Young investigator award winner: Validation of the mouse and rat disc as mechanical models of the human lumbar disc

Study Design. Measure the mechanical properties of the mouse and rat disc in compression and torsion. Objectives. Validate mouse and rat disc as a biomechanical model of the human disc by comparing the normalized properties in compression and torsion loading. Summary of Background Data. Rodents have been widely used as models to study disc degeneration; however, mechanical assessments of the rodent disc have been limited. Mouse and rat disc mechanical properties have not been determined. Methods. Mechanically test mouse and rat motion segments from both the lumbar and the caudal levels in axial compression and torsion. Normalize the stiffness using disc geometry and compare with human motion segment stiffness taken from the literature. Compare lumbar and caudal levels with each other within each species, and test for correlation between mechanics and body weight. Results. The average compression stiffness, normalized by geometry, was 2 - 4 MPa and compared well with human motion segment stiffness in compression ( 3 - 9 MPa). The average torsion stiffness, normalized by disc geometry, was 5 - 11 MPa and compared well with human motion segment stiffness in torsion ( 2 - 9 MPa). Differences between the lumbar and caudal levels were observed. For the caudal tail, no correlation between body weight and any compression property was observed, but for the lumbar spine, some correlations were observed. Conclusions. This study provides validation for the mouse and rat disc as a mechanical model of the human disc. Correlations between lumbar spine properties and animal body weight provide support for the use of quadruped animal lumbar spines as mechanical models of the bipedal human spine. The differences between lumbar and tail mechanics need further exploration. These findings are important in light of the extensive use of the rodent in disc studies and the expected future utility of genetically engineered mice.