Posterior Forearm & Wrist

The back of the forearm is the simplest compartment in the body to innervate — every muscle in it belongs to the radial nerve — and the wrist it moves is the most anatomically crowded two centimeters in the limb, where nine tendons and a nerve share one tunnel, twelve more tendons pass through six roofed compartments, and a small bone with a backwards blood supply dies quietly after an ordinary fall.

What you should be able to do: list the superficial and deep extensors with attachments, innervation and action, and state the innervation rule for the whole compartment; explain where the radial nerve divides and why a posterior interosseous lesion gives finger drop without numbness while a spiral groove lesion gives wrist drop with numbness; recite the six extensor retinaculum compartments in order with their contents; give the boundaries, floor, roof and contents of the anatomical snuffbox and say what a tender snuffbox means; name the eight carpal bones in both rows; describe the radiocarpal, midcarpal, intercarpal and distal radioulnar joints and the triangular fibrocartilage complex, and say which joint contributes most to each wrist movement; give the boundaries and the full contents of the carpal tunnel and of Guyon canal, and name what is deliberately excluded from each; and work through scaphoid fracture, Colles and Smith fractures, lunate dislocation, carpal tunnel syndrome, Guyon canal compression, de Quervain tenosynovitis, lateral epicondylitis, posterior interosseous nerve syndrome and ganglion cyst.

Orientation — how the compartment is built

The forearm is divided into an anterior (flexor–pronator) compartment and a posterior (extensor–supinator) compartment by three structures: the interosseous membrane between radius and ulna, the subcutaneous posterior border of the ulna (palpable along its whole length, the only place where bone reaches the skin from elbow to wrist), and a lateral intermuscular septum of deep antebrachial fascia running from that fascia to the radius.

The anterior compartment, the cubital fossa, the brachial plexus, the arteries of the arm and the osteology of the radius and ulna are treated with the arm and the anterior forearm; this section assumes them and builds on the back of the limb. Two facts frame everything that follows:

The posterior compartment is a single nerve territory. Every muscle behind the interosseous membrane is supplied by the radial nerve or by its deep branch / posterior interosseous nerve. There is no exception and no shared supply — unlike the anterior forearm, where the median nerve takes most muscles but the ulnar nerve claims flexor carpi ulnaris and the medial half of flexor digitorum profundus.
The compartment has two layers separated by a plane the nerve uses. The deep branch of the radial nerve pierces supinator and emerges on the back of the forearm as the posterior interosseous nerve, then runs distally in the plane between the superficial and deep extensor groups, on the interosseous membrane, with the posterior interosseous artery. That plane is the surgical approach to the posterior forearm and the reason a proximal radius fracture endangers the nerve.

Functionally the compartment does four things: it extends the wrist (which is what lets the long flexors generate grip — try to make a fist with the wrist flexed and the grip collapses), it extends the fingers, it abducts and extends the thumb, and it supinates.

Pearl. Wrist extension is the unsung half of grip. The long finger flexors cross the wrist, so a flexed wrist puts them on slack and lengthens their antagonists — grip strength falls by more than half. This is the mechanism of the weak grip in a radial nerve palsy: the finger flexors are intact and median- and ulnar-innervated, but with a dropped wrist they cannot work. Hold the wrist extended for the patient and grip returns immediately.

The common extensor origin

The lateral epicondyle of the humerus gives origin to a shared tendon, the common extensor tendon (common extensor origin), which is the counterpart of the common flexor origin on the medial epicondyle. Four superficial extensors take origin from it:

Extensor carpi radialis brevis
Extensor digitorum
Extensor digiti minimi
Extensor carpi ulnaris (which also arises from the posterior border of the ulna by an aponeurosis)

Supinator and anconeus also take partial origin from the lateral epicondyle, and the radial collateral and anular ligaments blend into the deep surface of the tendon.

Two muscles that are usually counted as superficial extensors do not arise from the epicondyle at all — brachioradialis and extensor carpi radialis longus arise more proximally from the lateral supracondylar ridge of the humerus. Together with extensor carpi radialis brevis they form the “mobile wad” of three, the radial group of muscles that can be lifted as one mass off the lateral forearm.

⭐ High yield — Learn the origin split precisely: lateral supracondylar ridge gives brachioradialis and extensor carpi radialis longus; the lateral epicondyle gives extensor carpi radialis brevis and the rest of the superficial group. That difference is why extensor carpi radialis brevis, not longus, is the muscle degenerated in tennis elbow, and why the two radial wrist extensors sit at slightly different levels in the forearm.

Superficial extensor group

Muscle	Origin	Insertion	Innervation	Action
Brachioradialis	Proximal two thirds of the lateral supracondylar ridge of the humerus	Lateral surface of the distal radius, just proximal to the styloid process	Radial n. (C5, C6) — in the arm, before the nerve divides	Flexes the elbow, most powerfully with the forearm in midprone (thumb-up) position; no meaningful effect on the wrist
Extensor carpi radialis longus	Distal third of the lateral supracondylar ridge	Dorsal surface of the base of the 2nd metacarpal	Radial n. (C6, C7) — before the nerve divides	Extends and abducts (radially deviates) the hand at the wrist; assists elbow flexion weakly
Extensor carpi radialis brevis	Common extensor origin, lateral epicondyle	Dorsal surface of the base of the 3rd metacarpal	Deep branch of the radial n. (C7, C8), usually taken before the nerve enters supinator	Extends and abducts the hand at the wrist; the principal wrist extensor in grip
Extensor digitorum	Common extensor origin	Via four tendons into the extensor expansions of digits 2–5	Posterior interosseous n. (C7, C8)	Extends digits 2–5, chiefly at the metacarpophalangeal joints (interphalangeal extension secondarily, and mostly delegated to lumbricals and interossei); assists wrist extension
Extensor digiti minimi	Common extensor origin	Extensor expansion of digit 5, joining the extensor digitorum tendon	Posterior interosseous n. (C7, C8)	Extends the little finger at the metacarpophalangeal joint; allows independent extension of digit 5
Extensor carpi ulnaris	Common extensor origin (humeral head) and posterior border of the ulna (ulnar head)	Medial tubercle of the base of the 5th metacarpal	Posterior interosseous n. (C7, C8)	Extends and adducts (ulnar deviates) the hand at the wrist; a key stabilizer of the ulnar side of the wrist
Anconeus (transitional — posterior arm by position, often listed with this group)	Lateral epicondyle, posterior aspect	Lateral olecranon and superior posterior surface of the ulna	Radial n. (C7, C8, T1) — branch given in the arm	Assists triceps in elbow extension; stabilizes the elbow and abducts the ulna during pronation

Grouping the actions is faster than memorizing each muscle in isolation:

Movement	Muscles
Wrist extension	Extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris (assisted by extensor digitorum, extensor digiti minimi and extensor indicis)
Wrist abduction (radial deviation)	Extensor carpi radialis longus and brevis — working with flexor carpi radialis and abductor pollicis longus from the front
Wrist adduction (ulnar deviation)	Extensor carpi ulnaris — working with flexor carpi ulnaris from the front
Digit 2–5 extension	Extensor digitorum, plus extensor digiti minimi (digit 5) and extensor indicis (digit 2)
Thumb extension / abduction	Extensor pollicis longus (interphalangeal joint), extensor pollicis brevis (metacarpophalangeal joint), abductor pollicis longus (carpometacarpal joint)
Supination	Supinator (unresisted, elbow extended) and biceps brachii (powerful, elbow flexed)

Notice the couples: pure wrist extension requires the radial and ulnar extensors to fire together so their deviating components cancel. Denervate extensor carpi ulnaris alone and the wrist still extends — but it swings into radial deviation as it does so. That single observation is the clinical signature of a posterior interosseous lesion.

