Modelling the failure precursor mechanism of lamellar fibrous tissues, example of the annulus fibrosus

Comprehensive analysis of damage evolution in human annulus fibrosus: Numerical exploration of mechanical sensitivity to biological age-dependent alteration

BACKGROUND AND OBJECTIVE

The annulus fibrosus is an essential part of the intervertebral disc, critical for its structural integrity. Mechanical deterioration in this component can lead to complete disc failure, particularly through tears development, with radial tears being the most common. These tears are often the result of both mechanical and biological factors. This study aims to numerically investigate the mechanisms of radial failure in the annulus tissue, taking into account the mechanical and age-dependent biological damage origins. A newly developed microstructure-based model was upgraded to predict damage evolution in the different annulus regions.

METHODS

The study employs a computational model to predict mechanical failures in various annulus regions, using experimental data for comparison. The model incorporates age-dependent microstructural changes to evaluate the effects of biological aging on the mechanical behavior. It specifically includes a detailed analysis of the temporal changes in circumferential rigidity and failure strain of the annulus.

RESULTS

The model demonstrated a strong ability to replicate the experimental responses of the different annulus regions to failure. It revealed that age-related microstructural changes significantly impact the rigidity and failure response of the annulus, particularly in the posterior regions and as well the anterior inner side. These changes increase susceptibility to rupture with aging. A correlation was also observed between the composition of collagen fibers, water content, and the annulus transversal response in both radial and axial directions.

CONCLUSION

The findings challenge previous assumptions, showing that age-dependent microstructural changes have a notable effect on the annulus mechanical properties. The computational model closely aligns with experimental observations, underscoring the determinant role of oriented collagen fibers in radial failure. This study enhances the understanding of annulus failure and provides a foundation for further research on the impact of aging on disc mechanical integrity and failure.

A data-driven microstructure-based model for predicting circumferential behavior and failure in degenerated human annulus fibrosus

The degeneration of the intervertebral disc annulus fibrosus poses significant challenges in understanding and predicting its mechanical behavior. In this article, we present a novel approach, enriched with detailed insights into microstructure and degeneration progression, to accurately predict the mechanics of the degenerated human annulus. Central to this framework is a fully three-dimensional continuum-based model that integrates hydration state and multiscale structural features, including proteoglycan macromolecules and interpenetrating collagen fibrillar networks across various hierarchical levels within the multi-layered lamellar/inter-lamellar soft tissue, capable of sustaining deformation-induced damage. To ensure accurate and comprehensive predictions of the degenerated annulus mechanical behavior, we establish a data-driven correlation between disc degeneration grade and individual age, which influences the composition and mechanical integrity of annulus constituents while accounting for regional variations. The methodology includes a thorough identification of age- and grade-related evolutions of model inputs, followed by a detailed quantitative evaluation of the model predictive capabilities, with a focus on circumferential behavior and failure. The model successfully replicates experimental data, accurately capturing stiffness, transverse response (Poisson's ratio), and ultimate properties across different annulus regions, while also accommodating the modulation of the age/grade relationship. The reduction rates between normal and severe degeneration align reasonably well with experimental data, with the inner region exhibiting the largest decrease in stiffness (34.63 %) and no significant change observed in the outer region. Failure stress drops considerably in both regions (49.86 % in the inner and 45.33 % in the outer), while failure strain decreases by 36.39 % in the outer and 24.74 % in the inner. Our findings demonstrate that the proposed framework significantly enhances the predictive accuracy of annulus mechanics across a spectrum of degeneration levels, from normal to severely degenerated states. This approach promises improved predictive accuracy, deeper insights into disc health and injury risk, and a robust foundation for further research on the impact of degeneration on disc integrity. STATEMENT OF SIGNIFICANCE: Understanding and predicting the mechanical behavior of degenerated human annulus fibrosus remains a significant challenge due to the complex interplay of structural, biochemical, and age-related factors. This study presents a microstructure-based approach to address this challenge by integrating hydration state, detailed structural features across hierarchical scales, and deformation-induced damage and failure, alongside age-related changes and degeneration grade factors. This approach enables accurate simulations of annulus mechanics across regions, with model results thoroughly compared to available data, reinforcing its applicability in capturing degeneration effects. By capturing the intricate interactions between microstructure and mechanical behavior in degenerated discs, the model lays a strong foundation for improving clinical assessments and guiding future treatment strategies for disc-related conditions.

