Anatomy of the arm along with the nerves and their course

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Anatomy of the Arm

Source: Gray's Anatomy for Students

Overview

The arm is the region of the upper limb between the shoulder and the elbow. Its superior aspect communicates medially with the axilla; inferiorly, important neurovascular structures pass through the cubital fossa anterior to the elbow joint.

The arm is divided into two compartments by medial and lateral intermuscular septa, which pass from each side of the humerus to the surrounding sleeve of deep fascia:

Anterior (flexor) compartment — muscles that flex the elbow
Posterior (extensor) compartment — muscle that extends the elbow

Cross-section of the arm showing anterior and posterior compartments, deep fascia, humerus, and intermuscular septa

Fig. 7.59 — Arm: proximal/distal relationships (A) and transverse section (B)

Skeleton — The Humerus

The sole skeletal support of the arm is the humerus. Key surface landmarks relevant to nerves and vessels:

Radial groove (spiral groove) — a shallow diagonal groove on the posterior surface that houses the radial nerve and profunda brachii artery
Medial and lateral borders — attachment points for the medial and lateral intermuscular septa
Lateral supraepicondylar ridge — attachment for brachioradialis and extensor carpi radialis longus

Muscles

Anterior Compartment

Fig. 7.64 — Anterior compartment muscles

Muscle	Origin	Insertion	Nerve	Spinal Level	Function
Coracobrachialis	Apex of coracoid process	Medial midshaft of humerus	Musculocutaneous	C5, C6, C7	Flexion of arm at glenohumeral joint
Biceps brachii	Long head: supraglenoid tubercle; Short head: coracoid process	Radial tuberosity + bicipital aponeurosis	Musculocutaneous	C5, C6	Powerful elbow flexion + forearm supination
Brachialis	Anterior surface of humerus (medial & lateral surfaces) + adjacent septa	Tuberosity of ulna	Musculocutaneous (predominantly); radial nerve (lateral portion)	C5, C6	Flexion of forearm at elbow

Posterior Compartment

Muscle	Origin	Insertion	Nerve	Spinal Level	Function
Triceps brachii	Long head: infraglenoid tubercle of scapula; Medial head: posterior humerus (below radial groove); Lateral head: posterior humerus (above radial groove)	Olecranon	Radial nerve	C6, C7, C8	Extension of forearm at elbow; long head also extends and adducts at shoulder

Nerves of the Arm

Five major nerves traverse the arm, all derived from the brachial plexus (C5–T1):

Fig. 7.68 — Nerves of the arm

1. Musculocutaneous Nerve (C5, C6, C7)

Origin: Lateral cord of the brachial plexus

Course:

Leaves the axilla and enters the arm by piercing the coracobrachialis muscle
Passes diagonally down the arm in the plane between the biceps brachii and brachialis muscles
Emerges lateral to the biceps tendon at the elbow
Penetrates deep fascia → continues as the lateral cutaneous nerve of the forearm

Motor supply in arm:

All three anterior compartment muscles: coracobrachialis, biceps brachii, brachialis

Sensory supply:

Skin on the lateral surface of the forearm (as the lateral cutaneous nerve of the forearm)

2. Median Nerve (C6–T1)

Origin: Formed by union of lateral (C6, C7) and medial (C8, T1) cords

Course:

Enters the arm at the inferior margin of the teres major muscle
Runs vertically down the medial side of the anterior compartment
Proximal arm: lies lateral to the brachial artery
Distal arm: crosses to the medial side of the brachial artery
Passes anterior to the elbow joint → enters the forearm

Branches in the arm: None of significance (a branch to pronator teres may arise just above the elbow)

Sensory: No cutaneous branches in the arm

3. Ulnar Nerve (C8, T1)

Origin: Medial cord of the brachial plexus

Course:

