Hydraulic permeability of meniscus fibrocartilage measured via direct permeation: Effects of tissue anisotropy, water volume content, and compressive strain

A comprehensive review on modeling aspects of infusion-based drug delivery in the brain

Brain disorders represent an ever-increasing health challenge worldwide. While conventional drug therapies are less effective due to the presence of the blood-brain barrier, infusion-based methods of drug delivery to the brain represent a promising option. Since these methods are mechanically controlled and involve multiple physical phases ranging from the neural and molecular scales to the brain scale, highly efficient and precise delivery procedures can significantly benefit from a comprehensive understanding of drug-brain and device-brain interactions. Behind these interactions are principles of biophysics and biomechanics that can be described and captured using mathematical models. Although biomechanics and biophysics have received considerable attention, a comprehensive mechanistic model for modeling infusion-based drug delivery in the brain has yet to be developed. Therefore, this article reviews the state-of-the-art mechanistic studies that can support the development of next-generation models for infusion-based brain drug delivery from the perspective of fluid mechanics, solid mechanics, and mathematical modeling. The supporting techniques and database are also summarized to provide further insights. Finally, the challenges are highlighted and perspectives on future research directions are provided. STATEMENT OF SIGNIFICANCE: Despite the immense potential of infusion-based drug delivery methods for bypassing the blood-brain barrier and efficiently delivering drugs to the brain, achieving optimal drug distribution remains a significant challenge. This is primarily due to our limited understanding of the complex interactions between drugs and the brain that are governed by principles of biophysics and biomechanics, and can be described using mathematical models. This article provides a comprehensive review of state-of-the-art mechanistic studies that can help to unravel the mechanism of drug transport in the brain across the scales, which underpins the development of next-generation models for infusion-based brain drug delivery. More broadly, this review will serve as a starting point for developing more effective treatments for brain diseases and mechanistic models that can be used to study other soft tissue and biomaterials.

On the characteristics of natural hydraulic dampers: An image-based approach to study the fluid flow behaviour inside the human meniscal tissue

The meniscal tissue is a layered material with varying properties influenced by collagen content and arrangement. Understanding the relationship between structure and properties is crucial for disease management, treatment development, and biomaterial design. The internal layer of the meniscus is softer and more deformable than the outer layers, thanks to interconnected collagen channels that guide fluid flow. To investigate these relationships, we propose an integrated approach that combines Computational Fluid Dynamics (CFD) with Image Analysis (CFD-IA). We analyze fluid flow in the internal architecture of the human meniscus across a range of inlet velocities (0.1 mm/s to 1.6 m/s) using high-resolution 3D micro-computed tomography scans. Statistical correlations are observed between architectural parameters (tortuosity, connectivity, porosity, pore size) and fluid flow parameters (Re number distribution, permeability). Some channels exhibit Re values of 1400 at an inlet velocity of 1.6 m/s, and a transition from Darcy's regime to a non-Darcian regime occurs around an inlet velocity of 0.02 m/s. Location-dependent permeability ranges from 20-32 Darcy. Regression modelling reveals a strong correlation between fluid velocity and tortuosity at high inlet velocities, as well as with channel diameter at low inlet velocities. At higher inlet velocities, flow paths deviate more from the preferential direction, resulting in a decrease in the concentration parameter by an average of 0.4. This research provides valuable insights into the fluid flow behaviour within the meniscus and its structural influences. 3D models and image stack are available to download at https://doi.org/10.5281/zenodo.10401592. STATEMENT OF SIGNIFICANCE: The meniscus is a highly porous soft tissue with remarkable properties of load transfer and energy absorption. We give insight on the mechanism of energy absorption from high resolution uCT scans, never presented before, and a new method which combine CFD and image. The structure is similar to a sandwich structure with a stiff outside layer and a soft internal layer made of collagen channels oriented in a preferential direction guiding the fluid flow, enabling it to accommodate deformation and dissipate energy, making it a potentially optimized damping system. We investigate architectural/ fluid flow parameters- fluid regimes relationship, which is of interest of the readers working on designing suitable biomimetic systems that can be adopted for replacement.

