Finite element analysis of the equine distal limb transfixation cast
Transfixation casting is a method of managing distal limb fractures in the horse. It has similarities to external skeletal fixation including the use of transcortical pins and the complications that occur as a result of concentrated stresses at the bone-pin interface. Currently, the major challenges facing equine surgeons when using a transfixation cast are pin loosening, secondary pin hole fracture and excessive stress reduction distal to the transcortical pins during healing. The equine distal limb transfixation cast was modeled from a computed tomography scan of a representative third metacarpal bone of the horse. Finite element analysis was performed to evaluate the impact of pin parameters on bone-pin interface stresses and strains. The parameters were pin diameter, number, type (half or full, threaded or smooth), spacing, orientation, location within the bone and pin material. The model was also used to determine the effect of the cast-pin interface attachment, and to determine the effect of increasing fracture tissue stiffness or external foot contact pressure, on bone-pin interface stresses. A general approach to transcortical pin selection was developed based on the total pin area moment of inertia of pins in the cast. Pin diameter and pin number had the most profound influence on bone-pin interface stresses. Cast-pin interface attachment influenced the bone-pin interface stresses and modeling a fixed pin end position underestimated bone-pin interface stresses. Increasing distal contact pressure and tissue modulus decreased bone-pin interface stresses and their distribution around pin holes. The results of this study will assist equine surgeons in improving transfixation casting in the horse by employing methods that help to minimize complications associated with the bone-pin interface that are currently limiting the clinical success of this technique.
Nauman, Purdue University.
Biomedical engineering|Surgery|Veterinary services
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