Diffusion coefficients of articular cartilage for different CT and MRI contrast agents

Evaluation of a lanthanide nanoparticle-based contrast agent for microcomputed tomography of porous channels in subchondral bone

Osteoarthritis (OA) is a chronic joint disease that causes disability and pain. The osteochondral interface is a gradient tissue region that plays a significant role in maintaining joint health. It has been shown that during OA, increased neoangiogenesis creates porous channels at the osteochondral interface allowing the transport of molecules related to OA. Importantly, the connection between these porous channels and the early stages of OA development is still not fully understood. Microcomputed tomography (microCT) offers the ability to image the porous channels at the osteochondral interface, however, a contrast agent is necessary to delineate the different X-ray attenuations of the tissues. In this study BaYbF₅ -SiO₂ nanoparticles are synthesized and optimized as a microCT contrast agent to obtain an appropriate contrast attenuation for subsequent segmentation of structures of interest, that is, porous channels, and mouse subchondral bone. For this purpose, BaYbF₅ nanoparticles were synthesized and coated with a biocompatible silica shell (SiO₂ ). The optimized BaYbF₅ -SiO₂ 27 nm nanoparticles exhibited the highest average microCT attenuation among the biocompatible nanoparticles tested. The BaYbF₅ -SiO₂ 27 nm nanoparticles increased the mean X-ray attenuation of structures of interest, for example, porous channel models and mouse subchondral bone. The BaYbF₅ -SiO₂ contrast attenuation was steady after diffusion into mouse subchondral bone. In this study, we obtained for the first time, the average microCT attenuation of the BaYbF₅ -SiO₂ nanoparticles into porous channel models and mouse subchondral bone. In conclusion, BaYbF₅ -SiO₂ nanoparticles are a potential contrast agent for imaging porous channels at the osteochondral interface using microCT.

Contrast solution properties and scan parameters influence the apparent diffusivity of computed tomography contrast agents in articular cartilage

The inability to detect early degenerative changes to the articular cartilage surface that commonly precede bulk osteoarthritic degradation is an obstacle to early disease detection for research or clinical diagnosis. Leveraging a known artefact that blurs tissue boundaries in clinical arthrograms, contrast agent (CA) diffusivity can be derived from computed tomography arthrography (CTa) scans. We combined experimental and computational approaches to study protocol variations that may alter the CTa-derived apparent diffusivity. In experimental studies on bovine cartilage explants, we examined how CA dilution and transport direction (absorption versus desorption) influence the apparent diffusivity of untreated and enzymatically digested cartilage. Using multiphysics simulations, we examined mechanisms underlying experimental observations and the effects of image resolution, scan interval and early scan termination. The apparent diffusivity during absorption decreased with increasing CA concentration by an amount similar to the increase induced by tissue digestion. Models indicated that osmotically-induced fluid efflux strongly contributed to the concentration effect. Simulated changes to spatial resolution, scan spacing and total scan time all influenced the apparent diffusivity, indicating the importance of consistent protocols. With careful control of imaging protocols and interpretations guided by transport models, CTa-derived diffusivity offers promise as a biomarker for early degenerative changes.

Optical Coherence Elastography as a Tool for Studying Deformations in Biomaterials: Spatially-Resolved Osmotic Strain Dynamics in Cartilaginous Samples

This paper presents a recently developed variant of phase-resolved Optical Coherence Elastography (OCE) enabling non-contact visualization of transient local strains of various origins in biological tissues and other materials. In this work, we demonstrate the possibilities of this new technique for studying dynamics of osmotically-induced strains in cartilaginous tissue impregnated with optical clearing agents (OCA). For poroelastic water-containing biological tissues, application of non-isotonic OCAs, various contrast additives, as well as drug solutions administration, may excite transient spatially-inhomogeneous strain fields of high magnitude in the tissue bulk, initiating mechanical and structural alterations. The range of the strain reliably observed by OCE varied from ±10-3 to ±0.4 for diluted and pure glycerol, correspondingly. The OCE-technique used made it possible to reveal previously inaccessible details of the complex spatio-temporal evolution of alternating-sign osmotic strains at the initial stages of agent diffusion. Qualitatively different effects produced by particular hydrophilic OCAs, such as glycerol and iohexol, are discussed, as well as concentration-dependent differences. Overall, the work demonstrates the unique abilities of the new OCE-modality in providing a deeper insight in real-time kinetics of osmotically-induced strains relevant to a broad range of biomedical applications.

Validation of watershed-based segmentation of the cartilage surface from sequential CT arthrography scans

BACKGROUND

This study investigated the utility of a 2-dimensional watershed algorithm for identifying the cartilage surface in computed tomography (CT) arthrograms of the knee up to 33 minutes after an intra-articular iohexol injection as boundary blurring increased.

METHODS

A 2D watershed algorithm was applied to CT arthrograms of 3 bovine stifle joints taken 3, 8, 18, and 33 minutes after iohexol injection and used to segment tibial cartilage. Thickness measurements were compared to a reference standard thickness measurement and the 3-minute time point scan.

RESULTS

77.2% of cartilage thickness measurements were within 0.2 mm (1 voxel) of the thickness calculated in the reference scan at the 3-minute time point. 42% fewer voxels could be segmented from the 33-minute scan than the 3-minute scan due to diffusion of the contrast agent out of the joint space and into the cartilage, leading to blurring of the cartilage boundary. The traced watershed lines were closer to the location of the cartilage surface in areas where tissues were in direct contact with each other (cartilage-cartilage or cartilage-meniscus contact).

CONCLUSIONS

The use of watershed dam lines to guide cartilage segmentation shows promise for identifying cartilage boundaries from CT arthrograms in areas where soft tissues are in direct contact with each other.

Anionic Contrast-Enhanced MicroCT Imaging Correlates with Biochemical and Histological Evaluations of Osteoarthritic Articular Cartilage

This study addressed difficulties in evaluating osteoarthritis (OA) progression in species with thin cartilage. Feasibility of using short, nonequilibrium contrast-enhanced micro-computed tomography (CE-μCT) to evaluate the physical and biochemical properties of cartilage was investigated. A preliminary in vitro study using CE-μCT study was performed using bovine osteochondral blocks with intact, mildly damaged (fibrillated), or severely damaged (delaminated) cartilage. Delamination of the superficial zone resulted in elevated apparent density compared with intact cartilage after 10 minutes of anionic contrast exposure (P < 0.01). OA was induced by unilateral meniscal destabilization in n = 20 sheep divided into: early phase OA (n = 9) and late phase OA (n = 11), while n = 4 remained as naive controls. In vivo anionic nonequilibrium contrast CT of the operated stifle was conducted in the early phase sheep 13 weeks postoperatively using clinical resolution CT. Cartilage visibility in the contrasted leg was significantly improved compared with the noncontrasted contralateral stifle (P < 0.05). Animals were sacrificed at 3 months (early phase) or 12 months (late phase) for additional ex vivo CE-μCT, and correlative tests with biochemical and histological measures. Concentration of sulfated glycosaminoglycan (sGAG) significantly varied between control, early, and late phase OA (P < 0.005) and showed a negative (r = -0.56) relationship with apparent density in the medial tibial plateau (R2 = 0.28, P < 0.001). Histologically, parameters in proteoglycan and cartilage surface structure correlated with increasing attenuation. While previous studies have shown that CE-CT increases the apparent density of proteoglycan-depleted cartilage, we concluded that superficial zone disruption also contributes to this phenomenon.

