Composition and transport properties of human ankle and knee cartilage

Effect of molecular weight and tissue layer on solute partitioning in the knee meniscus

Objective

Knee meniscus tissue is partly vascularized, meaning that nutrients must be transported through the extracellular matrix of the avascular portion to reach resident cells. Similarly, drugs used as therapeutic agents to treat meniscal pathologies rely on transport through the tissue. The driving force of diffusive transport is the gradient of concentration, which depends on molecular solubility. The meniscus is organized into a core region sandwiched between the tibial and femoral superficial layers. Structural differences exist across meniscal regions; therefore, regional differences in solubility are also hypothesized.

Methods

Samples from the core, tibial and femoral layers were obtained from 5 medial and 5 lateral porcine menisci. The partition coefficient (K) of fluorescein, 3 ​kDa and 40 ​kDa dextrans in the layers of the meniscus was measured using an equilibration experiment. The effect of meniscal compartment, layer, and solute molecular weight on K was analyzed using a three-way ANOVA.

Results

K ranged from a high of ∼2.9 in fluorescein to a low of ∼0.1 in 40 ​kDa dextran and was inversely related to the solute molecular weight across all tissue regions. Tissue layer only had a significant effect on partitioning of 40k Dex solute, which was lower in the tibial surface layer relative to the core (p ​= ​0.032).

Conclusion

This study provides insight into depth-dependent partitioning in the meniscus, indicating the limiting effect of the meniscus superficial layer on solubility increases with solute molecular size. This illustrates how the surface layers could potentially reduce the effectiveness of drug delivery therapies incorporating large molecules (>40 ​kDa).

Strain-Dependent Diffusivity of Small and Large Molecules in Meniscus

Due to lack of full vascularization, the meniscus relies on diffusion through the extracellular matrix to deliver small (e.g., nutrients) and large (e.g., proteins) to resident cells. Under normal physiological conditions, the meniscus undergoes up to 20% compressive strains. While previous studies characterized solute diffusivity in the uncompressed meniscus, to date, little is known about the diffusive transport under physiological strain levels. This information is crucial to fully understand the pathophysiology of the meniscus. The objective of this study was to investigate strain-dependent diffusive properties of the meniscus fibrocartilage. Tissue samples were harvested from the central portion of porcine medial menisci and tested via fluorescence recovery after photobleaching to measure diffusivity of fluorescein (332 Da) and 40 K Da dextran (D40K) under 0%, 10%, and 20% compressive strain. Specifically, average diffusion coefficient and anisotropic ratio, defined as the ratio of the diffusion coefficient in the direction of the tissue collagen fibers to that orthogonal, were determined. For all the experimental conditions investigated, fluorescein diffusivity was statistically faster than that of D40K. Also, for both molecules, diffusion coefficients significantly decreased, up to ∼45%, as the strain increased. In contrast, the anisotropic ratios of both molecules were similar and not affected by the strain applied to the tissue. This suggests that compressive strains used in this study did not alter the diffusive pathways in the meniscus. Our findings provide new knowledge on the transport properties of the meniscus fibrocartilage that can be leveraged to further understand tissue pathophysiology and approaches to tissue restoration.

EFFECTS OF SOLUTE SIZE AND TISSUE COMPOSITION ON MOLECULAR AND MACROMOLECULAR DIFFUSIVITY IN HUMAN KNEE CARTILAGE

Objective

Articular cartilage is an avascular tissue. Accordingly, diffusivity represents a fundamental transport mechanism for nutrients and other molecular signals regulating its cell metabolism and maintenance of the extracellular matrix. Understanding how solutes spread into articular cartilage is crucial to elucidating its pathologies, and to designing treatments for repair and restoration of its extracellular matrix. As in other connective tissues, diffusivity in articular cartilage may vary depending both its composition and the specific diffusing solute. Hence, this study investigated the roles of solute size and tissue composition on molecular diffusion in knee articular cartilage.

Design

FRAP tests were conducted to measure diffusivity of five molecular probes, with size ranging from ~332Da to 70,000Da, in human knee articular cartilage. The measured diffusion coefficients were related to molecular size, as well as water and glycosaminoglycan (GAG) content of femoral and tibial condyle cartilage.

Results

Diffusivity was affected by molecular size, with the magnitude of the diffusion coefficients decreasing as the Stokes radius of the probe increased. The values of diffusion coefficients in tibial and femoral samples were not significantly different from one another, despite the fact that tibial samples exhibited significantly higher water content and lower GAG content of the femoral specimens. Water content did not affect diffusivity. In contrast, diffusivities of large molecules were sensitive to GAG content.

Conclusions

This study provides new knowledge on the mechanisms of diffusion in articular cartilage. Our findings can be leveraged to further investigate osteoarthritis and to design treatments for cartilage restoration or replacement.

Comparative Tribology: Articulation-induced rehydration of cartilage across species

Articular cartilage is a robust tissue that facilitates load distribution and wear-free articulation in diarthrodial joints. These biomechanical capabilities are fundamentally tied to tissue hydration, whereby high interstitial fluid pressures and fluid load support facilitate the maintenance of low tissue strains and frictions. Our recent ex vivo studies of cartilage sliding biomechanics using the convergent stationary contact area (cSCA) configuration, first introduced by Dowson and colleagues, unexpectedly demonstrated that sliding alone can promote recovery of interstitial pressure and lubrication lost to static compression through a mechanism termed 'tribological rehydration.' Although exclusively examined in bovine stifle cartilage to date, we hypothesized that tribological rehydration, i.e., the ability to recover/modulate tissue strains and lubrication through sliding, is a universal behavior of articular cartilage. This study aimed to establish if, and to what extent, sliding-induced tribological rehydration is conserved in articular cartilage across a number of preclinical animal species/models and diarthrodial joints. Using a comparative approach, we found that articular cartilage from equine, bovine, ovine, and caprine stifles, and porcine stifle, hip, and tarsal joints all exhibited remarkably consistent sliding speed-dependent compression/strain recovery and lubrication behaviors under matched contact stresses (0.25 MPa). All cartilage specimens tested supported robust, tribological rehydration during high-speed sliding (>30 mm/s), which as a result of competitive recovery of interstitial lubrication, promoted remarkable decreases in kinetic friction during continuous sliding. The conservation of tribological rehydration across mammalian quadruped articular cartilage suggests that sliding-induced recovery of interstitial hydration represents an important tissue adaptation and largely understudied contributor to the biomechanics of cartilage and joints.

