A transversely isotropic biphasic finite element model of the meniscus

Finite element analysis of meniscus contact mechanical behavior based on kinematic simulation of abnormal gait

The meniscus plays a crucial role in the proper functioning of the knee joint, and when it becomes damaged, partial removal or replacement is necessary to restore proper function. Understanding the stress and deformation of the meniscus during various movements is essential for developing effective materials for meniscus repair. However, accurately estimating the contact mechanics of the knee joint can be challenging due to its complex shape and the dynamic changes it undergoes during movement. To address this issue, the open-source software SCONE can be used to establish a kinematics model that monitors the different states of the knee joint during human motion and obtains relevant gait kinematics data. To evaluate the stress and deformation of the meniscus during normal human movement, values of different states in the movement gait can be selected for finite element analysis (FEA) of the knee joint. This analysis enables researchers to assess changes in the meniscus. To evaluate meniscus damage, it is necessary to obtain changes in its mechanical behavior during abnormal movements. This information can serve as a reference for designing and optimizing the mechanical performance of materials used in meniscus repair and replacement.

Structure-Mechanics Principles and Mechanobiology of Fibrocartilage Pericellular Matrix: A Pivotal Role of Type V Collagen

The pericellular matrix (PCM) is the immediate microniche surrounding resident cells in various tissue types, regulating matrix turnover, cell-matrix cross-talk and disease initiation. This study elucidated the structure-mechanical properties and mechanobiological functions of the PCM in fibrocartilage, a family of connective tissues that sustain complex tensile and compressive loads in vivo. Studying the murine meniscus as the model tissue, we showed that fibrocartilage PCM contains thinner, random collagen fibrillar networks that entrap proteoglycans, a structure distinct from the densely packed, highly aligned collagen fibers in the bulk extracellular matrix (ECM). In comparison to the ECM, the PCM has a lower modulus and greater isotropy, but similar relative viscoelastic properties. In Col5a1 +/- menisci, the reduction of collagen V, a minor collagen localized in the PCM, resulted in aberrant fibril thickening with increased heterogeneity. Consequently, the PCM exhibited a reduced modulus, loss of isotropy and faster viscoelastic relaxation. This disrupted PCM contributes to perturbed mechanotransduction of resident meniscal cells, as illustrated by reduced intracellular calcium signaling, as well as upregulated biosynthesis of lysyl oxidase and tenascin C. When cultured in vitro, Col5a1 +/- meniscal cells synthesized a weakened nascent PCM, which had inferior properties towards protecting resident cells against applied tensile stretch. These findings underscore the PCM as a distinctive microstructure that governs fibrocartilage mechanobiology, and highlight the pivotal role of collagen V in PCM function. Targeting the PCM or its molecular constituents holds promise for enhancing not only meniscus regeneration and osteoarthritis intervention, but also addressing diseases across various fibrocartilaginous tissues.

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.

Region- and layer-specific investigations of the human menisci using SHG imaging and biaxial testing

In this paper, we examine the region- and layer-specific collagen fiber morphology via second harmonic generation (SHG) in combination with planar biaxial tension testing to suggest a structure-based constitutive model for the human meniscal tissue. Five lateral and four medial menisci were utilized, with samples excised across the thickness from the anterior, mid-body, and posterior regions of each meniscus. An optical clearing protocol enhanced the scan depth. SHG imaging revealed that the top samples consisted of randomly oriented fibers with a mean fiber orientation of 43.3 ^o . The bottom samples were dominated by circumferentially organized fibers, with a mean orientation of 9.5 ^o . Biaxial testing revealed a clear anisotropic response, with the circumferential direction being stiffer than the radial direction. The bottom samples from the anterior region of the medial menisci exhibited higher circumferential elastic modulus with a mean value of 21 MPa. The data from the two testing protocols were combined to characterize the tissue with an anisotropic hyperelastic material model based on the generalized structure tensor approach. The model showed good agreement in representing the material anisotropy with a mean r ² = 0.92.

Tibiofemoral Cartilage Contact Pressures in Athletes During Landing: A Dynamic Finite Element Study

Cartilage defects are common in the knee joint of active athletes and remain a problem as a strong risk factor for osteoarthritis. We hypothesized that landing during sport activities, implication for subfailure ACL loading, would generate greater contact pressures (CP) at the lateral knee compartment. The purpose of this study is to investigate tibiofemoral cartilage CP of athletes during landing. Tibiofemoral cartilage contact pressures (TCCP) under clinically relevant anterior cruciate ligament subfailure external loadings were predicted using four dynamic explicit finite element (FE) models (2 males and 2 females) of the knee. Bipedal landing from a jump for five cases of varying magnitudes of external loadings (knee abduction moment, internal tibial torque, and anterior tibial shear) followed by an impact load were simulated. Lateral TCCP from meniscus (area under meniscus) and from femur (area under femur) increased by up to 94% and %30 respectively when external loads were incorporated with impact load in all the models compared to impact-only case. In addition, FE model predicted higher CP in lateral compartment by up to 37% (11.87 MPa versus 8.67 MPa) and 52% (20.19 MPa versus 13.29 MPa) for 90% and 50% percentile models, respectively. For the same percentile populations, CPs were higher by up to 25% and 82% in smaller size models than larger size models. We showed that subfailure ACL loadings obtained from previously conducted in vivo study led to high pressures on the tibiofemoral cartilage. This knowledge is helpful in enhancing neuromuscular training for athletes to prevent cartilage damage.

Meniscal Salvage: Where We Are Today

The menisci are fibrocartilaginous semilunar structures in the knee that provide load support. Injury to the meniscus alters its load sharing and biomechanical profile. Knee arthroscopy with meniscus débridement is the most common orthopaedic surgical procedure done in the United States. The current goals of meniscal surgery are to preserve native meniscal tissue and maintain structural integrity. Meniscal preservation is critical to maintain the normal mechanics and homeostasis of the knee; however, it is not always feasible because of the structure's poor blood supply and often requires removal of irreparable tissue with meniscectomy. Efforts have increasingly focused on the promotion of meniscal healing and the replacement of damaged menisci with allografts, scaffolds, meniscal implants, or substitutes. The purpose of this article was to review current and future meniscal salvage treatments such as meniscus transplant, synthetic arthroplasty, and possible bioprinted meniscus to allow patients to maintain quality of life, limit pain, and delay osteoarthritis.

Viscoelastic and equilibrium shear properties of human meniscus: Relationships with tissue structure and composition

The meniscus is crucial in maintaining the knee function and protecting the joint from secondary pathologies, including osteoarthritis. Although most of the mechanical properties of human menisci have been characterized, to our knowledge, its dynamic shear properties have never been reported. Moreover, little is known about meniscal shear properties in relation to tissue structure and composition. This is crucial to understand mechanisms of meniscal injury, as well as, in regenerative medicine, for the design and development of tissue engineered scaffolds mimicking the native tissue. Hence, the objective of this study was to characterize the dynamic and equilibrium shear properties of human meniscus in relation to its anisotropy and composition. Specimens were prepared from the axial and the circumferential anatomical planes of medial and lateral menisci. Frequency sweeps and stress relaxation tests yielded storage (G') and loss moduli (G″), and equilibrium shear modulus (G). Correlations of moduli with water, glycosaminoglycans (GAGs), and collagen content were investigated. The meniscus exhibited viscoelastic behavior. Dynamic shear properties were related to tissue composition: negative correlations were found between G', G″ and G, and meniscal water content; positive correlations were found for G' and G″ with GAG and collagen (only in circumferential samples). Circumferential samples, with collagen fibers orthogonal to the shear plane, exhibited superior dynamic mechanical properties, with G' ~70 kPa and G″ ~10 kPa, compared to those of the axial plane ~15 kPa and ~1 kPa, respectively. Fiber orientation did not affect the values of G, which ranged from ~50 to ~100 kPa.

