High-resolution measurements of the multilayer ultra-structure of articular cartilage and their translational potential

Investigating cartilage-related diseases by polarization-resolved second harmonic generation (P-SHG) imaging

Establishing quantitative parameters for differentiating between healthy and diseased cartilage tissues by examining collagen fibril degradation patterns facilitates the understanding of tissue characteristics during disease progression. These findings could also complement existing clinical methods used to diagnose cartilage-related diseases. In this study, cartilage samples from normal, osteoarthritis (OA), and rheumatoid arthritis (RA) tissues were prepared and analyzed using polarization-resolved second harmonic generation (P-SHG) imaging and quantitative image texture analysis. The enhanced molecular contrast obtained from this approach is expected to aid in distinguishing between healthy and diseased cartilage tissues. P-SHG image analysis revealed distinct parameters in the cartilage samples, reflecting variations in collagen fibril arrangement and organization across different pathological states. Normal tissues exhibited distinct χ33/χ31 values compared with those of OA and RA, indicating collagen type transition and cartilage erosion with chondrocyte swelling, respectively. Compared with those of normal tissues, OA samples demonstrated a higher degree of linear polarization, suggesting increased tissue birefringence due to the deposition of type-I collagen in the extracellular matrix. The distribution of the planar orientation of collagen fibrils revealed a more directional orientation in the OA samples, associated with increased type-I collagen, while the RA samples exhibited a heterogeneous molecular orientation. This study revealed that the imaging technique, the quantitative analysis of the images, and the derived parameters presented in this study could be used as a reference for disease diagnostics, providing a clear understanding of collagen fibril degradation in cartilage.

[Current MR imaging of cartilage in the context of knee osteoarthritis (part 1) : Principles and sequences]

Magnetic resonance imaging (MRI) is the clinical method of choice for cartilage imaging in the context of degenerative and nondegenerative joint diseases. The MRI-based definitions of osteoarthritis rely on the detection of osteophytes, cartilage pathologies, bone marrow edema and meniscal lesions but currently a scientific consensus is lacking. In the clinical routine proton density-weighted, fat-suppressed 2D turbo spin echo sequences with echo times of 30-40 ms are predominantly used, which are sufficiently sensitive and specific for the assessment of cartilage. The additionally acquired T1-weighted sequences are primarily used for evaluating other intra-articular and periarticular structures. Diagnostically relevant artifacts include magic angle and chemical shift artifacts, which can lead to artificial signal enhancement in cartilage or incorrect representations of the subchondral lamina and its thickness. Although scientifically validated, high-resolution 3D gradient echo sequences (for cartilage segmentation) and compositional MR sequences (for quantification of physical tissue parameters) are currently reserved for scientific research questions. The future integration of artificial intelligence techniques in areas such as image reconstruction (to reduce scan times while maintaining image quality), image analysis (for automated identification of cartilage defects), and image postprocessing (for automated segmentation of cartilage in terms of volume and thickness) will significantly improve the diagnostic workflow and advance the field further.

In situ self-assembled organoid for osteochondral tissue regeneration with dual functional units

The regeneration of hierarchical osteochondral units is challenging due to difficulties in inducing spatial, directional and controllable differentiation of mesenchymal stem cells (MSCs) into cartilage and bone compartments. Emerging organoid technology offers new opportunities for osteochondral regeneration. In this study, we developed gelatin-based microcryogels customized using hyaluronic acid (HA) and hydroxyapatite (HYP), respectively for inducing cartilage and bone regeneration (denoted as CH-Microcryogels and OS-Microcryogels) through in vivo self-assembly into osteochondral organoids. The customized microcryogels showed good cytocompatibility and induced chondrogenic and osteogenic differentiation of MSCs, while also demonstrating the ability to self-assemble into osteochondral organoids with no delamination in the biphasic cartilage-bone structure. Analysis by mRNA-seq showed that CH-Microcryogels promoted chondrogenic differentiation and inhibited inflammation, while OS-Microcryogels facilitated osteogenic differentiation and suppressed the immune response, by regulating specific signaling pathways. Finally, the in vivo engraftment of pre-differentiated customized microcryogels into canine osteochondral defects resulted in the spontaneous assembly of an osteochondral unit, inducing simultaneous regeneration of both articular cartilage and subchondral bone. In conclusion, this novel approach for generating self-assembling osteochondral organoids utilizing tailor-made microcryogels presents a highly promising avenue for advancing the field of tissue engineering.

Recent development in multizonal scaffolds for osteochondral regeneration

Osteochondral (OC) repair is an extremely challenging topic due to the complex biphasic structure and poor intrinsic regenerative capability of natural osteochondral tissue. In contrast to the current surgical approaches which yield only short-term relief of symptoms, tissue engineering strategy has been shown more promising outcomes in treating OC defects since its emergence in the 1990s. In particular, the use of multizonal scaffolds (MZSs) that mimic the gradient transitions, from cartilage surface to the subchondral bone with either continuous or discontinuous compositions, structures, and properties of natural OC tissue, has been gaining momentum in recent years. Scrutinizing the latest developments in the field, this review offers a comprehensive summary of recent advances, current hurdles, and future perspectives of OC repair, particularly the use of MZSs including bilayered, trilayered, multilayered, and gradient scaffolds, by bringing together onerous demands of architecture designs, material selections, manufacturing techniques as well as the choices of growth factors and cells, each of which possesses its unique challenges and opportunities.

Strategic outline of interventions targeting extracellular matrix for promoting healthy longevity

The extracellular matrix (ECM), composed of interlinked proteins outside of cells, is an important component of the human body that helps maintain tissue architecture and cellular homeostasis. As people age, the ECM undergoes changes that can lead to age-related morbidity and mortality. Despite its importance, ECM aging remains understudied in the field of geroscience. In this review, we discuss the core concepts of ECM integrity, outline the age-related challenges and subsequent pathologies and diseases, summarize diagnostic methods detecting a faulty ECM, and provide strategies targeting ECM homeostasis. To conceptualize this, we built a technology research tree to hierarchically visualize possible research sequences for studying ECM aging. This strategic framework will hopefully facilitate the development of future research on interventions to restore ECM integrity, which could potentially lead to the development of new drugs or therapeutic interventions promoting health during aging.

