Compressive strength tests were performed on the L4 and L5 vertebral bodies from each rat using an Instron materials testing machine (Model 5544, Canton, MA). The vertebral endplates were potted in bone cement using a previously described method that resulted in 2 parallel loading surfaces necessary to perform a uniform compression test on individual rodent vertebrae (Steinke et al., 1999). The specimens were prepared so that the posterior elements of the vertebra did not contact the loading platforms. Compression testing was performed at a crosshead speed of 0.5 mm/min to eliminate any strain rate effects. A 100 kg load-cell was used to monitor the compressive load and a precision sensor was used to measure the axial deformation of the specimen. The load-deformation data was analyzed to obtain the compressive strength of the vertebrae, defined as the maximum load sustained before failure.