Molecular Signaling Interactions and Transport at the Osteochondral Interface: A Review

Articular joints are comprised of different tissues, including cartilage and bone, with distinctive structural and mechanical properties. Joint homeostasis depends on mechanical and biological integrity of these components and signaling exchanges between them. Chondrocytes and osteocytes actively sense, integrate, and convert mechanical forces into biochemical signals in cartilage and bone, respectively. The osteochondral interface between the bone and cartilage allows these tissues to communicate with each other and exchange signaling and nutritional molecules, and by that ensure an integrated response to mechanical stimuli. It is currently not well known how molecules are transported between these tissues. Measuring molecular transport in vivo is highly desirable for tracking cartilage degeneration and osteoarthritis progression. Since transport of contrast agents, which are used for joint imaging, also depend on diffusion through the cartilage extracellular matrix, contrast agent enhanced imaging may provide a high resolution, non-invasive method for investigating molecular transport in the osteochondral unit. Only a few techniques have been developed to track molecular transport at the osteochondral interface, and there appear opportunities for development in this field. This review will describe current knowledge of the molecular interactions and transport in the osteochondral interface and discuss the potential of using contrast agents for investigating molecular transport and structural changes of the joint.

Contrast enhanced computed tomography for real-time quantification of glycosaminoglycans in cartilage tissue engineered constructs

Tissue engineering and regenerative medicine are two therapeutic strategies to treat, and to potentially cure, diseases affecting cartilaginous tissues, such as osteoarthritis and cartilage defects. Insights into the processes occurring during regeneration are essential to steer and inform development of the envisaged regenerative strategy, however tools are needed for longitudinal and quantitative monitoring of cartilage matrix components. In this study, we introduce a contrast-enhanced computed tomography (CECT)-based method using a cationic iodinated contrast agent (CA4+) for longitudinal quantification of glycosaminoglycans (GAG) in cartilage-engineered constructs. CA4+ concentration and scanning protocols were first optimized to ensure no cytotoxicity and a facile procedure with minimal radiation dose. Chondrocyte and mesenchymal stem cell pellets, containing different GAG content were generated and exposed to CA4+. The CA4+ content in the pellets, as determined by micro computed tomography, was plotted against GAG content, as measured by 1,9-dimethylmethylene blue analysis, and showed a high linear correlation. The established equation was used for longitudinal measurements of GAG content over 28 days of pellet culture. Importantly, this method did not adversely affect cell viability or chondrogenesis. Additionally, the CA4+ distribution accurately matched safranin-O staining on histological sections. Hence, we show proof-of-concept for the application of CECT, utilizing a positively charged contrast agent, for longitudinal and quantitative imaging of GAG distribution in cartilage tissue-engineered constructs. STATEMENT OF SIGNIFICANCE: Tissue engineering and regenerative medicine are promising therapeutic strategies for different joint pathologies such as cartilage defects or osteoarthritis. Currently, in vitro assessment on the quality and composition of the engineered cartilage mainly relies on destructive methods. Therefore, there is a need for the development of techniques that allow for longitudinal and quantitative imaging and monitoring of cartilage-engineered constructs. This work harnesses the electrostatic interactions between the negatively-charged glycosaminoglycans (GAGs) and a positively-charged contrast agent for longitudinal and non-destructive quantification of GAGs, providing valuable insight on GAG development and distribution in cartilage engineered constructs. Such technique can advance the development of regenerative strategies, not only by allowing continuous monitoring but also by serving as a pre-implantation screening tool.

Diffusion of neutral solutes within human osteoarthritic cartilage: Effect of loading patterns

Objective

Variation of the solute diffusion within articular cartilage is an important feature of osteoarthritis (OA) progression. For in vitro study of monitoring of the diffusion process, it is essential to simulate physiological conditions as much as possible. Our objective was to investigate the effects of loading patterns on diffusion processes of neutral solutes within osteoarthritic cartilage.

Methods

Osteochondral plugs were harvested from human tibial plateaus and separated into three OA stages according to modified Mankin scoring system. The samples were subjected to static or cyclic compression using a carefully designed loading device. Contrast-enhanced micro-computed tomography (CEμCT) was applied to acquire image sequences while the cartilage was being compressed. The apparent diffusion maps and diffusion coefficients were analysed, as well as histological and stereological assessments of the plugs.

Results

The diffusion of neutral solutes was significantly affected by the loading patterns. For OA cartilage with early and middle stages, cyclic loading accelerated contrast agent infiltration compared with static loading. However, for late-stage OA samples, no acceleration of diffusion was observed in the first 2 ​h because of the insufficient resilience of compressed cartilage. The accumulation of neutral solutes in an upward invasive fissure also suggested that solutes could penetrate into the fissure under cyclic loading.

Conclusions

To our knowledge, this is the first study to combine the cyclic compression and CEμCT scanning in the diffusion testing of human OA cartilage. This loading pattern could simulate the physiological conditions and reduce the time to reach solute equilibrium within cartilage. The diffusion data may contribute to joint drug-injection therapies for early OA.

The translational potential of this article

The combination of cyclic loading and CEμCT scanning enabled diffusion analysis of osteoarthritic cartilage under different compressions. A comprehensive evaluation of OA cartilage and subchondral bone may benefit from this technique. The diffusion data provide theoretical support and reference for intra-articular injection of drugs.

Molecular transport in articular cartilage - what have we learned from the past 50 years?

Developing therapeutic molecules that target chondrocytes and locally produced inflammatory factors within arthritic cartilage is an active area of investigation. The extensive studies that have been conducted over the past 50 years have enabled the accurate prediction and reliable optimization of the transport of a wide variety of molecules into cartilage. In this Review, the factors that can be used to tune the transport kinetics of therapeutics are summarized. Overall, the most crucial factor when designing new therapeutic molecules is solute size. The diffusivity and partition coefficient of a solute both decrease with increasing solute size as indicated by molecular mass or by hydrodynamic radius. Surprisingly, despite having an effective pore size of ~6 nm, molecules of ~16 nm radius can diffuse through the cartilage matrix. Alteration of the shape or charge of a solute and the application of physiological loading to cartilage can be used to predictably improve solute transport kinetics, and this knowledge can be used to improve the development of therapeutic agents for osteoarthritis that target the cartilage.

