Effects of repetitive loading on the integrity of porous coatings

In spite of the increasing numbers of porous coated total hip femoral components implanted each year, little is known about the shear strength or fatigue performance of the interface between coating and implant substrate. The appearance of loose beads in clinical follow-up roentgenograms suggests that shear failure of bead welds does occur and that the coating could be a weak link in the implant-bone or implant-cement-bone system. In this study, a test specimen design and test regimen for quasistatic and fatigue testing of implant-porous coating shear properties was developed. Both cobalt-chromium (CoCr) alloy and titanium (Ti) bead-substrate interfaces of various surface designs were studied. Results showed that static bead-substrate interface properties for both titanium and CoCr specimens were superior to published values of bone-bead shear strengths. By comparison, fatigue tests showed that the strength of bead welds on a smooth substrate was inadequate, with a preferential failure site being between beads and substrate rather than between beads and cement or beads and bone. However, implant surface contours or a bead recess were found to protect the bead welds, and specimens with those features successfully withstood ten million dynamic loading cycles.