Adaptable test bench for ASTM-compliant permeability measurement of porous scaffolds for tissue engineering

Intrinsic permeability describes the ability of a porous medium to be penetrated by a fluid. Considering porous scaffolds for tissue engineering (TE) applications, this macroscopic variable can strongly influence the transport of oxygen and nutrients, the cell seeding process, and the transmission of fluid forces to the cells, playing a crucial role in determining scaffold efficacy. Thus, accurately measuring the permeability of porous scaffolds could represent an essential step in their optimization process. In literature, several methods have been proposed to characterize scaffold permeability. Most of the currently adopted approaches to assess permeability limit their applicability to specific scaffold structures, hampering protocols standardization, and ultimately leading to incomparable results among different laboratories. The content of novelty of this study is in the proposal of an adaptable test bench and in defining a specific testing protocol, compliant with the ASTM International F2952-22 guidelines, for reliable and repeatable measurements of the intrinsic permeability of TE porous scaffolds. The developed permeability test bench (PTB) exploits the pump-based method, and it is composed of a modular permeability chamber integrated within a closed-loop hydraulic circuit, which includes a peristaltic pump and pressure sensors, recirculating demineralized water. A specific testing protocol was defined for characterizing the pressure drop associated with the scaffold under test, while minimizing the effects of uncertainty sources. To assess the operational capabilities and performance of the proposed test bench, permeability measurements were conducted on PLA scaffolds with regular (PS) and random (RS) micro-architecture and on commercial bovine bone matrix-derived scaffolds (CS) for bone TE. To validate the proposed approach, the scaffolds were as well characterized using an alternative test bench (ATB) based on acoustic measurements, implementing a blind randomized testing procedure. The consistency of the permeability values measured using both the test benches demonstrated the reliability of the proposed approach. A further validation of the PTB's measurement reliability was provided by the agreement between the measured permeability values of the PS scaffolds and the theory-based predicted permeability value. Once validated the proposed PTB, the performed measurements allowed the investigation of the scaffolds' transport properties. Samples with the same structure (guaranteed by the fused-deposition modeling technique) were characterized by similar permeability values, and CS and RS scaffolds showed permeability values in agreement with the values reported in the literature for bovine trabecular bone. In conclusion, the developed PTB and the proposed testing protocol allow the characterization of the intrinsic permeability of porous scaffolds of different types and dimensions under controlled flow regimes, representing a powerful tool in view of providing a reliable and repeatable framework for characterizing and optimizing scaffolds for TE applications.

Older age and the presence of intrameniscal signs are risk factors for nonsurgical treatment failure of symptomatic intact discoid lateral meniscus

PURPOSE

The treatment for symptomatic intact discoid lateral meniscus (DLM) is controversial and the long-term clinical outcome remains unknown. The purpose of this study was to analyze the overall failure rate of nonsurgical treatment for symptomatic intact DLM and identify the risk factors for nonoperative management failure.

METHODS

Consecutive patients who underwent nonsurgical treatment for symptomatic intact DLM at our hospital from 2014 to 2017 were retrospectively reviewed. Patients were divided into Group A (failure group) and Group B (nonfailure group) based on overall failure criteria: conversion to surgery, progression of a tear on MRI re-examination, or severely abnormal International Knee Documentation Committee (IKDC) scores. Statistical analyses between the two groups were performed for demographic and radiographic characteristics. Multivariate regression analysis was used to determine the risk factors associated with worse outcomes.

RESULTS

One-hundred and four knees in 96 patients were included in this study. After a mean follow-up of 76.9 ± 11.1 months, 25 knees (24.0%) met the overall failure criteria. Multivariate regression analysis demonstrated that age and the presence of intrameniscus signals increased the risk of nonoperative management failure. The clinical criterion of age > 37.5 years combined with the imaging criterion of the presence of intrameniscal signals predicted conservative treatment failure of symptomatic intact DLM with a sensitivity of 0.87 and a specificity of 0.91.

CONCLUSION

Twenty-five (24.0%) knees that underwent nonsurgical treatment met the overall failure criteria after a mean follow-up of 76.9 months. With increased age and the presence of intrameniscal signals, the nonoperative results become worse.