Osteoarthritis: New Strategies for Transport and Drug Delivery Across Length Scales

Osteoarthritis (OA) is the fourth leading cause of disability in adults. Yet, few viable pharmaceutical options exist for pain abatement and joint restoration, aside from joint replacement at late and irreversible stages of the disease. From the first onset of OA, as joint pain increases, individuals with arthritis increasingly reach for drug delivery solutions, from taking oral glycosaminoglycans (GAGs) bought over the counter from retail stores (e.g., Costco) to getting injections of viscous, GAG-containing synovial fluid supplement in the doctor's office. Little is known regarding the efficacy of delivery mode and/or treatment by such disease-modulating agents. This Review addresses the interplay of mechanics and biology on drug delivery to affected joints, which has profound implications for molecular transport in joint health and (patho)physiology. Multiscale systems biology approaches lend themselves to understand the relationship between the cell and joint health in OA and other joint (patho)physiologies. This Review first describes OA-related structural and functional changes in the context of the multilength scale anatomy of articular joints. It then summarizes and categorizes, by size and charge, published molecular transport studies, considering changes in permeability induced through inflammatory pathways. Finally, pharmacological interventions for OA are outlined in the context of molecular weights and modes of drug delivery. Taken together, the current state-of-the-art points to a need for new drug delivery strategies that harness systems-based interactions underpinning molecular transport and maintenance of joint structure and function at multiple length scales from molecular agents to cells, tissues, and tissue compartments which together make up articular joints. Cutting edge and cross-length and -time scale imaging represents a key discovery enabling technology in this process.

Spatially patterned microribbon-based hydrogels induce zonally-organized cartilage regeneration by stem cells in 3D

Regenerating cartilage with biomimetic zonal organization, which is critical for tissue function, remains a great challenge. The objective of this study was to evaluate the potential of spatially-patterned, multi-compositional, macroporous, extracellular matrix-based microribbon (µRB) µRB scaffolds to regenerate cartilage with biochemical, mechanical, and morphological zonal organization by mesenchymal stem cells (MSCs) compared to conventional multi-layer nanoporous hydrogels. MSCs were seeded in either trilayer microribbon (µRB) or hydrogel (HG) scaffolds that were composed of layered biomaterial compositions that had been chosen for their ability to differentiate MSCs into chondrocytes with zonal properties. To mimic the aligned collagen morphology in the superficial layer of native cartilage, an additional experimental group added MSC-laden aligned µRBs to the surface of the superficial layer of a µRB trilayer. Tuning µRB alignment and compositions in different zones led to zonal-specific responses of MSCs to create neocartilage with zonal biochemical, morphological, and mechanical properties, while trilayer HGs led to minimal cartilaginous deposition overall. Trilayer µRBs created neocartilage exhibiting significant increases in compressive modulus (up to 456 kPa) and > 4-fold increase in sGAG production from superficial to deep zones. Aligned gelatin µRBs in the superficial zone further enhanced biomimetic mimicry of the produced neocartilage by leading to robust collagen deposition and superficial zone protein production. STATEMENT OF SIGNIFICANCE: Regenerating cartilage with zonal organization using mesenchymal stem cells (MSCs) remains a great challenge. We developed a spatially-patterned, gradient, macroporous, trilayer microribbon (µRB) scaffold that we used to engineer MSC-based neocartilage with zonal trends that match native cartilage in many aspects, including collagen, sGAG, superficial zone protein, and compressive moduli. This is in direct contrast to conventional trilayer nanoporous hydrogels which led to minimal cartilage deposition and weak mechanical properties. It took only 21 days for MSC-seeded trilayer µRB scaffolds to reach cartilage-mimicking compressive moduli without requiring high cell seeding density, which has never been reported before. While this paper focuses on cartilage zonal organization, gradient µRB scaffolds can be used to repair other tissue interfaces such as osteochondral defects.

A Synthetic Gene Circuit for Self-Regulating Delivery of Biologic Drugs in Engineered Tissues

IMPACT STATEMENT

We engineered a synthetic transcription system based on nuclear factor kappa-light-chain-enhancer of activated B cells signaling that can attenuate the effects of the inflammatory cytokine interleukin (IL)-1α in a self-regulating manner. This system responds in a time- and dose-dependent manner to rapidly produce therapeutic levels of IL-1 receptor antagonist (IL-1Ra). The use of lentiviral gene therapy allows this system to be utilized through different transduction methods and in different cell types for a variety of applications. Broadly, this approach may be applicable in developing autoregulated biologic systems for tissue engineering and drug delivery in a range of disease applications.

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.

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.

Subchondral drilling for articular cartilage repair: a systematic review of translational research

Articular cartilage defects may initiate osteoarthritis. Subchondral drilling, a widely applied clinical technique to treat small cartilage defects, does not yield cartilage regeneration. Various translational studies aiming to improve the outcome of drilling have been performed; however, a robust systematic analysis of its translational evidence was still lacking. Here, we performed a systematic review of the outcome of subchondral drilling for knee cartilage repair in translational animal models. A total of 12 relevant publications studying 198 animals was identified, detailed study characteristics were extracted, and methodological quality and risk of bias were analyzed. Subchondral drilling led to improved repair outcome compared with defects that were untreated or treated with abrasion arthroplasty for cartilage repair in multiple translational models. Within the 12 studies, considerable subchondral bone changes were observed, including subchondral bone cysts and intralesional osteophytes. Furthermore, extensive alterations of the subchondral bone microarchitecture appeared in a temporal pattern in small and large animal models, together with specific topographic aspects of repair. Moreover, variable technical aspects directly affected the outcomes of osteochondral repair. The data from this systematic review indicate that subchondral drilling yields improved short-term structural articular cartilage repair compared with spontaneous repair in multiple small and large animal models. These results have important implications for future investigations aimed at an enhanced translation into clinical settings for the treatment of cartilage defects, highlighting the importance of considering specific aspects of modifiable variables such as improvements in the design and reporting of preclinical studies, together with the need to better understand the underlying mechanisms of cartilage repair following subchondral drilling.

Bioinspired microstructures of chitosan hydrogel provide enhanced wear protection

We describe the fabrication of physical chitosan hydrogels exhibiting a layered structure. This bilayered structure, as shown by SEM and confocal microscopy, is composed of a thin dense superficial zone (SZ), covering a deeper zone (DZ) containing microchannels orientated perpendicularly to the SZ. We show that such structure favors diffusion of macromolecules within the hydrogel matrix up to a critical pressure, σ_c, above which channels were constricted. Moreover, we found that the SZ provided a higher wear resistance than the DZ which was severely damaged at a pressure equal to the elastic modulus of the gel. The coefficient of friction (CoF) of the SZ remained independent of the applied load with μ_SZ = 0.38 ± 0.02, while CoF measured at DZ exhibited two regimes: an initial CoF close to the value found on the SZ, and a CoF that decreased to μ_DZ = 0.18 ± 0.01 at pressures higher than the critical pressure σ_c. Overall, our results show that internal structuring is a promising avenue in controlling and improving the wear resistance of soft materials such as hydrogels.

