Trabecular microfracture precedes cortical shell failure in the rat caudal vertebra under cyclic overloading

Microscopic tissue damage has been observed in otherwise healthy cancellous bone in humans and is believed to contribute to bone fragility and increased fracture risk. Animal models to study microscopic tissue damage and repair in cancellous bone would be useful, but it is currently not clear how loads applied to a whole animal bone are related to the amount and type of resulting microdamage in cancellous bone. In the current study we determine the relationship between applied cyclic compressive overloading and the resulting amount of microdamage in isolated rat tail vertebrae, a bone that has been used previously for in vivo loading experiments. Rat caudal vertebrae (C7-C9, n = 22) were potted in bone cement and subjected to cyclic compressive loading from 0 to 260 N. Loading was terminated in the secondary and tertiary phases of the creep-fatigue curve using custom data-monitoring software. In cancellous bone, trabecular microfracture was the primary form of microdamage observed with few microcracks. Trabecular microfracture prevalence increased with the amount of cyclic loading and occurred in nine out of 10 specimens loaded into the tertiary phase. Only small amounts of microdamage were observed in the cortical shell of the vertebrae, demonstrating that, under axial cyclic loading, damage occurs primarily in regions of cancellous bone before overt fracture of the bone (macroscopic cracks in the cortical shell). These experiments in isolated rat tail vertebrae suggest that it may be possible to use an animal model to study the generation and repair of microscopic tissue damage in cancellous bone.