Biomechanical response of lumbar intervertebral disc in daily sitting postures: a poroelastic finite element analysis

This study aims to establish and validate a poroelastic L4-L5 finite element model to evaluate the effect of different sitting postures and their durations on the mechanical responses of the disc. During the sustained loading conditions, the height loss, fluid loss and von-Mises stress gradually increased, but the intradiscal pressure decreased. The varying rates of aforementioned parameters were more significant at the initial loading stage and less so at the end. The predicted values in the flexed sitting posture were significantly greater than other postures. The extended sitting posture caused an obvious von-Mises stress concentration in the posterior region of the inter-lamellar matrix. From the biomechanical perspective, prolonged sitting may pose a high risk of lumbar disc degeneration, and therefore adjusting the posture properly in the early stage of sitting time may be useful to mitigate that. Additionally, upright sitting is a safer posture, while flexed sitting posture is more harmful.

Modeling multiaxial damage regional variation in human annulus fibrosus

In the present article, a fully three-dimensional human annulus fibrosus model is developed by considering the regional variation of the complex structural organization of collagen network at different scales to predict the regional anisotropic multiaxial damage of the intervertebral disc. The model parameters are identified using experimental data considering as elementary structural unit, the single annulus lamellae stretched till failure along the micro-sized collagen fibers. The multi-layered lamellar/inter-lamellar annulus model is constructed by considering the effective interactions between adjacent layers and the chemical-induced volumetric strain. The regional dependent model predictions are analyzed under various loading modes and compared to experimental data when available. The stretching along the circumferential and radial directions till failure serves to check the predictive capacities of the annulus model. Model results under simple shear, biaxial stretching and plane-strain compression are further presented and discussed. Finally, a full disc model is constructed using the regional annulus model and simulations are presented to assess the most likely failed areas under disc axial compression. STATEMENT OF SIGNIFICANCE: The damage in annulus soft tissues is a complex multiscale phenomenon due to a complex structural arrangement of collagen network at different scales of hierarchical organization. A fully three-dimensional constitutive representation that considers the regional variation of the structural complexity to estimate annulus multiaxial mechanics till failure has not yet been developed. Here, a model is developed to predict deformation-induced damage and failure of annulus under multiaxial loading histories considering as time-dependent physical process both chemical-induced volumetric effects and damage accumulation. After model identification using single lamellae extracted from different disc regions, the model predictability is verified for various multiaxial elementary loading modes representative of the spine movement. The heterogeneous mechanics of a full human disc model is finally presented.

Historical Review of Combined Experimental and Computational Approaches for Investigating Annulus Fibrosus Mechanics

Intervertebral disc research has sought to develop a deeper understanding of spine biomechanics, the complex relationship between disc health and back pain, and the mechanisms of spinal injury and repair. To do so, many researchers have focused on characterizing tissue-level properties of the disc, where the roles of tissue subcomponents can be more systematically investigated. Unfortunately, experimental challenges often limit the ability to measure important disc tissue- and subtissue-level behaviors, including fiber-matrix interactions, transient nutrient and electrolyte transport, and damage propagation. Numerous theoretical and numerical modeling frameworks have been introduced to explain, complement, guide, and optimize experimental research efforts. The synergy of experimental and computational work has significantly advanced the field, and these two aspects have continued to develop independently and jointly. Meanwhile, the relationship between experimental and computational work has become increasingly complex and interdependent. This has made it difficult to interpret and compare results between experimental and computational studies, as well as between solely computational studies. This paper seeks to explore issues of model translatability, robustness, and efficient study design, and to propose and motivate potential future directions for experimental, computational, and combined tissue-level investigations of the intervertebral disc.

Percutaneous posterolateral approach for the simulation of a far-lateral disc herniation in an ovine model

PURPOSE

This work describes a minimally invasive damage model for ovine lumbar discs via partial nucleotomy using a posterolateral approach.

METHODS

Two cadavers were dissected to analyze the percutaneous corridor. Subsequently, 28 ovine had their annulus fibrosus punctured via awl penetration under fluoroscopic control and nucleus pulposus tissue removed via rongeur. Efficacy was assessed by animal morbidity, ease of access to T12-S1 disc spaces, and production of a mechanical injury as verified by discography, radiography, and histology.

RESULTS

T12-S1 were accessible with minimal nerve damage morbidity. Scar tissue sealed the disc puncture site in all animals within 6 weeks, withstanding 1 MP of intradiscal pressure. Partial nucleotomy led to a significant reduction in intervertebral disk height and an increased histological degeneration score.

CONCLUSION

Inducing a reproducible injury pattern of disc degeneration required minimal time, effort, and equipment. The posterolateral approach allows operation on several discs within a single surgery and multiple animal surgeries within a single day.

A Novel Method for Repeatable Failure Testing of Annulus Fibrosus

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.