The effect of screw holes on reducing the flexural rigidity of long bones was confirmed and quantified in six intact canine femora. Each femur was first tested in its intact state by multidirectional nondestructive four point bending. Six bicortical screw holes were drilled and tapped into the anterolateral aspect of each bone, and all six bones were retested in the drilled state. The results from each pair of tests was used to calculate a minimum normalized flexural rigidity for each bone. The mean for all six bones was 0.82 (SEM ±0.01), and the direction of this minimum value was observed to coincide with the direction of the screw holes. Drilling the screw holes had a significant effect of reducing the flexural rigidity of the femora. Therefore, the researcher wishing to measure the strength of long bones in bending need only test in the direction of the screw holes to be certain that the weakest direction has been chosen.
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