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Anatomy · Model structures

Wrist and finger anatomy

This page lists the 141 structures of the MSK Atlas interactive 3D model of the right wrist and middle finger, with the teaching note for each one and, where the card gives them, origin, insertion and function.

The structures are split between the two plates of the model, Wrist and Finger, and grouped by category as in the model’s list. In the model you can rotate, zoom, show or hide categories and tap a structure to open its card. The name of each structure opens the model with that structure already selected, isolated with the bones and centered.

Open the 3D wrist model Open the 3D finger model

Wrist 102 structures

Open the 3D wrist model

Bones 16

Radius (distal end)

Scaphoid and lunate facets; radial inclination about 22°, palmar tilt about 11°. Dorsally, Lister tubercle separates the 2nd extensor compartment from the 3rd; on the ulnar side the sigmoid notch articulates with the ulnar head (DRUJ).

Ulna (head and styloid)

The head articulates with the sigmoid notch of the radius; the TFCC attaches to the fovea and the base of the styloid. Positive ulnar variance predisposes to ulnocarpal impaction (in the model it is close to neutral).

Scaphoid

Links the two carpal rows. Retrograde blood supply from the distal pole: fractures of the proximal pole and waist are at risk of nonunion and avascular necrosis. The palmar tubercle gives attachment to the flexor retinaculum.

Lunate

Keystone of the proximal row, stabilized by the scapholunate and lunotriquetral ligaments. Site of Kienböck disease; its alignment with the capitate is assessed on the lateral radiograph (DISI/VISI).

Triquetrum

Articulates with the lunate, hamate and pisiform. Dorsal avulsion fracture (dorsal chip) is common after falls on the outstretched hand.

Pisiform

Sesamoid bone in the flexor carpi ulnaris tendon; continues into the pisohamate and pisometacarpal ligaments. Ulnar boundary of the Guyon canal.

Trapezium

Saddle joint with the base of the 1st metacarpal (trapeziometacarpal joint, site of thumb basal joint arthritis). The palmar ridge gives attachment to the flexor retinaculum; the adjacent groove carries the FCR tendon.

Trapezoid

Small bone wedged between the trapezium, capitate, scaphoid and base of the 2nd metacarpal; rarely fractured.

Capitate

The largest carpal bone, at the center of the distal row; its head articulates with the lunate and scaphoid (midcarpal joint). Base for the 3rd metacarpal.

Hamate

The hook (hamulus) forms the radial boundary of the Guyon canal and the ulnar boundary of the carpal tunnel; hook fractures are typical in golf, tennis and baseball. Articulates with the 4th and 5th metacarpals.

1st metacarpal

Saddle-shaped base on the trapezium; Bennett fracture-dislocation and Rolando fracture at the base.

2nd metacarpal

Base wedged between the trapezium, trapezoid and capitate; insertion of the ECRL (dorsal) and FCR (palmar).

3rd metacarpal

Base with a dorsal styloid process (carpal boss); insertion of the ECRB.

4th metacarpal

The thinnest; articulates with the capitate and hamate.

5th metacarpal

ECU insertion at the base. The neck is the typical site of boxer's fracture.

Proximal phalanx of thumb

Base with the palmar plate and the sesamoids of the MCP joint; on the ulnar side the ulnar collateral ligament (Stener lesion).

Cartilage 4

Cartilage of the radius

Covers the scaphoid and lunate fossae and the sigmoid notch. Reconstructed on the surfaces facing the real bones.

Cartilage of the ulnar head

Covers the part of the head that articulates with the sigmoid notch and the distal dome, in contact with the TFCC.

Ligaments and TFCC 17

Triangular fibrocartilage complex (TFCC)

Fibrocartilage disc between the ulnar head and the carpus, from the margin of the sigmoid notch of the radius to the fovea and the base of the ulnar styloid. Its dorsal and palmar margins (radioulnar ligaments) stabilize the DRUJ; the thin center is the site of degenerative perforations. Lesions graded with the Palmer classification (1 traumatic, 2 degenerative).

