A biomechanical comparison of equine third metacarpal condylar bone fragment compression and screw pushout strength between headless tapered variable pitch and AO cortical bone screws.
OBJECTIVE
To compare bone fragment compression and the mechanical pushout strength and stiffness of 6.5-mm Acutrak Plus (AP) and 4.5-mm AO cortical (AO) bone screws after stabilization of a simulated equine third metacarpal (MC3) bone complete lateral condylar fracture.


METHODS
In vitro biomechanical paired study of screw insertion variables, bone fragment compression, and screw pushout tests using a bone screw stabilized simulated lateral condylar fracture model.


METHODS
Six pairs of cadaveric equine MC3s.


METHODS
Metacarpi were placed in a fixture and centered on a biaxial load cell in a materials testing system to measure torque, compressive force, and time for drilling, tapping, and screw insertion. Fragment compression was measured with a pressure-sensing device placed between the simulated fracture fragments during screw insertion for fragment stabilization. Subsequently, screws were pushed out of the stabilized bone fragments in a single cycle to failure. A paired t test was used to assess differences between site preparation, screw insertion, fragment compression, and screw pushout variables, with significance set at P <.05.


RESULTS
Measured drilling variables were comparable for AO and AP specimens. However, the AP tap had significantly greater insertion torque and force. Mean maximum screw insertion torque was significantly greater for AO screws. For fragment compression, AP screws generated 65% and 44% of the compressive pressure and force, respectively, of AO screws. AP screws tended to have higher overall pushout strength. Pushout stiffness was similar between both screw types.


CONCLUSIONS
The 6.5-mm tapered AP screw generated less interfragmentary compressive pressure and force but had similar pushout stiffness. Evaluation of failure patterns demonstrated that AP screws had greater pushout strength compared with 4.5-mm AO screws for fixation of a simulated complete lateral condylar fracture.


CONCLUSIONS
The 6.5-mm tapered AP screw should provide ample holding strength but would provide less interfragmentary compression than 4.5-mm AO screws for repair of complete lateral condylar fractures in horses.
Galuppo
Larry D
LD
J. D. Wheat Veterinary Orthopedic Research Laboratory, School of Veterinary Medicine, University of California, One Shields Avenue, Davis, CA 95616, USA.
Stover
Susan M
SM
Jensen
David G
DG
D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000662:veterinary*; D002102:Cadaver; D004867:Equipment Design / Q000662:veterinary; D005593:Fracture Fixation, Internal / Q000295:instrumentation / Q000379:methods / Q000662:veterinary*; D050723:Fractures, Bone / Q000000981:diagnostic imaging / Q000601:surgery / Q000662:veterinary*; D006736:Horses / Q000293:injuries / Q000601:surgery*; D008663:Metacarpus / Q000293:injuries / Q000601:surgery*; D011859:Radiography