Cartilage mechanical response under dynamic compression at physiological stress levels following collagenase digestion

How Do Cartilage Lubrication Mechanisms Fail in Osteoarthritis? A Comprehensive Review

Cartilage degeneration is a characteristic of osteoarthritis (OA), which is often observed in aging populations. This degeneration is due to the breakdown of articular cartilage (AC) mechanical and tribological properties primarily attributed to lubrication failure. Understanding the reasons behind these failures and identifying potential solutions could have significant economic and societal implications, ultimately enhancing quality of life. This review provides an overview of developments in the field of AC, focusing on its mechanical and tribological properties. The emphasis is on the role of lubrication in degraded AC, offering insights into its structure and function relationship. Further, it explores the fundamental connection between AC mechano-tribological properties and the advancement of its degradation and puts forth recommendations for strategies to boost its lubrication efficiency.

Enzymatic digestion does not compromise sliding-mediated cartilage lubrication

Articular cartilage's remarkable low-friction properties are essential to joint function. In osteoarthritis (OA), cartilage degeneration (e.g., proteoglycan loss and collagen damage) decreases tissue modulus and increases permeability. Although these changes impair lubrication in fully depressurized and slowly slid cartilage, new evidence suggests such relationships may not hold under biofidelic sliding conditions more representative of those encountered in vivo. Our recent studies using the convergent stationary contact area (cSCA) configuration demonstrate that articulation (i.e., sliding) generates interfacial hydrodynamic pressures capable of replenishing cartilage interstitial fluid/pressure lost to compressive loading through a mechanism termed tribological rehydration. This fluid recovery sustains in vivo-like kinetic friction coefficients (µk<0.02 in PBS and <0.005 in synovial fluid) with little sensitivity to mechanical properties in healthy tissue. However, the tribomechanical function of compromised cartilage under biofidelic sliding conditions remains unknown. Here, we investigated the effects of OA-like changes in cartilage mechanical properties, modeled via enzymatic digestion of mature bovine cartilage, on its tribomechanical function during cSCA sliding. We found no differences in sliding-driven tribological rehydration behaviors or µk between naïve and digested cSCA cartilage (in PBS or synovial fluid). This suggests that OA-like cartilage retains sufficient functional properties to support naïve-like fluid recovery and lubrication under biofidelic sliding conditions. However, OA-like cartilage accumulated greater total tissue strains due to elevated strain accrual during initial load application. Together, these results suggest that elevated total tissue strains-as opposed to activity-mediated strains or friction-driven wear-might be the key biomechanical mediator of OA pathology in cartilage. STATEMENT OF SIGNIFICANCE: Osteoarthritis (OA) decreases cartilage's modulus and increases its permeability. While these changes compromise frictional performance in benchtop testing under low fluid load support (FLS) conditions, whether such observations hold under sliding conditions that better represent the joints' dynamic FLS conditions in vivo is unclear. Here, we leveraged biofidelic benchtop sliding experiments-that is, those mimicking joints' native sliding environment-to examine how OA-like changes in mechanical properties effect cartilage's natural lubrication. We found no differences in sliding-mediated fluid recovery or kinetic friction behaviors between naïve and OA-like cartilage. However, OA-like cartilage experienced greater strain accumulation during load application, suggesting that elevated tissue strains (not friction-driven wear) may be the primary biomechanical mediator of OA pathology.

Engineered dual affinity protein fragments to bind collagen and capture growth factors

Collagen type I lacks affinity for growth factors (GFs) and yet it is clinically used to deliver bone morphogenic protein 2 (BMP-2), a potent osteogenic growth factor. To mitigate this lack of affinity, supra-physiological concentrations of BMP-2 are loaded in collagen sponges leading to uncontrolled BMP-2 leakage out of the material. This has led to important adverse side effects such as carcinogenesis. Here, we design recombinant dual affinity protein fragments, produced in E. Coli, which contain two regions, one that spontaneously binds to collagen and a second one that binds BMP-2. By adding the fragment to collagen sponges, BMP-2 is sequestered enabling solid phase presentation of BMP-2. We demonstrate osteogenesis in vivo with ultra-low doses of BMP-2. Our protein technology enhances the biological activity of collagen without using complex chemistries or changing the manufacturing of the base material and so opens a pathway to clinical translation.

Effects of tissue degradation by collagenase and elastase on the biaxial mechanics of porcine airways

BACKGROUND

Common respiratory illnesses, such as emphysema and chronic obstructive pulmonary disease, are characterized by connective tissue damage and remodeling. Two major fibers govern the mechanics of airway tissue: elastin enables stretch and permits airway recoil, while collagen prevents overextension with stiffer properties. Collagenase and elastase degradation treatments are common avenues for contrasting the role of collagen and elastin in healthy and diseased states; while previous lung studies of collagen and elastin have analyzed parenchymal strips in animal and human specimens, none have focused on the airways to date.

