The interosseous muscle tendons insert on both the proximal phalanx base (producing MCP flexion and deviation) and the lateral bands of the extensor expansion (producing IP extension). Their dual insertion allows them to produce MCP flexion simultaneously with IP extension — the intrinsic plus position. In intrinsic contracture (from Volkmann ischaemia, inflammatory arthritis, or overly tight intrinsic repair), the fingers are held in this position and passive IP flexion is restricted when the MCP is extended.
MCP abduction/adduction; contribution to MCP flexion and IP extension through the extensor expansion
The intrinsic tightness test distinguishes intrinsic contracture from joint contracture: if passive PIP flexion improves when the MCP is flexed (relaxing the intrinsic tendons), intrinsic tightness is confirmed. Intrinsic release by dividing the intrinsic tendon lateral to the extensor hood is performed for severe contracture.
Interosseous tendon shortening producing the intrinsic-plus posture with restriction of passive PIP flexion in MCP extension, managed with intrinsic release.
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