Osteology

Orientation: S-shaped long bone; medial two-thirds convex anteriorly, lateral third concave anteriorly. Lies horizontally at the root of the neck.

Side determination:

• Medial (sternal) end is large, rounded, bulbous.
• Lateral (acromial) end is flat.
• Inferior surface has a groove for subclavius and roughened attachment areas.
• Whichever end is bulbous and which surface faces down/back determines side.

Major landmarks:

• Sternal end: articulates with manubrium (sternoclavicular joint) and 1st costal cartilage.
• Acromial end: articulates with acromion (acromioclavicular joint).
• Conoid tubercle: posterior border of lateral third, attachment for conoid ligament.
• Trapezoid line: lateral to conoid tubercle, attachment for trapezoid ligament.
• Subclavian groove: inferior surface, medial third, for subclavius muscle.
• Impression for costoclavicular ligament: inferior surface of sternal end.

Muscle attachments:

• Sternocleidomastoid (clavicular head), superior surface, medial third.
• Pectoralis major (clavicular head), anterior surface, medial two-thirds.
• Deltoid: anterior surface, lateral third.
• Trapezius: posterior surface, lateral third.
• Subclavius: subclavian groove.
• Sternohyoid: posterior surface, medial end.

Ligament attachments:

• Coracoclavicular ligament (conoid + trapezoid parts), inferior surface, lateral third.
• Costoclavicular ligament: inferior surface, sternal end.
• Interclavicular ligament and sternoclavicular capsule: sternal end.

Articulations: Sternoclavicular joint (medially), acromioclavicular joint (laterally).

Applied anatomy:

• Most commonly fractured bone in the body; typically at the junction of the middle and lateral thirds, the weakest point (transition between the two curvatures, and site where the bone changes from tubular to flattened cross-section).
• Medial fragment is pulled superiorly by sternocleidomastoid; lateral fragment drops due to the weight of the limb, and is pulled medially by pectoralis major.
• Subclavian vessels and the brachial plexus lie directly posterior/inferior, at risk in displaced fractures.

Orientation: Flat, triangular bone on the posterolateral thoracic wall, overlying ribs 2–7.

Side determination:

• Glenoid cavity points laterally.
• Spine is posterior and superior.
• Subscapular fossa (concave, costal surface) faces anteriorly against the ribs.

Major landmarks:

• Spine of scapula: divides posterior surface into supraspinous and infraspinous fossae; ends laterally as the acromion.
• Acromion: articulates with clavicle.
• Coracoid process: anterior projection below the clavicle; important muscular and ligamentous attachment site.
• Glenoid cavity (fossa): shallow, pear-shaped, articulates with humeral head.
• Supraglenoid tubercle: origin of long head of biceps.
• Infraglenoid tubercle: origin of long head of triceps.
• Suprascapular notch: medial to coracoid base, transmits suprascapular nerve (artery passes over the superior transverse scapular ligament, nerve passes under it, mnemonic "Army over, Navy under").
• Spinoglenoid notch: lateral, transmits suprascapular nerve and vessels to the infraspinous fossa.
• Superior, medial (vertebral), and lateral (axillary) borders; superior, inferior angles.

Muscle attachments:

• Supraspinatus: supraspinous fossa.
• Infraspinatus: infraspinous fossa.
• Subscapularis: subscapular fossa.
• Teres minor: upper two-thirds, lateral border (dorsal surface).
• Teres major: lower third, lateral border, and inferior angle.
• Deltoid: spine (posterior lip) and acromion.
• Trapezius: spine (superior lip) and acromion.
• Long head of biceps brachii: supraglenoid tubercle.
• Long head of triceps brachii: infraglenoid tubercle.
• Coracobrachialis, short head of biceps: coracoid process (tip).
• Pectoralis minor: coracoid process (medial border/superior surface).
• Levator scapulae, rhomboid minor/major: medial border.
• Serratus anterior: costal surface, medial border.
• Omohyoid: superior border, near suprascapular notch.