Scapholunate ligament

C-shaped: dorsal portion (the strongest, main stabilizer), proximal membranous portion and palmar portion. Rupture leads to scapholunate diastasis, DISI deformity and, over time, SLAC arthritis.

Lunotriquetral ligament

The palmar portion is the strongest. Injury may be associated with VISI instability and TFCC pathology.

Radioscaphocapitate ligament

From the radial styloid, palmar to the scaphoid waist, to the capitate; acts as a fulcrum for scaphoid rotation.

Long radiolunate ligament

From the palmar rim of the radius to the lunate, ulnar to the radioscaphocapitate; between the two lies the space of Poirier, a palmar weak point.

Ulnolunate ligament

From the palmar radioulnar ligament of the TFCC to the lunate, not from the ulna; part of the ulnocarpal complex.

Ulnotriquetral ligament

From the palmar TFCC to the triquetrum; part of the ulnocarpal complex together with the ulnolunate.

Ulnocapitate ligament

From the ulnar fovea, superficial to the ulnolunate and ulnotriquetral, to the capitate; with the radioscaphocapitate it forms the arcuate ligament, distal to the space of Poirier.

Palmar capsule and intermediate fibrous zone

Between the pronator quadratus and the palmar rim, the radius is covered by a fibrous zone that continues into the radiocarpal capsule, of which the palmar ligaments are thickenings. The FPL and FDP glide over it: a plate placed on or beyond the watershed line can damage them.

Dorsal radiocarpal ligament

From the dorsal rim of the radius, running obliquely ulnarward to the triquetrum (dorsal radiotriquetral). With the dorsal intercarpal ligament it forms a horizontal "V".

Dorsal intercarpal ligament

From the triquetrum, transversely to the scaphoid and trapezoid; secondary stabilizer of the scapholunate joint.

Pisohamate ligament

Continuation of the FCU from the pisiform to the hook of the hamate: forms the floor of the Guyon canal.

Retinacula 3

Flexor retinaculum (transverse carpal ligament)

From the scaphoid tubercle and the trapezium ridge to the pisiform and the hook of the hamate: roof of the carpal tunnel and origin of the thenar and hypothenar muscles. On the radial side a deep layer closes the FCR's own tunnel. On ultrasound, assess its palmar bowing.

Extensor retinaculum

From the lateral border of the radius to the triquetrum and pisiform; its septa to the radius form six compartments: I APL-EPB, II ECRL-ECRB, III EPL, IV EDC-EIP, V EDM, VI ECU. It does not attach to the ulna, so it does not restrict pronation and supination. Shown here without the septa.

Palmar carpal ligament (roof of the Guyon canal)

Thickening of the antebrachial fascia over the ulnar nerve and artery: from the pisiform and the FCU it blends radially with the flexor retinaculum. Distally it continues with the palmaris brevis (not shown).

Flexors 7

Flexor carpi radialis (FCR)

At the wrist it runs in its own tunnel in the trapezium groove, outside the carpal tunnel; the median nerve lies ulnar to it, the radial artery radial.

Origin
Medial epicondyle of the humerus (common flexor tendon).
Insertion
Base of the 2nd metacarpal (palmar surface).
Function
Flexes and abducts the wrist (radial deviation).

Palmaris longus and palmar aponeurosis

Superficial to the retinaculum, in the midline. Absent in about 15% of people; used as a tendon graft. The aponeurosis is the site of Dupuytren disease.

Origin
Medial epicondyle of the humerus (common flexor tendon).
Insertion
Superficial surface of the flexor retinaculum and palmar aponeurosis.
Function
Flexes the wrist and tenses the palmar aponeurosis.

Flexor carpi ulnaris (FCU)

The pisiform is a sesamoid bone in its tendon; the ulnar nerve and artery run beneath and radial to the tendon.

Origin
Humeral head: medial epicondyle. Ulnar head: olecranon and posterior border of the ulna.
Insertion
Pisiform and, through the pisohamate and pisometacarpal ligaments, hamate and base of the 5th metacarpal.
Function
Flexes and adducts the wrist (ulnar deviation).

Flexor digitorum superficialis (FDS)

Four tendons in the carpal tunnel, in two layers: 3rd and 4th superficial, 2nd and 5th deep.

