MOVEMENTS OF THE HAND AND WRIST
No single articulation in the hand is an isolated mechanical entity – a mobile balance is achieved throughout the hand. The wrist influences the position of the MCP, which affects the position of the PIP, which in turn affects the DIP. The dynamic balance between antagonistic muscles forms the active influence on this “kinetic chain”, while the passive factors include the ligaments.
There are three primary movements – rotation, flexion/extension, and radial/ulnar deviation.
– the radius rotates around a moving distal ulna, which rotates in the opposite direction; the axis of rotation is not fixed.
– there is about 80o of flexion and about the same of extension – this represents rotation and gliding of the carpus on the distal radius.
– this motion is distributed between radiocarpal and midcarpal motion
– there is physiological ulnar deviation at rest
– there is about 40o of ulnar deviation, 15o of radial deviation, also distributed between the radiocarpal and midcarpal joints
The concept of “variable geometry” of the proximal carpal row
– radial deviation induces flexion of the scaphoid as the trapezium approaches the radius. Through the dorsal aspect of the scapholunate ligament, this motion s transmitted sequentially to the lunate and triquetrum, which flex approximately 25o.
– as the carpus ulnar deviates, the proximal row extends and supinates. The scaphoid can be observed to extend. The hamate migrates proximally, forcing the triquetrum to displace volarly and extend, bringing the lunate into extension, and hence the scaphoid.
– thus, from the ulnar side, there is an extension moment on the lunate exerted by the triquetrum. From the radial side, there is a flexion moment on the lunate exerted by the scaphoid.
– if the scapholunate ligament is disrupted, the lunate, influenced by the triquetrum, will eventually adopt a more extended position with a scapholunate angle >60o, and will not flex with radial deviation.
– if the lunatotriquetral ligament is disrupted, the lunate, influenced by the scaphoid, will eventually adopt a more flexed position with a scapholunate angle