Ligament attachments:

• Coracoacromial ligament: coracoid process to acromion.
• Coracoclavicular ligament: coracoid process.
• Glenohumeral ligaments and glenoid labrum: margin of glenoid cavity.
• Superior transverse scapular ligament: bridges suprascapular notch.

Articulations: Glenohumeral joint (glenoid cavity), acromioclavicular joint (acromion).

Applied anatomy:

• Winged scapula: long thoracic nerve injury paralyses serratus anterior; medial border protrudes posteriorly, especially on pushing against resistance.
• Scapular fractures are uncommon (well protected by surrounding muscle) and usually indicate high-energy trauma.
• Suprascapular nerve entrapment at the suprascapular or spinoglenoid notch can cause supraspinatus/infraspinatus weakness.

Orientation: Long bone of the arm; head faces medially, superiorly, and posteriorly.

Side determination:

• Head is medial; capitulum (lateral distal condyle) is lateral.
• Radial groove runs obliquely, posteroinferior to superior on the shaft.
• Deltoid tuberosity is on the lateral surface.

Major landmarks:

• Head: articulates with glenoid cavity; roughly one-third of a sphere.
• Anatomical neck: groove immediately distal to head, site of capsular attachment.
• Greater tubercle: lateral, site of insertion for supraspinatus, infraspinatus, teres minor (superior to inferior facets).
• Lesser tubercle: anterior, insertion for subscapularis.
• Intertubercular (bicipital) sulcus: between tubercles, transmits the long head of biceps tendon; bordered by pectoralis major (lateral lip), latissimus dorsi (floor), teres major (medial lip).
• Surgical neck: narrowing below the tubercles; common fracture site; axillary nerve and posterior circumflex humeral vessels are closely applied here.
• Deltoid tuberosity: lateral surface, mid-shaft, insertion of deltoid.
• Radial (spiral) groove: posterior surface, oblique; transmits radial nerve and profunda brachii vessels.
• Medial and lateral supracondylar ridges: distal shaft, converge on the epicondyles.
• Medial epicondyle: larger, more prominent; common flexor origin; ulnar nerve passes directly posterior to it.
• Lateral epicondyle: common extensor origin.
• Capitulum: lateral, rounded, articulates with radial head.
• Trochlea: medial, pulley-shaped, articulates with trochlear notch of ulna.
• Coronoid fossa (anterior, above trochlea), radial fossa (anterior, above capitulum), olecranon fossa (posterior, above trochlea), accommodate corresponding processes in elbow flexion/extension.

Muscle attachments:

• Supraspinatus, infraspinatus, teres minor: greater tubercle.
• Subscapularis: lesser tubercle.
• Pectoralis major: lateral lip of intertubercular sulcus.
• Latissimus dorsi: floor of intertubercular sulcus.
• Teres major: medial lip of intertubercular sulcus.
• Deltoid: deltoid tuberosity.
• Coracobrachialis: medial shaft, mid-humerus.
• Brachialis: distal anterior shaft.
• Triceps (lateral and medial heads): posterior shaft, above and below the radial groove respectively.
• Common flexor origin (pronator teres, FCR, palmaris longus, FCU, FDS): medial epicondyle.
• Common extensor origin (ECRB, EDC, EDM, ECU, supinator part): lateral epicondyle.
• Brachioradialis, extensor carpi radialis longus: lateral supracondylar ridge.

Important nerve relationships (high-yield):

• Axillary nerve: surgical neck (with posterior circumflex humeral vessels); injury from surgical neck fractures or anterior shoulder dislocation.
• Radial nerve: radial groove, mid-shaft posteriorly; injury from mid-shaft humeral fractures causes wrist drop.
• Ulnar nerve: posterior to medial epicondyle; injury from medial epicondyle fractures causes ulnar claw/sensory loss.
• Median nerve and brachial artery: anterior to elbow, related to supracondylar region; at risk in supracondylar fractures.

