Functional characterization of normal and degraded bovine meniscus: rate-dependent indentation and friction studies

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

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

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.

A review of osteoarthritis signaling intervention using small-molecule inhibitors

Numerous small-molecule inhibitors (SMIs) have been approved as adjuvant or first-line therapies for malignancies. Based on cancer treatment using SMIs, next-generation SMIs that can be used to optimize the therapeutic index, overcome drug resistance, and establish combination therapies are in development. Osteoarthritis (OA) is the most common chronic joint disease with senescence, and there are various approaches to OA treatment; however, the gold standard treatment is controversial. Therefore, in this manuscript, we demonstrated the potential of using SMIs in OA treatment and described the general strategies for using SMIs in OA treatment.

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.

Ultrashort Time to Echo Magnetic Resonance Evaluation of Calcium Pyrophosphate Crystal Deposition in Human Menisci

OBJECTIVES

In human menisci, we aimed to investigate whether calcium pyrophosphate crystal deposition (CPPD) affects biomechanical and quantitative MR properties, and their zonal distribution.

MATERIALS AND METHODS

From 9 cadaveric knees, sectioned triangular meniscus pieces were harvested. Samples were classified into "normal" or "CPPD" groups based upon visual inspection. Micro computed tomography scan verified CPPD. Using magnetic resonance imaging, ultrashort echo time (UTE) T2* and spin echo (SE) T2, quantitative values in 3 zones (red, red-white, and white) were determined. Using biomechanical test, indentation forces in the same zones were determined. Effects of CPPD and meniscal zone on indentation force and quantitative MR values were compared.

RESULTS

On UTE MRI scans, CPPD-affected menisci exhibited punctate dark regions, found mostly (92%) in avascular white and red-white zones. Indentation forces were significantly higher for CPPD samples in the red-white (all P < 0.02) and white (all P < 0.004) zones but not in the vascular red zone (all P > 0.2). Similarly, UTE T2* red zone values were similar between both groups (~6.6 milliseconds, P = 0.8), whereas in the red-white and white zones, CPPD samples had significantly lower values (~5.1 milliseconds, P = 0.005 to 0.007). In contrast, SE T2 values showed no difference with CPPD (P = 0.12 to 0.16). UTE T2*, but not SE T2, correlated significantly with indentation force (R = -0.29, P = 0.009).

CONCLUSIONS

Dark CPP deposits were detectable on UTE images featuring high signal intensity from surrounding meniscal tissue. Preliminary results indicate that CPP deposits were almost exclusively found in the avascular zones. Compared with normal, CPPD menisci featured higher indentation stiffness and lower UTE T2* values in the affected zones.

Effects of tissue culture on the biomechanical properties of porcine meniscus explants

BACKGROUND

The meniscus is critical for the normal functioning of the knee joint. The specific aim of this study was to validate an in vitro culture model of meniscus explants for testing the impact of culture conditions on meniscus biomechanical properties. We hypothesized that culturing menisci in the presence of intermediate and high concentration of serum would have a positive effect on the compressive stiffness of the meniscus.

METHODS

Unconstrained microindentation testing was performed on porcine meniscus explants cultured with varying concentrations 1%, 5%, or 10% of fetal bovine serum media. Meniscus explants that were not cultured were used as a control. These tests quantified the Young's Modulus of the listed groups of cultured and uncultured explant tissues.

FINDINGS

The Young's modulus for 10% cultured explants were significantly higher compared to the control, 1%, and 5% cultured meniscus explants. There was no statistical significance when the Young's modulus between control, 1%, and 5% cultured explants were compared.

INTERPRETATION

These results suggest that low concentrations of serum do not impart an anabolic effect on meniscus tissue explant biomechanical properties.

The Meniscus in Normal and Osteoarthritic Tissues: Facing the Structure Property Challenges and Current Treatment Trends

The treatment of meniscus injuries has recently been facing a paradigm shift toward the field of tissue engineering, with the aim of regenerating damaged and diseased menisci as opposed to current treatment techniques. This review focuses on the structure and mechanics associated with the meniscus. The meniscus is defined in terms of its biological structure and composition. Biomechanics of the meniscus are discussed in detail, as an understanding of the mechanics is fundamental for the development of new meniscal treatment strategies. Key meniscal characteristics such as biological function, damage (tears), and disease are critically analyzed. The latest technologies behind meniscal repair and regeneration are assessed.

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.

