| Shoulder girdle | |
|---|---|
 Human shoulder girdle orpectoral girdle | |
| Details | |
| Identifiers | |
| Latin | cingulum pectorale |
| TA98 | A01.1.00.020 |
| TA2 | 361 |
| FMA | 23217 |
| Anatomical terms of bone | |
Theshoulder girdle orpectoral girdle is the set ofbones in theappendicular skeleton which connects to thearm on each side. In humans, it consists of theclavicle andscapula; in thosespecies with three bones in the shoulder, it consists of the clavicle, scapula, andcoracoid. Somemammalian species (such as thedog and thehorse) have only the scapula.
The pectoral girdles are to the upper limbs as thepelvic girdle is to the lower limbs; the girdles are the part of theappendicular skeleton that anchor theappendages to the axial skeleton.
In humans, the only true anatomicaljoints between the shoulder girdle and the axial skeleton are thesternoclavicular joints on each side. No anatomical joint exists between each scapula and therib cage; instead themuscular connection or physiological joint between the two permits great mobility of the shoulder girdle compared to the compactpelvic girdle; because theupper 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 theforelimb and thethorax, the only attachment being muscular.
The shoulder girdle is the anatomical mechanism that allows for all upper arm and shoulder movement in humans.[1]
The shoulder girdle consists of five muscles that attach to theclavicle andscapula and allow for the motion of thesternoclavicular joint (connection between sternum and clavicle) andacromioclavicular joint (connection between clavicle and scapula).[2] The five muscles that comprise the function of the shoulder girdle are thetrapezius muscle (upper, middle, and lower),levator scapulae muscle,rhomboid muscles (major and minor),serratus anterior muscle, andpectoralis minor muscle.[3]
The shoulder girdle is a complex of 5 joints that can be divided into two groups. 3 of these joints are trueanatomical joints, while 2 arephysiological ("false") joints.[explain 1] Within each group, the joints are mechanically linked so that both groups simultaneously contribute to the different movements of the shoulder to variable degrees.[4]: 20
In the first group, the scapulohumeral orglenohumeral joint is the anatomical joint mechanically linked to the physiological subdeltoid orsuprahumeral joint (the "second shoulder joint") so that movements in the suprahumeral joint results in movements in theglenohumeral joint.
In the second group, the scapulocostal orscapulothoracic joint is the important physiological joint that can not function without the two anatomical joints in the group, theacromioclavicular andsternoclavicular joints, i.e. they join both ends of theclavicle.[4]: 20
Theglenohumeral joint is the articulation between thehead of the humerus and theglenoid cavity of thescapula. It is aball and socket type ofsynovial joint with three rotatory and three translatory degrees of freedom. The glenohumeral joint allows foradduction,abduction,medial andlateral rotation,flexion andextension of the arm.
Theacromioclavicular joint is the articulation between theacromion process of the scapula and thelateral end of theclavicle. It is aplane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
Thesternoclavicular joint is the articulation of themanubrium of thesternum and the firstcostal cartilage with themedial end of theclavicle. It is asaddle 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
The scapulocostal joint (also known as the scapulothoracic joint) is aphysiological joint formed by an articulation of theanteriorscapula and theposteriorthoracic rib cage. It is musculotendinous in nature and is formed predominantly by thetrapezius,rhomboids andserratus anterior muscles. Thepectoralis minor also plays a role in its movements. The gliding movements at the scapulocostal joint areelevation,depression,retraction,protraction and superior and inferior rotation of the scapula. Disorders of the scapulocostal joint are not very common and usually restricted tosnapping scapula.[5]
The suprahumeral joint (also known as the subacromial joint) is a physiological joint formed by an articulation of thecoracoacromial ligament and thehead 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 thesubacromial bursa and thetendon ofsupraspinatus. 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.
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 15 centimetres (5.9 in) 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.[4]: 40
The scapula can be elevated and depressed from the neutral position to a total range of 10 to 12 centimetres (3.9 to 4.7 in); 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 10 to 12 centimetres (3.9 to 4.7 in) and the lateral angle 5 to 6 centimetres (2.0 to 2.4 in).[4]: 40
Shoulders are a common place for tissue injuries, especially if the person plays overhead sports such astennis,volleyball,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.[6] 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, ligamentous sprains such asAC joint or GH ligaments,rotator cuff injuries, different labral tears, and other acute or chronic conditions related to shoulder anatomy.
Shoulder girdle pain can beacute orchronic and be due to a number of causes.Inflammation or injury of associatedtendons,bone,muscles,nerves,ligaments, andcartilage can all cause pain. Also, past injury compensation, and stress can result in complicated shoulder pain.[7]
In humans,winged scapula is a condition in which the shoulder blade protrudes from a person's back in an abnormal position.
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The pectoral girdle demonstrates an enormous variation inamniotes (vertebrates adapted to a full terrestrial life), both among extant species and along evolutionary lines, and determininghomologies for individual pectoral elements is difficult. Except for the sternum, these elements (along with thepelvic girdle) were, however, present in earlybony fishes before there were even limbs, arising from their ancestral external armor plates. In digitlesschoanates, thecleithrum, clavicle, andinterclavicle aredermal and linked to the caudal part of the head while the humerus articulates with a smallscapulocoracoid 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.[8] As for the sternum, it also came from the fusion of the inner ends of the pectoral girdles intetrapods, subsequently growing between theribs.
Indinosaurs, the main bones of the pectoral girdle were thescapula (shoulder blade) and thecoracoid, both of which directly articulated with theclavicle. The clavicle was present insaurischian dinosaurs but largely absent inornithischian dinosaurs. The place on the scapula where it articulated with thehumerus (upper bone of the forelimb) is the called theglenoid. The scapula served as the attachment site for a dinosaur's back and forelimb muscles.
Chimpanzees are far better atbrachiation 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.[9]
A specimen of theplacodermKolymaspis demonstrates that the shoulder girdle evolved from gill arches present in ancient vertebrates.[10]
