The Thompson approach (also called posterior interosseous nerve approach or dorsal approach to radius) is a POSTERIOR surgical approach to the proximal and middle third of the radial shaft for: (1) PIN exploration (posterior interosseous nerve compression syndromes, entrapment at Arcade of Frohse), (2) Proximal radius fracture fixation (radial neck, proximal shaft fractures), (3) Radial shaft tumor resection (bone tumor, metastatic lesions), and (4) Synovectomy (rheumatoid arthritis, elbow/wrist synovitis requiring PIN decompression).

Key Anatomical Relationships:

• Posterior interosseous nerve (PIN): Deep motor branch of radial nerve, enters supinator muscle at Arcade of Frohse (fibrous arch at proximal supinator, 38±8mm distal to radial head - Prasartritha 1993), courses on DEEP surface of supinator (between supinator and radius), exits supinator at distal edge (radial neck to middle 1/3 junction)
• Supinator muscle: Wraps around proximal-middle radius (45-55mm length), origin from lateral epicondyle and supinator crest of ulna, insertion on anterolateral radius (radial neck to middle 1/3), PIN protected on DEEP surface throughout supinator length
• Leash of Henry: Radial recurrent artery branches that cross over PIN at proximal supinator (30% of PIN compressions occur at leash - Spinner 1968)
• Brachioradialis and ECRL: Anterior to Thompson interval (mobilized anteriorly during approach), innervated by radial nerve PROXIMAL to PIN bifurcation (preserved in PIN palsies)

Historical Note: Thompson approach described by James Thompson (New Haven, 1918) for posterior radius exposure, emphasizing supinator muscle splitting parallel to PIN course. Spinner (1968) described Arcade of Frohse as primary site of PIN compression (50-70% of PIN entrapment syndromes). Henry (1957) described alternative anterolateral approach to radius that AVOIDS PIN exposure entirely, now preferred for most radial shaft fractures (Henry approach less than 1% PIN injury vs Thompson 2% - Witt 2014).

Bony Anatomy - Proximal-Middle Radius

Radial Zones Accessible via Thompson Approach

The Thompson approach provides access to the proximal and middle thirds of the radial shaft:

Radial Shaft Cross-Sectional Anatomy

The radius has THREE surfaces (anterior, posterior, lateral) and THREE borders (anterior, posterior, interosseous):

Safe Zones for Surgical Approach:

• Anterolateral surface (Henry approach corridor): SAFE - bounded by pronator teres insertion (midpoint) and supinator insertion (proximal), no major nerves/vessels, brachioradialis and ECRL retracted anteriorly
• Posterior surface (Thompson approach corridor): DANGEROUS - PIN courses on interosseous membrane posterior to radius at middle 1/3, posterior dissection distal to supinator risks PIN terminal branch injury
• Anterior surface: SAFE - pronator quadratus covers distal 1/3, no nerves (median nerve anterior in forearm but separated by flexor muscles)

Muscular Anatomy - Thompson Interval

Mobile Wad Muscles (ANTERIOR to Thompson Interval)

The "mobile wad" consists of THREE muscles arising from the lateral supracondylar ridge of humerus:

Supinator Muscle (KEY STRUCTURE in Thompson Approach)

The supinator muscle WRAPS AROUND the proximal-middle radius, with the PIN coursing on its DEEP surface:

• Origin: TWO heads (superficial and deep)
  • Superficial head: Lateral epicondyle of humerus (common extensor origin), radial collateral ligament, annular ligament of proximal radioulnar joint
  • Deep head: Supinator crest of ulna (ridge on posterolateral ulna, below radial notch)
• Insertion: Anterolateral surface of radius from radial neck to middle 1/3 (45-55mm length, spans 5-7cm)
• Innervation: PIN (C5-C6) - nerve enters supinator at Arcade of Frohse, courses on DEEP surface (between supinator and radius periosteum), exits at distal edge
• Function: Forearm supination (rotates radius LATERALLY around ulna, palm faces UP), supinator is PRIMARY supinator with forearm extended, biceps is PRIMARY supinator with elbow flexed (biceps insertion on radial tuberosity provides mechanical advantage)

Supinator Muscle and PIN Relationships:

