Controlled enzymatic matrix degradation for integrative cartilage repair: effects on viable cell density and proteoglycan deposition

Photo-Cross-Linkable, Injectable, and Highly Adhesive GelMA-Glycol Chitosan Hydrogels for Cartilage Repair

Hydrogels provide a promising platform for cartilage repair and regeneration. Although hydrogels have shown some efficacy, they still have shortcomings including poor mechanical properties and suboptimal integration with surrounding cartilage. Herein, hydrogels that are injectable, cytocompatible, mechanically robust, and highly adhesive to cartilage are developed. This approach uses GelMA-glycol chitosan (GelMA-GC) that is crosslinkable with visible light and photoinitiators (lithium acylphosphinate and tris (2,2'-bipyridyl) dichlororuthenium (II) hexahydrate ([RuII(bpy)3 ]2+ and sodium persulfate (Ru/SPS)). Ru/SPS-cross-linked hydrogels have higher compressive and tensile modulus, and most prominently higher adhesive strength with cartilage, which also depends on inclusion of GC. Tensile and push-out tests of the Ru/SPS-cross-linked GelMA-GC hydrogels demonstrate adhesive strength of ≈100 and 46 kPa, respectively. Hydrogel precursor solutions behave in a Newtonian manner and are injectable. After injection in focal bovine cartilage defects and in situ cross-linking, this hydrogel system remains intact and integrated with cartilage following joint manipulation ex vivo. Cells remain viable (>85%) in the hydrogel system and further show tissue regeneration potential after three weeks of in vitro culture. These preliminary results provide further motivation for future research on bioadhesive hydrogels for cartilage repair and regeneration.

Articular Joint-Simulating Mechanical Load Activates Endogenous TGF-β in a Highly Cellularized Bioadhesive Hydrogel for Cartilage Repair

BACKGROUND

The treatment of osteochondral defects (OCDs) constitutes a major problem for orthopaedic surgeons. The altered mechanics and the cell types, with associated soluble factors derived from the exposed subchondral bone, are likely responsible for the mechanically and structurally inferior articular cartilage subsequently obtained as a repair tissue. There is therefore an unmet clinical need for bioresponsive biomaterials that allow cell delivery, reduce cell infiltration from the bone marrow, and support chondrogenesis in the presence of joint mechanical loading.

PURPOSE

To develop a cell-laden injectable biomaterial, with bioadhesive properties, low cell invasion, and good mechanoresilience, in which simulated joint loading could induce tissue maturation through the production and activation of transforming growth factor beta 1 (TGF-β1).

STUDY DESIGN

Controlled laboratory study.

METHODS

Human bone marrow-derived mesenchymal stromal/stem cells were encapsulated in tyramine-modified hyaluronic acid (HA-Tyr) hydrogels, with crosslinking initiated by the addition of horseradish peroxidase (HRP) and various concentrations of hydrogen peroxide (H₂O₂; 0.3-2 mM). Cytocompatibility and biomechanical and adhesive properties were analyzed by live/dead staining, rheology, and push-out test, respectively. For multiaxial loading, cell-laden hydrogels were subjected to 10% compression superimposed onto a 0.5-N preload and shear loading (±25°) at 1 Hz for 1 hour per day and 5 times a week for 4 weeks. TGF-β1 production and activation were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

The viscoelastic properties of the cell-laden HA-Tyr hydrogels, as crosslinked with different ratios of HRP and H₂O₂, were demonstrated for a range of cell densities and HRP/H₂O₂ concentrations. In the absence of serum supplementation, cell invasion into HA-Tyr hydrogels was minimal to absent. The bonding strength of HA-Tyr to articular cartilage compared favorably with clinically used fibrin gel.

CONCLUSION

HA-Tyr hydrogels can be mechanically conditioned to induce activation of endogenous TGF-b1 produced by the embedded cells. HA-Tyr hydrogels function as cell carriers supporting biomechanically induced production and activation of TGF-β1 and as bioadhesive materials with low cell invasion, suggesting that they hold promise as a novel biomaterial for OCD repair strategies.

CLINICAL RELEVANCE

Leveraging physiological joint mechanics to support chondrogenic graft maturation in an optimized mechanosensitive hydrogel in the absence of exogenous growth factors is of highest interest for OCD repair.

Trypsin Pre-Treatment Combined With Growth Factor Functionalized Self-Assembling Peptide Hydrogel Improves Cartilage Repair in Rabbit Model

The objective of this study was to improve cartilage repair and integration using self-assembling KLD hydrogel functionalized with platelet-derived growth factor-BB and heparin-binding insulin-like growth factor-1 with associated enzymatic trypsin pre-treatment of the native cartilage. Bilateral osteochondral defects were created at the central portion of the femoral trochlear groove of 48 skeletally mature, white New Zealand rabbits. One limb received a randomly assigned treatment and the contralateral limb served as the control. Treated defects were exposed to trypsin for 2 min and filled with self-assembling KLD hydrogel only, or associated to growth factors. All control limbs received KLD hydrogel alone or received only trypsin but not hydrogel. Ninety days post-defect creation, the rabbits were euthanized and magnetic resonance imaging, radiography, macroscopic evaluation, histology, and immunohistochemistry of the joint and repaired tissue were performed. Mixed model analyses of variance were utilized to assess the outcome parameters and individual comparisons were performed using Least Square Means procedure and differences with p-value < 0.05 were considered significant. Trypsin enzymatic pre-treatment improved cellular morphology, cluster formation and subchondral bone reconstitution. Platelet-derived growth factor-BB improved subchondral bone healing and basal integration. Heparin-binding insulin-like growth factor-1 associated with platelet-derived growth factor improved tissue and cell morphology. The authors conclude that self-assembling KLD hydrogel functionalized with platelet-derived growth factor and heparin-binding insulin-like growth factor-1 with associated enzymatic pre-treatment of the native cartilage with trypsin resulted in an improvement on the cartilage repair process. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2307-2315, 2019.

