EXPERIMENTALLY DETERMINED MICROCRACKING AROUND A CIRCULAR HOLE IN A FLAT-PLATE OF BONE - COMPARISON WITH PREDICTED STRESSES

We examined the microcracking (damage) in the vicinity of a circular h ole in bovine femoral bone specimens. The stresses near the hole were derived by a finite element analysis model using the bone's elastic co nstants and yield stresses, which were determined from a series of mec hanical tests specifically for the type of bone under examination. The spatial occurrence and distribution of microcracking was compared to the patterns of the predicted maximum principal stress, the von Mises stress, and the strain energy density function (all implicated by vari ous workers as stimuli for bone remodelling) and to the predictions de rived by the use of two engineering criteria for anisotropic yield und er mixed mode of stress. The predictions for stresses and the strain e nergy density were all very similar, making it impossible to claim tha t any of them is superior to the others. However, empirical examinatio n of the results of the Hencky-von Mises and Tsai-Wu anisotropic yield criteria showed that the Tsai-Wu criterion approximated reasonably th e pattern of microcracking around the hole. We suggest that, in the li ght of the considerable damage observed in the vicinity of stress conc entrators, similar damage in irregular material interfaces (i.e. near orthopaedic implants) would require the re-examination of the theories concerning bone remodelling so as to account for the possibility of o ccurrence of damage and the quantification of its magnitude and likely effect. The presence of considerable microdamage in bone long before it fails sue-pests that damage-based criteria are more likely to be su ccessful predictors of bone remodelling behaviour than would stress or strain-based criteria.