Updated: 9/23/2018

MCP Dislocations

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Introduction
  • Overview
    • MCP dislocations are a dislocation of the metacarpophalangeal joint, usually dorsal, caused by a fall and hyperextension of the MCP joint
      • treatment is closed reduction unless soft tissue interposition blocks reduction, in which case open reduction is indicated.
  • Epidemiology
    • location
      • dorsal dislocations are most common
      • index finger is most commonly involved
        • thumb is second most common digit involved
  • Pathophysiology
    • mechanism of injury
      • usually a fall on outstretched hand leading to hyperextension of MCP joint
        • leads to avulsion of the volar plate from metacarpal neck
  • Associated conditions
    • metacarpal and phalanx fractures
      • fractures of the base of proximal phalanx or metacarpal head
        • seen in up to 50%
Anatomy
  • MCP Joint Osteology
    • a condyloid joint
      • proximal phalanx
        • contains shallow and concave surface
          • congruent with metacarpal head
      • metacarpal head
        • is cam shaped
    • sesamoids
      • embedded in the volar plate
      • attached to flexor pollicis brevis and abductor pollicis brevis in thumb
  • MCP Joint Ligaments
    • proper collateral ligaments
      • are the primary stabilizer of the MCP joint
      • originate from the dorsal aspect of metacarpal head
      • insert on the volar aspect of base of proximal phalanx
      • are tight in flexion
    • accessory collateral ligaments
      • originate volar to the proper collateral ligaments
      • insert on the volar plate
      • are tight in extension
    • volar plate
      • stabilizes the volar aspect of MCP joint
      • comprised of thick fibrocartilaginous portion distally and loose membranous portion proximally
        • allows for hyperextension of MCP joint
      • forms volar recess proximally
        • allows for flexion of proximal phalanx
    • dorsal capsule
      • a weak stabilizer of the dorsal aspect of MCP joint
      • thin and loose in structure
    • sagittal bands
      • supports the extrinsic tendons
      • originates from the extensor hood
      • attaches volarly with transverse metacarpal ligament
Classification
  • Anatomic
    • volar
      • results from hyperextension or hyperflexion injury
    • dorsal
      • more common
      • results from hyperextension injury
  • Complexity
    • simple (subluxation)
      • no interposition of volar plate and/or sesamoids
      • base of proximal phalanx remains in contact with metacarpal head
    • complex (complete)
      • interposition of volar plate and/or sesamoids
      • metacarpal head becomes entrapped by 
        • displaced natatory ligaments distally
        • superficial transverse metacarpal ligament proximally
      • Kaplan's lesion (rare) 
        • most common in index finger
        • metacarpal head buttonholes into palm (volarly)
        • volar plate is interposed between base of proximal phalanx and metacarpal head
Presentation
  • Physical exam
    • deformity seen on inspection depends on type of dislocation
      • dorsal dislocation
        • simple
          • hyperextension of proximal phalanx on metacarpal head
          • flexion of PIP joint
        • complex
          • bayonet positioning of proximal phalanx (dorsal to metacarpal shaft)
          • skin dimpling in proximal palmar crease
      • volar dislocation
        • extensor lag present
        • dorsal skin depression found proximal to base of proximal phalanx
Imaging
  • Radiographs
    • recommended views
      • AP
      • lateral
        • best view to see dislocation
      • oblique
    • findings
      • complex dislocation
        • joint space widening may indicate interposition of volar plate 
        • entrapment of sesamoid in MCP joint is diagnostic of complex dislocation
Treatment
  • Nonoperative
    • closed reduction
      • indications
        • simple dislocations
  • Operative
    • open reduction
      • indications
        • complex dislocations
Techniques
  • Closed Reduction
    • dorsal dislocation
      • reduction technique
        • apply direct pressure over dorsal aspect of proximal phalanx with the wrist in flexion to take tension off the intrinsic and extrinsic flexors
          • avoid longitudinal traction during closed reduction as it may pull volar plate into joint and convert to irreducible
      • immobilization
        • early ROM and dorsal blocking splint following successful reduction
    • volar dislocation
      • reduction technique
        • apply direct pressure over volar aspect of proximal phalanx with MCP in flexion
      • immobilization
        • immobilize in 30° of flexion for 2 weeks, then active ROM in dorsal blocking splint
  • Open reduction
    • approach
      • dorsal approach
        • approach
          • midline incision
        • soft tissue 
          • split extensor tendon and joint capsule longitudinally
            • in thumb, develop interval between EPL and EPB
          • may be able to push volar plate out with freer elevator
            • usually need to split volar plate to remove from joint
        • pros
          • decreased risk of injury to neurovascular bundle
          • easier to address metacarpal head fractures
      • volar approach 
        • approach
          • oblique incision
            • important to incise skin only to avoid injury to neurovascular bundle
        • soft tissue
          • release A1 pulley to expose volar plate
          • push volar plate and surrounding ligaments/tendons out with freer elevator
        • pros
          • provides better access to volar plate and surrounding ligaments/tendons
        • cons
          • risk of injury to neurovascular bundle
          • difficult to address osteochondral fractures
    • soft tissue reduction
      • identify and reduce soft tissue blocking reduction
        • dorsal dislocations
          • in thumb
            • FPL tendon displaces ulnarly to create a noose with radially displaced intrinsics 
          • index finger
            • flexor tendon displaces ulnarly and lumbrical displaces radially which tighten around metacarpal neck 
          • in small finger
            • flexor tendons and lumbrical displace radially and the abductor digiti minimi and flexor digiti minimi ulnarly
        • volar dislocations
          • interposition of dorsal capsule in joint
          • avulsion and entrapment of distal insertion of volar plate or collateral ligament
          • distal and volar displacement of tendinous juncture connecting 4th and 5th EDC tendons in small finger
          • entrapment of 1st dorsal interossei in thumb
Complications
  • Joint stiffness
    • due to soft tissue trauma at time of injury or prolonged immobilization
  • Post-traumatic arthritis or osteonecrosis
    • due to repeated attempts at closed reduction, prolonged dislocation, traumatic open reduction
  • Premature physeal closure
    • rare
 

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