Enzyme Pretreatment plus Locally Delivered HB-IGF-1 Stimulate Integrative Cartilage Repair In Vitro

IMPACT STATEMENT

A critical attribute for the long-term success of cartilage defect repair is the strong integration between the repair tissue and the surrounding native tissue. Current approaches utilized by physicians fail to achieve this attribute, leading to eventual relapse of the defect. This article demonstrates the concept of a simple, clinically viable approach for enhancing tissue integration via the combination of a safe, transient enzymatic treatment with a locally delivered, retained growth factor through an in vitro hydrogel/cartilage explant model.

Cells, soluble factors and matrix harmonically play the concert of allograft integration

Implantation of allograft tissues has massively grown over the last years, especially in the fields related to sports medicine. Beside the fact that often no autograft option exists, autograft related disadvantages as donor-site morbidity and prolonged operative time are drastically reduced with allograft tissues. Despite the well documented clinical success for bone allograft procedures, advances in tissue engineering raised the interest in meniscus, osteochondral and ligament/tendon allografts. Notably, their overall success rates are constantly higher than 80%, making them a valuable treatment option in orthopaedics, especially in knee surgery. Complications reported for allografting procedures are a small risk of disease transmission, immunologic rejection, and decreased biologic incorporation together with nonunion at the graft-host juncture and, rarely, massive allograft resorption. Although allografting is a successful procedure, improved techniques and biological knowledge to limit these pitfalls and maximize graft incorporation are needed. A basic understanding of the biologic processes that affect the donor-host interactions and eventual incorporation and remodelling of various allograft tissues is a fundamental prerequisite for their successful clinical use. Further, the importance of the interaction of immunologic factors with the biologic processes involved in allograft incorporation has yet to be fully dissected. Finally, new tissue engineering techniques and use of adjunctive growth factors, cell based and focused gene therapies may improve the quality and uniformity of clinical outcomes. The aim of this review is to shed light on the biology of meniscus, osteochondral and ligament/tendon allograft incorporation and how collection and storage techniques may affect graft stability and embodiment.Level of evidence V.

Enhancing integration of articular cartilage grafts via photochemical bonding

The integration of osteochondral grafts to native articular cartilage is critical as the lack of graft integration may lead to continued tissue degradation, poor load transfer and inadequate nutrient transport. Photochemical bonding promotes graft integration by activating a photosensitizer at the interface via a light source and avoids negative effects associated with other bonding techniques. We hypothesized that the bond strength depends on photosensitizer type and concentration in addition to light exposure. Photochemical bonding was evaluated using methylene blue (MB), a cationic phenothiazine photosensitizer, and two phthalocyanine photosensitizers, Al(III) phthalocyanine chloride tetrasulfonic acid (CASPc) and aluminum phthalocyanine chloride (AlPc). Exposure was altered by varying irradiation time for a fixed irradiance or by varying irradiance with a fixed irradiation time. MB was ineffective at producing bonding at the range of concentrations tested while CASPc produced a peak twofold bond strength increase over controls. AlPc produced substantial bonding at all concentrations with a peak 3.9-fold bond strength increase over controls. Parametric tests revealed that bond strength depended primarily on the total energy delivered to the bonding site rather than the rate of light delivery or light irradiance. Bond strength persisted for 1 week of in-vitro culture, which warrants further exploration for clinical applications. These studies indicate that photochemical bonding is a viable strategy for enhancing articular cartilage graft integration. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2406-2415, 2018.

Current strategies for integrative cartilage repair

Osteoarthritis (OA) is a degenerative joint condition characterized by painful cartilage lesions that impair joint mobility. Current treatments such as lavage, microfracture, and osteochondral implantation fail to integrate newly formed tissue with host tissues and establish a stable transition to subchondral bone. Similarly, tissue-engineered grafts that facilitate cartilage and bone regeneration are challenged by how to integrate the graft seamlessly with surrounding host cartilage and/or bone. This review centers on current approaches to promote cartilage graft integration. It begins with an overview of articular cartilage structure and function, as well as degenerative changes to this relationship attributed to aging, disease, and trauma. A discussion of the current progress in integrative cartilage repair follows, focusing on graft or scaffold design strategies targeting cartilage-cartilage and/or cartilage-bone integration. It is emphasized that integrative repair is required to ensure long-term success of the cartilage graft and preserve the integrity of the newly engineered articular cartilage. Studies involving the use of enzymes, choice of cell source, biomaterial selection, growth factor incorporation, and stratified versus gradient scaffolds are therefore highlighted. Moreover, models that accurately evaluate the ability of cartilage grafts to enhance tissue integrity and prevent ectopic calcification are also discussed. A summary and future directions section concludes the review.