Control of optical transparency and infrared laser heating of costal cartilage via injection of iohexol

Infrared (IR) laser impact has no analogues for rapid and safe cartilage reshaping. For better penetration of radiation optical clearing agents (OCAs) can be applied. In present work, the effect of low-osmolality agent iohexol on costal cartilage is studied. Specifically, it is shown that ½ of total increase of optical transparency occurs in 20 minutes of immersion. Maximally, cartilage transparency on 1560 nm can be increased in 1.5 times. Injection of iohexol results in increased tissue hygroscopicity, lower drying rate and higher percentage of bound water. Effective diffusion coefficients of water liberation at 21°C are (5.3 ± 0.4) × 10^-7 and (3.3 ± 0.1) × 10^-7 cm² /s for untreated and iohexol-modified tissue, respectively. Raman spectroscopy of irradiated iohexol solution reveals its photo and thermo-stability under clinically used IR laser energies up to 350 W/cm² for exposure times of several seconds. At energies higher than 500 W/cm² [Correction added on 5 September 2018, after first online publication: This unit has been changed] decomposition of iohexol occurs rapidly through formation of molecular iodine and fluorescent residue.

Early Fracture Healing is Delayed in the Col1a2+/G610C Osteogenesis Imperfecta Murine Model

Osteogenesis imperfecta (OI) is a rare heritable skeletal dysplasia mainly caused by type I collagen abnormalities and characterized by bone fragility and susceptibility to fracture. Over 85% of the patients carry dominant mutations in the genes encoding for the collagen type I α1 and α2 chains. Failure of bone union and/or presence of hyperplastic callus formation after fracture were described in OI patients. Here we used the Col1a2^+/G610C mouse, carrying in heterozygosis the α2(I)-G610C substitution, to investigate the healing process of an OI bone. Tibiae of 2-month-old Col1a2^+/G610C and wild-type littermates were fractured and the healing process was followed at 2, 3, and 5 weeks after injury from fibrous cartilaginous tissue formation to its bone replacement by radiography, micro-computed tomography (µCT), histological and biochemical approaches. In presence of similar fracture types, in Col1a2^+/G610C mice an impairment in the early phase of bone repair was detected compared to wild-type littermates. Smaller callus area, callus bone surface, and bone volume associated to higher percentage of cartilage and lower percentage of bone were evident in Col1a2^+/G610C at 2 weeks post fracture (wpf) and no change by 3 wpf. Furthermore, the biochemical analysis of collagen extracted from callus 2 wpf revealed in mutants an increased amount of type II collagen, typical of cartilage, with respect to type I, characteristic of bone. This is the first report of a delay in OI bone fracture repair at the modeling phase.

How can 50 years of solute transport data in articular cartilage inform the design of arthritis therapeutics?

OBJECTIVE

For the last half century, transport of nutrients and therapeutics in articular cartilage has been studied with various in vitro systems that attempt to model in vivo conditions. However, experimental technique, tissue species, and tissue storage condition (fresh/frozen) vary widely and there is debate on the most appropriate model system. Additionally, there is still no clear overarching framework with which to predict solute transport properties based on molecular characteristics. This review aims to develop such a framework, and to assess whether experimental procedure affects trends in transport data.

METHODS

Solute data from 31 published papers that investigated transport in healthy articular cartilage were obtained and analyzed for trends.

RESULTS

Here, we show that diffusivity of spherical and globular solutes in cartilage can be predicted by molecular weight (MW) and hydrodynamic radius via a power-law relationship. This relationship is robust for many solutes, spanning 5 orders of magnitude in MW and was not affected by variations in cartilage species, age, condition (fresh/frozen), and experimental technique. Traditional models of transport in porous media exhibited mixed effectiveness at predicting diffusivity in cartilage, but were good in predicting solute partition coefficient.

CONCLUSION

Ultimately, these robust relationships can be used to accurately predict and improve transport of solutes in adult human cartilage and enable the development of better optimized arthritis therapeutics.

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.

The Effect of Antibody Size and Mechanical Loading on Solute Diffusion Through the Articular Surface of Cartilage

Because of the heterogeneous nature of articular cartilage tissue, penetration of potential therapeutic molecules for osteoarthritis (OA) through the articular surface (AS) is complex, with many factors that affect transport of these solutes within the tissue. Therefore, the goal of this study is to investigate how the size of antibody (Ab) variants, as well as application of cyclic mechanical loading, affects solute transport within healthy cartilage tissue. Penetration of fluorescently tagged solutes was quantified using confocal microscopy. For all the solutes tested, fluorescence curves were obtained through the articular surface. On average, diffusivities for the solutes of sizes 200 kDa, 150 kDa, 50 kDa, and 25 kDa were 3.3, 3.4, 5.1, and 6.0 μm2/s from 0 to 100 μm from the articular surface. Diffusivities went up to a maximum of 16.5, 18.5, 20.5, and 23.4 μm2/s for the 200 kDa, 150 kDa, 50 kDa, and 25 kDa molecules, respectively, from 225 to 325 μm from the surface. Overall, the effect of loading was very significant, with maximal transport enhancement for each solute ranging from 2.2 to 3.4-fold near 275 μm. Ultimately, solutes of this size do not diffuse uniformly nor are convected uniformly, through the depth of the cartilage tissue. This research potentially holds great clinical significance to discover ways of further optimizing transport into cartilage and leads to effective antibody-based treatments for OA.

Biomaterials and Tissue Biomechanics: A Match Made in Heaven?

Biomaterials and tissue biomechanics have been traditionally separate areas of research with relatively little overlap in terms of methodological approaches. Recent advances in both fields on the one hand and developments in fabrication techniques and design approaches on the other have prepared the ground for joint research efforts by both communities. Additive manufacturing and rational design are examples of the revolutionary fabrication techniques and design methodologies that could facilitate more intimate collaboration between biomaterial scientists and biomechanists. This editorial article highlights the various ways in which the research on tissue biomechanics and biomaterials are related to each other and could benefit from each other’s results and methodologies.

Contrast-enhanced μCT of the intervertebral disc: A comparison of anionic and cationic contrast agents for biochemical and morphological characterization

The objective of this study was to quantify and compare the contrast-enhancing properties of the anionic contrast agent ioxaglate/Hexabrix, and cationic contrast agent CA⁴⁺ for biochemical and morphological characterization of the intervertebral disc (IVD) via μCT. Optimal contrast agent concentrations were determined by incubating rat lumbar IVDs in dilutions of Hexabrix-320 (20%, 30%, 40%, and 50%) and CA⁴⁺ (10, 20, 30, and 40 mg I/ml). μCT imaging was performed at 70 kVp, 114 μA, and 250 ms integration time, 12 μm voxel size. The kinetics of contrast enhancement were quantified with cumulative incubations for 0.5, 1, 2, 12, 16, 20, and 24 h using both agents. Agreement in morphological quantification was assessed via serial scans of the same IVDs. Correlation of attenuation to glycosaminoglycan (GAG) content was determined by enzymatic digestion of IVDs, subsequent μCT imaging, and GAG quantification via dimethylmethylene blue assay. Forty percent Hexabrix and 30 mg I/ml CA⁴⁺ were chosen as optimal concentrations. Hexabrix enabled greater delineation of the IVD from surrounding tissues, and CA⁴⁺ had the lowest uptake in surrounding soft tissue. Twenty-four hour incubation was sufficient for >99% equilibration of both agents. A high level of agreement was observed in the quantification of IVD volume (ICC = 0.951, r = 0.997) and height (ICC = 0.947, r = 0.991). Both agents exhibited strong linear correlations between μCT attenuation and GAG content (Hexabrix: r = -0.940; CA⁴⁺ : r = 0.887). Both agents enable biochemical and morphological quantification of the IVD via contrast-enhanced μCT and are effective tools for preclinical characterization. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1067-1075, 2017.

