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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