Enters the arm with the median nerve and axillary artery, lying medial to the axillary artery in the proximal arm
At mid-arm, it pierces the medial intermuscular septum → moves into the posterior compartment
Lies anterior to the medial head of the triceps brachii
Passes posterior to the medial epicondyle of the humerus (the "funny bone" groove)
Enters the anterior compartment of the forearm

Branches in the arm: None

4. Radial Nerve (C5–T1)

Origin: Posterior cord of the brachial plexus

Course:

Enters the arm by crossing the inferior margin of the teres major muscle, initially posterior to the brachial artery
Accompanied by the profunda brachii artery, enters the posterior compartment via the triangular interval
Passes diagonally (medial → lateral) through the posterior compartment, lying in the radial groove directly on bone
Pierces the lateral intermuscular septum → enters the anterior compartment
Lies between the brachialis (medially) and brachioradialis (laterally)
Enters the forearm anterior to the lateral epicondyle, just deep to brachioradialis

Branches in the arm:

Motor:

Triceps brachii (all three heads; the medial head branch arises before entering the posterior compartment and descends with the ulnar nerve)
Brachioradialis
Extensor carpi radialis longus
Lateral portion of brachialis

Cutaneous:

Inferior lateral cutaneous nerve of the arm — skin over the inferolateral arm
Posterior cutaneous nerve of the forearm — skin over the posterior forearm

Both cutaneous branches penetrate the lateral head of triceps + overlying fascia to reach the skin.

Radial nerve in the arm — showing its course in the radial groove, profunda brachii artery, triangular interval, inferior lateral cutaneous nerve, posterior cutaneous nerve of forearm, and branch to medial head of triceps

Fig. 7.69 — Radial Nerve in the Arm

5. Axillary Nerve (C5, C6)

Origin: Posterior cord of the brachial plexus

Course (passes through the arm's immediate proximal region):

Exits the axilla posteriorly through the quadrangular space (along with the posterior circumflex humeral artery)
Winds around the surgical neck of the humerus
Supplies the deltoid and teres minor muscles + skin over the lateral shoulder (superior lateral cutaneous nerve of arm)

(Technically supplies structures at the shoulder but is relevant context because it branches just proximal to the arm.)

Brachial Plexus Architecture (Summary Diagram)

Brachial plexus from roots C5–T1 through trunks, divisions, and cords to terminal branches — cadaveric dissection + color-coded schematic

Brachial plexus: roots → trunks → divisions → cords → terminal branches

Terminal Branch	Cord of Origin	Root Values
Musculocutaneous	Lateral	C5, C6, C7
Median	Lateral + Medial	C6–T1
Ulnar	Medial	C8, T1
Radial	Posterior	C5–T1
Axillary	Posterior	C5, C6

Arteries and Veins

Brachial Artery

Begins as continuation of the axillary artery at the lower border of teres major
Runs in the anterior compartment, moving from medial (proximal) to midline (distal)
Terminates just distal to the elbow joint → divides into radial and ulnar arteries
Key branch: profunda brachii artery — largest branch, enters the posterior compartment with the radial nerve through the triangular interval

Veins

Brachial veins (deep, paired): flank the brachial artery
Basilic vein (superficial): ascends medially, penetrates deep fascia mid-arm, joins the axillary vein
Cephalic vein (superficial): ascends on the anterolateral arm → passes through the anterior axillary wall to the axillary vein

Clinical Correlates

Radial Nerve Injury (Midshaft Humerus Fracture)

The radial nerve is tightly bound with the profunda brachii artery in the radial groove. A midshaft humeral fracture can stretch or transect the nerve here, producing:

Wrist drop — loss of wrist extension (paralysis of posterior forearm extensors)
Sensory loss over the dorsum of the hand

Ulnar Nerve at Medial Epicondyle

As the ulnar nerve passes posterior to the medial epicondyle, it is superficial and vulnerable to trauma. Injury here causes:

Loss of intrinsic hand muscle function (supplied in the forearm and hand)
"Claw hand" pattern + sensory loss over the little finger and medial half of the ring finger

Musculocutaneous Nerve Injury

Rare in isolation; causes weakness of elbow flexion and loss of biceps/brachioradialis reflex + sensory loss over the lateral forearm.