An Experimental and Finite Element Protocol to Investigate the Transport of Neutral and Charged Solutes across Articular Cartilage

Osteoarthritis (OA) is a debilitating disease that is associated with degeneration of articular cartilage and subchondral bone. Degeneration of articular cartilage impairs its load-bearing function substantially as it experiences tremendous chemical degradation, i.e. proteoglycan loss and collagen fibril disruption. One promising way to investigate chemical damage mechanisms during OA is to expose the cartilage specimens to an external solute and monitor the diffusion of the molecules. The degree of cartilage damage (i.e. concentration and configuration of essential macromolecules) is associated with collisional energy loss of external solutes while moving across articular cartilage creates different diffusion characteristics compared to healthy cartilage. In this study, we introduce a protocol, which consists of several steps and is based on previously developed experimental micro-Computed Tomography (micro-CT) and finite element modeling. The transport of charged and uncharged iodinated molecules is first recorded using micro-CT, which is followed by applying biphasic-solute and multiphasic finite element models to obtain diffusion coefficients and fixed charge densities across cartilage zones.

Topographical and depth-dependent glycosaminoglycan concentration in canine medial tibial cartilage 3 weeks after anterior cruciate ligament transection surgery-a microscopic imaging study

BACKGROUND

Medical imaging has become an invaluable tool to diagnose damage to cartilage. Depletion of glycosaminoglycans (GAG) has been shown to be one of the early signs of cartilage degradation. In order to investigate the topographical changes in GAG concentration caused by the anterior cruciate ligament transection (ACLT) surgery in a canine model, microscopic magnetic resonance imaging (µMRI) and microscopic computed tomography (µCT) were used to measure the GAG concentration with correlation from a biochemical assay, inductively coupled plasma optical emission spectroscopy (ICP-OES), to understand where the topographical and depth-dependent changes in the GAG concentration occur.

METHODS

This study used eight knee joints from four canines, which were examined 3 weeks after ACLT surgery. From right (n=3) and left (n=1) medial tibias of the ACLT and the contralateral side, two ex vivo specimens from each of four locations (interior, central, exterior and posterior) were imaged before and after equilibration in contrast agents. The cartilage blocks imaged using µMRI were approximately 3 mm × 5 mm and were imaged before and after eight hours submersion in a gadolinium (Gd) contrast agent with an in-plane pixel resolution of 17.6 µm² and an image slice thickness of 1 mm. The cartilage blocks imaged using µCT were approximately 2 mm × 1 mm and were imaged before and after 24 hours submersed in ioxaglate with an isotropic voxel resolution of 13.4 µm³. ICP-OES was used to quantify the bulk GAG at each topographical location.

RESULTS

The pre-contrast µMRI and µCT results did not demonstrate significant differences in GAG between the ACLT and contralateral cartilage at all topographical locations. The post-contrast µMRI and µCT results demonstrated topographically similar significant differences in GAG concentrations between the ACLT and contralateral tibia. Using µMRI, the GAG concentrations (mg/mL) were measured for the ACLT and contralateral respectively, the exterior (54.0±3.6; 70.4±4.3; P=0.001) and interior (54.9±5.9; 71.0±5.9; P=0.029) demonstrated significant differences, but not for the central (61.0±12.0; 67.4±7.2; P=0.438) or posterior (61.6±6.3; 70.3±4.4; P=0.097) locations. Using µCT, the GAG concentrations (mg/mL) were measured for the ACLT and contralateral respectively, the exterior (68.8±0.4; 87.7±4.1; P=0.023) and interior (60.5±9.1; 82.6±8.7; P=0.039) demonstrated significant differences, but not for the central (53.5±5.5; 59.1±25.6; P=0.684) or posterior (52.3±6.2; 61.5±12.7; P=0.325) locations. The depth-dependent GAG (mg/mL) profiles showed significant differences in µMRI for the transitional zone (TZ) [exterior (28.1±4.7; 47.0±8.6; P=0.01) and interior (32.6±4.8; 43.8±8.7; P=0.025)], radial zone (RZ) 1 [exterior (49.6±4.8; 71.5±5.8; P=0.001) and interior (49.4±7.4; 66.7±6.8; P=0.041)], and RZ 2 [exterior (74.9±4.7; 91.8±2.9; P=0.001) and interior (77.1±6.0; 94.8±4.5; P=0.015)], and in µCT for the superficial zone (SZ) [interior (20.6±1.2; 40.4±5.4; P=0.004)], TZ [exterior (45.6±12.0; 61.8±0.5; P=0.049) and interior (36.3±11.7; 60.8±2.0; P=0.019)], and RZ 1 [exterior (61.1±4.1; 85.3±5.6; P=0.039) and interior (53.9±4.9; 78.0±5.1; P=0.041)] for the ACLT and contralateral, respectively. ICP-OES measured significant differences in GAG were found for the exterior (42.1±19.6; 65.3±16.2; P=0.017), central (43.4±4.4; 65.3±10.6; P=0.0111), and interior (46.8±5.6; 61.7±7.3; P=0.0445) but not for the posterior (52.6±12.1; 59.0±2.6; P=0.9252) medial tibia locations compared for the ACLT and contralateral, respectively.

CONCLUSIONS

The detection and correlation between the three techniques show a topographic depth-dependency on the initial GAG loss in injured cartilage. This topographic and high resolution investigation of ACLT cartilage demonstrated the potential of using µMRI and µCT to study and help diagnose cartilage with very early stages of osteoarthritis.

Cationic Contrast Agent Diffusion Differs Between Cartilage and Meniscus

Contrast enhanced computed tomography (CECT) is a non-destructive imaging technique used for the assessment of composition and structure of articular cartilage and meniscus. Due to structural and compositional differences between these tissues, diffusion and distribution of contrast agents may differ in cartilage and meniscus. The aim of this study is to determine the diffusion kinematics of a novel iodine based cationic contrast agent (CA(2+)) in cartilage and meniscus. Cylindrical cartilage and meniscus samples (d = 6 mm, h ≈ 2 mm) were harvested from healthy bovine knee joints (n = 10), immersed in isotonic cationic contrast agent (20 mgI/mL), and imaged using a micro-CT scanner at 26 time points up to 48 h. Subsequently, normalized X-ray attenuation and contrast agent diffusion flux, as well as water, collagen and proteoglycan (PG) contents in the tissues were determined. The contrast agent distributions within cartilage and meniscus were different. In addition, the normalized attenuation and diffusion flux were higher (p < 0.05) in cartilage. Based on these results, diffusion kinematics vary between cartilage and meniscus. These tissue specific variations can affect the interpretation of CECT images and should be considered when cartilage and meniscus are assessed simultaneously.

