Scapula

General features.–The scapula is a large, flattened, triangular bone, which lies on the posterolateral aspect of the chest wall, covering parts of the second to the seventh ribs. It presents for examination costal and dorsal surfaces, upper, lateral and medial borders, inferior, superior and lateral angles and three bony processes, viz. the spine, its continuation the acromion, and the coracoid process. The lateral angle is truncated and bears the glenoid cavity for articulation with the head of the humerus. This part of the bone may be regarded as the head and it is connected to the plate-like body by an inconspicuous neck. The long axis of the scapula is nearly vertical and the relatively featureless costal surface can easily be distinguished from the dorsal surface, which is interrupted by the shelf-like projection of the spine (fig. 402). The bone is very much thickened in the immediate neighborhood of the lateral border, which runs from the inferior angle below, to the glenoid cavity above. The student now possesses sufficient information to enable him to determine correctly whether a given scapula belongs to the right or the left side of the body.

The costal surface (fig. 400), which is directed medially and forwards when the arm is by the side, is slightly hollowed out, especially in its upper part. Near the lateral border it presents a longitudinal, rounded ridge, stout and salient in the neighbourhood of the neck, but becoming less prominent below, which is separated from the lateral border by a narrow, grooved area. The dorsal surface (fig. 401) is divided into a smaller, supraspinous fossa and a larger, infraspinous fossa by the spine of the scapula. These two fossa communicate with each other through the spinoglenoid notch, which lies between the free, lateral border of the spine and the dorsal aspect of the neck of the bone. A flattened strip, for muscular attachments, marks the dorsal surface along the lateral border.

The lateral border (axillary border) of the scapula forms a sharp, roughened ridge, ^* which runs sinuously from the inferior angle to the glenoid cavity. At its upper end it widens into a rough, somewhat triangular, area, which is termed the infraglenoid tubercle (fig. 402). Throughout its whole length the lateral border is thickly covered with muscles and cannot be felt satisfactorily in the living subject. The medial border (vertebral border) of the scapula extends from the inferior to the superior angle. In its lower two-thirds, i.e. where it forms the medial boundary of the infraspinous fossa, this border can easily be felt through the skin, but its upper third is more deeply placed and cannot be palpated in the normal subject. The superior border, thin and sharp, is the shortest of the three borders. At its anterolateral end it is separated from the root of the coracoid process by the suprascapular notch, which is converted into a foramen by the suprascapular ligament.

^* The lateral border is often described as a thick border, but such a description includes in the border the grooved, lateral part of the costal surface and, frequently, the flattened strip along the lateral part of the dorsal surface. The actual border, however, is clearly defined on the bone as a narrow, roughened ridge.

The inferior angle lies over the seventh rib, or sometimes over the seventh intercostal space. It can be felt through the skin and the muscles which cover it and, when the arm is raised above the head, it can be seen to pass forwards round the chest wall. The superior angle is placed at the junction of the superior and medial borders, and is obscured by the muscles which cover it. The lateral angle is truncated and broadened. It constitutes the head of the bone. On its free surface it bears the glenoid cavity, for articulation with the head of the humerus in the shoulder-joint. When the arm is by the side the glenoid cavity is directed forwards and laterally and slightly upwards. Very gently hollowed out, it forms a far from stable socket for the humeral head. It is narrow above and wider below, and is therefore pear-shaped in outline. Immediately above the glenoid cavity a small roughened area encroaches on the root of the coracoid process and is termed the supraglenoid tubercle. The neck of the scapula is the constriction immediately adjoining the head. It can be identified most easily on its inferior and dorsal aspects. Ventrally, it can be regarded as running from the infraglenoid tubercle to the anterior margin of the suprascapular notch.

Figure 400
The spine of the scapula (fig. 401) forms a shelf–like elevation on the upper part of the dorsal surface of the bone, and is somewhat triangular in shape. Its lateral border is free, thick and rounded and helps to bound the spinoglenoid notch. Its anterior border joins the dorsal surface of the scapula along a line which runs laterally and slightly upwards from the junction of the upper and middle thirds of the medial border. It should be noted that the plate-like body of the bone is bent along this line, and this fact accounts for the concavity of the upper part of the costal surface. The third border constitutes the crest of the spine, which is subcutaneous throughout its whole extent. At its medial end the crest expands into a smooth, triangular area. Elsewhere the upper and lower edges and the surface of the crest are roughened for muscular attachments. The upper surface of the spine widens as it is traced laterally, and is slightly hollowed out. The lower surface is overhung by the crest at its medial, narrow end, but is gently convex in its wider, lateral portion. Both surfaces display large vascular foramina running towards the junction of the free, lateral border with the body of the bone.

