The loosening at the PMMA/cancellous bone interface associated with an internal total joint prosthesis was studied as a fracture mechanics event.

The morphological characteristics of this interface suggests that the interface flaws should be modelled as no-slip cracks. Basing on this model, mathematical problems of such interfacial cracks in various loading configurations were formulated using integral transform techniques. Coupled dual integral equations were resulted which were then converted to a singular integral equation of the Cauchy type. These singular equations were solved numerically. The various stress intensity factors and the crack opening displacements were evaluated. Among the problems solved were the edge crack problem and the problems of an internal crack under pure tension, pure shear and combined loading. It was found that if the interface crack is entirely interlocked in the tangential direction, the K_I expression is identical to that for the corresponding one-material case.

The interfacial fracture toughness for the PMMA/cancellous bone interface was also determined experimentally using the compact tension specimen testing configuration. The interfacial fracture toughness was found dependent upon the failure modes which in turn depends on the degree of interfacial interdigitation. No significant difference was found between specimens from different bone locations. The most striking observation was that the crack wasobserve to initiate and propagate at a much lower load level than the maximum which can be carried by the specimen, suggesting a possible strengthening mechanism of the interface structure as the crack moves along and also allowing relative motions at the interface even before the ultimate failure load is reached. A possible interfacial failure mechanism is also given.