Transport of Iodine Is Different in Cartilage and Meniscus

Contrast enhanced computed tomography (CECT) has been proposed for diagnostics of cartilage and meniscus injuries and degeneration. As both tissues may be imaged simultaneously, CECT could provide a method for comprehensive evaluation of knee joint health. Since the composition and structure of cartilage and meniscus are different, we hypothesize that transport characteristics of anionic contrast agents also differ between the tissues. This would affect interpretation of CECT images and warrants investigation. To clarify this, we aimed to determine the transport kinematics of anionic iodine (q = -1, M = 126.9 g/mol), assumed to not be significantly affected by the steric hindrance, thus providing faster transport than large molecule contrast agents (e.g., ioxaglate). Cylindrical samples (d = 6 mm, h = 2 mm) were prepared from healthy bovine (n = 10) patella and meniscus, immersed in isotonic phosphate-buffered NaI solution (20 mgI/mL), and subsequently imaged with a micro-CT at 20 time points up to 23 h. Subsequently, normalized attenuation and contrast agent flux, as well as water, collagen, and proteoglycan (PG) contents in the tissues were determined. Normalized attenuation at equilibrium was higher (p = 0.005) in meniscus. Contrast agent flux was lower (p = 0.005) in the meniscus at 10 min, but higher (p < 0.05) between 30 and 120 min. In both tissues, contrast agent distribution at equilibrium suggested an inverse agreement with the depth-wise PG distribution. In conclusion, iodine transport into cartilage and meniscus was different, especially between the first 2 hours after the immersion. This is an important finding which should be considered during simultaneous CECT of cartilage and meniscus.

Molecular transport in collagenous tissues measured by gel electrophoresis

Molecular transport in tissues is important for drug delivery, nutrient supply, waste removal, cell signaling, and detecting tissue degeneration. Therefore, the objective of this study was to investigate gel electrophoresis as a simple method to measure molecular transport in collagenous tissues. The electrophoretic mobility of charged molecules in tissue samples was measured from relative differences in the velocity of a cationic dye passing through an agarose gel in the absence and presence of a tissue section embedded within the gel. Differences in electrophoretic mobility were measured for the transport of a molecule through different tissues and tissue anisotropy, or the transport of different sized molecules through the same tissue. Tissue samples included tendon and fibrocartilage from the proximal (tensile) and distal (compressive) regions of the bovine flexor tendon, respectively, and bovine articular cartilage. The measured electrophoretic mobility was greatest in the compressive region of the tendon (fibrocartilage), followed by the tensile region of tendon, and lowest in articular cartilage, reflecting differences in the composition and organization of the tissues. The anisotropy of tendon was measured by greater electrophoretic mobility parallel compared with perpendicular to the predominate collagen fiber orientation. Electrophoretic mobility also decreased with increased molecular size, as expected. Therefore, the results of this study suggest that gel electrophoresis may be a useful method to measure differences in molecular transport within various tissues, including the effects of tissue type, tissue anisotropy, and molecular size.

An ultrasmall and metabolizable PEGylated NaGdF4:Dy nanoprobe for high-performance T(1)/T(2)-weighted MR and CT multimodal imaging

Lanthanide-based multimodal probes with high sensitivity, simple synthesis strategy, and good biocompatibility promise new applications for clinical diagnosis. However, today's challenge is not only to develop high-performance multimodal probes for more accurate and reliable diagnosis, but also to understand the fate of these probes in vivo. In this context, a novel PEGylated Dy-doped NaGdF4 nanoprobe (PEG-NaGdF4:Dy) was designed and fabricated as a T1/T2-weighted MRI/CT imaging agent. This nanoprobe has a distinct longitudinal relaxivity (r1 = 5.17 mM(-1) s(-1)), relatively high transverse relaxivity (r2 = 10.64 mM(-1) s(-1)), and exhibits strong X-ray attenuation properties (44.70 HU L g(-1)) in vitro. Furthermore, T1/T2-weighted MRI/CT imaging in vivo confirmed that this PEG-NaGdF4:Dy nanoprobe could lead to a significant contrast enhancement effect on liver, spleen and kidney at 24 h post injection. The MTT assay, histological analysis, and biodistribution investigation demonstrated that this multifunctional nanoprobe possessed relatively low cytotoxicity, negligible tissue damage and could be completely excreted out of the body of mice as time prolonged. Therefore, the present PEG-NaGdF4:Dy nanoprobe has the potential for the development of multifunctional T1/T2-weighted MRI/CT imaging to provide more comprehensive and accurate diagnosis information.

Diffusion of MRI and CT contrast agents in articular cartilage under static compression

Cartilage has a limited capacity for self-repair and focal damage can eventually lead to complete degradation of the tissue. Early diagnosis of degenerative changes in cartilage is therefore essential. Contrast agent-based computed tomography and magnetic resonance imaging provide promising tools for this purpose. However, the common assumption in clinical applications that contrast agents reach steady-state distributions within the tissue has been of questionable validity. Characterization of nonequilibrium diffusion of contrast agents rather than their equilibrium distributions may therefore be more effective for image-based cartilage assessment. Transport of contrast agent through the extracellular matrix of cartilage can be affected by tissue compression due to matrix structural and compositional changes including reduced pore size and fluid content. We therefore investigate the effects of static compression on diffusion of three common contrast agents: sodium iodide, sodium diatrizoate, and gadolinium diethylenetriamine-pentaacid (Gd-DTPA). Results showed that static compression was associated with significant decreases in diffusivities for sodium iodide and Gd-DTPA, with similar (but not significant) trends for sodium diatrizoate. Molecular mass of contrast agents affected diffusivities as the smallest one tested, sodium iodide, showed higher diffusivity than sodium diatrizoate and Gd-DTPA. Compression-associated cartilage matrix alterations such as glycosaminoglycan and fluid contents were found to correspond with variations in contrast agent diffusivities. Although decreased diffusivity was significantly correlated with increasing glycosaminoglycan content for sodium iodide and Gd-DTPA only, diffusivity significantly increased for all contrast agents by increasing fluid fraction. Because compounds based on iodine and gadolinium are commonly used for computed tomography and magnetic resonance imaging, present findings can be valuable for more accurate image-based assessment of variations in cartilage composition associated with focal injuries.

Contrast enhanced imaging of human meniscus using cone beam CT

OBJECTIVE

Meniscal injuries can lead to mechanical overloading of articular cartilage and eventually to knee osteoarthritis. The objective was to evaluate the potential of contrast enhanced computed tomography (CECT) to image contrast agent (CA) diffusion in human menisci with a clinical cone beam CT scanner.