Time and dose-dependent effects of chondroitinase ABC on growth of engineered cartilage

Tissue engineering techniques have been effective in developing cartilage-like tissues in vitro. However, many scaffold-based approaches to cultivating engineered cartilage have been limited by low collagen production, an impediment for attaining native functional load-bearing tensile mechanical properties. Enzymatic digestion of glycosaminoglycans (GAG) with chondroitinase ABC (chABC) temporarily suppresses the construct's GAG content and compressive modulus and increases collagen content. Based on the promising results of these early studies, the aim of this study was to further promote collagen deposition through more frequent chABC treatments. Weekly dosing of chABC at a concentration of 0.15 U/mL resulted in a significant cell death, which impacted the ability of the engineered cartilage to fully recover GAG and compressive mechanical properties. In light of these findings, the influence of lower chABC dosage on engineered tissue (0.004 and 0.015 U/mL) over a longer duration (one week) was investigated. Treatment with 0.004 U/mL reduced cell death, decreased the recovery time needed to achieve native compressive mechanical properties and GAG content, and resulted in a collagen content that was 65 % greater than the control. In conclusion, the results of this study demonstrate that longer chABC treatment (one week) at low concentrations can be used to improve collagen content in developing engineered cartilage more expediently than standard chABC treatments of higher chABC doses administered over brief durations.

Effect of chondroitinase ABC on adhesion and behavior of synovial membrane-derived mesenchymal stem cells in rabbit partial-thickness chondral defects

Transplanted cells may have difficulty attaching to the surface of partial-thickness chondral lesions because of the anti-adhesive properties of the proteoglycan rich matrix. Therefore, the current study attempts to evaluate the effect of chondroitinase ABC (chABC) on the adhesion and behavior of transplanted synovial membrane-derived mesenchymal stem cells (SDSCs) in rabbit partial-thickness chondral defects. In ex vivo adhesion experiments, chABC treatment (0.1 U/ml) was increased in SDSC attachment to the cartilage explants, and significantly diminished by pretreatment with neutralizing antibody against fibronectin. In the in vivo experiments, 1 day and 4 weeks after the chABC treatment (0.1 and 1 U/ml), the immunoreactivity (IR) against CS-56 (intact chondroitin sulfate antibody) was markedly decreased; however, the IR of 2B6 (stub of the chondroitin 4-sulfate chain), 3B3 (stub of the chondroitin 6-sulfate chain), and fibronectin was increased. At 12 weeks, this IR returned to normal except in the high-dose chABC-treated group (1 U/ml). Furthermore, the attachment of SDSCs to the chondral defects after chABC treatment was increased at 7 days compared with that in the chondral defects pretreated with saline. However, the tissue repaired by SDSCs was negatively stained for type II collagen at 12 weeks. In conclusion, these results showed that the exposure to fibronectin by chABC treatment enhances the attachment of SDSCs to partial-thickness chondral defects. However, the tissue regenerated by SDSCs showed lack of hyaline cartilage regeneration. Thus, to understand the fate of transplanted MSCs in cartilage defect is very important for successful cell therapies.

Chondrocyte repopulation of the zone of death induced by osteochondral harvest

OBJECTIVE

Harvesting osteochondral grafts results in a zone of chondrocyte death (ZCD) in and around the periphery of the graft, creating a barrier for chondrocytes to migrate to the graft periphery, thus limiting cartilage-to-cartilage healing. The purpose of this study was to repopulate the induced ZCD through the combined effects of collagenase treatment and delivery of a chemotactic agent.

DESIGN

In bovine cartilage, the ZCD induced by the OATS™ osteochondral harvesting system was determined, followed by a corresponding collagenase treatment to penetrate the developed ZCD. The chemotactic potential of platelet derived growth factor (PDGF-bb), insulin-like growth factor I (IGF-I), and basic fibroblast growth factor (bFGF) (2.5-100 ng/mL) was then assessed using a modified Boyden chamber assay to select an appropriate agent to induce chondrocyte migration. Afterwards, the combined effects of collagenase treatment and chondrocyte chemotaxis on the repopulation of an induced ZCD were examined in cartilage explants over a 4-week-period.

RESULTS

The OATS™ osteochondral harvesting system induced a significant ZCD (173 μm, 95% CI: [72-274 μm]) in the grafts. Chondrocyte chemotaxis was induced by all agents investigated at concentrations greater than 25 ng/mL. After 4 weeks in culture, collagenase treatment alone reduced the ZCD by approximately 40% relative to untreated explants. Coupling the collagenase treatment with 25 ng/mL IGF-I reduced the ZCD by approximately 80% relative to untreated explants, and 65% relative to explants treated only with collagenase.

CONCLUSION

Treating cartilage explants with collagenase and 25 ng/mL IGF-I resulted in a decreased ZCD after a 4-week-period, and increased chondrocyte density within the induced ZCD.