Gray's Anatomy for Students, Chapter 7 (Upper Limb), pp. 870–882

Anatomy of forearm and nerves supply along with route

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Anatomy of the Forearm

Source: Gray's Anatomy for Students, Chapter 7

Overview

The forearm extends between the elbow joint and the wrist joint.

Proximally: most structures pass through the cubital fossa (anterior to the elbow). The exception is the ulnar nerve, which passes posterior to the medial epicondyle.
Distally: structures pass through, or anterior to, the carpal tunnel. The exception is the radial artery, which passes dorsally around the wrist.

Forearm anatomy — proximal/distal relationships (A) and transverse cross-section showing anterior compartment, posterior compartment, radius, ulna, and interosseous membrane (B)

Fig. 7.79 — Forearm: relationships and cross-section

Skeleton

Two parallel bones form the framework:

Bone	Position	Proximal Dimension	Distal Dimension
Radius	Lateral	Small (articulates with capitulum)	Large (forms wrist joint)
Ulna	Medial	Large (trochlear notch, olecranon)	Small (ulnar styloid)

The interosseous membrane connects adjacent borders of radius and ulna and, together with the lateral intermuscular septum and deep fascia along the posterior ulnar border, separates the two compartments.

Compartments

	Anterior (Flexor) Compartment	Posterior (Extensor) Compartment
Function	Flex wrist/digits, pronate	Extend wrist/digits, supinate
Nerve	Median + Ulnar (partial)	Radial nerve (all muscles)
Layers	Superficial, Intermediate, Deep	Superficial, Deep

Muscles — Anterior (Flexor) Compartment

Rule: All anterior forearm muscles are innervated by the median nerve, except:

Flexor carpi ulnaris (FCU) → ulnar nerve

Medial half of flexor digitorum profundus (ring + little fingers) → ulnar nerve

Superficial Layer (all from medial epicondyle)

Muscle	Insertion	Nerve	Level	Function
Pronator teres	Middle lateral surface of radius	Median	C6, C7	Pronation; accessory elbow flexor
Flexor carpi radialis	Base of metacarpal II (± III)	Median	C6, C7	Wrist flexion + abduction
Palmaris longus (absent ~15%)	Flexor retinaculum + palmar aponeurosis	Median	C7, C8	Accessory wrist flexor
Flexor carpi ulnaris	Pisiform → hamate → metacarpal V	Ulnar	C7, C8	Wrist flexion + adduction

Intermediate Layer

Muscle	Origin	Insertion	Nerve	Level	Function
Flexor digitorum superficialis (FDS)	Medial epicondyle (humero-ulnar head) + anterior radius (radial head)	Middle phalanges, digits 2–5	Median	C7, C8, T1	Flexes PIP and MCP joints of digits 2–5; accessory wrist flexor

Deep Layer

Muscle	Origin	Insertion	Nerve	Level	Function
Flexor digitorum profundus (FDP) — lateral half (index/middle)	Anterior + medial surface of ulna + interosseous membrane	Distal phalanges, digits 2–5	Anterior interosseous nerve (branch of median)	C8, T1	Flexes DIP joints
Flexor digitorum profundus (FDP) — medial half (ring/little)	Same	Same	Ulnar nerve	C8, T1	Flexes DIP joints
Flexor pollicis longus	Anterior surface of radius + interosseous membrane	Distal phalanx of thumb	Anterior interosseous nerve	C8, T1	Flexes thumb IP joint
Pronator quadratus	Anterior surface of distal ulna	Anterior surface of distal radius	Anterior interosseous nerve	C8, T1	Pronation

Muscles — Posterior (Extensor) Compartment

Rule: All posterior forearm muscles are innervated by the radial nerve (superficial branch/posterior interosseous nerve).

