IN-VITRO BIOMECHANICAL AND HISTOLOGICAL ASSESSMENT OF PILOT HOLE DIAMETER FOR POSITIVE-PROFILE EXTERNAL SKELETAL FIXATION PINS IN CANINE TIBIAE

This study was conducted to evaluate the effect of pilot hole (PH) dia meter (0, 1.5, 2.0, 2.7, 3.1, 3.3, 3.5, and 3.7 mm) on the biomechanic al and microstructural performance of positive-profile threaded extern al skeletal fixation pins (3.18 mm inner diameter, 3.97 mm outer diame ter) using cadaveric canine tibiae. Eight pins per pilot hole diameter (four pins per bone) were used to assess differences in end-insertion al torque and pin pull-out strength. Histological evaluation of eight bicortical pin tracts per pilot hole diameter was accomplished using c omputer-interfaced videomicroscopy on specimens processed using a bulk -staining technique. Compared with no predrill, use of 2.7 mm PH incre ased end-insertional torque and pull-out strength by 25% and 13.5%, re spectively. No significant differences were observed in biomechanical variables for the PH diameter range of 2.0 to 3.1 mm. Compared with no predrill, use of a 3.1 mm PH increased thread area by 18%. Microfract uring around the threads decreased as PH diameter increased. Damage to the interface at the entry and exit sites of both near and far cortic es also decreased as PH diameter increased. It was concluded that pred rilling a PH whose diameter approximates, but does not exceed the inne r diameter of the positive profile pin will not only improve initial p in stability compared with no predrilling, but it will also reduce mic rostructural damage that may lead to excessive bone resorption and pre mature pin loosening. (C) Copyright 1996 by The American College of Ve terinary Surgeons