Aim:The intervertebral disc plays an important role in the mobility of the spine. Degeneration, main cause of back pain, leads to changes in its mechanical properties. Their quantification is realised by mechanical tests and approximation of the experimental curves by mathematical models. However the variability of data is problematic. The purpose of this study is to determine the influence of the mathematical model and experimental protocols on the mechanical properties computation.
Materials and methods: Samples of the annulus (AF) and nucleus (NP) (12 per protocol) were tested in unconfined compression stress-relaxation experiments according to 6 different protocols, where for each protocol, the initial swelling of the samples and the preload applied are different. The data were analyzed by three mathematical models: viscoelastic, Poro-Visco-Elastic Biphasic (BPVE), and Linear Biphasic reinforced with a fiber network. The ability of the three models to predict the reaction force of the disc was tested, and the computed values, such as Young s modulus, Poisson s ratio and permeability, were compared statistically.
Results : Permeabilities of 4,99e-14±3,84e-14 m4/N.s (AF), 3,31e-14±2,11e-14 m4/N.s (NP), Young's moduli of 7,81±4,62 kPa (AF), 8,81±3,88 kPa (NP), Poisson's ratio of 0,47±0,03(AF), 0,49±0,04(NP) were calculated. The data analysis shows that the presence of initial swelling and preload is essential.
Conclusion: In unconfined compression of the annulus and nucleus, the viscoelastic and the BPVE mathematical models are the most efficient to evaluate the young's modulus and the permeability respectively. The protocols including an initial swelling and a preload at 5 or 10% are the most relevant protocols.
Relevance: Accurate determination of the mechanical behavior of the intervertebral discs is essential for the modeling of spine diseases and the simulation of orthopedic or surgical treatment.