Posterior compartment muscles — brachioradialis (anterior view) and superficial extensors (posterior view)

Fig. 7.90 — Posterior compartment muscles

Superficial Layer (common origin: lateral epicondyle / supraepicondylar ridge)

Muscle	Insertion	Nerve	Level	Function
Brachioradialis	Lateral distal radius (radial styloid)	Radial nerve (before bifurcation)	C5, C6	Elbow flexion (accessory; strongest mid-prone)
Extensor carpi radialis longus (ECRL)	Base of metacarpal II	Radial nerve (before bifurcation)	C6, C7	Wrist extension + abduction
Extensor carpi radialis brevis (ECRB)	Base of metacarpal III	Deep branch radial nerve	C7, C8	Wrist extension + abduction
Extensor digitorum	Extensor hoods, digits 2–5	Posterior interosseous nerve	C7, C8	Extends digits + wrist
Extensor digiti minimi	Extensor hood of little finger	Posterior interosseous nerve	C7, C8	Extends little finger
Extensor carpi ulnaris	Base of metacarpal V	Posterior interosseous nerve	C7, C8	Wrist extension + adduction
Anconeus	Olecranon + posterior ulna	Radial nerve	C7, C8, T1	Accessory elbow extensor

Deep Layer

Muscle	Origin	Insertion	Nerve	Level	Function
Supinator	Lateral epicondyle + radial collateral/anular ligaments + supinator crest of ulna	Lateral radius (superior to anterior oblique line)	Posterior interosseous nerve	C6, C7	Supination
Abductor pollicis longus (APL)	Posterior ulna + radius + interosseous membrane	Lateral base of metacarpal I	Posterior interosseous nerve	C7, C8	Thumb abduction
Extensor pollicis brevis (EPB)	Posterior radius + interosseous membrane	Base of proximal phalanx of thumb	Posterior interosseous nerve	C7, C8	Extends thumb MCP joint
Extensor pollicis longus (EPL)	Posterior ulna + interosseous membrane	Base of distal phalanx of thumb	Posterior interosseous nerve	C7, C8	Extends all thumb joints
Extensor indicis	Posterior ulna + interosseous membrane	Extensor hood of index finger	Posterior interosseous nerve	C7, C8	Extends index finger independently

Nerves of the Forearm

1. Median Nerve (C6–T1)

Nerves of the anterior forearm — median nerve, ulnar nerve, radial nerve with deep and superficial branches, anterior interosseous nerve, palmar branches, dorsal branch of ulnar nerve

Fig. 7.89 — Nerves of the anterior forearm

Entry: Leaves the cubital fossa by passing between the two heads of pronator teres (humeral + ulnar heads), then passes between the humero-ulnar and radial heads of flexor digitorum superficialis.

Course:

Runs straight down the forearm in the fascial plane on the deep surface of FDS
In the distal forearm, moves around the lateral side of FDS → becomes superficial between the tendons of palmaris longus (medially) and flexor carpi radialis (laterally)
Exits the forearm through the carpal tunnel deep to the flexor retinaculum

Branches in the forearm:

Branch	Course	Supply
Muscular branches	Arise just distal to elbow, medially from the nerve	Pronator teres, FCR, palmaris longus, FDS
Anterior interosseous nerve (AIN)	Arises between heads of pronator teres → descends with anterior interosseous artery along the interosseous membrane	FPL, lateral half of FDP (index + middle), pronator quadratus; terminates as articular branches to distal radioulnar joint and wrist
Palmar branch	Arises just proximal to flexor retinaculum → passes superficial to the retinaculum	Skin over base and central palm (spared in carpal tunnel syndrome)

2. Ulnar Nerve (C8, T1)

Entry: Passes posterior to the medial epicondyle → enters the anterior forearm by passing through the triangular gap between the humeral and ulnar heads of flexor carpi ulnaris (FCU)

Course:

