Problem of Pin Breakage in Equine Transfixation Pin Casting: Biomechanical Ex Vivo Testing of Four Different Pins.

The research studied the fatigue resistance of four different types of pins used in equine transfixation pin casting, specifically focusing on a newly designed pin with a thread run-out design. The results revealed that all pins failed at clinically relevant conditions, with a standard smooth pin proving most resilient to fatigue failure.

Research Methodology

• The study was conducted on 24 pairs of horse cadaveric third metacarpal bones (MC3). Each bone pair was fitted with a transfixation pin placed horizontally towards the end of the bone (distal metaphysis).
• The research utilised a simplified model to mimic the real-world biomechanical conditions faced during equine transfixation pin casting.
• The researchers tested the fatigue endurance of four different pins including a new 6.3/8.0-mm Imex Duraface pin with a thread run-out design (ITROP), a standard 6.1-mm smooth Steinmann pin (SSP), a Securos 6.2-mm positive-profile pin (SPPP) and a 6.3-mm Imex positive-profile pin (IPPP).
• The comparison was performed under cyclic loading conditions until the failure in axial compression of MC3, simulating the strains the pins would encounter while in use.

Results and Observations

• All tested pins broke at load levels and cycle numbers that are relevant in clinical situations, showing that under certain conditions, all pins could fail.
• The standard smooth Steinmann pin (SSP) demonstrated a significantly higher resistance to fatigue, enduring more cycles before failure when compared to the new pin with thread run-out design (ITROP). This was statistically significant with a p-value of 0.0025.
• There were no significant differences in the number of cycles to failure when comparing the ITROP with SPPP, and ITROP with IPPP. This suggests that the new thread run-out design didn’t exhibit higher resistance to cyclic fatigue.

Conclusions

• The study concluded that a thread run-out design doesn’t necessarily increase resistance to pin breakage under cyclic loading conditions. This contradicts the expectation that the new pin design would provide superior performance.
• The smooth Steinmann pin was the most resistant to cyclic failure in the given tests, but it demonstrated more lateromedial displacement and cortical wear-out. Despite this, its higher fatique resistance could outweigh previously reported drawbacks such as pin loosening and reduced resistance to axial extraction. Therefore, it might still be a preferred choice despite its drawbacks.