Effects of some technological aspects on the fatigue strength of a cementless hip stem

Four prototype cementless hip stems were tested following the ISO 7206 protocol for the assessment of the endurance properties and compared with a cast Cr-Co-Mo (ASTM F75) commercially available stem which was used as reference design. All the tested stems were similar in shape and size, but with some substantial differences. The first was made of forged Ti6A14V alloy (ASTM F136). The second, made of the same material, featured a central hole intended to reduce the bending stiffness of the stem itself. The third was identical to the second but for a small tooling notch in one of the fillets of the hole. The fourth was similar to the first but had a coating of sintered titanium beads in the proximal part. All of these modifications were made to evaluate the effect of fatigue strength of intentional or unintentional features commonly found in commercial stems. The forged Ti6A14V allow was found to be substantially stronger than the cast ASTM F75 Cr-Co-Mo alloy. However, tooling notches or sintered coatings were found to dramatically reduce this strength. Thus, the Ti6A14V alloy calls for an accurate design process, especially when complex shapes or sintered structures are required.