Descends along the medial side of the forearm between FCU and FDP
In the distal two-thirds, the ulnar artery lies lateral to the nerve
Distally lies tucked under the anterolateral lip of the FCU tendon
Exits by passing superficial to the flexor retinaculum, lateral to the pisiform bone

Branches in the forearm:

Branch	Course	Supply
Muscular branches	Arise soon after entering the forearm	FCU + medial half of FDP (ring + little fingers)
Palmar branch	Arises in the middle of the forearm → passes superficially into the hand	Skin over the medial palm
Dorsal branch	Arises in the distal forearm → passes posteriorly deep to the FCU tendon	Skin over the posteromedial dorsum of hand + posterior surfaces of medial 1½ digits

3. Radial Nerve — in the Forearm (C5–T1)

The radial nerve bifurcates into deep and superficial branches under the margin of brachioradialis in the lateral wall of the cubital fossa.

Radial nerve in the forearm — deep branch, superficial branch, posterior interosseous nerve, branches to muscles (anterior and posterior views)

Fig. 7.92 — Radial nerve and posterior interosseous nerve in the forearm

Before bifurcation (at cubital fossa):

Branches to brachioradialis and extensor carpi radialis longus

A. Deep Branch (→ Posterior Interosseous Nerve)

Course:

Originates in the lateral cubital fossa, deep to brachioradialis
Innervates extensor carpi radialis brevis
Passes between the superficial and deep layers of the supinator muscle, winding around the lateral aspect of the proximal radius
Supplies supinator muscle during its passage through it
Emerges from the distal edge of supinator as the posterior interosseous nerve (PIN)
Lies between the superficial and deep layers of posterior forearm muscles
Terminates as articular branches to the wrist, passing deep to extensor pollicis longus

Muscles supplied by PIN: extensor carpi radialis brevis, supinator, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, APL, EPB, EPL, extensor indicis

B. Superficial Branch

Course:

Purely sensory
Descends down the anterolateral forearm deep to brachioradialis, in close association with the radial artery
Approximately two-thirds of the way down the forearm → passes laterally and posteriorly around the radial side of the forearm, deep to the brachioradialis tendon
Enters the hand to supply skin over the posterolateral surface (dorsum of lateral 3½ digits to PIP level)

4. Lateral Cutaneous Nerve of the Forearm (C5, C6)

Origin: Continuation of the musculocutaneous nerve after it emerges lateral to the biceps tendon at the elbow.

Course: Pierces deep fascia lateral to the biceps tendon → descends superficially along the lateral forearm to the wrist.

Supply: Skin over the lateral surface of the forearm (both anterior and posterior aspects laterally).

Summary: Nerve Supply of the Forearm

Nerve	Anterior Compartment Muscles	Posterior Compartment Muscles	Cutaneous
Median	Pronator teres, FCR, palmaris longus, FDS, FPL, lateral FDP, pronator quadratus	None	Skin of central/lateral palm (palmar branch)
Ulnar	FCU, medial FDP	None	Medial palm; posteromedial dorsum of hand (palmar + dorsal branches)
Radial	Brachioradialis, ECRL (before bifurcation)	All posterior muscles via PIN	Posterolateral dorsum of hand + lateral 3½ fingers (superficial branch)
Lateral cutaneous n. of forearm	None	None	Lateral forearm skin

Arteries

Artery	Course	Territory
Radial artery	Anterolateral forearm, medial to brachioradialis → passes dorsally around wrist	Radial side muscles; radial pulse lateral to FCR tendon
Ulnar artery	Medial forearm between FCU + FDP → enters hand lateral to pisiform, superficial to flexor retinaculum	Ulnar side muscles; dominant supply to palm via superficial palmar arch
Anterior interosseous artery	Anterior surface of interosseous membrane → perforates membrane distally	Deep anterior muscles; perforating branches to posterior compartment
Posterior interosseous artery	Passes over proximal interosseous membrane into posterior compartment	Superficial extensors; joins dorsal carpal arch

Clinical Correlates

Anterior Interosseous Nerve (AIN) Palsy

Caused by fractures, penetrating wounds, forearm surgery, or tight cast. Loss of FPL + lateral FDP + pronator quadratus → inability to flex the IP joint of the thumb and DIP joint of the index finger → loss of the "OK sign" (pinch posture). No sensory deficit (AIN is purely motor).

