Simulation of inhomogeneous rather than homogeneous poroelastic tissue material properties within disc annulus and nucleus better predicts cervical spine response: a C3-T1 finite element model analysis under compression and moment loadings

Characterizing the Baseline Regional Biphasic Mechanical Properties of Cervical Intervertebral Discs

PURPOSE

While the regional viscoelastic biomechanical properties of lumbar intervertebral disc tissues are well documented, equivalent tissue-level characterizations for human cervical discs remain unexplored. This study aimed to quantify biphasic mechanical properties of the nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP) in cervical discs.

METHODS

A previously established confined compression testing technique was used to measure swelling pressure, equilibrium aggregate modulus, and hydraulic permeability in cervical NP, AF, and CEP tissues. Specimen-specific porosity was also assessed and correlated with these properties. A finite element model was used to simulate unconfined compression.

RESULTS

Swelling pressure (154.50 ± 89.47 kPa) and aggregate modulus (0.677 ± 0.671 MPa) were significantly higher in the CEP compared to the NP (p = 0.0308 and p = 0.0227, respectively) or AF (p = 0.0338 for aggregate modulus), with no significant differences observed between NP and AF. Permeability did not differ significantly among regions. Porosity showed negative correlations with both swelling pressure (r = - 0.55, p = 0.0006) and aggregate modulus (r = - 0.53, p = 0.001). Finite element analysis revealed a relatively uniform von Mises stress distribution between NP and AF, with higher magnitudes concentrated in the CEP.

CONCLUSION

Cervical NP and AF exhibit relatively homogeneous biomechanical properties, whereas the CEP is found to have greater stiffness and swelling pressure. These findings indicate unique tissue-level adaptations in cervical discs to support greater mobility. These data could also inform future studies investigating region-specific degeneration and aging effects on cartilaginous tissue function in cervical discs and enhance the representation of viscoelasticity in computational modeling of the IVD.

Proposal and validation of polyconvex strain-energy function for biological soft tissues

BACKGROUND

Mechanical simulations for biological tissues are effective technology for development of medical equipment, because it can be used to evaluate mechanical influences on the tissues. For such simulations, mechanical properties of biological tissues are required. For most biological soft tissues, stress tends to increase monotonically as strain increases.

OBJECTIVE

Proposal of a strain-energy function that can guarantee monotonically increasing trend of biological soft tissue stress-strain relationships and applicability confirmation of the proposed function for biological soft tissues.

METHOD

Based on convexity of invariants, a polyconvex strain-energy function that can reproduce monotonically increasing trend was derived. In addition, to confirm its applicability, curve-fitting of the function to stress-strain relationships of several biological soft tissues was performed.

RESULTS

A function depending on the first invariant alone was derived. The derived function does not provide such inappropriate negative stress in the tensile region provided by several conventional strain-energy functions.

CONCLUSIONS

The derived function can reproduce the monotonically increasing trend and is proposed as an appropriate function for biological soft tissues. In addition, as is well-known for functions depending the first invariant alone, uniaxial-compression and equibiaxial-tension of several biological soft tissues can be approximated by curve-fitting to uniaxial-tension alone using the proposed function.

The effect of cervical intervertebral disc degeneration on the motion path of instantaneous center of rotation at degenerated and adjacent segments: A finite element analysis

BACKGROUND

The motion path of instantaneous center of rotation (ICR) is a crucial kinematic parameter to dynamically characterize cervical spine intervertebral patterns of motion; however, few studies have evaluated the effect of cervical disc degeneration (CDD) on ICR motion path. The purpose of this study was to investigate the effect of CDD on the ICR motion path of degenerated and adjacent segments.

METHOD

A validated nonlinear three-dimensional finite element (FE) model of a healthy adult cervical spine was used. Progressive degeneration was simulated with six FE models by modifying intervertebral disc height and material properties, anterior osteophyte size, and degree of endplate sclerosis at the C5-C6 level. All models were subjected to a pure moment of 1 Nm and a compressive follower load of 73.6 N to simulate physical motion. ICR motion paths were compared among different models.

RESULTS

The normal FE model results were consistent with those of previous studies. In degenerative models, average ICR motion paths shifted significantly anterior at the degenerated segment (β = 0.27 mm; 95% CI: 0.22, 0.32) and posterior at the proximal adjacent segment (β = -0.09 mm; 95% CI: -0.15, -0.02) than those of the normal model.

CONCLUSION

CDD significantly affected ICR motion paths at the degenerated and proximal adjacent segments. The changes at adjacent segments may be a result of compensatory mechanisms to maintain the balance of the cervical spine. Surgical treatment planning should take into account the restoration of ICR motion path to normal. These findings could provide a basis for prosthesis design and clinical practice.

Commentary: Biomechanics of various surgical procedures for the treatment of multilevel cervical spine

Hussain M, Nassr A, Natarajan RN, et al. Corpectomy versus discectomy for the treatment of multilevel cervical spine pathology: a finite element model analysis. Spine J 2012;12:401-8 (in this issue).