The effects of testing methods on the flexural fatigue life of human cortical bone

A flexural model of four-point bending fatigue that has been experimentally validated for human cortical bone under load control was used to determine how load and displacement control testing affects the fatigue behavior of human cortical bone in three-point and symmetric four-point bending. Under load control, it was predicted that three-point bending produced no signifi cant differences in fatigue life when compared to four-point bending. Howev er, three-point bending produced less stiffness loss with increasing cycles than four-point bending. In four-point bending, displacement control was p redicted to produce about one and a half orders of magnitude greater fatigu e life when compared to load control. This prediction agrees with experimen tal observations of equine cannon bone tested in load and displacement cont rol (Gibson et al., 1998). Displacement controlled three-point bending was found to produce approximately a 25% greater fatigue life when compared to load control. The prediction of longer fatigue life under displacement cont rol may have clinical relevance for the repair of damaged bone. The model c an also be adapted to other geometric configurations, including modeling of whole long bones, and with appropriate fatigue data, other cortical bone t ypes. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.