Origin
Humeroulnar head: medial epicondyle and coronoid process. Radial head: anterior surface of the radius.
Insertion
Sides of the middle phalanx of the 2nd–5th fingers (Camper chiasm).
Function
Flexes the PIP joints and, secondarily, the MCP joints and the wrist.

Flexor digitorum profundus (FDP)

On the floor of the carpal tunnel; in the palm it gives origin to the lumbricals. 2nd–3rd fingers innervated by the anterior interosseous nerve (median), 4th–5th by the ulnar nerve. Distal avulsion is jersey finger.

Origin
Anterior and medial surfaces of the ulna and interosseous membrane.
Insertion
Palmar base of the distal phalanx of the 2nd–5th fingers.
Function
Flexes the DIP joints and, secondarily, the PIP and MCP joints and the wrist.

Flexor pollicis longus (FPL)

The most radial tendon in the carpal tunnel, in its own sheath (radial bursa).

Origin
Anterior surface of the radius and interosseous membrane.
Insertion
Palmar base of the distal phalanx of the thumb.
Function
Flexes the IP joint of the thumb and, secondarily, the MCP joint.

Pronator quadratus

The deepest muscle of the palmar forearm. It ends just proximal to the watershed line; the overlying fat (Parona space) is obliterated in occult distal radius fractures.

Origin
Distal quarter of the palmar surface of the ulna.
Insertion
Distal quarter of the palmar surface of the radius, up to the anterior margin.
Function
Main pronator of the forearm; stabilizes the distal radioulnar joint.

Extensors 10

Brachioradialis

An elbow flexor even though it is innervated by the radial nerve. The superficial branch of the radial nerve emerges from under its margin.

Origin
Lateral supracondylar ridge of the humerus.
Insertion
Base of the radial styloid process.
Function
Flexes the elbow, especially in midpronation.

Extensor carpi radialis longus (ECRL)

2nd compartment. 4–8 cm above the wrist it is crossed by the APL and EPB: intersection syndrome.

Origin
Distal third of the lateral supracondylar ridge of the humerus.
Insertion
Base of the 2nd metacarpal (dorsal surface).
Function
Extends and abducts the wrist (radial deviation).

Extensor carpi radialis brevis (ECRB)

2nd compartment, radial to Lister tubercle. Its origin is the main site of lateral epicondylitis.

Origin
Lateral epicondyle of the humerus (common extensor tendon).
Insertion
Base of the 3rd metacarpal (dorsal surface).
Function
Extends the wrist.

Abductor pollicis longus (APL)

1st compartment, often with multiple slips. With the EPB: De Quervain tenosynovitis (sometimes with an intracompartmental septum) and palmar boundary of the snuffbox.

Origin
Posterior surfaces of the radius and ulna and interosseous membrane.
Insertion
Base of the 1st metacarpal (radial side).
Function
Abducts and extends the thumb at the trapeziometacarpal joint.

Extensor pollicis brevis (EPB)

1st compartment, dorsal to the APL; palmar boundary of the anatomical snuffbox.

Origin
Posterior surface of the radius and interosseous membrane.
Insertion
Dorsal base of the proximal phalanx of the thumb.
Function
Extends the MCP joint of the thumb.

Extensor pollicis longus (EPL)

3rd compartment: uses Lister tubercle as a pulley. Delayed rupture after distal radius fracture, even when nondisplaced. Dorsal boundary of the snuffbox.

Origin
Posterior surface of the ulna and interosseous membrane.
Insertion
Dorsal base of the distal phalanx of the thumb.
Function
Extends the IP joint of the thumb; adducts and retropulses it.

Extensor digitorum (EDC)

4th compartment with the EIP; the posterior interosseous nerve runs along its floor. The tendons are linked by the juncturae tendinum.

Origin
Lateral epicondyle of the humerus (common extensor tendon).
Insertion
Extensor apparatus of the 2nd–5th fingers (middle and distal phalanges).
Function
Extends the fingers and the wrist.

Extensor indicis (EIP)

4th compartment, deep to the EDC; at the MCP joint it lies ulnar to the EDC tendon of the index finger. Used for tendon transfer after EPL rupture.

