Screw fixation in lag fashion of equine cadaveric metacarpal and metatarsal condylar bone specimens: a biomechanical comparison of shaft and cortex screws.
OBJECTIVE
To compare acute fixation stability and insertion effort of cortex bone screws with and without a shaft inserted in lag fashion in equine metacarpal (metatarsal, MC(T)III) bone.


METHODS
Screw types with independent variables of screw diameter (4.5 or 5.5 mm) and shaft type (without shaft, with 20-mm shaft, or with 25-mm shaft) were studied. Bone specimens cut from distal equine MC(T)III condyles were used. After screw insertion in lag fashion into 2 bone blocks with an instrumented device, shear tests were conducted in a mechanical testing machine. Outcome variables of peak insertion torque, insertion energy, stiffness. yield strength, and displacement at 3 kN of load were compared.


RESULTS
The effects of screw design were substantial. Screws with shaft were 30% to 40% stiffer and 60% to 70% stronger than screws without shaft. Screws with shaft could tolerate 80 to 95 kg more force than screws without shaft before yielding. At 3 kN load, the displacement with screws with shaft was 55% to 60% of that with screws without shaft. Screws with a long shaft tended to perform better than those with a short shaft. There was no difference in the shear stiffness, shear yield strength, or shear displacement between the 2 screw diameters. Although larger diameter screws required more insertion effort, and screws with a short shaft required the most insertion energy, these differences were small.


CONCLUSIONS
Cortex screws with a long shaft of 4.5- or 5.5-mm diameter provide better stability in equine MC(T)III condyle bone with less insertion effort compared with those with a short shaft or no shaft.


CONCLUSIONS
Cortex bone screws with a shaft inserted in lag fashion should be considered for the fixation of equine MC(T)III condylar fractures.
Rahm
C
C
Veterinary Surgery Clinic, University of Zürich, Switzerland.
Ito
K
K
Auer
J
J
D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000662:veterinary*; D002102:Cadaver; D005592:Fracture Fixation / Q000379:methods / Q000662:veterinary*; D050723:Fractures, Bone / Q000601:surgery / Q000662:veterinary*; D006736:Horses / Q000293:injuries* / Q000601:surgery; D008663:Metacarpus / Q000293:injuries* / Q000601:surgery; D008682:Metatarsal Bones / Q000293:injuries* / Q000601:surgery