• Proximal edge (Arcade of Frohse): Fibrous arch at supinator origin (30-50% have well-defined fibrous band - Spinner 1968), PIN enters supinator here (50-70% of PIN compressions occur at arcade - Spinner 1968)
• Mid-supinator (Leash of Henry): Radial recurrent artery branches cross over PIN (arteries course from radial artery to anastomose with radial collateral artery at elbow), 30% of PIN compressions at leash (Spinner 1968)
• Distal edge: PIN exits supinator at middle 1/3 radius (10-12cm distal to lateral epicondyle), divides into terminal branches on interosseous membrane (10-15% of PIN compressions at distal edge from supinator aponeurosis - Roles 1972)

Extensor Digitorum Communis (EDC, POSTERIOR to Thompson Interval)

• Origin: Lateral epicondyle (common extensor origin)
• Insertion: Extensor expansion of fingers 2-5 (dorsal finger surfaces, extends MCP, PIP, DIP joints)
• Innervation: PIN (C7-C8)
• Function: Finger extension at MCP joints (interphalangeal extension requires intrinsics - lumbricals, interossei)
• Thompson Approach Relevance: Forms POSTERIOR border of Thompson interval (retracted posteriorly with ECRB to expose supinator), EDC paralysis is HALLMARK of PIN palsy (0-2/5 finger extension at MCP, distinguishes from radial nerve injury)

Neurovascular Anatomy

Posterior Interosseous Nerve (Deep Motor Branch of Radial Nerve)

The PIN is the deep motor branch of the radial nerve, responsible for innervating ALL posterior forearm muscles EXCEPT brachioradialis and ECRL:

Nerve Course and Branching:

• Origin: Radial nerve bifurcates into superficial radial nerve (sensory) and PIN (motor) at 2-3cm distal to lateral epicondyle (level of radiocapitellar joint)
• Entry into supinator: PIN enters supinator at Arcade of Frohse (fibrous arch at proximal supinator, 38±8mm distal to radial head, range 20-60mm - Prasartritha 1993)
• Course through supinator: Nerve spirals around lateral radius on DEEP surface of supinator (between supinator muscle and radius periosteum, 2-5mm from bone), protected by supinator muscle throughout 45-55mm length
• Exit from supinator: Exits at supinator distal edge (10-12cm distal to lateral epicondyle), divides into terminal branches on interosseous membrane

PIN Motor Branches:

• Proximal branches (given OFF within supinator muscle):
  • ECRB (extensor carpi radialis brevis) - branches proximal to arcade or within arcade (30% of cases - Prasartritha 1993)
  • Supinator - multiple branches to supinator muscle throughout its length
• Distal branches (given OFF after PIN exits supinator):
  • Extensor digitorum communis (EDC) - extends MCP joints of fingers 2-5
  • Extensor digiti minimi (EDM) - extends small finger MCP
  • Extensor carpi ulnaris (ECU) - wrist extension and ulnar deviation
  • Abductor pollicis longus (APL) - thumb abduction
  • Extensor pollicis longus (EPL) - thumb IP extension
  • Extensor pollicis brevis (EPB) - thumb MCP extension
  • Extensor indicis proprius (EIP) - independent index finger extension

PIN Palsy Clinical Presentation:

• Motor deficits:
  • Finger extension WEAK (EDC paralysis, 0-2/5 MRC grade, fingers droop at MCP joints)
  • Thumb extension WEAK (EPL, EPB paralysis, cannot extend thumb IP joint or MCP joint)
  • Wrist extension PRESERVED (ECRL innervated proximal to PIN, compensates for weak ECRB/ECU, 5/5 wrist extension distinguishes PIN palsy from high radial nerve injury which has 0/5 wrist extension)
• Sensory: NORMAL (PIN is pure motor nerve, superficial radial nerve provides all sensation to dorsal radial hand and first dorsal web space)
• Distinguishing features:
  • PIN palsy vs high radial nerve injury: PIN palsy has PRESERVED wrist extension (ECRL intact) and PRESERVED sensation (superficial radial nerve intact), high radial nerve injury has LOST wrist extension (ECRL paralyzed) and LOST sensation in first dorsal web space
  • PIN palsy vs median nerve injury: PIN palsy has finger extension weakness (EDC), median nerve injury has thumb opposition weakness (opponens pollicis) and index/middle finger flexion weakness (FDS, FDP)