Proteoglycans contribute locally to swelling, but globally to compressive mechanics, in intact cervine medial meniscus

Loss of charged proteoglycans in the knee meniscus, which aid in the support of compressive loads by entraining water, is an effect of degeneration and is often associated with osteoarthritis. In healthy menisci, proteoglycan content is highest in the inner white zone and decreases towards the peripheral red zone. We hypothesized that loss of proteoglycans would reduce both osmotic swelling and compressive stiffness, spatially localized to the avascular white zone of the meniscus. This hypothesis was tested by targeted enzymatic digestion of proteoglycans using hyaluronidase in intact cervine medial menisci. Mechanics were quantified by creep indentation on the femoral surface. Osmotic swelling changes were assessed by measuring collagen fiber crimp period in the radial-axial plane in the lamellar layer along both the tibial and femoral contacting surfaces. All measurements were made in the inner, middle, and outer zones of the anterior, central, and posterior regions. Mechanical measurements showed variation in creep behavior with anatomical location, along with spatially uniform decreases in viscosity (average of 21%) and creep stiffness (average of 15%) with hyaluronidase treatment. Lamellar collagen crimp period was significantly decreased (average of 27%) by hyaluronidase, indicating a decrease in osmotic swelling, with the largest decreases seen in locations with the highest proteoglycan content. Taken together, these results suggest that while proteoglycans have localized effects on meniscus swelling, the resulting effect on compressive properties is distributed throughout the tissue.

Variations in the attachments of the medial meniscal anterior horn: a descriptive cadaveric study

PURPOSE

Meniscal tears are common in Kenya, with prevalence rates ranging from 45 to 78% of intracapsular knee pathology. Diagnosis of these injuries relies on the use of both clinical signs and symptoms as well as radiological investigations. In a few instances, radiological detection could be difficult, partly because of variant attachment patterns of the medial meniscal anterior horn. Some of these unusual attachments of the anterior horn of the medial meniscus could even be mistaken for meniscal tears. There is also evidence that these variations differ from population to population. This study, therefore, aimed to determine the variant bony and ligamentous attachments of the medial meniscal anterior horn in a sample Kenyan population.

METHODS

The study was conducted at the Department of Human Anatomy, University of Nairobi. Thirty-one male and female unpaired medial menisci were obtained from cadaveric specimen. The bony and ligamentous attachments were identified and recorded and photomacrographs taken.

RESULTS

The bony attachments different from the classical textbook attachment accounted for 54.8% of the medial meniscal anterior horns. The anterior intermeniscal ligament was present in 62.3% while 16.2% showed attachment to the anterior cruciate ligament. Twenty-nine percent (29%) of the medial menisci studied did not have any ligamentous attachments.

CONCLUSIONS

The bony and ligamentous attachments of the medial meniscal anterior horn are highly variable presenting unique diagnostic and therapeutic challenges. A new classification of ligamentous attachments is thus proposed.

Evaluating the effects of material properties of artificial meniscal implant in the human knee joint using finite element analysis

Artificial meniscal implants may replace severely injured meniscus and restore the normal functionality of the knee joint. Implant material stiffness and shape influence the longevity of implantations. This study, using 3D finite element analysis, aimed to evaluate the effects of material stiffness variations of anatomically shaped artificial meniscal implant in the knee joint. Finite element simulations were conducted on five different cases including intact knee, medial meniscectomized knee, and the knee joint with the meniscal implant with three distinct material stiffness. Cartilage contact pressures, compression stresses, shear stresses, and implant kinematics (medial-lateral and posterior-anterior displacement) were evaluated for an axial compressive load of 1150 N at full extension. Compared to the meniscectomized knee, the knee joint with the meniscal implant induced lower peak cartilage contact pressure and reduced the cartilage regions loaded with contact pressures greater than the peak cartilage contact pressure induced by the intact knee. Results of the current study also demonstrate that cartilage contact pressures and implant displacement are sensitive to the implant material stiffness. The meniscal implant with a stiffness of 11 MPa restores the intact knee contact mechanics, thereby reducing the risk of physiological damage to the articular cartilage.

