Increased Insertion Torque Delays Pin–Bone Interface Loosening in External Fixation With Tapered Bone Screws

OBJECTIVE: To test the null hypothesis that osseomechanical integration is not related to the maximum insertion torque of tapered external fixation pins. DESIGN: Prospective in vivo study in a functionally loading ovine model. In 12 animals, tapered commercial external fixation pins were inserted at predefined locations with measured insertion torques and extraction torque measured at 10 weeks postoperatively. SETTING: Unrestricted stall activity under veterinary supervision. INTERVENTIONS: Under general anesthesia and aseptic conditions, mid-diaphyseal tibial osteotomies were created and a 3-mm gap width stabilized with a custom-made, high-precision, single-sided external fixator, in compliance with United Kingdom government regulations [Animals (Scientific Procedures) Act 1986]. MAIN OUTCOME MEASUREMENTS: Primary pin site stability and interface load were assessed by measuring maximum insertion torque (Nm). At a 10-week postoperative end point, osseomechanical stability was assessed by measuring the extraction torque and a pin performance index determined from the insertion/extraction torque ratio. RESULTS: A positive correlation was found between extraction torque and insertion torque (R2 = 0.322, P < 10(-6)). All pins with an insertion torque equal to or greater than 7 Nm had a measurable extraction torque, as did 98% of the pins with an insertion torque above 5 Nm. Extraction torque decreased both as a function of pin site position by the postoperative end point. High insertion torques were found to enhance end point stability in both diaphyseal and metaphyseal bone. CONCLUSION: The data from this study indicate that tapered external fixation pins should be inserted with a high torque to enhance the long-term integrity of the pin-bone interface.