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Scapula bone

The scapula, commonly known as the shoulder blade, is a large, flat, triangular bone situated in the upper back. It forms a vital component of the pectoral girdle, providing attachment sites for numerous muscles involved in shoulder and upper limb movement. Its complex anatomy and relationships with surrounding structures make it crucial to both anatomical study and clinical practice.

Overview of the Scapula

Definition and General Description

The scapula is a paired, flat bone located on the posterior aspect of the thoracic wall, extending from the second to the seventh ribs. It connects the upper limb to the axial skeleton through articulations with the clavicle and humerus. The bone serves as a strong yet flexible structure that supports shoulder motion and stability, acting as both a site of muscle attachment and a lever for limb movement.

Location and Orientation

Each scapula lies on the postero-lateral surface of the thoracic cage, covering portions of the second to seventh ribs. It is oriented obliquely, with its medial border nearly parallel to the vertebral column and its lateral angle directed outward to articulate with the humerus at the glenoid cavity. The scapula is tilted slightly forward, ensuring the glenoid cavity faces anterolaterally, facilitating a wide range of upper limb movements.

Historical and Anatomical Significance

The term “scapula” originates from Latin, referring to the shoulder or shoulder blade. Anatomically, the scapula plays an essential role in maintaining the functional integrity of the shoulder girdle, enabling coordinated movements such as elevation, rotation, and abduction of the arm. Its structure has been studied extensively due to its involvement in various orthopedic and neuromuscular conditions affecting the upper extremity.

Gross Anatomy

Shape and Position

The scapula is triangular in shape with three borders, three angles, and two main surfaces. It lies against the posterior thoracic wall and moves dynamically with shoulder motion. The bone’s thin, flat structure provides both strength and mobility, adapting well to the complex demands of the shoulder joint.

Borders and Angles

The borders and angles of the scapula define its geometry and serve as key anatomical landmarks for muscle and ligament attachment.

Superior border: The shortest and thinnest border, extending between the superior angle and the base of the coracoid process. It features the suprascapular notch, which transmits the suprascapular nerve.
Medial (vertebral) border: Extends from the superior to the inferior angle, running parallel to the vertebral column. It provides attachment to the levator scapulae, rhomboid minor, and rhomboid major muscles.
Lateral (axillary) border: Thick and slightly curved, extending from the inferior angle to the glenoid cavity. It gives rise to the long head of the triceps brachii and forms part of the glenoid rim.
Superior angle: Located at the junction of the superior and medial borders, serving as an attachment for the levator scapulae muscle.
Inferior angle: The most inferior point of the bone, formed by the junction of the medial and lateral borders. It serves as a key landmark for scapular movements such as rotation and is the attachment site for the teres major muscle.
Lateral angle: The thickest part of the scapula, bearing the glenoid cavity that articulates with the head of the humerus.

Surfaces

Costal (anterior) surface: Concave and smooth, facing the thoracic wall. It contains the subscapular fossa, which provides origin to the subscapularis muscle. Numerous ridges on this surface accommodate muscle tendons and fascial attachments.
Dorsal (posterior) surface: Convex and divided by the spine of the scapula into the supraspinous and infraspinous fossae. These fossae serve as sites of origin for the supraspinatus and infraspinatus muscles, respectively.

Processes and Notches

Spine of the scapula: A prominent ridge that runs transversely across the dorsal surface, separating the supraspinous and infraspinous fossae. It terminates laterally in the acromion process.
Acromion process: The lateral extension of the spine that projects over the shoulder joint. It articulates with the clavicle at the acromioclavicular joint and provides attachment for the deltoid and trapezius muscles.
Coracoid process: A hook-like projection on the superior border of the scapula. It serves as an attachment for several muscles, including the pectoralis minor, short head of biceps brachii, and coracobrachialis, as well as for coracoacromial and coracoclavicular ligaments.
Suprascapular notch: A depression on the superior border near the root of the coracoid process that allows passage of the suprascapular nerve beneath the superior transverse scapular ligament.