DESIGN

Isolated human menisci (n = 26) were imaged using magnetic resonance imaging (MRI) and CECT in situ. Diffusion of anionic CA into the meniscus was imaged for up to 30 h. The results of CECT were compared with water, collagen and proteoglycan (PG) contents, biomechanical properties, age and histological and MR images of the samples.

RESULTS

Diffusion of CA required over 25 h to reach equilibrium. The CA partition (the CA concentration in the tissue divided by that in the bath) at the 40 min time point correlated significantly with that at the 30 h time point in both lateral (r = 0.706, P = 0.007) and medial (r = 0.669, P = 0.012) menisci. Furthermore, CA partition in meniscus after 30 h of diffusion agreed qualitatively with the distribution of PGs.

CONCLUSION

The cross-sectional distribution of CA was consistent with that reported in a previous μCT study on bovine meniscus. The time required to reach diffusion equilibrium was found impractical for clinical applications. However, based on the present results, shorter delay between injection and imaging (e.g., 40 min) could be feasible in clinical diagnostics of meniscal pathologies.

Transport of Neutral Solute Across Articular Cartilage: The Role of Zonal Diffusivities

Transport of solutes through diffusion is an important metabolic mechanism for the avascular cartilage tissue. Three types of interconnected physical phenomena, namely mechanical, electrical, and chemical, are all involved in the physics of transport in cartilage. In this study, we use a carefully designed experimental-computational setup to separate the effects of mechanical and chemical factors from those of electrical charges. Axial diffusion of a neutral solute (Iodixanol) into cartilage was monitored using calibrated microcomputed tomography (micro-CT) images for up to 48 hr. A biphasic-solute computational model was fitted to the experimental data to determine the diffusion coefficients of cartilage. Cartilage was modeled either using one single diffusion coefficient (single-zone model) or using three diffusion coefficients corresponding to superficial, middle, and deep cartilage zones (multizone model). It was observed that the single-zone model cannot capture the entire concentration-time curve and under-predicts the near-equilibrium concentration values, whereas the multizone model could very well match the experimental data. The diffusion coefficient of the superficial zone was found to be at least one order of magnitude larger than that of the middle zone. Since neutral solutes were used, glycosaminoglycan (GAG) content cannot be the primary reason behind such large differences between the diffusion coefficients of the different cartilage zones. It is therefore concluded that other features of the different cartilage zones such as water content and the organization (orientation) of collagen fibers may be enough to cause large differences in diffusion coefficients through the cartilage thickness.

Micro-poromechanics model of fluid-saturated chemically active fibrous media

We have developed a micromechanics based model for chemically active saturated fibrous media that incorporates fiber network microstructure, chemical potential driven fluid flow, and micro-poromechanics. The stress-strain relationship of the dry fibrous media is first obtained by considering the fiber behavior. The constitutive relationships applicable to saturated media are then derived in the poromechanics framework using Hill's volume averaging. The advantage of this approach is that the resultant continuum model accounts for the discrete nature of the individual fibers while retaining a form suitable for porous materials. As a result, the model is able to predict the influence of micro-scale phenomena, such as the fiber pre-strain caused by osmotic effects and evolution of fiber network structure with loading, on the overall behavior and in particular, on the poromechanics parameters. Additionally, the model can describe fluid-flow related rate-dependent behavior under confined and unconfined conditions and varying chemical environments. The significance of the approach is demonstrated by simulating unconfined drained monotonic uniaxial compression under different surrounding fluid bath molarity, and fluid-flow related creep and relaxation at different loading-levels and different surrounding fluid bath molarity. The model predictions conform to the experimental observations for saturated soft fibrous materials. The method can potentially be extended to other porous materials such as bone, clays, foams and concrete.

Effect of mechanical convection on the partitioning of an anionic iodinated contrast agent in intact patellar cartilage

To determine if mechanical convection accelerates partitioning of an anionic contrast agent into cartilage while maintaining its ability to reflect the glycosaminoglycan (GAG) content in contrast-enhanced computed tomography (CECT) of cartilage. Bovine patellae (N = 4) were immersed in iothalamate and serially imaged over 24 h of passive diffusion at 34°C. Following saline washing for 14 h, each patella was serially imaged over 2.5 h of mechanical convection by cyclic compressive loading (120N, 1 Hz) while immersed in iothalamate at 34°C. After similar saline washing, each patella was sectioned into 15 blocks (n = 60) and contrast concentration per time point as well as GAG content were determined for each cartilage block. Mechanical convection produced 70.6%, 34.4%, and 16.4% higher contrast concentration at 30, 60, and 90 min, respectively, compared to passive diffusion (p < 0.001) and boosted initial contrast flux 330%. The correlation between contrast concentration and GAG content was significant at all time points and correlation coefficients improved with time, reaching R(2)  = 0.60 after 180 min of passive diffusion and 22.5 min of mechanical convection. Mechanical convection significantly accelerated partitioning of a contrast agent into healthy cartilage while maintaining strong correlations with GAG content, providing an evidence-based rationale for adopting walking regimens in CECT imaging protocols.

Tantalum oxide nanoparticles for the imaging of articular cartilage using X-ray computed tomography: visualization of ex vivo/in vivo murine tibia and ex vivo human index finger cartilage

The synthesis and characterization of tantalum oxide (Ta2O5) nanoparticles (NPs) as new X-ray contrast media for microcomputed tomography (μCT) imaging of articular cartilage are reported. NPs, approximately 5-10 nm in size, and possessing distinct surface charges, were synthesized using phosphonate (neutral), ammonium (cationic), and carboxylate (anionic) ligands as end functional groups. Assessment of a cartilage defect in a human cadaver distal metacarpophalangeal (MCP) joint with the ammonium nanoparticles showed good visualization of damage and preferential uptake in areas surrounding the defect. Finally, an optimized nontoxic cationic NP contrast agent was evaluated in an in vivo murine model and the cartilage was imaged. These nanoparticles represent a new type of contrast agent for imaging articular cartilage, and the results demonstrate the importance of surface charge in the design of nanoparticulate agents for targeting the surface or interior zones of articular cartilage.

Matrix fixed charge density modulates exudate concentration during cartilage compression

Electrolyte filtration arises due to the presence of fixed charges in cartilage extracellular matrix glycosaminoglycans (GAGs). Commonly assumed negligible, it can be important for design and interpretation of streaming potential measurements and modeling assumptions. To quantify the scale of this phenomenon, chloride ion concentration in exudate of compressed cartilage was measured by Mohr's titration and explant GAG content was colorimetrically assayed. Pilot studies indicated that an appropriate strain rate for experiments was 8 × 10(-3) s(-1) to eliminate concerns of exudate evaporation and explant damage (at low and high strain rates, respectively). Exudate chloride concentration of explants equilibrated in 1× PBS was significantly (p < 0.05) lower than the bath chloride concentration at strains of 37.5, 50, and 62.5%, with clear dependence on strain magnitude. Exudate chloride concentration was also significantly lower than that of the bath when 50% strain was applied after equilibration in 0.5, 1, and 2× PBS, with a trend for an increase in this relative difference with decreasing bath concentration (p = 0.065 between 0.5 and 2× PBS). Decreasing exudate chloride concentration correlated negatively with increasing postcompression GAG concentration. No difference between exudate chloride concentration and bath chloride concentration was ever observed for compression of uncharged agarose gel controls. Findings show that exudate from compressed cartilage is dilute relative to the bath due to the presence of matrix fixed charges, and this difference can generate diffusion potentials external to the explant, which may affect streaming potential measurements particularly under conditions of low strain rates and high strains.