⭐ High yield — brachioradialis, the “beer-drinker’s muscle.” It is anatomically a posterior compartment, radial nerve muscle, yet it flexes the elbow — it crosses in front of the elbow axis because it originates above the joint and runs down the front of the lateral forearm. It works best in midpronation, the position of lifting a glass toward the mouth, which is where the nickname comes from. Every part of that sentence is testable: compartment (posterior), nerve (radial, C5–C6), action (elbow flexion), optimal position (neutral forearm). It is also the one muscle preserved in a low radial nerve lesion that lets you distinguish levels — and it is the muscle whose reflex (the brachioradialis or supinator jerk) tests C6.

Deep extensor group

The deep group lies against the interosseous membrane and the posterior surfaces of the radius and ulna. All five are supplied by the posterior interosseous nerve. Three of them — abductor pollicis longus, extensor pollicis brevis and extensor pollicis longus — are the outcropping muscles: they emerge obliquely from beneath the superficial group in the distal third of the forearm and cross over the radial wrist extensors, and their tendons form the anatomical snuffbox.

Muscle	Origin	Insertion	Innervation	Action
Supinator	Two heads: lateral epicondyle, radial collateral and anular ligaments (superficial head); supinator crest and fossa of the ulna (deep head)	Wraps around to the lateral, posterior and anterior surfaces of the proximal third of the radius, above the pronator teres insertion	Deep branch of the radial n. (C6, C7) — branches given as the nerve enters or pierces the muscle	Supinates the forearm — the prime mover for slow, unresisted supination with the elbow extended
Abductor pollicis longus	Posterior surfaces of the proximal ulna, middle radius and intervening interosseous membrane	Base of the 1st metacarpal, lateral side (often with a slip to the trapezium)	Posterior interosseous n. (C7, C8)	Abducts and extends the thumb at the carpometacarpal joint; assists wrist abduction (radial deviation)
Extensor pollicis brevis	Posterior surface of the distal radius and adjacent interosseous membrane (distal to abductor pollicis longus)	Base of the proximal phalanx of the thumb	Posterior interosseous n. (C7, C8)	Extends the thumb at the metacarpophalangeal joint and at the carpometacarpal joint
Extensor pollicis longus	Posterior surface of the middle third of the ulna and adjacent interosseous membrane	Base of the distal phalanx of the thumb	Posterior interosseous n. (C7, C8)	Extends the thumb at the interphalangeal joint (and at the metacarpophalangeal and carpometacarpal joints); retroposes the thumb — lifts it off a flat surface
Extensor indicis	Posterior surface of the distal ulna and adjacent interosseous membrane	Extensor expansion of digit 2, on the ulnar side of the extensor digitorum tendon	Posterior interosseous n. (C7, C8)	Extends the index finger independently; assists wrist extension. Its independence is why it points

Pearl. Root levels for supinator vary between standard sources — some texts give C5–C6, others C7–C8, and C6–C7 is a common compromise. What is not disputed is the nerve: the deep branch of the radial nerve, which supplies supinator before or as it pierces it. That ordering matters clinically — a compression at the distal edge of supinator can weaken every other posterior interosseous muscle while sparing supination, because the branches to supinator were already given off upstream.

Pearl. Two muscles insert on the thumb metacarpal versus its phalanges, and the naming follows the joint each crosses: abductor pollicis longus → 1st metacarpal (carpometacarpal joint), extensor pollicis brevis → proximal phalanx (metacarpophalangeal joint), extensor pollicis longus → distal phalanx (interphalangeal joint). So a patient who cannot lift the thumb interphalangeal joint but can still extend the metacarpophalangeal joint has an isolated extensor pollicis longus problem — the classic rupture at Lister tubercle.

The radial nerve in the forearm — where it divides and what that costs

The radial nerve (C5–T1) leaves the spiral groove of the humerus, pierces the lateral intermuscular septum about 10 cm above the lateral epicondyle, and enters the front of the arm in the groove between brachialis medially and brachioradialis / extensor carpi radialis longus laterally. It gives its branches to brachioradialis and extensor carpi radialis longus here, still in the arm.

At the level of the lateral epicondyle, anterior to the radiocapitellar joint, it divides into two terminal branches with a clean division of labor:

	Superficial branch of the radial n.	Deep branch → posterior interosseous n.
Modality	Purely sensory	Purely motor (with articular afferents)
Course	Runs distally deep to brachioradialis, lying on the lateral side of the radial artery in the middle third of the forearm; leaves the artery, passes deep to the brachioradialis tendon and pierces the deep fascia about 7–9 cm above the wrist; crosses the roof of the anatomical snuffbox subcutaneously	Winds laterally around the radial neck and pierces supinator, passing between its two heads under the arcade of Frohse; emerges on the back of the forearm as the posterior interosseous nerve, descends on the interosseous membrane between the superficial and deep extensor layers with the posterior interosseous artery, and ends at the wrist in a small swelling (pseudoganglion) supplying the carpal joints
Supplies	Skin of the dorsolateral hand and the dorsum of the lateral three and a half digits, as far distally as the proximal interphalangeal joints only — the nail beds and distal phalanges are median and ulnar territory	Every remaining posterior compartment muscle: supinator (proximally), extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis. Extensor carpi radialis brevis is usually taken by the deep branch just before it enters supinator

⭐ High yield — the innervation rule, stated exactly. Every muscle of the posterior compartment of the forearm is supplied by the radial nerve or its posterior interosseous branch. Split it at the elbow: before the division, the radial nerve proper takes brachioradialis, extensor carpi radialis longus, and usually extensor carpi radialis brevis (by the deep branch, before it enters supinator). After the division and after the nerve passes through supinator, the posterior interosseous nerve takes everything else. Learn the three that escape the tunnel and you have automatically learned the eight that do not.

Why the split matters: two radial lesions that look nothing alike.

