Validation of a novel testing machine for the investigation of the biomechanical properties of lumbar vertebrae in an osteoporotic rat model

Abstract

Background: For the investigation of the biomechanical properties of bone, various testing devices have been described. However, only a limited number have been developed to test the vertebral body of small animals. The aim of this study was to develop and validate a new bone testing device, which investigates the different biomechanical properties in small-animal vertebrae as a whole, three-dimensional unit, respecting its anatomical structure.

Methods: Thirty-five twelve-week-old female Sprague Dawley rats were utilized. Group 1 was composed of 17 rats with a normal bone metabolism without osteoporosis, while Group 2 consisted of 18 rats with manifest osteoporosis, 8 weeks after ovariectomy. The 5th lumbar vertebra of each animal was tested using the new bone testing device. This device has the ability to be adjusted to the slanted nature of each individual vertebral body and fix the vertebra in a natural position to allow for a non-dislocating axial force application. The device is designed to respect the anatomical three-dimensional shape of the vertebral body, thus avoiding the application of non-anatomic, non-physiological forces and thus preventing a distortion of the biomechanical testing results. The parameters investigated were stiffness, yield load, maximum load and failure load, and the results were compared to current literature values.

Results: The conduction of the biomechanical bone testing of the vertebral bodies with the new device was conductible without any instances of dislocation of the vertebrae or machine malfunctions. Significant differences were found for stiffness, maximum load and failure load between groups, with a lower value in the osteoporotic rats in each parameter tested. The yield load was also lower in the osteoporotic group, however not significantly. The values achieved correlate with those in current literature.

Conclusions: This study demonstrates that the newly developed testing machine is easy to handle and produces valid data sets for testing biomechanical bone parameters of whole vertebral bodies in an established small animal model. Therefore, it can be utilized, also as reference data, to test different structural properties and changes in vertebral bone, for example, in different metabolic settings or under the influence of different pharmaceutical entities in further studies.

Keywords: Axial force application; Biomechanical analysis; Biomechanics; Bone; Compression testing; Lumbar vertebrae; Osteoporosis; Rat vertebrae; Rat vertebral body; Small animal model; Spine; Vertebral body; Vertebral fracture.

Fig. 1

Depiction of the novel vertebral testing device in counterclockwise rotation from left to right starting with a slanted frontal viewpoint and ending with a viewpoint of the left side

Fig. 2

Depiction of the upper and lower stamp of the new biomechanical testing device. Top right and top left image: a test-vertebra mounted in the testing machine with the pin inserted into the vertebral foramen, the base plate placed onto the lower punch and the top punch lowered onto the top plate. Note in the top left image, the natural incline of the vertebra which is addressed by the slanted nature of the top stamp. Lower image: lower punch with lower stamp with an exemplary description of the different ways of adjustment in all directions. Both systems used together ensure a perfect fit for each individual vertebra

Fig. 3

Test set-up. Fixed novel testing device with vertebral body specimen aligned under the Zwick Testing machine

Fig. 4

Exemplary graphical visualization of the force recorded during the biomechanical testing procedure. Plotted were the distance travelled by the stamp in millimeters (mm) against the force exerted in Newton (N). Derived from the data were the Stiffness (S), the yield load (yL), the maximum load (mL) and the failure load (fL). The first part, up to the yield load, depicts elastic (reversible) deformation. The second part, from the yield load on, depicts plastic (irreversible) deformation

Fig. 5

Results for stiffness, yield load, maximum load and failure load of the biomechanical analysis using the newly developed biomechanical testing machine for non-osteoporotic healthy bone (SHAM = Group 1) and osteoporotic bone (OVX = Group 2) in rat lumbar vertebrates. The statistically significant differences are marked with and asterisk (*). N: Newton, mm: millimeter