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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MUN ORTHOPEDICS

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 vsarthroscopically-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