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  • A compressive neuropathy of the posterior interosseous nerve (PIN) with pain only 
    • no motor or sensory dysfunction, and EMG/NCS is not useful
  • Pathophysiology
    • involves same sites of compression as PIN syndrome, which include (from proximal to distal) 
      • fibrous bands anterior to radiocapitellar joint
      • radial recurrent vessels (leash of Henry 
      • medial edge of ECRB
      • proximal aponeurotic/tendinous edge of the supinator (arcade of Frohse
        • most frequent site of entrapment of the PIN
        • normal radial tunnel pressure 50mmHg
        • with supinator stretch (forced wrist flexion) pressure increases to 250mmHg
      • distal edge of the superficial layer of the supinator
    • risks
      • constant prono-supination with 1kg force and elbow in 0°-45° flexion
  • Associated conditions
    • lateral epicondylitis 
      • RTS is difficult to distinguish from lateral epicondylitis and coexists in 5% of patients 
  • Radial Tunnel
    • 5cm in length
    • from the level of the radiocapitellar joint, extending distally past the proximal edge of the supinator
    • boundaries
      • lateral
        • brachioradialis
        • ECRL
        • ECRB 
      • medial
        • biceps tendon
        • brachialis
      • floor
        • capsule of the radiocapitellar joint
  • PIN 
    • origin
      • PIN is a branch of the radial nerve that provides motor innervation to the extensor compartment
    • course
      • passes between the two heads of origin of the supinator muscle  
      • direct contact with the radial neck osteology
      • passes over abductor pollicis longus muscle origin to reach interosseous membrane
      • transverses along the posterior interosseous membrane
    • innervation 
      • motor
        • common extensors
          •  ECRB (often from radial nerve proper, but can be from PIN) 
          •  Extensor digitorum communis (EDC) 
          •  Extensor digiti minimi (EDM) 
          •  Extensor carpi ulnaris (ECU) 
        • deep extensors
          •  Supinator 
          •  Abductor pollicis longus (APL) 
          •  Extensor pollicus brevis (EPB) 
          •  Extensor pollicus longus (EPL) 
          •  Extensor indicis proprius (EIP) 
      • sensory
        • sensory fibers to dorsal wrist capsule
          • provided by terminal branch which is located on the floor of the 4th extensor compartment
        • no cutaneous innervation
  • Symptoms
    • deep aching pain in dorsoradial proximal forearm q
      • from lateral elbow to wrist
      • increases during forearm rotation and lifting activities
    • muscle weakness
      • because of pain and not muscle denervation
  • Physical exam
    • tenderness
      • over mobile wad over the supinator arch
      • maximal tenderness is 3-5cm distal to lateral epicondyle
        • more distal than lateral epicondylitis
    • provocative tests
      • resisted long finger extension test
        • reproduces pain at radial tunnel (weakness because of pain)
      • resisted supination test (with elbow and wrist in extension)
        • reproduces pain at radial tunnel (weakness because of pain)
      • passive pronation with wrist flexion
        • reproduces pain at radial tunnel
        • passive stretch of supinator muscle increases pressure inside radial tunnel to 250mmHg (normal 50mmHg)
      • radial tunnel injection test
        • diagnostic if injection leads to a PIN palsy and relieves pain
    • sensory
      • may have paresthesias in the first dorsal web space
    • motor
      • no motor manifestations
  • MRI
    • usually negative
    • indications
      • to identify muscle changes in muscles innervated by PIN
        • denervation edema/atrophy within the supinator/extensor  
      • to evaluate compression sites 
        • may show thickened edge of ECRB, prominent radial recurrent vessels (leash of Henry), swelling of PIN   
      • to identify other causes of entrapment (rare)
        • tumors, ganglia, radiocapitellar synovitis, bicipital bursitis,  radial head fractures and dislocations
  • Electrodiagnostic studies
    • EMG/NCV are inconclusive because
      • PIN carries unmyelinated Group IV fibers (C-fibers, nociception) and small myelinated Group IIA afferent fibers (temperature) 
      • pressure on these fibers produces pain
      • these fibers cannot be evaluated by EMG/NCV
      • the large myelinated fibers of PIN remain normal, producing normal EMG/NCV
  • Diagnostic injection
    • injection of local anesthetic (LA) into the area of localized tenderness
    • ensure that LA does not spread to lateral epicondyle
Differential Diagnosis
  • Lateral epicondylitis
    • both conditions coexist in 5% of patients
    • in lateral epicondylitis, tenderness is directly over the lateral epicondyle
    • in RTS, tenderness is 3-5cm distal to the lateral epicondyle
  • Cervical radiculopathy at C6-7
    • electrodiagnostic studies may show denervation 
  • Nonoperative
    • activity modification, temporary splinting, NSAIDS
      • indications
        • first line of treatment for at least one year
      • technique of activity modification
        • avoid prolonged elbow extension with forearm pronation and wrist flexion
    • corticosteroid injection
      • indications
        • both diagnostic and therapeutic
      • outcomes
        • 70% improvement at 6 weeks
        • 60% pain free at 2 years
  • Operative
    • radial tunnel release    
      • indications
        • extensive nonoperative treatment fails
      • outcomes
        • surgical release has disappointing results
          • only 50-90% good to excellent results 
          • delayed maximal recovery of up to 9-18 months
        • lower success rate in the following groups
          • concomitant multiple entrapment neuropathies (60%)
          • concomitant lateral epicondylitis (40%)
          • workers compensation patients (30%)
  • Radial tunnel release 
    • approach
      • dorsal approaches to the PIN
        • 3 planes have been described
          • between ECRB and EDC  
          • between brachioradialis and ECRL
          • transmuscular brachioradialis-splitting
      • anterior approach to the PIN
        • between brachioradialis and biceps
    • technique
      • release arcade of Frohse
      • release distal edge of supinator 
      • release fibrous bands superficial to the radiocapitellar joint
    • outcomes
      • success rate of surgical decompression is 70-90% 

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