METHODS

Specimens were extracted from the proximal and distal airways, namely the trachea, large bronchi, and small bronchi to facilitate evaluations of material heterogeneity, and subjected to biaxial planar loading in the circumferential and axial directions to assess airway anisotropy. Next, samples were subjected to collagenase and elastase enzymatic treatment and tensile tests were repeated. Airway tissue mechanical properties pre- and post-treatment were comprehensively characterized via measures of initial and ultimate moduli, strain transitions, maximum stress, hysteresis, energy loss, and viscoelasticity to gain insights regarding the specialized role of individual connective tissue fibers and network interactions.

RESULTS

Enzymatic treatment demonstrated an increase in airway tissue compliance throughout loading and resulted in at least a 50% decrease in maximum stress overall. Strain transition values led to significant anisotropic manifestation post-treatment, where circumferential tissues transitioned at higher strains compared to axial counterparts. Hysteresis values and energy loss decreased after enzymatic treatment, where hysteresis reduced by almost half of the untreated value. Anisotropic ratios exhibited axially led stiffness at low strains which transitioned to circumferentially led stiffness when subjected to higher strains. Viscoelastic stress relaxation was found to be greater in the circumferential direction for bronchial airway regions compared to axial counterparts.

CONCLUSION

Targeted fiber treatment resulted in mechanical alterations across the loading range and interactions between elastin and collagen connective tissue networks was observed. Providing novel mechanical characterization of elastase and collagenase treated airways aids our understanding of individual and interconnected fiber roles, ultimately helping to establish a foundation for constructing constitutive models to represent various states and progressions of pulmonary disease.

Glycosaminoglycan depletion increases energy dissipation in articular cartilage under high-frequency loading

High-frequency material behavior of cartilage at macroscopic lengths is not widely understood, despite a wide range of frequencies and contact lengths experienced in vivo. For example, cartilage at different stages of matrix integrity can experience high-frequency loading during traumatic impact, making high-frequency behavior relevant in the context of structural failure. Therefore, this study examined macroscopic dissipative and mechanical responses of intact and glycosaminoglycan (GAG)-depleted cartilage under previously unexplored high-frequency loading. These dynamic responses were complemented with the evaluation of quasi-static responses. A custom dynamic mechanical analyzer was used to obtain dynamic behavior, and stress relaxation testing was performed to obtain quasi-static behavior. Under high-frequency loading, cartilage energy dissipation increased with GAG depletion and decreased with strain; dynamic modulus exhibited opposite trends. Similarly, under quasi-static loading, equilibrium modulus and relaxation time of cartilage decreased with GAG depletion. The increased energy dissipation after GAG depletion under high-frequency loading was likely due to increased viscoelastic dissipation. These findings broaden our understanding of fundamental properties of cartilage as a function of solid matrix integrity in an unprecedented loading regime. They also provide a foundation for analyzing energy dissipation associated with cartilage failure induced by traumatic impact.

Effect of collagenase-gelatinase ratio on the mechanical properties of a collagen fibril: a combined Monte Carlo-molecular dynamics study

Loading in cartilage is supported primarily by fibrillar collagen, and damage will impair the function of the tissue, leading to pathologies such as osteoarthritis. Damage is initiated by two types of matrix metalloproteinases, collagenase and gelatinase, that cleave and denature the collagen fibrils in the tissue. Experimental and modeling studies have revealed insights into the individual contributions of these two types of MMPs, as well as the mechanical response of intact fibrils and fibrils that have experienced random surface degradation. However, no research has comprehensively examined the combined influences of collagenases and gelatinases on collagen degradation nor studied the mechanical consequences of biological degradation of collagen fibrils. Such preclinical examinations are required to gain insights into understanding, treating, and preventing degradation-related cartilage pathology. To develop these insights, we use sequential Monte Carlo and molecular dynamics simulations to probe the effect of enzymatic degradation on the structure and mechanics of a single collagen fibril. We find that the mechanical response depends on the ratio of collagenase to gelatinase—not just the amount of lost fibril mass—and we provide a possible mechanism underlying this phenomenon. Overall, by characterizing the combined influences of collagenases and gelatinases on fibril degradation and mechanics at the preclinical research stage, we gain insights that may facilitate the development of targeted interventions to prevent the damage and loss of mechanical integrity that can lead to cartilage pathology.

Combined enzymatic degradation of proteoglycans and collagen significantly alters intratissue strains in articular cartilage during cyclic compression

As degenerative joint diseases such as osteoarthritis (OA) progress, the matrix constituents, particularly collagen fibrils and proteoglycans, become damaged, therefore deteriorating the tissue's mechanical properties. This study aims to further the understanding of the effect of degradation of the different cartilage constituents on the mechanical loading environment in early stage OA. To this end, intact, collagen- and proteoglycan-depleted cartilage plugs were cyclically loaded in axial compression using an experimental model simulating in vivo cartilage-on-cartilage contact conditions in a micro-MRI scanner. Depletion of collagen and proteoglycans was achieved through enzymatic degradation with collagenase and chondroitinase ABC, respectively. Using a displacement-encoded imaging sequence (DENSE), strains were computed and compared in intact and degraded samples. The results revealed that, while degradation with one or the other enzyme had little effect on the contact strains, degradation with a combination of both enzymes caused an increase in the means and variance of the transverse, axial and shear strains, particularly in the superficial zone of the cartilage. This effect indicates that the balance between cartilage matrix constituents plays an essential role in maintaining the mechanical properties of the tissue, and a disturbance in this balance leads to a decrease of the load bearing capacity associated with degenerative joint diseases such as OA.

