ARTICLE ON ANATOMY OF THE UPPER LIMB (SCAPULAR OR SHOULDER BLADE)

INTRODUCTION

In anatomy, the scapula (plural scapulae or scapulas; also known as shoulder bone, shoulder blade or wing bone) 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 either side of the body being roughly a mirror image of the other. In early Roman times, people thought the bone resembled a trowel, a small shovel.

In compound terms, the prefix omo- is used for the shoulder blade in Latin medical terminology. The prefix is derived from ὦμος (ōmos), the Ancient Greek word for shoulder and is cognate with the Latin (h)umerus.

The scapula forms the back 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

The scapula is a wide, flat bone lying on the thoracic wall that provides an attachment for three groups of muscles: intrinsic, extrinsic, and stabilising and rotating muscles. The intrinsic muscles of the scapula include the muscles of the rotator cuff—the subscapularis, teres minor, supraspinatus, and infraspinatus. These muscles attach to the surface of the scapula and are responsible for the internal and external rotation of the shoulder 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. These attach to the medial, superior, and inferior borders of the scapula.

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. The scapula has two surfaces, three borders, three angles, and three processes.

SURFACES

Front or Subscapular fossa

The front of the scapula (also known as the costal or ventral surface) has a broad concavity called the subscapular fossa, to which the subscapularis muscle attaches. The medial two-thirds of the fossa have 3 longitudinal oblique ridges, and another thick ridge adjoins the lateral border; they run outward and upward. The ridges give attachment to the tendinous insertions, and the surfaces between them to the fleshy fibers, of the subscapularis muscle. 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.

The costal surface superior of the scapula is the origin of 1st digitation for the serratus anterior origin.

Back

The back of the scapula (also called the dorsal or posterior surface) is arched from above downward, and is subdivided into two unequal parts by the spine of the scapula. The portion above the spine is called the supraspinous fossa, and that below it the infraspinous fossa. The two fossae are connected by the spinoglenoid notch, situated lateral to the root of the spine.

• 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.

There is a ridge on the outer part of the back of the scapula. This runs from the lower part of the glenoid cavity, downward and backward to the vertebral border, about 2.5 cm above the inferior angle. Attached to the ridge is a fibrous septum, which separates the infraspinatus muscle from the Teres major and Teres minor muscles. The upper two-thirds of the surface between the ridge and the axillary border is narrow, and is crossed near its center by a groove for the scapular circumflex vessels; the Teres minor attaches here.

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.

Side

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.

Angles

There are 3 angles:

The superior angle of the scapula or medial angle, is covered by the trapezius muscle. This angle is formed by the junction of the superior and medial borders of the scapula. The superior angle is located at the approximate level of the second thoracic vertebra. The superior angle of the scapula is thin, smooth, rounded, and inclined somewhat lateralward, and gives attachment to a few fibers of the levator scapulae muscle.

The inferior angle of the scapula is the lowest part of the scapula and is covered by the latissimus dorsi muscle. It moves forwards round the chest when the arm is abducted. The inferior angle is formed by the union of the medial and lateral borders of the scapula. It is thick and rough and its posterior or back surface affords attachment to the teres major and often to a few fibers of the latissimus dorsi. The anatomical plane that passes vertically through the inferior angle is named the scapular line.

The lateral angle of the scapula or glenoid angle also known as the head of the scapula is the thickest part of the scapula. It is broad and bears the glenoid cavity on its articular surface which is directed forward, laterally and slightly upwards, and articulates with the head of the humerus. The inferior angle is broader below than above and its vertical diameter is the longest. The surface is covered with cartilage in the fresh state; and its margins, slightly raised, give attachment to a fibrocartilaginous structure, the glenoidal labrum, which deepens the cavity. At its apex is a slight elevation, the supraglenoid tuberosity, to which the long head of the biceps brachii is attached.

The neck of the scapula is the slightly constricted portion which surrounds the head and is more distinct below and behind than above and in front.

Borders

There are three borders of the scapula:

• The superior border is the shortest and thinnest; it is concave, and extends from the superior angle to the base of the coracoid process. It is referred to as the cranial border in animals.

At its lateral part is a deep, semicircular notch, the scapular notch, formed partly by the base of the coracoid process. This notch is converted into a foramen by the superior transverse scapular ligament, and serves for the passage of the suprascapular nerve; sometimes the ligament is ossified.

The adjacent part of the superior border affords attachment to the omohyoideus.

• 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.

It begins above at the lower margin of the glenoid cavity, and inclines obliquely downward and backward to the inferior angle.

Immediately below the glenoid cavity is a rough impression, the infraglenoid tuberosity, about 2.5 cm (1 in). in length, which gives origin to the long head of the triceps brachii; in front of this is a longitudinal groove, which extends as far as the lower third of this border, and affords origin to part of the subscapularis.

The inferior third is thin and sharp, and serves for the attachment of a few fibers of the teres major behind, and of the subscapularis in front.

• The medial border (also called the vertebral border or medial margin) is the longest of the three borders, and extends from the superior angle to the inferior angle. In animals it is referred to as the dorsal border.

Four muscles attach to the medial border. Serratus anterior has a long attachment on the anterior lip. Three muscles insert along the posterior lip, the levator scapulae (uppermost), rhomboid minor (middle), and to the rhomboid major (lower middle).

DEVELOPMENT

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

FUNCTIONS AND ATTACHMENT OF THE SCAPULAR

 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 the scapular muscles. The scapula can perform six actions:

• Elevation: upper trapezius and levator scapulae
• Depression: lower trapezius
• Retraction (adduction): rhomboids and middle trapezius
• Protraction (abduction): serratus anterior
• Upward rotation: upper and middle trapezius
• Downward rotation: rhomboids

CLINICAL CORRELATIONS OR SIGNIFICANCE

Fracture of the Scapular

Because of its sturdy structure and protected location, fractures of the scapula are uncommon. When they do occur, they are an indication that severe chest trauma has occurred. Scapular fractures involving the neck of the scapula have two patterns. One (rare) type of fracture is through the anatomical neck of the scapula. The other more common type of fracture is through the surgical neck of the scapula. The surgical neck exits medial to the coracoid process.

An abnormally protruding inferior angle of the scapula is known as a winged scapula and can be caused by paralysis of the serratus anterior muscle. In this condition the sides of the scapula nearest the spine are positioned outward and backward. The appearance of the upper back is said to be wing-like. In addition, any condition causing weakness of the serratus anterior muscle may cause scapular "winging".

IMPINGEMENT SYNDROME

The scapula plays an important role in shoulder impingement syndrome.

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. 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. 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.