STUDIES ON THE EXTERNAL FIXATOR PIN-BONE INTERFACE - THE EFFECT OF PIN DESIGN AND PIN COOLING IN AN IN-VIVO SHEEP TIBIA MODEL

The purpose of this study was to examine the effect of pin design and drill/pin cooling on the biological response of bone in the near (entr y) and far (exit) cortices of mature intact sheep tibiae. Two pins wit h markedly different characteristics were used; the AO Schanz screw, a traditional pyramidal tipped design, and the fluted, tapered end Howm edica Apex pin. Our hypothesis was that cooling of hardware would redu ce thermal necrosis of bone at the pin-bone interface (PBI). When the effect of pin design was controlled for, the cooled pins resulted in b etter pin bone interfaces at the near cortex but not the far cortex. I t is possible that the drill bits and pins warmed significantly after traversing the more vascular bone marrow space, thus the cooling effec t was lost before drilling the far cortex. Secondly, the Howmedica Ape x pin resulted in more bone at the pin bone interface than the AO Scha nz pin indicating that the former's more advanced design follows the p ilot hole and creates threads more accurately, resulting in a better p in-bone fit and less mechanical damage.