Integrated gradient tissue-engineered osteochondral scaffolds: Challenges, current efforts and future perspectives

The osteochondral defect repair has been most extensively studied due to the rising demand for new therapies to diseases such as osteoarthritis. Tissue engineering has been proposed as a promising strategy to meet the demand of simultaneous regeneration of both cartilage and subchondral bone by constructing integrated gradient tissue-engineered osteochondral scaffold (IGTEOS). This review brought forward the main challenges of establishing a satisfactory IGTEOS from the perspectives of the complexity of physiology and microenvironment of osteochondral tissue, and the limitations of obtaining the desired and required scaffold. Then, we comprehensively discussed and summarized the current tissue-engineered efforts to resolve the above challenges, including architecture strategies, fabrication techniques and in vitro/in vivo evaluation methods of the IGTEOS. Especially, we highlighted the advantages and limitations of various fabrication techniques of IGTEOS, and common cases of IGTEOS application. Finally, based on the above challenges and current research progress, we analyzed in details the future perspectives of tissue-engineered osteochondral construct, so as to achieve the perfect reconstruction of the cartilaginous and osseous layers of osteochondral tissue simultaneously. This comprehensive and instructive review could provide deep insights into our current understanding of IGTEOS.

Preparation and properties of a photocrosslinked MCl n -doped PDMA-g-PSMA hydrogel

To treat damaged joint areas, photocrosslinked hydrophobically associating PDMA-g-PSMA hydrogels can act as mild and easily regulated materials due to their rich pore structure, which have been widely applied in articular cartilage replacement research. In this study, the effects of ADS-MCl _n (ADS-NaCl, ADS-MgCl₂ and ADS-CaCl₂) doping systems on the micro morphology, mechanical, self-healing, and friction properties and cytotoxicity of PDMA-g-PSMA hydrogels were studied. The results showed that the solubilization behavior of the ADS-MCl _n ionic micelles affected the hydrophobic association stability, thereby changing the toughness, self-healing and friction properties of the hydrogel. Ca²⁺-doping resulted in the crystallization and precipitation of the anionic surfactants, destroying the solubilization ability of the ionic micelles for the hydrophobic units, and thus hydrogels with high hardness, low toughness and no self-healing function were obtained. Doping with Na⁺ greatly improved the dissolving power of the ADS micelles for SMA, yielding PDMA-g-PSMA hydrogels with good mechanical strength and good self-healing ability. However, in this case, a drawback is that the Na⁺-doped system will lose its components during the swelling process, leading to the degradation of its self-healing performance. Interestingly, Mg²⁺ doping resulted in the formation of highly stable ADS micellar aggregates, and then PDMA-g-PSMA hydrogels with a lower friction coefficient (0.023), less wear (35.0 mg), higher elongation at break and 100% self-healing efficiency were obtained. The hydrogel products obtained from the three doping systems all exhibited good biocompatibility. Our research provides important guidelines for the design and preparation of anti-friction artificial articular cartilage.

Recent Advances in "Functional Engineering of Articular Cartilage Zones by Polymeric Biomaterials Mediated with Physical, Mechanical, and Biological/Chemical Cues"

Articular cartilage (AC) plays an unquestionable role in joint movements but unfortunately the healing capacity is restricted due to its avascular and acellular nature. While cartilage tissue engineering has been lifesaving, it is very challenging to remodel the complex cartilage composition and architecture with gradient physio-mechanical properties vital to proper tissue functions. To address these issues, a better understanding of the intrinsic AC properties and how cells respond to stimuli from the external microenvironment must be better understood. This is essential in order to take one step closer to producing functional cartilaginous constructs for clinical use. Recently, biopolymers have aroused much attention due to their versatility, processability, and flexibility because the properties can be tailored to match the requirements of AC. This review highlights polymeric scaffolds developed in the past decade for reconstruction of zonal AC layers including the superficial zone, middle zone, and deep zone by means of exogenous stimuli such as physical, mechanical, and biological/chemical signals. The mimicked properties are reviewed in terms of the biochemical composition and organization, cell fate (morphology, orientation, and differentiation), as well as mechanical properties and finally, the challenges and potential ways to tackle them are discussed.

Quantitative evaluation of very early cartilage damage in experimental osteoarthritis using scanning electron microscopy

OBJECTIVE

To describe parametric changes observed using scanning electron microscopy (SEM) in very early stages in posttraumatic osteoarthritis (OA) models in mice.

METHODS

Mice (5/group) had their knees subjected to anterior cruciate ligament transection (ACLT), ACLT plus meniscectomy (MNCT) or sham surgery, sacrificed after 3, 7 or 14 days, had the articular cartilage evaluated under optical microscopy using Osteoarthritis Research Society International (OARSI) parameters as well as cartilage thickness, roughness, and a damage index using SEM.

RESULTS

Alterations of the cartilage under optical microscopy were not significantly relevant among groups. SEM analysis revealed reduction of femoral and tibial cartilage thickness in ACLT and MNCT groups at 7 and 14 days, with increased cartilage roughness in MNCT group as early as 3 days postsurgery, being sustained up to 14 days. Articular damage index was significantly higher at 14 days post surgery in ACLT and MNCT vs control groups.

CONCLUSION

This is the first demonstration of very early quantitative changes in the cartilage of mice subjected to posttraumatic experimental OA using SEM, revealing increased roughness and thickness as early as 3 days post surgery. These changes may be used as early surrogates for later joint damage in experimental OA.

Characterization of costal cartilage allografts

Background

Human costal cartilage remains widely used in the reconstruction of soft tissues, particularly within the field of plastic and orthopaedic surgery. The biologic expense of using autologous human costal cartilage has become superseded by the increasingly common use of irradiated costal cartilage allografts. To date, there has been no histologic investigation of such costal cartilage allografts. This study aims to characterize the histologic variations that exist between different costal cartilage specimens, and to quantify this between specimens in spite of their common anatomical derivation.

Methods

Twenty‐five specimens of cadaveric human costal cartilage were obtained from Australian Biotechnologies. Each specimen was irradiated, sectioned and stained with Haematoxylin and Eosin, Masson's trichrome and tetrachrome stains. After being analysed under light microscopy, specimen dimensions, chondrocyte counts and mineral content was quantified and measured.

Results

The median specimen diameter was 8.20 mm, with an interquartile range (IQR) of 1.59 mm. The median measurement from the superficial to basal chondrocyte layer was 1409.91 μm (IQR = 885.59 μm), and the median measurement from superficial to calcified zone was 4146.26 μm (IQR 1441.83 μm). The median chondrocyte area was 442.74 μm² (IQR = 2622.72 μm²) with their total chondrocyte count ranging from 289 to 591 chondrocytes per square millimetre. The median percentages of collagen and mineral content were 45.17% and 71.82%, respectively (IQR = 20.48%, 14.75%).

