IN-VITRO PULLOUT STRENGTH OF SCREWS INSERTED IN ADULT EQUINE 3RD METACARPAL BONE AFTER OVERDRILLING A 4.5-MM THREADED INSERTION HOLE

Objective - To determine and compare the in vitro pullout strength of 5.5-mm cortical versus 6.5-mm cancellous bone screws inserted in the d iaphysis and metaphysis of adult equine third metacarpal (MCIII) bones , in threaded 4.5-mm cortical bone screw insertion holes that were the n overdrilled with a 4.5-mm drill bit to provide information relevant to the selection of a replacement screw if a 4.5-mm cortical screw is stripped. Study Design - In vitro pullout tests of 5.5-mm cortical and 6.5-mm cancellous screws in equine MCIII bones. Sample Population - T wo independent cadaver studies each consisting of 14 adult equine MCII I bones. Methods - Two 4.5-mm cortical screws were placed either in th e middiaphysis (study 1) or distal metaphysis (study 2) of MCIII bones . The holes were then overdrilled with a 4.5-mm drill bit and had eith er a 5.5-mm cortical or a 6.5-mm cancellous screw inserted; screw pull out tests were performed at a rate of 0.04 mm/second until screw or bo ne failure occurred. Results - In diaphyseal bone, the screws failed i n all tests. Tensile breaking strength for 5.5-mm cortical screws (997 .5 +/- 49.3 kg) and 6.5-mm cancellous screws (931.6 +/- 19.5 kg) was n ot significantly different. In metaphyseal bone, the bone failed in al l tests. The holding power for 6.5-mm cancellous screws (39.1 +/- 4.9 kg/mm) was significantly greater than 5.5-mm cortical screws (23.5 +/- 3.5 kg/mm) in the metaphysis. There was no difference in the tensile breaking strength of screws in the diaphysis between proximal and dist al screw holes; however, the holding power was significantly greater i n the distal, compared with the proximal, metaphyseal holes. Conclusio ns - Although tensile breaking strength was not different between 5.5- mm cortical and 6.5-mm cancellous screws in middiaphyseal cortical bon e, holding power of 6.5-mm cancellous screws was greater than 5.5-mm c ortical screws in metaphyseal bone of adult horses. Clinical Relevance - If a 4.5-mm cortical bone screw strips in MCIII diaphyseal bone of adult horses, either a 5.5-mm cortical or 6.5-mm cancellous screw, how ever, would have equivalent pullout strengths. A 6.5-mm cancellous scr ew, however, would provide greater holding power than a 5.5-mm cortica l screw in metaphyseal bone. (C) Copyright 1998 by The American Colleg e of Veterinary Surgeons.