Rheological Characterization of Bovine Intervertebral Discs: A Model for Human Discs

Human spinal discs cannot be reliably used to study their mechanical properties, as obtainable discs are typically degenerate and have large variability. Bovine discs, obtained from mature runt cow tails, have been hypothesized to have similar mechanical properties to human spinal discs. The caudal discs are readily available, easily extractable, and provide improved consistency. Characterizing the mechanical properties of human spinal discs, especially the nucleus pulposus (NP), is desirable to aid in the development of replacement methods for spinal injuries such as herniation. Our study is unique as it focuses on mechanical tests that simulate the experiences of human spinal discs rather than typical material characterization tests such as stress-relaxation. The nucleus pulposus of bovine caudal spinal discs were tested unconfined using dynamic oscillatory shear, low-strain dynamic oscillatory compression and steady compression. This study is the first to report dynamic oscillatory shear and compression data for bovine intervertebral discs. The shear moduli obtained for bovine NP tissue at 1 Hz were 69.2 ± 27.4 kPa (G’) and 19.1 ± 6.6 kPa (G”). These values are higher than those previously reported for human lumbar NP [1]. The elastic moduli of 84.9 ± 11 kPa (E’) and 27.5 ± 6 kPa (E”), obtained from the dynamic oscillatory compression at 1 Hz, are not significantly different from values previously reported for human lumbar discs [2]. Steady compression, which applied a larger deformation to the disc, provided a complex elastic modulus of 41 ± 15 kPa. This value cannot be compared to human lumbar values as steady compression has not yet been performed. The reproducibility seen and favorable comparison to available human compression data confirms the viability of bovine caudal discs as a model for human spinal discs.

[1] Iatridis, J. C., Setton, L. A., Weidenbaum, M., Mow, V. C. (1997). The viscoelastic behavior of the non-degenerate human lumbar nucleus pulposus in shear, Biomechanics, 30(10), 1005-1013.

[2] Freeman, A. L., Butterman, G. R., Beaubien, B. P., Rochefort, W. E. (2013). Compressive properties of fibrous repair tissue compared to nucleus and annulus. Biomechanics, 46(10), 1714-1721.