This paper is concerned with the effect of stress concentrations on the failure of two materials of major importance in the field of biomechanics: bone and bone cement. Stress concentrations such as holes and notches are often introduced into our bones during orthopaedic surgery: examples are holes drilled to accommodate screws in fracture fixation, and pieces of bone removed for biopsies. Other workers have shown that the monotonic strength of whole bones is significantly reduced by the presence of circular holes: two interesting observations are that the size of the hole has a significant effect and that the size dependence is much greater for bones loaded in bending than those loaded in torsion [
]. The present work aimed to predict these effects. Bone cement, which is the polymeric material PMMA, is used widely in orthopaedic surgery to assist in the fixation of implants such as the artificial hip and knee joints. Fatigue cracking of this material is responsible for a large proportion of failures of these joint prostheses. Cement layers tend to contain defects, including small spherical pores of entrapped air and larger defects which are essentially casting defects. These features act as stress concentrations which can initiate fatigue cracking [
]. Much effort has been expended to try to reduce the amount of porosity in bone cement: the work described here attempted to quantify the effect of individual defect size and shape on high cycle fatigue strength.