Posterior Interosseous Nerve (PIN) Compression

The PIN can be compressed as it passes through the supinator muscle (arcade of Frohse). Results in weakness of finger/thumb extension with no wrist drop (ECRL is spared, as it is innervated before bifurcation) and no sensory loss (the superficial branch carries sensation).

Carpal Tunnel Syndrome

The median nerve is compressed under the flexor retinaculum. The palmar branch (arising proximal to the retinaculum, passing superficial to it) is spared, so thenar skin sensation is preserved while lateral 3½ fingers are affected.

Cubital Tunnel Syndrome

Ulnar nerve compression at the medial epicondyle/between the FCU heads. Results in weakness of intrinsic hand muscles (in addition to FCU + medial FDP), sensory loss over the medial 1½ fingers, and "claw hand" of the ring and little fingers.

Gray's Anatomy for Students, Chapter 7 (Upper Limb) — Forearm section, pp. 892–910

Flexor retinaculum

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Flexor Retinaculum

Source: Gray's Anatomy for Students, Chapter 7

Definition

The flexor retinaculum (also called the transverse carpal ligament) is a thick, strong band of connective tissue that bridges across the anterior aspect of the wrist, converting the concave arch of the carpal bones into the carpal tunnel — a fibro-osseous channel through which the long flexor tendons and median nerve pass from the forearm into the hand.

Attachments (4 bony points)

The retinaculum spans between the medial and lateral sides of the base of the carpal arch:

Side	Bony Attachment
Lateral (radial)	Tubercle of the scaphoid + tubercle of the trapezium
Medial (ulnar)	Pisiform + hook of the hamate

Anterior view of the wrist showing the flexor retinaculum, pisiform, tubercle of scaphoid, FCR tendon, FCU tendon, and the recurrent branch of the median nerve supplying the thenar eminence

Fig. 7.124 — Position of the flexor retinaculum and recurrent branch of the median nerve

Surface Landmarks

Proximal margin — an imaginary line between the pisiform (palpable at the distal end of the FCU tendon) and the tubercle of the scaphoid (palpable at the distal end of the FCR tendon)
Distal margin — deep to the junction of the anterior margin of the thenar eminence and the hypothenar eminence, near the base of the palm

The Carpal Tunnel

The carpal arch (concavity formed by the carpal bones) + the flexor retinaculum anteriorly = the carpal tunnel.

Roof (anterior wall): Flexor retinaculum
Floor and sides: Carpal bones (scaphoid, lunate, triquetrum, pisiform, trapezium, trapezoid, capitate, hamate)

The retinaculum holds the tendons against the bony plane and prevents bowstringing during wrist flexion.

Contents of the Carpal Tunnel (9 tendons + 1 nerve)

Everything passing deep to (through) the flexor retinaculum:

Structure	Notes
Flexor digitorum superficialis (FDS) tendons ×4	Enclosed in a common synovial sheath with FDP
Flexor digitorum profundus (FDP) tendons ×4	Enclosed in the same common synovial sheath as FDS
Flexor pollicis longus (FPL) tendon ×1	Enclosed in its own separate synovial sheath
Median nerve	Positioned anterior to the tendons (most superficial structure in the tunnel)

Total: 9 tendons + 1 nerve

Synovial sheaths around the tendons facilitate free gliding movement within the tunnel.

Carpal tunnel cross-section (A) and axial MRI (B) and coronal MRI (C) — showing the flexor retinaculum as the roof, median nerve anteriorly, FDS and FDP tendons in common sheath, FPL tendon separately, and carpal arch as the floor. Extensor tendons are shown posteriorly on the dorsal aspect.

