Fixation strength of tapered versus bicylindrical hydroxyapatite-coated external fixation pins: an animal study

The purpose of this study was to comparatively measure the bone-pin interface strength in two types of hydroxyapatite-coated pins that differed in shape, diameter, and implantation technique. Both pin types are commonly used for tibia fixation. Thirty bicylindrical 5/4-mm stainless-steel pins (Group A) and 30 tapered 6/5-mm stainless-steel pins (Group B) were implanted in the left tibiae of 10 adult sheep. A monolateral fixator was mounted on the pins after implantation. A 5-mm-long bone segment was removed from the tibial mid-diaphysis to ensure high load at the bone-pin interface. Pin insertion torque was 830 +/- 446 N/mm in Group A and 3415 +/- 743 N/mm in Group B (p<0.0001, one-way ANOVA). Six weeks after surgery, the sheep were euthanized and pin extraction torque measured. Pin extraction torque was 1237 +/- 499 N/mm in Group A and 3367 +/- 1512 N/mm in Group B (p<0.0001, Mann-Whitney test). Morphological analysis performed at 60x magnification showed a direct bone-pin contact in both groups. Despite challenging biomechanical conditions, this study showed both pin types to be well-fixed and osteointegrated. Therefore, both pin types are recommended for tibial fixation. However, the tapered pins showed higher extraction torque, an important result for external fixation treatments.