A biomechanical and histological analysis of standard versus hydroxyapatite-coated pins for external fixation

This sheep study was designed to make a comparative evaluation of two external fixation pin types each with and without hydroxyapatite (HA) coating. The two pins had different taper, pitch, and self drilling capabilities. Twenty Orthofix standard, self-tapping pins (group A), 20 Orthofix HA-coated, self-tapping pins (group B), 20 X-caliber, self-drilling, self-tapping pins (group C), and 20 X-caliber HA-coated, self-drilling, self-tapping pins (group D) were selected. Four pins were implanted in the right femurs of 20 adult sheep that were euthanized at 6 weeks. Mean pin insertion torque was 2745 +/- 822 Nmm in group A, 2726 +/- 784 Nmm in group B, 2818 +/- 552 Nmm in group C, and 2657 +/- 732 Nmm in group D (ns). Mean pin extraction torque was 1567 +/- 541 Nmm in group A, 2524 +/- 838 Nmm in group B, 1650 +/- 650 Nmm in group C, and 2517 +/- 726 Nmm in group D. HA-coated pins (group B and D) had a significantly greater mean pin extraction torque compared to similar uncoated pins (group A and C) (p < 0.0005). Histological analysis showed good osteointegration of the two coated pin types. This study shows that HA-coating is more important for optimal pin fixation than the particular combination of design parameters used in each pin type.