The mechanism of the intrinsic-minus finger: a biomechanical study.

To demonstrate in the cadaver model that the sequence of extension of the flexed metacarpophalangeal and proximal interphalangeal joints of the intrinsic-minus finger can be explained on the basis of moment ratios about these joints, the engineering concept of free body analysis was applied. Intrinsic-minus fingers, i.e., fingers of a cadaver in which all forces exerted by the intrinsic muscles were removed, were observed to hyperextend maximally at the metacarpophalangeal joint before extension of the proximal interphalangeal joint began. Mathematical calculation of moments provides an explanation of this sequence. This study confirmed that, when an equilibrium of forces at the hyperextended metacarpophalangeal joint is reached, the proximal interphalangeal joint is incompletely extended. These forces include: (1) the proximal pull of the extensor tendon; (2) the distal loading of the extensor tendon by the flexor tendons; (3) the force preventing spanning of the laminae.