	Spiral groove (mid-shaft humerus / “Saturday night palsy”)	Posterior interosseous nerve (arcade of Frohse)
Wrist	True wrist drop — all three wrist extensors are denervated, the wrist hangs flexed	No wrist drop. Extensor carpi radialis longus and brevis are supplied above the lesion, so the wrist still extends — but extensor carpi ulnaris is out, so it extends into radial deviation
Fingers and thumb	Cannot extend the metacarpophalangeal joints of digits 2–5 or the thumb	Finger drop — loss of metacarpophalangeal extension of digits 2–5 and of thumb extension and abduction, with the interphalangeal joints still extendable by the intact lumbricals and interossei
Sensation	Numb over the posterior forearm (posterior cutaneous n. of the forearm) and the dorsal first web space (superficial radial branch)	Completely normal — the superficial branch has already left the trunk proximal to the lesion. This is the single most useful discriminator
Elbow	Triceps spared if the lesion is below the branches to it (usual); brachioradialis weak	Triceps, brachioradialis and elbow flexion all normal; supination may be preserved if the lesion is at the distal supinator edge
Typical cause	Humeral shaft fracture, prolonged compression of the arm over a chair back or during anesthesia, crutch use higher up	Compression at the arcade of Frohse, radial head fracture or dislocation (Monteggia), rheumatoid synovitis of the elbow, lipoma or ganglion at the radial neck, forceful repetitive supination

⭐ High yield — Finger drop without numbness and without wrist drop = posterior interosseous nerve. Wrist drop with numbness over the dorsal first web space = radial nerve in the spiral groove. Sensory testing at the dorsal web space between thumb and index is the fastest bedside way to localize a radial nerve lesion.

The radial tunnel is the space through which the radial nerve and its deep branch pass at the elbow, from the level of the radiocapitellar joint to the distal border of supinator. Five recognized compression points, proximal to distal:

Fibrous bands anterior to the radiocapitellar joint capsule
The leash of Henry — the fan of recurrent radial vessels crossing the nerve
The sharp tendinous medial border of extensor carpi radialis brevis
The arcade of Frohse — the thickened, often tendinous proximal edge of the superficial head of supinator; the most common site
The distal edge of supinator

Radial tunnel syndrome and posterior interosseous nerve syndrome are compression at the same anatomic tunnel with different presentations: radial tunnel syndrome is pain without weakness, posterior interosseous nerve syndrome is weakness (finger drop) with little or no pain. Both are sensory-normal.

Pearl. Radial tunnel syndrome versus lateral epicondylitis is decided by where the finger hurts. Lateral epicondylitis is maximally tender on the lateral epicondyle itself; radial tunnel syndrome is maximally tender 3–5 cm distal to it, over the supinator mass, and is aggravated by resisted supination with the elbow extended and by resisted middle finger extension. They coexist often enough that the second diagnosis is missed when tennis elbow treatment fails.

The extensor expansion — where the extensor tendons end

Over the dorsum of each finger the extensor digitorum tendon flattens into a triangular aponeurosis, the extensor expansion or extensor hood, which wraps around the sides of the metacarpophalangeal joint and is anchored to the deep transverse metacarpal ligament and the volar plate.

A central band (central slip) continues distally to insert on the base of the middle phalanx — the extensor of the proximal interphalangeal joint.
Two lateral bands pass on either side of the proximal interphalangeal joint, converge over the middle phalanx and insert as the terminal tendon on the base of the distal phalanx — the extensor of the distal interphalangeal joint.
The lumbricals and interossei insert into the lateral bands. This is why intrinsic hand muscles that flex the metacarpophalangeal joints simultaneously extend the interphalangeal joints, and why the fingers of a hand with a posterior interosseous palsy can still straighten at the interphalangeal joints even though they hang at the knuckles.
Juncturae tendinum are oblique fibrous bands linking the extensor digitorum tendons on the back of the hand. They limit independent extension (especially of the ring finger), and they can mask a proximal extensor digitorum laceration by transmitting pull from a neighboring tendon.

Two deformities follow directly from the anatomy:

Deformity	Lesion	Posture
Mallet finger (baseball finger)	Avulsion or rupture of the terminal extensor tendon at the base of the distal phalanx, usually from forced flexion of an extended fingertip	Distal interphalangeal joint held flexed and cannot be actively extended
Boutonnière deformity	Rupture of the central slip at the base of the middle phalanx; the lateral bands slip volar to the proximal interphalangeal axis and become flexors of it	Proximal interphalangeal joint flexed, distal interphalangeal joint hyperextended — the head of the phalanx buttonholes through the split hood
Swan-neck deformity	Volar plate laxity or intrinsic tightness — the reverse arrangement, classically in rheumatoid disease	Proximal interphalangeal joint hyperextended, distal interphalangeal joint flexed

The extensor retinaculum and its six compartments

The extensor retinaculum is a strong oblique thickening of the deep antebrachial fascia, roughly 2–3 cm wide, on the back of the wrist. It runs from the anterolateral border of the distal radius, obliquely downward and medially, to the styloid process of the ulna, the triquetrum and the pisiform. Note that its ulnar attachment is to carpal bones, not to the ulna alone — this is what allows the radius and hand to rotate around the fixed ulna in pronation and supination without the retinaculum unwinding.

From its deep surface, fibrous septa pass to ridges on the dorsal surfaces of the radius and ulna, converting the space beneath it into six osseofibrous compartments (tunnels). Each contains tendons wrapped in their own synovial sheath, which extends from roughly 2 cm proximal to the retinaculum to the level of the metacarpal bases.

The retinaculum has one job and one consequence: it prevents bowstringing of the extensor tendons when the wrist is extended, and by confining tendons in unyielding tunnels it makes them vulnerable to stenosing tenosynovitis when the sheath swells.

The six compartments, radial to ulnar — memorize this table as a unit.

#	Contents	Tendons	Landmark / clinical hook
1	Abductor pollicis longus and extensor pollicis brevis	2	Most radial, over the radial styloid; forms the anterior (radial) border of the anatomical snuffbox. Site of de Quervain tenosynovitis
2	Extensor carpi radialis longus and extensor carpi radialis brevis	2	Lies radial to Lister tubercle. Where compartment 1 crosses over it, 4–6 cm proximal to the wrist, is the site of intersection syndrome
3	Extensor pollicis longus	1	Runs on the ulnar side of Lister tubercle and hooks around it at about 45°, then crosses superficial to compartment 2 to form the posterior (ulnar) border of the snuffbox. Site of attritional EPL rupture
4	Extensor digitorum (4 tendons) and extensor indicis	5	The largest compartment; the posterior interosseous nerve terminal branch lies on its floor. Common site of dorsal wrist ganglia and of rheumatoid tenosynovitis
5	Extensor digiti minimi	1	Sits directly over the distal radioulnar joint — useful landmark for finding that joint, and why extensor digiti minimi ruptures first in rheumatoid arthritis with caput ulnae
6	Extensor carpi ulnaris	1	In the groove between the ulnar head and the ulnar styloid, held by its own subsheath (part of the triangular fibrocartilage complex). Subluxes with forced supination — a cause of ulnar-sided wrist pain and snapping

⭐ High yield — The tendon count from radial to ulnar is 2 · 2 · 1 · 2 · 1 · 1. Counting extensor indicis with extensor digitorum makes compartment 4 hold five tendons but two muscles; the classic count of 2-2-1-2-1-1 counts muscles, not tendons. Also anchor the whole map to Lister tubercle, the palpable dorsal tubercle of the radius: compartment 2 is radial to it, compartment 3 is ulnar to it, and the extensor pollicis longus turns around it like a rope around a pulley.

