Fatigue Damage in Bone: Links to Adaptation

Abstract

This paper reviews the evidence for a link between fatigue damage in bone, which is caused by mechanical loading, and the physiological processes of remodelling and adaptation, whereby bone is deposited and removed by specialised cells. On a theoretical level, this link is an appealing one because it provides a direct pathway between the tendency to failure and those processes which mitigate against failure. Experimentally, evidence is available in terms of the observed link between remodelling systems (BMUs) and fatigue microcracks. Further evidence comes from the relationship between adaptation phenomena (bone resorption and deposition) and parameters such as applied cyclic stress, strain and frequency. Similar relationships exist between these parameters and fatigue variables such as crack growth rate and number of cycles to failure. This paper describes the development of a theoretical model which describes the rate of growth of a crack as a function of cyclic stress intensity and material microstructure, along the lines previously used for engineering materials. This model is useful because it is able to predict a number of different experimental phenomena, including: number of cycles to failure; reduction in material stiffness and changes to the number and length of cracks in the material. Scatter in these data can also be predicted using stochastic quantities. This model provides the theoretical basis necessary for the consideration of fatigue as a predictor of adaptation and remodelling. Initial predictions from the model are shown: it is capable of predicting various features of the experimental data, but the levels of cyclic strain predicted to induce bone deposition and resorption are higher than found experimentally.