3D bioprinting of a biomimetic meniscal scaffold for application in tissue engineering

Appropriate biomimetic scaffolds created via 3D bioprinting are promising methods for treating damaged menisci. However, given the unique anatomical structure and complex stress environment of the meniscus, many studies have adopted various techniques to take full advantage of different materials, such as the printing combined with infusion, or electrospining, to chase the biomimetic meniscus, which makes the process complicated to some extent. Some researchers have tried to tackle the challenges only by 3D biopringting, while its alternative materials and models have been constrained. In this study, based on a multilayer biomimetic strategy, we optimized the preparation of meniscus-derived bioink, gelatin methacrylate (GelMA)/meniscal extracellular matrix (MECM), to take printability and cytocompatibility into account together. Subsequently, a customized 3D bioprinting system featuring a dual nozzle + multitemperature printing was used to integrate the advantages of polycaprolactone (PCL) and meniscal fibrocartilage chondrocytes (MFCs)-laden GelMA/MECM bioink to complete the biomimetic meniscal scaffold, which had the best biomimetic features in terms of morphology and components. Furthermore, cell viability, mechanics, biodegradation and tissue formation in vivo were performed to ensure that the scaffold had sufficient feasibility and functionality, thereby providing a reliable basis for its application in tissue engineering.

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We have optimized the preparation of meniscus-derived bioink with good printability and cytocompatibility.

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A customized printing system for biomimetic meniscus, the dual-nozzle + multitemperature printing system was developed.

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We have achieved multilayer meniscal biomimetic strategy, especially the best biomimetics of morphology and components.

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Focusing on application prospect, we designed a few experiments to verity the feasibility and functionality of the scaffold.

Constitutive modeling of menisci tissue: a critical review of analytical and numerical approaches

Menisci are fibrocartilaginous disks consisting of soft tissue with a complex biomechanical structure. They are critical determinants of the kinematics as well as the stability of the knee joint. Several studies have been carried out to formulate tissue mechanical behavior, leading to the development of a wide spectrum of constitutive laws. In addition to developing analytical tools, extensive numerical studies have been conducted on menisci modeling. This study reviews the developments of the most widely used continuum models of the meniscus mechanical properties in conjunction with emerging analytical and numerical models used to study the meniscus. The review presents relevant approaches and assumptions used to develop the models and includes discussions regarding strengths, weaknesses, and discrepancies involved in the presented models. The study presents a comprehensive coverage of relevant publications included in Compendex, EMBASE, MEDLINE, PubMed, ScienceDirect, Springer, and Scopus databases. This review aims at opening novel avenues for improving menisci modeling within the framework of constitutive modeling through highlighting the needs for further research directed toward determining key factors in gaining insight into the biomechanics of menisci which is crucial for the elaborate design of meniscal replacements.

Early degeneration of the meniscus revealed by microbiomechanical alteration in a rabbit anterior cruciate ligament transection model

Background

The microbiomechanical properties of the meniscus influence the cell response to the surrounding biomechanical environment ​and are beneficial to understand meniscus repairing and healing. To date, however, this information remains ambiguous. This study aims to characterise the microbiomechanical properties of the meniscus after degeneration in a rabbit anterior cruciate ligament transection (ACLT) model and to analyse the corresponding histology at the macroscale and chemical composition.

Methods

Twenty New Zealand white rabbits were used. Menisci were collected from the knee joints 4 and 8 weeks after the ACLT and from those of the corresponding control groups. The central portions of both medial and lateral menisci were investigated using atomic force microscopy, histological study, and an energy-dispersive spectrometer. The evaluation was conducted regionally within the inner, middle, and outer sites from the top layer (facing the femoral surface) to the bottom layer (facing the tibial surface) in both the lateral and medial menisci to obtain the site-dependent properties.

Results

At 4 weeks after surgery, the dynamic elastic modulus at the microlevel increased significantly at both the top and bottom layers compared with the intact meniscus (P ​= ​0.021). At 8 weeks after surgery, the stiffening occurred in all regions (P ​= ​0.030). The medial meniscus showed greater change than the lateral meniscus. All these microbiomechanical alterations occurred before the histological findings at the macroscale.

Conclusion

The microbiomechanical properties in the meniscus changed significantly after ACLT and were site dependent. Their alterations occurred before the histological changes of degeneration were observed.

The Translational Potential of this Article

The results of our study indicated that degeneration promoted meniscus stiffening. Thus, they provide a better understanding of the disease process affecting the meniscus. Our results might be beneficial to understand how mechanical forces distribute throughout the healthy and pathologic joint. They indicate the possibility of early diagnosis using a minimally invasive arthroscopic tool, as well as they might guide treatment to the healthy and pathologic meniscus and joint.

Molecular and macromolecular diffusion in human meniscus: relationships with tissue structure and composition

OBJECTIVE

To date, the pathophysiology of the meniscus has not been fully elucidated. Due to the tissue's limited vascularization, nutrients and other molecular signals spread through the extracellular matrix via diffusion or convection (interstitial fluid flow). Understanding transport mechanisms is crucial to elucidating meniscal pathophysiology, and to designing treatments for repair and restoration of the tissue. Similar to other fibrocartilaginous structures, meniscal morphology and composition may affect its diffusive properties. The objective of this study was to investigate the role of solute size, and tissue structure and composition on molecular diffusion in meniscus tissue.

DESIGN

Using a custom FRAP technique developed in our lab, we measured the direction-dependent diffusivity in human meniscus of six different molecular probes of size ranging from ∼300Da to 150,000Da. Diffusivity measurements were related to sample water content. SEM images were used to investigate collagen structure in relation to transport mechanisms.

RESULTS

Diffusivity was anisotropic, being significantly faster in the direction parallel to collagen fibers when compared the orthogonal direction. This was likely due to the unique structural organization of the tissue presenting pores aligned with the fibers, as observed in SEM images. Diffusion coefficients decreased as the molecular size increased, following the Ogston model. No significant correlations were found among diffusion coefficients and water content of the tissue.

CONCLUSIONS

This study provides new knowledge on the mechanisms of molecular transport in meniscal tissue. The reported results can be leveraged to further investigate tissue pathophysiology and to design treatments for tissue restoration or replacement.

