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Introduction
  • Sagittal band (SB) rupture leads to dislocation of the extensor tendon post
    • also known as "boxer's knuckle"
  • Epidemiology
    • demographics
      • more common in pugilists
        • index and middle finger in professionals
        • ring and little finger in amateurs
    • location
      • the middle finger is most commonly involved
        • index 14%
        • middle 48%
        • ring 7%
        • little 31%
      • the radial SB is more commonly involved
        • radial:ulnar = 9:1
  • Mechanisms
    • traumatic
      • forceful resisted flexion or extension 
      • laceration of extensor hood  
      • direct blow to MCP joint
    • atraumatic
      • inflammatory (e.g. rheumatoid arthritis)  
      • spontaneously during routine activities
  • Associated conditions
    • MCP joint collateral ligament injuries
Anatomy
  • Extensor mechanism comprises
    • tendons
      • EDC/EIP/EDM
      • lumbricals
      • interossei
    • retinacular system
      • sagittal bands
        • the sagittal bands are part of a closed cylindrical tube (or girdle) that surrounds the metacarpal head and MCP along with the palmar plate  
        • origin
          • volar plate and intermetacarpal ligament at the metacarpal neck
        • insertion 
          • extensor mechanism (curving around radial and ulnar side of MCP joint)
      • retinacular ligaments
      • triangular ligament 
  • Sagittal band
    • function 
      • the SB is the primary stabilizer of the extensor tendon at the MCP joint   
        • juncturae tendinum are the secondary stabilizers  
      • resists ulnar deviation of the tendon, especially during MCP flexion
      • prevents tendon bowstringing during MCP joint hyperextension
    • biomechanics
      • ulnar sagittal band
        • partial or complete sectioning does not lead to extensor tendon dislocation
      • radial sagittal band
        • distal sectioning does not produce extensor tendon instability
        • complete sectioning leads to extensor dislocation
        • sectioning of 50% of the proximal SB leads to extensor tendon subluxation
      • extensor tendon 
        • instability after sectioning is greater with wrist flexion  
        • instability after sectioning is greater in the central digits (than border digits)
          • the least stable tendon is the middle finger
          • the most stable tendon is the little finger
            • junctura tendinum stabilize the small finger 
Classification
 
Rayan and Murray Classification of Closed SB Injury
Type Description Image
Type I SB injury without extensor tendon instability
 
Type II SB injury with tendon subluxation

Type III SB injury with tendon dislocation
 
Presentation
  • Symptoms 
    • MCP soreness
  • Physical exam
    • tendon snapping
    • ulnar deviation of the digits at the MCP joint (rheumatoid arthritis)
    • inability to initiate extension
    • pseudo-triggering
    • extensor tendon dislocation into intermetacarpal gully
      • most unstable during MCP flexion with wrist flexed 
      • least unstable during MCP flexion with wrist extended
    • provocative test
      • pain when extending MCP joint against resistance (with both IP joints extended) 
Imaging
  • Radiographs
    • required views
      • hand PA, lateral, oblique
    • optional view
      • Brewerton view
        • AP with dorsal surface of fingers touching the cassette and MCP joints flexed 45deg
      • stress view
        • to rule out collateral ligament avulsion/injury
    • findings
      • exclude mechanical/bony pathology limiting extension, or predisposing to sagittal band rupture
      • may show dropped fingers and ulnar deviation in rheumatoid arthritis  
  • Ultrasound (dynamic)
    • indications
      • when swelling obscures the physical exam
    • findings
      • subluxation of EDC tendon relative to metacarpal head on MCP flexion  
  • MRI
    • indications
      • to establish diagnosis of SB disruption (radial or ulnar SB)
      • may show underlying etiology e.g. synovitis in rheumatoid arthritis
    • views
      • axial images at the level of the long MCP 
      • with MCP joint flexed for maximum EDC tendon displacement 
    • findings
      • poor definition, focal discontinuity and focal thickening in acute injury
      • subluxation of extensor tendon in radial direction due ulnar SB defec
      • dislocation of extensor tendon into ulnar intermetacarpal gully radial SB defect  
Differentials
  • MCP joint collateral ligament injury
  • EDC tendon rupture
  • Trigger finger
  • Junctura tendinum disruption
  • Congenital sagittal band deficiency
  • MCP joint arthritis
Treatment
  • Nonoperative
    • extension splint for 4-6 weeks  
      • indications
        • acute injuries (within one week)
  • Operative
    • direct repair (Kettlekamp)  
      • indications
        • chronic injuries (more than one week) where primary repair is possible
        • professional athlete 
    • extensor centralization procedure
      • indications
        • chronic injuries (more than one week) where primary repair is NOT possible
        • professional athlete
Techniques
  • Extensor Centralization Procedures
    • various techniques described including
      • trapdoor flap  
        • ulnar based partial thickness capsular flap created
        • tendon placed deep to flap
        • flap resutured to capsule
      • Kilgore tendon slip
        • distally based slip of EDC tendon on radial side
        • looped around radial collateral ligament
        • sutured to itself after tensioning to centralize tendon
      • Carroll tendon slip  
        • distally based slip of EDC tendon on ulnar side
        • routed deep to affected tendon and around radial collateral ligament
        • sutured to itself after tensioning to centralize tendon
      • McCoy tendon slip  
        • proximally based slip of EDC tendon
        • looped around lumbrical insertion
        • sutured to itself after tensioning to centralize tendon
      • Watson EDC tendon transfer post 
        • distally based slip of EDC tendon slip  
        • looped under deep transverse metacarpal ligament
        • weaved to remaining EDC tendon after tensioning to centralize tendon
      • Wheeldon junctural reinforcement  
        • for a middle finger radial SB rupture, the juncturae tendinum (JT) of the ring finger is  divided close to the ring finger, 
        • bring JT over the extensor tendon
        • attach JT to the torn SB  
      • fascial strips or free tendon graft
 

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