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Updated: 5/22/2021

Anomalous Extensor Tendon

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  • Introduction
    • Definition
      • variations of extensor tendons of the hand
      • usually discovered incidentally during surgery for other reasons (e.g. ganglion excision)
  • Epidemiology
    • Incidence
      • common
        • 1-3% in cadaveric studies
  • Etiology
    • Mechanism
      • symptoms arise because of increased muscle volume within small muscle compartment
        • pain from synovitis or ischemia
  • Anatomy
    • Normal EIP
      • occupies 4th dorsal extensor compartment (8-10mm wide)
      • ratio of 1:1 for muscle:tendon length
      • origin - posterior surface of distal third of ulna and adjacent interosseous membrane
      • insertion - dorsal expansion of index finger on ulnar side of EDC
  • Classification
      • Anomalous Extensor Muscle Forms
      • Anomalous extensor indicis proprius (aEIP)
      • Most common cause of symptoms
      • Extensor digitorum brevis manus (EDBM)
      • Less common cause of symptoms because muscle belly is proximal to extensor retinaculum
      • Extensor medii proprius (EMP)
      • Extensor indicis et medii communis (EIMC)
      • EIP and EIMC unlikely to be symptomatic because of narrow width
  • Presentation
    • Symptoms
      • usually asymptomatic
        • discovered incidentally during surgery (e.g. ganglion removal)
      • mass on the dorsum of the hand
      • intermittent dorsal wrist pain if muscle bellies impinge on and occupy the narrow dorsal compartments of the wrist
    • Physical Exam
      • inspection
        • mass does not transilluminate
        • moves with movement of local muscles (flexion and extension of hand and wrist(
        • becomes firmer with grasp
      • provocative tests
        • resisted extension triggers pain
  • Imaging
    • MRI
      • indications
        • exclude other more common conditions e.g. ganglion
      • findings
        • mass is isointense with muscle tissue
        • anomalous extensor indicis proprius (aEIP)
        • extensor digitorum brevis manus (EDBM)
        • extensor medii proprius (EMP)
        • extensor indicis et medii communis (EIMC)
  • Differential
    • Ganglion cysts 
    • Synovitis
      • both produce dorsal wrist pain
  • Treatment
    • Nonoperative
      • observation
        • indications
          • first line treatment
    • Operative
      • surgical decompression of 4th dorsal compartment (aEIP) or reduction of muscle belly (EDBM)
        • indications
          • failed conservative treatment, and symptoms, signs and imaging point to anomalous muscle, with no associated conditions (e.g. ganglion)
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