In vivo Layer-specific Mechanical Characterization of Porcine Stomach Tissue using Ultrasound Elastography

This paper presents in vivo mechanical characterization of the muscularis, submucosa and mucosa of the porcine stomach wall under large deformation loading. This is important for the development of gastrointestinal pathology-specific surgical intervention techniques. The study is based on testing the cardiac and fundic glandular stomach regions using a custom-developed compression elastography setup. Particular attention has been paid to elucidate the heterogeneity and anisotropy of tissue response. A Fung hyperelastic material model has been used to model the mechanical response of each tissue layer. A univariate analysis comparing the initial shear moduli of the three layers indicates that the muscularis (5.69±4.06 kPa) is the stiffest followed by the submucosa (3.04±3.32 kPa) and the mucosa (0.56±0.28 kPa). The muscularis is found to be strongly distinguishable from the mucosa tissue in the cardiac and fundic region based on a multivariate discriminant analysis. The cardiac muscularis is observed to be stiffer than the fundic muscularis tissue (shear moduli of 7.96±3.82 kPa vs. 3.42±2.96 kPa), more anisotropic (anisotropic parameter of 2.21±0.77 vs. 1.41±0.38), and strongly distinguishable from its fundic counterpart. Finally, a univariate comparison of the in vivo and ex vivo initial shear moduli for each layer shows that the muscularis and submucosa tissues are softer while in vivo, but the mucosa tissue is stiffer while in vivo. The mechanical properties highlight the inhomogeneity and anisotropy of multilayer stomach tissue.

Short cracks in knee meniscus tissue cause strain concentrations, but do not reduce ultimate stress, in single-cycle uniaxial tension

Tears are central to knee meniscus pathology and, from a mechanical perspective, are crack-like defects (cracks). In many materials, cracks create stress concentrations that cause progressive local rupture and reduce effective strength. It is currently unknown if cracks in meniscus have these consequences; if they do, this would have repercussions for management of meniscus pathology. The objective of this study was to determine if a short crack in meniscus tissue, which mimics a preclinical meniscus tear, (a) causes crack growth and reduces effective strength, (b) creates a near-tip strain concentration and (c) creates unloaded regions on either side of the crack. Specimens with and without cracks were tested in uniaxial tension and compared in terms of macroscopic stress-strain curves and digital image correlation strain fields. The strain fields were used as an indicator of stress concentrations and unloaded regions. Effective strength was found to be insensitive to the presence of a crack (potential effect < 0.86 s.d.; β = 0.2), but significant strain concentrations, which have the potential to lead to long-term accumulation of tissue or cell damage, were observed near the crack tip.

FEBio: History and Advances

The principal goal of the FEBio project is to provide an advanced finite element tool for the biomechanics and biophysics communities that allows researchers to model mechanics, transport, and electrokinetic phenomena for biological systems accurately and efficiently. In addition, because FEBio is geared toward the research community, the code is designed such that new features can be added easily, thus making it an ideal tool for testing novel computational methods. Finally, because the success of a code is determined by its user base, integral goals of the FEBio project have been to offer support and outreach to our community; to provide mechanisms for dissemination of results, models, and data; and to encourage interaction between users. This review presents the history of the FEBio project, from its initial developments through its current funding period. We also present a glimpse into the future of FEBio.

Micromechanical anisotropy and heterogeneity of the meniscus extracellular matrix

To understand how the complex biomechanical functions of the meniscus are endowed by the nanostructure of its extracellular matrix (ECM), we studied the anisotropy and heterogeneity in the micromechanical properties of the meniscus ECM. We used atomic force microscopy (AFM) to quantify the time-dependent mechanical properties of juvenile bovine meniscus at deformation length scales corresponding to the diameters of collagen fibrils. At this scale, anisotropy in the elastic modulus of the circumferential fibers, the major ECM structural unit, can be attributed to differences in fibril deformation modes: uncrimping when normal to the fiber axis, and laterally constrained compression when parallel to the fiber axis. Heterogeneity among different structural units is mainly associated with their variations in microscale fiber orientation, while heterogeneity across anatomical zones is due to alterations in collagen fibril diameter and alignment at the nanoscale. Unlike the elastic modulus, the time-dependent properties are more homogeneous and isotropic throughout the ECM. These results enable a detailed understanding of the meniscus structure-mechanics at the nanoscale, and can serve as a benchmark for understanding meniscus biomechanical functions, documenting disease progression and designing tissue repair strategies.

STATEMENT OF SIGNIFICANCE

Meniscal damage is a common cause of joint injury, which can lead to the development of post-traumatic osteoarthritis among young adults. Restoration of meniscus function requires repairing its highly heterogeneous and complex extracellular matrix. Employing AFM, this study quantifies the anisotropic and heterogeneous features of the meniscus ECM structure and mechanics. The micromechanical properties are interpreted within the context of the collagen fibril nanostructure and its variation with tissue anatomical locations. These results provide a fundamental structure-mechanics knowledge benchmark, against which, repair and regeneration strategies can be developed and evaluated with respect to the specialized structural and functional complexity of the native tissue.

Meniscus, articular cartilage and nucleus pulposus: a comparative review of cartilage-like tissues in anatomy, development and function

The degradation of cartilage in the human body is impacted by aging, disease, genetic predisposition and continued insults resulting from daily activity. The burden of cartilage defects (osteoarthritis, rheumatoid arthritis, intervertebral disc damage, knee replacement surgeries, etc.) is daunting in light of substantial economic and social stresses. This review strives to broaden the scope of regenerative medicine and tissue engineering approaches used for cartilage repair by comparing and contrasting the anatomical and functional nature of the meniscus, articular cartilage (AC) and nucleus pulposus (NP). Many review papers have provided detailed evaluations of these cartilages and cartilage-like tissues individually but none have comprehensively examined the parallels and inconsistencies in signaling, genetic expression and extracellular matrix composition between tissues. For the first time, this review outlines the importance of understanding these three tissues as unique entities, providing a comparative analysis of anatomy, ultrastructure, biochemistry and function for each tissue. This novel approach highlights the similarities and differences between tissues, progressing research toward an understanding of what defines each tissue as distinctive. The goal of this paper is to provide researchers with the fundamental knowledge to correctly engineer the meniscus, AC and NP without inadvertently developing the wrong tissue function or biochemistry.

Current concepts on structure-function relationships in the menisci

The menisci are intricately organized structures that perform many tasks in the knee. We review their structure and function and introduce new data about their tibial and femoral surfaces. As the femur and tibia approach each other when the knee is bearing load, circumferential tension develops in the menisci, enabling the transmission of compressive load between the femoral and tibial cartilage layers. A low shear modulus is necessary for the tissue to adapt its shape to the changing radius of the femur as that bone moves relative to the tibia during joint articulation. The organization of the meniscus facilitates its functions. In the outer region of the menisci, intertwined collagen fibrils, fibers, and fascicles with predominantly circumferential orientation are prevalent; these structures are held together by radial tie fibers and sheets. Toward the inner portion of the menisci, there is more proteoglycan and the structure becomes more cartilage-like. The transition between these structural forms is gradual and seamless. The flexible roots, required for rigid body motion of the menisci, meld with both the tibia and the outer portion of the menisci to maintain continuity for resistance to the circumferential tension. Our new data demonstrate that the femoral and tibial surfaces of the menisci are structurally analogous to the surfaces of articular cartilage, enabling consistent modes of lubrication and load transfer to occur at the interfacing surfaces throughout motion. The structure and function of the menisci are thus shown to be strongly related to one another: form clearly complements function.

