Effect of Increasing Fracture Site Stiffness on Bone–Pin Interface Stress in the Equine Distal Limb Transfixation Cast: A Finite Element Analysis

Introduction: Transfixation pin casts are used to treat phalangeal fractures in the horse. An understanding of how changing fracture site stiffness within a transfixation cast affect the stresses distal to the transcortical pins, at the fracture site, and at the BPI, is required. We hypothesized that increasing tissue stiffness at the fracture site would increase stress in bone distal to the pins and decrease the BPI stress.

Materials and Methods: An FE model of the equine transfixation cast with transcortical pins positioned in the distal third metacarpal bone was generated. The stiffness of the tissues was represented by a composite tissue block within the cast. The composite stiffness was altered to approximate various time points during fracture healing.

Results: FE analysis showed that increasing the composite tissue stiffness resulted in a decrease in the maximum von Mises stress at the BPI, a decrease in the maximum and minimum principal stress at the BPI and an increase in von Mises stress distal to the transcortical pins.

Discussion/Conclusion: Healing at the fracture site is expected to alter BPI stresses and fracture site stresses over time. These changes support the approach of not reducing the number of pins present in a distal limb transfixation cast during healing as a method to dynamize the fracture site since this is expected to occur as a direct result of the altered stress environment due to healing and local changes in tissue stiffness.

Acknowledgment: This work was supported by the State of Indiana, Purdue University College of Veterinary Medicine Research account funded by the Total Wagers tax.