Scapula

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shoulder blade
Pectoral girdle front diagram.svg
Scapula - posterior view2.png
The upper picture is an anterior view of the thorax and shoulder girdle. The lower picture is a posterior view of the thorax (scapula shown in red.)
Latin Scapula
(omo)
Gray's p.202
MeSH Scapula
TA A02.4.01.001
FMA FMA:13394
Anatomical terms of bone

In anatomy, the scapula (plural scapulae) or 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 the scapula on the left side of the body being roughly a mirror image of the right scapula. In early Roman times, people thought the bone resembled a trowel, a small shovel. The shoulder blade is also called omo in Latin medical terminology.

The scapula forms the posterior (back) located part of the shoulder girdle. In humans, it is a flat bone, roughly triangular in shape, placed on a posterolateral aspect of the thoracic cage.

Structure

Surfaces

Costal (Front, Ventral, Anterior)

The costal or ventral surface [Fig. 1] presents a broad concavity, the subscapular fossa.

The medial two-thirds of this fossa are marked by several oblique ridges, which run lateralward and upward. The ridges give attachment to the tendinous insertions, and the surfaces between them to the fleshy fibers, of the subscapularis. The lateral third of the fossa is smooth and covered by the fibers of this muscle.

At the upper part of the fossa is a transverse depression, where the bone appears to be bent on itself along a line at right angles to and passing through the center of the glenoid cavity, forming a considerable angle, called the subscapular angle; this gives greater strength to the body of the bone by its arched form, while the summit of the arch serves to support the spine and acromion.

Figure 1 : Left scapula. Costal surface.
Left scapula - close-up - animation - stop at anterior surface.gif Gray202.png Scapula ant numbered.png 1. Subscapular fossa
2. Glenoid cavity
3. Coracoid process
4. Acromion
5. Superior border
6. Scapular notch
7. Superior angle
8. Medial border
9. Inferior angle
10. Lateral border
11. Infraglenoid tubercle


Dorsal (Back, Posterior)

The dorsal surface [Fig. 2] is arched from above downward, and is subdivided into two unequal parts by the spine; the portion above the spine is called the supraspinous fossa, and that below it the infraspinous fossa.

  • The supraspinous fossa, the smaller of the two, is concave, smooth, and broader at its vertebral than at its humeral end; its medial two-thirds give origin to the Supraspinatus.
  • The infraspinous fossa is much larger than the preceding; toward its vertebral margin a shallow concavity is seen at its upper part; its center presents a prominent convexity, while near the axillary border is a deep groove which runs from the upper toward the lower part. The medial two-thirds of the fossa give origin to the Infraspinatus; the lateral third is covered by this muscle.

The dorsal surface is marked near the axillary border by an elevated ridge, which runs from the lower part of the glenoid cavity, downward and backward to the vertebral border, about 2.5 cm above the inferior angle.

The ridge serves for the attachment of a fibrous septum, which separates the Infraspinatus from the Teres major and Teres minor.

The surface between the ridge and the axillary border is narrow in the upper two-thirds of its extent, and is crossed near its center by a groove for the passage of the scapular circumflex vessels; it affords attachment to the Teres minor.

The costal surface superior of the scapula is the origin of 1st digitation for the serratus anterior origin. The broad and narrow portions above alluded to are separated by an oblique line, which runs from the axillary border, downward and backward, to meet the elevated ridge: to it is attached a fibrous septum which separates the Teres muscles from each other.

Its lower third presents a broader, somewhat triangular surface, the Inferior angle of the scapula, which gives origin to the Teres major, and over which the Latissimus dorsi glides; frequently the latter muscle takes origin by a few fibers from this part.

Figure 2 : Left scapula. Dorsal surface.
Left scapula - close-up - animation - stop at posterior surface.gif Gray203.png Scapula post numbered.png 1. Supraspinatous fossa
2. Spine
3. Infraspinatous fossa
4. Superior border
5. Superior angle
6. Medial border
7. Inferior angle
8. Lateral border
9. Lateral angle
10. Acromion
11. Coracoid process
12. Origin of teres major muscle
13. Origin of teres minor muscle


Borders

There are three borders of the scapula:

  • The axillary border (or "lateral border") is the thickest of the three. It begins above at the lower margin of the glenoid cavity, and inclines obliquely downward and backward to the inferior angle. It is referred to as the caudal border in animals.
  • The vertebral border (or "medial border") is the longest of the three, and extends from the medial to the inferior angle. It is referred to as the dorsal border in animals.

Angles

There are 3 angles

  1. The superior angle is covered by trapezius.
  2. The inferior angle is covered by latissimus dorsi. It moves forwards round the chest when the arm is abducted.
  3. The lateral or glenoid angle is broad and bears the glenoid cavity or fossa, which is directed forward, laterally and slightly upwards.