Origin
Posterior surface of the distal ulna and interosseous membrane.
Insertion
Extensor apparatus of the index finger.
Function
Extends the index finger independently.

Extensor digiti minimi (EDM)

5th compartment, over the distal radioulnar joint; often bifid. In rheumatoid arthritis it is the first to rupture over the ulnar head (Vaughan-Jackson).

Origin
Lateral epicondyle of the humerus (common extensor tendon).
Insertion
Extensor apparatus of the little finger.
Function
Extends the little finger.

Extensor carpi ulnaris (ECU)

6th compartment, in the ulnar groove, with its own subsheath that contributes to TFCC stability. Subluxation can be assessed with dynamic ultrasound in supination.

Origin
Lateral epicondyle and posterior border of the ulna.
Insertion
Base of the 5th metacarpal (ulnar side).
Function
Extends and adducts the wrist (ulnar deviation).

Hand muscles 10

Abductor pollicis brevis

The most superficial thenar muscle (recurrent motor branch of the median nerve). Atrophy is a late sign of carpal tunnel syndrome.

Origin
Flexor retinaculum, scaphoid and trapezium.
Insertion
Base of the proximal phalanx of the thumb (radial side).
Function
Abducts the thumb; contributes to opposition.

Flexor pollicis brevis

Superficial head innervated by the median nerve, deep head by the ulnar nerve (variable).

Origin
Superficial head: flexor retinaculum and trapezium. Deep head: trapezoid and capitate.
Insertion
Base of the proximal phalanx of the thumb (radial side).
Function
Flexes the MCP joint of the thumb.

Opponens pollicis

Deep to the abductor pollicis brevis (recurrent motor branch of the median nerve).

Origin
Flexor retinaculum and tubercle of the trapezium.
Insertion
Radial border of the 1st metacarpal.
Function
Opposes the thumb.

Adductor pollicis

Deep, in the first web space (deep branch of the ulnar nerve). The radial artery passes between its two heads; its aponeurosis is interposed in the Stener lesion.

Origin
Oblique head: capitate and bases of the 2nd–3rd metacarpals. Transverse head: 3rd metacarpal.
Insertion
Base of the proximal phalanx of the thumb (ulnar side).
Function
Adducts the thumb; stabilizes the thumb-index pinch.

Abductor digiti minimi

The most ulnar and superficial hypothenar muscle (deep branch of the ulnar nerve).

Origin
Pisiform and FCU tendon.
Insertion
Base of the proximal phalanx of the little finger (ulnar side).
Function
Abducts the little finger.

Flexor digiti minimi brevis

Hypothenar muscle, radial to the abductor (deep branch of the ulnar nerve).

Origin
Hook of the hamate and flexor retinaculum.
Insertion
Base of the proximal phalanx of the little finger.
Function
Flexes the MCP joint of the little finger.

Opponens digiti minimi

The deepest hypothenar muscle; the deep branches of the ulnar nerve and artery pass through it.

Origin
Hook of the hamate and flexor retinaculum.
Insertion
Ulnar border of the 5th metacarpal.
Function
Draws forward and rotates the 5th metacarpal (opposition of the little finger).

Lumbricals

Four, arising from the FDP tendons. 1st–2nd innervated by the median nerve, 3rd–4th by the ulnar nerve.

Origin
FDP tendons in the palm.
Insertion
Radial margin of the extensor apparatus of the corresponding finger.
Function
Flex the MCP joints and extend the interphalangeal joints.

Dorsal interossei

Four, bipennate, between the metacarpals (deep branch of the ulnar nerve).

Origin
Facing surfaces of adjacent metacarpals.
Insertion
Base of the proximal phalanx and extensor apparatus of the 2nd, 3rd and 4th fingers.
Function
Abduct the fingers away from the axis of the middle finger (DAB).

Palmar interossei

Three, unipennate (deep branch of the ulnar nerve).

Origin
Palmar surface of the 2nd, 4th and 5th metacarpals.
Insertion
Base of the proximal phalanx and extensor apparatus of the 2nd, 4th and 5th fingers.
Function
Adduct the fingers toward the axis of the middle finger (PAD).