Near infrared spectroscopic assessment of developing engineered tissues: correlations with compositional and mechanical properties

Articular cartilage degeneration causes pain and reduces the mobility of millions of people annually. Regeneration of cartilage is challenging, due in part to its avascular nature, and thus tissue engineering approaches for cartilage repair have been studied extensively. Current techniques to assess the composition and integrity of engineered tissues, including histology, biochemical evaluation, and mechanical testing, are destructive, which limits real-time monitoring of engineered cartilage tissue development in vitro and in vivo. Near infrared spectroscopy (NIRS) has been proposed as a non-destructive technique to characterize cartilage. In the current study, we describe a non-destructive NIRS approach for assessment of engineered cartilage during development, and demonstrate correlation of these data to gold standard mid infrared spectroscopic measurements, and to mechanical properties of constructs. Cartilage constructs were generated using bovine chondrocyte culture on polyglycolic acid (PGA) scaffolds for six weeks. BMP-4 growth factor and ultrasound mechanical stimulation were used to provide a greater dynamic range of tissue properties and outcome variables. NIR spectra were collected daily using an infrared fiber optic probe in diffuse reflectance mode. Constructs were harvested after three and six weeks of culture and evaluated by the correlative modalities of mid infrared (MIR) spectroscopy, histology, and mechanical testing (equilibrium and dynamic stiffness). We found that specific NIR spectral absorbances correlated with MIR measurements of chemical composition, including relative amount of PGA (R = 0.86, p = 0.02), collagen (R = 0.88, p = 0.03), and proteoglycan (R = 0.83, p = 0.01). In addition, NIR-derived water content correlated with MIR-derived proteoglycan content (R = 0.76, p = 0.04). Both equilibrium and dynamic mechanical properties generally improved with cartilage growth from three to six weeks. In addition, significant correlations between NIRS-derived parameters and mechanical properties were found for constructs that were not treated with ultrasound (PGA (R = 0.71, p = 0.01), water (R = 0.74, p = 0.02), collagen (R = 0.69, p = 0.04), and proteoglycan (R = 0.62, p = 0.05)). These results lay the groundwork for extension to arthroscopic engineered cartilage assessment in clinical studies.

Chondrocyte and mesenchymal stem cell derived engineered cartilage exhibits differential sensitivity to pro-inflammatory cytokines

Tissue engineering is a promising approach for the repair of articular cartilage defects, with engineered constructs emerging that match native tissue properties. However, the inflammatory environment of the damaged joint might compromise outcomes, and this may be impacted by the choice of cell source in terms of their ability to operate anabolically in an inflamed environment. Here, we compared the response of engineered cartilage derived from native chondrocytes and mesenchymal stem cells (MSCs) to challenge by TNFα and IL-1β in order to determine if either cell type possessed an inherent advantage. Compositional (extracellular matrix) and functional (mechanical) characteristics, as well as the release of catabolic mediators (matrix metalloproteinases [MMPs], nitric oxide [NO]) were assessed to determine cell- and tissue-level changes following exposure to IL-1β or TNF-α. Results demonstrated that MSC-derived constructs were more sensitive to inflammatory mediators than chondrocyte-derived constructs, exhibiting a greater loss of proteoglycans and functional properties at lower cytokine concentrations. While MSCs and chondrocytes both have the capacity to form functional engineered cartilage in vitro, this study suggests that the presence of an inflammatory environment is more likely to impair the in vivo success of MSC-derived cartilage repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2901-2910, 2018.

A device for high-throughput monitoring of degradation in soft tissue samples

This work describes the design and validation of a novel device, the High-Throughput Degradation Monitoring Device (HDD), for monitoring the degradation of 24 soft tissue samples over incubation periods of several days inside a cell culture incubator. The device quantifies sample degradation by monitoring its deformation induced by a static gravity load. Initial instrument design and experimental protocol development focused on quantifying cartilage degeneration. Characterization of measurement errors, caused mainly by thermal transients and by translating the instrument sensor, demonstrated that HDD can quantify sample degradation with <6 μm precision and <10 μm temperature-induced errors. HDD capabilities were evaluated in a pilot study that monitored the degradation of fresh ex vivo human cartilage samples by collagenase solutions over three days. HDD could robustly resolve the effects of collagenase concentration as small as 0.5 mg/ml. Careful sample preparation resulted in measurements that did not suffer from donor-to-donor variation (coefficient of variance <70%). Due to its unique combination of sample throughput, measurement precision, temporal sampling and experimental versality, HDD provides a novel biomechanics-based experimental platform for quantifying the effects of proteins (cytokines, growth factors, enzymes, antibodies) or small molecules on the degradation of soft tissues or tissue engineering constructs. Thereby, HDD can complement established tools and in vitro models in important applications including drug screening and biomaterial development.

