Mechanical considerations in impaction bone grafting

In impaction grafting of contained bone defects after revision joint arthro plasty the graft behaves as a friable aggregate and its resistance to compl ex forces depends on grading, normal load and compaction. Bone mills in cur rent use produce a distribution of particle sizes more uniform than is desi rable for maximising resistance to shear stresses. We have performed experiments in vitro using morsellised allograft bone fro m the femoral head which have shown that its mechanical properties improve with increasing normal load and with increasing shear strains (strain harde ning). The mechanical strength also increases with increasing compaction en ergy, and with the addition of bioglass particles to make good the deficien cy in small and very small fragments. Donor femoral heads may be milled whi le frozen without affecting the profile of the particle size. Osteoporotic femoral heads provide a similar grading of sizes, although fewer particles are obtained from each specimen. Our findings have implications for current practice and for the future development of materials and techniques.