Mechanical Integrity of a Decellularized and Laser Drilled Medial Meniscus

Since the meniscus has limited capacity to self-repair, creating a long-lasting meniscus replacement may help reduce the incidence of osteoarthritis (OA) after meniscus damage. As a first step toward this goal, this study evaluated the mechanical integrity of a decellularized, laser drilled (LD) meniscus as a potential scaffold for meniscal engineering. To evaluate the decellularization process, 24 porcine menisci were processed such that one half remained native tissue, while the other half was decellularized in sodium dodecyl sulphate (SDS). To evaluate the laser drilling process, 24 additional menisci were decellularized, with one half remaining intact while the other half was LD. Decellularization did not affect the tensile properties, but had significant effects on the cyclic compressive hysteresis and unconfined compressive stress relaxation. Laser drilling decreased the Young's modulus and instantaneous stress during unconfined stress relaxation and the circumferential ultimate strength during tensile testing. However, the losses in mechanical integrity in the LD menisci were generally smaller than the variance observed between samples, and thus, the material properties for the LD tissue remained within a physiological range. In the future, optimization of laser drilling patterns may improve these material properties. Moreover, reseeding the construct with cells may further improve the mechanical properties prior to implantation. As such, this work serves as a proof of concept for generating decellularized, LD menisci scaffolds for the purposes of meniscal engineering.

Passive strain-induced matrix synthesis and organization in shape-specific, cartilaginous neotissues

Tissue-engineered musculoskeletal soft tissues typically lack the appropriate mechanical robustness of their native counterparts, hindering their clinical applicability. With structure and function being intimately linked, efforts to capture the anatomical shape and matrix organization of native tissues are imperative to engineer functionally robust and anisotropic tissues capable of withstanding the biomechanically complex in vivo joint environment. The present study sought to tailor the use of passive axial compressive loading to drive matrix synthesis and reorganization within self-assembled, shape-specific fibrocartilaginous constructs, with the goal of developing functionally anisotropic neotissues. Specifically, shape-specific fibrocartilaginous neotissues were subjected to 0, 0.01, 0.05, or 0.1 N axial loads early during tissue culture. Results found the 0.1-N load to significantly increase both collagen and glycosaminoglycan synthesis by 27% and 67%, respectively, and to concurrently reorganize the matrix by promoting greater matrix alignment, compaction, and collagen crosslinking compared with all other loading levels. These structural enhancements translated into improved functional properties, with the 0.1-N load significantly increasing both the relaxation modulus and Young's modulus by 96% and 255%, respectively, over controls. Finite element analysis further revealed the 0.1-N uniaxial load to induce multiaxial tensile and compressive strain gradients within the shape-specific neotissues, with maxima of 10.1%, 18.3%, and -21.8% in the XX-, YY-, and ZZ-directions, respectively. This indicates that strains created in different directions in response to a single axis load drove the observed anisotropic functional properties. Together, results of this study suggest that strain thresholds exist within each axis to promote matrix synthesis, alignment, and compaction within the shape-specific neotissues. Tailoring of passive axial loading, thus, presents as a simple, yet effective way to drive in vitro matrix development in shape-specific neotissues toward more closely achieving native structural and functional properties.

Effect of Static Compressive Strain, Anisotropy, and Tissue Region on the Diffusion of Glucose in Meniscus Fibrocartilage

Osteoarthritis (OA) is a significant socio-economic concern, affecting millions of individuals each year. Degeneration of the meniscus of the knee is often associated with OA, yet the relationship between the two is not well understood. As a nearly avascular tissue, the meniscus must rely on diffusive transport for nutritional supply to cells. Therefore, quantifying structure–function relations for transport properties in meniscus fibrocartilage is an important task. The purpose of the present study was to determine how mechanical loading, tissue anisotropy, and tissue region affect glucose diffusion in meniscus fibrocartilage. A one-dimensional (1D) diffusion experiment was used to measure the diffusion coefficient of glucose in porcine meniscus tissues. Results show that glucose diffusion is strain-dependent, decreasing significantly with increased levels of compression. It was also determined that glucose diffusion in meniscus tissues is anisotropic, with the diffusion coefficient in the circumferential direction being significantly higher than that in the axial direction. Finally, the effect of tissue region was not statistically significant, comparing axial diffusion in the central and horn regions of the tissue. This study is important for better understanding the transport and nutrition-related mechanisms of meniscal degeneration and related OA in the knee.

Biological augmentation and tissue engineering approaches in meniscus surgery

PURPOSE

The purpose of this review was to evaluate the role of biological augmentation and tissue engineering strategies in meniscus surgery. Although clinical (human), preclinical (animal), and in vitro tissue engineering studies are included here, we have placed additional focus on addressing preclinical and clinical studies reported during the 5-year period used in this review in a systematic fashion while also providing a summary review of some important in vitro tissue engineering findings in the field over the past decade.

METHODS

A search was performed on PubMed for original works published from 2009 to March 31, 2014 using the term "meniscus" with all the following terms: "scaffolds," "constructs," "cells," "growth factors," "implant," "tissue engineering," and "regenerative medicine." Inclusion criteria were the following: English-language articles and original clinical, preclinical (in vivo), and in vitro studies of tissue engineering and regenerative medicine application in knee meniscus lesions published from 2009 to March 31, 2014.

RESULTS

Three clinical studies and 18 preclinical studies were identified along with 68 tissue engineering in vitro studies. These reports show the increasing promise of biological augmentation and tissue engineering strategies in meniscus surgery. The role of stem cell and growth factor therapy appears to be particularly useful. A review of in vitro tissue engineering studies found a large number of scaffold types to be of promise for meniscus replacement. Limitations include a relatively low number of clinical or preclinical in vivo studies, in addition to the fact there is as yet no report in the literature of a tissue-engineered meniscus construct used clinically. Neither does the literature provide clarity on the optimal meniscus scaffold type or biological augmentation with which meniscus repair or replacement would be best addressed in the future. There is increasing focus on the role of mechanobiology and biomechanical and biochemical cues in this process, however, and it is hoped that this may lead to improvements in this strategy.

CONCLUSIONS

There appears to be significant potential for biological augmentation and tissue engineering strategies in meniscus surgery to enhance options for repair and replacement. However, there are still relatively few clinical studies being reported in this regard. There is a strong need for improved translational activities and infrastructure to link the large amounts of in vitro and preclinical biological and tissue engineering data to clinical application.

LEVEL OF EVIDENCE

Level IV, systematic review of Level I-IV studies.

Material properties of individual menisci and their attachments obtained through inverse FE-analysis

Meniscal properties for computational methods have already been proposed. However, it is well known that there is high intra subject variability in the material properties of soft tissues and that disruption of the fiber network alters the biomechanics of the meniscus. Therefore, the objective of this study was to establish a non invasive method to determine the material properties of the individual menisci and their attachments using inverse FE-analyses. In a previous study, the 3D displacements of the meniscus and its attachments under axial joint loads were determined for intact porcine knees. To simulate the experimental response in individual FE-analyses (n=5), an anisotropic, hyperelastic meniscus matrix was embedded in a poroelastic model. During a particle swarm optimization, the difference between the force applied to the meniscus during the experiment and the femoral surface reaction force of the FE model at equilibrium was minimized by varying four material parameters. Afterwards, a prediction error was determined to describe how well the material parameter fit to each of the three displacement directions. Additionally, the stresses occurring in the meniscus were evaluated. The error of the material parameter optimization was on average 6.5±4.4%. The best fitting material parameter combination revealed an error of 1.2%. The highest stresses occurred in the region between the pars intermedia and posterior horn of the meniscus. The individual material properties of the meniscus were successfully obtained with a combination of previously reported, noninvasively measured 3D displacements and inverse FE-analyses. The methodology presented in this study is a promising contribution to the detection of degeneration within the meniscus.