Diffusion of ionic and non-ionic contrast agents in articular cartilage with increased cross-linking--contribution of steric and electrostatic effects

OBJECTIVE

To investigate the effect of threose-induced collagen cross-linking on diffusion of ionic and non-ionic contrast agents in articular cartilage.

DESIGN

Osteochondral plugs (Ø=6mm) were prepared from bovine patellae and divided into two groups according to the contrast agent to be used in contrast enhanced computed tomography (CECT) imaging: (I) anionic ioxaglate and (II) non-ionic iodixanol. The groups I and II contained 7 and 6 sample pairs, respectively. One of the paired samples served as a reference while the other was treated with threose to induce collagen cross-linking. The equilibrium partitioning of the contrast agents was imaged after 24h of immersion. Fixed charge density (FCD), water content, contents of proteoglycans, total collagen, hydroxylysyl pyridinoline (HP), lysyl pyridinoline (LP) and pentosidine (Pent) cross-links were determined as a reference.

RESULTS

The equilibrium partitioning of ioxaglate (group I) was significantly (p=0.018) lower (-23.4%) in threose-treated than control samples while the equilibrium partitioning of iodixanol (group II) was unaffected by the threose-treatment. FCD in the middle and deep zones of the cartilage (p<0.05) and contents of Pent and LP (p=0.001) increased significantly due to the treatment. However, the proteoglycan concentration was not systematically altered after the treatment. Water content was significantly (-3.5%, p=0.007) lower after the treatment.

CONCLUSIONS

Since non-ionic iodixanol showed no changes in partition after cross-linking, in contrast to anionic ioxaglate, we conclude that the cross-linking induced changes in charge distribution have greater effect on diffusion compared to the cross-linking induced changes in steric hindrance.

A review of the combination of experimental measurements and fibril-reinforced modeling for investigation of articular cartilage and chondrocyte response to loading

The function of articular cartilage depends on its structure and composition, sensitively impaired in disease (e.g. osteoarthritis, OA). Responses of chondrocytes to tissue loading are modulated by the structure. Altered cell responses as an effect of OA may regulate cartilage mechanotransduction and cell biosynthesis. To be able to evaluate cell responses and factors affecting the onset and progression of OA, local tissue and cell stresses and strains in cartilage need to be characterized. This is extremely challenging with the presently available experimental techniques and therefore computational modeling is required. Modern models of articular cartilage are inhomogeneous and anisotropic, and they include many aspects of the real tissue structure and composition. In this paper, we provide an overview of the computational applications that have been developed for modeling the mechanics of articular cartilage at the tissue and cellular level. We concentrate on the use of fibril-reinforced models of cartilage. Furthermore, we introduce practical considerations for modeling applications, including also experimental tests that can be combined with the modeling approach. At the end, we discuss the prospects for patient-specific models when aiming to use finite element modeling analysis and evaluation of articular cartilage function, cellular responses, failure points, OA progression, and rehabilitation.

Bath Concentration of Anionic Contrast Agents Does Not Affect Their Diffusion and Distribution in Articular Cartilage In Vitro

OBJECTIVE

Differences in contrast agent diffusion reflect changes in composition and structure of articular cartilage. However, in clinical application the contrast agent concentration in the joint capsule varies, which may affect the reliability of contrast enhanced cartilage tomography (CECT). In the present study, effects of concentration of x-ray contrast agents on their diffusion and equilibrium distribution in cartilage were investigated.

DESIGN

Full-thickness cartilage discs (d = 4.0 mm, n = 120) were detached from bovine patellae (n = 24). The diffusion of various concentrations of ioxaglate (5, 10, 21, 50 mM) and iodide (30, 60, 126, 300 mM) was allowed only through the articular surface. Samples were imaged with a clinical peripheral quantitative computed tomography scanner before immersion in contrast agent, and after 1, 5, 9, 16, 25, and 29 hours in the bath.

RESULTS

Diffusion and partition coefficients were similar between different contrast agent concentrations. The diffusion coefficient of iodide (473 ± 133 µm(2)/s) was greater (P ≤ 0.001) than that of ioxaglate (92 ± 46 µm(2)/s). In full-thickness cartilage, the partition coefficient (at 29 h) of iodide (71 ± 5%) was greater (P ≤ 0.02 with most concentrations) than that of ioxaglate (62 ± 6%).

CONCLUSIONS

Significant differences in partition and diffusion coefficient of two similarly charged (-1) contrast agents were detected, which shows the effect of steric interactions. However, the increase in solute concentration did not increase its partition coefficient. In clinical application, it is important that contrast agent concentration does not affect the interpretation of CECT imaging.

Contrast-Enhanced Micro-Computed Tomography in Evaluation of Spontaneous Repair of Equine Cartilage

OBJECTIVE

Contrast-enhanced computed tomography (CECT) has been introduced for the evaluation of cartilage integrity. Furthermore, CECT enables imaging of the structure and density of subchondral bone. In this laboratory study, we investigate the potential of microCECT to simultaneously image cartilage and subchondral bone for the evaluation of tissue healing.

DESIGN

Osteochondral lesions (Ø = 6 mm) were surgically created in equine intercarpal joints (n = 7). After spontaneous healing for 12 months, the horses were sacrificed and osteochondral plugs (Ø = 14 mm), including the repair cartilage and adjacent intact tissue, were harvested. The nonfibrillar and fibrillar moduli and the permeability of cartilage were determined using indentation testing. Contrast agent diffusion into the samples was imaged for 36 hours using high-resolution CT. Results from CECT, mechanical testing, and microscopic analyses were compared and correlated.

RESULTS

The contrast agent diffusion coefficient showed a significant (P < 0.05) difference between the repair and adjacent intact tissue. MicroCECT revealed altered (P < 0.05) bone volume fraction, mineral density, and microstructure of subchondral bone at the repair site. The contrast agent diffusion coefficient correlated with the moduli of the nonfibrillar matrix (R = -0.662, P = 0.010), collagen fibril parallelism index (R = -0.588, P = 0.035), and glycosaminoglycan content (R = -0.503, P = 0.067). The repair cartilage was mechanically and structurally different from adjacent intact tissue (P < 0.05).

CONCLUSIONS

MicroCECT enabled simultaneous quantitative evaluation of subchondral bone and monitoring of cartilage repair, distinguishing quantitatively the repair site from the adjacent intact tissue. As the only technique able to simultaneously image cartilage and determine subchondral bone mineral density and microstructure, CECT has potential clinical value.