T1ρ mapping of articular cartilage grafts after autologous osteochondral transplantation for osteochondral lesions of the talus: A longitudinal evaluation

BACKGROUND

Clinical results of autologous osteochondral transplantation (AOT) for treatment of osteochondral lesions of the talus have been mixed. T1ρ imaging can be used to noninvasively detect early cartilage degeneration.

PURPOSE OR HYPOTHESIS

To quantitatively assess, by means of T1ρ imaging, changes over time in the biochemical health of grafted cartilage after AOT for osteochondral lesions of the talus.

STUDY TYPE

Retrosepctive case series.

POPULATION

The study group comprised nine patients who underwent AOT for an osteochondral lesion of the talus and in whom T1ρ mapping was performed 1 and 2 years postoperatively.

FIELD STRENGTH/SEQUENCE

3 Tesla. T1ρ-weighted turbo field echo.

ASSESSMENT

The mean T1ρ value of full-thickness cartilage at the repair site and that of full-thickness cartilage elsewhere in the same image (far-field cartilage) were determined. Clinical assessment was based on the American Orthopaedic Foot & Ankle Society (AOFAS) scale. Correlation between the T1ρ ratios (grafted-to-far-field cartilage T1ρ values) and clinical outcomes was examined.

STATISTICAL TESTS

Mixed effects model. Pearson correlation analysis.

RESULTS

At 1 year, a significant difference existed between the mean T1ρ value of the grafted cartilage (57.0 ± 7.7 ms) and that of the far-field cartilage (41.8 ± 4.6 ms) (P < 0.001). At 2 years, the mean T1ρ value of the grafted cartilage (49.1 ± 6.4 ms) was significantly lower than that at 1 year (P = 0.011). Moderate negative correlation was found between the 1-year T1ρ ratio and 1-year AOFAS score (r = -0.60) and between the 2-year T1ρ ratio and 2-year AOFAS score (r = -0.50).

DATA CONCLUSION

Our observation of substantial restoration of the proteoglycan content of the grafted cartilage approximately 2 years after AOT for osteochondral lesions of the talus indicates that the content changes gradually and that the cartilage reparation process is slower than previously believed.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 3 J. MAGN. RESON. IMAGING 2018;48:398-403.

Arthroscopic Treatment of Osteochondral Lesions of the Talus With Allograft Cartilage Matrix

BACKGROUND

This prospective study evaluated the intermediate-term outcomes of operatively treating primary osteochondral lesions of the talus (OLT) of 1.5 cm² or smaller with arthroscopic excision, microfracture, and allograft cartilage extracellular matrix (ECM).

METHODS

Between 2012 and 2015, 30 consecutive patients received allograft cartilage ECM at their microfractured OLT of 1.5 cm² or smaller after failing nonoperative treatment. Preoperative and postoperative function and pain were graded using the Foot and Ankle Ability Measure (FAAM) and a visual analog scale (VAS), respectively. Postoperative imaging was assessed for osteochondral healing and degenerative changes. This included computed tomography (CT) at 6 months from surgery. Data regarding postoperative complications were recorded. All 30 patients that received allograft cartilage ECM for their OLT were evaluated for final follow-up at a mean of 20.2 months.

RESULTS

Mean FAAM increased from 51.4 preoperatively to 89.3 of 100 at final follow-up. Mean VAS decreased from 8.1 preoperatively to 1.7 of 10 at final follow-up. These differences between preoperative and postoperative function and pain were statistically significant ( P < .05). At 6 months from surgery, 2 patients (6.7%) received CT scans that revealed incomplete chondral formation at their OLT. At 19 months after surgery, a separate patient (3.3%) developed ankle arthritic changes.

CONCLUSION

Treating OLTs with allograft cartilage ECM has not been previously reported. Using allograft cartilage ECM resulted in a high rate of improvement in ankle function and pain in patients with OLTs. These findings are important as allograft cartilage ECM could be of significant benefit for patients with OLTs.

LEVEL OF EVIDENCE

Level III, prospective comparative series.

Effect of Strain, Region, and Tissue Composition on Glucose Partitioning in Meniscus Fibrocartilage

A nearly avascular tissue, the knee meniscus relies on diffusive transport for nutritional supply to cells. Nutrient transport depends on solute partitioning in the tissue, which governs the amount of nutrients that can enter a tissue. The purpose of the present study was to investigate the effects of mechanical strain, tissue region, and tissue composition on the partition coefficient of glucose in meniscus fibrocartilage. A simple partitioning experiment was employed to measure glucose partitioning in porcine meniscus tissues from two regions (horn and central), from both meniscal components (medial and lateral), and at three levels of compression (0%, 10%, and 20%). Partition coefficient values were correlated to strain level, water volume fraction, and glycosaminoglycan (GAG) content of tissue specimens. Partition coefficient values ranged from 0.47 to 0.91 (n = 48). Results show that glucose partition coefficient is significantly (p < 0.001) affected by compression, decreasing with increasing strain. Furthermore, we did not find a statistically significant effect of tissue when comparing medial versus lateral (p = 0.181) or when comparing central and horn regions (p = 0.837). There were significant positive correlations between tissue water volume fraction and glucose partitioning for all groups. However, the correlation between GAG content and partitioning was only significant in the lateral horn group. Determining how glucose partitioning is affected by tissue composition and loading is necessary for understanding nutrient availability and related tissue health and/or degeneration. Therefore, this study is important for better understanding the transport and nutrition-related mechanisms of meniscal degeneration.

Differences in joint morphology between the knee and ankle affect the repair of osteochondral defects in a rabbit model

Background

Although differences in the results of the bone marrow stimulation technique between the knee and ankle have been reported, a detailed mechanism for those differences has not been clarified. The purpose of this study was to examine whether morphological differences between the knee and ankle joint affect the results of drilling as treatment for osteochondral defects in a rabbit model.

Methods

Osteochondral defects were created at the knee and ankle joint in the rabbit. In the knee, osteochondral defects were created at the medial femoral condyle (MFC) and patellar groove (PG). At the ankle, defects were created in the talus at either a covered or uncovered area by the tibial plafond. After creating the osteochondral defect, drilling was performed. At 4, 8, and 12 weeks after surgery, repair of the osteochondral defects were evaluated histologically. The proliferation of rabbit chondrocytes and proteoglycan release of cartilage tissue in response to IL-1β were analyzed in vitro in both joints.

Results

At 8 weeks after surgery, hyaline cartilage repair was observed in defects at the covered area of the talus and the MFC. At 12 weeks, hyaline cartilage with a normal thickness was observed for the defect at the covered area of the talus, but not for the defect at the MFC. At 12 weeks, subchondral bone formation progressed and a normal contour of subchondral bone was observed on CT in the defect at the covered area of the talus. No significant differences in chondrocyte proliferation rate and proteoglycan release were detected between the knee and ankle in vitro.

