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Posterior Interosseous Nerve Compression Syndrome

- See: PIN

- Inciting causes:
    - radiocapitellar joint ganglions and synovitis
    - congenital tightness of ligamentous arcade of Frohse;
    - include poorly placed screws for fracture fixation
           - PIN is vulnerable during ORIF of proximal radius;
           - in 25% of pts, PIN lies in direct contact w/ periosteum of radius just dorsal to the biciptial tuberosity;
           - fixation device applied to radius, which might have its proximal screw at level of bicipital tuberosity, could cause PIN
                     syndrome by entrapment beneath plate;
           - to avoid this complication, the nerve should be directly exposed;
    - idiopathic compression syndrome:
           - sites of compression:
                  - fibrous bands anterior to the radial head at the entrance of radial tunnel (uncommon cause of compression);
                  - radial recurrent vessels (leash of Henry);
                  - tendinous origin of ECRB: the ECRL is more superficial and is not a source of compression;
                  - arcade of Froshe:
                        - tendinous proximal border of supinator (arcade of Frohse):
                        - this is the most common location of nerve compression in radial tunnel syndrome;
                        - lies deep to the extensor carpi radialis brevis
                        - references: 
                              - The arcade of Frohse and its relationship to posterior interosseous nerve paralysis.
                              - Anatomic and morphometric study of the arcade of Frohse in cadavers
                  - distal edge of the supinator at exit: this is the least common site of compression;
           - reference:
                  - Radial nerve entrapment at the elbow: surgical anatomy.
                  - Peripheral nerve compression.  

- Diff Dx: of Post Interosseous Nerve Syndrome:
    - C7 radiculopathy:
          - unlike PIN, there will be weakness of triceps and wrist flexors;
    - lateral epicondylitis (ECRB)
          - it is often misdiagnosed as resistant tennis elbow or PIN Syndrome;
          - unlike tennis elbow, there is tenderness about 4 cm distal to the lateral humeral epicondyle;
    - distal PIN syndrome:
          - pts w/ distal posterior interosseous nerve syndrome have pain with repetitive dorsiflexion & tenderness centered over
                     the 4th extensor compartment;
    - trigger finger (no passive movement possible);
    - extensor tendon rupture:
          - may be differentiated by tenodesis effect of passive flexion of wrist: if the tendons are intact, the digits will extend (ie.,
                    tenodesis effect is lost);
    - failure of digit extension from chronic dislocation of MCP (see MP joint in RA)
          - pt can maintain extension achieved passively
          - Bouvier's Test

- Exam:
    - following muscles are intact with PIN syndrome:
         - BR, ECRL , often ECRB, & supinator;
    - pts commonly have tenderness over lateral epicondyle & almost always have tenderness more distally over the arcade of Froshe;
    - pain is almost always experienced w/ resisted supination of the forearm and frequently w/ resisted pronation;
          - full pronation of forearm produces pressure on PIN by sharp tendinous edge of the origin of ECRB  muscle;
          - PIN may be compressed by the tendinous origin of the ECRB;
          - active supination from a pronated position (tightening supinator) along w/ wrist flexion (which tighens the ECRB) may
                     reproduce the patient's symptoms;
    - most will have pain w/ resisted extension of extension of middle finger;
    - pt will unable to extend thumb or other digits at MCP joints;
          - w/ complete palsy, pts will continue to have wrist extension (ECU) but they are unable to extend wrist at neutral or in
                   ulnar deviation;
          - they can extend the digits at the interphalangeal joints, but not at MP joints;
    - pain is relieved by blocking the posterior interosseous nerve 3 cm proximal to the wrist joint;
          - performed by injecting approx 1 cm ulnar to Lister's tubercle;

- Injection Test:
    - lidocaine injection 4 finger breadths distal to the lateral epicondyle will result in temporary PIN palsy and, in the case of PIN
               syndrome, will result in temporary relief of pain;
    - w/ lateral epicondylitis, the patient should note pain relief following injection at the origin of the ECRB tendon (which is usually
               more proximal than the site of injection for PIN compression);

- Succinylcholine test:
    - diagnostic aid for differentiation of loss of function due to nerve injury from muscle or tendon rupture;
    - succinylcholine paralyzes normal skeletal muscles by blocking transmission at myoneural junction;
           - in denervated muscle, however, sustained muscle contractions are seen lasting several minutes (so-called denervation
    - w/ muscle or tendon disruption, fasciculations last only seconds;

- Operative Decompression

Anatomic dissections relating the posterior interosseous nerve to the carpus, and the etiology of dorsal wrist ganglion pain.

The terminal branch of posterior interosseous nerve: a useful donor for digital nerve grafting.

Posterior interosseous nerve palsies

Posterior interosseous nerve: an anatomic study of potential nerve grafts.

A study of the posterior interosseous nerve (PIN) and the radial tunnel in 30 Thai cadavers.