Effect of diameter of the drill hole on torque of screw insertion and pushout strength for headless tapered compression screws in simulated fractures of the lateral condyle of the equine third metacarpal bone.
OBJECTIVE
To compare variables for screw insertion, pushout strength, and failure modes for a headless tapered compression screw inserted in standard and oversize holes in a simulated lateral condylar fracture model.


METHODS
6 pairs of third metacarpal bones from horse cadavers.


METHODS
Simulated lateral condylar fractures were created, reduced, and stabilized with a headless tapered compression screw by use of a standard or oversize hole. Torque, work, and time for drilling, tapping, and screw insertion were measured during site preparation and screw implantation. Axial load and displacement were measured during screw pushout. Effects of drill hole size on variables for screw insertion and screw pushout were assessed by use of Wilcoxon tests.


RESULTS
Drill time was 59% greater for oversize holes than for standard holes. Variables for tapping (mean maximum torque, total work, positive work, and time) were 42%, 70%, 73%, and 58% less, respectively, for oversize holes, compared with standard holes. Variables for screw pushout testing (mean yield load, failure load, failure displacement, and failure energy) were 40%, 40%, 47%, and 71% less, respectively, for oversize holes, compared with standard holes. Screws could not be completely inserted in 1 standard and 2 oversize holes.


CONCLUSIONS
Enlarging the diameter of the drill hole facilitated tapping but decreased overall holding strength of screws. Therefore, holes with a standard diameter are recommended for implantation of variable pitch screws whenever possible. During implantation, care should be taken to ensure that screw threads follow tapped bone threads.
Carpenter
Ryan S
RS
JD Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California, Davis, CA 95616, USA.
Galuppo
Larry D
LD
Stover
Susan M
SM
D000818:Animals; D001863:Bone Screws / Q000662:veterinary*; D002102:Cadaver; D005552:Forelimb / Q000601:surgery; D050723:Fractures, Bone / Q000601:surgery / Q000662:veterinary*; D006734:Horse Diseases / Q000601:surgery*; D006736:Horses; D050279:Metacarpal Bones / Q000293:injuries* / Q000473:pathology / Q000601:surgery*; D019415:Torque