Fossae and Important Features

Supraspinous fossa: Located above the spine of the scapula; serves as the origin for the supraspinatus muscle.
Infraspinous fossa: Found below the spine; provides origin to the infraspinatus muscle.
Subscapular fossa: Occupies the anterior surface of the scapula; provides origin to the subscapularis muscle.
Glenoid cavity: A shallow, pear-shaped depression on the lateral angle of the scapula that articulates with the head of the humerus to form the glenohumeral joint.
Supraglenoid and infraglenoid tubercles: Small projections above and below the glenoid cavity, serving as origins for the long head of the biceps brachii and long head of the triceps brachii, respectively.

Osteological Landmarks

Attachments for Muscles

The scapula serves as an essential anchoring site for multiple muscles that control the movement and stabilization of the shoulder joint. These attachments are distributed across its borders, angles, fossae, and processes, providing a structural framework for dynamic motion.

Supraspinous fossa: Origin of the supraspinatus muscle.
Infraspinous fossa: Origin of the infraspinatus muscle.
Subscapular fossa: Origin of the subscapularis muscle.
Lateral border: Gives origin to the teres minor and part of the long head of the triceps brachii.
Inferior angle: Attachment for the teres major muscle.
Medial border: Attachment for the serratus anterior (anteriorly) and the rhomboids and levator scapulae (posteriorly).
Spine and acromion process: Provide insertion for the trapezius and origin for the deltoid muscle.
Coracoid process: Attachment for the short head of biceps brachii, coracobrachialis, and pectoralis minor.

Ligamentous Attachments

Several ligaments attach to the scapula, contributing to the stability of the shoulder girdle and limiting excessive movement. These ligaments connect the scapula to the clavicle, coracoid, and acromion processes, forming key structural connections.

Coracoacromial ligament: Extends between the acromion and coracoid process, forming an arch over the shoulder joint.
Coracoclavicular ligament: Comprising the conoid and trapezoid parts, it connects the clavicle to the coracoid process, maintaining the integrity of the acromioclavicular joint.
Superior transverse scapular ligament: Bridges the suprascapular notch, converting it into a foramen for the suprascapular nerve.
Inferior transverse scapular ligament: Stretches across the spinoglenoid notch, stabilizing the suprascapular artery and nerve.

Articulations

The scapula articulates with neighboring bones and forms both anatomical and functional joints that enable smooth movement of the upper limb.

Glenohumeral joint: A ball-and-socket synovial joint between the glenoid cavity of the scapula and the head of the humerus. It allows a wide range of motion, including rotation, abduction, and flexion.
Acromioclavicular joint: A plane-type synovial joint between the acromion of the scapula and the lateral end of the clavicle. It provides gliding movements and maintains alignment of the shoulder girdle.
Scapulothoracic articulation: A functional joint formed by the scapula gliding over the thoracic wall, enabling scapular rotation, elevation, and protraction.

Muscle Attachments

Muscles Originating from the Scapula

Numerous muscles arise from the scapula, contributing to the control and movement of the shoulder and arm. These muscles are critical for joint stabilization, power generation, and coordination of upper limb motion.

Supraspinatus: Originates from the supraspinous fossa; initiates abduction of the arm.
Infraspinatus: Arises from the infraspinous fossa; responsible for lateral rotation of the arm.
Subscapularis: Originates from the subscapular fossa; acts as a medial rotator of the arm.
Teres minor: Originates from the lateral border; assists in lateral rotation and stabilization of the humeral head.
Teres major: Arises from the inferior angle; functions in adduction and medial rotation of the arm.
Deltoid: Originates partially from the spine and acromion; responsible for abduction and flexion of the shoulder.
Coracobrachialis: Originates from the coracoid process; assists in flexion and adduction of the arm.
Biceps brachii (short head): Arises from the coracoid process; flexes the elbow and supinates the forearm.
Biceps brachii (long head): Arises from the supraglenoid tubercle; contributes to shoulder and elbow flexion.

Muscles Inserting into the Scapula

Several muscles insert into the scapula, enabling movements such as elevation, depression, and retraction. These insertions are crucial for stabilizing the scapula during upper limb activity.