Mapping the spatiotemporal evolution of solute transport in articular cartilage explants reveals how cartilage recovers fluid within the contact area during sliding

The interstitial fluid within articular cartilage shields the matrix from mechanical stresses, reduces friction and wear, enables biochemical processes, and transports solutes into and out of the avascular extracellular matrix. The balanced competition between fluid exudation and recovery under load is thus critical to the mechanical and biological functions of the tissue. We recently discovered that sliding alone can induce rapid solute transport into buried cartilage contact areas via a phenomenon termed tribological rehydration. In this study, we use in situ confocal microscopy measurements to track the spatiotemporal propagation of a small neutral solute into the buried contact area to clarify the fluid mechanics underlying the tribological rehydration phenomenon. Sliding experiments were interrupted by periodic static loading to enable scanning of the entire contact area. Spatiotemporal patterns of solute transport combined with tribological data suggested pressure driven flow through the extracellular matrix from the contact periphery rather than into the surface via a fluid film. Interestingly, these testing interruptions also revealed dynamic, repeatable and history-independent fluid loss and recovery processes consistent with those observed in vivo. Unlike the migrating contact area, which preserves hydration by moving faster than interstitial fluid can flow, our results demonstrate that the stationary contact area can maintain and actively recover hydration through a dynamic competition between load-induced exudation and sliding-induced recovery. The results demonstrate that sliding contributes to the recovery of fluid and solutes by cartilage within the contact area while clarifying the means by which it occurs.

Near-Infrared Spectroscopy Predicts Compositional and Mechanical Properties of Hyaluronic Acid-Based Engineered Cartilage Constructs

Hyaluronic acid (HA) has been widely used for cartilage tissue engineering applications. However, the optimal time point to harvest HA-based engineered constructs for cartilage repair is still under investigation. In this study, we investigated the ability of a nondestructive modality, near-infrared spectroscopic (NIR) analysis, to predict compositional and mechanical properties of HA-based engineered cartilage constructs. NIR spectral data were collected from control, unseeded constructs, and twice per week by fiber optic from constructs seeded with chondrocytes during their development over an 8-week period. Constructs were harvested at 2, 4, 6, and 8 weeks, collagen and sulfated glycosaminoglycan content measured using biochemical assays, and the mechanical properties of the constructs evaluated using unconfined compression tests. NIR absorbances associated with the scaffold material, water, and engineered cartilage matrix, were identified. The NIR-determined matrix absorbance plateaued after 4 weeks of culture, which was in agreement with the biochemical assay results. Similarly, the mechanical properties of the constructs also plateaued at 4 weeks. A multivariate partial least square model based on NIR spectral input was developed to predict the moduli of the constructs, which resulted in a prediction error of 10% and R value of 0.88 for predicted versus actual values of dynamic modulus. Furthermore, the maximum increase in moduli was calculated from the first derivative of the curve fit of NIR-predicted and actual moduli values over time, and both occurred at ∼2 weeks. Collectively, these data suggest that NIR spectral data analysis could be an alternative to destructive biochemical and mechanical methods for evaluation of HA-based engineered cartilage construct properties.

Quantifying Cartilage Biomechanical Properties Using a Linearized Frequency-Domain Method

Articular cartilage function relies on its unique mechanical behavior. Cartilage mechanics have been described by several analytic models, whose parameters are usually estimated by fitting their constitutive equations to stress-relaxation data. This procedure can be long and is prone to experimental and fitting errors. Τhis study describes a novel methodology for estimating the biomechanical properties of cartilage samples based on their linearized frequency response, derived by applying a series of small-amplitude harmonic displacements superimposed to a bias strain. The proposed methodology, denoted as linearized frequency-domain method (LFM), was demonstrated by quantifying the effects of collagenase and hyaluronidase on cartilage, where it provided robust cartilage parameter estimates that overall agreed well with estimates obtained by stress-relaxation analysis. LFM was also applied to unveil the strain-dependent nature of porcine cartilage biomechanical parameters. Results showed that increasing the bias strain from 5% to 15% caused a significant decrease in cartilage permeability but did not have significant effect on the compression modulus and the Poisson's ratio. Apart from cartilage, LFM can potentially quantify the strain-dependent nature of tissues and biomaterials, thereby enhance tissue-level understanding on organ physiology and pathology, lead to better computational tissue models, and guide tissue engineering research.