Conclusion

These findings emphasize the histologic and biochemical degree of variation that exists between specimens of human cadaveric costal cartilage on a microscopic level. This has the potential to influence the selection of costal cartilage allografts for reconstructive purposes.

The Role of Inflammasomes in Osteoarthritis and Secondary Joint Degeneration Diseases

Osteoarthritis is age-related and the most common form of arthritis. The main characteristics of the disease are progressive loss of cartilage and secondary synovial inflammation, which finally result in pain, joint stiffness, and functional disability. Similarly, joint degeneration is characteristic of systemic inflammatory diseases such as rheumatoid arthritis and gout, with the associated secondary type of osteoarthritis. Studies suggest that inflammation importantly contributes to the progression of the disease. Particularly, cytokines TNFα and IL-1β drive catabolic signaling in affected joints. IL-1β is a product of inflammasome activation. Inflammasomes are inflammatory multiprotein complexes that propagate inflammation in various autoimmune and autoinflammatory conditions through cell death and the release of inflammatory cytokines and damage-associated molecule patterns. In this article, we review genetic, marker, and animal studies that establish inflammasomes as important drivers of secondary arthritis and discuss the current evidence for inflammasome involvement in primary osteoarthritis. The NLRP3 inflammasome has a significant role in the development of secondary osteoarthritis, and several studies have provided evidence of its role in the development of primary osteoarthritis, while other inflammasomes cannot be excluded. Inflammasome-targeted therapeutic options might thus provide a promising strategy to tackle these debilitating diseases.

New imaging tools for mouse models of osteoarthritis

Osteoarthritis (OA) is a chronic degenerative disease characterized by a disruption of articular joint cartilage homeostasis. Mice are the most commonly used models to study OA. Despite recent reviews, there is still a lack of knowledge about the new development in imaging techniques. Two types of modalities are complementary: those that assess structural changes in joint tissues and those that assess metabolism and disease activity. Micro MRI is the most important imaging tool for OA research. Automated methodologies for assessing periarticular bone morphology with micro-CT have been developed allowing quantitative assessment of tibial surface that may be representative of the whole OA joint changes. Phase-contrast X-ray imaging provides in a single examination a high image precision with good differentiation between all anatomical elements of the knee joint (soft tissue and bone). Positron emission tomography, photoacoustic imaging, and fluorescence reflectance imaging provide molecular and functional data. To conclude, innovative imaging technologies could be an alternative to conventional histology with greater resolution and more efficiency in both morphological analysis and metabolism follow-up. There is a logic of permanent adjustment between innovations, 3R rule, and scientific perspectives. New imaging associated with artificial intelligence may add to human clinical practice allowing not only diagnosis but also prediction of disease progression to personalized medicine.

3D Printed Multiphasic Scaffolds for Osteochondral Repair: Challenges and Opportunities

Osteochondral (OC) defects are debilitating joint injuries characterized by the loss of full thickness articular cartilage along with the underlying calcified cartilage through to the subchondral bone. While current surgical treatments can provide some relief from pain, none can fully repair all the components of the OC unit and restore its native function. Engineering OC tissue is challenging due to the presence of the three distinct tissue regions. Recent advances in additive manufacturing provide unprecedented control over the internal microstructure of bioscaffolds, the patterning of growth factors and the encapsulation of potentially regenerative cells. These developments are ushering in a new paradigm of 'multiphasic' scaffold designs in which the optimal micro-environment for each tissue region is individually crafted. Although the adoption of these techniques provides new opportunities in OC research, it also introduces challenges, such as creating tissue interfaces, integrating multiple fabrication techniques and co-culturing different cells within the same construct. This review captures the considerations and capabilities in developing 3D printed OC scaffolds, including materials, fabrication techniques, mechanical function, biological components and design.

High Resolution and Labeling Free Studying the 3D Microstructure of the Pars Tensa-Annulus Unit of Mice

Hearing loss is a serious illness affecting people’s normal life enormously. The acoustic properties of a tympanic membrane play an important role in hearing, and highly depend on its geometry, composition, microstructure and connection to the surrounding annulus. While the conical geometry of the tympanic membrane is critical to the sound propagation in the auditory system, it presents significant challenges to the study of the 3D microstructure of the tympanic membrane using traditional 2D imaging techniques. To date, most of our knowledge about the 3D microstructure and composition of tympanic membranes is built from 2D microscopic studies, which precludes an accurate understanding of the 3D microstructure, acoustic behaviors and biology of the tissue. Although the tympanic membrane has been reported to contain elastic fibers, the morphological characteristic of the elastic fibers and the spatial arrangement of the elastic fibers with the predominant collagen fibers have not been shown in images. We have developed a 3D imaging technique for the three-dimensional examination of the microstructure of the full thickness of the tympanic membranes in mice without requiring tissue dehydration and stain. We have also used this imaging technique to study the 3D arrangement of the collagen and elastic fibrillar network with the capillaries and cells in the pars tensa-annulus unit at a status close to the native. The most striking findings in the study are the discovery of the 3D form of the elastic and collagen network, and the close spatial relationships between the elastic fibers and the elongated fibroblasts in the tympanic membranes. The 3D imaging technique has enabled to show the 3D waveform contour of the collagen and elastic scaffold in the conical tympanic membrane. Given the close relationship among the acoustic properties, composition, 3D microstructure and geometry of tympanic membranes, the findings may advance the understanding of the structure—acoustic functionality of the tympanic membrane. The knowledge will also be very helpful in the development of advanced cellular therapeutic technologies and 3D printing techniques to restore damaged tympanic membranes to a status close to the native.

Enzymatic Machinery of Ubiquitin and Ubiquitin-Like Modification Systems in Chondrocyte Homeostasis and Osteoarthritis

PURPOSE OF REVIEW

To date, a vast amount of information regarding ubiquitination (Ub) and ubiquitylation-like (Ubl) modification-related mechanisms has been reported in the context of skeletal cell homeostasis and diseases. In this review, we mainly focus on recent findings regarding the contribution of enzymatic machinery that directly adds or removes Ub and Ubl modifications from protein targets in chondrocyte homeostasis and osteoarthritis (OA) development.