The acromion

The acromion forms the summit of the shoulder, and is a large, somewhat triangular or oblong process, flattened from behind forward, projecting at first laterally, and then curving forward and upward, so as to overhang the glenoid cavity.

Figure 3 : Left scapula. Lateral surface.
Left scapula - close-up - animation - stop at lateral surface.gif Gray205 left scapula lateral view.png LeftScapulaLateral.jpg 1. Coracoid process
2. Glenoid cavity
3. Supraspinatous fossa
4. Acromion
5. Infraspinatous fossa
6. Inferior angle
7. Lateral border


Development

The larger part of the scapula undergoes membranous ossification.1 Some of the outer parts of the scapula are cartilaginous at birth, and would therefore undergo endochondral ossification.2

The head, processes, and the thickened parts of the bone, contain cancellous tissue; the rest consists of a thin layer of compact tissue.

The central part of the supraspinatus fossa and the upper part of the infraspinatous fossa, but especially the former, are usually so thin in humans as to be semitransparent; occasionally the bone is found wanting in this situation, and the adjacent muscles are separated only by fibrous tissue.

Figure 5 : Plan of ossification of the scapula. From seven centers.

The scapula is ossified from 7 or more centers: one for the body, two for the coracoid process, two for the acromion, one for the vertebral border, and one for the inferior angle.

Ossification of the body begins about the second month of fetal life, by an irregular quadrilateral plate of bone forming, immediately behind the glenoid cavity. This plate extends to form the chief part of the bone, the scapular spine growing up from its dorsal surface about the third month. Ossification starts as membranous ossification before birth.34 After birth, the cartilaginous components would undergo endochondral ossification.

At birth, a large part of the scapula is osseous, but the glenoid cavity, the coracoid process, the acromion, the vertebral border, and the inferior angle are cartilaginous.

From the 15th to the 18th month after birth, ossification takes place in the middle of the coracoid process, which as a rule becomes joined with the rest of the bone about the 15th year.

Between the 14th and 20th years, the remaining parts ossify in quick succession, and usually in this order; first, in the root of the coracoid process, in the form of a broad scale; secondly, near the base of the acromion; thirdly, in the inferior angle and contiguous part of the vertebral border; fourthly, near the outer end of the acromion; fifthly, in the vertebral border.

The base of the acromion is formed by an extension from the spine; the two nuclei of the acromion unite, and then join with the extension from the spine. The upper third of the glenoid cavity is ossified from a separate center (sub coracoid), which appears between the 10th and 11th years and joins between the 16th and the 18th years.

Further, an epiphysial plate appears for the lower part of the glenoid cavity, and the tip of the coracoid process frequently has a separate nucleus. These various epiphyses are joined to the bone by the 25th year.

Failure of bony union between the acromion and spine sometimes occurs (see os acromiale), the junction being effected by fibrous tissue, or by an imperfect articulation; in some cases of supposed fracture of the acromion with ligamentous union, it is probable that the detached segment was never united to the rest of the bone.

Function

The following muscles attach to the scapula:

Muscle Direction Region
Pectoralis Minor insertion coracoid process
Coracobrachialis origin coracoid process
Serratus Anterior insertion medial border
Triceps Brachii (long head) origin infraglenoid tubercle
Biceps Brachii (short head) origin coracoid process
Biceps Brachii (long head) origin supraglenoid tubercle
Subscapularis origin subscapular fossa
Rhomboid Major insertion medial border
Rhomboid Minor insertion medial border
Levator Scapulae insertion medial border
Trapezius insertion spine of scapula
Deltoid origin spine of scapula
Supraspinatus origin supraspinous fossa
Infraspinatus origin infraspinous fossa
Teres Minor origin lateral border
Teres Major origin lateral border
Latissimus Dorsi (a few fibers, attachment may be absent) origin inferior angle
Omohyoid origin superior border

Movements

Movements of the scapula are brought about by scapular muscles:

Elevation, Depression, Protraction (abduction) Retraction (adduction) Upward (lateral) rotation, Downward (medial) rotation, Anterior Tipping, and Posterior Tipping

Clinical relevance

Injury

Because of its sturdy structure and protected location, scapular fractures are uncommon; when they do occur, they are an indication that severe chest trauma has occurred.5

A winged scapula is a condition in which the medial border (the side nearest the spine) of a person's scapula is abnormally positioned outward and backward. The resulting appearance of the upper back is said to be wing-like because the inferior angle of the shoulder blade protrudes backward rather than lying mostly flat. In addition, any condition causing weakness of the serratus anterior muscle may cause scapular "winging".

