The shoulder girdle or pectoral girdle is the set of

bones 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, a ...

in the appendicular skeleton which connects to the

arm In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between th ...

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; 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 appropriate s ...

with three bones in the shoulder, it consists of the clavicle, scapula, and

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 ...

. Some mammalian species (such as the

dog The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...

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 The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The p ...

is to the lower limbs; the girdles are the parts of the appendicular skeleton that anchor the appendages 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 The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a sem ...

; instead the

muscular Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...

connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compact

; because the upper limb 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 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 cre ...

, 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

and scapula and allow for the motion of the

sternoclavicular joint The sternoclavicular joint or sternoclavicular articulation is a synovial saddle joint between the manubrium of the sternum, and the clavicle, as well as the first rib. The joint possesses a joint capsule, and an articular disk, and is reinfo ...

(connection between sternum and clavicle) and

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 ...

(connection between clavicle and scapula). The five muscles that comprise the function of the shoulder girdle are the

trapezius muscle 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 ...

(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 The rhomboid muscles (), often simply called the rhomboids, are rhombus-shaped muscles associated with the scapula. There are two rhomboid muscles on each side of the upper back: * Rhomboid major muscle * Rhomboid minor muscle The large rhom ...

(major and minor),

serratus anterior muscle The serratus anterior is a muscle that originates on the surface of the 1st to 8th ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula. The serratus anterior acts to pull the scapula fo ...

, and pectoralis minor muscle.

Joints

The shoulder girdle is a complex of 5 joints that can be divided into two groups. 3 of these joints are true anatomical joints, while 2 are physiological ("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 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 and

s, i.e. they join both ends of the

Glenohumeral joint

The glenohumeral joint is the articulation between the

head of 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 ...

and the

glenoid cavity 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 ...

of the scapula. It is a

ball and socket The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number of ...

type of synovial joint with three rotatory and three translatory degrees of freedom. The glenohumeral joint allows for

adduction Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

, abduction, medial and

lateral rotation Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

flexion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

and extension of the arm.

Acromioclavicular joint

The

is the articulation between the

acromion process In human anatomy, the acromion (from Greek: ''akros'', "highest", ''ōmos'', "shoulder", plural: acromia) is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint. The acr ...

of the scapula and the

lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction * Lateral cricoarytenoid muscle * Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

end of the

. It is a plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.

Sternoclavicular joint

The

is the articulation of the manubrium of the

sternum 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. Sha ...

and the first

costal cartilage The costal cartilages are bars of hyaline cartilage that serve to prolong the ribs forward and contribute to the elasticity of the walls of the thorax. Costal cartilage is only found at the anterior ends of the ribs, providing medial extension. ...

with the medial end of the

. 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 k ...

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 joint formed by an articulation of the anterior scapula and the posterior thoracic rib cage. It is musculotendinous in nature and is formed predominantly by the trapezius, 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 § Ver ...

, depression, retraction,

protraction Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...

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 and the

. 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 The subacromial bursa is the synovial cavity located just below the acromion, which communicates with the subdeltoid bursa in most individuals, forming the so-called subacromial-subdeltoid bursa (SSB). The SSB bursa is located deep to the deltoid ...

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 ...

supraspinatus The supraspinatus (plural ''supraspinati'') is a relatively small muscle of the upper back that runs from the supraspinous fossa superior portion of the scapula (shoulder blade) to the greater tubercle of the humerus. It is one of the four rotat ...

. 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.

, ligamentous sprains such as AC joint 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 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,

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 Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of musc ...

nerves A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

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 l ...

, and cartilage can all cause pain. Also, past injury compensation, and stress can result in complicated shoulder pain.

Disorders

In humans,

winged scapula A winged scapula (scapula alata) is a skeletal medical condition in which the shoulder blade protrudes from a person's back in an abnormal position. In rare conditions it has the potential to lead to limited functional activity in the upper ex ...

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 amniotes (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

) were, however, present in early bony fishes before there were even limbs, arising from their ancestral external armor plates. In digitless choanates the

cleithrum The cleithrum (plural cleithra) is a membrane bone which first appears as part of the skeleton in primitive bony fish, where it runs vertically along the scapula. Its name is derived from Greek κλειθρον = "key (lock)", by analogy with "cla ...

, clavicle, and

interclavicle An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...

are dermal and linked to the caudal part of the head while the humerus articulates with a small

scapulocoracoid The scapulocoracoid is the unit of the pectoral girdle that contains the coracoid and scapula. The coracoid itself is a beak-shaped bone that is commonly found in most vertebrates with a few exceptions. The scapula is commonly known as the ''shoulde ...

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 The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a semi- ...

In dinosaurs

In dinosaurs the main bones of the pectoral girdle were the scapula (shoulder blade) and the

, both of which directly articulated with the

. The clavicle was present in

saurischian Saurischia ( , meaning "reptile-hipped" from the Greek ' () meaning 'lizard' and ' () meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia), classified by their hip structure. Saurischia and Ornithis ...

dinosaurs but largely absent in

ornithischian Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek st ...

dinosaurs. The place on the scapula where it articulated with the humerus (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

Chimpanzees are far better at brachiation 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.

In humans

Joints

Glenohumeral joint

Acromioclavicular joint

Sternoclavicular joint

Scapulocostal joint

Suprahumeral joint

Movements

Injury

Disorders

Other animals

In dinosaurs

In primates

See also

Notes

References

Further reading