Evaluation of transfixation cast constructs in horses

Transfixation pin casts have been used to manage a variety of different equine fracture configurations, but are particularly useful in comminuted fractures of the distal limb. The objectives of this study were to investigate strain at the bone-pin interface, the cast, and the fracture site, as well as load transfer between the bone and cast in different equine transfixation pin cast configurations. Three transfixation pin cast configurations (5 forelimbs per group) were evaluated: Construct 1: Two, 6.3-mm diameter pins spaced 4-cm apart in the cannon bone; Construct 2: Two, 6.3-mm diameter pins spaced 5-cm apart; Construct 3: Four, 4.8-mm diameter pins spaced 2-cm apart. Strain gauges were attached to the cast, cannon bone, and adjacent to a simulated fracture in the proximal phalanx. Limbs were subjected to single cycle compressive loading to failure as well as cyclic loading that simulated 6 weeks of wearing a cast. A simplified finite element (FE) model of Construct 1 and 3 was used to further evaluate strain and load transfer between the bone and cast during load to failure and cyclic loading. The results indicated that there was no difference in strain between the two 2-pin constructs in load to failure or cyclic loading. Relative to the 2-pin constructs, the 4-pin construct had less strain at the bone-pin interface and more strain in the cast, indicating that more load is transferred to the cast with the 4-pin construct. In-line with these findings, FE analyses indicated that the 4-pin system had less bone strain at the bone-pin interface, less strain adjacent to the fracture site, and less load transferred to the bone. These results suggest that the 4-pin cast is more effective at unloading the fractured bone