Increased insertion torque delays pin-bone interface loosening in external fixation with tapered bone screws

To test the null hypothesis that osseomechanical integration is not related to the maximum insertion torque of tapered external fixation pins.

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.

Unrestricted stall activity under veterinary supervision.

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].

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.

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.

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.