THE EFFECT OF EXTERNAL FIXATOR PIN GEOMETRY AND DYNAMIC LOADING ON BONE REMODELING AT THE PIN-BONE INTERFACE (PBI), IN AN IN-VIVO SHEEP TIBIA MODEL

The purpose of this study was to study the effects of pin design and d ynamic loading on the biological response of bone at the pin-bone inte rface (PBI). An electro-pneumatic pin-loading device was used to mimic the forces that would be experienced by external fixator pins, during patient ambulation following repair of a fracture with external skele tal fixation. The remodelling response was assessed histologically wit h fluorescence microscopy of plastic-embedded undecalcified pin-bone s ections, and surface-stained histomorphometry. There were no significa nt differences between the AO Schanz and Howmedica Apex 5.0 mm diamete r self-tapping pin designs when they were inserted in 4.5 mm diameter predrilled pilot holes. When pin type was controlled for, significant differences (p<0.05) existed between loaded and control groups as well as between entry (near) and exit (far) cortices. Less bone resorption was observed in the non-loaded group in comparison to the dynamically loaded group, and bone resorption in the near cortex consistently exc eeded the amount of bone resorption in the far cortex for both treatme nt groups.