Tensile fatigue strength and endurance limit of human meniscus

Modeling Fatigue Failure of Cartilage and Fibrous Biological Tissues Using Constrained Reactive Mixture Theory

Fatigue failure in biological soft tissues plays a critical role in the etiology of chronic soft tissue injuries and diseases such as osteoarthritis (OA). Understanding failure mechanisms is hindered by the decades-long timescales over which damage takes place. Analyzing the factors contributing to fatigue failure requires the help of validated computational models developed for soft tissues. This study presents a framework for fatigue failure of fibrous biological tissues based on reaction kinetics, where the composition of intact and fatigued material regions can evolve via degradation and breakage over time, in response to energy-based fatigue and damage criteria. Using reactive constrained mixture theory, material region mass fractions are governed by the axiom of mass balance. Progression of fatigue is controlled by an energy-based reaction rate, with user-selected probability functions defining the damage propensity of intact and fatigued material regions. Verification of this reactive theory, which is implemented in the open-source FEBio finite element software, is provided in this study. Validation is also demonstrated against experimental data, showing that predicted damage can be linked to results from biochemical assays. The framework is also applied to study fatigue failure during frictional contact of cartilage. Simulating previous experiments suggests that frictional effects slightly increase fatigue progression, but the main driver is cyclic compressive contact loading. This study demonstrated the ability of theoretical models to complement and extend experimental findings, advancing our understanding of the time progression of fatigue in biological tissues.

Articular cartilage fatigue causes frequency-dependent softening and crack extension

Soft biological polymers, such as articular cartilage, possess exceptional fracture and fatigue resistance, offering inspiration for the development of novel materials. However, we lack a detailed understanding of changes in cartilage material behavior and of crack propagation following cyclic compressive loading. We investigated the structure and mechanical behavior of cartilage as a function of loading frequency and number of cycles. Microcracks were initiated in cartilage samples using microindentation, then cracks were extended under cyclic compression. Thickness, apparent stiffness, energy dissipation, phase angle, and crack length were measured to determine the effects of cyclic loading at two frequencies (1 Hz and 5 Hz). To capture the fatigue-induced material response (thickness, stiffness, energy dissipation, and phase angle), material properties were compared between pre-and-post diagnostic tests. The findings indicate that irreversible structural damage (reduced thickness), cartilage softening (reduced apparent stiffness), and reduced energy dissipation (including phase angle) increased with an increase in the number of cycles. Higher frequency loading resulted in less reduction in energy dissipation, phase angle, and thickness change. Crack lengths, quantified through brightfield imaging, increased with number of cycles and frequency. This study sheds light on the complex response of cartilage under cyclic loading resulting in softening, structural damage, and altered dynamic behavior. The findings provide better understanding of failure mechanisms in cartilage and thus may help in diagnosis and treatment of osteoarthritis.

Non-invasive regional parameter identification of degenerated human meniscus

Accurate identification of local changes in the biomechanical properties of the normal and degenerative meniscus is critical to better understand knee joint osteoarthritis onset and progression. Ex-vivo material characterization is typically performed on specimens obtained from different locations, compromising the tissue's structural integrity and thus altering its mechanical behavior. Therefore, the aim of this in-silico study was to establish a non-invasive method to determine the region-specific material properties of the degenerated human meniscus. In a previous experimental magnetic resonance imaging (MRI) study, the spatial displacement of the meniscus and its root attachments in mildly degenerated (n = 12) and severely degenerated (n = 12) cadaveric knee joints was determined under controlled subject-specific axial joint loading. To simulate the experimental response of the lateral and medial menisci, individual finite element models were created utilizing a transverse isotropic hyper-poroelastic constitutive material formulation. The superficial displacements were applied to the individual models to calculate the femoral reaction force in an inverse finite element analysis. During particle swarm optimization, the four most sensitive material parameters were varied to minimize the error between the femoral reaction force and the force applied in the MRI loading experiment. Individual global and regional parameter sets were identified. In addition to in-depth model verification, prediction errors were determined to quantify the reliability of the identified parameter sets. Both compressibility of the solid meniscus matrix (+141 %, p ≤ 0.04) and hydraulic permeability (+53 %, p ≤ 0.04) were significantly increased in the menisci of severely degenerated knees compared to mildly degenerated knees, irrespective of the meniscus region. By contrast, tensile and shear properties were unaffected by progressive knee joint degeneration. Overall, the optimization procedure resulted in reliable and robust parameter sets, as evidenced by mean prediction errors of <1 %. In conclusion, the proposed approach demonstrated high potential for application in clinical practice, where it might provide a non-invasive diagnostic tool for the early detection of osteoarthritic changes within the knee joint.