LEVEL OF EVIDENCE

III.

Comparison of constitutive models for meniscus and their effect on the knee joint biomechanics during gait

Mechanical behavior of meniscus can be modeled using constitutive material models of varying complexity, such as isotropic elastic or fibril reinforced poroelastic (FRPE). However, the FRPE material is complex to implement, computationally demanding in 3D geometries, and simulation is time-consuming. Hence, we aimed to quantify the most suitable and efficient constitutive model of meniscus for simulation of cartilage responses in the knee joint during walking. We showed that simpler constitutive material models can reproduce similar cartilage responses to a knee model with the FRPE meniscus, but only knee models that consider orthotropic elastic meniscus can also reproduce meniscus responses adequately.

Heterogeneity of dynamic shear properties of the meniscus: A comparison between tissue core and surface layers

Damage to the meniscus has been associated with excessive shear loads. Aimed at elucidating meniscus pathophysiology, previous studies have investigated the shear properties of the meniscus fibrocartilaginous core. However, the meniscus is structurally inhomogeneous, with an external cartilaginous envelope (tibial and femoral surface layers) wrapping the tissue core. To date, little is known about the shear behavior of the surface layers. The objective of this study was to measure the dynamic shear properties of the surface layers and derive empirical relations with their composition. Specimens were harvested from tibial and femoral surface layers and core of porcine menisci (medial and lateral, n = 10 each). Frequency sweep tests yielded complex shear modulus (G*) and phase shifts (δ). Mechanical behavior of regions was described by a generalized Maxwell model. Correlations between shear moduli with water and glycosaminoglycans content of the tissue regions were investigated. The femoral surface had the lowest shear modulus, when compared to core and tibial regions. A 3-relaxation times Maxwell model satisfactorily interpreted the shear behavior of all tissue regions. Inhomogeneous tissue composition was also observed, with water content in the surface layers being higher when compared with tissue core. Water content negatively correlated with shear properties in all regions. The lower measured shear properties in the femoral layer may explain the higher prevalence of meniscal tears on the superior surface of the tissue. The heterogenous behavior of the tissue in shear provides insight into meniscus pathology and has important implications for efforts to tissue engineer replacement tissues.

On the fractional transversely isotropic functionally graded nature of soft biological tissues: Application to the meniscal tissue

This paper focuses on the origin of the poroelastic anisotropic behaviour of the meniscal tissue and its spatially varying properties. We present confined compression creep test results on samples extracted from three parts of the tissue (Central body, Anterior horn and Posterior horn) in three orientations (Circumferential, Radial and Vertical). We show that a poroelastic model in which the fluid flow evolution is ruled by non-integer order operators (fractional Darcy's law) provides accurate agreement with the experimental creep data. The model is validated against two additional sets of experimental data: stress relaxation and fluid loss during the consolidation process measured as weight reduction. Results show that the meniscus can be considered as a transversely isotropic poroelastic material. This behaviour is due to the fluid flow rate being about three times higher in the circumferential direction than in the radial and vertical directions in the body region of the meniscus. The 3D fractional poroelastic model is implemented in the finite element software to estimate the weight loss during the confined compression tests.

Effect of molecular weight and tissue layer on solute partitioning in the knee meniscus

Objective

Knee meniscus tissue is partly vascularized, meaning that nutrients must be transported through the extracellular matrix of the avascular portion to reach resident cells. Similarly, drugs used as therapeutic agents to treat meniscal pathologies rely on transport through the tissue. The driving force of diffusive transport is the gradient of concentration, which depends on molecular solubility. The meniscus is organized into a core region sandwiched between the tibial and femoral superficial layers. Structural differences exist across meniscal regions; therefore, regional differences in solubility are also hypothesized.

Methods

Samples from the core, tibial and femoral layers were obtained from 5 medial and 5 lateral porcine menisci. The partition coefficient (K) of fluorescein, 3 ​kDa and 40 ​kDa dextrans in the layers of the meniscus was measured using an equilibration experiment. The effect of meniscal compartment, layer, and solute molecular weight on K was analyzed using a three-way ANOVA.

