Volar subluxation of the metacarpophalangeal (MCP) joint involves partial dislocation of the joint in which the proximal phalanz slips away from the metacarpal head and moves in the palmar direction. The middle and distal phalanges follow the movement of the proximal phalanx.
Appreciate the force relationship
of the extensor digitorum (ED) communis tendon to the front-to-back axis for
MCP abduction and adduction with ulnar drift. In its normal anatomical position
the extensor digitorum communis has no moment for abduction and adduction because
it pulls directly through this front-to-back axis. After slipping ulnarly, the
tendon develops a moment at this axis and tends to adduct the index finger toward
the long finger. (With the long finger involved, MCP abduction results. OT practitioners
use the terms ulnar deviation and ulnar drift of the MCP to prevent confusion
over abduction and adduction of the different fingers.) The ED continues to
extend the finger, but its moment arm decreases as it slips palmarly, falling
off the "top" (most dorsal aspect) of the MCP joint.
In some cases of long-standing ulnar drift the proximal phalanx slips palmarly off the metacarpal head. This happens for a number of mechanical reasons, including support failure in the dorsal structures. Since this semi-flexed resting position places the lumbricals and interossei on slack, intrinsic contracture develops and worsens MCP volar subluxation.
Because people tire of having to passively range their MCP joints into full extension using the other hand, many people with this problem later develop shortening of the lumbricals and interossei - which are placed on the slack by the resting position of MCP flexion. This is "intrinsic +" hand.
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