Trapezius: Inserts along the spine of the scapula and acromion; elevates and retracts the scapula.
Levator scapulae: Inserts at the superior angle and medial border; elevates the scapula.
Rhomboid major: Inserts along the medial border; retracts and rotates the scapula downward.
Rhomboid minor: Inserts at the root of the scapular spine; assists in scapular retraction.
Serratus anterior: Inserts along the costal surface of the medial border; protracts and rotates the scapula upward, maintaining its position against the thoracic wall.
Pectoralis minor: Inserts at the coracoid process; draws the scapula forward and downward during movement of the upper limb.

Blood Supply and Venous Drainage

Arterial Supply

The scapula receives its arterial blood from a rich anastomotic network formed by branches of the subclavian and axillary arteries. This collateral circulation ensures consistent perfusion even if one of the primary vessels becomes compromised. The arteries supplying the scapula contribute to the scapular anastomosis, which plays a vital role in maintaining blood flow to the upper limb.

Suprascapular artery: A branch of the thyrocervical trunk (from the subclavian artery), it supplies the supraspinous and infraspinous regions of the scapula.
Dorsal scapular artery: Originating from the subclavian artery or its branches, it runs along the medial border of the scapula and supplies the rhomboid and levator scapulae muscles.
Subscapular artery: A branch of the third part of the axillary artery, it gives rise to the circumflex scapular artery, which supplies the subscapular fossa and the posterior surface of the scapula.
Circumflex scapular artery: Passes through the triangular space to anastomose with the suprascapular and dorsal scapular arteries, forming a key part of the scapular collateral network.

Venous Drainage

Venous return from the scapula parallels its arterial supply. Veins draining the scapular region merge into larger veins associated with the subclavian and axillary systems, ensuring efficient removal of deoxygenated blood.

Suprascapular vein: Accompanies the suprascapular artery and drains into the external jugular or subclavian vein.
Dorsal scapular vein: Drains into the subclavian vein, accompanying the dorsal scapular artery along the medial border.
Subscapular vein: Empties into the axillary vein, draining the subscapular region and posterior surface of the scapula.

Scapular Anastomosis

The scapular anastomosis is a vital arterial network that provides collateral circulation between the subclavian and axillary arteries. This anastomotic arrangement ensures continuous perfusion to the shoulder and upper limb, even during arterial obstruction or movement-induced compression.

Formed mainly by the suprascapular, dorsal scapular, and circumflex scapular arteries.
Provides an alternative route for blood flow if the axillary artery is compressed or occluded.
Ensures vascular continuity between anterior and posterior shoulder regions.

Nerve Supply

Motor Innervation

The scapula and its associated muscles receive motor innervation from several nerves that arise from the brachial plexus. These nerves coordinate the complex range of movements of the shoulder and upper limb, ensuring stability and precision in muscle contraction.

Suprascapular nerve: Arises from the upper trunk of the brachial plexus (C5–C6); supplies the supraspinatus and infraspinatus muscles and passes through the suprascapular notch.
Dorsal scapular nerve: Originates from the C5 root of the brachial plexus; supplies the rhomboid major, rhomboid minor, and levator scapulae.
Subscapular nerves: Derived from the posterior cord of the brachial plexus; the upper and lower subscapular nerves supply the subscapularis and teres major muscles, respectively.
Axillary nerve: Arises from the posterior cord (C5–C6); supplies the deltoid and teres minor muscles, and contributes to shoulder joint sensation.
Long thoracic nerve: Originates from roots C5–C7; innervates the serratus anterior, essential for scapular protraction and upward rotation.

Sensory Innervation

Sensory innervation to the scapular region is derived primarily from cutaneous branches of cervical and thoracic nerves. These nerves provide sensation to the overlying skin and contribute to proprioceptive feedback from the shoulder region.