The acromion projects freely from the lateral end of the spine, with which it is continuous. Its long axis, however, does not coincide with that of the spine, but makes an angle with it of rather more than 90 degrees. The lower border of the crest of the spine becomes continuous with the lateral border of the acromion at the acromial angle, which forms a subcutaneous, bony landmark. The medial border of the acromion is short and is marked anteriorly by a small, oval facet, directed upwards and medially, for articulation with the lateral end of the clavicle. The lateral border, tip and upper surface of the acromion can all be felt through the skin without difficulty. The corticoid process (fig. 400) springs from the upper aspect of the head of the scapula and is bent sharply so as to project forwards and slightly laterally. When the arm is by the side, the corticoid process points almost straight forwards and its slightly enlarged tip can be felt through the skin, although it is covered by the anterior fibers of the deltoid muscle. The supraglenoid tubercle marks the root of the corticoid process where it adjoins the upper part of the glenoid cavity. Another impression is placed on the dorsal aspect of the corticoid process at the point where it changes direction. This gives attachment to the conoid part of the coracoclavicuIar ligament, which will be mentioned again in connection with the clavicle.

Particular features.-The costal surface gives origin to the subscapularis muscle (fig. 400), which arises from nearly the whole of this aspect including the grooved area immediately adjoining the lateral border and excluding the area adjoining the neck of the bone. Small intramuscular tendons are attached to the roughened ridges which subdivide this surface incompletely into a number of smooth areas. The anterior aspect of the neck is separated from the subscapularis muscle by a bursal protrusion of the synovial membrane of the shoulder-joint. Near the inferior angle a somewhat oval area gives insertion to the lower five or six digitations of the serratus anterior. The remainder of the muscle is inserted into a narrow strip along the ventral aspect of the medial border, which is wider above, where it receives the large first digitation. The rounded, longitudinal ridge near the lateral border corresponds to a localised thickening of the bone and provides a lever of the necessary strength to enable the serratus anterior to rotate the scapula forwards. In this movement the upper limb is raised from the side and carried above the head against the action of gravity, and it is this latter factor which calls for the provision of a substantial lever.

On the dorsal surface, the supraspinous fossa in its medial two-thirds gives origin to the supraspinatus, and its margins give attachment to the fascia which covers the muscle. The flattened strip adjoining the lateral border gives origin in its upper two-thirds to the teres minor muscle and is grooved, near its upper end, by the circumflex scapular vessels, which pass between the muscle and the bone as they enter the infraspinous fossa (fig. 401). The lower limit of the origin of the teres minor is indicated by an oblique ridge, which runs from the lateral border to the neighbourhood of the inferior angle and cuts off a somewhat oval area for the origin of the teres major muscle. The dorsal aspect of the inferior angle may give origin to a small slip which joins the deep surface of the latissimus dorsi. With the exception of an area near the neck of the bone, the rest of the infraspinous fossa, which is hollowed out laterally but convex medially, gives origin to the infraspinatus muscle. The strong infraspinatus fascia passes on to the teres minor and the teres major and sends fascial partitions between them to reach the bone along the ridges which mark the limits of their attachments.

The lateral (axillary) border separates the origin of the subscapularis from the origins of the teres minor anti the teres major. These muscles project laterally beyond it, and together with the latissimus dorsi cover it so completely that it cannot be felt through the skin. The infraglenoid tubercle gives origin to the long head of the triceps muscle. The medial (vertebral) border is thin and often angled opposite the root of the spine. A narrow strip extends from the superior tangle to the root of the spine and gives insertion to the levator scapula muscle. Below this, and opposite the root of the spine, the rhomboideus minor gains insertion. The remainder of the border provides insertion for the rhomboideus major, but the precise character of the insertion is unusual. The ventral aspect of the medial border presents a flattened strip throughout its whole extent for the insertion of the serratus anterior (fig. 400). This strip is wider above, where it receives the first digitation of the muscle, and again below where it forms an oval area on the costal aspect of the inferior angle for the lower five, or six, digitations.

Figure 401
The upper border of the scapula is thin and sharp. Near the suprascapular notch it gives origin to the inferior belly of the omohyoid muscle, and the posterior limit of the notch gives attachment to the suprascapular ligament, which is sometimes ossified. The foramen completed by the ligament transmits the suprascapular nerve to the supraspinous fossa, whereas the suprascapular (transverse scapular) vessels pass dorsally above the ligament. The inferior angle of the scapula is covered on its dorsal aspect, by the upper border of the latissimus dorsi muscle which frequently receives a small slip of origin from this part of the bone. The superior angle of the scapula is covered by the upper part of the trapezius muscle. The truncated lateral angle bears the glenoid cavity, the margins of which give attachment to the glenoidal labrum. Its surface iscovered with a layer of hyaline articular cartilage, which is thinnest at the center of the cavity and thickest at its periphery . Its anterior margin gives attachment to the glenohumeral ligaments (p. 452). When the arm is by the side the cavity is directed forwards, and sotnewhat laterally and upwards. When the arm is raised above the head it is directed almost straight upwards.

