Three-dimensional characterization of resorption cavity size and location in human vertebral trabecular bone.

The number and size of resorption cavities in cancellous bone are believed to influence rates of bone loss, local tissue stress and strain and potentially whole bone strength. Traditional two-dimensional approaches to measuring resorption cavities in cancellous bone report the percent of the bone surface covered by cavities or osteoclasts, but cannot measure cavity number or size. Here we use three-dimensional imaging (voxel size 0.7×0.7×5.0 µm) to characterize resorption cavity location, number and size in human vertebral cancellous bone from nine elderly donors (7 male, 2 female, ages 47-80 years). Cavities were 30.10 ± 8.56 µm in maximum depth, 80.60 ± 22.23∗10(3) µm(2) in surface area and 614.16 ± 311.93∗10(3) µm(3) in volume (mean ± SD). The average number of cavities per unit tissue volume (N.Cv/TV) was 1.25 ± 0.77 mm(-3). The ratio of maximum cavity depth to local trabecular thickness was 30.46 ± 7.03% and maximum cavity depth was greater on thicker trabeculae (p<0.05, r(2)=0.14). Half of the resorption cavities were located entirely on nodes (the intersection of two or more trabeculae) within the trabecular structure. Cavities that were not entirely on nodes were predominately on plate-like trabeculae oriented in the cranial-caudal (longitudinal) direction. Cavities on plate-like trabeculae were larger in maximum cavity depth, cavity surface area and cavity volume than cavities on rod-like trabeculae (p<0.05). We conclude from these findings that cavity size and location are related to local trabecular microarchitecture.

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.