Arteries 8

Radial artery and deep palmar arch

In the forearm between the brachioradialis and the FCR, where the radial pulse is felt. At the wrist it turns dorsally under the APL and EPB, crosses the anatomical snuffbox, pierces the first interosseous space and forms the deep palmar arch. Allen test before blood sampling or cannulation.

Ulnar artery and superficial palmar arch

Under the FCU, radial to the ulnar nerve; it crosses the Guyon canal and forms the superficial palmar arch, distal to the deep arch. Thrombosis or aneurysm in hypothenar hammer syndrome.

Common palmar digital arteries

From the superficial palmar arch to the 2nd–4th web spaces, where they divide into the proper digital arteries; in addition, the proper digital artery of the ulnar border of the little finger.

Palmar metacarpal arteries

From the deep palmar arch, on the interossei, they join the common digital arteries: anastomoses between the two arches.

Veins 5

Cephalic vein

From the radial side of the dorsal venous network it passes over the snuffbox and ascends along the radial border of the forearm. Close to the superficial branch of the radial nerve: take care during cannulation at the wrist.

Basilic vein

From the ulnar side of the dorsal venous network it ascends along the ulnar border of the forearm toward the elbow.

Dorsal venous network of hand

Subcutaneous arch over the dorsum of the metacarpals that receives the dorsal metacarpal veins; it continues radially into the cephalic vein and ulnarly into the basilic vein.

Median antebrachial vein

Arises from the superficial palmar venous plexus and ascends subcutaneously on the palmar surface of the forearm; variable course, usually ending in the basilic or median cubital vein at the elbow.

Dorsal metacarpal veins

From the fingers they drain into the dorsal venous network; the thumb vein drains into the cephalic vein, the ulnar marginal vein into the basilic vein.

Nerves 9

Median nerve

Mixed nerve. In the forearm between the FDS and FDP; at the wrist between the FCR and PL, then in the carpal tunnel under the flexor retinaculum. Motor to the forearm flexors and pronators (except the FCU and the FDP of the 4th–5th fingers), the superficial thenar muscles (APB, OP, superficial head of FPB) and the 1st–2nd lumbricals; sensory to the palmar surface of the thumb, index, middle and radial half of the ring finger. On ultrasound, an area at the tunnel inlet above 9–10 mm² is suspicious, above 12 mm² more specific; wrist-to-forearm ratio above 1.4.

Recurrent motor branch of median nerve

Motor to the APB, OP and superficial head of the FPB. It arises at the tunnel outlet, usually from the radial side, and curves back into the thenar muscles; transligamentous variants are at risk during surgical release.

Common palmar digital nerves (median)

Mainly sensory, with motor branches to the 1st–2nd lumbricals. Three branches, deep to the superficial palmar arch, to the 1st–3rd web spaces: palmar sensation of the thumb, index, middle and radial half of the ring finger.

Ulnar nerve

Mixed nerve. Under the FCU, ulnar to the artery; in the Guyon canal it divides into a superficial branch (sensory to the little finger and ulnar half of the ring finger) and a deep branch (motor to most intrinsic muscles). The deficit depends on the zone of compression (Gross).

Superficial branch of ulnar nerve

Mainly sensory: palmar surface of the little finger and the ulnar half of the ring finger; motor only to the palmaris brevis. From the Guyon canal it descends superficial to the hypothenar muscles.

Deep branch of ulnar nerve

Motor to the hypothenar muscles, interossei, 3rd–4th lumbricals, adductor pollicis and deep head of the FPB. It winds around the hook of the hamate and follows the deep palmar arch; compression (Gross zone 2) causes a purely motor deficit.

Dorsal cutaneous branch of ulnar nerve

Sensory to the ulnar dorsum of the hand, the little finger and half of the ring finger. It arises 5–7 cm above the wrist and turns dorsally under the FCU; it is spared in Guyon canal compression.

Palmar cutaneous branch of median nerve

Sensory to the skin of the thenar eminence and the central palm. It passes over the retinaculum, outside the tunnel: this is why thenar sensation is spared in carpal tunnel syndrome.

