Human pectoral girdle
|Anatomical terms of bone|
The pectoral girdle or shoulder girdle is the set of bones which connects the upper limb to the axial skeleton on each side. It consists of the clavicle and scapula in humans and, in those species with three bones in the pectoral girdle, the coracoid. Some mammalian species (e.g. the dog and the horse) have only the scapula.
In humans, the only true anatomical joints between the shoulder girdle and 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 — i.e. because the upper limb is not usually involved in weight bearing, its stability has been sacrificed in exchange for greater mobility.
The pectoral girdle is a complex of five joints that can be divided into two groups. Three of these joints are true anatomical joints while two are physiological ("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. 
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 latter results in movements in the former. 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 sternoclavicular joints, i.e. the join both ends of the clavicle. 
The glenohumeral joint is the articulation between the head of the humerus and the glenoid cavity of the scapula. It is a ball and socket type of synovial joint. The glenohumeral joint allows for adduction, abduction, medial and lateral rotation, flexion and extension of the arm.
The acromioclavicular joint is the articulation between the acromion process of the scapula and the lateral end of the clavicle. It is a plane type of synovial joint. The acromion of the scapula rotates on the acromial end of the clavicle.
The sternoclavicular joint is the articulation of the manubrium of the sternum and the first costal cartilage with the medial end of the clavicle. It is a saddle 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 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, 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.
The suprahumeral joint (also known as the subacromial joint) is a physiological joint formed by an articulation of the coracoacromial ligament 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 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.
From its neutral position, the pectoral 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. 
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 rotate 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). 
The pectoral girdle demonstrates an enormous variation in amniotes (animals adapted to a 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 were, however, present in early bony fishes before there were even limbs. In digitless tetrapods 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 more derived tetrapods the dermal part of the girdle was gradually reduced and the scapulocoracoid split into a dorsal scapula and a ventral coracoid. 
In dinosaurs the main bones of the pectoral girdle were the scapula (shoulder blade) and the coracoid, both of which directly articulated with the clavicle. The clavicle was present in saurischian dinosaurs but largely absent in ornithischian 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.
Chimpanzees are far better adapted to brachiation than modern 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.
- Physiological joints are called "false" joints because they lack the characteristics of "true" anatomcial joints, such as ligaments and a capsule, but they are still joints because the gliding movements they provide play an important biomechanical role. (Roy 2009, Focused anatomy)
- Kapandji 1982, p. 20
- Arend CF. Ultrasound of the Shoulder. Master Medical Books, 2013. Free section on snapping scapula available at ShoulderUS.com
- Kapandji 1982, p. 40
- Vickaryous & Hall 2006, Introduction, see also Fig. 1
- Bramble, Dennis; Lieberman, Daniel (23 September 2004). "Endurance running and the evolution of Homo". Nature 432: 345–352. doi:10.1038/nature03052. Retrieved 14 November 2014.
- Kapandji, Ibrahim Adalbert (1982). The Physiology of the Joints: Volume One Upper Limb (5th ed.). New York: Churchill Livingstone.
- Roy, André (June 2009). "Rotator Cuff Disease Clinical Presentation". MedScape. Retrieved July 2011.
- Vickaryous, Matthew K; Hall, Brian K (March 2006). "Homology of the reptilian coracoid and a reappraisal of the evolution and development of the amniote pectoral apparatus". Journal of Anatomy 208 (3): 263–285. doi:10.1111/j.1469-7580.2006.00542.x.