The Scapula

The scapula is a large, flat, triangular bone on the posterolateral aspect of the thoracic wall, overlying ribs 2–7. It connects to the humerus via the glenohumeral joint and to the axial skeleton via the clavicle at the acromioclavicular joint.

Bony Features

Surfaces (2)

Surface	Key Feature
Costal (ventral/anterior)	Subscapular fossa — shallow concavity where subscapularis attaches; serratus anterior attaches along medial edge
Dorsal (posterior)	Divided by the spine of the scapula into supraspinous fossa (supraspinatus origin) above and infraspinous fossa (infraspinatus origin, teres minor/major) below; the two fossae converge at the spinoglenoid notch

Borders (3)

Superior border — concave; has the suprascapular notch (covered by the transverse scapular ligament); the inferior belly of omohyoid originates near its lateral end
Medial (vertebral) border — slightly convex, thickened; site of serratus anterior insertion along its anterior costal surface
Lateral (axillary) border — thickened; contains the infraglenoid tubercle inferiorly (long head of triceps origin)

Angles (3)

Superior angle — levator scapulae inserts here
Inferior angle — the most mobile point; marks the level of T7–T8; serratus anterior holds it against the thorax
Lateral angle — bears the glenoid cavity (see below)

Processes (3)

Spine — triangular ridge on dorsal surface; trapezius attaches to its superior surface, deltoid to its inferior surface
Acromion — continuous with the lateral spine, overhangs the glenohumeral joint; articulates with the clavicle; may fail to fuse (os acromiale, 7–15%)
Coracoid process — projects anterosuperiorly; coracobrachialis + short head of biceps brachii arise from its tip; pectoralis minor from its medial aspect

Glenoid Cavity

Shallow, comma-shaped socket at the lateral angle
~6–8 cm² in adults, retroverted ~4–8°
Supraglenoid tubercle — long head of biceps brachii (intra-articular tendon)
Infraglenoid tubercle — long head of triceps brachii (extra-articular)

Scapula posterior view with landmarks including spine, supraspinous/infraspinous fossae, glenoid neck, and suprascapular notch

Muscle Attachments (17 total)

The scapula has 17 muscular attachments divided into axial stabilizers and scapulohumeral movers.

Muscle attachments of scapula, shoulder, and arm — anterior and posterior views

Posterior view scapula with labeled muscles, landmarks, and suprascapular notch

Axial Stabilizers (attach scapula to spine/chest wall)

Muscle	Origin → Insertion on Scapula	Action
Trapezius (descending)	Occiput/nuchal lig → lateral clavicle & spine	Elevates, upwardly rotates scapula
Trapezius (horizontal)	C7–T3 → acromion & lateral spine	Retracts scapula
Trapezius (ascending)	T3–T12 → medial spine	Retracts + depresses
Levator scapulae	C1–C4 transverse processes → superior angle	Elevates + rotates
Rhomboid minor/major	C6–C7 / T1–T4 spinous processes → medial border	Retraction + elevation
Serratus anterior	Ribs 1–10 → anterior surface of medial border	Protraction; upward rotation; holds medial border to thorax
Pectoralis minor	Ribs 3–5 → coracoid process	Protraction + depression

Scapulohumeral Muscles (rotator cuff + others)

Muscle	Origin	Action
Supraspinatus	Supraspinous fossa	Initiates abduction, assists ER
Infraspinatus	Infraspinous fossa	External rotation
Teres minor	Dorsolateral border	External rotation
Teres major	Inferior angle (posterior)	IR, adduction, extension
Subscapularis	Subscapular fossa	Internal rotation
Deltoid	Spine + acromion	Abduction (middle), flex/ext (anterior/posterior)
Biceps brachii (LH/SH)	Supraglenoid tubercle / coracoid	Flexion, supination
Triceps brachii (LH)	Infraglenoid tubercle	Extension
Coracobrachialis	Coracoid tip	Flexion/adduction of arm

Neurovascular Structures

Suprascapular nerve (C5–C6, from superior trunk of brachial plexus):

Passes under the transverse scapular ligament through the suprascapular notch → supplies supraspinatus
Winds around the spinoglenoid notch under the spinoglenoid ligament → supplies infraspinatus
The suprascapular artery passes over the transverse ligament (mnemonic: "navy over the bridge, army under")

Dorsal scapular nerve (C5): supplies levator scapulae and rhomboids, runs ~1–2 cm medial to the vertebral border

Long thoracic nerve (C5–C7): runs on the anterior surface of serratus anterior along the lateral thoracic wall — damage causes winged scapula (serratus anterior paralysis → medial border and inferior angle lift away from thorax)

Spinal accessory nerve (CN XI): innervates trapezius; travels with the superficial branch of the transverse cervical artery

Scapulothoracic Motion

The scapula moves through three axes (YXZ Euler sequence):

Internal/external rotation (Y-axis) = protraction / retraction
Upward/downward tilt (Z-axis, parallel to spine) = elevation / depression
Lateral/medial rotation (X-axis) = glenoid moving superolaterally or inferomedially

With full arm elevation the scapula elevates 10–12 cm and the inferior angle translates laterally 10–15 cm with ~45–60° of tilt. The classic scapulohumeral rhythm approximates 2:1 glenohumeral to scapulothoracic motion.

Clinical Relevance

Condition	Mechanism	Sign
Winged scapula	Long thoracic nerve injury (e.g., mastectomy, chest wall surgery, viral neuritis) → serratus anterior paralysis	Medial border + inferior angle protrude posteriorly, worsened on arm elevation or pushing against wall
Suprascapular nerve entrapment	Compression at suprascapular notch (e.g., ligament ossification in 10%) or spinoglenoid notch (ganglion cyst)	Shoulder pain + weakness of supraspinatus/infraspinatus; posterior shoulder atrophy
Os acromiale	Non-fusion of acromial ossification centers by age 25	7–15% incidence; can cause impingement or chronic shoulder pain
Scapular fractures	High-energy trauma (direct blow, humeral head impact, dislocation, violent muscular contracture) — associated with thoracic injuries; body and upper scapula most frequently affected

Sources: Gray's Anatomy for Students; Imaging Anatomy Vol. 3 (Bones, Joints, Vessels & Nerves); Rockwood & Green's Fractures in Adults, 10th ed.; Miller's Review of Orthopaedics, 9th ed.

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