Failure analysis of in vivo fractured ceramic femoral heads

Abstract

Ceramics are more and more used for hip joint implants due to their excellent biocompatibility, longevity, corrosion and wear resistance. Despite these excellent material properties, their fracture toughness is low compared to metals. Various studies reported the probability of in vivo fracture occurring on an implanted ceramic femoral head is in the range of 0.004–0.05%. Besides material flaws and overloading, improper handling during implantation may induce fractures of the ceramic ball head on the long term. Even though the exact cause of failure could not be clarified in most of the cases, there is strong suspicion that the presence of biological contaminants may constitute the primary cause of failure. Asymmetric wear traces observed on fragments of in vivo fractured alumina femoral head endorse the assumption that a non-uniform load was introduced in the ball head. This study focuses on the influence of blood and bone chips located in the stem–ball interface and of scratches on the metal tapers on the strength of alumina ball heads. Mechanical tests on 26 samples according to ISO 7206-10 were performed to identify the effect of contaminants and altered taper on the static fracture load. The experimental study showed that relatively small contaminants induce considerable decreases of the static fracture load. A blood drop of 30 mg reduced the fracture load by more than 60% while a bone chip as small as 4 mg reduced it to less than half the reference fracture load. In a third experimental series, the change of fracture load with a 5–10 mm scratch on the stem was found to be in the range of the standard deviation and thus no clear influence could be observed. Since any alteration of the interface between metal taper and ceramic ball head through contaminants yields a non-uniform load introduction leading to a reduction of the static fracture load, an absolute clean interface is of primary importance.