Wrist biomechanics

Biomechanics of the Wrist

Scaphoid Mechanics

  • The scaphoid spans both the proximal and distal carpal rows.
  • There is a distal flexion moment and a proximal extension moment, with the radioscaphocapitate ligament serving as the fulcrum.
  • In the case of a scaphoid waist fracture, these forces lead to a humpback deformity.

Mechanics of SLAC & SNAC Wrist

Scapholunate Advanced Collapse (SLAC)
- Scapholunate Ligament (SL) Disruption: The scaphoid flexes while the lunate extends, creating an imbalance. - Scaphoid Flexion: Initially dynamic, then static, the scaphoid impinges on the radius, causing scaphostyloid arthritis (Stage 1). - Progression: Impingement leads to scaphofacet arthritis (Stage 2), and with ongoing axial force from the capitate, the carpal height decreases, eventually causing capitolunate arthritis and carpal collapse (Stage 3).

Scaphoid Nonunion Advanced Collapse (SNAC)
- The scaphoid and lunate become uncoupled through a fracture, leading to a humpback deformity. - Flexed Distal Pole: Causes impingement on the radius, leading to scaphostyloid arthritis (Stage 1 SNAC). - Extended Proximal Pole: Impinges on the capitate, resulting in scaphocapitate arthritis (Stage 2 SNAC). - Stage 3 SNAC: The capitate impinges on the rim of the extended lunate, causing capitolunate arthritis and carpal collapse.

Distal Radioulnar Joint (DRUJ) Mechanics

  • The ulna remains fixed while the radius rotates.
  • The ulna’s smaller radius of curvature compared to the shallow sigmoid notch allows a wide range of motion and varied contact pressures.
  • Pronation: Causes proximal and volar translation of the radius.
  • Supination: Causes distal and dorsal translation of the radius.
  • Proximal translation of the radius is limited by the interosseous membrane.

Triangular Fibrocartilage Complex (TFCC)

  • The TFCC is the main static stabilizer of the DRUJ, providing a stable platform for movement and linking the DRUJ to the ulnar carpus.
  • TFCC components:
    • Dorsal and volar radio-ulnar ligaments
    • Ulno-carpal ligaments (ulno-capitate, ulno-lunate, and ulno-triquetral ligaments)
    • Meniscal homologue
    • Cartilage disc
    • ECU subsheath
  • Dynamic stabilization of the DRUJ is provided by the pronator quadratus (PQ) and extensor carpi ulnaris (ECU).

The Paralysed Hand and Tendon Transfers

Principles of Tendon Transfer

  1. The joint across which the transfer is performed should be passively mobile.
  2. Functional gain must outweigh the potential loss.
  3. The motor used should have sufficient power (expect a loss of one grade).
  4. The motor should have adequate excursion.

Ideal characteristics: - The line of pull should be straight. - Only one motor per joint should be necessary. - The transfer should be synergistic. - The hand must retain sensibility.

Typical Upper Limb Tendon Transfers

Radial Nerve Palsy (wrist and thumb extension needed for power grip): - Pronator Teres (PT) to Extensor Carpi Radialis Brevis (ECRB) (preferred over ECRL for being more central). - Palmaris Longus (PL) to Extensor Pollicis Longus (EPL). - Flexor Carpi Radialis (FCR) to Extensor Digitorum Communis (EDC).

Ulnar Nerve Low Lesion (restoration of intrinsic function and thumb adduction for pinch grip): - Flexor Digitorum Superficialis (FDS) to lateral bands (prevents clawing) or tenodesis of FDS. - Extensor Indicis Proprius (EIP) to Adductor Pollicis (ADP).

Ulnar Nerve High Lesion (requires restoration of functions in FCU and FDP): - Sew FDP tendons together to allow unified function. - Palmaris Longus (PL) or FCR to Flexor Carpi Ulnaris (FCU). - FDS to lateral bands. - EIP to ADP.

Median Nerve Low Lesion (restoration of thumb opposition): - EIP or PL to Abductor Pollicis Brevis (APB).

Median Nerve High Lesion (focus on restoring PT, FDP, FPL, and APB): - EIP to APB. - Sew FDP tendons to the ulnar FDP. - ECRL to Flexor Pollicis Longus (FPL). - Biceps or ECU to the radial aspect of the radius for pronation.

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