Extensor pollicis longus rupture at Lister tubercle. The tendon changes direction sharply around the tubercle inside a tight tunnel, and at that point it lies in a relative watershed zone of poor vascularity. Two settings produce rupture:

After a distal radius fracture — classically a nondisplaced or minimally displaced Colles-type fracture, with rupture appearing weeks to a few months later. The paradox is real: the displaced fracture is often treated operatively and decompresses the tunnel, whereas the nondisplaced one is left in a cast while callus and swelling strangle the tendon.
Rheumatoid arthritis, from tenosynovitis and attrition on a roughened dorsal radius.

Presentation: painless, sudden loss of the ability to lift the thumb off a flat surface (loss of retroposition) and to extend the thumb interphalangeal joint. The thumb can still be extended at the metacarpophalangeal joint by extensor pollicis brevis and abducted by abductor pollicis longus, so the deficit is easy to underestimate. Repair is by tendon transfer, usually of extensor indicis — which the hand can spare because extensor digitorum still extends the index finger.

The anatomical snuffbox

A triangular depression on the dorsolateral wrist, best seen when the thumb is fully extended and abducted, so named because ground tobacco was placed there to be sniffed. It is the single most useful surface landmark on the back of the wrist.

Boundary	Structure
Anterior (lateral / radial) border	Tendons of abductor pollicis longus and extensor pollicis brevis — retinaculum compartment 1
Posterior (medial / ulnar) border	Tendon of extensor pollicis longus — retinaculum compartment 3
Proximal border (base)	Styloid process of the radius
Distal border (apex)	Base of the 1st metacarpal and the trapeziometacarpal joint
Floor	From proximal to distal: distal radius, scaphoid, trapezium, base of the 1st metacarpal. The scaphoid and trapezium are the answer expected on an exam. The tendons of extensor carpi radialis longus and brevis cross the floor obliquely
Roof	Skin and superficial fascia, crossed by the superficial branch of the radial nerve and by the origin of the cephalic vein from the dorsal venous network
Contents	Radial artery, crossing the floor obliquely from the anterior wrist toward the first dorsal interosseous space, where it dives between the two heads of the first dorsal interosseous muscle to become the deep palmar arch. It gives the dorsal carpal branch here

⭐ High yield — Three things are palpable or clinically usable in the snuffbox: the radial pulse (in the box itself, not only at the front of the wrist), the scaphoid in its floor, and the superficial radial nerve in its roof — which is why placing an intravenous cannula in the snuffbox vein risks a painful superficial radial neuritis, and why percussion of a swollen nerve there can produce a Tinel sign (Wartenberg syndrome, or “handcuff neuropathy”: a compressive neuropathy of the superficial radial nerve as it emerges between the brachioradialis and extensor carpi radialis longus tendons — pure numbness of the dorsal thumb and web space with no weakness).

The carpus — eight bones in two rows

The carpal bones lie in two rows of four, described lateral (radial, thumb side) to medial (ulnar) in each row, proximal row first:

Row	Lateral → medial
Proximal row	Scaphoid · Lunate · Triquetrum · Pisiform
Distal row	Trapezium · Trapezoid · Capitate · Hamate

Mnemonic: “Some Lovers Try Positions That They Can’t Handle.” Read it with the palm facing you and start at the thumb.

Facts about individual bones that get asked:

Bone	What to know
Scaphoid	Boat-shaped; the largest bone of the proximal row and the only carpal that bridges both rows, which is why it fails under load. Most frequently fractured carpal bone. Its tubercle is palpable at the base of the thenar eminence and gives lateral attachment to the flexor retinaculum. Forms the floor of the snuffbox
Lunate	Crescent-shaped, the keystone of the proximal row; most frequently dislocated carpal bone (anteriorly, into the carpal tunnel). Avascular necrosis of the lunate is Kienböck disease
Triquetrum	Pyramidal; articulates with the articular disc of the triangular fibrocartilage complex rather than directly with the ulna, and carries the pisiform on its palmar surface
Pisiform	A sesamoid bone in the tendon of flexor carpi ulnaris; sits on the palmar surface of the triquetrum. Palpable at the base of the hypothenar eminence. Forms the medial wall of Guyon canal and gives medial attachment to the flexor retinaculum
Trapezium	Has a saddle-shaped facet for the 1st metacarpal — the joint that gives the thumb its opposition, and the joint that wears out in basal thumb arthritis. Its tubercle (crest) is a lateral attachment of the flexor retinaculum; a groove on it carries the flexor carpi radialis tendon
Trapezoid	Small, wedge-shaped; articulates with the 2nd metacarpal. The least frequently injured carpal bone
Capitate	The largest carpal bone, central; its head sits in the concavity of the scaphoid and lunate. Articulates chiefly with the 3rd metacarpal. It is the axis around which the carpus rotates
Hamate	Wedge-shaped, with a palmar hook (hamulus) that forms the lateral (radial) wall of Guyon canal and a medial attachment of the flexor retinaculum. The hook fractures from a golf club or bat handle and can injure the adjacent ulnar nerve and artery

Pearl. The four bony attachments of the flexor retinaculum are the same four palpable landmarks that define the carpal arch: laterally the tubercle of the scaphoid and the tubercle (crest) of the trapezium; medially the pisiform and the hook of the hamate. Learn them as two lateral and two medial and both the carpal tunnel and Guyon canal fall into place.

The wrist joints

“The wrist” in clinical use is a stack of joints working together. Four levels matter.

