Magnitude of loads influences the site of failure of highly curved bones

The structure and material properties of bones along with applied boundary conditions determine the region of peak stresses, where fracture is expected to occur. As the site of peak stresses is not influenced by the magnitude of applied load, the fracture site is not expected to change during fatigue loading of whole bone at different loads. However, in a highly curved bone such as the rat ulna, the magnitude of applied loads was found to influence the fracture site. Fatigue loading was conducted under load control on intact rat forearms and on excised ulnae. The distance to the site of failure from the proximal olecranon process of ulnae was determined. In intact forearms, the site of failure demonstrated a linear progression distally, towards sites with lower moment of inertia (or sites exhibiting lower section modulus). Intact rat forearms and excised ulnae loaded to failure at low loads fractured 2–3 mm distal to where they failed when applying high loads. This indicates a shift in the site of failure by approximately 10% of whole bone length just by varying the applied load magnitude. The site of failure in excised ulnae was similar when loading at 2 Hz or at 4 Hz, suggesting that this was frequency independent in this range and indicating that strain rate was not an important contributing factor. Creep loading of excised ulnae also demonstrated similar changes in the site of failure, indicating that magnitude of loads, and not type of loading were important in determining the site of failure. This has important implications with regards to the volume of bone that undergoes damage under physiological loading, before it fails.