Dual Function of Glucosamine in Gelatin/Hyaluronic Acid Cryogel to Modulate Scaffold Mechanical Properties and to Maintain Chondrogenic Phenotype for Cartilage Tissue Engineering

Glucosamine (GlcN) fulfills many of the requirements as an ideal component in scaffolds used in cartilage tissue engineering. The incorporation of GlcN in a gelatin/hyaluronic acid (GH) cryogel scaffold could provide biological cues in maintaining the phenotype of chondrocytes. Nonetheless, substituting gelatin with GlcN may also decrease the crosslinking density and modulate the mechanical properties of the cryogel scaffold, which may be beneficial as physical cues for chondrocytes in the scaffold. Thus, we prepared cryogel scaffolds containing 9% GlcN (GH-GlcN9) and 16% GlcN (GH-GlcN16) by carbodiimide-mediated crosslinking reactions at -16 °C. The crosslinking density and the mechanical properties of the cryogel matrix could be tuned by adjusting the content of GlcN used during cryogel preparation. In general, incorporation of GlcN did not influence scaffold pore size and ultimate compressive strain but increased porosity. The GH-GlcN16 cryogel showed the highest swelling ratio and degradation rate in hyaluronidase and collagenase solutions. On the contrary, the Young's modulus, storage modulus, ultimate compressive stress, energy dissipation level, and rate of stress relaxation decreased by increasing the GlcN content in the cryogel. The release of GlcN from the scaffolds in the culture medium of chondrocytes could be sustained for 21 days for GH-GlcN16 in contrast to only 7 days for GH-GlcN9. In vitro cell culture experiments using rabbit articular chondrocytes revealed that GlcN incorporation affected cell proliferation, morphology, and maintenance of chondrogenic phenotype. Overall, GH-GlcN16 showed the best performance in maintaining chondrogenic phenotype with reduced cell proliferation rate but enhanced glycosaminoglycans (GAGs) and type II collagen (COL II) secretion. Quantitative real-time polymerase chain reaction also showed time-dependent up-regulation of cartilage-specific marker genes (COL II, aggrecan and Sox9) for GH-GlcN16. Implantation of chondrocytes/GH-GlcN16 constructs into full-thickness articular cartilage defects of rabbits could regenerate neocartilage with positive staining for GAGs and COL II. The GH-GlcN16 cryogel will be suitable as a scaffold for the treatment of articular cartilage defects.

Anatomic Mesenchymal Stem Cell-Based Engineered Cartilage Constructs for Biologic Total Joint Replacement

Cartilage has a poor healing response, and few viable options exist for repair of extensive damage. Hyaluronic acid (HA) hydrogels seeded with mesenchymal stem cells (MSCs) polymerized through UV crosslinking can generate functional tissue, but this crosslinking is not compatible with indirect rapid prototyping utilizing opaque anatomic molds. Methacrylate-modified polymers can also be chemically crosslinked in a cytocompatible manner using ammonium persulfate (APS) and N,N,N',N'-tetramethylethylenediamine (TEMED). The objectives of this study were to (1) compare APS/TEMED crosslinking with UV crosslinking in terms of functional maturation of MSC-seeded HA hydrogels; (2) generate an anatomic mold of a complex joint surface through rapid prototyping; and (3) grow anatomic MSC-seeded HA hydrogel constructs using this alternative crosslinking method. Juvenile bovine MSCs were suspended in methacrylated HA (MeHA) and crosslinked either through UV polymerization or chemically with APS/TEMED to generate cylindrical constructs. Minipig porcine femoral heads were imaged using microCT, and anatomic negative molds were generated by three-dimensional printing using fused deposition modeling. Molded HA constructs were produced using the APS/TEMED method. All constructs were cultured for up to 12 weeks in a chemically defined medium supplemented with TGF-β3 and characterized by mechanical testing, biochemical assays, and histologic analysis. Both UV- and APS/TEMED-polymerized constructs showed increasing mechanical properties and robust proteoglycan and collagen deposition over time. At 12 weeks, APS/TEMED-polymerized constructs had higher equilibrium and dynamic moduli than UV-polymerized constructs, with no differences in proteoglycan or collagen content. Molded HA constructs retained their hemispherical shape in culture and demonstrated increasing mechanical properties and proteoglycan and collagen deposition, especially at the edges compared to the center of these larger constructs. Immunohistochemistry showed abundant collagen type II staining and little collagen type I staining. APS/TEMED crosslinking can be used to produce MSC-seeded HA-based neocartilage and can be used in combination with rapid prototyping techniques to generate anatomic MSC-seeded HA constructs for use in filling large and anatomically complex chondral defects or for biologic joint replacement.

The contribution of collagen fibers to the mechanical compressive properties of the temporomandibular joint disc

OBJECTIVE

The Temporomandibular Joint (TMJ) disc is a fibrocartilaginous structure located between the mandibular condyle and the temporal bone, facilitating smooth movements of the jaw. The load-bearing properties of its anisotropic collagenous network have been well characterized under tensile loading conditions. However, recently it has also been speculated that the collagen fibers may contribute dominantly in reinforcing the disc under compression. Therefore, in this study, the structural-functional role of collagen fibers in mechanical compressive properties of TMJ disc was investigated.