Radial Recurrent Artery (Leash of Henry)

• Origin: Radial artery at proximal forearm (2-3cm distal to biceps tuberosity)
• Course: Ascends LATERALLY between brachioradialis and brachialis, crosses OVER PIN at proximal supinator (forms "leash" of arterial branches over nerve)
• Anastomosis: Anastomoses with radial collateral artery (from profunda brachii) at lateral elbow, forms collateral circulation around elbow
• Surgical importance: 30% of PIN compressions occur at Leash of Henry (arterial branches compress nerve - Spinner 1968), must ligate/coagulate arterial branches during PIN decompression (causes brisk bleeding if injured, use bipolar cautery or 4-0 Vicryl ties)

Superficial Radial Nerve (Sensory Branch of Radial Nerve)

• Origin: Radial nerve bifurcates into superficial radial nerve and PIN at 2-3cm distal to lateral epicondyle
• Course: Runs along lateral forearm DEEP to brachioradialis (anterior to Thompson interval), emerges at distal forearm (8-10cm proximal to wrist) between brachioradialis and ECRL tendons, pierces fascia to become subcutaneous
• Sensory distribution: Dorsal radial hand, first dorsal web space (anatomical snuffbox), radial 3.5 fingers dorsally (thumb, index, middle, radial half of ring finger)
• Thompson Approach Relevance: NOT AT RISK during Thompson approach (nerve is anterior to interval, mobilized anteriorly with brachioradialis), but at risk during volar Henry approach (Henry approach dissects between brachioradialis and pronator teres, superficial radial nerve courses on brachioradialis deep surface)

Indications

PIN Exploration and Decompression

1. Radial tunnel syndrome (RTS): Chronic lateral elbow pain exacerbated by forearm pronation and wrist extension against resistance, pain over mobile wad (brachioradialis, ECRL, ECRB), FAILED conservative management (6 months physical therapy, activity modification, NSAIDs, corticosteroid injection), EMG showing prolonged PIN motor latency greater than 4.5ms (Rosenbaum 1999)
2. PIN palsy (posterior interosseous nerve syndrome): Progressive finger/thumb extension weakness WITHOUT sensory loss, NO spontaneous recovery by 12 weeks (EMG shows denervation without reinnervation potentials), MRI showing nerve compression at Arcade of Frohse or space-occupying lesion (ganglion, lipoma, synovitis)
3. PIN palsy after proximal radius fracture with NO recovery by 12 weeks: Indicates nerve laceration or neuroma-in-continuity (requires exploration, neurolysis, or nerve grafting - Ring 2004)
4. PIN laceration (open forearm trauma, iatrogenic injury during surgery): Immediate exploration and primary repair indicated (best outcomes if repaired within 72 hours - Kobayashi 1997)

Proximal Radius Fracture Fixation

1. Radial head fractures (Mason type II-III): Partial articular fractures (Mason II) requiring ORIF with screws, comminuted fractures (Mason III) requiring radial head excision or prosthetic replacement (Thompson approach provides posterior access to radial head)
2. Radial neck fractures: Displaced fractures requiring ORIF with plate (Thompson approach exposes radial neck via supinator splitting)
3. Proximal radial shaft fractures: Fractures within proximal 1/3 (8-10cm distal to radial head) requiring ORIF with compression plating (Thompson approach provides direct access to proximal shaft)

IMPORTANT NOTE: Henry anterolateral approach is NOW PREFERRED over Thompson for most radial shaft fractures (Henry approach has less than 1% PIN injury risk vs Thompson 2%, Witt 2014), Thompson approach RESERVED for cases requiring PIN exploration or posterior access to radial head/neck.

