...
(shoulder blade) and the coracoid, both of which directly articulated 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 r ...
. The clavicle was present in saurischian dinosaurs but largely absent in ornithischian dinosaurs. The place on the scapula where it articulated with the humerus
The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...
(upper bone of the forelimb) is the called the glenoid. The scapula served as the attachment site for a dinosaur's back and forelimb muscles.
In primates
Chimpanzee
The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative t ...
s are far better at brachiation
Brachiation (from "brachium", Latin for "arm"), or arm swinging, is a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms. During brachiation, the body is alternately supported under each forelimb ...
than humans. Their clavicles possess a cranially oriented twist on the acromial end, conducive to better force transfer through it - a very important function in arboreal locomotion. Chimpanzee scapulas also possess a considerably larger supraspinous fossa, allowing for a larger supraspinatus muscle. Through the process of evolution, humans have lost the Atlantoclavicularis muscle, originating on the atlas of the vertebral column, and inserting onto the acromial clavicle. This muscle acts to elevate the clavicle.
See also
* Shoulder
* Human skeletal changes due to bipedalism
Notes
References
Further reading
*
{{Authority control
Skeletal system