A model for studying human articular cartilage integration in vitro

One of the major obstacles hindering cartilage repair is the integration of the reparative cartilage with the recipient cartilage. The purpose of this study was to develop an in vitro model that can be conveniently applied to simulate and improve the integration of tissue engineered cartilage with native articular cartilage. This model, a cartilage integration construct, consists of a cartilage explant and isolated chondrocytes. The explant was anchored to agarose gel on a culture plate as agarose gelation at 4 degrees C to seal the gap between the bottom of the explant and culture plate surface. Isolated chondrocytes were added and confined in the defect created in the center of the explant. After 4 weeks of culture, neocartilage containing proteoglycans and type II collagen was formed. Minimal integration occurred between the neocartilage and the cartilage explant, resembling the failure of cartilage integration manifested in experimental and clinical cartilage repair. In this model, agarose gel anchors the explant onto culture plate by altering temperatures and effectively prevents "leakage" of the isolated chondrocytes from the defect of the explant. This model provides a convenient simulation of the cartilage integration process in vitro and has applications in studies of cartilage integration and cartilage tissue engineering.

Native Chondrocyte Viability during Cartilage Lesion Progression: Normal to Surface Fibrillation

OBJECTIVE

Early surgical intervention for articular cartilage disease is desirable before full-thickness lesions develop. As early intervention treatments are designed, native chondrocyte viability at the treatment site before intervention becomes an important parameter to consider. The purpose of this study is to evaluate native chondrocyte viability in a series of specimens demonstrating the progression of articular cartilage lesions to determine if the chondrocyte viability profile changes during the evolution of articular cartilage disease to the level of surface fibrillation.

DESIGN

Osteochondral specimens demonstrating various degrees of articular cartilage damage were obtained from patients undergoing knee total joint replacement. Three groups were created within a patient harvest based on visual and tactile cues commonly encountered during surgical intervention: group 1, visually and tactilely intact surfaces; group 2, visually intact, tactilely soft surfaces; and group 3, surface fibrillation. Confocal laser microscopy was performed following live/dead cell viability staining.

RESULTS

Groups 1 to 3 demonstrated viable chondrocytes in all specimens, even within the fibrillated portions of articular cartilage, with little to no evidence of dead chondrocytes. Chondrocyte viability profile in articular cartilage does not appear to change as disease lesion progresses from normal to surface fibrillation.

CONCLUSIONS

Fibrillated partial-thickness articular cartilage lesions are a good therapeutic target for early intervention. These lesions retain a high profile of viable chondrocytes and are readily diagnosed by visual and tactile cues during surgery. Early intervention should be based on matrix failure rather than on more aggressive procedures that further corrupt the matrix and contribute to chondrocyte necrosis of contiguous untargeted cartilage.

A nanofibrous cell-seeded hydrogel promotes integration in a cartilage gap model

The presence of a defect in mature articular cartilage can lead to degenerative changes of the joint. This is in part caused by the inability of cartilage to regenerate tissue that is capable of spanning a fissure or crack. In this study, we hypothesized that introduction of a biodegradable cell-seeded nanofibrous hydrogel, Puramatrix(), into a cartilage gap would facilitate the generation of a mechanically stable interface. The effects of chondrocyte incorporation within the hydrogel and supplementation with transforming growth factor-beta3 (TGFbeta3), a known regulator of cell growth and differentiation, on cartilage integration were examined mechanically and histologically as a function of cell density and incubation time. When supplemented with TGFbeta3, the cell-seeded hydrogel exhibited abundant matrix generation within the hydrogel and a corresponding increase in maximum push-out stress as compared to all other groups. Furthermore, initial cell seeding density affected interfacial strength in a time-dependent manner. This study suggests that a cell-seeded TGFbeta3-supplemented hydrogel can encourage integration between two opposing pieces of articular cartilage.

Effects of introducing cultured human chondrocytes into a human articular cartilage explant model

Articular cartilage (AC) heals poorly and effective host-tissue integration after reconstruction is a concern. We have investigated the ability of implanted chondrocytes to attach at the site of injury and to be incorporated into the decellularized host matrix adjacent to a defect in an in vitro human explant model. Human osteochondral dowels received a standardized injury, were seeded with passage 3 chondrocytes labelled with PKH 26 and compared with two control groups. All dowels were cultured in vitro, harvested at 0, 7, 14 and 28 days and assessed for chondrocyte adherence and migration into the region of decellularized tissue adjacent to the defects. Additional evaluation included cell viability, general morphology and collagen II production. Seeded chondrocytes adhered to the standardized defect and areas of lamina splendens disruption but did not migrate into the adjacent acellular region. A difference was noted in viable-cell density between the experimental group and one control group. A thin lattice-like network of matrix surrounded the seeded chondrocytes and collagen II was present. The results indicate that cultured human chondrocytes do indeed adhere to regions of AC matrix injury but do not migrate into the host tissue, despite the presence of viable cells. This human explant model is thus an effective tool for studying the interaction of implanted cells and host tissue.

Photochemical approaches for bonding of cartilage tissues

OBJECTIVE

The objective of this study was to evaluate photochemical bonding as an approach for adhering live cartilage tissues across a repair interface in a manner that may lead to enhanced integration.

DESIGN

Photochemical bonding of both meniscal fibrocartilage and articular cartilage was explored using an anionic, hydrophilic phthalocyanine photosensitizer. Variations on surface preparations and irradiation parameters were explored using overlapped tissue strips and tested using a modified single-lap shear test. Durability of the photochemically induced bonds and cellular viability were examined in an in vitro cartilage defect model for up to 1 week in culture, with bond strength assessed via push-out test.