RECENT FINDINGS

Mechanisms that promote homeostasis of articular chondrocytes are crucial for maintaining the integrity of articular joints to prevent osteoarthritis development. Articular chondrocytes are postmitotic cells that continuously produce and remodel cartilage matrix. In addition, the long lifespan of chondrocytes makes them susceptible to accumulating cellular damage. Ub and the evolutionarily conserved Ubl modifications, such as SUMOylation, ATGylation, and UFMylation, play important roles in promoting chondrocyte homeostasis, including regulating cell signaling and protein stability, resolving cellular stresses and inflammation, and maintaining differentiation and survival of chondrocytes. Uncovering new components/functions of Ub/Ubl modification machinery may provide novel drug targets to treat OA.

3D-bioprinted gradient-structured scaffold generates anisotropic cartilage with vascularization by pore-size-dependent activation of HIF1α/FAK signaling axis

Articular cartilage injury is one of the most common diseases in orthopedics, which seriously affects patients' life quality, the development of a biomimetic scaffold that mimics the multi-layered gradient structure of native cartilage is a new cartilage repair strategy. It has been shown that scaffold topography affects cell attachment, proliferation, and differentiation; the underlying molecular mechanism of cell-scaffold interaction is still unclear. In the present study, we construct an anisotropic gradient-structured cartilage scaffold by three-dimensional (3D) bioprinting, in which bone marrow stromal cell (BMSC)-laden anisotropic hydrogels micropatterns were used for heterogeneous chondrogenic differentiation and physically gradient synthetic poly (ε-caprolactone) (PCL) to impart mechanical strength. In vitro and in vivo, we demonstrated that gradient-structured cartilage scaffold displayed better cartilage repair effect. The heterogeneous cartilage tissue maturation and blood vessel ingrowth were mediated by a pore-size-dependent mechanism and HIF1α/FAK axis activation. In summary, our results provided a theoretical basis for employing 3D bioprinting gradient-structured constructs for anisotropic cartilage regeneration and revealed HIF1α/FAK axis as a crucial regulator for cell-material interactions, so as to provide a new perspective for cartilage regeneration and repair.

Graphene Oxide-Doped Gellan Gum-PEGDA Bilayered Hydrogel Mimicking the Mechanical and Lubrication Properties of Articular Cartilage

Articular cartilage (AC) is a specialized connective tissue able to provide a low-friction gliding surface supporting shock-absorption, reducing stresses, and guaranteeing wear-resistance thanks to its structure and mechanical and lubrication properties. Being an avascular tissue, AC has a limited ability to heal defects. Nowadays, conventional strategies show several limitations, which results in ineffective restoration of chondral defects. Several tissue engineering approaches have been proposed to restore the AC's native properties without reproducing its mechanical and lubrication properties yet. This work reports the fabrication of a bilayered structure made of gellan gum (GG) and poly (ethylene glycol) diacrylate (PEGDA), able to mimic the mechanical and lubrication features of both AC superficial and deep zones. Through appropriate combinations of GG and PEGDA, cartilage Young's modulus is effectively mimicked for both zones. Graphene oxide is used as a dopant agent for the superficial hydrogel layer, demonstrating a lower friction than the nondoped counterpart. The bilayered hydrogel's antiwear properties are confirmed by using a knee simulator, following ISO 14243. Finally, in vitro tests with human chondrocytes confirm the absence of cytotoxicity effects. The results shown in this paper open the way to a multilayered synthetic injectable or surgically implantable filler for restoring AC defects.

Perlecan in Pericellular Mechanosensory Cell-Matrix Communication, Extracellular Matrix Stabilisation and Mechanoregulation of Load-Bearing Connective Tissues

In this study, we review mechanoregulatory roles for perlecan in load-bearing connective tissues. Perlecan facilitates the co-acervation of tropoelastin and assembly of elastic microfibrils in translamellar cross-bridges which, together with fibrillin and elastin stabilise the extracellular matrix of the intervertebral disc annulus fibrosus. Pericellular perlecan interacts with collagen VI and XI to define and stabilize this matrix compartment which has a strategic position facilitating two-way cell-matrix communication between the cell and its wider extracellular matrix. Cues from the extracellular matrix are fed through this pericellular matrix back to the chondrocyte, allowing it to perceive and respond to subtle microenvironmental changes to regulate tissue homeostasis. Thus perlecan plays a key regulatory role in chondrocyte metabolism, and in chondrocyte differentiation. Perlecan acts as a transport proteoglycan carrying poorly soluble, lipid-modified proteins such as the Wnt or Hedgehog families facilitating the establishment of morphogen gradients that drive tissue morphogenesis. Cell surface perlecan on endothelial cells or osteocytes acts as a flow sensor in blood and the lacunar canalicular fluid providing feedback cues to smooth muscle cells regulating vascular tone and blood pressure, and the regulation of bone metabolism by osteocytes highlighting perlecan's multifaceted roles in load-bearing connective tissues.

Wear Behavior Characterization of Hydrogels Constructs for Cartilage Tissue Replacement

This paper aims to characterize the wear behavior of hydrogel constructs designed for human articular cartilage replacement. To this purpose, poly (ethylene glycol) diacrylate (PEGDA) 10% w/v and gellan gum (GG) 1.5% w/v were used to reproduce the superior (SUP) cartilage layer and PEGDA 15% w/v and GG 1.5% w/v were used to reproduce the deep (DEEP) cartilage layer, with or without graphene oxide (GO). These materials (SUP and DEEP) were analyzed alone and in combination to mimic the zonal architecture of human articular cartilage. The developed constructs were tested using a four-station displacement control knee joint simulator under bovine calf serum. Roughness and micro-computer tomography (µ-CT) measurements evidenced that the hydrogels with 10% w/v of PEGDA showed a worse behavior both in terms of roughness increase and loss of uniformly distributed density than 15% w/v of PEGDA. The simultaneous presence of GO and 15% w/v PEGDA contributed to keeping the hydrogel construct's characteristics. The Raman spectra of the control samples showed the presence of unreacted C=C bonds in all the hydrogels. The degree of crosslinking increased along the series SUP < DEEP + SUP < DEEP without GO. The Raman spectra of the tested hydrogels showed the loss of diacrylate groups in all the samples, due to the washout of unreacted PEGDA in bovine calf serum aqueous environment. The loss decreased along the series SUP > DEEP + SUP > DEEP, further confirming that the degree of photo-crosslinking of the starting materials plays a key role in determining their wear behavior. μ-CT and Raman spectroscopy proved to be suitable techniques to characterize the structure and composition of hydrogels.