Conclusions

Our results demonstrate that the covered areas of the talus show early and sufficient osteochondral repair compared to that of the knee and the uncovered areas of the talus. These results suggest that the congruent joint shows better subchondral repair prior to cartilage repair compared to that of the incongruent joint.

Human talar and femoral cartilage have distinct mechanical properties near the articular surface

Talar osteochondral lesions (OCL) frequently occur following injury. Surgical interventions such as femoral condyle allogeneic or autogenic osteochondral transplant (AOT) are often used to treat large talar OCL. Although AOT aims to achieve OCL repair by replacing damaged cartilage with mechanically matched cartilage, the spatially inhomogeneous material behavior of the talar dome and femoral donor sites have not been evaluated or compared. The objective of this study was to characterize the depth-dependent shear properties and friction behavior of human talar and donor-site femoral cartilage. To achieve this objective, depth-dependent shear modulus, depth-dependent energy dissipation and coefficient of friction were measured on osteochondral cores from the femur and talus. Differences between anatomical regions were pronounced near the articular surface, where the femur was softer, dissipated more energy and had a lower coefficient of friction than the talus. Conversely, shear modulus near the osteochondral interface was nearly indistinguishable between anatomical regions. Differences in energy dissipation, shear moduli and friction coefficients have implications for graft survival and host cartilage wear. When the biomechanical variation is combined with known biological variation, these data suggest the use of caution in transplanting cartilage from the femur to the talus. Where alternatives exist in the form of talar allograft, donor-recipient mechanical mismatch can be greatly reduced.

Nutrient Channels Aid the Growth of Articular Surface-Sized Engineered Cartilage Constructs

Symptomatic osteoarthritic lesions span large regions of joint surfaces and the ability to engineer cartilage constructs at clinically relevant sizes would be highly desirable. We previously demonstrated that nutrient transport limitations can be mitigated by the introduction of channels in 10 mm diameter cartilage constructs. In this study, we scaled up our previous system to cast and cultivate 40 mm diameter constructs (2.3 mm overall thickness); 4 mm diameter and channeled 10 mm diameter constructs were studied for comparison. Furthermore, to assess whether prior results using primary bovine cells are applicable for passaged cells-a more clinically realistic scenario-we cast constructs of each size with primary or twice-passaged cells. Constructs were assessed mechanically for equilibrium compressive Young's modulus (EY), dynamic modulus at 0.01 Hz (G*), and friction coefficient (μ); they were also assessed biochemically, histologically, and immunohistochemically for glycosaminoglycan (GAG) and collagen contents. By maintaining open channels, we successfully cultured robust constructs the size of entire human articular cartilage layers (growing to ∼52 mm in diameter, 4 mm thick, mass of 8 g by day 56), representing a 100-fold increase in scale over our 4 mm diameter constructs, without compromising their functional properties. Large constructs reached EY of up to 623 kPa and GAG contents up to 8.9%/ww (% of wet weight), both within native cartilage ranges, had G* >2 MPa, and up to 3.5%/ww collagen. Constructs also exhibited some of the lowest μ reported for engineered cartilage (0.06-0.11). Passaged cells produced tissue of lower quality, but still exhibited native EY and GAG content, similar to their smaller controls. The constructs produced in this study are, to our knowledge, the largest engineered cartilage constructs to date which possess native EY and GAG, and are a testament to the effectiveness of nutrient channels in overcoming transport limitations in cartilage tissue engineering.

An analysis of changes in in vivo cartilage thickness of the healthy ankle following dynamic activity

Abnormal cartilage loading after injury is believed to be an important factor leading to post-traumatic ankle osteoarthritis. Due to the viscoelastic behavior of cartilage, it is possible to measure localized cartilage strains from changes in thickness following dynamic activities. However, there are limited data characterizing in vivo cartilage mechanics under physiological loading conditions in the healthy ankle. Therefore, the objective of this study was to directly measure in vivo cartilage strains in the healthy ankle joint in response to a dynamic hopping exercise. Ten healthy subjects with no history of ankle injury underwent magnetic resonance imaging before and after a single-leg hopping exercise. Bony and articular cartilage surfaces were created from these images using solid modeling software. Pre-exercise and post-exercise models were then registered to each other, and site-specific cartilage strains (defined as the normalized changes in cartilage thickness) were calculated at grid points spanning the articular surfaces. The effects of both location and exercise on strain were tested using a two-way repeated measures analysis of variance. We did not detect any significant interaction effect between location and exercise for either tibial or talar cartilage. However, hopping resulted in significant decreases in tibial (p<0.05) and talar (p<0.05) cartilage thicknesses, corresponding to strains of 3% and 2%, respectively. Additionally, pre-exercise cartilage thickness varied significantly by location in the talus (p<0.05), but not in the tibia. These strain data may provide important baseline information for future studies investigating altered biomechanics in those at high risk for the development of post-traumatic ankle osteoarthritis.

Optimizing nutrient channel spacing and revisiting TGF-beta in large engineered cartilage constructs

Cartilage tissue engineering is a promising approach to treat osteoarthritis. However, current techniques produce tissues too small for clinical relevance. Increasingly close-packed channels have helped overcome nutrient transport limitations in centimeter-sized chondrocyte-agarose constructs, yet optimal channel spacings to recapitulate native cartilage compositional and mechanical properties in constructs this large have not been identified. Transient active TGF-β treatment consistently reproduces native compressive Young׳s modulus (EY) and glycosaminoglycan (GAG) content in constructs, but standard dosages of 10ng/mL exacerbate matrix heterogeneity. To ultimately produce articular layer-sized constructs, we must first optimize channel spacing and investigate the role of TGF-β in the utility of channels. We cultured ∅10mm constructs with 0, 12, 19, or 27 nutrient channels (∅1mm) for 6-8 weeks with 0, 1, or 10ng/mL TGF-β; subsequently we analyzed them mechanically, biochemically, and histologically. Constructs with 12 or 19 channels grew the most favorably, reaching EY=344±113kPa and GAG and collagen contents of 10.8±1.2% and 2.2±0.2% of construct wet weight, respectively. Constructs with 27 channels had significantly less deposited GAG than other groups. Channeled constructs given 1 or 10ng/mL TGF-β developed similar properties. Without TGF-β, constructs with 0 or 12 channels exhibited properties that were indistinguishable, and lower than TGF-β-supplemented constructs. Taken together, these results emphasize that nutrient channels are effective only in the presence of TGF-β, and indicate that spacings equivalent to 12 channels in ∅10mm constructs can be employed in articular-layer-sized constructs with reduced dosages of TGF-β.