Mechanics- and Behavior-Related Temporomandibular Joint Differences

Fatigue of temporomandibular joint (TMJ) tissues reflects the effects of magnitude (energy density; ED) and frequency of loading (jaw muscle duty factor; DF). This observational study measured these variables and tested for differences in mechanobehavior scores (MBS = ED2 × DF) and component variables in subjects with and without TMJ disc displacement (±D). In accordance with Institutional Review Board and STROBE guidelines, written informed consent was obtained, and examination and imaging protocols identified eligible adult subjects. Specifically, magnetic resonance imaging was used to assign subjects' TMJs to ±D groups. Subjects were trained to record in-field jaw muscle activities, from which DFs (percentage of recording time) were determined. EDs (mJ/mm3) were estimated using modeled TMJ loads and in vivo dynamic stereometry. Multivariate analysis of variance, post hoc independent t tests, and K-means cluster analysis identified significant group differences (P < 0.05). Of 242 individuals screened, 65 females (TMJs: 78 +D, 52 -D) and 53 males (TMJs: 39 +D, 67 -D) participated. Subjects produced 312 daytime and 319 nighttime recordings of average duration 6.0 ± 0.2 h and 7.6 ± 0.1 h, respectively, and 219 (114 right, 105 left) intact dynamic stereometry recordings. Average EDs were 2-fold and significantly larger in +D than -D TMJs (P < 0.0001). DFs were on average 3-fold larger during the daytime versus nighttime for both masseter and temporalis muscles and 1.8- and 3.0-fold larger for the masseter versus temporalis muscle during the daytime and nighttime, respectively. Daytime masseter MBSs for +D TMJs in females were the largest overall at 621 ± 212 (mJ/mm3)2% and 2- to 43-fold larger versus -D TMJs in both sexes during daytime and nighttime. Cluster analysis (P < 0.0001) identified groups 2 and 3, which comprised 87% +D TMJs and had average MBSs 21-fold larger than group 1. The results show MBS as a potential biomarker to predict homeostasis versus progression or reversal of degenerative TMJ structural changes.

An experimental study of the heterogeneity and anisotropy of porcine meniscal ultimate tensile strength

Characterizing the ultimate tensile strength (UTS) of the meniscus is critical in studying knee damage and pathology. This study aims to determine the UTS of the meniscus with an emphasis on its heterogeneity and anisotropy. We performed tensile tests to failure on the menisci of six month old Yorkshire pigs at a low strain rate. Specimens from the anterior, middle and posterior regions of the meniscus were tested in the radial and circumferential directions. Then the UTS was obtained for each specimen and the data were analyzed statistically, leading to a comprehensive view of the variations in porcine meniscal strength. The middle region has the highest average strength in the circumferential (43.3 ± 4.7 MPa) and radial (12.6 ± 2.2 MPa) directions. This is followed by the anterior and posterior regions, which present similar average values (about 34.0MPa) in circumferential direction. The average strength of each region in the radial direction is approximately one-fourth to one-third of the value in the circumferential direction. This study is novel as it is the first work to focus on the experimental methods to investigate the heterogeneity and anisotropy only for porcine meniscus.

Age-dependent changes in collagen crosslinks reduce the mechanical toughness of human meniscus

The mechanical resilience of the knee meniscus is provided by a group of structural proteins in the extracellular matrix. Aging can alter the quantity and molecular structure of these proteins making the meniscus more susceptible to debilitating tears. In this study, we determined the effect of aging on the quantity of structural proteins and collagen crosslinks in human lateral meniscus, and examined whether the quantity of these molecules was predictive of tensile toughness (area under the stress-strain curve). Two age groups were tested: a young group under 40 and an older group over 65 years old. Using mass spectrometry, we quantified the abundance of proteins and collagen crosslinks in meniscal tissue that was adjacent to the dumbbell-shaped specimens used to measure uniaxial tensile toughness parallel or perpendicular to the circumferential fiber orientation. We found that the enzymatic collagen crosslink deoxypyridinoline had a significant positive correlation with toughness, and reductions in the quantity of this crosslink with aging were associated with a loss of toughness in the ground substance and fibers. The non-enzymatic collagen crosslink carboxymethyl-lysine increased in quantity with aging, and these increases corresponded to reductions in ground substance toughness. For the collagenous (Types I, II, IV, VI, VIII) and non-collagenous structural proteins (elastin, decorin, biglycan, prolargin) analyzed in this study, only the quantity of collagen VIII was predictive of toughness. This study provides valuable insights on the structure-function relationships of the human meniscus, and how aging causes structural adaptations that weaken the tissue's mechanical integrity.