Superficial branch of radial nerve

Sensory to the radial dorsum of the hand, the thumb and the proximal part of the index and middle fingers. It emerges from under the brachioradialis about 8–9 cm from the styloid and runs subcutaneously over the 1st compartment. Wartenberg syndrome; at risk from tight bracelets and cephalic vein cannulation.

Fat 1

Pronator quadratus fat pad

Thin fat layer between the pronator quadratus and the deep flexors (Parona space). On the lateral radiograph, displacement or loss of the fat stripe indicates edema or hematoma, for example in an occult distal radius fracture (indirect sign, of limited accuracy).

Tendon sheaths 9

Common flexor sheath (ulnar bursa)

Envelops the FDS and FDP in the carpal tunnel; it continues with the sheath of the little finger. Infections can spread to the radial bursa and form a "horseshoe abscess".

FCR sheath

In the FCR's own tunnel, along the trapezium groove; site of tenosynovitis and palmar ganglia.

Anatomical spaces 2

Carpal tunnel (space)

Floor and walls formed by the carpal bones, roof by the flexor retinaculum. Contains the median nerve, 4 FDS tendons, 4 FDP tendons and the FPL. The narrowest point is at the level of the hook of the hamate.

Guyon canal (space)

Between the pisiform and the hook of the hamate; floor: flexor retinaculum and pisohamate ligament; roof: palmar carpal ligament. Contains the ulnar nerve and artery (the artery radial to the nerve). Gross zones: I proximal to the bifurcation (mixed deficit), II deep branch (motor), III superficial branch (sensory).

Skin 1

Skin

Semi-transparent skin outline, useful as a reference for surface landmarks.

Finger 39 structures

Open the 3D finger model

Bones 4

3rd metacarpal (head)

The head is eccentric, wider palmarly: this is why the MCP collateral ligaments tighten in flexion (cam effect). Hence the "intrinsic plus" immobilization position, with the MCP joints flexed and the interphalangeal joints extended.

Proximal phalanx

On the palmar surface the lateral ridges give attachment to the A2 pulley. The base articulates with the metacarpal head, the bicondylar head with the middle phalanx (PIP joint).

Middle phalanx

The central slip inserts dorsally at the base; the two halves of the FDS insert on the sides. The A4 pulley lies at mid-shaft.

Distal phalanx

The terminal tendon inserts at the dorsal base (mallet finger), the FDP at the palmar base (jersey finger). The ungual tuberosity supports the fingertip pulp.

Cartilage 1

Articular cartilage

Surfaces of the MCP, PIP and DIP joints, reconstructed on the real bones.

Ligaments and palmar plates 9

Palmar (volar) plate of MCP joint

Fibrocartilaginous distally and membranous proximally, to allow flexion. It is continuous laterally with the deep transverse metacarpal ligament; the A1 pulley arises from it.

Palmar (volar) plate of PIP joint

Attached distally to the base of the middle phalanx, proximally by the "checkrein" ligaments. It prevents hyperextension: in dorsal dislocation it can avulse a palmar fragment from the base of P2.

Radial collateral ligaments of PIP joint (proper and accessory)

The proper ligament is taut throughout the range of motion; the accessory ligament runs to the palmar plate. PIP sprains, most often of the radial collateral ligament, are common in ball sports; on ultrasound, assess continuity and varus or valgus stress.

Ulnar collateral ligaments of PIP joint (proper and accessory)

The proper ligament is taut throughout the range of motion; the accessory ligament runs to the palmar plate. PIP sprains, most often of the radial collateral ligament, are common in ball sports; on ultrasound, assess continuity and varus or valgus stress.

Pulleys 8

A1 pulley

On the palmar plate of the MCP joint. It is the site of trigger finger (stenosing tenosynovitis): on ultrasound, a thickened, hypoechoic pulley, tenosynovitis and dynamic tendon catching. It is the target of injection and percutaneous release.

A2 pulley

The longest and strongest, on the proximal half of the proximal phalanx. With A4 it is the most important against bowstringing. Rupture is typical in climbers: on ultrasound the tendon-to-bone distance increases, best assessed in flexion against resistance.

