Using a three-dimensional analysis, the internal forces in the joints
and soft tissues of the thumb during pinch and grasp were calculated. To do
this, mechanical equivalents were ascribed to the anatomical system, the
joint orientation and tendon locations were determined from biplanar
roentgenograms of five marked normal cadaver specimens, and the magnitudes
of forces in the tendons, intrinsic muscles, joint contact surfaces, and
constraining ligaments were calculated based on assumed loads applied to
the tip of the thumb in various types of pinch and grasp. These results are
the direct extension of a two-dimensional analysis that proved inadequate
for the determination of static tendon and joint forces. Extrinsic and
intrinsic tendons of the thumb were found to sustain forces of up to 10.0
and thirty kilograms during pinch, producing five kilograms of force at the
thumb tip and forces of as much as fifty kilograms during grasp. The joint
compression (contact) forces averaged three kilograms of force at the
interphalangeal joint, 5.4 kilograms at the metacarpophalangeal joint, and
12.0 kilograms at the carpometacarpal joint during simple pinch (one
kilogram of applied force). Compression forces of as much as 120 kilograms
may occur at the carpometacarpal joint during strong grasp.