Results

K ranged from a high of ∼2.9 in fluorescein to a low of ∼0.1 in 40 ​kDa dextran and was inversely related to the solute molecular weight across all tissue regions. Tissue layer only had a significant effect on partitioning of 40k Dex solute, which was lower in the tibial surface layer relative to the core (p ​= ​0.032).

Conclusion

This study provides insight into depth-dependent partitioning in the meniscus, indicating the limiting effect of the meniscus superficial layer on solubility increases with solute molecular size. This illustrates how the surface layers could potentially reduce the effectiveness of drug delivery therapies incorporating large molecules (>40 ​kDa).

Tensile energy dissipation and mechanical properties of the knee meniscus: relationship with fiber orientation, tissue layer, and water content

Introduction: The knee meniscus distributes and dampens mechanical loads. It is composed of water (∼70%) and a porous fibrous matrix (∼30%) with a central core that is reinforced by circumferential collagen fibers enclosed by mesh-like superficial tibial and femoral layers. Daily loading activities produce mechanical tensile loads which are transferred through and dissipated by the meniscus. Therefore, the objective of this study was to measure how tensile mechanical properties and extent of energy dissipation vary by tension direction, meniscal layer, and water content. Methods: The central regions of porcine meniscal pairs (n = 8) were cut into tensile samples (4.7 mm length, 2.1 mm width, and 0.356 mm thickness) from core, femoral and tibial components. Core samples were prepared parallel (circumferential) and perpendicular (radial) to the fibers. Tensile testing consisted of frequency sweeps (0.01-1Hz) followed by quasi-static loading to failure. Dynamic testing yielded energy dissipation (ED), complex modulus (E*), and phase shift (δ) while quasi-static tests yielded Young's Modulus (E), ultimate tensile strength (UTS), and strain at UTS (ε_UTS). To investigate how ED is influenced by the specific mechanical parameters, linear regressions were performed. Correlations between sample water content (φ_w) and mechanical properties were investigated. A total of 64 samples were evaluated. Results: Dynamic tests showed that increasing loading frequency significantly reduced ED (p < 0.05). Circumferential samples had higher ED, E*, E, and UTS than radial ones (p < 0.001). Stiffness was highly correlated with ED (R² > 0.75, p < 0.01). No differences were found between superficial and circumferential core layers. ED, E*, E, and UTS trended negatively with φ_w (p < 0.05). Discussion: Energy dissipation, stiffness, and strength are highly dependent on loading direction. A significant amount of energy dissipation may be associated with time-dependent reorganization of matrix fibers. This is the first study to analyze the tensile dynamic properties and energy dissipation of the meniscus surface layers. Results provide new insights on the mechanics and function of meniscal tissue.

Biomechanical properties of porcine meniscus as determined via AFM: Effect of region, compartment and anisotropy

The meniscus is a fibrocartilaginous tissue that plays an essential role in load transmission, lubrication, and stabilization of the knee. Loss of meniscus function, through degeneration or trauma, can lead to osteoarthritis in the underlying articular cartilage. To perform its crucial function, the meniscus extracellular matrix has a particular organization, including collagen fiber bundles running circumferentially, allowing the tissue to withstand tensile hoop stresses developed during axial loading. Given its critical role in preserving the health of the knee, better understanding structure-function relations of the biomechanical properties of the meniscus is critical. The main objective of this study was to measure the compressive modulus of porcine meniscus using Atomic Force Microscopy (AFM); the effects of three key factors were investigated: direction (axial, circumferential), compartment (medial, lateral) and region (inner, outer). Porcine menisci were prepared in 8 groups (= 2 directions x 2 compartments x 2 regions) with n = 9 per group. A custom AFM was used to obtain force-indentation curves, which were then curve-fit with the Hertz model to determine the tissue's compressive modulus. The compressive modulus ranged from 0.75 to 4.00 MPa across the 8 groups, with an averaged value of 2.04±0.86MPa. Only direction had a significant effect on meniscus compressive modulus (circumferential > axial, p = 0.024), in agreement with earlier studies demonstrating that mechanical properties in the tissue are anisotropic. This behavior is likely the result of the particular collagen fiber arrangement in the tissue and plays a key role in load transmission capability. This study provides important information on the micromechanical properties of the meniscus, which is crucial for understanding tissue pathophysiology, as well as for developing novel treatments for tissue repair.

