MUN ORTHOPEDICS

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 : “high”position 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 transloc’n

• 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 Instability:Classification

• Type 1 – dynamic– Neg Xray;+ve Watson:+ve cine

• Type 2 – static– +ve plain films

• Type 3 – degenerative

• Type 4 – secondary– Kienbock’s ; SNAC

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Scapholunate Instability:Radiographs

• Scapholunate gap >2mm

• 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 Instability:Treatment

• Acute (0-3 wks) : open repair vs arthroscopically-assisted PCP x 8wks

• Chronic (>4 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 Instability:Arthrosis

• 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,Madelung’s

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Ulnar Translocation

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Carpal Instability:Unresolved 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