DESIGN

Intact porcine TMJ discs were enzymatically digested with collagenase to disrupt the collagenous network of the cartilage. The digested and non-digested articular discs were analyzed mechanically, biochemically and histologically in five various regions. These tests included: (1) cyclic compression tests, (2) biochemical quantification of collagen and glycosaminoglycan (GAG) content and (3) visualization of collagen fibers' alignment by polarized light microscopy (PLM).

RESULTS

The instantaneous compressive moduli of the articular discs were reduced by as much as 50-90% depending on the region after the collagenase treatment. The energy dissipation properties of the digested discs showed a similar tendency. Biochemical analysis of the digested samples demonstrated an average of 14% and 35% loss in collagen and GAG, respectively. Despite the low reduction of collagen content the PLM images showed considerable perturbation of the collagenous network of the TMJ disc.

CONCLUSIONS

The results indicated that even mild disruption of collagen fibers can lead to substantial mechanical softening of TMJ disc undermining its reinforcement and mechanical stability under compression.

High mesenchymal stem cell seeding densities in hyaluronic acid hydrogels produce engineered cartilage with native tissue properties

Engineered cartilage based on adult mesenchymal stem cells (MSCs) is an alluring goal for the repair of articular defects. However, efforts to date have failed to generate constructs with sufficient mechanical properties to function in the demanding environment of the joint. Our findings with a novel photocrosslinked hyaluronic acid (HA) hydrogel suggest that stiff gels (high HA concentration, 5% w/v) foster chondrogenic differentiation and matrix production, but limit overall functional maturation due to the inability of the formed matrix to diffuse away from the point of production and form a contiguous network. In the current study, we hypothesized that increasing the MSC seeding density would decrease the required diffusional distance, and so expedite the development of functional properties. To test this hypothesis bovine MSCs were encapsulated at seeding densities of either 20,000,000 or 60,000,000 cells ml(-1) in 1%, 3%, and 5% (w/v) HA hydrogels. Counter to our hypothesis the higher concentration HA gels (3% and 5%) did not develop more rapidly with increased MSC seeding density. However, the biomechanical properties of the low concentration (1%) HA constructs increased markedly (nearly 3-fold with a 3-fold increase in seeding density). To ensure that optimal nutrient access was delivered, we next cultured these constructs under dynamic culture conditions (with orbital shaking) for 9 weeks. Under these conditions 1% HA seeded at 60,000,000 MSCs ml(-1) reached a compressive modulus in excess of 1 MPa (compared with 0.3-0.4 MPa for free swelling constructs). This is the highest level we have reported to date in this HA hydrogel system, and represents a significant advance towards functional stem cell-based tissue engineered cartilage.

Improved cartilage repair via in vitro pre-maturation of MSC-seeded hyaluronic acid hydrogels

Functional repair of focal cartilage defects requires filling the space with neotissue that has compressive properties comparable to native tissue and integration with adjacent host cartilage. While poor integration is a common complication with current clinical treatments, reports of tissue engineering advances in the development of functional compressive properties rarely include analyses of their potential for integration. Our objective was thus to assess both the maturation and integration of mesenchymal stem cell (MSC)-laden hyaluronic acid (HA) hydrogels in an in vitro cartilage defect model. Furthermore, we considered the effects of an initial period of pre-maturation as well as various material formulations to maximize both construct compressive properties and integration strength. MSCs were encapsulated in 1%, 3% and 5% methacrylated HA (MeHA) or 2% agarose (Ag) and gelled directly (in situ) within an in vitro cartilage defect or were formed and then pre-cultured for 4 weeks before implantation. Results showed that the integration strength of pre-cultured repair constructs was equal to (1% MeHA) or greater than (2% Ag) the integration of in situ repaired cartilage. Moreover, MSC chondrogenesis and maturation was restricted by the in situ repair environment with constructs maturing to a much lesser extent than pre-matured constructs. These results indicate that construct pre-maturation may be an essential element of functional cartilage repair.

Cartilage matrix formation by bovine mesenchymal stem cells in three-dimensional culture is age-dependent

BACKGROUND

Cartilage degeneration is common in the aged, and aged chondrocytes are inferior to juvenile chondrocytes in producing cartilage-specific extracellular matrix. Mesenchymal stem cells (MSCs) are an alternative cell type that can differentiate toward the chondrocyte phenotype. Aging may influence MSC chondrogenesis but remains less well studied, particularly in the bovine system.

QUESTIONS/PURPOSES

The objectives of this study were (1) to confirm age-related changes in bovine articular cartilage, establish how age affects chondrogenesis in cultured pellets for (2) chondrocytes and (3) MSCs, and (4) determine age-related changes in the biochemical and biomechanical development of clinically relevant MSC-seeded hydrogels.

METHODS

Native bovine articular cartilage from fetal (n = 3 donors), juvenile (n = 3 donors), and adult (n = 3 donors) animals was analyzed for mechanical and biochemical properties (n = 3-5 per donor). Chondrocyte and MSC pellets (n = 3 donors per age) were cultured for 6 weeks before analysis of biochemical content (n = 3 per donor). Bone marrow-derived MSCs of each age were also cultured within hyaluronic acid hydrogels for 3 weeks and analyzed for matrix deposition and mechanical properties (n = 4 per age).