RESULTS

Meniscal tissue strips bonded with no surface treatment, but cartilage strips required enzymatic treatment with chondroitinase-ABC to effectively bond. More aggressive removal of glycosaminoglycans at the interface led to increased bond strengths. Bond strength achieved with a 10min irradiation of treated tissue was on the order of that previously achieved through several weeks of culture. In the defect model, photochemical bonds between a tissue annulus and a press-fit tissue core were maintained for 1 week in culture without substantial increases in cell death near the bonded interface.

CONCLUSIONS

With appropriate treatment parameters, photochemical bonding rapidly produced a stable structural interface between cartilage tissue samples and may be a promising strategy for enhancing initial attachment in cartilage repair strategies.

Characterization of engineered tissue construct mechanical function by magnetic resonance imaging

Non-invasive magnetic resonance imaging (MRI) is a technology that enables the characterization of multiple physical phenomena in living and engineered tissues. The mechanical function of engineered tissues is a primary endpoint for the successful regeneration of many biological tissues, such as articular cartilage, spine and heart. Here we demonstrate the application of MRI to characterize the mechanical function of engineered tissue. Phase contrast-based methods were demonstrated to characterize detailed deformation fields throughout the interior of native and engineered tissue, using an articular cartilage defect model as a study system. MRI techniques revealed that strain fields varied non-uniformly, depending on spatial position. Strains were highest in the tissue constructs compared to surrounding native cartilage. Tissue surface geometry corresponded to strain fields observed within the tissue interior near the surface. Strain fields were further evaluated with respect to the spatial variation in the concentration of glycosaminoglycans ([GAG]), critical proteoglycans in the extracellular matrix of cartilage, as determined by gadolinium-enhanced imaging. [GAG] also varied non-uniformly, depending on spatial position and was lowest in the tissue constructs compared to the surrounding cartilage. The use of multiple MRI techniques to assess tissue mechanical function provides complementary data and suggests that deformation is related to tissue geometry, underlying extracellular matrix constituents and the lack of tissue integration in the model system studied. Specialized and advanced MRI phase contrast-based methods are valuable for the detailed characterization and evaluation of mechanical function of tissue-engineered constructs.

The interface of functional biotribology and regenerative medicine in synovial joints

Biotribology is the science of biological surfaces in sliding contact encompassing the concepts of friction, wear, and lubrication of interacting surfaces. This bioscience field has emerged from the classical field of tribology and is of paramount importance to the normal function of numerous tissues, including articular cartilage, blood vessels, heart, tendons, ligaments, and skin. Surprisingly, relatively little attention has been given to the restoration of surface characteristics in the fields of tissue engineering and regenerative medicine-the science of design and manufacture of new tissues for the functional restoration of impaired or diseased organs that depend on inductive signals, responding stem cells, and extracellular matrix scaffolding. Analogous to ancient civilizations (c. 3000 B.C.) that introduced wheeled vehicles, sledges for transporting heavy blocks, and lubricants, modern biotribologists must aim to restore surface characteristics to regenerated tissues and develop novel biomaterials with optimal tribological properties. The objective of this article is to highlight the significance of functional biotribology in the physiology of body surfaces and provide a comprehensive overview of unresolved issues and controversies as it relates to regenerative medicine. Specific attention is placed on the molecular basis of lubrication, mechanical and biochemical regulation of lubricating molecules, and the need to study wear processes in articular cartilage, especially in light of degenerative diseases, such as osteoarthritis. Surface engineering of replacement tissues exhibiting low friction and high wear resistance is examined using articular cartilage as an illustrative model system.

In vitro model of full-thickness cartilage defect healing

Integration of the host-graft interface is implicated as one of the significant reasons for lack of complete healing in osteochondral grafting procedures for the treatment of cartilage lesions. We developed an in vitro model of cartilage healing in an osteochondral setting to study the effect of developmental age and collagenase treatment. Circular full-thickness vertical surgical incisions were made in the cartilaginous portion of cylindrical bovine osteochondral specimens. Two age groups were selected: Young (1-2 months old) and Older (6-8 months old). Cartilage integration across the surgical incisions was assessed by histologic analysis and by mechanical push-out testing at 2 and 4 weeks in culture. Histologic integration as well as peak push-out shear stress was significantly higher in older calf cartilage than in the young calf. Collagenase pretreatment in the older calf samples increased push-out strength at 4 weeks. Histologic integration correlated well with the mechanical push-out strength. Developmental age and time after injury affected the response to collagenase pretreatment. This osteochondral cartilage integration model can be useful to study factors that modulate healing of surgical replacement procedures in vitro, which may aid the development of newer approaches to promote the healing of cartilage defects.

