Moment Arms of the Intrinsic Muscles at the PIP Joint of the Middle Finger are Independent of the Position of the MCP Joint

Studies of muscle moment arms have classically followed a method of superposition whereby the measurement or derivation of moment arm magnitudes at any given joint assume independence of the position of joints proximal to the joint under study. This is particularly important for muscles crossing multiple joints. However, because of the nature of the soft tissue anatomy that defines the paths of the intrinsic muscles of the hand inserting into the extensor hood (also known as the dorsal expansion), this assumption may not be true for the intrinsic muscle moment arms at the Proximal Interphalangeal (PIP) Joint. These are muscles that insert into the “hood” of the dorsal expansion at the metacarpophalangeal (MCP) joint of the fingers acting as flexors at the MCP joint and extensors at the PIP joint. The gliding and flexible nature of these tissues bring into question the validity of the assumption that the extension moment arm of the intrinsic muscles at the PIP joint are independent of the position of the MCP joint. Indeed the fact that some of the intrinsic muscles exhibit an increasing (bowstringing) moment arm at the MCP joint with increased flexion infers that the paths of muscles acting through the dorsal expansion do change with respect to phalanx rotation. Does this changing path then affect the extension moment arm of the intrinsics acting at the PIP joint? This project investigates this question with the hypothesis that the extension moment arms of the second lumbrical (2Lum), the second and third dorsal interosseous (2DI and 3DI) at the middle finger PIP joint are significantly different with the middle MCP joint flexed, neutral (at extension) and hyperextended.