Scapula

The scapula is the flat, triangular shoulder blade that lies on the upper back and carries the socket of the shoulder joint.

At a glance

English: Scapula
Latin (TA2): Scapula
System: Bone
Region: Upper limb
Side: Paired, left and right

About the scapula

The scapula, or shoulder blade, is a flat triangular bone lying on the back of the upper chest wall, roughly over the second to seventh ribs. It links the arm to the clavicle and, through muscle rather than a true joint, to the trunk. Its free gliding over the ribs widens the range of the shoulder well beyond what the shoulder joint alone allows.

The bone has three borders (medial, lateral and superior) and three angles. The superior angle sits near the top of the medial border, the inferior angle lies at the lower tip, and the lateral angle is thickened to carry the glenoid cavity, a shallow oval socket for the head of the humerus. Small tubercles above and below the glenoid give origin to the long heads of biceps and triceps. The front surface is a gently hollowed subscapular fossa facing the ribs. The back is divided by a prominent spine into a small supraspinous fossa above and a larger infraspinous fossa below; the spine ends laterally in the flat acromion, which forms the point of the shoulder and meets the clavicle. The coracoid process curls forwards from the upper border, and just medial to its base the suprascapular notch, bridged by a ligament, passes the suprascapular nerve.

Many muscles act on or arise from the scapula. Supraspinatus, infraspinatus, teres minor and subscapularis form the rotator cuff around the shoulder joint. Trapezius, levator scapulae, the rhomboids, serratus anterior and pectoralis minor move the bone itself, raising, lowering, pulling it back towards the spine, drawing it forwards round the chest and rotating the glenoid upwards or downwards.

Fractures are uncommon because the bone is well padded by muscle, so a fracture usually signals a high-energy injury to the chest. Weakness of serratus anterior, often from injury to the long thoracic nerve, lets the medial border lift away from the ribs, giving a winged scapula. Poor timing of the scapular muscles, called scapular dyskinesis, can let the acromion press on the rotator cuff tendons during overhead movement and contribute to impingement. An acromion whose growth centre never fuses with the spine is known as os acromiale.

Function and supply

Function: Provides the socket of the shoulder joint and, by gliding and rotating on the chest wall, positions that socket so the arm can reach overhead.
Nerve supply: The suprascapular nerve passes through the suprascapular notch and around the spinoglenoid notch; the dorsal scapular, long thoracic, subscapular and axillary nerves supply the muscles that attach.
Blood supply: The suprascapular, dorsal scapular and circumflex scapular arteries, which anastomose around the bone.

Parts of the scapula

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Acromial angleAngulus acromiiThe corner where the outer edge of the acromion turns into the crest of the scapular spine. It is a sharp point easily felt at the back of the shoulder and is often used as a measuring landmark.
AcromionThe broad, flat end of the scapular spine that arches over the top of the shoulder joint and forms the point of the shoulder. It meets the clavicle in front and gives attachment to deltoid and trapezius.
Clavicular facetFacies articularis clavicularisA small oval joint surface on the inner edge of the acromion. It meets the outer end of the clavicle to form the acromioclavicular joint, which is often injured in falls onto the shoulder.
Coracoid processProcessus coracoideusA curved, finger-like hook projecting forwards from the top of the neck of the scapula, felt below the outer clavicle. Pectoralis minor, coracobrachialis and the short head of biceps attach to it, as do strong ligaments to the clavicle.
Costal surface of scapulaFacies costalis scapulaeThe front of the blade, which faces the ribs and is slightly hollowed. Most of it is taken up by the subscapular fossa, and serratus anterior attaches along its medial edge.
Deltoid tubercleTuberculum deltoideumA small rough rise on the lower lip of the scapular spine, near its medial end. It marks where the hindmost fibres of deltoid begin.
Glenoid fossaFossa glenoideaThe shallow, pear-shaped hollow on the lateral angle that receives the head of the humerus. It is deepened by a fibrocartilage rim, the labrum, but remains much smaller than the head it holds, which is why the shoulder dislocates easily.
Glenoid process of scapulaProcessus glenoideus scapulaeThe thick projection at the lateral angle that carries the glenoid fossa on its end. It narrows slightly at its base, where it joins the blade through the neck.
Inferior angle of scapulaAngulus inferior scapulaeThe lowest point of the shoulder blade, where the medial and lateral borders meet, usually level with the seventh rib. Teres major arises near it, and its forward swing round the chest wall shows how the scapula rotates when the arm is raised.
Infraglenoid tubercleTuberculum infraglenoideumA rough area just below the glenoid fossa, at the top of the lateral border. The long head of triceps brachii arises from it.
Infraspinous fossaFossa infraspinataThe large hollow on the back of the blade below the spine. It is filled by the infraspinatus muscle, one of the rotator cuff, with teres minor arising along its outer part.
Lateral angle of scapulaAngulus lateralis scapulaeThe thickest corner of the bone, at the top of the lateral border. It carries the glenoid process and fossa, so it is the part that bears the arm.
Lateral border of scapulaMargo lateralis scapulaeThe thick edge running from below the glenoid fossa down to the inferior angle, facing the armpit. Teres minor and teres major arise along its back, and its thickness makes it a lever that resists bending.
Medial border of scapulaMargo medialis scapulaeThe longest edge, running parallel to the spine of the backbone from the superior to the inferior angle. Levator scapulae, the rhomboids and serratus anterior attach along it, and a winged scapula shows when serratus anterior fails.
Neck of scapulaCollum scapulaeThe slightly narrowed band of bone around the base of the glenoid process. It is clearest at the back and below and is a site where fractures of the shoulder blade occur.
Posterior surface of scapulaFacies posterior scapulaeThe back of the blade, curved from top to bottom and split by the spine into a smaller upper and a larger lower hollow. It carries supraspinatus, infraspinatus and teres minor.
Scapular notchIncisura scapulaeA U-shaped dip in the upper border just medial to where the coracoid springs from the bone. A ligament bridges it into a foramen, with the suprascapular nerve passing below and the suprascapular artery usually above.
Spine of scapulaSpina scapulaeA raised shelf of bone crossing the back of the scapula obliquely and ending in the acromion. Its crest gives attachment to trapezius above and deltoid below, and it is easily felt under the skin.
Spinoglenoid notchIncisura spinoglenoideaThe gap between the outer edge of the scapular spine and the back of the glenoid neck, linking the two posterior fossae. The suprascapular nerve and vessels pass through it to reach infraspinatus, and a cyst or compression here weakens only that muscle.
Subscapular fossaFossa subscapularisThe broad, shallow hollow on the front of the scapula. The subscapularis muscle, the largest of the rotator cuff, arises from most of it.
Superior angle of scapulaAngulus superior scapulaeThe upper inner corner, where the superior and medial borders meet, lying under trapezius at about the level of the second rib. Levator scapulae inserts at and just below it.
Superior border of scapulaMargo superior scapulaeThe shortest and thinnest edge, stretching outwards from the superior angle as far as the coracoid. The scapular notch lies at its outer end, and the inferior belly of omohyoid arises near it.
Supraglenoid tubercleTuberculum supraglenoideumA small rough point just above the glenoid fossa, close to where the coracoid arises. The long head of biceps brachii arises from it and runs over the head of the humerus inside the joint.
Supraspinous fossaFossa supraspinataThe smaller, smooth hollow on the back of the scapula above the spine. It holds the supraspinatus muscle, whose tendon passes under the acromion and is often worn or torn.

Anthromica

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At a glance

About the scapula

Function and supply

Related structures

See the scapula where it really sits, with every layer of the body around it.