Dynamic pinch tolerance of the phalanges and interphalangeal joints.

OBJECTIVES: This article presents force and displacement tolerances for pinch loading of the bones and joints of the digits.

METHODS: Dynamic (54 mm/s) pinch loading was applied in a dorsal-palmar direction to all digits (fingers and thumb) on matched pairs of hands from 8 adult human cadavers. Opposed 3.1-mm thick, 1.0-mm radius of curvature aluminum pinching surfaces were used to represent the geometry of moving surfaces in an automobile, such as doors, lift gates, or window edges. Injury timing was determined using acoustic sensors. The applied force and the distance between the pinching surfaces at the time of an acoustic burst were recorded.

RESULTS: The force tolerance ranged from 245 N for the distal phalanx of the fifth digit (little finger) to 1,155 N for the middle phalanx of the second digit (index finger), with a general trend toward increasing force tolerance for more proximal anatomical structures. The distance between pinching surfaces (gap) at the time of injury ranged from 4.5 mm for the distal interphalangeal joint of the fifth digit to 9.1 mm for the proximal phalanx of the third digit (middle finger). Orthopedic trauma found during necropsy of the test specimens included tuft fractures, transverse fractures, cortical crush injuries, a longitudinal fracture, and tendon injuries.

CONCLUSIONS: A value of approximately 35% of the initial dorsal-palmar dimension of the phalanx is reasonably representative of the gap tolerance for all digits and loading sites considered here. Although additional testing is required to quantify variability in tolerance, the tests here indicate that moving surfaces having geometry similar to that considered here would not be expected to injure a digit if the opposing surfaces come no closer together than 35% of the digit's dorsal-palmar dimension.