Geometrical Properties and Torsional Fatigue Life of a Tibial Interlocking Intramedullary Nail Segment

OBJECTIVES:The first goal was to present details of a methodology for selecting the cross-sectional geometry and associated dimensions for a tibial interlocking intramedullary (IM) nail segment, given a set of candidate geometries. The second goal was to study the effect of four variables on the torsional fatigue life of specimens that constitute a segment of a tibial IM nail. METHODS:For the cross-sectional geometry study, the geometric element modeling and analysis method was used to obtain the areas (A) and moments of inertia (I and J) of each of the cross-sectional geometries in the set considered. In the fatigue tests, a specimen was subjected to a load at the end of a lever arm, through an actuator, at a fixed frequency. RESULTS:On the basis of the values of the A, I, and J and consideration of compatibility with the biological envelope, the hourglass cross-section geometry was selected as the best compromise. The cannulation, fillet radius, and main radius of this hourglass were determined to be 3.96, 1.02, and 4.06 millimeters, respectively. Fatigue test specimens fabricated from Ti-6Al-4V and having three sets of additional characteristics performed equally well as those fabricated from 316L stainless steel. These sets of characteristics are hourglass cross-section and mill annealed only; hourglass cross-section and mill annealed, followed by machining and anodization to give a gray coating; and cloverleaf cross-section and mill annealed, followed by machining and anodization to give a gray coating. CONCLUSION:Tibial IM nails with hourglass cross-sectional shape and fabricated of Ti-6Al-4V or Ti-13Nb-13Zr alloy, having appropriate mechanical, thermal, and surface treatments, may be competitive with the current "gold standard" (namely cloverleaf cross-section and fabricated of 316L stainless steel in the cold-worked state).