Supraclavicular nerves: Branches of the cervical plexus (C3–C4) that supply the upper region of the shoulder and supraclavicular area.
Cutaneous branches of intercostal nerves: Provide sensation to the skin covering the posterior and lateral thoracic regions near the scapula.
Posterior rami of thoracic spinal nerves: Supply the deep muscles and skin overlying the vertebral and medial scapular borders.

Functional Relevance

The coordination of motor and sensory innervation ensures that scapular movement remains smooth and well-regulated. Damage to these nerves, such as in the case of long thoracic or suprascapular nerve injury, can result in winged scapula or weakness in shoulder elevation, highlighting the importance of neural integrity for normal shoulder function.

Relations and Neighboring Structures

Anterior Relations

The anterior or costal surface of the scapula lies adjacent to the thoracic wall, separated by the subscapularis muscle and a thin film of loose connective tissue that facilitates smooth gliding motion. The serratus anterior muscle intervenes between the scapula and the ribs, forming a dynamic interface for scapulothoracic movement.

The subscapularis muscle covers the anterior surface, filling the subscapular fossa.
The serratus anterior separates the scapula from the thoracic wall, particularly over ribs two to seven.
The thoracic cage forms the bony base beneath the scapula, accommodating respiratory motion during scapular gliding.
The subscapular bursa lies between the scapula and serratus anterior, reducing friction during shoulder movements.

Posterior Relations

The posterior surface of the scapula is subcutaneous in most of its extent, especially along the spine and acromion, making these landmarks easily palpable. Numerous muscles attach to the dorsal aspect, forming the bulk of the shoulder contour.

The trapezius overlies the superior portion of the scapular spine and acromion process.
The deltoid muscle covers the lateral border and acromion, forming the rounded prominence of the shoulder.
The supraspinatus and infraspinatus muscles occupy their respective fossae on the posterior surface.
The rhomboids and levator scapulae lie medially, bridging the scapula with the vertebral column.

Superior and Inferior Relations

Superiorly, the scapula is related to the clavicle and associated ligaments forming the acromioclavicular joint. Inferiorly, it lies adjacent to the muscles of the posterior thoracic wall, maintaining structural continuity with the trunk.

Superiorly: Related to the clavicle, coracoclavicular ligaments, and suprascapular artery and nerve passing near the superior border.
Inferiorly: Related to the latissimus dorsi and teres major muscles, which attach near the inferior angle.
Lateral relation: Involves the glenohumeral joint and the humeral head, allowing articulation and free movement of the upper limb.

Development and Ossification

Embryological Origin

The scapula develops from mesenchymal condensations derived from the paraxial mesoderm and lateral plate mesoderm. It begins forming during the fifth week of embryonic life and contributes to the pectoral girdle that supports the upper limb. The body of the scapula originates from intramembranous ossification, while its processes form through endochondral ossification.

Ossification Centers

The scapula has one primary ossification center and several secondary centers that appear and fuse over time to form the mature bone. These centers contribute to different parts of the scapula and help establish its complex shape.

Primary center: Appears around the eighth week of intrauterine life for the body of the scapula.
Secondary centers: Appear for the acromion, coracoid process, glenoid cavity, inferior angle, and medial border during later childhood and adolescence.

Timeline of Ossification

The sequence and timing of ossification vary among individuals but generally follow a predictable developmental pattern. The primary center forms early, whereas secondary centers appear progressively during postnatal growth.

The coracoid process ossifies during the first year after birth and fuses with the body around puberty.
The acromion ossifies between 14 and 20 years of age, occasionally showing delayed fusion or persistent separate ossification (os acromiale).
The glenoid cavity and inferior angle ossify during adolescence, with complete fusion by the third decade.

Fusion and Maturation

Complete fusion of all ossification centers occurs by approximately 22 to 25 years of age, resulting in a fully mature scapula. Failure of fusion of secondary centers, particularly in the acromion or coracoid, can lead to anatomical variations that may predispose individuals to shoulder pain or impingement syndromes.

