Wrist Biomechanics and Carpal Instability

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Wrist Biomechanicsand Carpal Instability
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Wrist Biomechanics

• Anatomy
• Kinematics
• Force transmission

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Anatomy

• 8 bones
• Complex interlocking shapes
• Intrinsic and extrinsic ligaments

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Wrist ligaments
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Wrist ligaments

• Volar stronger than dorsal
• Double V shape with weak area space of Poirier
• Important interosseous ligaments are SLIL and
  LTIL
• Dorsal ligaments tend to converge on triquetrum

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Kinematics

• Three axes of motion
• FEM 90 70 degrees
• Flex/ext split between radiocarpal midcarpal
• RUD 20 50 degrees
• PSM 90 90 degrees

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Axes of Motion
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Kinematics

• Rows
• Columns (Navarro)
• Oval ring
• Longitudinal columns (Weber)
• Link Joint

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Link Joint
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Kinematics

• Rows
• Proximal and Distal with scaphoid as a bridge
• Motion within and between rows
• Columns
• Central(flex/ext) lunate,capitate,hamate
• Lateral (mobile) scaphoid,trapezoid,trapezium
• Medial (rotation) triquetrum

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Kinematics

• Center of rotation head of capitate

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Kinematics

• Radial deviation scaphoid flexes proximal pole
  goes dorsal pulling lunate into palmar flexion
• Ulnar deviation scaphoid extends proximal pole
  goes volar pulling lunate into dorsiflexion

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Kinematics

• Triquetrohamate helicoid joint
• Ulnar deviation low position distal and
  dorsiflexed pulling lunate into dorsiflexion
• Radial deviation highposition proximal and
  palmar flexed pulling lunate into palmar flexion

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Force Transmission

• Principal force transmission is through capitate
  lunate and proximal pole of scaphoid
• 75 radius 25 ulna

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Classification of Carpal Instability

• CID (dissociative)
• DISI
• VISI
• CIND (non-dissociative)
• Radiocarpal,Midcarpal,Ulnar translocn
• CIC (complex)
• Perilunate Dislocation

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Progressive periLunate Instability

• Stage I scapholunate instability
• Stage II capitate dislocation
• Stage III triquetral dislocation
• Stage IV lunate dislocation
• Spectrum of injury

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PLI
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Mechanism of injury

• Impact on thenar side of wrist causes
  hyperextension , ulnar deviation and intercarpal
  supination
• Progressive damage around lunate
• Bony or ligamentous

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Normal wrist
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Volar Intercalated SegmentInstability
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Dorsal Intercalated SegmentInstability
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Gilula lines
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Carpal Angles
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Carpal Height

• L2/L1 0.54
• New ratio L2/capitate 1.57

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Scapholunate Instability

• Most common form
• Rarely diagnosed acutely
• Local tenderness
• Scaphoid shift(Watson)
• Associated with other injuries eg distal radius

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Scapholunate InstabilityClassification

• Type 1 dynamic
• Neg Xrayve Watsonve cine
• Type 2 static
• ve plain films
• Type 3 degenerative
• Type 4 secondary
• Kienbocks SNAC

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Scapholunate InstabilityRadiographs

• Scapholunate gap gt2mm
• Foreshortened scaphoid
• Cortical ring sign
• Taliesnik,s V sign
• Lack of parallelism?

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Scapholunate Instability
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DISI
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Scapholunate Instability
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Scapholunate InstabilityTreatment

• Acute (0-3 wks) open repair vs
  arthroscopically-assisted PCP x 8wks
• Chronic (gt4 wks) repair reconstruction
• STT
• Blatt
• SLC

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Scapholunate instability
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Acute repair SLIL
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Blatt Capsulodesis
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STT Fusion
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STT Arthrodesis
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Scapholunate InstabilityArthrosis

• SLAC
• PRC
• Arthrodesis
• RSL

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Triquetrolunate instabliity

• Limited understanding of ulnar side
• TL or TH ??
• Ulnar pain post injury
• Click
• ve ballottement test
• Beware ulnar impaction syndrome
• Conservative Rx rarely need limited fusion

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VISI
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Perilunate Dislocation

• Perilunate Lunate are same basic injury
• Still missed in ER
• Rx of choice open reduction repair of
  ligaments/bones
• Dorsal and volar approach
• Late fusion or PRC

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Lesser and Greater arcs
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Perilunate Dislocation
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Perilunate repair
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Ulnar Translocation

• Rare
• Difficult to treat
• Non-traumatic causes RA,Madelungs

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Ulnar Translocation
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Carpal InstabilityUnresolved Issues

• Role of arthroscopy
• Method of reconstruction SLIL eg bone-tendon-bone
• Ulnar side pathomechanics
• Role of MRI

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Grade III
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Grade IV