Joint	Type	Articulation	Movements
Distal radioulnar	Pivot synovial	Head of the ulna with the ulnar notch of the radius; the two are bound distally by the articular disc of the triangular fibrocartilage complex	Pronation and supination — the radius swings around a relatively fixed ulna. No contribution to wrist flexion or deviation
Radiocarpal (the wrist joint proper)	Ellipsoid (condyloid) synovial	Proximally: the distal articular surface of the radius and the articular disc of the triangular fibrocartilage complex. Distally: the proximal surfaces of the scaphoid, lunate and triquetrum	Flexion–extension and abduction–adduction (radial and ulnar deviation); circumduction as their combination. No active rotation
Midcarpal	Compound / modified plane synovial	Between the proximal and distal carpal rows — an S-shaped interlocking line, with the head of the capitate and the hamate in the socket of the scaphoid, lunate and triquetrum laterally the scaphotrapeziotrapezoid articulation	Contributes substantially to flexion and to radial deviation; moves as a functional unit with the radiocarpal joint
Intercarpal	Plane synovial	Between adjacent bones within each row	Small gliding movements that augment and distribute wrist motion. The intercarpal, midcarpal and the carpometacarpal joints of digits 2–5 share a single continuous synovial cavity — infection or injected contrast spreads freely between them

⭐ High yield — the ulna does not reach the carpus. The articular disc of the triangular fibrocartilage complex is interposed between the head of the ulna and the triquetrum, so the ulna has no direct articulation with any carpal bone. Contrast with the radius, which carries roughly 80% of the axial load from hand to forearm and articulates directly with the scaphoid and lunate. This is why a fall on the outstretched hand fractures the distal radius, not the distal ulna, and why the term “ulnocarpal joint” refers to an articulation through the disc rather than bone on bone.

Ligaments of the wrist.

Palmar radiocarpal ligaments — strong, running from the radius obliquely downward and medially to the carpal bones. Their oblique direction means the hand follows the radius during supination and they are the reason the carpus is dragged with the radius as the forearm rotates.
Dorsal radiocarpal ligaments — the same arrangement behind, weaker.
Radial collateral ligament — radial styloid to the scaphoid, limiting ulnar deviation.
Ulnar collateral ligament — ulnar styloid to the triquetrum and pisiform, limiting radial deviation.
Intrinsic (interosseous) ligaments — the scapholunate and lunotriquetral ligaments bind the proximal row. Rupture of the scapholunate ligament produces a widened scapholunate gap (“Terry Thomas sign”) and progressive carpal collapse.

The triangular fibrocartilage complex (TFCC). Not a single structure but a group centered on the wedge-shaped articular disc, which runs from the ulnar notch of the radius to the base of the ulnar styloid. The complex also includes the dorsal and palmar radioulnar ligaments, the meniscus homolog, the ulnolunate and ulnotriquetral ligaments, the ulnar collateral ligament, and the subsheath of extensor carpi ulnaris. Its three jobs:

Stabilize the distal radioulnar joint through the whole arc of pronation–supination — its most important function.
Transmit and cushion axial load on the ulnar side of the wrist, carrying roughly 20% of the force from hand to forearm.
Separate the ulna from the carpus, forming the concave proximal surface of the ulnocarpal articulation.

TFCC injury presents as ulnar-sided wrist pain, clicking with rotation, weak grip and pain on the ulnar grind or press test; it follows a fall on a pronated extended hand, a distraction injury to the ulnar wrist, or repetitive loading in racquet and stick sports. Degenerative tears are far more likely with a positive ulnar variance (an ulna long relative to the radius), which increases ulnocarpal load.

Ranges of motion and which joint provides them. Figures vary with the source and with whether the fingers are flexed (long flexors and extensors are tightened by extremes and limit the wrist), but the ranking is consistent and testable.

Movement	Typical range	Main contributor	Limiting factor
Flexion	~80–90°	Midcarpal joint contributes the larger share	Tension in the dorsal radiocarpal ligaments and the extensor tendons
Extension	~70–85°	Radiocarpal joint contributes the larger share	Tension in the palmar radiocarpal ligaments; bony contact of the dorsal carpus on the radius
Adduction (ulnar deviation)	~30–45° — the greater of the two deviations	Mostly the radiocarpal joint	Radial collateral ligament
Abduction (radial deviation)	~15–20° — the lesser	Mostly the midcarpal joint	Impaction of the scaphoid against the radial styloid, which projects further distally than the ulnar styloid
Pronation / supination	~85–90° each	Proximal and distal radioulnar joints — not the wrist at all	Interosseous membrane, radioulnar ligaments and the antagonist muscles

Why radial deviation is so limited: the radial styloid extends about 1 cm more distally than the ulnar styloid and physically blocks the scaphoid. Remember that when reading a wrist radiograph — loss of that normal radial inclination is one of the measurements used to judge a distal radius fracture.

Pearl. Flexion exceeds extension and ulnar deviation exceeds radial deviation, and the two are combined in the natural arc of hand use — the “dart-thrower’s motion” runs from extension with radial deviation to flexion with ulnar deviation. It occurs mostly at the midcarpal joint, which is why splints that block radiocarpal motion after a scaphoid or ligament repair can still allow a surprising amount of functional hand use.

The carpal tunnel

The carpus is not flat: the four bony landmarks named above make it an arch, concave anteriorly. Bridging the arch is the flexor retinaculum (transverse carpal ligament), a thick fibrous band about 2–3 cm square, converting the carpal groove into the carpal tunnel.

Boundary	Structure
Roof (anterior)	Flexor retinaculum (transverse carpal ligament)
Floor and side walls (posterior and lateral)	The carpal arch — the concave palmar surfaces of the carpal bones and the interosseous ligaments binding them
Lateral (radial) attachments of the retinaculum	Tubercle of the scaphoid and tubercle (crest) of the trapezium
Medial (ulnar) attachments of the retinaculum	Pisiform and hook of the hamate

Contents — nine tendons and one nerve, ten structures in all:

Structure	Number	Note
Flexor digitorum superficialis tendons	4	Lie in two rows within the tunnel — the tendons to digits 3 and 4 lie superficial to those for digits 2 and 5
Flexor digitorum profundus tendons	4	Lie in a single deeper plane, on the floor of the tunnel
Flexor pollicis longus tendon	1	The most lateral (radial) content, in its own radial bursa
Median nerve	1	The most superficial and most radial structure, lying immediately deep to the retinaculum — which is exactly why it is the structure that gets compressed

The eight digital flexor tendons share a single common flexor (ulnar) synovial sheath; flexor pollicis longus has its own radial bursa, which usually communicates with the sheath of the thumb. That communication explains how an infected thumb pulp can track proximally into the wrist and, if the two bursae communicate, across into the little finger — a “horseshoe abscess.”

⭐ High yield — what is NOT in the carpal tunnel. Flexor carpi radialis runs in its own separate compartment, in a groove on the trapezium formed by a split in the lateral attachment of the retinaculum — deep to the retinaculum but outside the tunnel proper. Also outside: palmaris longus (superficial to the retinaculum, and often the surface landmark for it), the ulnar nerve and ulnar artery (superficial to the retinaculum in Guyon canal), flexor carpi ulnaris (inserting on the pisiform before the tunnel), and the palmar cutaneous branch of the median nerve (superficial to the retinaculum).

Carpal tunnel syndrome is the most common entrapment neuropathy in the body: compression of the median nerve in a tunnel with rigid walls and a fixed volume, so that any increase in the contents — most often thickening of the flexor tendon synovial sheaths from repetitive use — raises pressure on the nerve.

