A Novel Method for Repeatable Failure Testing of Annulus Fibrosus

Abstract

Tears in the annulus fibrosus (AF) of the intervertebral disk can result in disk herniation and progressive degeneration. Understanding AF failure mechanics is important as research moves toward developing biological repair strategies for herniated disks. Unfortunately, failure mechanics of fiber-reinforced tissues, particularly tissues with fibers oriented off-axis from the applied load, is not well understood, partly due to the high variability in reported mechanical properties and a lack of standard techniques ensuring repeatable failure behavior. Therefore, the objective of this study was to investigate the effectiveness of midlength (ML) notch geometries in producing repeatable and consistent tissue failure within the gauge region of AF mechanical test specimens. Finite element models (FEMs) representing several notch geometries were created to predict the location of bulk tissue failure using a local strain-based criterion. FEM results were validated by experimentally testing a subset of the modeled specimen geometries. Mechanical testing data agreed with model predictions (∼90% agreement), validating the model's predictive power. Two of the modified dog-bone geometries ("half" and "quarter") effectively ensured tissue failure at the ML for specimens oriented along the circumferential-radial and circumferential-axial directions. The variance of measured mechanical properties was significantly lower for notched samples that failed at the ML, suggesting that ML notch geometries result in more consistent and reliable data. In addition, the approach developed in this study provides a framework for evaluating failure properties of other fiber-reinforced tissues, such as tendons and meniscus.

Multiscale composite model of fiber-reinforced tissues with direct representation of sub-tissue properties

“…Our findings also suggest that obtaining consistent bulk tissue properties along the circumferential direction may be possible by using specimens with large aspect ratios and a smaller width, which agrees with recent work on meniscus, tendons, and ligaments (Wren et al 2001;Peloquin et al 2016;Creechley et al 2017). Interestingly, Adams and Green (1993) and Werbner et al (2017) both observed an increase in modulus as the midlength width relative to the grip width decreased. While the midlength-to-grip width ratio was not varied in this study, differences in trends may be due to a difference in fiber engagement, which can be directly evaluated with the SEP model, but not the HOM model.…”

“…For example, to study bulk tissue mechanics, model calibration would be conducted based on multi-lamellar test data from a dataset obtained from a single loading modality, often limiting the model's ability to accurately predict tissue mechanics under other loading modalities (Bass et al 2004). Moreover, findings from this study and experimental observations suggest that this curve-fitting approach is limited to specimens with a specific geometry constrained by a particular boundary condition, further restricting the predictive power and the robustness of the model (Adams and Green 1993;Sun et al 2005;Jacobs et al 2013;Werbner et al 2017). Additionally, while these models are widely used to understand contributions of sub-tissue properties to bulk tissue mechanics, it has been difficult to establish relationships between model parameters and tissue physical properties (e.g., collagen stiffness) or biochemical compositions (e.g., cross-links) as model parameters can be nonunique and are purely mathematical coefficients without physical significance (Yin and Elliott 2005;Eskandari et al 2019).…”

“…To further demonstrate the predictive power of the SEP model, we evaluated the relationship between specimen geometry and AF tensile modulus, based on experimental observations that reported modulus sensitivity to specimen width (Adams and Green 1993;Werbner et al 2017). A multivariate linear regression model was used to characterize AF tensile modulus as a function of specimen geometry, where specimen length and width were investigated as main factors and aspect ratio was evaluated as an interaction term.…”

“…Following model validation, the effect of specimen geometry on AF bulk mechanics was investigated, because experimental observations noted that modulus was sensitive to specimen geometry (Adams and Green 1993;Lechner et al 2000;Werbner et al 2017). Additional uniaxial multi-lamellar SEP models were created along the circumferential direction (n = 50 models; Fig.…”

“…So far, most bulk tissue-scale FEMs employ homogenization theory, where every model element includes a combined and homogenized description of tissue subcomponents (e.g., fibers and the matrix) (Bensoussan et al 1978;Sanchez-Palencia and Zaoui 1987;Jones 1999;Yin and Elliott 2005). This approach has allowed researchers to study three-dimensional stress and strain distributions, which has been valuable for predicting peak strains at failure (Eberlein et al 2001) and for directing experimental protocol designs (Jacobs et al 2013;Werbner et al 2017). Unfortunately, homogenization of tissue subcomponents does not accurately represent the heterogeneous architecture of native tissues, where fibers and the extrafibrillar matrix are distinct materials that occupy separate volumes.…”