Creep bulging deformation of intervertebral disc under axial compression

The internal hydrostatic pressure (IHP) of the intervertebral disc is the functional and physiological basis of the spine. Disc bulging is a direct effect of increased IHP and can be used to evaluate the IHP without destroying the structure of the disc. Disc tissue engineering is a developing field but more data on the properties of normal discs are required for evaluation of possible graft materials. However, very little data is available concerning bulge distribution along the normal disc surface under creep. Methods: Fifteen motion segment specimens of ovine IVD were used to analyze axial creep, and disc bulging deformations of 5 markers on the surface were measured and analyzed. Findings: The maximum radial bulging rate was 2.78%±1.09% and the position at which the maximum radial deformation occurred was found to be below the midline of the disc during all levels of loading. The results showed that deformations occurred in the order vertical, radial, circumferential. Interpretation: Disc bulging during creep is a very important biomechanical response, affecting spinal functions. The deformation regularities of the disc surface were identified and may help supply important basic data for disc tissue engineering.