Meniscus replacement: Influence of geometrical mismatches on chondroprotective capabilities

The chondroprotective success of meniscal transplantation is variable. Poorly controlled factors such as a geometrical mismatch of the implant may be partly responsible. Clinical data, animal studies and cadaver experiments suggest that smaller transplants perform better than oversized, but clear evidence is lacking. The hypothesis of this study is that smaller menisci outperform larger ones because they distribute stresses more effectively at those particular locations that receive the highest loads. Consequently, collagen in the adjacent cartilage is protected from damage due to overstraining. Experimentally it is not possible to measure load distribution and collagen strain inside articular cartilage (AC). Therefore, a numerical model was used to determine the mechanical conditions throughout the depth of the AC. Meniscus implants with different sizes and mechanical properties were evaluated. These were compared with healthy and with meniscectomized joints. To account for the time-dependent behavior 600s of loading was simulated; results were visualized after 1s and 600s. Simulations showed that AC's strains strongly depended on implant size and loading duration. They depended less on the stiffness of the implant material. With an oversized implant, collagen strains were particularly large in the femoral AC initially and further increased upon sustained loading. The severest compressive strains occurred after sustained loading in the meniscectomized joint. Strains with an undersized meniscus were comparable to a perfectly sized implant. In conclusion, these results support the hypothesis that an undersized implant may outperform an oversized one because it distributes stresses better in the most intensely loaded joint area.

Biomechanical properties of murine meniscus surface via AFM-based nanoindentation

This study aimed to quantify the biomechanical properties of murine meniscus surface. Atomic force microscopy (AFM)-based nanoindentation was performed on the central region, proximal side of menisci from 6- to 24-week old male C57BL/6 mice using microspherical tips (Rtip≈5µm) in PBS. A unique, linear correlation between indentation depth, D, and response force, F, was found on menisci from all age groups. This non-Hertzian behavior is likely due to the dominance of tensile resistance by the collagen fibril bundles on meniscus surface that are mostly aligned along the circumferential direction. The indentation resistance was calculated as both the effective modulus, Eind, via the isotropic Hertz model, and the effective stiffness, Sind = dF/dD. Values of Sind and Eind were found to depend on indentation rate, suggesting the existence of poro-viscoelasticity. These values do not significantly vary with anatomical sites, lateral versus medial compartments, or mouse age. In addition, Eind of meniscus surface (e.g., 6.1±0.8MPa for 12 weeks of age, mean±SEM, n=13) was found to be significantly higher than those of meniscus surfaces in other species, and of murine articular cartilage surface (1.4±0.1MPa, n=6). In summary, these results provided the first direct mechanical knowledge of murine knee meniscus tissues. We expect this understanding to serve as a mechanics-based benchmark for further probing the developmental biology and osteoarthritis symptoms of meniscus in various murine models.

An optimized transversely isotropic, hyper-poro-viscoelastic finite element model of the meniscus to evaluate mechanical degradation following traumatic loading

Inverse finite element (FE) analysis is an effective method to predict material behavior, evaluate mechanical properties, and study differences in biological tissue function. The meniscus plays a key role in load distribution within the knee joint and meniscal degradation is commonly associated with the onset of osteoarthritis. In the current study, a novel transversely isotropic hyper-poro-viscoelastic constitutive formulation was incorporated in a FE model to evaluate changes in meniscal material properties following tibiofemoral joint impact. A non-linear optimization scheme was used to fit the model output to indentation relaxation experimental data. This study is the first to investigate rate of relaxation in healthy versus impacted menisci. Stiffness was found to be decreased (p=0.003), while the rate of tissue relaxation increased (p=0.010) at twelve weeks post impact. Total amount of relaxation, however, did not change in the impacted tissue (p=0.513).

Mechanobiology of the meniscus

The meniscus plays a critical biomechanical role in the knee, providing load support, joint stability, and congruity. Importantly, growing evidence indicates that the mechanobiologic response of meniscal cells plays a critical role in the physiologic, pathologic, and repair responses of the meniscus. Here we review experimental and theoretical studies that have begun to directly measure the biomechanical effects of joint loading on the meniscus under physiologic and pathologic conditions, showing that the menisci are exposed to high contact stresses, resulting in a complex and nonuniform stress-strain environment within the tissue. By combining microscale measurements of the mechanical properties of meniscal cells and their pericellular and extracellular matrix regions, theoretical and experimental models indicate that the cells in the meniscus are exposed to a complex and inhomogeneous environment of stress, strain, fluid pressure, fluid flow, and a variety of physicochemical factors. Studies across a range of culture systems from isolated cells to tissues have revealed that the biological response of meniscal cells is directly influenced by physical factors, such as tension, compression, and hydrostatic pressure. In addition, these studies have provided new insights into the mechanotransduction mechanisms by which physical signals are converted into metabolic or pro/anti-inflammatory responses. Taken together, these in vivo and in vitro studies show that mechanical factors play an important role in the health, degeneration, and regeneration of the meniscus. A more thorough understanding of the mechanobiologic responses of the meniscus will hopefully lead to therapeutic approaches to prevent degeneration and enhance repair of the meniscus.

FE analysis of stress and displacements occurring in the bony chain of leg

AIMS

The aim of this study was to assess how the stress shielding can influence the integrity and resistance of bones.

METHODS

With this purpose a complete FE model of the human leg was realised. A load of 700 N has been applied at the top of pelvis and the feet, at the tip, was rigidly fixed.

RESULTS

Obtained results reveal interesting consequences deriving by taking into account the complete bony chain.

CONCLUSION

A comparison among the literature data and our models can furnish a complete vision of the global spreading of the forces along the various bony components.

Relationship of gene expression in the injured human meniscus to body mass index: a biologic connection between obesity and osteoarthritis

OBJECTIVE

Higher body mass index (BMI) increases the risk of meniscus injury and knee osteoarthritis (OA). However, it is unknown if and how obesity affects meniscus biology. We analyzed transcriptome-wide gene expression profiles of injured human menisci to test the hypothesis that meniscal gene expression signatures relate to patient BMI.

METHODS

Meniscus samples were obtained from patients undergoing arthroscopic partial meniscectomy. Transcriptome-wide analysis of gene expression followed by validation of selected transcripts by QuantiGene Plex assay was performed. Correlations of gene expression with BMI and relative fold changes (≥1.5-fold) in 3 BMI categories (lean [BMI 18.5-24.9 kg/m(2) ], overweight [BMI 25.0-29.9 kg/m(2) ], and obese [BMI >30.0 kg/m(2) ]) were analyzed, and integrated functional classifications were probed computationally.