RESULTS

Articular cartilage mechanical properties and collagen content increased with age. We observed robust matrix accumulation in three-dimensional pellet culture by fetal chondrocytes with diminished collagen-forming capacity in adult chondrocytes. Chondrogenic induction of MSCs was greater in fetal and juvenile cell pellets. Likewise, fetal and juvenile MSCs in hydrogels imparted greater matrix and mechanical properties.

CONCLUSIONS

Donor age and biochemical microenvironment were major determinants of both bovine chondrocyte and MSC functional capacity.

CLINICAL RELEVANCE

In vitro model systems should be evaluated in the context of age-related changes and should be benchmarked against human MSC data.

Freeze-thaw treatment effects on the dynamic mechanical properties of articular cartilage

BACKGROUND

As a relatively non-regenerative tissue, articular cartilage has been targeted for cryopreservation as a method of mitigating a lack of donor tissue availability for transplant surgeries. In addition, subzero storage of articular cartilage has long been used in biomedical studies using various storage temperatures. The current investigation studies the potential for freeze-thaw to affect the mechanical properties of articular cartilage through direct comparison of various subzero storage temperatures.

METHODS

Both subzero storage temperature as well as freezing rate were compared using control samples (4°C) and samples stored at either -20°C or -80°C as well as samples first snap frozen in liquid nitrogen (-196°C) prior to storage at -80°C. All samples were thawed at 37.5°C to testing temperature (22°C). Complex stiffness and hysteresis characterized load resistance and damping properties using a non-destructive, low force magnitude, dynamic indentation protocol spanning a broad loading rate range to identify the dynamic viscoelastic properties of cartilage.

RESULTS

Stiffness levels remained unchanged with exposure to the various subzero temperatures. Hysteresis increased in samples snap frozen at -196°C and stored at -80°C, though remained unchanged with exposure to the other storage temperatures.

CONCLUSIONS

Mechanical changes shown are likely due to ice lens creation, where frost heave effects may have caused collagen damage. That storage to -20°C and -80°C did not alter the mechanical properties of articular cartilage shows that when combined with a rapid thawing protocol to 37.5°C, the tissue may successfully be stored at subzero temperatures.

Homologous structure-function relationships between native fibrocartilage and tissue engineered from MSC-seeded nanofibrous scaffolds

Understanding the interplay of composition, organization and mechanical function in load-bearing tissues is a prerequisite in the successful engineering of tissues to replace diseased ones. Mesenchymal stem cells (MSCs) seeded on electrospun scaffolds have been successfully used to generate organized tissues that mimic fibrocartilages such as the knee meniscus and the annulus fibrosus of the intervertebral disc. While matrix deposition has been observed in parallel with improved mechanical properties, how composition, organization, and mechanical function are related is not known. Moreover, how this relationship compares to that of native fibrocartilage is unclear. Therefore, in the present work, functional fibrocartilage constructs were formed from MSC-seeded nanofibrous scaffolds, and the roles of collagen and glycosaminoglycan (GAG) in compressive and tensile properties were determined. MSCs deposited abundant collagen and GAG over 120 days of culture, and these extracellular molecules were organized in such a way that they performed similar mechanical functions to their native roles: collagen dominated the tensile response while GAG was important for compressive properties. GAG removal resulted in significant stiffening in tension. A similar stiffening response was observed when GAG was removed from native inner annulus fibrosus, suggesting an interaction between collagen fibers and their surrounding extrafibrillar matrix that is shared by both engineered and native fibrocartilages. These findings strongly support the use of electrospun scaffolds and MSCs for fibrocartilage tissue engineering, and provide insight on the structure-function relations of both engineered and native biomaterials.

Postnatal development of collagen structure in ovine articular cartilage

Background

Articular cartilage (AC) is the layer of tissue that covers the articulating ends of the bones in diarthrodial joints. Across species, adult AC shows an arcade-like structure with collagen predominantly perpendicular to the subchondral bone near the bone, and collagen predominantly parallel to the articular surface near the articular surface. Recent studies into collagen fibre orientation in stillborn and juvenile animals showed that this structure is absent at birth. Since the collagen structure is an important factor for AC mechanics, the absence of the adult Benninghoff structure has implications for perinatal AC mechanobiology. The current objective is to quantify the dynamics of collagen network development in a model animal from birth to maturity. We further aim to show the presence or absence of zonal differentiation at birth, and to assess differences in collagen network development between different anatomical sites of a single joint surface. We use quantitative polarised light microscopy to investigate properties of the collagen network and we use the sheep (Ovis aries) as our model animal.

Results

Predominant collagen orientation is parallel to the articular surface throughout the tissue depth for perinatal cartilage. This remodels to the Benninghoff structure before the sheep reach sexual maturity. Remodelling of predominant collagen orientation starts at a depth just below the future transitional zone. Tissue retardance shows a minimum near the articular surface at all ages, which indicates the presence of zonal differentiation at all ages. The absolute position of this minimum does change between birth and maturity. Between different anatomical sites, we find differences in the dynamics of collagen remodelling, but no differences in adult collagen structure.