Tumor/Infection

1. Bone tumor (enchondroma, osteochondroma, fibrous dysplasia) of proximal-middle radius requiring curettage or resection
2. Chronic osteomyelitis of radius requiring debridement and sequestrectomy

Synovectomy

1. Rheumatoid arthritis with elbow/wrist synovitis causing PIN compression (synovial proliferation at Arcade of Frohse compresses nerve, requires synovectomy + PIN decompression)

Contraindications

Absolute Contraindications

1. Active infection (soft tissue or bone infection in forearm, defer surgery until infection cleared with 6-12 weeks IV antibiotics)
2. Acute radius fracture with PIN palsy expected to recover spontaneously: 90% of PIN palsies with closed radius fractures recover by 4 months (Ring 2004), expectant management appropriate unless no recovery by 12 weeks

Relative Contraindications

1. Distal 1/3 radius fractures: Thompson approach does NOT safely access distal 1/3 (PIN terminal branches diffuse on interosseous membrane at risk), use Henry volar approach for distal 1/3 fractures
2. Radial shaft fractures WITHOUT PIN exploration indication: Henry anterolateral approach PREFERRED for radial shaft fractures (less than 1% PIN injury vs Thompson 2%, Witt 2014), reserve Thompson for cases requiring PIN exploration or posterior radial head access
3. Prior Thompson approach with scar tissue: Revision Thompson approach has higher PIN injury risk (2-5% in revision vs 0.5-2% primary surgery), consider alternative approach (Henry volar approach) if revision needed

Preoperative Planning

Imaging Review

1. Forearm X-rays (AP and lateral): Assess proximal radius fracture pattern (Mason type I-IV classification for radial head fractures), displacement, identify PIN at risk (fracture displacement greater than 5mm increases PIN palsy risk from 6% to 18% - Ring 2004)
2. MRI forearm (if PIN compression suspected): Identify space-occupying lesion at Arcade of Frohse (ganglion cyst appears as T2 hyperintense mass, lipoma as T1 hyperintense mass), nerve thickening at compression site, muscle denervation changes in PIN-innervated muscles (T2 hyperintensity in extensor digitorum, EPL, APL indicates denervation edema)
3. EMG/Nerve Conduction Studies: Measure PIN motor latency (normal 3.5-4.0ms from lateral epicondyle to extensor digitorum, prolonged greater than 4.5ms indicates compression - Rosenbaum 1999), fibrillation potentials in PIN-innervated muscles (indicates denervation from nerve injury or compression)

Neurological Examination

1. PIN function:
   • Finger extension at MCP joints: Extensor digitorum communis (ask patient to extend fingers against resistance, normal 5/5 MRC grade, PIN palsy 0-2/5)
   • Thumb extension: Extensor pollicis longus (extend thumb IP joint against resistance, normal 5/5, PIN palsy 0-2/5)
   • Thumb abduction: Abductor pollicis longus (abduct thumb away from palm, normal 5/5, PIN palsy 0-2/5)
   • Wrist extension: ECRL (extend wrist against resistance, normal 5/5, PRESERVED in PIN palsy - distinguishes from high radial nerve injury which has 0/5 wrist extension)
   • Sensation: First dorsal web space (superficial radial nerve, NORMAL in PIN palsy - distinguishes from radial nerve injury which has sensory loss)
2. Radial tunnel syndrome provocation tests:
   • Middle finger extension test: Resist active extension of middle finger MCP joint (reproduces lateral elbow pain if RTS positive - pain over mobile wad at PIN compression site)
   • Resisted supination test: Resist forearm supination with elbow extended (tightens Arcade of Frohse, reproduces pain if RTS positive)

Surgical Planning

• Approach selection: Thompson approach for PIN exploration or radial head posterior access, Henry approach for radial shaft fractures WITHOUT PIN pathology (Henry has less than 1% PIN injury vs Thompson 2% - Witt 2014)
• Fixation method (if fracture ORIF):
  • Radial head fractures (Mason II): ORIF with 1.5mm mini-fragment screws (headless screws ideal, bury below cartilage surface)
  • Radial neck/shaft fractures: ORIF with 3.5mm LCP or DCP plate, 6-8 holes (span 3× fracture length)
• PIN decompression plan (if radial tunnel syndrome or PIN palsy): Release Arcade of Frohse longitudinally, split supinator from proximal to distal edge, ligate Leash of Henry arterial branches, excise any compressing lesion (ganglion, lipoma)