High seeding density of human chondrocytes in agarose produces tissue-engineered cartilage approaching native mechanical and biochemical properties

Animal cells have served as highly controllable model systems for furthering cartilage tissue engineering practices in pursuit of treating osteoarthritis. Although successful strategies for animal cells must ultimately be adapted to human cells to be clinically relevant, human chondrocytes are rarely employed in such studies. In this study, we evaluated the applicability of culture techniques established for juvenile bovine and adult canine chondrocytes to human chondrocytes obtained from fresh or expired osteochondral allografts. Human chondrocytes were expanded and encapsulated in 2% agarose scaffolds measuring ∅3-4mm×2.3mm, with cell seeding densities ranging from 15 to 90×10(6)cells/mL. Subsets of constructs were subjected to transient or sustained TGF-β treatment, or provided channels to enhance nutrient transport. Human cartilaginous constructs physically resembled native human cartilage, and reached compressive Young's moduli of up to ~250kPa (corresponding to the low end of ranges reported for native knee cartilage), dynamic moduli of ~950kPa (0.01Hz), and contained 5.7% wet weight (%/ww) of glycosaminoglycans (≥ native levels) and 1.5%/ww collagen. We found that the initial seeding density had pronounced effects on tissue outcomes, with high cell seeding densities significantly increasing nearly all measured properties. Transient TGF-β treatment was ineffective for adult human cells, and tissue construct properties plateaued or declined beyond 28 days of culture. Finally, nutrient channels improved construct mechanical properties, presumably due to enhanced rates of mass transport. These results demonstrate that our previously established culture system can be successfully translated to human chondrocytes.

Comparison of Osteochondral Autografts and Allografts for Treatment of Recurrent or Large Talar Osteochondral Lesions

BACKGROUND

The purpose of this study was to prospectively evaluate and compare the long-term clinical and radiographic outcomes of using osteochondral autograft and allograft to manage either recurrent or large osteochondral lesions of the talar dome (OLT) in a single surgeon's practice.

METHODS

Between January 2008 and January 2014, a total of 40 patients presented with either a recurrent OLT that failed initial arthroscopic treatment (ie, excision, curettage, debridement, and micro-fracture) or a primary OLT greater than 1.5 cm2 that had undergone no prior surgery. Before surgery, 20 patients were randomized to receive osteochondral autograft plugs (Arthrex, Naples, FL) from the ipsilateral superolateral distal femoral condyle whereas the remaining 20 were randomized to receive osteochondral allograft plugs from a fresh size-matched donor talus (Joint Restoration Foundation, Centennial, CO, and Arthrex, Naples, FL), but 4 of these were excluded that received a hemi-talus allograft with internal fixation. Preoperative and postoperative function and pain was graded using the Foot and Ankle Ability Measures (FAAM) scoring system and a Visual Analog Scale (VAS) of pain, respectively. Radiographs were assessed for osteochondral graft healing, joint congruency, and degenerative changes. Data regarding postoperative complications and revision surgeries were also recorded.

RESULTS

Of the 20 patients who received osteochondral autograft, the mean FAAM score increased from 54.4 preoperatively to 85.5 at the time of final follow-up. The mean VAS pain score decreased from 7.9 of 10 preoperatively to 2.2 of 10 at final follow-up. Two patients (10%) that received osteochondral autograft, 1 for a recurrent OLT of 1.3 cm2 and 1 for a primary OLT of 2.0 cm2, developed a symptomatic nonunion at the entire graft site. Both of these patients had their autograft converted to talar allograft plugs and achieved full osteochondral healing. At the time of final follow-up, no patients who received osteochondral autograft developed ankle degenerative changes or knee complications. The mean FAAM score of the 16 patients who received osteochondral allograft plugs increased from 55.2 preoperatively to 80.7 at the time of final follow-up. This postoperative score was lower than that of the osteochondral autograft group, but not to a statistically significant degree (P = .25). The mean VAS pain score decreased from 7.8 of 10 preoperatively to 2.7 of 10 at final follow-up. This postoperative score was higher than that of the osteochondral autograft group but not to a statistically significant degree (P = .15). Three patients (18.8%) that received osteochondral talar allograft, 2 for recurrent OLTs less than 1.5 cm2 and 1 for a primary OLT of 2.2 cm2, developed a symptomatic nonunion at the entire graft site. Two of these 3 patients had their allograft converted to osteochondral autograft plugs harvested from the ipsilateral superolateral distal femoral condyle and achieved full osteochondral healing. At the time of final follow-up, 1 of these 16 (6.3%) patients who received talar allograft as OLT treatment had developed asymptomatic anterior ankle arthritis upon radiographs.

CONCLUSION

Using fresh talar osteochondral allograft provided results that were comparable to the use of distal femoral osteochondral autograft for treating recurrent or large OLTs. Although the use of allograft avoided the risk of knee complications when harvesting autograft from the distal femur, fresh talar allograft may have lower healing rates than osteochondral autograft.

LEVEL OF EVIDENCE

Level II, comparative case series.

Effect of Static Compressive Strain, Anisotropy, and Tissue Region on the Diffusion of Glucose in Meniscus Fibrocartilage

Osteoarthritis (OA) is a significant socio-economic concern, affecting millions of individuals each year. Degeneration of the meniscus of the knee is often associated with OA, yet the relationship between the two is not well understood. As a nearly avascular tissue, the meniscus must rely on diffusive transport for nutritional supply to cells. Therefore, quantifying structure–function relations for transport properties in meniscus fibrocartilage is an important task. The purpose of the present study was to determine how mechanical loading, tissue anisotropy, and tissue region affect glucose diffusion in meniscus fibrocartilage. A one-dimensional (1D) diffusion experiment was used to measure the diffusion coefficient of glucose in porcine meniscus tissues. Results show that glucose diffusion is strain-dependent, decreasing significantly with increased levels of compression. It was also determined that glucose diffusion in meniscus tissues is anisotropic, with the diffusion coefficient in the circumferential direction being significantly higher than that in the axial direction. Finally, the effect of tissue region was not statistically significant, comparing axial diffusion in the central and horn regions of the tissue. This study is important for better understanding the transport and nutrition-related mechanisms of meniscal degeneration and related OA in the knee.

Topographical and sex variations in the T2 relaxation times of articular cartilage in the ankle joints of healthy young adults using 3.0T MRI

PURPOSE

To prospectively evaluate topographical and sex variations in the T2 relaxation values of tibiotalar cartilage (TTC) of healthy young adults.

MATERIALS AND METHODS

Sagittal 8-echo multiecho spin-echo T2 maps of TTC were acquired using a 3.0T MR in 25 male and 25 female healthy young adult participants. Quantitative measurements of T2 values in tibial cartilage (TBC) and talar cartilage (TLC) were obtained from three zones (anterior zone, AZ; middle zone, MZ; and posterior zone, PZ) and from four compartments (medial compartment, MC; midmedial compartment, MMC; midlateral compartment, MLC; and lateral compartment, LC) of TBC and TLC in the sagittal plane.