Slippery when wet: The free surface of the articular cartilage

The free surface of the articular cartilage must withstand compressive and shearing forces, maintain a low friction coefficient and allow oxygen and metabolites to reach the underlying matrix. In many ways it is critical to the physiology of the whole tissue and its disruption always involves deep pathological alterations and loss of the joint integrity. Being very difficult to image with section-based conventional techniques, it was often described by previous research in conflicting terms or entirely overlooked. High-magnification face-on observations with high resolution scanning electron microscopy and with scanning probe microscopy revealed a very thin, delicate superficial layer rich in glycoconjugates, which may explain the very low friction coefficient of the tissue but which was very easily altered and/or dissolved in the preparation. Beneath this superficial sheet lies a thicker coat of thin, highly uniform, slightly wavy collagen fibrils lying parallel to the surface and mutually interconnected by a huge number of interfibrillar glycosaminoglycan bridges. These bridges and the collagen fibrils form an extended reticular structure able to redistribute tensile and compressive stress across a larger area of the surface and hence a greater volume of tissue.

3D bioprinting dual-factor releasing and gradient-structured constructs ready to implant for anisotropic cartilage regeneration

Cartilage injury is extremely common and leads to joint dysfunction. Existing joint prostheses do not remodel with host joint tissue. However, developing large-scale biomimetic anisotropic constructs mimicking native cartilage with structural integrity is challenging. In the present study, we describe anisotropic cartilage regeneration by three-dimensional (3D) bioprinting dual-factor releasing and gradient-structured constructs. Dual-factor releasing mesenchymal stem cell (MSC)-laden hydrogels were used for anisotropic chondrogenic differentiation. Together with physically gradient synthetic biodegradable polymers that impart mechanical strength, the 3D bioprinted anisotropic cartilage constructs demonstrated whole-layer integrity, lubrication of superficial layers, and nutrient supply in deep layers. Evaluation of the cartilage tissue in vitro and in vivo showed tissue maturation and organization that may be sufficient for translation to patients. In conclusion, one-step 3D bioprinted dual-factor releasing and gradient-structured constructs were generated for anisotropic cartilage regeneration, integrating the feasibility of MSC- and 3D bioprinting-based therapy for injured or degenerative joints.

The synovial surface of the articular cartilage

The articular cartilage has been the subject of a huge amount of research carried out with a wide array of different techniques. Most of the existing morphological and ultrastructural data on this tissue, however, were obtained either by light microscopy or by transmission electron microscopy. Both techniques rely on thin sections and neither allows a direct, face-on visualization of the free cartilage surface (synovial surface), which is the only portion subject to frictional as well as compressive forces. In the present research, high resolution visualization by scanning electron microscopy and by atomic force microscopy revealed that the collagen fibrils of the articular surface are exclusively represented by thin, uniform, parallel fibrils evocative of the heterotypic type IX-type II fibrils reported by other authors, immersed in an abundant matrix of glycoconjugates, in part regularly arranged in phase with the D-period of collagen. Electrophoresis of fluorophore-labeled saccharides confirmed that the superficial and the deeper layers are quite different in their glycoconjugate content as well, the deeper ones containing more sulfated, more acidic small proteoglycans bound to thicker, more heterogeneous collagen fibrils. The differences found between the synovial surface and the deeper layers are consistent with the different mechanical stresses they must withstand.

Polarized reflectance from articular cartilage depends upon superficial zone collagen network microstructure

Polarized reflectance from articular cartilage involves light scattering dependent on surface features, sub-surface optical properties, and collagen birefringence. To understand how surface roughness, zonal collagen microstructure, and chondrocyte organization contribute to polarized reflectance signals, experiments were conducted on bovine cartilage explants and osteochondral cores to compare polarized reflectance texture with split lines and relate these signals to cartilage zonal features and chondrocyte distribution. Texture parameter sensitivity to articular surface damage was determined from polarized reflectance maps and optimized to detect surface damage. Results indicate that polarized reflectance texture predominantly derives from the superficial zone collagen network, while the parameter average value also depends on surface roughness and total cartilage thickness.

Propagation of microcracks in collagen networks of cartilage under mechanical loads

OBJECTIVE

We recently demonstrated that low-energy mechanical impact to articular cartilage, usually considered non-injurious, can in fact cause microscale cracks (widths <30μm) in the collagen network of visually pristine human cartilage. While research on macro-scale cracks in cartilage and microcracks in bone abounds, how microcracks within cartilage initiate and propagate remains unknown. We quantified the extent to which microcracks initiate and propagate in the collagen network during mechanical loading representative of normal activities.

DESIGN

We tested 76 full-thickness, cylindrical osteochondral plugs. We imaged untreated specimens (pristine phase) via second harmonic generation and assigned specimens to three low-energy impact groups (none, low, high), and thereafter to three cyclic compression groups (none, low, high) which simulate walking. We re-imaged specimens in the post-impact and post-cyclic compression phases to identify and track microcracks.

RESULTS

Microcracks in the network of collagen did not present in untreated controls but did initiate and propagate under mechanical treatments. We found that the length and width of microcracks increased from post-impact to post-cyclic compression in tracked microcracks, but neither depth nor angle presented statistically significant differences.

CONCLUSIONS

The microcracks we initiated under low-energy impact loading increased in length and width during subsequent cyclic compression that simulated walking. The extent of this propagation depended on the combination of impact and cyclic compression. More broadly, the initiation and propagation of microcracks may characterize pathogenesis of osteoarthritis, and may suggest therapeutic targets for future studies.

Slope-based segmentation of articular cartilage using polarization-sensitive optical coherence tomography phase retardation image

A segmentation method based on phase retardation measurements from polarization-sensitive optical coherence tomography (PS-OCT) is developed to differentiate the structural zones of articular cartilage. The organization of collagen matrix in articular cartilage varies over the different structural zones, generating different tissue birefringence. Analyzing the slope of the accumulated phase retardation at different depths can detect the variation in tissue birefringence and be used to segment the structural zones. The method is validated on phantoms composed of layers of different materials. Articular cartilage samples from adult swine are segmented with the method. The characteristics in each segmented zone are also examined by histology and high-resolution second-harmonic generation imaging, showing distinctive properties that match with the anatomical structure of articular cartilage. The segmentation algorithm is also applied on PS-OCT images acquired at multiple illumination angles, where the angular dependence of tissue birefringence in the deep zone is detected. This method offers a noninvasive imaging approach to differentiating the structural zones of articular cartilage, as well as a quantification approach based on the phase retardation measurements of PS-OCT. This method has great potential in studying depth-related progression of cartilage degeneration.