Enhancing tissue integration in cartilage repair procedures

Arguably, the gold standard of biological repair of articular cartilage lesions is autologous chondrocyte transplantation. Although the clinical outcomes appear to range between good and excellent in most cases, there are, nevertheless, both clinical and biological challenges that remain to improve rehabilitation and clinical outcome. One of the major biological problems relates to tissue integration of the reparative tissue into the host tissue at a predictable level. Often within a single lesion, varying degrees of integration can be observed from total integration through to non-integration as one passes through the defect. Here we briefly review some of the literature relating to this problem and include some of our own data drawn from questions we have posed about the biological nature of cartilage/cartilage integration. The nature and status of the tissue that comprises the wound lesion edge is central to tissue integration, and controlling aspects of trauma and free-radical-induced cell death together with matrix synthesis are identified as two components that require further investigation. Interestingly, there appears to be a limited ability of chondrocytes to be able to infiltrate existing cartilage matrices and even to occupy empty chondrocyte lacunae. Proliferation as a result of blunt and sharp trauma shows differential responses. As expected, blunt trauma induces a greater proliferative burst than sharp trauma and is more widespread from the lesion edge. However, in the case of sharp trauma, the basal cells enter proliferation before surface zone chondrocytes, which is not the case in blunt wounds. Regulation of these and associated processes will be necessary in order to devise strategies that can predict successful integration in biological repair procedures.

Mechanisms of cartilage growth: modulation of balance between proteoglycan and collagen in vitro using chondroitinase ABC

OBJECTIVE

To examine the cartilage growth-associated effects of a disruption in the balance between the swelling pressure of glycosaminoglycans (GAGs) and the restraining function of the collagen network, by diminishing GAG content prior to culture using enzymatic treatment with chondroitinase ABC.

METHODS

Immature bovine articular cartilage explants from the superficial and middle layers were analyzed immediately or after incubation in serum-supplemented medium for 13 days. Other explants were treated with chondroitinase ABC to deplete tissue GAG and also either analyzed immediately or after incubation in serum-supplemented medium for 13 days. Treatment- and incubation-associated variations in tissue volume, contents of proteoglycan and collagen network components, and tensile mechanical properties were assessed.

RESULTS

Incubation in serum-supplemented medium resulted in expansive growth with a marked increase in tissue volume that was associated with a diminution of tensile integrity. In contrast, chondroitinase ABC treatment on day 0 led to a marked reduction of GAG content and enhancement of tensile integrity, and subsequent incubation led to maturational growth with minimal changes in tissue volume and maintenance of tensile integrity at the enhanced levels.

CONCLUSION

The data demonstrate that a manipulation of GAG content in articular cartilage explants can distinctly alter the growth phenotype of cartilage. This may have practical utility for tissue engineering and cartilage repair. For example, the expansive growth phenotype may be useful to fill cartilage defects, while the maturational growth phenotype may be useful to induce matrix stabilization after filling defect spaces.

Injectable biomaterials and vertebral endplate treatment for repair and regeneration of the intervertebral disc

The objectives of augmentation of the nucleus pulposus following disc removal are to prevent disc height loss and the associated biomechanical and biochemical changes. Flowable materials may be injected via a small incision, allowing minimally invasive access to the disc space. Fluids can interdigitate with the irregular surgical defects and may even physically bond to the adjacent tissue. Injectable biomaterials allow for incorporation and uniform dispersion of cells and/or therapeutic agents. Injectable biomaterials have been developed that may act as a substitute for the disc nucleus pulposus. Our work has focused on the evaluation of a recombinant protein copolymer consisting of amino acid sequence blocks derived from silk and elastin structural proteins as an injectable biomaterial for augmentation of the nucleus pulposus. This implant, NuCoretrade mark Injectable Nucleus is being developed by Spine Wave (Shelton, CT). The NuCoretrade mark material is comprised of a solution of the protein polymer and a polyfunctional cross-linking agent. The material closely mimics the protein content, water content, pH and complex modulus of the natural nucleus pulposus. Extensive mechanical testing, biocompatibility and toxicology testing have been performed on the material. Characterization studies indicate that the NuCoretrade mark Injectable Nucleus is able to restore the biomechanics of the disc following a microdiscectomy. Extensive biomaterial characterization shows the material to be non-toxic and biocompatible. The mechanical properties of the material mimic those of the natural nucleus pulposus. Thus NuCoretrade mark Injectable Nucleus is suitable to replace the natural nucleus pulposus following a discectomy procedure. Human clinical evaluation is underway in a multi center clinical study on the use of the material as an adjunct to microdiscectomy. Further clinical studies of the use of NuCoretrade mark Injectable Nucleus for treatment of early stage degenerative disc disease are planned in the near future. On-going efforts are characterizing the use of the material as a cell delivery vehicle for disc repair and reconstruction. Related development efforts are exploring methods for repair and regeneration of the cartilage endplate that are implemented to enhance the host-implant interface. Prior to the introduction of the above-mentioned biomaterial, our work proposes to utilize a process for the treatment of the vertebral endplates. The goal of this process is to restore the endplates as closely as possible to their natural state prior to disease or degeneration. The nature of the treatment will depend upon the form of the endplate degeneration and on the type of scaffolding that is intended to be introduced in the nuclear cavity. Endplate therapy is a potential means of enhancing biomaterial integration and cell survival, but remains a long-term and currently untested methodology.

Injectable cartilage tissue engineering

Cartilage is the tissue that lines the surface of bones in articulating joints, allowing painless joint movement. Cartilage loss is an increasingly significant problem, particularly with the ageing of active baby boomers, with few efficacious treatments available at present. Tissue engineering is a field that has evolved over recent years to combat tissue loss by providing a living tissue equivalent or substitute that can mimic the properties of the lost tissue. The general strategy of tissue engineering is to place cells on a biomaterial scaffold that is designed to promote cell function and form new tissue. This review describes the status of materials that are available as injectable scaffolds for tissue engineering and the numerous cell types that can be applied to cartilage repair, including cells derived from cartilage and stem cells. The current state of injectable cartilage tissue engineering and the hurdles that remain for widespread clinical application are discussed.

