In vitro comparison of two centrally threaded, positive-profile transfixation pin designs for use in third metacarpal bones in horses

Abstract

Objective - To compare heat generation during insertion, pullout strength, and associated microdamage between a self-tapping positive profile transfixation pin (STTP) and nontapping positive profile transfixation pin (NTTP). Sample Population - 30 pairs of third metacarpal bones (MC3s) from adult equine cadavers. Procedures - One MC3 of each pair was assigned to the STTP group; the other was assigned to the NTTP group. The assigned pin was inserted into the diaphysis in a lateral to medial direction. Bone temperature increase during pilot-hole drilling and pin insertion was recorded at 1 mm from the final thread position with wire thermocouples at cis and trans cortices. Resistance to axial extraction before and after cyclic loading was measured in a material testing device, and microstructural damage caused by transfixation pin insertion was assessed with scanning electron microscopy. Results - The STTP group developed a significant increase in bone temperature, compared with the NTTP group. No significant difference was found between the mean maximal pull-out strength of the STTP and the NTTP in both non-cyclic-loaded and cyclic-loaded groups. Microdamage to the bone-pin interface was lower when the STTP versus the NTTP was used, but more bone debris was apparent after inserting the STTP. Conclusion and Clinical Relevance - Because of the significant increase in temperature generation and debris accumulation despite similar pullout strengths and lesser microfracture formation, the STTP likely poses a higher risk of bone necrosis and potential loosening than the NTTP. This might be corrected by redesign of the tapping aspect of the STTP.