A3 pulley

Thin, on the palmar plate of the PIP joint.

A4 pulley

At the middle of the middle phalanx. With A2 it is essential for flexor efficiency; often injured together with A3 in climbers.

A5 pulley

Thin and inconstant, on the palmar plate of the DIP joint.

Flexor tendons 2

Flexor digitorum superficialis (FDS)

At the level of the proximal phalanx it splits into two slips that encircle the FDP and rejoin dorsal to it (Camper chiasm), inserting on the sides of the middle phalanx. Flexes the PIP joint.

Flexor digitorum profundus (FDP)

Passes through the FDS and inserts at the palmar base of the distal phalanx; flexes the DIP joint. Avulsion (jersey finger, most common in the ring finger) is graded with the Leddy-Packer classification by level of retraction.

Extensor apparatus 9

Extensor digitorum (EDC) and central slip

Over the dorsum of the MCP joint it is centered by the sagittal bands and flattens into the extensor hood; on the proximal phalanx it gives off the lateral slips and continues as the central slip, which inserts on the dorsal tubercle at the base of the middle phalanx. Central slip rupture (Elson test) leads to the boutonnière deformity.

Sagittal bands

From the extensor tendon to the palmar plate and the deep transverse metacarpal ligament: they keep the tendon centered over the metacarpal head. Injury, most often radial at the middle finger (boxer’s knuckle), causes ulnar subluxation of the tendon, visible with dynamic ultrasound in flexion.

Extensor hood

Dorsal fibrous sheet distal to the sagittal bands, over the MCP joint and the base of the proximal phalanx: the EDC and the expansions of the interossei and lumbrical converge on it, with fibers running obliquely toward the midline. It continues into the lateral bands; through it the intrinsic muscles extend the interphalangeal joints.

Lateral bands

Each arises on the proximal phalanx from the fusion of the intrinsic expansion (interossei and lumbrical) with the lateral slip of the EDC; the radial band receives the lumbrical tendon. They run dorsal to the PIP axis and lateral to the central slip, join over the middle phalanx and merge into the terminal tendon. They slide palmarly in boutonnière and dorsally in swan-neck deformity.

Triangular ligament

Dorsal sheet over the middle phalanx, distal to the central slip, which joins the two lateral bands and prevents their palmar subluxation; its failure is part of the boutonnière deformity.

Terminal tendon

From the fusion of the lateral bands to the dorsal base of the distal phalanx. Rupture or bony avulsion: mallet finger.

Transverse retinacular ligaments

At the PIP joint they connect the lateral bands to the flexor sheath and the palmar plate, preventing their dorsal subluxation; their laxity favors swan-neck deformity.

Dorsal interosseous tendons

Each interosseous ends in a tendon that passes dorsal to the deep transverse metacarpal ligament and palmar to the MCP axis; it then divides: a short slip inserts on the lateral tubercle at the base of the proximal phalanx, an oblique slip runs to the extensor hood and the lateral band.

Intrinsic muscles 2

Lumbrical (2nd)

Arises from the FDP and inserts on the radial lateral band: flexes the MCP joint and extends the interphalangeal joints. Each finger has only one lumbrical, always on the radial side; on the ulnar side of the middle finger the band receives only the dorsal interosseous. "Lumbrical plus finger" is paradoxical PIP extension during attempted flexion, after FDP injury.

2nd and 3rd dorsal interossei

They insert on the base of the proximal phalanx and on the extensor hood; they abduct the middle finger radially and ulnarly. Innervated by the deep branch of the ulnar nerve.

Flexor sheath 1

Flexor synovial sheath

Envelops the FDS and FDP from the A1 region to the DIP joint, beneath the pulley system. In infectious tenosynovitis the Kanavel signs are seen; on ultrasound, effusion and thickened synovium.

Arteries 1

Nerves 1

Proper palmar digital nerves

One on each side, palmar to the artery. In the middle finger they derive from the median nerve (2nd and 3rd common digital nerves); at risk in lateral wounds and during percutaneous A1 release.

Skin 1

Skin

Semi-transparent skin outline, useful for surface landmarks and creases.