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  • Characterized by paradoxical extension of the IP joints while attempting to flex the fingers
  • Epidemiology
    • location
      • most common in middle finger (2nd lumbrical)
        • FDP 3, 4, 5 share a common muscle belly
          • cannot independently flex 2 digits without pulling on the third
        • index finger has independent FDP belly
          • when making a fist following FDP2 transection, it is possible to only contract FDS2 (and not FDP2), thus avoiding paradoxical extension
  • Pathophysiology
    • mechanism
      • FDP disruption distal to the origin of the lumbicals (most common)
        • can be due to
          • FDP transection
          • FDP avulsion  
      • DIP amputation 
      • amputation through middle phalanx shaft 
      • "too long" tendon graft
    • pathoanatomy 
      • lumbricals originate from FDP
      • with FDP laceration, FDP contraction leads to pull on lumbricals
      • lumbricals pull on lateral bands leading to PIP and DIP extension of involved digit
      • with the middle finger, when the FDP is cut distally, the FDP shifts ulnarly (because of the pull of the 3rd lumbrical origin)(bipennate)
        • this leads to tightening of the middle finger lumbrical (2nd lumbrical, unipennate), and amplifies the "lumbrical plus" effect
  • Lumbricals  
    • 1st and 2nd lumbricals 
      • unipennate 
      • median nerve
      • originate from radial side of FDP2 and FDP3 respectively
    • 3rd and 4th lumbricals
      • bipennate
      • ulnar nerve
      • 3rd lumbrical originates from FDP 3 & 4
      • 4th lumbrical originates from FDP 4 & 5
    • all insert on radial side of extensor expansion
  • History
    • recent volar digital laceration (FDP transection) or sudden axial traction on flexed digit (FDP avulsion)
  • Symptoms
    • notices that when attempting to grip an object or form a fist, 1 digit sticks out or gets caught on clothes
  • Physical exam  post
    • paradoxical IP extension with grip (fingers extend while holding a beer can) 
  • Operative
    • tenodesis of FDP to terminal tendon or reinsertion to distal phalanx
      • indications
        •  FDP lacerations
        • do NOT suture flexor-extensor mechanisms over bone
    • lumbrical release
      • indications
        • if FDP is retracted or segmental loss makes it impossible to fix
        • NOT done in the acute setting as it does not occur  consistently enough to warrant routine lumbrical sectioning acutely
      • contraindications
        • do not transect lumbricals 1 & 2 if there is concomitant ulnar nerve palsy
        • with ulnar nerve paralysis, the interosseous muscles are also lost
        • (interosseus muscles extend the IP joints)
      • technique
        • transect at base of flexor sheath (in the palm)

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