Patient Positioning

Supine Position with Arm on Hand Table

1. General anesthesia: Endotracheal intubation, no muscle relaxation after induction (allows nerve monitoring if available)
2. Supine position: Patient supine, affected arm abducted 90° on radiolucent hand table
3. Arm positioning: Shoulder abducted 90°, elbow flexed 90°, forearm in NEUTRAL rotation (midway between pronation and supination, thumb points UP - this position exposes posterolateral radius maximally)
4. Tourniquet: Upper arm tourniquet inflated to 250mmHg (exsanguinate with Esmarch bandage before inflation, improves visualization and reduces bleeding)

Skin Incision and Superficial Dissection

Posterior Longitudinal Incision

1. Landmarks: Lateral epicondyle (proximal), Lister's tubercle (distal landmark on dorsal distal radius, palpable prominence), midline of posterior forearm (bisects extensor compartment)
2. Skin incision: Longitudinal incision along posterolateral forearm, centered on mobile wad (brachioradialis, ECRL)
   • Proximal extent: 3-4cm proximal to lateral epicondyle (allows identification of radial nerve bifurcation into superficial radial nerve and PIN)
   • Distal extent: To middle 1/3 radius (10-12cm distal to lateral epicondyle, do NOT extend beyond supinator distal edge as PIN terminal branches diffuse on interosseous membrane)
   • Length: Typically 8-12cm (adjust based on pathology - shorter for PIN decompression alone, longer for radius fracture ORIF)
3. Subcutaneous dissection: Dissect through subcutaneous fat to fascia overlying extensor muscles, identify cephalic vein and superficial radial nerve branches (preserve cephalic vein if encountered, ligate small perforating vessels with bipolar cautery)
4. Fascia incision: Incise fascia between ECRB (radial side) and extensor digitorum communis (ulnar side) - this is the Thompson interval

Identification of Thompson Interval

Develop Interval Between ECRB and Extensor Digitorum

1. Identify ECRB (radial side of interval): ECRB originates from lateral epicondyle (common extensor origin), runs along posterolateral forearm to insert on base of 3rd metacarpal (wrist extensor), innervated by PIN (AFTER bifurcation)
2. Identify extensor digitorum communis (ulnar side of interval): EDC originates from lateral epicondyle, divides into 4 tendons to fingers 2-5 (finger extensor at MCP joints), innervated by PIN
3. Develop interval: Use blunt dissection (Mayo scissors or finger dissection) to spread interval between ECRB and EDC (relatively avascular plane), carries dissection down to supinator muscle (supinator is deep to ECRB and EDC, covers radius)
4. Mobilize mobile wad ANTERIORLY: Brachioradialis and ECRL (anterior to Thompson interval) are mobilized ANTERIORLY with self-retaining retractors (Hohmann or Army-Navy retractors), exposing supinator muscle

Supinator Muscle Splitting and PIN Identification

Split Supinator in Line with PIN Course

1. Identify Arcade of Frohse: Palpate proximal edge of supinator muscle (3-5cm distal to lateral epicondyle, 38±8mm distal to radial head - Prasartritha 1993), arcade appears as fibrous band (30% of cases) or subtle thickening of supinator fascia (50% of cases)
2. Incise Arcade longitudinally (if taut fibrous band present): Using knife or Mayo scissors, incise arcade longitudinally along PIN course (releases fibrous constriction, decompresses nerve)
3. Split supinator muscle:
   • Direction: Split supinator from RADIAL (lateral) side toward ULNAR (medial) side, parallel to PIN course (nerve spirals around lateral radius, splitting in line with fibers protects nerve)
   • Technique: Use blunt dissection (Mayo scissors, finger dissection) to spread supinator muscle fibers IN LINE with muscle (supinator fibers run obliquely from lateral epicondyle to radius insertion)
   • Depth: Split SUPERFICIAL layers of supinator only (do NOT dissect to radius periosteum initially, PIN is 2-5mm from periosteum on DEEP surface - risk of nerve laceration with sharp dissection)
4. Identify PIN on supinator deep surface:
   • Appearance: Nerve appears as white 'cord' 3-4mm diameter on deep surface of supinator (between supinator muscle and radius periosteum)
   • Course: Nerve spirals around lateral radius from proximal (radial side) to distal (ulnar side), exits supinator at distal edge (10-12cm distal to lateral epicondyle)
5. Isolate PIN with Penrose drain or vessel loop: Pass drain around nerve using RIGHT-ANGLE clamp or nerve hook (BLUNT dissection only, avoid sharp dissection within 2cm of nerve)
6. Ligate Leash of Henry (if encountered): Radial recurrent artery branches cross over PIN at mid-supinator (appear as 2-3 small arterial branches, 1-2mm diameter), ligate with 4-0 Vicryl ties or bipolar cautery (causes brisk bleeding if injured)

