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 a nd size, but with some substantial differences. The first was made of forged Ti6Al4V alloy (ASTM F136). The second, made of the same materia l, featured a central hole intended to reduce the bending stiffness of the stem itself. The third was identical to the second but for a smal l tooling notch in one of the fillets of the hole. The fourth was simi lar to the first but had a coating of sintered titanium beads in the p roximal part. All of these modifications were made to evaluate the eff ect of fatigue strength of intentional or unintentional features commo nly found in commercial stems. The forged Ti6Al4V alloy was found to b e substantially stronger than the cast ASTM F75 Cr-Co-Mo alloy. Howeve r, tooling notches or sintered coatings were found to dramatically red uce this strength. Thus, the Ti6Al4V alloy calls for an accurate desig n process, especially when complex shapes or sintered structures are r equired. (C) 1995 John Wiley and Sons, Inc.