Impingement Syndrome

The scapula has been found to play an important role in shoulder impingement syndrome.6 It is a wide, flat bone lying on the thoracic wall that provides an attachment for three different groups of muscles. The intrinsic muscles of the scapula include the muscles of the rotator cuff- the subscapularis, teres minor, supraspinatus, and infraspinatus.7 These muscles attach to the surface of the scapula and are responsible for the internal and external rotation of the glenohumeral joint, along with humeral abduction. The extrinsic muscles include the biceps, triceps, and deltoid muscles and attach to the coracoid process and supraglenoid tubercle of the scapula, infraglenoid tubercle of the scapula, and spine of the scapula. These muscles are responsible for several actions of the glenohumeral joint. The third group, which is mainly responsible for stabilization and rotation of the scapula, consists of the trapezius, serratus anterior, levator scapulae, and rhomboid muscles and attach to the medial, superior, and inferior borders of the scapula. Each of these muscles has their own role in proper shoulder function and must be in balance with each other in order to avoid shoulder pathology. Abnormal scapular function is called scapular dyskinesis. One action the scapula performs during a throwing or serving motion is elevation of the acromion process in order to avoid impingement of the rotator cuff tendons.6 If the scapula fails to properly elevate the acromion, impingement may occur during the cocking and acceleration phase of an overhead activity. The two muscles most commonly inhibited during this first part of an overhead motion are the serratus anterior and the lower trapezius.8 These two muscles act as a force couple within the glenohumeral joint to properly elevate the acromion process, and if a muscle imbalance exists, shoulder impingement may develop.

In other animals

Scapulae, spine and ribs of Myotis lucifugus (Little Brown Bat).

In fish, the scapular blade is a structure attached to the upper surface of the articulation of the pectoral fin, and is accompanied by a similar coracoid plate on the lower surface. Although sturdy in cartilagenous fish, both plates are generally small in most other fish, and may be partially cartilagenous, or consist of multiple bony elements.9

In the early tetrapods, these two structures respectively became the scapula and a bone referred to as the procoracoid (commonly called simply the "coracoid", but not homologous with the mammalian structure of that name). In amphibians and reptiles (birds included), these two bones are distinct, but together form a single structure bearing many of the muscle attachments for the forelimb. In such animals, the scapula is usually a relatively simple plate, lacking the projections and spine that it possesses in mammals. However, the detailed structure of these bones varies considerably in living groups. For example, in frogs, the procoracoid bones may be braced together at the animal's underside to absorb the shock of landing, while in turtles, the combined structure forms a Y-shape in order to allow the scapula to retain a connection to the clavicle (which is part of the shell). In birds, the procoracoids help to brace the wing against the top of the sternum.9

In the fossil therapsids, a third bone, the true coracoid, formed just behind the procoracoid. The resulting three-boned structure is still seen in modern monotremes, but in all other living mammals, the procoracoid has disappeared, and the coracoid bone has fused with the scapula, to become the coracoid process. These changes are associated with the upright gait of mammals, compared with the more sprawling limb arrangement of reptiles and amphibians; the muscles formerly attached to the procoracoid are no longer required. The altered musculature is also responsible for the alteration in the shape of the rest of the scapula; the forward margin of the original bone became the spine and acromion, from which the main shelf of the shoulder blade arises as a new structure.9

In dinosaurs

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 serves as the attachment site for a dinosaur's back and forelimb muscles.

Gallery

Position of scapula (shown in red). Animation. 
Shape of scapula (left). Animation. 
Thorax seen from behind. 
Diagram of the human shoulder joint 
The scapular and circumflex arteries. 

See also

This article uses anatomical terminology; for an overview, see anatomical terminology.

References

  1. ^ "Ossification". Medcyclopaedia. GE. Archived from the original on 2012-02-05. 
  2. ^ http://www.bartleby.com/107/50.html
  3. ^ http://www.medcyclopaedia.com/library/topics/volume_vii/o/ossification.aspx
  4. ^ http://books.google.com/books?id=t70Ij7jHLIkC&pg=PA97&lpg=PA97&dq=%22membranous+ossification%22+scapula&source=web&ots=RUWL51g7YN&sig=tAu1oyrdRHvWKvRdaDkc0e7DZZk&hl=en&sa=X&oi=book_result&resnum=23&ct=result
  5. ^ Livingston DH, Hauser CJ (2003). "Trauma to the chest wall and lung". In Moore EE, Feliciano DV, Mattox KL. Trauma. Fifth Edition. McGraw-Hill Professional. p. 516. ISBN 0-07-137069-2. 
  6. ^ a b Kibler, BW. (1998). The role of the scapula in athletic shoulder function. The American Journal of Sports Medicine, 26(2), 325-337.
  7. ^ Marieb, E. (2005). Anatomy & Physiology (2nd ed.). San Francisco, CA: Pearson Benjamin Cummings.
  8. ^ Cools, A. , Dewitte, V. , Lanszweert, F. , Notebaert, D. , Roets, A. , et al. (2007). Rehabilitation of scapular muscle balance. The American Journal of Sports Medicine, 35(10), 1744.
  9. ^ a b c Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. pp. 186–187. ISBN 0-03-910284-X. 
  • Nickel, Schummer, & Seiferle; Lehrbuch der Anatomie der Haussäugetiere.

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