Hydraulic permeability and compressive properties of porcine and human synovium

The synovium is a multilayer connective tissue separating the intra-articular spaces of the diarthrodial joint from the extra-synovial vascular and lymphatic supply. Synovium regulates drug transport into and out of the joint, yet its material properties remain poorly characterized. Here, we measured the compressive properties (aggregate modulus, Young's modulus, and Poisson's ratio) and hydraulic permeability of synovium with a combined experimental-computational approach. A compressive aggregate modulus and Young's modulus for the solid phase of synovium were quantified from linear regression of the equilibrium confined and unconfined compressive stress upon strain, respectively (H_A = 4.3 ± 2.0 kPa, E_s = 2.1 ± 0.75, porcine; H_A = 3.1 ± 2.0 kPa, E_s = 2.8 ± 1.7, human). Poisson's ratio was estimated to be 0.39 and 0.40 for porcine and human tissue, respectively, from moduli values in a Monte Carlo simulation. To calculate hydraulic permeability, a biphasic finite element model's predictions were numerically matched to experimental data for the time-varying ramp and hold phase of a single increment of applied strain (k = 7.4 ± 4.1 × 10^-15 m⁴/N.s, porcine; k = 7.4 ± 4.3 × 10^-15 m⁴/N.s, human). We can use these newly measured properties to predict fluid flow gradients across the tissue in response to previously reported intra-articular pressures. These values for material constants are to our knowledge the first available measurements in synovium that are necessary to better understand drug transport in both healthy and pathological joints.

Anisotropy and inhomogeneity of permeability and fibrous network response in the pars intermedia of the human lateral meniscus

Within the human tibiofemoral joint, meniscus plays a key role due to its peculiar time-dependent mechanical characteristics, inhomogeneous structure and compositional features. To better understand the pathophysiological mechanisms underlying this essential component, it is mandatory to analyze in depth the relationship between its structure and the function it performs in the joint. Accordingly, the aim of this study was to evaluate the behavior of both solid and fluid phases of human meniscus in response to compressive loads, by integrating mechanical assessment and histological analysis. Cubic specimens were harvested from seven knee lateral menisci, specifically from anterior horn, pars intermedia and posterior horn; unconfined compressive tests were then performed according to three main loading directions (i.e., radial, circumferential and vertical). Fibril modulus, matrix modulus and hydraulic permeability of the tissue were thence estimated through a fibril-network-reinforced biphasic model. Tissue porosity and collagen fibers arrangement were assessed through histology for each region and related to the loading directions adopted during mechanical tests. Regional and strain-dependent constitutive parameters were finally proposed for the human lateral meniscus, suggesting an isotropic behavior of both the horns, and a transversely isotropic response of the pars intermedia. Furthermore, the histological findings supported the evidences highlighted by the compressive tests. Indeed, this study provided novel insights concerning the functional behavior of human menisci by integrating mechanical and histological characterizations and thus highlighting the key role of this component in knee contact mechanics and presenting fundamental information that can be used in the development of tissue-engineered substitutes. STATEMENT OF SIGNIFICANCE: This work presents an integration to the approaches currently used to model the mechanical behavior of the meniscal tissue. This study assessed in detail the regional and directional contributions of both the meniscal solid and fluid phases during compressive response, providing also complementary histological evidence. Within this updated perspective, both knee computational modeling and meniscal tissue engineering can be improved to have an effective impact on the clinical practice.