RESULTS

The obese versus overweight comparison resulted in the largest set of differences (565 transcripts) followed by obese versus lean (280 transcripts) and overweight versus lean (125 transcripts). Biologic reproducibility was confirmed by cluster analysis of expressed transcripts. Differentially regulated transcripts represented important functional classifications. Transcripts associated with oxygen transport, calcium ion binding, and cell homeostasis were elevated with BMI, while those related to extracellular matrix deposition, cell migration, and glucosamine metabolic processes were repressed. While these functional classifications may play key roles in cartilage/meniscus homeostasis, failure of extracellular matrix deposition and increase in calcium ion binding likely contribute to OA development following meniscal injury.

CONCLUSION

Our results indicate greater differences in gene expression between obese and overweight groups than between overweight and lean groups. This may indicate that there is a weight threshold at which injured meniscus responds severely to increased BMI. BMI-related changes in gene expression present a plausible explanation for the role of meniscal injury in OA development among obese patients.

The human meniscus: a review of anatomy, function, injury, and advances in treatment

Meniscal injuries are recognized as a cause of significant musculoskeletal morbidity. The menisci are vital for the normal function and long-term health of the knee joint. The purpose of this review is to provide current knowledge regarding the anatomy and biomechanical functions of the menisci, incidence, injury patterns and the advancements in treatment options of meniscal injury. A literature search was performed by a review of PubMed, Google Scholar, MEDLINE, and OVID for all relevant articles published between 1897 and 2014. This study highlights the anatomical and biomechanical characteristics of the menisci, which may be relevant to injury patterns and treatment options. An understanding of the normal anatomy and biomechanical functions of the knee menisci is a necessary prerequisite to understanding pathologies associated with the knee.

Restoration of the meniscus: form and function

Over the past 2 decades there has been a profound shift in our perception of the role of the meniscus in the knee joint. Orthopaedic opinion now favors salvaging and restoring the damaged meniscus where possible. Basic science is characterizing its form (anatomy) and functionality (biological and biomechanical) in an attempt to understand the effect of meniscal injury and repair on the knee joint as a whole. The meniscus is a complex tissue and has warranted extensive basic science, translational, and clinical research to identify techniques to augment healing and even replace the meniscus. The application of quantitative magnetic resonance image sequencing to the meniscus and articular cartilage of the affected compartment promises to add a quantifiable outcome measure to the body of clinical evidence that supports restoration of the meniscus. This article discusses the recent advances and outcomes in the pursuit of meniscal restoration with particular focus on the use of augmentation strategies in meniscal repair, meniscal imaging, and translational strategies.

Finite element modeling of soft tissues: material models, tissue interaction and challenges

BACKGROUND

Musculoskeletal soft tissues, such as articular cartilage, ligaments, knee meniscus and intervertebral disk, have a complex structure, which provides elasticity and capability to support and distribute the body loads. Soft tissues describe an inhomogeneous and multiphasic structure, and exhibit a nonlinear, time-dependent behavior. Their mechanical response is governed by a substance composed of protein fiber-rich and proteoglycan-rich extracellular matrix and interstitial fluid. Protein fibers (e.g. collagen) give the tissue direction dependent stiffness and strength. To investigate these complex biological systems, the use of mathematical tools is well established, alone or in combination with experimental in vitro and in vivo tests. However, the development of these models poses many challenges due to the complex structure and mechanical response of soft tissues.

METHODS

Non-systematic literature review.

FINDINGS

This paper provides a summary of different modeling strategies with associated material properties, contact interactions between articulating tissues, validation and sensitivity of soft tissues with special focus on knee joint soft tissues and intervertebral disk. Furthermore, it reviews and discusses some salient clinical findings of reported finite element simulations.

INTERPRETATION

Model studies extensively contributed to the understanding of functional biomechanics of soft tissues. Models can be effectively used to elucidate clinically relevant questions. However, users should be aware of the complexity of such tissues and of the capabilities and limitations of these approaches to adequately simulate a specific in vivo or in vitro phenomenon.

Current strategies in meniscal regeneration

The meniscus plays an important role in the biomechanics and tribology of the knee joint. Damage to or disease of the meniscus is now recognized to predispose to the development of osteoarthritis. Treatment of meniscal injury through arthroscopic surgery has become one of the most common orthopedic surgical procedures, and in the United States this can represent 10 to 20% of procedures related to the knee. The meniscus has a limited healing capacity constrained to the vascularized periphery and therefore, surgical repair of the avascular regions is not always feasible. Replacement and repair of the meniscus to treat injuries is being investigated using tissue engineering strategies. Promising as these approaches may be, there are, however, major barriers to overcome before translation to the clinic.

Atomic force microscopy reveals regional variations in the micromechanical properties of the pericellular and extracellular matrices of the meniscus

Regional variations in the composition and architecture of the extracellular matrix (ECM) and pericellular matrix (PCM) of the knee meniscus play important roles in determining the local mechanical environment of meniscus cells. In this study, atomic force microscopy was used to spatially map the mechanical properties of matched ECM and perlecan-labeled PCM sites within the outer, middle, and inner porcine medial meniscus, and to evaluate the properties of the proximal surface of each region. The elastic modulus of the PCM was significantly higher in the outer region (151.4 ± 38.2 kPa) than the inner region (27.5 ± 8.8 kPa), and ECM moduli were consistently higher than region-matched PCM sites in both the outer (320.8 ± 92.5 kPa) and inner (66.1 ± 31.4 kPa) regions. These differences were associated with a higher proportion of aligned collagen fibers and lower glycosaminoglycan content in the outer region. Regional variations in the elastic moduli and some viscoelastic properties were observed on the proximal surface of the meniscus, with the inner region exhibiting the highest moduli overall. These results indicate that matrix architecture and composition play an important role in the regional micromechanical properties of the meniscus, suggesting that the local stress-strain environment of meniscal cells may vary significantly among the different regions.

Biphasic finite element contact analysis of the knee joint using an augmented Lagrangian method

Biphasic contact analysis is essential to obtain a more complete understanding of soft tissue biomechanics; however, only a limited number of studies have addressed these types of problems. In this paper, a theoretically consistent biphasic finite element solution of the 2D axisymmetric human knee was developed, and an augmented Lagrangian method was used to enforce the biphasic continuity across the contact interface. The interaction between the fluid and solid matrices of the soft tissues of the knee joint, the stress and strain distributions within the meniscus, and the changes in stress and strain distributions in the articular cartilage of the femur and tibia after complete meniscectomy were investigated. It was found that (i) the fluid phase carries more than 60% of the load, which reinforces the need for the biphasic model for knee biomechanics; (ii) the inner third and outer two-thirds of the meniscus had different strain distributions; and (iii) the distributions of both maximum shear stress and maximum principal strain in articular cartilage changed after complete meniscectomy - with peak values increasing by over 350%.