Risk factors: repetitive forceful wrist work and vibration; pregnancy; hypothyroidism; rheumatoid arthritis; diabetes; acromegaly; obesity; long-term hemodialysis (amyloid deposition); previous distal radius fracture or lunate dislocation.

Symptoms and signs, in the order they appear:

Paresthesia, burning and aching pain in the palmar aspect of the lateral three and a half digits (thumb, index, middle, radial half of ring) — the median nerve digital territory. Classically worse at night, and relieved by shaking the hand (the “flick sign”).
Pain may radiate proximally into the forearm and even the arm, which misleads.
Weakness of thumb abduction and opposition as the recurrent (thenar) motor branch fails — the patient drops small objects and cannot pinch.
Wasting of the thenar eminence, a late and often irreversible sign; the thumb comes to lie in the plane of the palm (“ape hand”).

⭐ High yield — why the thenar SKIN is spared. The palmar cutaneous branch of the median nerve arises about 5 cm proximal to the flexor retinaculum and passes superficial to it, never entering the tunnel. It supplies the skin over the thenar eminence and the central palm. Therefore in carpal tunnel syndrome the digits are numb but the palm over the thenar eminence has normal sensation, even though the thenar muscles waste. Preserved palmar sensation with numb fingers points to the tunnel; numbness that includes the thenar palm points to a lesion proximal to the wrist — in the forearm, at the elbow, or a C6–C7 radiculopathy.

Provocative tests.

Test	How	Positive result
Phalen test	Hold the wrists in full flexion, backs of the hands pressed together, for 60 seconds	Reproduction of paresthesia in the median digits. Flexion narrows the tunnel and raises pressure on the nerve
Reverse Phalen	Full wrist extension held similarly	Same symptoms — extension also raises tunnel pressure
Tinel sign	Percuss over the flexor retinaculum at the distal wrist crease	Electric tingling shooting into the median digits; a sign of an irritable, regenerating or compressed nerve at that point
Durkan carpal compression	Direct thumb pressure over the tunnel for 30 seconds	Symptom reproduction; the most sensitive of the three in most series

Management runs from night wrist splints in neutral (the position of lowest tunnel pressure — not flexion), activity modification and corticosteroid injection, to carpal tunnel release, in which the flexor retinaculum is divided along its ulnar aspect. Two structures dictate the incision: the recurrent (thenar) motor branch of the median nerve, which usually arises just distal to the retinaculum and turns back into the thenar muscles, and the superficial palmar arch distally. Cutting toward the ulnar side of the retinaculum keeps the blade away from both.

Pearl. Nerve conduction studies confirm the diagnosis, but the anatomy predicts what they show — slowed conduction across the wrist with normal conduction proximal to it. And remember the crucial negative in the history: carpal tunnel syndrome never weakens the long flexors of the fingers, because those muscles are innervated in the forearm. A patient who cannot flex the distal interphalangeal joints of the index finger and thumb has an anterior interosseous problem, not a carpal tunnel one.

Guyon canal (the ulnar canal / ulnar tunnel)

A separate, smaller passage on the ulnar side of the wrist, superficial to the flexor retinaculum — which is precisely why the ulnar nerve escapes carpal tunnel syndrome and why a carpal tunnel release does not decompress it.

Boundary	Structure
Roof	Palmar carpal ligament (a superficial thickening of the antebrachial fascia), reinforced distally by palmaris brevis
Floor	Flexor retinaculum (transverse carpal ligament) with the pisohamate and pisometacarpal ligaments
Medial (ulnar) wall	Pisiform and the tendon of flexor carpi ulnaris
Lateral (radial) wall	Hook of the hamate
Contents	Ulnar nerve (medial) and ulnar artery (lateral to the nerve), plus accompanying veins. Within or just distal to the canal the nerve divides into a superficial (mainly sensory) branch to the palmar skin of the medial one and a half digits and a deep (purely motor) branch that curves around the hook of the hamate to supply the hypothenar muscles, all interossei, the medial two lumbricals, adductor pollicis and the deep head of flexor pollicis brevis

Ulnar canal (Guyon canal) syndrome is compression here. Causes: a ganglion (the most common), fracture of the hook of the hamate, repetitive pressure from handlebars (“cyclist’s palsy”), habitual use of a hand tool or crutch, an ulnar artery aneurysm or thrombosis (hypothenar hammer syndrome), or a lipoma.

⭐ High yield — Guyon canal versus cubital tunnel. Both are ulnar neuropathies; the discriminators are entirely anatomic.

Dorsal hand sensation. The dorsal cutaneous branch of the ulnar nerve leaves the trunk about 5 cm proximal to the wrist and never enters Guyon canal. A wrist lesion therefore spares sensation on the dorsomedial hand and dorsum of the medial one and a half digits; an elbow lesion does not. Numb dorsal hand = elbow. Normal dorsal hand with numb palmar little finger = wrist.
Forearm muscles. Flexor carpi ulnaris and the medial half of flexor digitorum profundus are supplied in the forearm. A wrist lesion leaves them intact; an elbow lesion weakens ulnar deviation of the wrist and flexion of the distal interphalangeal joints of the ring and little fingers.
The ulnar paradox. Because the wrist lesion spares flexor digitorum profundus, the ring and little fingers still flex at the distal interphalangeal joints while their metacarpophalangeal joints hyperextend from loss of the medial lumbricals — so the claw is more marked in the more distal (wrist) lesion. The higher elbow lesion looks deceptively better because the profundus is also out.
Purely motor or purely sensory presentations are common at the wrist depending on whether the deep branch, the superficial branch or the trunk is compressed — a pattern almost never seen at the elbow.