Pre-Clinical Translation of Second Harmonic Microscopy of Meniscal and Articular Cartilage Using a Prototype Nonlinear Microendoscope

Previous studies using nonlinear microscopy have demonstrated that osteoarthritis (OA) is characterized by the gradual replacement of Type II collagen with Type I collagen. The objective of this study was to develop a prototype nonlinear laser scanning microendoscope capable of resolving the structural differences of collagen in various orthopaedically relevant cartilaginous surfaces. The current prototype developed a miniaturized femtosecond laser scanning instrument, mounted on an articulated positioning system, capable of both conventional arthroscopy and second-harmonic laser-scanning microscopy. Its optical system includes a multi-resolution optical system using a gradient index objective lens and a customized multi-purpose fiber optic sheath to maximize the collection of backscattered photons or provide joint capsule illumination. The stability and suitability of the prototype arthroscope to approach and image cartilage were evaluated through preliminary testing on fresh, minimally processed, and partially intact porcine knee joints. Image quality was sufficient to distinguish between hyaline cartilage and fibrocartilage through unique Type I and Type II collagen-specific characteristics. Imaging the meniscus revealed that the system was able to visualize differences in the collagen arrangement between the superficial and lamellar layers. Such detailed in vivo imaging of the cartilage surfaces could obviate the need to perform biopsies for ex vivo histological analysis in the future, and provide an alternative to conventional external imaging to characterize and diagnose progressive and degenerative cartilage diseases such as OA. Moreover, this system is readily customizable and may provide a suitable and modular platform for developing additional tools utilizing femtosecond lasers for tissue cutting within the familiar confines of two or three portal arthroscopy techniques.

Effect of intra-articular administration of superparamagnetic iron oxide nanoparticles (SPIONs) for MRI assessment of the cartilage barrier in a large animal model

Early diagnosis of cartilage disease at a time when changes are limited to depletion of extracellular matrix components represents an important diagnostic target to reduce patient morbidity. This report is to present proof of concept for nanoparticle dependent cartilage barrier imaging in a large animal model including the use of clinical magnetic resonance imaging (MRI). Conditioned (following matrix depletion) and unconditioned porcine metacarpophalangeal cartilage was evaluated on the basis of fluorophore conjugated 30 nm and 80 nm spherical gold nanoparticle permeation and multiphoton laser scanning and bright field microscopy after autometallographic particle enhancement. Consequently, conditioned and unconditioned joints underwent MRI pre- and post-injection with 12 nm superparamagnetic iron oxide nanoparticles (SPIONs) to evaluate particle permeation in the context of matrix depletion and use of a clinical 1.5 Tesla MRI scanner. To gauge the potential pro-inflammatory effect of intra-articular nanoparticle delivery co-cultures of equine synovium and cartilage tissue were exposed to an escalating dose of SPIONs and IL-6, IL-10, IFN-γ and PGE₂ were assessed in culture media. The chemotactic potential of growth media samples was subsequently assessed in transwell migration assays on isolated equine neutrophils. Results demonstrate an increase in MRI signal following conditioning of porcine joints which suggests that nanoparticle dependent compositional cartilage imaging is feasible. Tissue culture and neutrophil migration assays highlight a dose dependent inflammatory response following SPION exposure which at the imaging dose investigated was not different from controls. The preliminary safety and imaging data support the continued investigation of nanoparticle dependent compositional cartilage imaging. To our knowledge, this is the first report in using SPIONs as intra-articular MRI contrast agent for studying cartilage barrier function, which could potentially lead to a new diagnostic technique for early detection of cartilage disease.

Assessment of doxorubicin-induced mouse testicular damage by the novel second-harmonic generation microscopy

Microtubules, maintaining a non-linear structure, are suitable for direct observation in living mammalian by second-harmonic imaging microscopy (SHIM) (a new kind of confocal microscopies). Testes constituted by vast seminiferous microtubules (SM), serve as good candidates for visualization by SHIM. This study employs the SHIM and Western-blot (WB) to assess the cellular-molecular levels of doxorubicin (Dox)-induced mouse testicular damage. The SHIM examination was able to clearly identify the integrity of normal architecture of the living mouse testis, namely, the anatomical features of SM, smooth muscle wall of SM, manchette microtubules, exoplasmic microtubules in Sertoli cells and interstitial connective tissue, as well as the destructive feature of SM in Dox-treated mice (n = 6 per group). By day 21 after Dox-treatment, the testicular weight and testicular length were significantly progressively decreased as Dox dosage was stepwise increased, i.e., 0/5/10/15/20 mg/kg/body-weight (BW) (all p<0.0001). The cross-section area of SM was significantly lower in Dox-treated (15 mg/kg-BW) mice than that in controls (p<0.001). The protein expression of vimentin was significantly progressively increased whereas the protein expression of β-tubulin/androgen-receptor was significantly progressively decreased in stepwise increased Dox dosage (all p<0.001). The protein expressions of inflammatory (MMP-9/IL-1β/TNF-α/iNOX), oxidative-stress (NOX-1/NOX-2/NOX-4/oxidized protein), apoptotic (mitochondrial-Bax/cleaved-caspase-3/PARP), fibrotic (Smad3/TGF-ß) mitochondrial/DNA-damaged (cytosolic cytochrome-C/γ-H2AX/ATM/KU70), and cell apoptotic/death (PTEN/p53) biomarkers were significantly higher in Dox-treated (15 mg/kg-BW) group than those in controls (all p<0.001). In conlusion, the dose-dependent Dox-caused mouse testicular damage can be not only detected by WB in molecular level but also clearly identified by SHIM in living mice.

MRI magic-angle effect in femorotibial cartilages of the red kangaroo

OBJECTIVE

Kangaroo knee cartilages are robust tissues that can support knee flexion and endure high levels of compressive stress. This study aimed to develop a detailed understanding of the collagen architecture in kangaroo knee cartilages and thus obtain insights into the biophysical basis of their function.

DESIGN

Cylindrical/square plugs from femoral and tibial hyaline cartilage and tibial fibrocartilage were excised from the knees of three adult red kangaroos. Multi-slice, multi-echo MR images were acquired at the sample orientations 0° and 55° ("magic angle") with respect to the static magnetic field. Maps of the transverse relaxation rate constant (R₂) and depth profiles of R₂ and its anisotropic component (R₂^A) were constructed from the data.