Is cartilage sGAG content related to early changes in cartilage disease? Implications for interpretation of dGEMRIC

OBJECTIVE

This study investigates sulphated glycosaminoglycans (sGAG) content changes in early osteoarthritis (OA), and whether contrast-enhanced magnetic resonance imaging (MRI) of cartilage in vitro may identify early event of OA pathology.

METHOD

Osteochondral plugs from patients with hip OA or femoral neck fracture (reference group) were collected and analysed by 1.5 T MRI with ΔR1 as a measure of cartilage contrast concentration. Cartilage hydration, contents of sGAG, cartilage oligomeric matrix protein (COMP), hydroxyproline, denatured collagen, and aggrecan TEGE(392) neoepitope were determined and histological grading was performed.

RESULTS

sGAG content correlated to ΔR1, although no difference in either of these parameters was detectable between OA and reference cartilage at 4 h of contrast equilibration. In contrast, biochemical analysis of other cartilage matrix constituents showed distinct alterations typical for early cartilage degradation in OA cartilage and with clear evidence for increased aggrecan turnover.

CONCLUSION

In the present in vitro study, cartilage sGAG content could not distinguish between early OA cartilage and reference cartilage. Given, that delayed gadolinium enhanced MRI of cartilage (dGEMRIC) indicates early events in the pathogenesis of OA in vivo, our results from the in vitro studies imply other, additional factors than cartilage sGAG content, e.g., alterations in diffusion or increased supply of contrast agent in the diseased joint. Alternatively, an altered dGEMRIC reflects later stages of OA, when sGAG content decreases. Further investigations are warranted, to understand variations in sGAG content in pathology, an essential background for interpreting dGEMRIC measurements.

Diffusion of Gd-DTPA²⁻ into articular cartilage

OBJECTIVES

The delayed Gadolinium-Enhanced MRI of Cartilage (dGEMRIC) technique is a method proposed for non-invasive measurement of cartilage glycosaminoglycan (GAG) content. In this method, gadopentetate (Gd-DTPA²⁻) is assumed to distribute in cartilage in inverse relation to the GAG distribution, thus allowing quantification of the GAG content. For accurate GAG quantification, the kinetics of Gd-DTPA²⁻ in articular cartilage is of critical importance. However, the diffusion of Gd-DTPA²⁻ has not been systematically studied over long time periods using MRI-feasible gadopentetate concentrations. Thus, the present study aims to investigate the diffusion of gadopentetate into cartilage in vitro in intact and enzymatically degraded cartilage.

METHODS

The diffusion of gadopentetate into bovine articular cartilage was investigated at 9.4 T over 18-h time period using repeated T(1) measurements in two models, (1) comparing intact and trypsin-treated tissue and (2) assessing the effect of penetration direction. The diffusion process was further assessed by determining the gadopentetate flux and diffusivity. The results were compared with histological and biochemical reference methods.

RESULTS AND CONCLUSIONS

The results revealed that passive diffusion of Gd-DTPA²⁻ was significantly slower than previously assumed, leading to overestimation of the GAG content at equilibrating times of few hours. Moreover, Gd-DTPA²⁻ distribution was found to depend not only on GAG content, but also on collagen content and diffusion direction. Interestingly, the dGEMRIC technique was found to be most sensitive to cartilage degradation in the early stages of diffusion process, suggesting that full equilibrium between gadopentetate and cartilage may not be required in order to detect cartilage degeneration.

Computed tomography detects changes in contrast agent diffusion after collagen cross-linking typical to natural aging of articular cartilage

OBJECTIVE

The effect of threose-induced collagen cross-linking on the mechanical and diffusive properties of cartilage was investigated in vitro. In particular, we investigated the potential of Contrast Enhanced Computed Tomography (CECT) to detect changes in articular cartilage after increased collagen cross-linking, which is an age-related phenomenon.

METHODS

Osteochondral plugs (Ø=6.0 mm, n=28) were prepared from intact bovine patellae (n=7). Two of the four adjacent samples, prepared from each patella, were treated with threose to increase the collagen cross-linking, while the other two specimen served as paired controls. One sample pair was mechanically tested and then mechanically injured using a material testing device. Contrast agent [ioxaglate (Hexabrix™)] diffusion was imaged in the other specimen pair for 25 h using CECT. Water fraction, collagen and proteoglycan content, collagen network architecture and the amount of cross-links [hydroxylysyl pyridinoline (HP), lysyl pyridinoline (LP) and pentosidine (Pent)] of the samples were also determined.

RESULTS

Cartilage collagen cross-linking, both Pent and LP, were significantly (P<0.001) increased due to threose treatment. CECT could detect the increased cross-links as the contrast agent penetration and the diffusion flux were significantly (P<0.05) lower in the threose treated than in untreated samples. The equilibrium modulus (+164%, P<0.05) and strain dependent dynamic modulus (+47%, P<0.05) were both significantly greater in the threose treated samples than in reference samples, but there was no association between the initial dynamic modulus and the threose treatment. The water fraction, proteoglycan and collagen contents, as well as collagen architecture, were not significantly altered by the threose treatment.

CONCLUSIONS

To conclude, the CECT technique was found to be sensitive at detecting changes in cartilage tissue due to increased collagen cross-linking. This is important since increased cross-linking has been proposed to be related to the increased injury susceptibility of tissue.

Detection of mechanical injury of articular cartilage using contrast enhanced computed tomography

OBJECTIVE

Osteoarthritic degeneration may be initiated by mechanical overloading of articular cartilage. Mechanical injury increases the permeability of tissue, thereby probably affecting the diffusion of contrast agents in articular cartilage. We investigated whether it is possible to detect acute cartilage injury by measuring contrast agent diffusion into articular cartilage using contrast enhanced computed tomography (CECT).

METHODS

Osteochondral plugs (Ø=6.0 mm, n=36) were prepared from intact bovine patellae (n=9). Two of the adjacent samples were injured by impact loading, using a drop tower, while the others served as paired controls. The samples were imaged before immersion in contrast agent solution [ioxaglate (Hexabrix™) or sodium iodide (NaI)] and 1, 3, 5, 7, 10, 15, 20 and 25 h after immersion using a MicroCT-instrument. Contrast agent content, diffusion coefficient and diffusion flux were determined for each sample.

RESULTS

Already after 1 h the penetration of contrast agents into cartilage was significantly (P<0.05) greater in the injured samples. The diffusion coefficient was not altered by the injury, which suggests that reaching the diffusion equilibrium takes the same time in injured and intact cartilage. However, the diffusion flux of ioxaglate through the articular surface was significantly higher in injured samples at 30-60 min after immersion.

CONCLUSIONS

To conclude, CECT could diagnose articular cartilage injuries, and determination of the diffusion flux of ioxaglate helped to detect tissue injury without waiting for the diffusion equilibrium. These results are encouraging, however, in vivo application of CECT is challenging and systematic further studies are needed to reveal its clinical potential.