The accumulation of fatigue microdamage in human cortical bone of two different ages in vitro

Objective. To analyse the development of damage during fatigue cycling of human bone. Design. Changes in compliance and the cycles to failure were monitored in cortical bone samples subjected to oscillating stress in vitro. Background. Previous studies produced mainly the relationship between the applied stress and the final cycles to failure (σ-Nfplots). However, cyclic stressing increases the compliance of the bone continuously, and causes a progressive mechanical/structural degradation. Recording this accumulation of damage allows one to know how close bone is to the point of failure; more importantly, it allows a more comprehensive modelling of fatigue processes in cortical bone. Methods. The occurrence of material damage was continuously monitored during the tests. The 20 specimens came from two female subjects, 27 and 56 years old. The range of the cyclic stresses was 58–130 MPa. Results. The damage was quantified with a graphical and an empirical/numerical method, and we have also microscopically observed the generation of internal microcracks. The range of cycles to failure was from 1 to 210000. Conclusions. It was observed that (i) the older tissue showed a lower fatigue strength than the younger one, (ii) both tissues sustained similar damage levels prior to failure, and (iii) they both showed a continuous accumulation of damage during the tests, the course of which depended on the level of stress.