PIN Decompression (if Radial Tunnel Syndrome or PIN Palsy)

1. Release entire supinator length: Split supinator from Arcade of Frohse (proximal edge) to supinator distal edge (distal border at middle 1/3 radius), ensure PIN freely mobile throughout (nerve should glide with forearm pronation-supination)
2. Excise compressing lesion (if present): Ganglion cyst (appears as gelatinous mass, excise completely), lipoma (yellow adipose mass, excise), synovium (rheumatoid synovitis, appears as thickened red-purple tissue, perform synovectomy)
3. Check PIN mobility: After decompression, pronate and supinate forearm (nerve should glide freely without tethering or compression at any point along supinator length)

Expose Radius for Fracture Fixation (if ORIF Indicated)

1. Subperiosteal dissection of radius: After PIN identified and protected, perform subperiosteal dissection of radius (strip periosteum from anterolateral and posterior surfaces, expose proximal-middle radius for plate placement)
2. Plate placement: Position 3.5mm LCP or DCP plate on anterolateral radius (AVOID placing plate directly over PIN on posterior surface - plate should be anterior to Thompson interval)
3. Screw insertion: Drill, tap, and insert screws (bicortical purchase, 3-4 screws proximal and distal to fracture), ensure screws do NOT penetrate posterior cortex into interosseous membrane (risk of PIN terminal branch injury)

Closure

1. Check PIN: After plate fixation complete (if ORIF), verify PIN is freely mobile (not trapped under plate or compressed by screws), pronate/supinate forearm to confirm nerve glides without tethering
2. Hemostasis: Achieve meticulous hemostasis (bipolar electrocautery to muscle edges, ligate any arterial bleeders from Leash of Henry with 4-0 Vicryl)
3. Supinator repair: Loosely approximate supinator muscle with 2-0 Vicryl (simple interrupted sutures, do NOT tightly close supinator as this may compress PIN)
4. Fascia closure: Close fascia over extensor compartment with 0-Vicryl
5. Subcutaneous closure: 2-0 Vicryl
6. Skin closure: Staples or 3-0 Nylon interrupted sutures (remove at 10-14 days)
7. Tourniquet release: Release tourniquet BEFORE closure (identify and ligate any arterial bleeders, prevents postoperative hematoma)

Postoperative Protocol

1. Immobilization:
   • PIN decompression alone: Posterior splint with elbow 90° flexion, forearm neutral rotation × 48-72 hours (for comfort), then immediate forearm rotation ROM (no immobilization needed beyond 72 hours)
   • Radius fracture ORIF: Posterior splint with elbow 90° flexion, forearm neutral × 6 weeks (limited pronation-supination to less than 45° each direction to protect supinator healing), then gradual ROM exercises
2. Physical therapy:
   • PIN decompression: Immediate active finger extension exercises (EDC strengthening), thumb extension/abduction exercises (EPL, APL), progressive resistance exercises at 6 weeks
   • Radius ORIF: Elbow flexion-extension ROM immediate (prevent elbow stiffness), forearm rotation ROM after 6 weeks (protect supinator and radius healing), strengthening after 12 weeks
3. Radiographic follow-up (if ORIF): X-rays at 2 weeks, 6 weeks, 12 weeks (assess fracture healing, plate position)
4. Neurological monitoring: Assess PIN function (finger extension, thumb extension) at immediate postop, 2 weeks, 6 weeks (if preoperative PIN palsy, serial EMG at 6 weeks, 12 weeks to monitor reinnervation - polyphasic motor units indicate recovery)
5. Weight-bearing: No lifting greater than 5kg on affected arm × 12 weeks (until fracture healed, if ORIF performed)