Compressive Properties and Hydraulic Permeability of Human Meniscus: Relationships With Tissue Structure and Composition

The meniscus is crucial in maintaining knee function and protecting the joint from secondary pathologies, including osteoarthritis. The meniscus has been shown to absorb up to 75% of the total load on the knee joint. Mechanical behavior of meniscal tissue in compression can be predicted by quantifying the mechanical parameters including; aggregate modulus (H) and Poisson modulus (ν), and the fluid transport parameter: hydraulic permeability (K). These parameters are crucial to develop a computational model of the tissue and for the design and development of tissue engineered scaffolds mimicking the native tissue. Hence, the objective of this study was to characterize the mechanical and fluid transport properties of human meniscus and relate them to the tissue composition. Specimens were prepared from the axial and the circumferential anatomical planes of the tissue. Stress relaxation tests yielded the H, while finite element modeling was used to curve fit for ν and K. Correlations of moduli with water and glycosaminoglycans (GAGs) content were investigated. On average H was found to be 0.11 ± 0.078 MPa, ν was 0.32 ± 0.057, and K was 2.9 ± 2.27 × 10^-15 m⁴N^-1s^-1. The parameters H, ν, and K were not found to be statistically different across compression orientation or compression level. Water content of the tissue was 77 ± 3.3% while GAG content was 8.79 ± 1.1%. Interestingly, a weak negative correlation was found between H and water content (R² ~ 34%) and a positive correlation between K and GAG content (R² ~ 53%). In conclusion, while no significant differences in transport and compressive properties can be found across sample orientation and compression levels, data trends suggest potential relationships between magnitudes of H and K, and GAG content.

Comparison of High-Hydrostatic-Pressure Decellularized Versus Freeze-Thawed Porcine Menisci

The meniscus functions as a load distributor and secondary stabilizer in the knee, and the loss of the meniscus increases the risk of osteoarthritis. Freeze-thawed menisci are used in clinical practice to replace defective menisci; however, the disadvantages of freeze-thawed tissues include disease transmission and immune rejection. In this study, we decellularized menisci using high hydrostatic pressure (HHP) and compared the decellularized menisci with freeze-thawed menisci. Porcine menisci were either pressurized at 1,000 MPa for 10 min and then washed with DNase solution or frozen at -80°C for 2 days and thawed. These menisci then underwent in vitro histological, biochemical, and biomechanical comparisons with native menisci. The HHP-treated and freeze-thawed menisci were also subcutaneously implanted in a pig, and later harvested for histological analysis. The numbers of histologically detected cells were significantly lower and the amount of biochemically detected DNA was approximately 100-fold lower in HHP-treated than in native and freeze-thawed menisci. The compression strength of the HHP-decellularized menisci was decreased after 1 and 50 cycles at 20% strain but was unchanged in the freeze-thawed menisci. After implantation, the numbers of multinucleated giant cells were significantly lower around the HHP-treated menisci than around the freeze-thawed menisci. Recellularization of the HHP-decellularized menisci was confirmed. Thus, although the HHP-decellularized menisci were mechanically inferior to the freeze-thawed meniscus in vitro, they were immunologically superior. Our study is the first to demonstrate the use of HHP for decellularization of the meniscus. © 2019 The Authors. Journal of Orthopaedic Research^® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2466-2475, 2019.

Biomechanical Stimulus Based Strategies for Meniscus Tissue Engineering and Regeneration

Meniscus injuries are very common in the knee joint. Treating a damaged meniscus continues to be a scientific challenge in sport medicine because of its poor self-healing potential and few clinical therapeutic options. Tissue engineering strategies are very promising solutions for repairing and regenerating a damaged meniscus. Meniscus is exposed to a complex biomechanical microenvironment, and it plays a crucial role in meniscal development, growth, and repairing. Over the past decades, increasing attention has been focused on the use of biomechanical stimulus to enhance biomechanical properties of the engineered meniscus. Further understanding the influence of mechanical stimulation on cell proliferation and differentiation, metabolism, relevant gene expression, and pro/anti-inflammatory responses may be beneficial to enhance meniscal repair and regeneration. On the one hand, this review describes some basic information about meniscus; on the other hand, we sum up the various biomechanical stimulus based strategies applied in meniscus tissue engineering and how these factors affect meniscal regeneration. We hope this review will provide researchers with inspiration on tissue engineering strategies for meniscus regeneration in the future.