Fig. 7.98 — Carpal Tunnel: structure, MRI axial, and MRI coronal

Structures Passing OUTSIDE (Superficial/Anterior to) the Retinaculum

These structures do not pass through the carpal tunnel:

Structure	Position	Notes
Ulnar nerve	Anterior to retinaculum (Guyon's canal)	Passes lateral to the pisiform
Ulnar artery	Anterior to retinaculum	Lies lateral to the ulnar nerve
Palmaris longus tendon	Anterior to retinaculum	Not enclosed in a synovial sheath
Palmar branch of median nerve	Anterior to retinaculum	Branches from median nerve proximal to retinaculum; supplies skin over central/lateral palm — spared in CTS
Radial artery	Passes posterolaterally, dorsally around the wrist	Does not pass through carpal tunnel

Special case — Flexor carpi radialis (FCR) tendon: The FCR tendon does pass through a separate compartment formed by the lateral aspect of the flexor retinaculum folding around a groove on the medial side of the tubercle of the trapezium. It is enclosed in its own synovial sheath and is technically within the retinaculum, but outside the main carpal tunnel.

Muscular Attachments from the Retinaculum

The flexor retinaculum serves as the origin for the thenar and hypothenar muscles:

Muscle Group	Muscles Originating from Retinaculum
Thenar muscles	Abductor pollicis brevis, flexor pollicis brevis (superficial head), opponens pollicis
Hypothenar muscles	Abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi

The palmar aponeurosis also has its apex continuous with the flexor retinaculum (when palmaris longus is absent, the aponeurosis anchors directly to it).

Recurrent (Motor) Branch of the Median Nerve

Immediately after emerging from the distal edge of the flexor retinaculum, the median nerve gives off its recurrent branch, which:

Curves back (recurs) over the distal margin of the retinaculum
Lies deep to skin and deep fascia over the anterior margin of the thenar eminence
Supplies all three thenar muscles: abductor pollicis brevis, flexor pollicis brevis, opponens pollicis
This branch is vulnerable during carpal tunnel release surgery — division causes permanent loss of thumb opposition

Clinical: Carpal Tunnel Syndrome (CTS)

Mechanism: Increased pressure within the carpal tunnel compresses the median nerve, causing venous congestion → nerve edema → ischemic axonal injury.

Common causes: Repetitive wrist use, rheumatoid arthritis (tendon sheath swelling), pregnancy (fluid retention), cysts from carpal joints, hypothyroidism, diabetes. In rare cases, a median artery variant may persist and run through the tunnel alongside the nerve.

Symptoms:

Pain, paraesthesia ("pins and needles"), and numbness in the distribution of the median nerve — lateral 3½ fingers (thumb, index, middle, radial half of ring)
Thenar wasting and weakness of pinch grip in advanced cases
Tinel's sign: tapping over the flexor retinaculum reproduces symptoms
Thenar skin is SPARED because the palmar branch of the median nerve branches proximal to the retinaculum and passes superficial to it

Treatment:

Conservative: splinting, anti-inflammatory measures, corticosteroid injection
Surgical: division of the flexor retinaculum (carpal tunnel decompression) — releases the roof, relieves pressure on the nerve

Summary Table

Feature	Detail
Structure	Thick connective tissue band (transverse carpal ligament)
Lateral attachments	Tubercle of scaphoid + tubercle of trapezium
Medial attachments	Pisiform + hook of hamate
Forms	Roof (anterior wall) of the carpal tunnel
Contents of tunnel	FDS ×4, FDP ×4, FPL ×1 tendons + median nerve
Passes anterior to it	Ulnar nerve, ulnar artery, palmaris longus, palmar branch of median n.
Muscle origins	Thenar muscles + hypothenar muscles
Key nerve through it	Median nerve (compressed in carpal tunnel syndrome)
Key nerve from it	Recurrent branch of median nerve → thenar muscles

Gray's Anatomy for Students, Chapter 7 (Upper Limb) — pp. 919–920

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