Conclusions

The collagen network in articular cartilage remodels between birth and sexual maturity from a network with predominant orientation parallel to the articular surface to a Benninghoff network. The retardance minimum near, but not at, the articular surface at all ages shows that a zonal differentiation is already present in the perinatal animals. In these animals, the zonal differentiation can not be correlated to the collagen network orientation. We find no difference in adult collagen structure in the nearly congruent metacarpophalangeal joint, but we do find differences in the dynamics of collagen network remodelling.

An in-vitro traumatic model to evaluate the response of myelinated cultures to sustained hydrostatic compression injury

While a variety of in-vitro models have been employed to investigate the response of load-bearing tissues to hydrostatic pressure, long-term studies are limited by the need to provide for adequate gas exchange during pressurization. Applying compression in vitro may alter the equilibrium of the system and thereby disrupt the gas exchange kinetics. To address this, several sophisticated compression chamber designs have been developed. However, these systems are limited in the magnitude of pressure that can be applied and may require frequent media changes, thereby eliminating critical autocrine and paracrine signaling factors. To better isolate the cellular response to long-term compression, we created a model that features continuous gas flow through the chamber during pressurization, and a negative feedback control system to rigorously control dissolved oxygen levels. Monitoring dissolved oxygen continuously during pressurization, we find that the ensuing response exhibits characteristics of a second- or higher-order system which can be mathematically modeled using a second-order differential equation. Finally, we use the system to model chronic nerve compression injuries, such as carpal tunnel syndrome and spinal nerve root stenosis, with myelinated neuron-Schwann cell co-cultures. Cell membrane integrity assay results show that co-cultures respond differently to hydrostatic pressure, depending on the magnitude and duration of stimulation. In addition, we find that myelinated Schwann cells proliferate in response to applied hydrostatic compression.

Effects of enzymatic digestion on compressive properties of rat intervertebral discs

Enzymatic treatments were applied to rat motion segments to establish structure-function relationships and determine mechanical parameters most sensitive to simulated remodeling and degeneration. Rat caudal and lumbar disc biomechanical behaviors were evaluated to improve knowledge of their similarities and differences due to their frequent use during in vivo models. Caudal motion segments were assigned to four groups: soaked (control), genipin treated, elastase treated, and collagenase treated. Fresh lumbar and caudal discs were also compared. The mechanical protocol involved five force-controlled loading stages: equilibration, cyclic compression-tension, quasi-static compression, frequency sweep, and creep. Crosslinking was found to have the greatest effect on IVD properties at resting stress. Elastin's role was greatest in tension and at higher force conditions, where GAG content was also a contributing factor. Collagenase treatment caused tissue compaction, which impacted mechanical properties at both high and low force conditions. Equilibration creep and cyclic compression-tension tests were the mechanical tests most sensitive to alterations in specific matrix constituents. Caudal and lumbar motion segments had many similarities but biomechanical differences suggested some distinctions in collagenous structure and water transport characteristics in addition to the geometric differences. Results provide a basis for interpreting biomechanical changes observed in animal model studies of degeneration and remodeling, and underscore the need to maintain and/or repair collagen integrity in IVD health and disease.

Macromer density influences mesenchymal stem cell chondrogenesis and maturation in photocrosslinked hyaluronic acid hydrogels

OBJECTIVE

Engineering cartilage requires that a clinically relevant cell type be situated within a 3D environment that supports cell viability, the production and retention of cartilage-specific extracellular matrix (ECM), and eventually, the establishment of mechanical properties that approach that of the native tissue. In this study, we investigated the ability of bone marrow derived mesenchymal stem cells (MSCs) to undergo chondrogenesis in crosslinked methacrylated hyaluronic acid hydrogels (MeHA) of different macromer concentrations (1, 2, and 5%).

DESIGN

Over a 6 week culture period under pro-chondrogenic conditions, we evaluated cartilage-specific gene expression, ECM deposition within constructs and released to the culture media, and mechanical properties in both compression and tension. Further, we examined early matrix assembly and long term histological features of the forming tissues, as well as the ability of macromolecules to diffuse within hydrogels as a function of MeHA macromer concentration.

RESULTS

Findings from this study show that variations in macromer density influence MSC chondrogenesis in distinct ways. Increasing HA macromer density promoted chondrogenesis and matrix formation and retention, but yielded functionally inferior constructs due to limited matrix distribution throughout the construct expanse. In 1% MeHA constructs, the equilibrium compressive modulus reached 0.12MPa and s-GAG content reached nearly 3% of the wet weight, values that matched or exceeded those of control agarose constructs and that are 25 and 50% of native tissue levels, respectively.

CONCLUSIONS

These data provide new insight into how early matrix deposition regulates long term construct development, and defines new parameters for optimizing the formation of functional MSC-based engineered articular cartilage using HA hydrogels.