Development and Morphology of Shoulder
Evidence points to the development of upper limb evolutionarily , from from the folds of epidermis in fish species Rhipidistian crossopterygian. The pectoral part developed in to the upper limb and the pelvic fin has developed into lower limb.(1)
As these pre historic fish evolved to become amphibians, they adapted to living on land without the buoyancy of water. This eventually led to freeing up of pectoral and pelvis parts to considerable distance away from central part of the body.
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| Evolutionary development of Shoulder Girdle(1) |
As mammals developed evolutionarily, they developed a well-developed clavicle and sternal medially (more central to body) and wide scapula laterally (peripheral aspect of body).The coracoid and scapular spine parts of scapular bone developed during this period of evolutionary development in mammals.
A number of variations happened in development of clavicle and scapula as functional demands of different mammals differed. Mammals which adapted to running and swimming lost the clavicle to further mobilise the scapula and thereby increase the thrust provided by shoulder to aid movements. Loss of clavicle in mammals, allowed the scapula to move forwards and backwards with the limb and thereby increased the stride length.Mammals adapted for swimming developed a wider scapula to allow for varied movements.
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| Animals cheetah have small clavicle which increases stride length and mobility |
Finally, shoulders modified for brachiating (including those of humans) developed a strong clavicle, a large coracoid, and a widened, strong scapula. The size of the infraspinous fossa has gradually enlarged over time relative to the length of the scapular spine. This adaptation allows the infraspinatus and teres minor to be more effective in their roles as depressors and external rotators of the humeral head.
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| Picture showing increase in infraspinous fossa enlargement with evolution. into Humans.(2) |
In animals the acromion process is not well developed but whereas in humans it is a massive structure. This development is a reflection of increasing role of deltoid muscle in shoulder function. As the deltoid attachment on acromion has broadened and its attachment shifted distally in humerus the mechanical advantage of deltoid has increased (2)
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| Fetus showing early evidence of Upper limb development |
In humans by 13 weeks of gestation, the rotator cuff tendons, coracoacromial ligament (CAL), and coracohumeral ligament are present. The acromion is cartilaginous and consistently has a gentle curve that conforms to the superior aspect of the humeral head, similar to a type II acromion. These data suggest that variations in acromial morphology are acquired.(2)
Histological studies have shown that curved acromion implicated in impingement syndrome shows evidence of traction phenomena establishing that these changes are acquired.
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| Acromial morpological patterns.(2) |
References :
2. Rockwood and Matsen’s The Shoulder: Expert Consult – Online and Print, 4e (Shoulder (Rockwood/Matsen)(2 Vol.))
Vertebrate Paleobiology and Paleoanthropology 2009, pp 65-75