Applied anatomy:

• Surgical neck fracture → axillary nerve injury → deltoid paralysis, regimental badge sensory loss.
• Mid-shaft fracture → radial nerve injury → wrist drop, sensory loss over first dorsal web space.
• Supracondylar fracture (common in children) → risk to brachial artery and median nerve; can cause Volkmann ischaemic contracture.
• Medial epicondyle fracture/avulsion → ulnar nerve injury.

Orientation: Lateral forearm bone; proximal end small (head), distal end large.

Side determination:

• Head is proximal and small; styloid process is distal and lateral.
• Radial tuberosity faces medially, just distal to the head.
• Concave anterior surface with interosseous border facing the ulna.

Major landmarks:

• Head: disc-shaped, articulates with capitulum (proximally) and radial notch of ulna (medially).
• Neck: narrowing below the head.
• Radial tuberosity: posteromedial, distal to neck; insertion of biceps brachii.
• Interosseous border: medial shaft, attachment of interosseous membrane.
• Pronator tuberosity: lateral shaft, mid-forearm; insertion of pronator teres.
• Distal end: carries the ulnar notch (medially, for distal radio-ulnar joint), dorsal (Lister) tubercle (posterior, grooves extensor tendons), and styloid process (lateral, distal-most point, larger and more distal than the ulnar styloid).
• Carpal articular surface: concave, articulates with scaphoid and lunate.

Muscle attachments:

• Biceps brachii: radial tuberosity.
• Supinator: proximal shaft (posterolateral).
• Pronator teres: pronator tuberosity, lateral shaft.
• Flexor digitorum superficialis, flexor pollicis longus: anterior shaft.
• Pronator quadratus: distal anterior shaft.
• Abductor pollicis longus, extensor pollicis brevis: posterior shaft (pass over it distally).
• Brachioradialis: base of styloid process.

Ligament attachments: Interosseous membrane (medial border), annular ligament (encircles head), radial collateral ligament of elbow (attaches to annular ligament, not directly to radius), triangular fibrocartilage complex (ulnar notch region).

Articulations: Elbow (capitulum), superior radio-ulnar (radial notch of ulna), inferior radio-ulnar (ulnar notch), wrist (scaphoid, lunate).

Applied anatomy:

• Colles fracture: distal radius fracture with dorsal displacement/angulation, classically from a fall on an outstretched hand ("dinner fork deformity").
• Radial head fracture: from fall on outstretched hand with forearm pronated; may limit pronation/supination.
• Radial styloid is a key surface landmark forming the lateral boundary of the anatomical snuffbox.

Orientation: Medial forearm bone; proximal end large (olecranon), distal end small (head).

Side determination:

• Olecranon and trochlear notch are proximal; head and styloid are distal (opposite pattern to radius).
• Trochlear notch faces anteriorly.

Major landmarks:

• Olecranon: proximal, posterior projection; forms the point of the elbow; receives triceps insertion.
• Coronoid process: anterior projection below the trochlear notch.
• Trochlear (semilunar) notch: formed by olecranon and coronoid process; articulates with the trochlea of humerus.
• Radial notch: lateral side of coronoid process; articulates with radial head.
• Ulnar tuberosity: anterior, distal to coronoid process; insertion of brachialis.
• Supinator crest: posterolateral shaft, below radial notch.
• Interosseous border: lateral shaft.
• Head: small, distal, rounded; articulates with ulnar notch of radius.
• Styloid process: posteromedial, distal-most point (does not participate in the wrist joint proper, separated from carpus by the triangular fibrocartilage complex).

Muscle attachments:

• Triceps brachii: olecranon.
• Anconeus: lateral surface of olecranon.
• Brachialis: ulnar tuberosity and coronoid process.
• Flexor digitorum superficialis, flexor carpi ulnaris, pronator teres (ulnar head): coronoid process region.
• Supinator: supinator crest.
• Flexor digitorum profundus: anteromedial shaft.
• Extensor carpi ulnaris, extensor digiti minimi, abductor pollicis longus, extensor pollicis longus: posterior shaft.
• Pronator quadratus: distal anterior shaft.