The spine of the scapula gives attachanent by its upper and lower surfaces to the supra- and infraspinatus muscles, respectively. The flattened, triangular area at its root lies opposite the spine of the third thoracic vertebra and is covered by the tendon of the trapezius, a bursa intervening to enable the tendon to play over this part of the bone. The lower border of the crest gives origin to the posterior fibers of the deltoid muscle, which often produce an elongated tubercle in this situation. The upper border of the crest receives the insertion of the lower fibers of the trapezius, which often encroach on the dorsal or subcutaneous aspect of the crest, especially near the root of the spine.

Figure 402
The acromion is subcutaneous over its dorsal surface, being covered only by the shin and superficial fascia. The lateral border, which is thick and irregular, and the tip of the process, as far round as the clavicular facet, give origin to the middle fibers of the deltoid muscle. The medial aspect of the tip gives attachment, below the deltoid, to the lateral end of the coraco-acromial ligament. The articular capsule of the acromioclavicular joint is attached around the margins of the clavicular facet. Behind the facet, the medial border of the acromion gives insertion to the horizontal fibers of the trapezius muscle. The inferior aspect of the acromion is relatively smooth, and together with the coraco-acromial ligament and the coracoid process forms a protective arch over the superior aspect of the shoulder-joint. The tendon of the supraspinatus passes below the overhanging acromion and is separated from it and from the deltoid muscle by the subacromial bursa.

The coracoid process lies below the clavicle at the junction of the lateral fourth with the rest of the bone and is connected to its under surface by the coraco-clavicular ligament. The attachment of the conoid part of the ligament has already been considered : the trapezoid part is attached to the upper aspect of the horizontal part of the process (fig. 400). The superior aspect of the process also receives the insertion of the pectoralis minor muscle. Its lateral border gives attachment to the wider medial end of the coraco-acromial ligament and, below that, to the coracohumeral ligament. The enlarged tip of the process gives origin to the coracobrachialis, medially, and to the short head of the biceps, laterally. It is covered, in life, by the anterior fibers of the deltoid muscle and can only be identified on deep pressure through the lateral border of the infraclavicular fossa at a distance of 2.5 cm. below the clavicle. The inferior aspect of the process is smooth and helps to complete the coraco-acromial arch. The anterior aspect of the root is crossed by the tendon of the subscapula.ris, and-its posterior aspect by the tendon of the supraspinatus muscle.

Structure.-The head, processes, and thickened parts of the scapula. contain: spongy subsfance ; the rest consists of a thin layer of compact bone. The central part of the supraspinous fossa and the greater part of the infraspinous fossa are thin ; occasionally the bone is wanting in these situations, the gaps being filled by fibrous tissue.

Ossification (fig. 403).-The scapula is ossified from eight or more centers : one for the body, two for the coracoid process, two for the acromion, one for the vertebral border, one for the inferior angle and one for the lower part of the rim of the glenoid cavity

Figure 403
The center for the both appears in the eighth week of fetal life, and spreads to form an irregular quadrilateral plate of bone, immediately behind the glenoid cavity. This plate forms the chief part of the bone, and the spine grows backwards from it about the third month. At birth, the glenoid cavity, coracoid process, acromion, medial border, and inferior angle are cartilaginous. In the first year of life, ossification begins in the middle of the coracoid process, and this process joins the rest of the bone about the fifteenth year. Between the tenth and twentieth years ossification of the remaining parts takes place, usually in the following order: first, in the root of the coracoid process (subcoracoid center) ; second, near the base of the acromion; third, in the inferior angle and contiguous part of the medial border; fourth, near the extremity of the acromion ; fifth, in the medial border. The base of the acromion is formed by an extension from the spine ; the rest of the acromion is ossified from two centers which unite, and then join the extension from the spine. The upper one-third of the glenoid cavity is ossified from the subcoracoid center, which appears between the tenth and eleventh, and joins between the sixteenth and the eighteenth, years. Further, a horseshoe-shaped epiphyseal plate appears for the lower part of the glenoid cavity about puberty; thicker at its peripheral than at its central margin, it converts the flat glenoid fossa of the child into the gently concave fossa of the adult. The tip of the coracoid process frequently has a separate center. These various epiphyses are joined to the bone by the twenty-fifth year.

 

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