Introduction: The pin-cast interface of transfixation pin casts (TPC) used in horses is thought
to be the weakest part of the construct, contributing to early pin loosening. Pin
deflection and stiffness were measured over time and compared between two methods
of cast application around the pins. We hypothesized that wrapping the cast material
around the pin limits deflection and that early cast failure results in an increase
in pin deflection and a decrease in construct stiffness.
Materials and Methods: Two transfixation pins were inserted in the distal metacarpus of two equine cadaver
limbs. Fiberglass cast material was applied by slipping the tape through the pin (construct
1) or wrapping the tape around the pin (construct 2). The constructs were cyclically
loaded by a servohydraulic load frame to a maximum compressive load of 2.7 kN for
a total of 91,260 cycles (equivalent to 20 days). Digital radiographs were taken at
specific intervals under maximum load. Image analysis was used for calculating maximum
pin bending and total stiffness. Paired t-tests were used for statistical analysis.
Results: Increasing pin deflections (p < 0.001) and decreasing stiffness (p < 0.001) are seen within 10 days of cast use for both cast wrapping methods. Construct
1 suffered pin deflections higher than construct 2 (p < 0.001).
Discussion/Conclusion: Failure at the pin-cast interface occurs in the first 10 days of use of a TPC leading
to a significant increase in pin bending, which clinically would contribute to early
pin loosening. Cast changes at 10-day intervals and wrapping the cast tape around
the pin should be considered to delay pin loosening.
Acknowledgments: This research project was funded by intramural funds from the College of Veterinary
Medicine at Kansas State University. The authors have no conflicts of interest to
disclose.