Ligament attachments: Interosseous membrane (lateral border), ulnar collateral ligament of elbow (medial coronoid/olecranon), annular ligament (attaches at radial notch margins).

Articulations: Elbow (trochlear notch, radial notch), distal radio-ulnar joint (head).

Applied anatomy:

• Olecranon fracture: often from a direct fall onto the elbow or forceful triceps contraction; can impair active elbow extension.
• Monteggia fracture: proximal ulna fracture with radial head dislocation.
• Ulnar shaft is subcutaneous along its length: easily palpated, common site of direct-blow fractures ("nightstick fracture").

Eight bones in two rows (mnemonic: "Some Lovers Try Positions That They Cannot Handle").

Proximal row (lateral to medial): Scaphoid, Lunate, Triquetrum, Pisiform (pisiform sits anterior to triquetrum, functioning as a sesamoid bone within FCU tendon).

Distal row (lateral to medial): Trapezium, Trapezoid, Capitate, Hamate.

Important individual landmarks:

• Scaphoid: boat-shaped; largest bone of the proximal row; forms the floor of the anatomical snuffbox; tubercle is a key palpable landmark.
• Lunate: moon-shaped; most commonly dislocated carpal bone.
• Triquetrum: pyramidal; articulates with the triangular fibrocartilage complex.
• Pisiform: small, pea-shaped sesamoid within flexor carpi ulnaris tendon; forms medial boundary of Guyon canal.
• Trapezium: articulates with the first metacarpal at a saddle joint, enabling thumb opposition.
• Trapezoid: smallest bone of the distal row.
• Capitate: largest carpal bone; central, head articulates with the lunate.
• Hamate: has a prominent hook (hamulus) projecting anteriorly; forms medial boundary of the carpal tunnel and lateral boundary of Guyon canal.

Scaphoid blood supply:

• Supplied predominantly by branches of the radial artery, entering retrograde: from distal to proximal.
• The proximal pole is the last to receive blood and has the poorest supply.
• Waist fractures (most common site) risk avascular necrosis of the proximal fragment because retrograde flow is interrupted.
• Clinically: snuffbox tenderness after a fall on an outstretched hand should raise suspicion of scaphoid fracture even with normal initial X-rays (occult fracture is common, repeat imaging or MRI if suspicion persists).

Carpal arch:

• The concave anterior surface of the carpal bones forms the carpal arch (carpal tunnel floor and walls).
• Bridged anteriorly by the flexor retinaculum, converting the arch into the carpal tunnel.
• Bony attachments of the retinaculum: scaphoid and trapezium tubercles (lateral), pisiform and hook of hamate (medial).

Five bones (I–V, thumb to little finger), each with a base (proximal, carpal articulation), shaft, neck, and head (distal, rounded, articulates with proximal phalanx).

• 1st metacarpal: shortest and stoutest; independently mobile at a saddle-shaped CMC joint (trapezium), enabling opposition; not part of a common CMC articulation like II–V.
• 2nd–5th metacarpal bases: articulate with each other and with the distal carpal row (relatively fixed, especially II and III).
• Metacarpal heads form the knuckles; collateral ligaments are taut in flexion (relevant to splinting position, MCP joints splinted in flexion to prevent collateral ligament shortening).
• Interossei attach along adjacent shafts.

Fourteen phalanges per hand: proximal, middle, and distal in digits II–V; the thumb (digit I) has only proximal and distal phalanges (no middle phalanx).

• Each phalanx has a base, shaft, and head (trochlea).
• Distal phalanges have a roughened distal tuberosity supporting the nail bed and pulp.
• Middle and proximal phalanx heads are pulley-shaped for hinge motion at PIP/DIP joints.
• Flexor and extensor tendon attachments (flexor digitorum superficialis to middle phalanx base, flexor digitorum profundus to distal phalanx base, extensor expansion to middle and distal phalanges) are covered under the Hand section.