The biomechanical properties of human menisci: A systematic review

Biomechanical characterization of meniscal tissue ex vivo remains a critical need, particularly for the development of suitable meniscus replacements or therapeutic strategies that target the native mechanical properties of the meniscus. To date, a huge variety of test configurations and protocols have been reported, making it extremely difficult to compare the respective outcome parameters, thereby leading to misinterpretation. Therefore, the purpose of this systematic review was to identify test-specific parameters that contribute to uncertainties in the determination of mechanical properties of the human meniscus and its attachments, which derived from common quasi-static and dynamic tests in tension, compression, and shear. Strong evidence was found that the determined biomechanical properties vary significantly depending on the specific test parameters, as indicated by up to tenfold differences in both tensile and compressive properties. Test mode (stress relaxation, creep, cyclic) and configuration (unconfined, confined, in-situ), specimen shape and dimensions, preconditioning regimes, loading rates, post-processing of experimental data, and specimen age and degeneration were identified as the most critical parameters influencing the outcome measures. In conclusion, this work highlights an unmet need for standardization and reporting guidelines to facilitate comparability and may prove beneficial for evaluating the mechanical properties of novel meniscus constructs. STATEMENT OF SIGNIFICANCE: The biomechanical properties of the human meniscus have been studied extensively over the past decades. However, it remains unclear to what extent both test protocol and specimen-related differences are responsible for the enormous variability in material properties. Therefore, this systematic review analyzes the biomechanical properties of the human meniscus in the context of the underlying testing protocol. The most sensitive parameters affecting the determination of mechanical properties were identified and critically discussed. Currently, it is of utmost importance for scientists evaluating potential meniscal scaffolds and biomaterials to have a control group rather than a direct comparison to the literature. Standardization of both test procedures and reporting requirements is needed to improve and accelerate the development of meniscal replacement constructs.

Custom-made meniscus biofabrication

Reconstructing the meniscus is not currently possible due to its intricate and heterogeneous structure. In this forum, we first discuss the shortcomings of current clinical strategies in meniscus repair. Then, we describe a new promising cell-based, ink-free 3D biofabrication technology to produce tailor-made large-scale functional menisci.

Volume Loss and Recovery in Bovine Knee Meniscus Loaded in Circumferential Tension

Load-induced volume change is an important aspect of knee meniscus function because volume loss creates fluid pressure, which minimizes friction and helps support compressive loads. The knee meniscus is unusual amongst cartilaginous tissues in that it is loaded not only in axial compression, but also in circumferential tension between its tibial attachments. Despite the physiologic importance of the knee meniscus' tensile properties, its volumetric strain in tension has never been directly measured, and predictions of volume strain in the scientific literature are inconsistent. In this study, we apply uniaxial tension to bovine knee meniscus and use biplanar imaging to directly observe the resulting three-dimensional volume change and unloaded recovery, revealing that tension causes volumetric contraction. Compression is already known to also cause contraction; therefore, all major physiologic loads compress and pressurize the meniscus, inducing fluid outflow. Although passive unloaded recovery is often described as slow relative to loaded loss, here we show that at physiologic strains the volume recovery rate in the meniscus upon unloading is faster than the rate of volume loss. These measurements of volumetric strain are an important step toward a complete theory of knee meniscus fluid flow and load support.

Experiments and hyperelastic modeling of porcine meniscus show heterogeneity at high strains

Constitutive modeling of the meniscus is critical in areas like knee surgery and tissue engineering. At low strain rates, the meniscus can be described using a hyperelastic model. Calibration of hyperelastic material models of the meniscus is challenging on many fronts due to material variability and friction. In this study, we present a framework to determine the hyperelastic material parameters of porcine meniscus (and similar soft tissues) using no-slip uniaxial compression experiments. Because of the nonhomogeneous deformation in the specimens, a finite element solution is required at each step of the iterative calibration process. We employ a Bayesian calibration approach to account for the inherent material variability and a Bayesian optimization approach to minimize the resulting cost function in the material parameter space. Cylindrical specimens of porcine meniscus from the anterior, middle and posterior regions are tested up to 30% compressive strain and the Yeoh form of hyperelastic strain energy density function is used to describe the material response. The results show that the Yeoh form is able to accurately describe the compressive response of porcine meniscus and that the Bayesian calibration and optimization approaches are able to calibrate the model in a computationally efficient manner while taking into account the inherent material variability. The results also show that the shear modulus or the initial stiffness is roughly uniform across the different areas of the meniscus, but there is significant spatial heterogeneity in the response at high strains. In particular, the middle region is considerably stiffer at high strains. This heterogeneity is important to consider in modeling the response of the meniscus for clinical applications.

Influence of Wire Geometry on the Mechanical Behavior of the TiNi Design

The present article is aimed at studying the deformation behavior of TiNi wire and knitted metal TiNi mesh under uniaxial tension and revealing the role of wire geometry on their main mechanical characteristics and mechanisms of deformation behavior. The temperature dependence curve of the electrical resistance indicates that a two-stage martensitic transformation of B2→R→B19′ is occurring, and is responsible for the superelasticity effect. The TEM results showed that at room temperature, the TiNi wire has a nanocrystalline structure composed of B2 austenite grains. A change in the deformation mechanism was established under the uniaxial tension, where the TiNi wire exhibits the effect of superelasticity, while the knitted metal TiNi mesh made from this wire is characterized by hyperelastic behavior. Fracturing of the knitted metal TiNi mesh requires significant loads of up to 3500 MPa compared to the fracture load of the TiNi wire. With the uniaxial tension of the wire, which maximally repeats the geometry of the wire in knitted metal mesh, an increase in mechanical characteristics was observed.