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To determine whether the fatigue properties of an interlocking nail (ILN) construct are influenced by metaphyseal or diaphyseal location of the locking bolt.Ex vivo mechanical investigation.Adult canine femora (n=19 pairs).Femora were implanted with a 6-mm diameter ILN. In 1 femur, the ILN was locked with a 2.7 mm bolt placed in the diaphysis; the ILN in the contralateral femur was locked with a bolt placed in the metaphysis. Constructs were tested to failure in axial loading (9 pairs) or torsion (10 pairs), with failure defined as displacement>2 mm or a total of 500,000 cycles for axial loading, and rotation>45° for torsional loading. Outcome measures included initial construct stiffness, number of cycles to failure, peak load, and peak torque. After testing, microradiography and histology were used to determine the location and nature of construct failure.Metaphyseal bolts failed at higher axial loads than diaphyseal bolts (P=.03), with bolt failure because of bending at the nail-bolt interface. All of the metaphyseal bolt constructs survived torsional testing whereas 9 of 10 diaphyseal bolt constructs failed catastrophically because of spiral fracture through the adjacent cortical bone.Placement of a locking bolt in metaphyseal bone extends fatigue life under axial loading and decreases the incidence of catastrophic failure under torsional loading.