Recent advances in computational mechanics of the human knee joint

Computational mechanics has been advanced in every area of orthopedic biomechanics. The objective of this paper is to provide a general review of the computational models used in the analysis of the mechanical function of the knee joint in different loading and pathological conditions. Major review articles published in related areas are summarized first. The constitutive models for soft tissues of the knee are briefly discussed to facilitate understanding the joint modeling. A detailed review of the tibiofemoral joint models is presented thereafter. The geometry reconstruction procedures as well as some critical issues in finite element modeling are also discussed. Computational modeling can be a reliable and effective method for the study of mechanical behavior of the knee joint, if the model is constructed correctly. Single-phase material models have been used to predict the instantaneous load response for the healthy knees and repaired joints, such as total and partial meniscectomies, ACL and PCL reconstructions, and joint replacements. Recently, poromechanical models accounting for fluid pressurization in soft tissues have been proposed to study the viscoelastic response of the healthy and impaired knee joints. While the constitutive modeling has been considerably advanced at the tissue level, many challenges still exist in applying a good material model to three-dimensional joint simulations. A complete model validation at the joint level seems impossible presently, because only simple data can be obtained experimentally. Therefore, model validation may be concentrated on the constitutive laws using multiple mechanical tests of the tissues. Extensive model verifications at the joint level are still crucial for the accuracy of the modeling.

The basic science of human knee menisci: structure, composition, and function

CONTEXT

Information regarding the structure, composition, and function of the knee menisci has been scattered across multiple sources and fields. This review contains a concise, detailed description of the knee menisci-including anatomy, etymology, phylogeny, ultrastructure and biochemistry, vascular anatomy and neuroanatomy, biomechanical function, maturation and aging, and imaging modalities.

EVIDENCE ACQUISITION

A literature search was performed by a review of PubMed and OVID articles published from 1858 to 2011.

RESULTS

This study highlights the structural, compositional, and functional characteristics of the menisci, which may be relevant to clinical presentations, diagnosis, and surgical repairs.

CONCLUSIONS

An understanding of the normal anatomy and biomechanics of the menisci is a necessary prerequisite to understanding the pathogenesis of disorders involving the knee.

Partial Meniscectomy Changes Fluid Pressurization in Articular Cartilage in Human Knees

Partial meniscectomy is believed to change the biomechanics of the knee joint through alterations in the contact of articular cartilages and menisci. Although fluid pressure plays an important role in the load support mechanism of the knee, the fluid pressurization in the cartilages and menisci has been ignored in the finite element studies of the mechanics of meniscectomy. In the present study, a 3D fibril-reinforced poromechanical model of the knee joint was used to explore the fluid flow dependent changes in articular cartilage following partial medial and lateral meniscectomies. Six partial longitudinal meniscectomies were considered under relaxation, simple creep, and combined creep loading conditions. In comparison to the intact knee, partial meniscectomy not only caused a substantial increase in the maximum fluid pressure but also shifted the location of this pressure in the femoral cartilage. Furthermore, these changes were positively correlated to the size of meniscal resection. While in the intact joint, the location of the maximum fluid pressure was dependent on the loading conditions, in the meniscectomized joint the location was predominantly determined by the site of meniscal resection. The partial meniscectomy also reduced the rate of the pressure dissipation, resulting in even larger difference between creep and relaxation times as compared to the case of the intact knee. The knee joint became stiffer after meniscectomy because of higher fluid pressure at knee compression followed by slower pressure dissipation. The present study indicated the role of fluid pressurization in the altered mechanics of meniscectomized knees.

Effects of perfusion and cyclic compression on in vitro tissue engineered meniscus implants

PURPOSE

The purpose of this study was to investigate the influence of continuous perfusion and mechanical stimulation on bone marrow stromal cells seeded on a collagen meniscus implant.

METHODS

Bone marrow aspirates from 6 donors were amplified in vitro. 10(6) human BMSC were distributed on a collagen meniscus implant. Scaffolds were cultured under static conditions (control) or placed into a bioreactor system where continuous perfusion (10 ml/min) or perfusion and mechanical stimulation (8 h of 10% cyclic compression at 0.5 Hz) were administered daily. After 24 h, 7 and 14 days, cell proliferation, synthesis of procollagen I and III peptide (PIP, PIIIP), histology, and the equilibrium modulus of the constructs were analyzed.

RESULTS

Proliferation demonstrated a significant increase over time in all groups (p < 0.001). PIP synthesis was found to increase from 0.1 ± 0.0 U/ml/g protein after 24 h to 2.0 ± 0.5 (perfusion), 3.8 ± 0.3 (mechanical stimulation), and 1.8 ± 0.2 U/ml/g protein (static control, lower than perfusion and mechanical stimulation, p < 0.05). These differences were also evident after 2 weeks (2.7 ± 0.3, 4.0 ± 0.6, and 1.8 ± 0.2 U/ml/g protein, p < 0.01); PIIIP synthesis was found to increase from 0.1 ± 0.0 U/ml/g protein after 24 h to 2.9 ± 0.7 (perfusion), 3.1 ± 0.9 (mechanical stimulation), and 1.6 ± 0.3 U/ml/g protein (controls) after 1 week and remained significantly elevated under the influence of perfusion and mechanical stimulation (p < 0.01) after 2 weeks. Mechanical stimulation increased the equilibrium modulus more than static culture and perfusion after 2 weeks (24.7 ± 7.6; 12.3 ± 3.7; 15.4 ± 2.6 kPa; p < 0.02).

CONCLUSION

Biomechanical stimulation and perfusion have impact on collagen scaffolds seeded with BMSCs. Cell proliferation can be enhanced using continuous perfusion and differentiation is fostered by mechanical stimulation.

Regional and fiber orientation dependent shear properties and anisotropy of bovine meniscus

Imaging of meniscal tissue reveals an extracellular matrix comprised of collagen fibrils arranged in circumferential bundles and radially aligned tie fibers, implicating structural material anisotropy. Biochemical analyses demonstrate regional disparities of proteoglycan content throughout the meniscal body, a constituent known to affect the shearing response of fibrocartilagenous tissue. Despite this phenomenological evidence and previous mechanical testing implicating otherwise, the meniscus if often modeled as a homogeneous, transversely isotropic material with little regard for regional specificity and material properties. The aim of this investigation was to determine if shear stress response homogeneity and directionality exists in and between bovine menisci with respect to anatomical location (medial and lateral), region (anterior, central, and posterior) and fiber orientation (parallel and perpendicular). Meniscus explants were subjected to lap shear strain at 0.002 s(-1) with the circumferential collagen fibers oriented parallel or perpendicular to the loading axis. Comparisons were made using a piecewise linear elastic analysis. The toe region shear modulus was calculated from the first observed linear region, between 3% and 13% strain and the extended shear modulus was established after 80% of the maximum shear strain. The posterior region was significantly different than the central for the extended shear modulus, correlating with known proteoglycan distribution. Observed shearing anisotropy led to the use of an anisotropic hyperelastic model based on a two-fiber family composite, previously used for arterial walls. The chosen model provided an excellent fit to the sample population for each region. These data can be utilized in the advancement of finite element modeling as well as biomimetic tissue engineered constructs.

