Small animal models of osteoarthritis are often used for evaluating the efficacy of pharmacologic treatments and cartilage repair strategies, but noninvasive techniques capable of monitoring matrix-level changes are limited by the joint size and the low radiopacity of soft tissues. Here we present a technique for the noninvasive imaging of cartilage at micrometer-level resolution based on detecting the equilibrium partitioning of an ionic contrast agent via microcomputed tomography. The approach exploits electrochemical interactions between the molecular charges present in the cartilage matrix and an ionic contrast agent, resulting in a nonuniform equilibrium partitioning of the ionic contrast agent reflecting the proteoglycan distribution. In an in vitro model of cartilage degeneration we observed changes in x-ray attenuation magnitude and distribution consistent with biochemical and histological analyses of sulfated glycosaminoglycans, and x-ray attenuation was found to be a strong predictor of sulfated glycosaminoglycan density. Equilibration with the contrast agent also permits direct in situ visualization and quantification of cartilage surface morphology. Equilibrium partitioning of an ionic contrast agent via microcomputed tomography thus provides a powerful approach to quantitatively assess 3D cartilage composition and morphology for studies of cartilage degradation and repair.noninvasive imaging ͉ proteoglycans ͉ cartilage degeneration ͉ osteoarthritis A nalysis of small-animal models is limited by the availability of quantitative evaluation techniques for studying the extracellular matrix (ECM) changes associated with osteoarthritis (OA) and cartilage repair. Histology is traditionally used to monitor the spatial distribution of matrix macromolecules but is time-consuming and subject to distortion artifacts and tissue damage, and it produces only semiquantitative analysis of 2D sections that may provide inaccurate 3D representations. Biochemical assays are available to quantify the amount and type of matrix macromolecules in cartilage, but these assays fail to provide their spatial distributions, particularly in small animals where the limited thickness and volume of cartilage make it difficult or impossible to extract samples from multiple regions. Additionally, longitudinal monitoring of changes with time are impossible because of the destructive nature of these histological and biochemical techniques.Proteoglycans (PGs) are a particularly appropriate target for studying OA and for evaluating the efficacy of cartilage defect repair. PGs comprise 5-10% of articular cartilage by wet mass (1) and are key regulators of its equilibrium and dynamic mechanical properties. This regulation is the result of interactions between ionic interstitial fluid and negatively charged sulfated glycosaminoglycans (sGAGs) attached to the PG backbone (2). The amount and distribution of PGs changes substantially during development (3), during degeneration and repair (4, 5), and in response to blunt trauma (6). Of particular...

Porous biomaterials designed to support cellular infiltration and tissue formation play a critical role in implant fixation and engineered tissue repair. The purpose of this Leading Opinion Paper is to advocate the use of high resolution 3D imaging techniques as a tool to quantify extracellular matrix formation and vascular ingrowth within porous biomaterials and objectively compare different strategies for functional tissue regeneration. An initial over-reliance on qualitative evaluation methods may have contributed to the false perception that developing effective tissue engineering technologies would be relatively straightforward. Moreover, the lack of comparative studies with quantitative metrics in challenging pre-clinical models has made it difficult to determine which of the many available strategies to invest in or use clinically for companies and clinicians, respectively. This paper will specifically illustrate the use of microcomputed tomography (micro-CT) imaging with and without contrast agents to nondestructively quantify the formation of bone, cartilage, and vasculature within porous biomaterials.

Articular cartilage undergoes matrix degradation and loss of mechanical properties when stimulated with proinflammatory cytokines such as interleukin-1 (IL-1). Aggrecanases and matrix metalloproteinases (MMPs) are thought to be principal downstream effectors of cytokine-induced matrix catabolism, and aggrecanase-or MMP-selective inhibitors reduce or block matrix destruction in several model systems. The objective of this study was to use metalloproteinase inhibitors to perturb IL-1-induced matrix catabolism in bovine cartilage explants and examine their effects on changes in tissue compression and shear properties. Explanted tissue was stimulated with IL-1 for up to 24 days in the absence or presence of inhibitors which were aggrecanase-selective, MMP-selective, or non-selective. Analysis of conditioned media and explant digests revealed that aggrecanase-mediated aggrecanolysis was delayed to varying extents with all inhibitor treatments, but that aggrecan release persisted. Collagen degradation was abrogated by MMP-and non-selective inhibitors and reduced by the aggrecanase inhibitor. The inhibitors delayed but did not reduce loss of the equilibrium compression modulus, whereas the loss of dynamic compression and shear moduli was delayed and reduced. The data suggest that non-metalloproteinase mechanisms participate in IL-1-induced matrix degradation and loss of tissue material properties.

Objective: To examine the relationships between biochemical composition and mechanical properties of articular cartilage explants during interleukin-1 (IL-1)-induced degradation and post-exposure recovery. Design: Bovine articular cartilage explants were cultured for up to 32 days with or without 20 ng/mL interleukin-1. The dynamic shear modulus |G*dyn| and equilibrium and dynamic unconfined compression moduli (Eequil and |E*dyn|) were measured at intervals throughout the culture period. In a subsequent recovery study, explants were cultured for 4 days with or without 20ng/mL IL-1 and for an additional 16 days in control media. The dynamic moduli |E*dyn| and |G*dyn| were measured at intervals during degeneration and recovery. Conditioned media and explant digests were assayed for sulfated glycosaminoglycans (sGAG) and collagen content. Results: Continuous IL-1 stimulation triggered progressive decreases in Eequil, |E*dyn|, and |G*dyn| concomitant with the sequential release of sGAG and collagen from the explants. Brief IL-1 exposure resulted in a short release of sGAG but not collagen, followed by a gradual and incomplete repopulation of sGAG. The temporary sGAG depletion was associated with decreases in both |E*dyn| and |G*dyn| which also recovered after removal of IL-1. During IL-1-induced degradation and post-exposure recovery, explant mechanical properties correlated well with tissue sGAG concentration. Conclusions: As previously shown for developing cartilages and engineered cartilage constructs, cytokine-induced changes in sGAG concentration (i.e., fixed charge density) are coincident with changes in compressive and shear properties of articular cartilage. Further, recovery of cartilage mechanical properties can be achieved by relief from proinflammatory stimuli and subsequent restoration of tissue sGAG concentration.