Neurovasculature of the posterior forearm and wrist

Structure	Origin	Course	Supplies
Radial n. (C5–T1)	Posterior cord of the brachial plexus	Enters the forearm anterior to the lateral epicondyle between brachialis and brachioradialis; divides there	Brachioradialis, extensor carpi radialis longus (and, by the deep branch, brevis) before dividing
Superficial branch of the radial n.	Radial n. at the lateral epicondyle	Deep to brachioradialis with the radial artery; pierces deep fascia 7–9 cm above the wrist; crosses the snuffbox roof	Sensory only — dorsolateral hand and dorsum of lateral three and a half digits to the level of the proximal interphalangeal joints
Deep branch of the radial n. / posterior interosseous n.	Radial n. at the lateral epicondyle	Pierces supinator at the arcade of Frohse; runs on the interosseous membrane between the extensor layers; ends in a pseudoganglion at the wrist	Motor only — all remaining posterior compartment muscles; articular branches to the carpal joints
Posterior cutaneous n. of the forearm	Radial n. in the arm, in the spiral groove	Descends subcutaneously down the back of the forearm	Skin of the posterior forearm. Its origin in the arm is why forearm numbness accompanies a spiral groove lesion but not a posterior interosseous one
Dorsal cutaneous branch of the ulnar n.	Ulnar n., ~5 cm proximal to the wrist	Passes deep to flexor carpi ulnaris onto the dorsum of the hand	Skin of the dorsomedial hand and dorsum of the medial one and a half digits. Spared in Guyon canal compression
Common interosseous a.	Ulnar a., just distal to the radial tuberosity	Short trunk; divides almost immediately	Gives the anterior and posterior interosseous arteries
Posterior interosseous a.	Common interosseous a.	Passes over the superior (free) border of the interosseous membrane into the posterior compartment; descends with the posterior interosseous nerve	The proximal posterior compartment; gives the recurrent interosseous artery to the anastomosis behind the elbow
Anterior interosseous a.	Common interosseous a.	Descends on the front of the interosseous membrane, then pierces the membrane in the distal forearm to reach the back	Deep anterior compartment; after piercing, it supplies the distal posterior compartment and joins the dorsal carpal arch — effectively taking over from the dwindling posterior interosseous artery
Radial a.	Brachial a. in the cubital fossa	Runs on the lateral forearm under brachioradialis; passes dorsally around the wrist through the anatomical snuffbox; dives between the heads of the first dorsal interosseous	Lateral forearm; becomes the deep palmar arch. Gives the dorsal carpal branch and the princeps pollicis and radialis indicis arteries
Dorsal carpal arch (rete)	Dorsal carpal branches of the radial and ulnar arteries, joined by the terminal posterior interosseous and the perforating anterior interosseous	Lies on the dorsal carpus deep to the extensor tendons	The carpal bones — including the scaphoid, through a branch entering its dorsal ridge distally — and gives the dorsal metacarpal arteries

Pearl. The posterior interosseous artery is large proximally and small distally; the anterior interosseous artery is the reverse, piercing the membrane to reinforce the back of the wrist. The practical consequence is that a posterior interosseous artery flap can be raised from the back of the forearm and rotated to cover a hand defect without sacrificing the radial or ulnar artery, because its distal territory is fed by the anterior interosseous perforator.

Clinical correlates

Scaphoid fracture — the fracture that hides.

Mechanism: a fall on the outstretched hand (FOOSH) with the wrist hyperextended and radially deviated. The scaphoid is levered against the dorsal rim of the radius and snaps, most often at its waist (the narrow middle, roughly 70–80% of cases).
Signs: tenderness in the anatomical snuffbox, tenderness over the scaphoid tubercle at the front of the wrist, pain on axial compression of the thumb, and pain with resisted pronation. Swelling and loss of the normal snuffbox hollow.
Blood supply — the key fact. The scaphoid is fed almost entirely by branches of the dorsal carpal branch of the radial artery that enter through small foramina on its dorsal ridge, distally, and then run retrograde — distal to proximal — inside the bone. A smaller vessel enters the distal tubercle to supply the distal pole only.
Consequence: a fracture through the waist or the proximal pole cuts the proximal fragment off from its blood supply. The result is avascular necrosis of the proximal fragment and nonunion, in as many as a third of waist fractures and the great majority of proximal pole fractures. Union takes months even when it happens.
Imaging: the initial radiograph is negative in up to a quarter of true scaphoid fractures, because the fracture line is undisplaced and the bone is obliquely oriented. Standard practice is to treat a clinically suspicious wrist as a fracture — thumb spica immobilization — and repeat imaging in 10–14 days, or obtain MRI or CT sooner. Calling a tender snuffbox “a sprain” because the film is clean is the classic error.
Late sequel: untreated nonunion collapses into scaphoid nonunion advanced collapse (SNAC) — progressive radioscaphoid then midcarpal arthritis and a permanently painful, stiff wrist.

⭐ High yield — Retrograde blood flow explains everything about the scaphoid. The vessel enters distally and runs proximally, so the more proximal the fracture, the worse the prognosis. Proximal pole fracture → highest risk of avascular necrosis. The identical principle governs the femoral head (retinacular vessels running proximally along the neck) and the talus — three bones, one rule.

Distal radius fractures — Colles versus Smith. The distal radius is the most commonly fractured bone in the adult upper limb; both patterns follow a fall, and the direction of displacement is decided by the position of the wrist at impact.

	Colles fracture	Smith fracture (reverse Colles)
Mechanism	Fall onto an extended (dorsiflexed), pronated outstretched hand	Fall onto a flexed (palmar-flexed) wrist, or a direct blow to the dorsum of the wrist
Displacement of the distal fragment	Dorsal (posterior) displacement and angulation, with radial shortening and impaction	Volar (anterior) displacement and angulation
Deformity	“Dinner fork” (silver fork) deformity — the wrist profile steps up dorsally like the bend of a fork	“Garden spade” deformity — the hand sits volar to the forearm axis
Typical patient	Older adult, often osteoporotic, low-energy fall — the most common fracture in women over 50	Younger patient with a higher-energy mechanism, or a fall on the back of the hand
Associations	Avulsion of the ulnar styloid in a majority; acute median nerve contusion or compression; later extensor pollicis longus rupture; malunion with loss of radial inclination and volar tilt; complex regional pain syndrome	Often unstable and frequently needs volar plate fixation; median nerve injury also possible

Two more named patterns are worth recognizing: a Barton fracture is an intra-articular distal radius fracture with subluxation of the carpus following the fragment (dorsal or volar), and a chauffeur (Hutchinson) fracture is an isolated fracture of the radial styloid, historically from a crank handle kicking back.

Pearl. Two ways to keep them straight. First by name: Colles → Caudal-to-dorsal displacement of the hand; the fork points up. Second by mechanism: whichever way the wrist was bent at impact, the distal fragment goes the other way — fall on an extended wrist and the fragment ends up dorsal, fall on a flexed wrist and it ends up volar.

Lunate dislocation and acute median nerve compression. A fall on a forcibly dorsiflexed wrist can drive the capitate backward and squeeze the lunate out of its seat. The lunate rotates and dislocates anteriorly (palmarly), into the carpal tunnel — the only carpal bone that regularly does so, and the most commonly dislocated carpal bone.

Consequence: the displaced lunate reduces the volume of the carpal tunnel and produces acute carpal tunnel syndrome — sudden severe median paresthesia in the lateral three and a half digits with rapid thenar weakness. This is a surgical emergency, unlike the chronic form.
Radiographic signs: on the lateral view the lunate tips volar and its cup empties — the “spilled teacup” sign. On the posteroanterior view the lunate looks triangular (“piece of pie”) instead of quadrilateral, and the smooth Gilula arcs of the carpal rows are broken.
Perilunate dislocation is the related injury in which the lunate stays articulated with the radius and the rest of the carpus dislocates dorsally around it. It sits on the same spectrum and is missed just as often.
Late sequel: avascular necrosis of the lunate — Kienböck disease — with progressive wrist pain, collapse and arthritis.