RESULTS

The R₂^A profiles confirmed the classic three-zone organisation of all cartilage samples. Femoral hyaline cartilage possessed a well-developed, thick superficial zone. Tibial hyaline cartilage possessed a very thick radial zone (80% relative thickness) that exhibited large R₂^A values consistent with highly ordered collagen. The R₂^A profile of tibial fibrocartilage exhibited a unique region near the bone (bottom 5-10%) consistent with elevated proteoglycan content ("attachment sub-zone").

CONCLUSIONS

Our observations suggest that the well-developed superficial zone of femoral hyaline cartilage is suitable for supporting knee flexion; the thick and well-aligned radial zone of tibial hyaline cartilage is adapted to endure high compressive stress; while the innermost part of the radial zone of tibial fibrocartilage may facilitate anchoring of the collagen fibres to withstand high shear deformation. These findings may inspire new designs for cartilage tissue engineering.

Three dimensional microstructural network of elastin, collagen, and cells in Achilles tendons

Similar to most biological tissues, the biomechanical, and functional characteristics of the Achilles tendon are closely related to its composition and microstructure. It is commonly reported that type I collagen is the predominant component of tendons and is mainly responsible for the tissue's function. Although elastin has been found in varying proportions in other connective tissues, previous studies report that tendons contain very small quantities of elastin. However, the morphology and the microstructural relationship among the elastic fibres, collagen, and cells in tendon tissue have not been well examined. We hypothesize the elastic fibres, as another fibrillar component in the extracellular matrix, have a unique role in mechanical function and microstructural arrangement in Achilles tendons. It has been shown that elastic fibres present a close connection with the tenocytes. The close relationship of the three components has been revealed as a distinct, integrated and complex microstructural network. Notably, a "spiral" structure within fibril bundles in Achilles tendons was observed in some samples in specialized regions. This study substantiates the hierarchical system of the spatial microstructure of tendon, including the mapping of collagen, elastin and tenocytes, with 3-dimensional confocal images. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1203-1214, 2017.

High-resolution study of the 3D collagen fibrillary matrix of Achilles tendons without tissue labelling and dehydrating

Knowledge of the collagen structure of an Achilles tendon is critical to comprehend the physiology, biomechanics, homeostasis and remodelling of the tissue. Despite intensive studies, there are still uncertainties regarding the microstructure. The majority of studies have examined the longitudinally arranged collagen fibrils as they are primarily attributed to the principal tensile strength of the tendon. Few studies have considered the structural integrity of the entire three-dimensional (3D) collagen meshwork, and how the longitudinal collagen fibrils are integrated as a strong unit in a 3D domain to provide the tendons with the essential tensile properties. Using second harmonic generation imaging, a 3D imaging technique was developed and used to study the 3D collagen matrix in the midportion of Achilles tendons without tissue labelling and dehydration. Therefore, the 3D collagen structure is presented in a condition closely representative of the in vivo status. Atomic force microscopy studies have confirmed that second harmonic generation reveals the internal collagen matrix of tendons in 3D at a fibril level. Achilles tendons primarily contain longitudinal collagen fibrils that braid spatially into a dense rope-like collagen meshwork and are encapsulated or wound tightly by the oblique collagen fibrils emanating from the epitenon region. The arrangement of the collagen fibrils provides the longitudinal fibrils with essential structural integrity and endows the tendon with the unique mechanical function for withstanding tensile stresses. A novel 3D microscopic method has been developed to examine the 3D collagen microstructure of tendons without tissue dehydrating and labelling. The study also provides new knowledge about the collagen microstructure in an Achilles tendon, which enables understanding of the function of the tissue. The knowledge may be important for applying surgical and tissue engineering techniques to tendon reconstruction.

Multimodal nonlinear optical microscopy reveals critical role of kinesin-1 in cartilage development

We developed a multimodal nonlinear optical (NLO) microscope system by integrating stimulated Raman scattering (SRS), second harmonic generation (SHG) and two-photon excited fluorescence (TPEF) imaging. The system was used to study the morphological and biochemical characteristics of tibial cartilage in a kinesin-1 (Kif5b) knockout mouse model. The detailed structure of fibrillar collagen in the extracellular matrix of cartilage was visualized by the forward and backward SHG signals, while high resolution imaging of chondrocytes was achieved by capturing endogenous TPEF and SRS signals of the cells. The results demonstrate that collagen fibrils in the superficial surface of the articular cartilage decreased significantly in the absence of Kif5b. The distorted morphology along with accumulated intracellular collagen was observed in the Kif5b-deficient chondrocytes, indicating the critical roles of kinesin-1 in the chondrocyte morphogenesis and collagen secretion. The study shows that multimodal NLO imaging method is an effective approach to investigate early development of cartilage.

Cyclic loading of human articular cartilage: The transition from compaction to fatigue

Osteoarthritis and articular cartilage injuries are common conditions in human joints and a frequent cause of pain and disability. Unfortunately, cartilage is avascular and has limited capabilities for self-repair. Despite the societal impact, there is little information on the dynamic process of cartilage degeneration. We performed a series of cyclic unconfined compression tests motivated by in vivo loading conditions and designed to generate mechanical fatigue. We examined the functional (both stress-stretch and creep) responses of the tissue after recovery from a specified number of loading cycles, as well as histology and second harmonic generation microscopy images. The effect of compaction was complimented by the effect of fatigue in our unconfined compression tests. A three-way, repeated-measures mixed model ANOVA showed significant differences between loading with a physiologically relevant low magnitude, and two more severe loading magnitudes, in terms of the resulting specimen stiffness, time to equilibrium and thickness. There was a statistically significant effect of loading frequency on a specimen's time to equilibrium and significant interaction of force and frequency on specimen thickness and time to equilibrium. Increasing the number of loading cycles significantly impacted a specimen's effective stiffness and resulting thickness. We attribute permanent loss of mechanical function under cyclic loading to rearrangement and disruption of the collagen network and resulting proteoglycan (PG) aggregation, as seen in histological and second harmonic generation images, as a result of induced mechanical fatigue.