Articulations and Movements

Scapulohumeral Rhythm

The scapulohumeral rhythm refers to the coordinated movement between the scapula and the humerus during arm elevation. It ensures smooth, efficient motion of the upper limb while maintaining stability of the glenohumeral joint. For every 3 degrees of arm elevation, approximately 2 degrees occur at the glenohumeral joint and 1 degree at the scapulothoracic articulation. This coordinated action is vital for complete range of motion in abduction and flexion of the shoulder.

Maintains optimal alignment between the humeral head and glenoid cavity.
Prevents impingement of soft tissues beneath the acromion.
Allows balanced distribution of movement between the scapula and humerus.

Movements at the Shoulder Girdle

The scapula participates in multiple movements that contribute to upper limb mobility. These movements occur at the scapulothoracic articulation through the coordinated action of surrounding muscles.

Elevation: Upward movement of the scapula, as in shrugging the shoulders. Muscles involved include the trapezius (upper fibers) and levator scapulae.
Depression: Downward movement of the scapula, aided by the lower fibers of the trapezius, pectoralis minor, and latissimus dorsi.
Protraction: Forward movement of the scapula along the thoracic wall, produced mainly by the serratus anterior and pectoralis minor.
Retraction: Backward movement of the scapula toward the vertebral column, produced by the trapezius (middle fibers) and rhomboid muscles.
Upward rotation: Movement of the inferior angle laterally and upward during arm elevation, performed by the serratus anterior and upper and lower fibers of the trapezius.
Downward rotation: Movement of the inferior angle medially and downward during arm lowering, carried out by the rhomboids, levator scapulae, and pectoralis minor.

Muscular Coordination in Movements

The precise motion of the scapula results from the synergistic action of multiple muscles working in harmony. This coordination ensures stable positioning of the glenoid cavity and prevents mechanical impingement during upper limb movements.

Force couple mechanism: The upward rotation during arm elevation results from the combined pull of the upper trapezius, lower trapezius, and serratus anterior.
Stabilizing muscles: The rhomboids and levator scapulae maintain scapular position during resisted motions or when carrying loads.
Dynamic stability: The rotator cuff muscles stabilize the humeral head while scapular stabilizers control glenoid positioning.

Surface Anatomy and Palpation

Palpable Landmarks

Due to its superficial position, several parts of the scapula are readily palpable through the skin. These bony landmarks assist clinicians and anatomists in assessing shoulder alignment, detecting deformities, and locating muscular attachments.

Spine of the scapula: Prominent ridge palpable across the posterior shoulder, marking the division between supraspinous and infraspinous fossae.
Acromion process: The most lateral and superior projection, easily felt at the tip of the shoulder.
Medial (vertebral) border: Palpable when the arm is crossed over the chest, extending from the superior to the inferior angle.
Inferior angle: The lowest point of the scapula, palpable at the level of the seventh thoracic vertebra and moves during scapular rotation.
Coracoid process: Palpable in the deltopectoral groove, deep to the clavicle and pectoralis major, serving as a key landmark in shoulder surgeries.
Suprascapular notch and glenoid cavity: Not directly palpable but located deep beneath the trapezius and deltoid, respectively.

Clinical Relevance of Surface Anatomy

Knowledge of scapular surface anatomy is essential for clinical assessment, imaging interpretation, and surgical interventions. The ability to identify scapular landmarks aids in diagnosis and localization of injuries.

Used as reference points during orthopedic examinations, such as assessing scapular winging or shoulder asymmetry.
Important for needle placement during injections into the subacromial or glenohumeral spaces.
Serves as a guide for radiographic positioning and detection of fractures or dislocations.
Essential in reconstructive and arthroscopic shoulder procedures to ensure proper anatomical orientation.

Applied Anatomy and Clinical Correlations

Fractures of the Scapula

Scapular fractures are relatively uncommon due to the bone’s protective position and mobility. When they occur, they are often associated with high-energy trauma such as road accidents or falls from height. Most fractures involve the body, neck, or acromion of the scapula and may accompany rib or clavicular fractures.