De Quervain tenosynovitis. Stenosing tenosynovitis of the first extensor compartment — thickening of the sheath and tendons of abductor pollicis longus and extensor pollicis brevis as they turn around the radial styloid.

Who: classically new mothers lifting an infant with the thumb abducted and wrist ulnar deviated, and anyone doing repetitive thumb abduction with wrist deviation — racquet sports, hammering, prolonged device use. Also idiopathic, and associated with pregnancy and rheumatoid disease.
Findings: pain and swelling over the radial styloid, sometimes a palpable thickened sheath or crepitus, weak pinch and difficulty gripping.
Finkelstein test: the patient flexes the thumb into the palm, closes the fingers over it, and the examiner ulnar-deviates the wrist. Sharp pain over the radial styloid is positive — the maneuver puts maximal excursion and tension on the two tendons in their tight tunnel. (The version in which the examiner grasps the thumb and pulls it ulnarward is strictly the Eichhoff test; both are used clinically under Finkelstein’s name.)
Differential: thumb carpometacarpal (basal joint) osteoarthritis — tenderness is more distal, at the trapeziometacarpal joint, with a positive grind test; and intersection syndrome, in which the first compartment muscles cross over the second compartment 4–6 cm proximal to the wrist and produce pain and squeaking crepitus proximal to the retinaculum.
Treatment: thumb spica splint, activity modification, corticosteroid injection into the sheath, and surgical release of the first compartment — at which the surgeon must look for a septum subdividing the compartment (present in a large minority) and must protect the superficial branch of the radial nerve in the roof.

Lateral epicondylitis (tennis elbow). Despite the name, this is a degenerative tendinosis rather than a true inflammation — angiofibroblastic change at the origin of the extensor carpi radialis brevis on the common extensor tendon, with the extensor digitorum origin sometimes involved.

Cause: repetitive, forceful use of the wrist extensors — a backhand stroke with poor technique, but far more often manual work, plumbing, carpentry or repeated gripping.
Findings: tenderness precisely over and just distal to the lateral epicondyle; pain reproduced by resisted wrist extension with the elbow held in extension, by resisted middle finger extension (Maudsley test, which loads extensor carpi radialis brevis selectively), and by gripping — the “painful handshake.”
Contrast: medial epicondylitis (golfer’s elbow) is the mirror lesion of the common flexor origin; radial tunnel syndrome is tender 3–5 cm distal to the epicondyle.

Ganglion (synovial) cyst. A mucin-filled outpouching of a joint capsule or a tendon sheath, connected to the joint by a one-way valvular stalk. It is the most common soft-tissue mass of the hand and wrist, and firmly benign.

Dorsal wrist ganglion — about 60–70%, arising from the scapholunate interval and presenting as a firm swelling that becomes prominent on wrist flexion.
Volar wrist ganglion — about 20%, arising from the radiocarpal or scaphotrapezial joint, sitting close to the radial artery. Perform an Allen test before excision.
Others: volar retinacular cysts at the A1 pulley (a firm bead at the base of a finger) and mucous cysts at the distal interphalangeal joint, which are associated with osteoarthritic osteophytes and can groove the nail.
Clues: smooth, firm, mobile, transilluminates, may fluctuate in size. Ultrasound confirms if needed.
Why it matters here: a ganglion is a leading cause of compressive neuropathy at both wrist tunnels — in Guyon canal and at the arcade of Frohse in the forearm — and a ganglion of the extensor tendon sheath can present exactly like an extensor compartment tenosynovitis.

Putting the wrist syndromes side by side.

Syndrome	Structure compressed / injured	Where	Signature finding
Carpal tunnel syndrome	Median n.	Beneath the flexor retinaculum	Numb lateral 3.5 digits with normal thenar palm skin; thenar wasting; positive Phalen and Tinel
Guyon canal syndrome	Ulnar n.	Between pisiform and hook of hamate, superficial to the retinaculum	Numb palmar medial 1.5 digits with normal dorsomedial hand; clawing; intrinsic weakness
Cubital tunnel syndrome	Ulnar n.	Behind the medial epicondyle	Numbness including the dorsomedial hand; weak flexor carpi ulnaris and profundus to digits 4–5; a less obvious claw
Posterior interosseous n. syndrome	Deep branch of radial n.	Arcade of Frohse in supinator	Finger drop, no numbness, no true wrist drop; wrist extends into radial deviation
Radial tunnel syndrome	Deep branch of radial n.	Same tunnel	Pain without weakness, maximal 3–5 cm distal to the lateral epicondyle
Wartenberg syndrome	Superficial branch of radial n.	Where it pierces fascia between brachioradialis and extensor carpi radialis longus	Numbness of the dorsal thumb and first web space only; no weakness at all
De Quervain tenosynovitis	Abductor pollicis longus and extensor pollicis brevis	Extensor compartment 1 at the radial styloid	Radial styloid pain, positive Finkelstein
Scaphoid fracture	Scaphoid	Waist, in the snuffbox floor	Snuffbox tenderness after a FOOSH with an initially normal radiograph
TFCC injury	Articular disc and its ligaments	Ulnar side of the wrist	Ulnar-sided pain and clicking with forearm rotation and grip

⭐ ⭐ Rapid recall for this section.

Posterior compartment = radial nerve territory, without exception. Three muscles before the split (brachioradialis, extensor carpi radialis longus, extensor carpi radialis brevis); eight after it, by the posterior interosseous nerve.
Brachioradialis: posterior compartment, radial nerve, flexes the elbow, strongest in midpronation, tests C6.
Six extensor compartments radial to ulnar: APL+EPB / ECRL+ECRB / EPL / ED+EI / EDM / ECU — count 2-2-1-2-1-1, anchored to Lister tubercle between 2 and 3.
Snuffbox: APL and EPB in front, EPL behind, scaphoid and trapezium in the floor, radial artery inside, superficial radial nerve and cephalic vein in the roof.
Carpals: Some Lovers Try Positions That They Can’t Handle. Scaphoid fractures, lunate dislocates, capitate is largest, pisiform is a sesamoid.
Radiocarpal joint: radius + articular disc against scaphoid, lunate, triquetrum. The ulna does not touch the carpus.
Carpal tunnel: 4 superficialis + 4 profundus + 1 flexor pollicis longus + median nerve = nine tendons and a nerve. Flexor carpi radialis and the ulnar nerve are outside.
Thenar skin is spared in carpal tunnel syndrome — the palmar cutaneous branch leaves 5 cm early and runs over the retinaculum.
Guyon canal: pisiform medially, hook of hamate laterally, palmar carpal ligament above, flexor retinaculum below; ulnar nerve and artery inside; the dorsal cutaneous branch is spared.
Scaphoid blood supply is retrograde from the dorsal carpal branch of the radial artery — proximal fractures die.
Colles = dorsal, dinner fork, fall on an extended wrist. Smith = volar, garden spade, fall on a flexed wrist.