The Repair Response to Osteochondral Implant Types in a Rabbit Model

Current treatments for damaged articular cartilage (i.e., shaving the articular surface, perforation or abrasion of the subchondral bone, and resurfacing with periosteal and perichondrial resurfacing) often produce fibrocartilage, or hyaline-appearing repair that is not sustained over time (Henche 1967, Ligament and Articular Cartilage Injuries. Springer-Verlag, New York, NY, pp. 157-164; Insall 1974, Clin. Orthop. 101: 61-67; Mitchell and Shepard 1976, J. Bone Joint Surg. [Am.] 58: 230-233; O'Driscoll et al. 1986, J. Bone Joint Surg. [Am.] 68: 1017-1035; 1989, Trans. Orthop. Res. Soc. 14: 145; Kim et al. 1991, J. Bone Joint Surg. [Am.] 73: 1301-1315). Autologous chondrocyte transplantation, although promising, requires two surgeries, has site-dependent and patient age limitations, and has unknown long-term donor site morbidity (Brittberg et al. 1994, N Engl. J. Med. 331: 889-895; Minas 2003, Orthopedics 26: 945-947; Peterson et al. 2003, J. Bone Joint Surg. Am. 85-A(Suppl. 2): S17-S24). Osteochondral allografts remain a widely used method of articular resurfacing to delay arthritic progression. The present study compared the histological response to four types of osteochondral implants in a rabbit model: autograft, frozen, freeze-dried, and fresh implants. Specimens implanted in the femoral groove were harvested at 6 and 12 weeks. Results showed similar restoration of the joint surface regardless of implant type, with a trend toward better repair at the later timepoint. As has been observed in other studies (Frenkel et al. 1997, J. Bone Joint Surg. 79B: 281-286; Toolan et al. 1998, J. Biomed. Mater. Res. 41: 244-250), each group in this study had at least one specimen in which a healthy-appearing surface on the implant was not well-integrated with host tissues. Although the differences were not statistically significant, freeze-dried implants at both timepoints had the best histological scores. The osteochondral grafts tested successfully restored the gross joint surface and congruity. At 12 weeks, no significant differences were observed between the various allografts and autologous osteochondral grafts.

Glycosaminoglycan degradation reduces mineralized tissue–titanium interfacial strength

Although the localization of the proteoglycan/glycosaminoglycan (GAG) complex at the bone-titanium implant interface has been implied, the role of proteoglycans on the establishment of bone-titanium integration is unknown. The hypothesis to be tested was that proteoglycans play an important role in establishing bone-titanium interfacial adhesion. The objective of this study is to investigate the effect of proteoglycan knockdown by GAG enzymatic degradation on the interfacial strength between mineralized tissue and titanium having different surface topographies. Rat bone marrow-derived osteoblastic cells were cultured on either a machined titanium disk or an acid-etched titanium disk. At day 21 of culture, one of the three following GAG degradation enzymes was added into the culture; chondroitinase AC, chondroitinase B, or keratanase. After 3 days of incubation (at day 24 of culture), the laser spallation technique was applied to the samples in order to assess the tissue-titanium interfacial strength. In this technique, a laser-generated stress wave is used to separate the tissue-titanium interface, and the interfacial strength is determined interferometrically by recording the transient free surface velocity of the tissue. Mineralized tissue cultured on the acid-etched titanium showed 20-30% higher tissue interfacial strength than that cultured on the machined titanium (p < 0.0001). For both the machined and acid-etched surface cultures, administration of the enzyme reduced the interfacial strength by 25-30% compared with the untreated control cultures (p < 0.0001). There were no differences in the effect among the three different enzymes tested. A nanoindentation study revealed that the enzyme treatment did not affect the elastic modulus of the mineralized tissue. Scanning electron microscopic and energy dispersive spectroscopic analyses revealed less post-spallation tissue remnant on the titanium substrates when treated with the enzymes. The tissue remnant was greater in amount on the acid-etched surface than on the machined surface. The results suggest that there exists not only mechanical interlocking but also biological interfacial adhesion between the mineralized tissue and titanium, in which the proteoglycan/GAG complex is involved.

Degree of Degeneration and Chondroitinase ABC Treatment of Human Articular Cartilage Affect Adhesion of Chondrocytes