Creep behavior of the intact and meniscectomy knee joints

The mechanical functions of the menisci may be partially performed through the fluid pressurization in articular cartilages and menisci. This creep behavior has not been investigated in whole knee joint modeling. A three-dimensional finite element knee model was employed in the present study to explore the fluid-flow dependent creep behaviors of normal and meniscectomy knees. The model included distal femur, tibia, fibula, articular cartilages, menisci and four major ligaments. Articular cartilage or meniscus was modeled as a fluid-saturated solid matrix reinforced by a nonlinear orthotropic and site-specific collagen network. A 300 N compressive force, equal to half of body weight, was applied to the knee in full extension followed by creep. The results showed that the fluid pressurization played a substantial role in joint contact mechanics. Menisci bore more loading as creep developed, leading to decreased stresses in cartilages. The removal of menisci not only changed the stresses in the cartilages, which was in agreement with published studies, but also altered the distribution and the rate of dissipation of fluid pressure in the cartilages. The high fluid pressures in the femoral cartilage moved from anterior to more central regions of the condyles after total meniscectomy. For both intact and meniscectomy joints, the fluid pressure level remained considerably high for thousands of seconds during creep, which lasted even longer after meniscectomy. For the femoral cartilage, the maximum principal stress was generally in agreement with the fiber direction, which indicated the essential role of fibers in load support of the tissue.

Dynamic loading enhances integrative meniscal repair in the presence of interleukin-1

OBJECTIVE

Meniscal tears are a common knee injury and increased levels of interleukin-1 (IL-1) have been measured in injured and degenerated joints. Studies have shown that IL-1 decreases the shear strength, cell accumulation, and tissue formation in meniscal repair interfaces. While mechanical stress and IL-1 modulate meniscal biosynthesis and degradation, the effects of dynamic loading on meniscal repair are unknown. The purpose of this study was to determine the effects of mechanical compression on meniscal repair under normal and inflammatory conditions.

EXPERIMENTAL DESIGN

Explants were harvested from porcine medial menisci. To simulate a full-thickness defect, a central core was removed and reinserted. Explants were loaded for 4h/day at 1 Hz and 0%-26% strain for 14 days in the presence of 0 or 100 pg/mL of IL-1. Media were assessed for matrix metalloproteinase (MMP) activity, aggrecanase activity, sulfated glycosaminoglycan (S-GAG) release, and nitric oxide (NO) production. After 14 days, biomechanical testing and histological analyses were performed.

RESULTS

IL-1 increased MMP activity, S-GAG release, and NO production, while decreasing the shear strength and tissue repair in the interface. Dynamic loading antagonized IL-1-mediated inhibition of repair at all strain amplitudes. Neither IL-1 treatment nor strain altered aggrecanase activity. Additionally, strain alone did not alter meniscal healing, except at the highest strain magnitude (26%), a level that enhanced the strength of repair.

CONCLUSIONS

Dynamic loading blocked the catabolic effects of IL-1 on meniscal repair, suggesting that joint loading through physical therapy may be beneficial in promoting healing of meniscal lesions under inflammatory conditions.

Implantation of autogenous meniscal fragments wrapped with a fascia sheath enhances fibrocartilage regeneration in vivo in a large harvest site defect

BACKGROUND

Concerning meniscal tissue regeneration, many investigators have studied the development of a tissue-engineered meniscus. However, the utility still remains unknown.

HYPOTHESIS

Implantation of autogenous meniscal fragments wrapped with a fascia sheath into the donor site meniscal defect may significantly enhance fibrocartilage regeneration in vivo in the defect.

STUDY DESIGN

Controlled laboratory study.

METHODS

Seventy-five mature rabbits were used in this study. In each animal, an anterior one-third of the right medial meniscus was resected. Then, the animals were divided into the following 3 groups of 25 rabbits each: In group 1, no treatment was applied to the meniscal defect. In group 2, the defect was covered with a fascia sheath. In group 3, after the resected meniscus was fragmented into small pieces, the fragments were grafted into the defect. Then, the defect with the meniscal fragments was covered with a fascia sheath. In each group, 5 rabbits were used for histological evaluation at 3, 6, and 12 weeks after surgery, and 5 rabbits were used for biomechanical evaluation at 6 and 12 weeks after surgery.

RESULTS

Histologically, large round cells in group 3 were scattered in the core portion of the meniscus-shaped tissue, and the matrix around these cells was positively stained by safranin O and toluisin blue at 12 weeks. The histological score of group 3 was significantly higher than that of group 1 and group 2. Biomechanically, the maximal load and stiffness of group 3 were significantly greater than those of groups 1 and 2.

CONCLUSION

This study clearly demonstrated that implantation of autogenous meniscal fragments wrapped with a fascia sheath into the donor site meniscal defect significantly enhanced fibrocartilage regeneration in vivo in the defect at 12 weeks after implantation in the rabbit.

CLINICAL RELEVANCE

This study proposed a novel strategy to treat a large defect after a meniscectomy.

Characterizing the mechanical contribution of fiber angular distribution in connective tissue: comparison of two modeling approaches

Modeling of connective tissues often includes collagen fibers explicitly as one of the components. These fibers can be oriented in many directions; therefore, several studies have considered statistical distributions to describe the fiber arrangement. One approach to formulate a constitutive framework for distributed fibers is to express the mechanical parameters, such as strain energy and stresses, in terms of angular integrals. These integrals represent the addition of the contribution of infinitesimal fractions of fibers oriented in a given direction. This approach leads to accurate results; however, it requires lengthy calculations. Recently, the use of generalized structure tensors has been proposed to represent the angular distribution in the constitutive equations of the fibers. Although this formulation is much simpler and fewer calculations are required, such structure tensors can only be used when all the fibers are in tension and the angular distribution is small. However, the amount of error introduced in these cases of non-tensile fiber loading and large angular distributions have not been quantified. Therefore, the objective of this study is to determine the range of values of angular distribution for which acceptable differences (less than 10%) between these two formulations are obtained. It was found, analytically and numerically, that both formulations are equivalent for planar distributions under equal-biaxial stretch. The comparison also showed, for other loading conditions, that the differences decrease when the fiber distribution is very small. Differences of less than 10% were usually obtained when the fiber distribution was very low (κ ≈ 0.03; κ ranges between 0 and 1/3, for aligned and isotropic distributed fibers, respectively). This range of angular distribution greatly limits the types of tissue that can be accurately analyzed using generalized structure tensors. It is expected that the results from this study guide the selection of a proper approach to analyze a particular tissue under a particular loading condition.

Tensile forces at the porcine anterior meniscal horn attachment

Tibiofemoral compression causes circumferential tension in the knee meniscus, which is transferred to the tibial bone at the anterior and posterior attachments. The objective of the study was to measure the resulting tensile forces at the horn attachment in a porcine model. The anterior horn attachment of the porcine medial meniscus (n = 10) was separated from the surrounding bone with a core reamer. A force transducer was installed such that tensile forces acting upon the now mobile horn attachment could be measured. The tibiofemoral joint was loaded in compression, starting at a preload of 30 N, with three 150-N increments, giving 180, 330, and 480 N load. Flexion angles of 0, 30, and 60 degrees were investigated. The average resultant tension at the horn attachment was 26.3, 40.6, and 55.4 N with full extension, 29.2, 47.8, and 62.2 N at 30 degrees flexion and 30.1, 49.6, and 68.1 N at 60 degrees flexion. The tibiofemoral compression had a significant effect on the tension (p < 0.001), whereas no influence of the flexion angle was found (p = 0.291). The study demonstrates that tibiofemoral compressive loads cause considerable tensile forces at the anterior meniscal horn attachment. The data are of interest for models of the repair or replacement of the knee menisci.