Viscoelastic properties of rabbit osteoarthritic menisci: A correlation with matrix alterations

The aim of this study was to evaluate the effect of early osteoarthritis (OA) on the viscoelastic properties of rabbit menisci and to correlate the mechanical alterations with the microstructural changes. Anterior Cruciate Ligament Transection (ACLT) was performed in six male New-Zealand White rabbits on the right knee joint. Six healthy rabbits served as controls. Menisci were removed six weeks after ACLT and were graded macroscopically. Indentation-relaxation tests were performed in the anterior and posterior regions of the medial menisci. The collagen fibre organization and glycosaminoglycan (GAG) content were assessed by biphotonic confocal microscopy and histology, respectively. OA menisci displayed severe macroscopic lesions compared with healthy menisci (p=0.009). Moreover, the instantaneous and equilibrium moduli, which were 2.9±1.0MPa and 0.60±0.18MPa in the anterior region of healthy menisci, respectively, decreased significantly (p=0.03 and p=0.004, respectively) in OA menisci by 55% and 57%, respectively, indicating a global decrease in meniscal stiffness in this region. The equilibrium modulus alone decreased significantly (p=0.04) in the posterior region, going from 0.60±0.18MPa to 0.26±012MPa. This induced a loss of tissue elasticity. These mechanical changes were associated in the posterior region with a structural disruption of the superficial layers, from which the tie fibres emanate, and with a decrease in the GAG content in the anterior region. Consequently, the circumferential collagen fibres of the deep zone were dissociated and the collagen bundles were less compact. Our results demonstrate the strong meniscal modifications induced by ACLT at an early stage of OA and highlight the relationship between structural and chemical matrix alterations and mechanical properties.

Meniscal biomechanical alterations in an ACLT rabbit model of early osteoarthritis

OBJECTIVE

The purpose of this study was to analyze the early biomechanical alterations of menisci during the early stage of osteoarthritis (OA) development and to correlate them with the chemical composition and matrix alteration. A particular focus was paid to pathological changes in glycosaminoglycan (GAG) content and collagen fiber architecture.

DESIGN

Menisci (n = 24) were removed from rabbits' knee joints 6 weeks following surgical anterior cruciate ligament transection (ACLT). Both the anterior and posterior regions of medial and lateral menisci were characterized using indentation tests, Raman microspectroscopy (RM), biphotonic confocal microscopy (BCM) and histology.

RESULTS

Mechanical and matrix alterations occurred in both regions of medial and lateral menisci. A significant decrease in the mechanical properties was observed in OA menisci, with a mean reduced modulus from 2.3 to 1.1 MPa. Microstructural observations revealed less organized and less compact collagen bundles in operated menisci than in contralateral menisci, as well as a loss of fiber tension. GAG content was increased in OA menisci, especially in the damaged areas. Neither changes in the secondary structure of collagen nor mineralization were detected through RM at this stage of OA.

CONCLUSION

ACLT led to a disorganization of the collagen framework at the early stage of OA development, which decreases the mechanical resistance of the menisci. GAG content increases in response to this degradation. All of these results demonstrate the strong correlation between matrix and mechanical alterations.

The distribution of superficial zone protein (SZP)/lubricin/PRG4 and boundary mode frictional properties of the bovine diarthrodial joint

The diarthrodial, knee joint is a remarkably efficient bearing system; articulating cartilage surfaces provide nearly frictionless performance with minimal wear. The low friction properties of the cartilage surfaces are due in part to the boundary lubricant, superficial zone protein (SZP); also known as lubricin or proteoglycan 4 (PRG4). In previous work, SZP localization and cartilage friction were examined across the femoral condyles. Studies in the literature have also individually investigated the other tissues that comprise the human knee and four-legged animal stifle joint, such as the meniscus or patella. However, comparisons between individual studies are limited due to the variable testing conditions employed. Friction is a system property that is dependent on the opposing articulating surface, entraining speed, and loading. A cross-comparison of the frictional properties and SZP localization across the knee/stifle joint tissues utilizing a common testing configuration is therefore needed. The objective of this investigation was to determine the friction coefficient and SZP localization of the tissues comprising the three compartments of the bovine stifle joint: patella, patellofemoral groove, femoral condyles, meniscus, tibial plateau, and anterior cruciate ligament. The boundary mode coefficient of friction was greater in tissues of the patellofemoral compartment than the lateral and medial tibiofemoral compartments. SZP immunolocalization followed this trend with reduced depth of staining and intensity in the patella and patellofemoral groove compared to the femoral condyles and tibial plateau. These results illustrate the important role of SZP in reducing friction in the tissues and compartments of the knee/stifle joint.

