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J Orthop Trauma. 2004 Oct;18(9):617-22.

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

Author information

1
Royal Veterinary College and Institute of Orthopaedics and Musculoskeletal Science, University College London, North Mymms Herts, United Kingdom.

Abstract

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.

PMID:
15448451
[Indexed for MEDLINE]
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