Body fractures: Usually minimally displaced due to muscular support and treated conservatively with immobilization.
Acromion fractures: May impair deltoid function or lead to impingement if displaced downward.
Glenoid fractures: Can disrupt the glenohumeral joint, requiring surgical fixation for joint stability.
Coracoid fractures: Associated with acromioclavicular injuries or traction from attached muscles.

Imaging techniques such as X-rays and CT scans are essential for diagnosis. Most scapular fractures heal well with conservative management, though complex intra-articular fractures may need surgical intervention.

Winged Scapula

Winged scapula is a condition characterized by the prominent projection of the medial border of the scapula away from the thoracic wall. It results from paralysis or weakness of the serratus anterior muscle due to injury to the long thoracic nerve.

Common causes include trauma, surgical injury (e.g., axillary dissection), or viral neuropathy.
Clinically, the medial border of the scapula protrudes posteriorly, especially when the patient pushes against a wall.
This deformity impairs abduction of the arm above the head due to defective upward rotation of the scapula.

Management depends on the underlying cause and may include physiotherapy, nerve repair, or muscle transfer surgery in chronic cases.

Scapulothoracic Bursitis and Snapping Scapula

Scapulothoracic bursitis, also known as snapping scapula syndrome, occurs due to inflammation of the bursae that lie between the scapula and the thoracic wall. It leads to pain, crepitus, or audible snapping during scapular motion.

Common causes include overuse injuries, postural deformities, or abnormal bony prominences.
Symptoms include localized pain and a grating sensation on shoulder movement.
Treatment involves rest, anti-inflammatory medications, physical therapy, and in refractory cases, surgical bursectomy or scapular resection.

Shoulder Girdle Dysfunction

Dysfunction of the shoulder girdle may arise from muscular imbalance, nerve injury, or degenerative changes affecting the scapula and its articulations. These can lead to limited range of motion, instability, and pain.

Muscular causes: Weakness of stabilizing muscles such as serratus anterior or trapezius leads to abnormal scapular motion (scapular dyskinesis).
Nerve-related causes: Involvement of the dorsal scapular, suprascapular, or long thoracic nerves alters muscle tone and control.
Degenerative changes: Osteophyte formation or arthritic changes in the acromioclavicular joint may impinge on nearby soft tissues.

Rehabilitation focuses on strengthening scapular stabilizers, correcting posture, and restoring coordinated movement patterns.

Radiological Anatomy

Radiographic evaluation of the scapula is vital for diagnosing fractures, dislocations, and joint pathologies. The bone can be visualized using different imaging modalities, each providing unique anatomical insights.

X-ray (anteroposterior and lateral views): Commonly used for detecting fractures and assessing joint integrity.
Computed tomography (CT): Offers detailed visualization of complex fractures and 3D reconstruction for surgical planning.
Magnetic resonance imaging (MRI): Useful for evaluating soft tissue attachments, bursae, and muscular injuries.
Ultrasound: Helpful in dynamic assessment of scapulothoracic motion and detecting bursitis or effusion.

Comparative and Evolutionary Anatomy

Scapula in Quadrupeds vs Humans

The scapula exhibits distinct structural adaptations across species based on locomotor requirements. In humans, it is oriented to facilitate a wide range of arm movements, while in quadrupeds it is optimized for support and propulsion during walking or running.

Feature	Human Scapula	Quadruped Scapula
Orientation	Placed posteriorly and laterally for free upper limb motion	Vertically positioned along the side of the thorax for limb support
Spine of scapula	Prominent and horizontal	More oblique or vertical to support weight transmission
Glenoid cavity	Faces laterally to articulate with the humerus	Faces downward and laterally to support forelimb extension
Coracoid process	Well developed, muscular attachment for pectoral muscles	Often fused or reduced in some species

Functional Adaptations

The evolutionary modifications of the scapula correspond to the functional demands of movement and posture. In primates, the flattened and broad scapula allows extensive rotation of the arm, enabling climbing and grasping. In contrast, terrestrial mammals possess narrow scapulae that optimize forelimb stabilization during locomotion. These differences illustrate how the scapula’s morphology evolved to balance strength, mobility, and energy efficiency in different species.

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