Posterior Interosseous Syndrome:

From: www.wheelessonline.com

- 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 the bicipital tuberosity, could cause
                    PIN syndrome by entrapment beneath the 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:
                         - this is the most common location of nerve compression in radial tunnel syndrome;
                         - lies deep to the extensor carpi radialis brevis
                         - tendinous proximal border of supinator (arcade of Frohse):
                   - 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.  RH Gelberman et al. JBJS. Vol 75-A. 1993. p 1854-1878.

- 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;