Fibrous cartilage of human menisci is less shock-absorbing and energy-dissipating than hyaline cartilage

PURPOSE

To test meniscal mechanical properties such as the dynamic modulus of elasticity E* and the loss angle δ at two loading frequencies ω at different locations of the menisci and compare it to E* and δ of hyaline cartilage in indentation mode with spherical indenters.

METHODS

On nine pairs of human menisci, the dynamic E*-modulus and loss angle δ (as a measure of the energy dissipation) were determined. The measurements were performed at two different strain rates (slow sinusoidal and fast single impact) to show the strain rate dependence of the material. The measurements were compared to previous similar measurements with the same equipment on human hyaline cartilage.

RESULTS

The resultant E* at fast indentation (median 1.16 MPa) was significantly higher, and the loss angle was significantly lower (median 10.2°) compared to slow-loading mode's E* and δ (median 0.18 MPa and 16.9°, respectively). Further, significant differences for different locations are shown. On the medial meniscus, the anterior horn shows the highest resultant dynamic modulus.

CONCLUSION

In dynamic measurements with a spherical indenter, the menisci are much softer and less energy-dissipating than hyaline cartilage. Further, the menisci are stiffer and less energy-dissipating in the middle, intermediate part compared to the meniscal base. In compression, the energy dissipation of meniscus cartilage plays a minor role compared to hyaline cartilage. At high impacts, energy dissipation is less than on low impacts, similar to cartilage.

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.

Role of interstitial fluid pressurization in TMJ lubrication

In temporomandibular joints (TMJs), the disc and condylar cartilage function as load-bearing, shock-absorbing, and friction-reducing materials. The ultrastructure of the TMJ disc and cartilage is different from that of hyaline cartilage in other diarthrodial joints, and little is known about their lubrication mechanisms. In this study, we performed micro-tribometry testing on the TMJ disc and condylar cartilage to obtain their region- and direction-dependent friction properties. Frictional tests with a migrating contact area were performed on 8 adult porcine TMJs at 5 different regions (anterior, posterior, central, medial, and lateral) in 2 orthogonal directions (anterior-posterior and medial-lateral). Some significant regional differences were detected, and the lateral-medial direction showed higher friction than the anterior-posterior direction on both tissues. The mean friction coefficient of condylar cartilage against steel was 0.027, but the disc, at 0.074, displayed a significantly higher friction coefficient. The 2 tissues also exhibited different frictional dependencies on sliding speed and normal loading force. Whereas the friction of condylar cartilage decreased with increased sliding speed and was independent of the magnitude of normal force, friction of the disc showed no dependence on sliding speed but decreased as normal force increased. Further analysis of the Péclet number and frictional coefficients suggested that condylar cartilage relies on interstitial fluid pressurization to a greater extent than the corresponding contact area of the TMJ disc.

An analytical model to predict interstitial lubrication of cartilage in migrating contact areas

For nearly a century, articular cartilage has been known for its exceptional tribological properties. For nearly as long, there have been research efforts to elucidate the responsible mechanisms for application toward biomimetic bearing applications. It is now widely accepted that interstitial fluid pressurization is the primary mechanism responsible for the unusual lubrication and load bearing properties of cartilage. Although the biomechanics community has developed elegant mathematical theories describing the coupling of solid and fluid (biphasic) mechanics and its role in interstitial lubrication, quantitative gaps in our understanding of cartilage tribology have inhibited our ability to predict how tribological conditions and material properties impact tissue function. This paper presents an analytical model of the interstitial lubrication of biphasic materials under migrating contact conditions. Although finite element and other numerical models of cartilage mechanics exist, they typically neglect the important role of the collagen network and are limited to a specific set of input conditions, which limits general applicability. The simplified approach taken in this work aims to capture the broader underlying physics as a starting point for further model development. In agreement with existing literature, the model indicates that a large Peclet number, Pe, is necessary for effective interstitial lubrication. It also predicts that the tensile modulus must be large relative to the compressive modulus. This explains why hydrogels and other biphasic materials do not provide significant interstitial pressure under high Pe conditions. The model quantitatively agrees with in-situ measurements of interstitial load support and the results have interesting implications for tissue engineering and osteoarthritis problems. This paper suggests that a low tensile modulus (from chondromalacia or local collagen rupture after impact, for example) may disrupt interstitial pressurization, increase shear stresses, and activate a condition of progressive surface damage as a potential precursor of osteoarthritis.