A micro-architecturally biomimetic collagen template for mesenchymal condensation based cartilage regeneration

The unique arcade-like orientation of collagen fibers enables cartilage to bear mechanical loads. In this study continuous-length aligned collagen threads were woven to emulate the interdigitated arcade structure of the cartilage. The weaving pattern provided a macropore network within which micromass cell pellets were seeded to take advantage of mesenchymal condensation driven chondrogenesis. Compression tests showed that the baseline scaffold had a modulus of 0.83±0.39MPa at a porosity of 80%. The modulus of pellet seeded scaffolds increased by 60% to 1.33±0.37MPa after 28days of culture, converging to the modulus of the native cartilage. The scaffolds displayed duress under displacement controlled low-cycle fatigue at 15% strain amplitude such that load reduction stabilized at 8% after 4500 cycles of loading. The woven structure demonstrated a substantial elastic recoil where 40% mechanical strain was close to completely recovered following unloading. A robust chondrogenesis was observed as evidenced by positive staining for GAGs and type II collagen and aggrecan. Dimethyl methylene blue and sircol assays showed GAGs and collagen productions to increase from 3.36±1.24 and 31.46±3.22 at day 3 to 56.61±12.12 and 136.70±12.29μg/μg of DNA at day 28 of culture. This woven collagen scaffold holds a significant potential for cartilage regeneration with shorter in vitro culture periods due to functionally sufficient mechanical robustness at the baseline. In conclusion, the mimicry of cartilage's arcade architecture resulted in substantial improvement of mechanical function while enabling one of the first pellet delivery platforms enabled by a macroporous network.

STATEMENT OF SIGNIFICANCE

Mesenchymal condensation is critical for driving chondrogenesis, making high density cell seeding a standard in cartilage tissue engineering. Efforts to date have utilized scaffold free delivery of MSCs in pellet form. This study developed a macroporous scaffold that is fabricated by weaving highly aligned collagen threads. The scaffold can deliver high density cell condensates while providing mechanical stiffness comparable to that of cartilage. The scaffold also mimicked the arcade-like orientation of collagen fibers in cartilage. A highly robust chondrogenesis was observed in this mesenchymal cell pellet delivery system. Baseline mechanical robustness of this scaffold system will enable delivery of cell pellets as early as three days.

Conventional and novel imaging modalities in osteoarthritis: current state of the evidence

PURPOSE OF REVIEW

Imaging modalities are currently an inseparable part of osteoarthritis diagnosis. In this review, we describe the current state of evidence regarding conventional and novel imaging modalities in evaluation of osteoarthritis. Modalities including radiography (qualitative and semi-quantitative assessments), ultrasonography, computed tomography [CT; conventional multidetector CT (MDCT), cone-beam CT (CBCT) and four-dimensional CT (4DCT)], MRI (MRI; semi-quantitative, quantitative and compositional) and PET and their applications are reviewed.

RECENT FINDINGS

Radiography is the modality of choice for initial assessment of osteoarthritis. However, due to its low sensitivity and specificity, numerous recent investigations have proposed MRI as a powerful addition to detect and grade osteoarthritis features, which are not apparent in radiography. Semi-quantitative MRI measurements are feasible to perform in routine clinical practice. Quantitative and compositional MRI measurements have extended the amount of information an MRI examination can provide regarding the three-dimensional shape and tissue composition of articular cartilage. 4DCT and CBCT are introduced as imaging examinations that may reveal biomechanical cartilage abnormalities in osteoarthritis joint by dynamic and weight-bearing evaluations, respectively. Recent PET studies may unveil the underlying metabolic activities that can be associated with osteoarthritis.

SUMMARY

In addition to the established role of radiographs, MRI is the advanced modality of choice for detection and quantification of various osteoarthritis features. 4DCT and CBCT may have specified applications when diagnosis of underlying motion abnormality or dynamic changes in weight-bearing situation is suspected. Future studies should elucidate the specific clinical applications of ultrasonography and PET.

Nonlinear optical microscopy of early stage (ICRS Grade-I) osteoarthritic human cartilage

In a synovial joint, the articular cartilage is directly affected during the progression of Osteoarthritis (OA). The characterization of early stage modification in extra-cellular matrix of cartilage is essential for detection as well as understanding the progression of disease. The objective of this study is to demonstrate the potential and capability of nonlinear optical microscopy for the morphological investigation of early stage osteoarthritic cartilage. ICRS Grade-I cartilage sections were obtained from the femoral condyle of the human knee. The surface of articular cartilage was imaged by second harmonic generation and two-photon excited fluorescence microscopy. Novel morphological features like microsplits and wrinkles were observed, which would otherwise not be visible in other clinical imaging modalities (e.g., CT, MRI, ultrasound and arthroscope. The presence of superficial layer with distinct collagen fibrils parallel to the articular surface in 4 specimens out of 14 specimens, indicates that different phases of OA within ICRS Grade-I can be detected by SHG microscopy. All together, the observed novel morphologies in early stage osteoarthritic cartilage indicates that SHG microscopy might be a significant tool for the assessment of cartilage disorder.

Characterization of the collagen component of cartilage repair tissue of the talus with quantitative MRI: comparison of T2 relaxation time measurements with a diffusion-weighted double-echo steady-state sequence (dwDESS)

OBJECTIVES

The purpose of this study was to characterize the collagen component of repair tissue (RT) of the talus after autologous matrix-induced chondrogenesis (AMIC) using quantitative T2 and diffusion-weighted imaging.

METHODS

Mean T2 values and diffusion coefficients of AMIC-RT and normal cartilage of the talus of 25 patients with posttraumatic osteochondral lesions and AMIC repair were compared in a cross-sectional design using partially spoiled steady-state free precession (pSSFP) for T2 quantification, and diffusion-weighted double-echo steady-state (dwDESS) for diffusion measurement. RT and cartilage were graded with modified Noyes and MOCART scores on morphological sequences. An association between follow-up interval and quantitative MRI measures was assessed using multivariate regression, after stratifying the cohort according to time interval between surgery and MRI.

RESULTS

Mean T2 of the AMIC-RT and cartilage were 43.1 ms and 39.1 ms, respectively (p = 0.26). Mean diffusivity of the RT (1.76 μm(2)/ms) was significantly higher compared to normal cartilage (1.46 μm(2)/ms) (p = 0.0092). No correlation was found between morphological and quantitative parameters. RT diffusivity was lowest in the subgroup with follow-up >28 months (p = 0.027).

CONCLUSIONS

Compared to T2-mapping, dwDESS demonstrated greater sensitivity in detecting differences in the collagen matrix between AMIC-RT and cartilage. Decreased diffusivity in patients with longer follow-up times may indicate an increased matrix organization of RT.

KEY POINTS

• MRI is used to assess morphology of the repair tissue during follow-up. • Quantitative MRI allows an estimation of biochemical properties of the repair tissue. • Differences between repair tissue and cartilage were more significant with dwDESS than T2 mapping.