RESULTS

The T2 values of AZ (for TBC, 29.31 msec; for TLC, 35.81 msec) and MZ (for TBC, 28.56 msec; for TLC, 36.12 msec) in males were significantly higher than those in females (for AZ of TBC, 26.99 msec; for AZ of TLC, 33.56 msec; for MZ of TBC, 25.88 msec; for MZ of TLC, 31.85 msec) (for TBC, AZ, P = 0.009, MZ, P = 0.002; for TLC, AZ, P = 0.047, MZ, P = 0.001). Except for MMC and MLC of TBC (MMC, P = 0.02, MLC, P = 0.03), TTC T2 values did not differ significantly between these compartment in either group (for TBC, MC, P = 0.27, LC, P = 0.37; for TLC, MC, P = 0.26, MMC, P = 0.08, MLC, P = 0.30, LC, P = 0.10).

CONCLUSION

We found significant sex and topographic variations among TTC T2 values of healthy young adults. Acknowledgment of the significant topographical and sex variations in cartilage T2 values may minimize misinterpretation of ankle joint TTC T2 mapping data.

High prevalence of contralateral ankle abnormalities in association with knee osteoarthritis and malalignment

OBJECTIVE

To evaluate ankle joint abnormalities in a knee osteoarthritis (OA) cohort.

METHODS

Participants (n = 159) with symptomatic and radiographic OA in at least one knee underwent technetium-99m methylene diphosphonate bone scan (scored 0-3) of the ankles and forefeet. Knee radiographs were graded for OA features of joint space narrowing (JSN) and osteophyte (OST). Ankle symptoms and history of ankle injury were assessed by self-report. Knee alignment was measured from a long-limb radiograph. Ankle radiographs were obtained on those who returned for follow-up (n = 138) and were graded for ankle tibiotalar JSN and OST.

DESIGN

Ankle scintigraphic abnormalities were frequent (31% of individuals, one-third bilateral). Ankle symptoms were reported by 23% of individuals and history of ankle injury by 24%. Controlling for gender, age, body mass index (BMI), and contralateral predictor, ankle scintigraphic abnormalities were associated with: ipsilateral ankle symptoms (P = 0.005); contralateral knee JSN (P = 0.001), knee OST (P = 0.006) and knee malalignment (P = 0.08); and history of ankle injury or surgery of either ankle (P < 0.0001). At follow-up, scintigraphic abnormalities of the ankle were strongly associated with presence of tibiotalar radiographic OA (P < 0.0001).

CONCLUSIONS

Although considered rare, we observed a high prevalence of radiographic features of ankle OA in this knee OA cohort. History of overt ankle injury did not appear to account for the majority of ankle abnormalities. These results are consistent with a probable kinematic association of knee OA pathology and contralateral ankle abnormalities and suggest that interventions targeting mechanical factors may be needed to prevent ankle OA in the setting of knee OA.

Relationship between anisotropic diffusion properties and tissue morphology in porcine TMJ disc

OBJECTIVE

To investigate the relationship between anisotropic solute diffusion properties and tissue morphology in porcine temporomandibular joint (TMJ) discs.

DESIGN

TMJ discs from eleven pigs aged 6-8 months were divided into five regions: anterior, intermediate, posterior, lateral, and medial. The transport properties and tissue morphology were investigated in three orthogonal orientations: anteroposterior (AP), mediolateral (ML), and superoinferior (SI). The anisotropic diffusivity of fluorescein (332 Da) in the right discs was determined by the fluorescence recovery after photobleaching (FRAP) protocols. The tissue morphology in the left discs was quantified by scanning electron microscopy.

RESULTS

The diffusivities of fluorescein in the TMJ disc were significantly anisotropic, except for the anterior region. In the medial, intermediate, and lateral regions, the diffusion along the fiber orientation (i.e., AP direction) was significantly faster than the diffusion in ML and SI directions. In the posterior region, the diffusion along the fiber orientation (i.e., ML direction) was significantly faster than the diffusion in AP and SI directions. The diffusion in the anterior region was mostly isotropic with the lowest degree of diffusion anisotropy, as well as collagen fiber alignment, likely due to the multi-directional fiber arrangement. The anterior region had the highest mean diffusivity [65.6 (49.3-81.8) μm(2)/s] in the disc, likely due to its high water content. The overall average diffusivity of fluorescein across the TMJ disc was 57.0 (43.0-71.0) μm(2)/s.

CONCLUSIONS

The solute diffusion in porcine TMJ discs was strongly anisotropic and inhomogeneous, which associated with tissue structure (i.e., collagen fiber alignment) and composition (e.g., water content).

An In Vivo Lapine Model for Impact-Induced Injury and Osteoarthritic Degeneration of Articular Cartilage

OBJECTIVE

In this study, we applied a spring-loaded impactor to deliver traumatic forces to articular cartilage in vivo. Based on our recent finding that a 0.28-J impact induces maximal catabolic response in adult bovine articular cartilage in vitro using this device, we hypothesize that this impact will induce the formation of a focal osteoarthritic defect in vivo.

DESIGN

The femoral condyle of New Zealand White rabbits was exposed and one of the following procedures performed: 0.28 J impact, anterior cruciate ligament transection, articular surface grooving, or no joint or cartilage destruction (control). After 24 hours, 4 weeks, or 12 weeks (n = 3 for each time point), wounds were localized with India ink, and tissue samples were collected and characterized histomorphometrically with Safranin O/Fast green staining and Hoechst 33342 nuclear staining for cell vitality.

RESULTS

The spring-loaded device delivered reproducible impacts with the following characteristics: impact area of 1.39 ± 0.11 mm(2), calculated load of 326 ± 47.3 MPa, time-to-peak of 0.32 ± 0.03 ms, and an estimated maximal displacement of 25.1% ± 4.5% at the tip apex. The impact resulted in immediate cartilage fissuring and cell loss in the surface and intermediate zones, and it induced the formation of a focal lesion at 12 weeks. The degeneration was defined and appeared more slowly than after anterior cruciate ligament transection, and more pronounced and characteristic than after grooving.

CONCLUSION

A single traumatic 0.28 J impact delivered with this spring-loaded impactor induces focal cartilage degeneration characteristic of osteoarthritis.

Nutrient transport in human annulus fibrosus is affected by compressive strain and anisotropy

The avascular intervertebral disc (IVD) receives nutrition via transport from surrounding vasculature; poor nutrition is believed to be a main cause of disc degeneration. In this study, we investigated the effects of mechanical deformation and anisotropy on the transport of two important nutrients--oxygen and glucose--in human annulus fibrosus (AF). The diffusivities of oxygen and glucose were measured under three levels of uniaxial confined compression--0, 10, and 20%--and in three directions--axial, circumferential, and radial. The glucose partition coefficient was also measured at three compression levels. Results for glucose and oxygen diffusivity in AF ranged from 4.46 × 10(-7) to 9.77 × 10(-6) cm(2)/s and were comparable to previous studies; the glucose partition coefficient ranged from 0.71 to 0.82 and was also similar to previous results. Transport properties were found to decrease with increasing deformation, likely caused by fluid exudation during tissue compression and reduction in pore size. Furthermore, diffusivity in the radial direction was lower than in the axial or circumferential directions, indicating that nutrient transport in human AF is anisotropic. This behavior is likely a consequence of the layered structure and unique collagen architecture of AF tissue. These findings are important for better understanding nutritional supply in IVD and related disc degeneration.

