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Kinematics of Wrist

- See: Carpal Instability

- Discussion:
    - scaphoid normally tends to flex under compressive load, and exerts a similar influence on lunate due to ligamentous attachments;
    - triquetrum tends to extend w/ compressive loading & will tend to move the lunate into extension;
    - hence lunate is in a state of dynamic balance between two antagonists;
          - when the dynamic balance is interrupted, the lunate will tend to flex with loss of ulnar support from the triquetrum or extend if there is loss of radial stability;
    - distal carpal row:
         - consisting of trapezium, trapezoid, capitate, and hamate;
         - during radial to ulnar deviation distal row translates palmarly to dorsally & rotates radial to ulnar;
          - total flexion and extension motion is divided equally between radio-carpal and mid carpal joints;
    - proximal carpal row:
          - consists of the scaphoid, lunate, and triquetrum;
          - there is greater carpal bone motion in proximal row versus distal row;
          - during radial to ulnar deviation, proximal row moves from flexion to extension, while the distal row translates palmarly to dorsally & rotates radial to ulnar;
          - total flexion and extension motion is divided equally between radio-carpal and mid carpal joints;
    - radial deviation
         - induces flexion of scaphoid as trapezium approaches radius;
                - scaphoid flexes to about 15 deg;
         - scaphoid palmar flexes which inturn influences the lunate to palmar flex due to the attachments of the scapholunate ligament;
                - lunate moves ulnarward to point where it rests on the TFCC;
         - proximal carpal row rotates into a physiologic palmar flexion;
    - ulnar deviation:
         - hamate rotates into low position, influencing triquetrum into dorsiflexion;
         - scaphoid becomes dorsiflexed as it is pulled into longitudinal attitude;
         - extension of scaphoid will tend to extend lunate (due to scaphoulnate lig);
         - lunate extends maximally to about 20 deg;

- Normal Carpal Alignment:
     - lunate and capitate are colinear;
     - scaphoid angled 45 deg (normal, 30 to 60 deg);

- Abnormal Alignment:
     - abnormal volar flexion of lunate and scaphoid with +/- 30 deg volar tilt is diagnostic of a VISI deformity;
     - DISI: abnormal dorsiflexion of lunate w/ vertical scaphoid;
     - when lunate slips into statically fixed position > 15 deg of flexion or 10 deg of extension there is VISI or DISI;
     - scapholunate alignment:
           - normally about 45 deg;
           - when > 70 deg there is scapholunate dissociation

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