The aim of the present study was to investigate whether degree of degeneration and chondroitinase ABC treatment of articular cartilage affect adhesion of chondrocytes to its surface. Human articular cartilage explants were harvested from patients undergoing total knee joint replacement arthroplasty, cut into discs of 0.5-0.8 mm thick and 3.75 mm in diameter, and then divided into three groups according to the degree of degeneration: G0, normal; G1, superficial fissures, and G2, deep fissures. These discs were stored at 70 degrees C until use. Chondrocytes (3 x 10(5) cell/mL in 100 microL of medium) isolated from articular cartilage by enzymatic digestion were transferred to a well plate containing cartilage discs and cultured for a maximum of 21 days. The cultures were divided into one of six groups according to the degree of degeneration and treatment of chondroitinase ABC of the disc. The morphologies of the disc surface and of cultured chondrocytes were assessed histologically. Chondrocyte adhesion, proliferation, and synthesis of proteoglycan and protein were analyzed. Chondrocyte adhesion significantly increased on the discs with higher degrees of degeneration, that is, maximum adhesion in G2, G1, and G0, subsequently. However, adhesion was not significantly different among the three groups at day 3. Chondrocyte adhesion on the chondroitinase ABC-treated disc significantly increased in comparison with that on the nontreated discs with the same degree of degeneration. However, proliferation of chondrocytes and synthesis of glycosaminoglycan and protein were not significant, regardless of degree of degeneration or chondroitinase ABC treatment. The present study demonstrated that chondrocyte adhesion on articular cartilage disc increased along with its degree of degeneration and with treatment of chondroitinase ABC, but that proliferation and synthesis of matrix of the cultured chondrocytes were not significantly influenced by both of them.

Advances in skeletal tissue engineering with hydrogels

OBJECTIVES

Tissue engineering has the potential to make a significant impact on improving tissue repair in the craniofacial system. The general strategy for tissue engineering includes seeding cells on a biomaterial scaffold. The number of scaffold and cell choices for tissue engineering systems is continually increasing and will be reviewed.

DESIGN

Multilayered hydrogel systems were developed to coculture different cell types and develop osteochondral tissues for applications including the temporomandibular joint.

EXPERIMENTAL VARIABLE

Hydrogels are one form of scaffold that can be applied to cartilage and bone repair using fully differentiated cells, adult and embryonic stem cells.

OUTCOME MEASURE

Case studies represent an overview of our laboratory's investigations.

RESULTS

Bilayered scaffolds to promote tissue development and the formation of more complex osteochondral tissues were developed and proved to be effective.

CONCLUSION

Tissue engineering provides a venue to investigate tissue development of mutant or diseased cells and potential therapeutics.

Inhibition of integrative cartilage repair by proteoglycan 4 in synovial fluid

OBJECTIVE

To determine the effects of the articular cartilage surface, as well as synovial fluid (SF) and its components, specifically proteoglycan 4 (PRG4) and hyaluronic acid (HA), on integrative cartilage repair in vitro.

METHODS

Blocks of calf articular cartilage were harvested, some with the articular surface intact and others without. Some of the latter types of blocks were pretreated with trypsin, and then with bovine serum albumin, SF, PRG4, or HA. Immunolocalization of PRG4 on cartilage surfaces was performed after treatment. Pairs of similarly treated cartilage blocks were incubated in partial apposition for 2 weeks in medium supplemented with serum and (3)H-proline. Following culture, mechanical integration between apposed cartilage blocks was assessed by measuring adhesive strength, and protein biosynthesis and deposition were determined by incorporated (3)H-proline.

RESULTS

Samples with articular surfaces in apposition exhibited little integrative repair compared with samples with cut surfaces in apposition. PRG4 was immunolocalized at the articular cartilage surface, but not in deeper, cut surfaces (without treatment). Cartilage samples treated with trypsin and then with SF or PRG4 exhibited an inhibition of integrative repair and positive immunostaining for PRG4 at treated surfaces compared with normal cut cartilage samples, while samples treated with HA exhibited neither inhibited integrative repair nor PRG4 at the tissue surfaces. Deposition of newly synthesized protein was relatively similar under conditions in which integration differed significantly.

CONCLUSION

These results support the concept that PRG4 in SF, which normally contributes to cartilage lubrication, can inhibit integrative cartilage repair. This has the desirable effect of preventing fusion of apposing surfaces of articulating cartilage, but has the undesirable effect of inhibiting integrative repair.

Improved cartilage integration and interfacial strength after enzymatic treatment in a cartilage transplantation model

The objective of the present study was to investigate whether treatment of articular cartilage with hyaluronidase and collagenase enhances histological and mechanical integration of a cartilage graft into a defect. Discs of 3 mm diameter were taken from 8-mm diameter bovine cartilage explants. Both discs and annulus were either treated for 24 hours with 0.1% hyaluronidase followed by 24 hours with 10 U/ml collagenase or left untreated (controls). Discs and annulus were reassembled and implanted subcutaneously in nude mice for 5 weeks. Integration of disc with surrounding cartilage was assessed histologically and tested biomechanically by performing a push-out test. After 5 weeks a significant increase in viable cell counts was seen in wound edges of the enzyme-treated group as compared with controls. Furthermore, matrix integration (expressed as a percentage of the total interface length that was connected; mean ± standard error) was 83 ± 15% in the treated samples versus 44 ± 40% in the untreated controls. In the enzyme-treated group only, picro-Sirius Red staining revealed collagen crossing the interface perpendicular to the wound surface. Immunohistochemical analyses demonstrated that the interface tissue contained cartilage-specific collagen type II. Collagen type I was found only in a small region of fibrous tissue at the level of the superficial layer, and collagen type III was completely absent in both groups. A significant difference in interfacial strength was found using the push-out test: 1.32 ± 0.15 MPa in the enzyme-treated group versus 0.84 ± 0.14 MPa in the untreated controls. The study shows that enzyme treatment of cartilage wounds increases histological integration and improves biomechanical bonding strength. Enzymatic treatment may represent a promising addition to current techniques for articular cartilage repair.