Varus ankle and osteochondral lesions of the talus

Considerable recent orthopedic literature is dedicated to either the surgical management of OLTs and correction of varus ankle alignment, but little is published on the combination of these 2 problems. We anticipate that future clinical and biomechanical research will address their simultaneous treatment; until then, we will continue to extrapolate from their independent management.

MR imaging of the articular cartilage of the knee and ankle

Cartilage abnormalities in the knee and ankle are a common source of pain and are often difficult to diagnose clinically or radiographically. MR imaging is a valuable tool for diagnosing and characterizing cartilage lesions of both the knee and ankle. An understanding of the appearance of cartilage, and an understanding of how and when to report cartilage injury in the knee and ankle based on current grading systems allows the radiologist to provide the most helpful reports to referring clinicians. This article presents the range of cartilage pathologies in the knee and ankle and provides clinically relevant guidelines.

Effects of in vivo exercise on ankle cartilage deformation and recovery in healthy volunteers: an experimental study

OBJECTIVE

To monitor ankle cartilage 3D volume changes after in vivo exercise and during recovery.

METHOD

Based on 3D MRI, 3D volumes of talar and tibial cartilage were calculated before and after 30 bilateral knee bends in 12 healthy volunteers. 3D volumes were calculated at five time points (one pre- and four post-scans) determining deformation and recovery for both cartilage plates of interest. Post-scans ran immediately after the exercise and were repeated according to a 15 min interval. 3D volumes were subjected to repeated measures GLM. Additionally, relative surface area use during deformation was compared between plates using a Wilcoxon Signed Ranks test and its correlation with deformation was investigated using Spearman's rho.

RESULTS

Mean 3D volume change percentages for talar cartilage after the exercise were: -10.41%, -8.18%, -5.61% and -3.90%. For tibial cartilage mean changes were: -5.97%, -5.75%, +0.89% and +1.51%. For talar cartilage changes were significant, except following 30 min post-exercise. For tibial cartilage no changes were significant. At all time points, no significant differences in relative volume changes between both cartilage plates existed. Although no significant differences in relative surface area use between plates were revealed, a moderate to strong correlation with deformation existed.

CONCLUSION

Ankle cartilage endures substantial deformation after in vivo loading that was restored within 30 min for the talus. Overall cartilage contact area involvement might be associated with cartilage quality maintenance in the upper ankle. Talar cartilage is suggested to play a critical role in intra-articular shock attenuation when compared to tibial cartilage.

Dynamic loading of immature epiphyseal cartilage pumps nutrients out of vascular canals

The potential influence of mechanical loading on transvascular transport in vascularized soft tissues has not been explored extensively. This experimental investigation introduced and explored the hypothesis that dynamic mechanical loading can pump solutes out of blood vessels and into the surrounding tissue, leading to faster uptake and higher solute concentrations than could otherwise be achieved under unloaded conditions. Immature epiphyseal cartilage was used as a model tissue system, with fluorescein (332 Da), dextran (3, 10, and 70 kDa) and transferrin (80 kDa) as model solutes. Cartilage disks were either dynamically loaded (± 10% compression over a 10% static offset strain, at 0.2 Hz) or maintained unloaded in solution for up to 20 h. Results demonstrated statistically significant solute uptake in dynamically loaded (DL) explants relative to passive diffusion (PD) controls for all solutes except unbound fluorescein, as evidenced by the DL:PD concentration ratios after 20 h (1.0 ± 0.2, 2.4 ± 1.1, 6.1 ± 3.3, 9.0 ± 4.0, and 5.5 ± 1.6 for fluorescein, 3, 10, and 70 kDa dextran, and transferrin). Significant uptake enhancements were also observed within the first 30s of loading. Termination of dynamic loading produced dissipation of enhanced solute uptake back to PD control values. Confocal images confirmed that solute uptake occurred from cartilage canals into their surrounding extracellular matrix. The incidence of this loading-induced transvascular solute pumping mechanism may significantly alter our understanding of the interaction of mechanical loading and tissue metabolism.

Human ankle cartilage deformation after different in vivo impact conditions

Recently, the general finding of increased ankle cartilage stiffness to loading has been challenged, suggesting the need for the investigation of different in vivo loading conditions. Therefore, the objectives of the present study were to determine ankle (talar) cartilage deformation after in vivo loading using 3D volume change calculation and to establish any difference in volume change between four weight-bearing exercises. The four exercises represented increasing impact (bilateral knee bends <unilateral knee bends <drop jumps) as well as two types of loading: dynamic and static loading (i.e. unilateral knee bends and unilateral static stance). Based on MRI, 3D reconstructions of talar cartilage were generated to determine 3D volumes before and after four exercises in 13 healthy subjects (bilateral and unilateral knee bends, static unilateral stance, drop jumps). Mean talar deformation (volume decrease) was 8.3% after bilateral knee bends (P = 0.001), 7.7% after unilateral knee bends (P = 0.020), 14.6% after unilateral static stance (P < 0.001), 12.5% after drop jumps (P = 0.001). Statistical analysis also revealed deformation to be significantly higher after unilateral static stance than after unilateral knee bends (P = 0.017). These results suggest that talar cartilage endures substantial deformation during in vivo loading characterized by more deformation (i.e. higher volume change) after static than after dynamic loading.

Anisotropic solute diffusion tensor in porcine TMJ discs measured by FRAP with spatial Fourier analysis

A new method solely based on spatial Fourier analysis (SFA) was developed to completely determine a two-dimensional (2D) anisotropic diffusion tensor in fibrous tissues using fluorescence recovery after photobleaching (FRAP). The accuracy and robustness of this method was validated using computer-simulated FRAP experiments. This method was applied to determine the region-dependent anisotropic diffusion tensor in porcine temporomandibular joint (TMJ) discs. The average characteristic diffusivity of 4 kDa FITC-Dextran across the disc was 26.05 ± 4.32 μm²/s which is about 16% of its diffusivity in water. In the anteroposterior direction, the anterior region (30.99 ± 5.93 μm²/s) had significantly higher characteristic diffusivity than the intermediate region (20.49 ± 5.38 μm²/s) and posterior region (20.97 ± 2.46 μm²/s). The ratio of the two principal diffusivities represents the anisotropy of the diffusion and ranged between 0.45 and 0.51 (1.0 = isotropic). Our results indicated that the solute diffusion in TMJ discs is inhomogeneous and anisotropic. These findings suggested that diffusive transport in the TMJ disc is dependent on tissue composition (e.g., water content) and structure (e.g., collagen orientation). This study provides a new method to quantitatively investigate the relationship between solute transport properties and tissue composition and structure.

