PULLOUT STRENGTH OF SCREWS IN FOAL 3RD METACARPAL BONE AFTER OVERDRILLING A 4.5 MM HOLE

The purpose of the study was to determine and to compare the in vitro pullout strength of 5.5 mm cortical versus 6.5 mm cancellous bone scre ws inserted in the diaphysis and metaphysis of foal third metacarpal ( MCIII) bones in threaded 4.5 mm cortical bone screw insertion holes th at were then overdrilled with a 4.5 mm drill bit. This information is relevant to the selection of a replacement screw if a 4.5 mm cortical screw is stripped during orthopaedic surgery. In vitro pullout tests w ere performed in two independent cadaver studies. each consisting of 1 2 foal MCIII bones. Two 4.5 mm cortical screws were placed either in t he mid-diaphysis (study 1) or distal metaphysis (study 2) of MCIII bon es. The holes were then overdrilled with a 4.5 mm bit and had either a 5.5 mm cortical or a 6.5 mm cancellous screw inserted; screw pullout tests were performed at a rate of 0.04 mm/s until screw or bone failur e occurred. The bone failed in all of the tests in the diaphyseal and metaphyseal bone. The holding power for 6.5 mm cancellous screws was s ignificantly (p <0.05) greater than for 5.5 mm cortical screws in both the diaphysis and metaphysis. There was not any difference in the hol ding power of screws in either the diaphysis or the metaphysis between proximal and distal screw holes. If a 4.5 mm cortical bone screw stri ps in MCIII diaphyseal or metaphyseal bone of foals, a 6.5 mm cancello us screw would provide greater holding power than a 5.5 mm cortical sc rew.