In vitro mechanical evaluation of three transfixation pin-cast constructs applied to equine forelimbs

OBJECTIVE To compare strain at the bone-pin and cast-pin interfaces among 3 transfixation pin-cast constructs applied to equine forelimbs. ANIMALS 15 forelimbs from 15 adult horses. PROCEDURES Limbs were randomly assigned to 1 of 3 constructs. Centrally threaded positive-profile pins were used for all constructs, and the most distal pin was placed just proximal to the epicondyles of the third metacarpal bone. Construct 1 consisted of two 6.3-mm-diameter pins spaced 4 cm apart at 30° to each other. Construct 2 was the same as construct 1 except the pins were placed 5 cm apart. Construct 3 consisted of four 4.8-mm-diameter pins spaced 2 cm apart and at 10° to one another. An osteotomy was created in the proximal phalanx. Strain gauges were attached to the cast and bone proximal to the pins and adjacent to the osteotomy. Limbs underwent compressive loading until failure. Simplified finite element models of constructs 1 and 3 were created to further evaluate strain and load transfer between the bone and cast. RESULTS Strain did not differ between constructs 1 and 2. Compared with the 2-pin constructs, construct 3 had less strain at the bone-pin interface and more strain at the cast-pin interface, which indicated a greater amount of load was transferred to the cast of the 4-pin construct than the cast of the 2-pin constructs. Finite element modeling supported those findings. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that the 4-pin construct was more effective in unloading the fractured bone than either 2-pin construct.