In vitro investigations into enhancement of tPA bioavailability in whole blood clots using pulsed-high intensity focused ultrasound exposures

Investigations were carried out on the manner by which pulsed-high intensity focused ultrasound (HIFU) enhances the effectiveness of tissue plasminogen activator (tPA) in whole blood clots, in vitro. Scanning electronic microscope (SEM) of the surface of the clots showed that the exposures increased exposed fibrin, as well as the number of openings to more interior regions. These findings were supported by fluorescent antibody labeling of tPA in frozen sections of clots treated post-HIFU. Here, improved accumulation at the surface and penetration of the tPA into the clots were observed in those treated with HIFU. Fluorescence recovery after photobleaching was also performed, indicating that the diffusion coefficient increased 6.3-fold for fluorescently labeled dextrans, comparable in size to tPA, in the HIFU-treated clots. Improved understanding of the manner by which pulsed--HIFU exposures can improve the effectiveness of thrombolytics will help optimize the exposures for this application and potentially facilitate translation to the clinic.

A review of the differences between normal and osteoarthritis articular cartilage in human knee and ankle joints

BACKGROUND

Osteoarthritis (OA) is the most common joint disease yet its pathophysiology is still poorly understood. It is more prevalent in some lower limb joints than others; in particular the knee is more commonly affected than the ankle. Research into articular cartilage and OA has primarily focussed on using animal models. However, it is apparent that articular cartilage differs between species, so more research is concentrating on human cartilage.

OBJECTIVE

This paper reviews recent studies that have been undertaken to elucidate the reasons for this, and to discover if the findings would alter the conception that articular cartilage is not capable of repair.

METHOD

Primary research papers into human knee and ankle cartilage published since 1997 have been reviewed.

RESULTS

Differences in the structure, metabolism, physical properties and response to trauma have been found, implying that ankle cartilage may be more resistant to damage.

CONCLUSIONS

More research is needed before definitive conclusions can be reached, but the findings so far suggest that OA should not be accepted as the inevitable outcome of joint injury and individuals and practitioners, such as podiatrists, may be able to use simple measures to prevent or delay its onset.

TRANSPORT PROPERTIES OF CARTILAGINOUS TISSUES

Cartilaginous tissues, such as articular cartilage and intervertebral disc, are avascular tissues which rely on transport for cellular nutrition. Comprehensive knowledge of transport properties in such tissues is therefore necessary in the understanding of nutritional supply to cells. Furthermore, poor cellular nutrition in cartilaginous tissues is believed to be a primary source of tissue degeneration, which may result in osteoarthritis (OA) or disc degeneration. In this mini-review, we present an overview of the current status of the study of transport properties and behavior in cartilaginous tissues. The mechanisms of transport in these tissues, as well as experimental approaches to measuring transport properties and results obtained are discussed. The current status of bioreactors used in cartilage tissue engineering is also presented.

Stimulation of ankle cartilage: other emerging technologies (cellular, electricomagnetic, etc.)

Advances in understanding age-related changes in articular cartilage, joint homeostasis, the natural healing process after cartilage injury, and improved standards for evaluation of a joint surface made the ultimate goal of cartilage repair a possibility. New strategies for enhancement of articular cartilages' limited healing potential and biologic regeneration include advances in tissue engineering and the use of electromagnetic fields. This article reviews developments in basic science and clinical research made with these emerging technologies concerning treatment of articular cartilage defects and treatment of osteoarthritis of the ankle.

Changes in pore morphology and fluid transport in compressed articular cartilage and the implications for joint lubrication

Cartilage sections were cut from the middle zone of pig knee articular cartilage and attached to substrates in two different kinds of newly designed 'pressure cells', one for fluorescence the other for NMR measurements. The fluorescence cell was filled with buffer solution containing fluorescently marked 70 kDa dextran which was allowed to diffuse into the cartilage pores. A second glass surface was then pressed down onto the thin cartilage sample under different loads (pressures), and the resulting compression (strain) and change in pore volume were measured as a function of time, simultaneously with measurements of the lateral diffusion and flow pattern of the dextran molecules using Fluorescence Recovery After Photobleaching (FRAP). Complementary experiments were made on the normal diffusion coefficients of pure electrolyte solutions (no dextran) in thicker cartilage sections with pulse-gradient NMR using a new pressure cell suitable for such measurements. Taken together our results show that the highly anisotropic structure of cartilage has a strong effect on the way fluid diffuses laterally and normally at different stages of compression. Our results also show how geometric constraints on a cartilage network and trapped high MW polymer such as HA during normal compressions are likely to affect both the normal and the lateral mobilities of polyelectrolytes and water.

Effects of interleukin-1 on calcium signaling and the increase of filamentous actin in isolated and in situ articular chondrocytes

OBJECTIVE

To determine whether interleukin-1 (IL-1) initiates transient changes in the intracellular concentration of [Ca2+]i and the organization of filamentous actin (F-actin) in articular chondrocytes.

METHODS

Articular chondrocytes within cartilage explants and enzymatically isolated chondrocytes were loaded with Ca(2+)-sensitive fluorescence indicators, and [Ca2+]i was measured using confocal fluorescence ratio imaging during exposure to 10 ng/ml IL-1alpha. Inhibitors of Ca2+ mobilization (Ca(2+)-free medium, thapsigargin [inhibitor of Ca-ATPases], U73122 [inhibitor of phospholipase C], and pertussis toxin [inhibitor of G proteins]) were used to determine the mechanisms of increased [Ca2+]i. Cellular F-actin was quantified using fluorescently labeled phalloidin. Toxin B was used to determine the role of the Rho family of small GTPases in F-actin reorganization.

RESULTS

In isolated cells on glass and in in situ chondrocytes within explants, exposure to IL-1 induced a transient peak in [Ca2+]i that was generally followed by a series of decaying oscillations. Thapsigargin, U73122, and pertussis toxin inhibited the percentage of cells responding to IL-1. IL-1 increased F-actin content in chondrocytes in a manner that was inhibited by toxin B.

CONCLUSION

Both isolated and in situ chondrocytes respond to IL-1 with transient increases in [Ca2+]i via intracellular Ca2+ release mediated by the phospholipase C and inositol trisphosphate pathways. The influx of Ca2+ from the extracellular space and the activation of G protein-coupled receptors also appear to contribute to these mechanisms. These findings suggest that Ca2+ mobilization may be one of the first signaling events in the response of chondrocytes to IL-1.