Scapula anatomy: the shoulder blade, its borders, fossae and processes

Also called

Shoulder blade

What it is

The shoulder blade, a flat triangular bone on the back of the thorax.

Location

Lies on the posterior chest wall over ribs 2 to 7, articulating with the humerus at the glenoid cavity and with the clavicle at the acromion.

Function

Provides a mobile base for the arm and attachment for muscles of the shoulder.

Body system

Skeletal · paired (left and right)

FMA ID

FMA13396 (Foundational Model of Anatomy)

Key facts

• Key features are the spine, acromion, coracoid process, glenoid cavity and inferior angle.
• The rotator cuff muscles attach to it.
• The suprascapular notch on its superior border, bridged by the superior transverse scapular ligament, transmits the suprascapular nerve.

Clinical relevance

Winged scapula results from injury to the long thoracic nerve.

The scapula lies on the posterior shoulder. It is a flat, triangular bone with a prominent ridge, the spine, running across its back surface. It is surrounded by muscles and does not form a joint with the ribs of the thoracic cage. (OpenStax Anatomy and Physiology 2e, 8.1: The Pectoral Girdle.)

It rests over the posterior surface of ribs two to seven. Its anterior (costal) surface faces the ribcage and its posterior surface faces outward under the skin and back muscles. (Kenhub: Scapula.)

The scapula has only two true joints: the glenohumeral (shoulder) joint between the glenoid fossa and the head of the humerus, and the acromioclavicular joint between the acromion and the clavicle. Through the clavicle it connects the upper limb to the trunk. (TeachMeAnatomy: The Scapula.)

The scapula has three borders. The superior border is the shortest and thinnest. The medial border is thin and runs parallel to the vertebral column, so it is also called the vertebral border. The lateral border, or axillary border, is the thickest and strongest and carries the glenoid cavity at its upper end. (Kenhub: Scapula.)

Three angles sit where the borders meet. The superior angle joins the superior and medial borders. The inferior angle joins the medial and lateral borders and is the lowest point of the bone. The lateral angle joins the superior and lateral borders and holds the glenoid cavity. (Kenhub: Scapula.)

The inferior angle is a muscle attachment point. The suprascapular notch lies on the superior border, lateral to its midpoint. (OpenStax Anatomy and Physiology 2e, 8.1: The Pectoral Girdle.)

Followed laterally, the notch sits just medial to the base of the coracoid process, a useful landmark when you look for it on a bone or an X-ray. (Kenhub: Scapula.)

The spine of the scapula is the most prominent feature of the posterior surface. It runs transversely and divides the back of the bone into two hollows: the supraspinous fossa above and the infraspinous fossa below. (TeachMeAnatomy: The Scapula.)

The supraspinous fossa is much smaller than the infraspinous fossa and gives origin to supraspinatus. The broad infraspinous fossa gives origin to infraspinatus. On the front, the large, shallow subscapular fossa covers most of the costal surface and gives origin to subscapularis. (TeachMeAnatomy: The Scapula.)

Laterally the spine continues as the acromion (acromial process), a flat projection that arches over the shoulder joint and forms the bony tip of the shoulder. The acromion articulates with the lateral end of the clavicle at the acromioclavicular joint and can be felt under the skin. (Kenhub: Scapula.)

The glenoid cavity (glenoid fossa) is the shallow socket at the lateral angle that receives the head of the humerus. A roughened supraglenoid tubercle just above it anchors the long head of biceps brachii, and an infraglenoid tubercle just below it anchors the long head of triceps brachii. (TeachMeAnatomy: The Scapula.)

The coracoid process is a hook-like projection shaped like a crow's beak. It projects anteriorly from the superior border, curves laterally, and lies just below the lateral part of the clavicle. (OpenStax Anatomy and Physiology 2e, 8.1: The Pectoral Girdle.)

Pectoralis minor, coracobrachialis and the short head of biceps brachii attach to the coracoid process. Three ligaments also arise from it: the coracohumeral ligament to the humerus, the coracoclavicular ligament to the clavicle and the coracoacromial ligament to the acromion. (Kenhub: Scapula.)

The coracoacromial ligament, the acromion and the coracoid process together form the coracoacromial arch above the head of the humerus. This arch stops the humeral head moving upward out of the glenoid cavity. (Kenhub: Coracoacromial ligament.)

The scapula gives attachment to 17 muscles, which is why it can move so freely while still acting as a base for the arm. (TeachMeAnatomy: The Scapula.)

Muscles that originate from the scapula include deltoid, supraspinatus, infraspinatus, subscapularis, teres minor, teres major, coracobrachialis, both heads of biceps brachii, the long head of triceps brachii and omohyoid. Latissimus dorsi has an inconstant slip from the inferior angle. Muscles that insert on the scapula include trapezius, levator scapulae, rhomboid major, rhomboid minor, serratus anterior and pectoralis minor. (Kenhub: Scapula.)

Four of these, supraspinatus, infraspinatus, teres minor and subscapularis, are the rotator cuff. They arise from the scapula, attach to the tubercles of the humerus and blend with the shoulder joint capsule. (OpenStax Anatomy and Physiology 2e, 9.6: Anatomy of Selected Synovial Joints.)

The scapula slides on the chest wall. Elevation lifts it upward, as in a shrug, and depression lowers it. Protraction moves it forward around the chest, as when pushing, and retraction pulls it back toward the vertebral column. (OpenStax Anatomy and Physiology 2e, 9.5: Types of Body Movements.)

Elevation is produced by trapezius, levator scapulae and the rhomboids. Retraction is produced by the rhomboids and the middle and lower fibres of trapezius. Protraction is produced by serratus anterior and pectoralis minor. Upward rotation, needed to lift the arm above 90 degrees, comes from trapezius and serratus anterior. (Kenhub: Scapula.)

The acromion forms from several ossification centres that normally fuse into one bone; studies give the age of complete fusion as anywhere from about 14 to 25 years. If they fail to fuse, the separate piece is called an os acromiale. In a population study of 3,050 adults scanned with MRI, 1.9% had one, about a quarter of them on both sides, and it was more common in men. (Fischer CS et al. Os acromiale: prevalence and associated patient-related factors. Int Orthop 2022.)

An os acromiale is usually found by chance, but it can cause shoulder pain through movement at the unfused junction or impingement, and surgeons need to know about it before shoulder operations. (Fischer CS et al. Os acromiale: prevalence and associated patient-related factors. Int Orthop 2022.)

Winging of the scapula is the most common disorder of scapular movement. Injury to the long thoracic nerve, which supplies serratus anterior, lets the medial border and inferior angle lift away from the chest wall. Injury to the dorsal scapular nerve causes milder winging, and injury to the spinal accessory nerve (which supplies trapezius) leaves the scapula depressed and rotated. (Kenhub: Scapula.)

Scapular fractures are uncommon, making up about 0.5 to 1% of all fractures. They usually follow high-energy direct trauma such as a road collision or crush injury and are often accompanied by other serious chest injuries. (Kenhub: Scapula.)

Most scapular fractures do not need fixation, because the thick muscles around the bone hold the fragments in place while they heal. (TeachMeAnatomy: The Scapula.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Subscapularis · muscular
• Infraspinatus muscle · muscular
• Dorsal scapular artery · cardiovascular
• Teres major · muscular
• Teres minor · muscular
• Circumflex scapular artery · cardiovascular
• Spinal part of deltoid · muscular
• Thoracodorsal artery · cardiovascular
• Subscapular artery · cardiovascular
• Axillary artery · cardiovascular