Collagen fibrils are differently organized in weight-bearing and not-weight-bearing regions of pig articular cartilage

Rolling-sliding load decreases the loss of chondrocyte viability and the mechanical properties of cartilage explants preserved in vitro

The viability of cartilage explants preserved in vitro decreases with time, which limits its use for transplantation. The effect of mechanical stimulation to cartilage explants in vitro is unknown. In this study, we observed the effects of mechanical stimulation on chondrocyte viability and the mechanical properties of cartilage explants preserved in vitro using a rolling-sliding loading device designed by us, and the optimal stimulation protocol was established. A cylindrical osteochondral mass drilled on the femoral condyle of a healthy pig was divided into two groups (loading group and control group), and changes in the chondrocyte survival rate, matrix composition and cartilage biomechanical properties was observed at different time points. Additionally, the mRNA expression of the apoptosis-related proteins caspase-3/Bax/Bcl-2, the cytoskeletal proteins actin/vimentin, and the matrix-related protein MMP13 were detected. The loading group exhibited delayed collagen and aggrecan degeneration and improved chondrocyte viability for three days. Protein and mRNA detection showed that apoptotic factors such as caspase-3 and Bax decreased rapidly in cartilage tissue after loading. The cytoskeletal proteins actin and vimentin showed no significant changes in mRNA expression in the control group, but was significantly higher in the loading group. MMP-13 mRNA expression was significantly higher in both the control group and loading group. Overall, this study suggests that suitable mechanical stimulation decreases the loss of chondrocyte viability and the mechanical properties of cartilage explants in vitro and improves cartilage preservation.

Sex- and age-dependence of region- and layer-specific knee cartilage composition (spin-spin-relaxation time) in healthy reference subjects

Compositional measures of articular cartilage are accessible in vivo by magnetic resonance imaging (MRI) based relaxometry and cartilage spin-spin transverse relaxation time (T2) has been related to tissue hydration, collagen content and orientation, and mechanical (functional) properties of articular cartilage. The objective of the current study was therefore to evaluate subregional variation, and sex- and age-differences, in laminar (deep and superficial) femorotibial cartilage T2 relaxation time in healthy adults. To this end, we studied the right knees of 92 healthy subjects from the Osteoarthritis Initiative reference cohort (55 women, 37 men; age range 45-78 years; BMI 24.4±3.1) without knee pain, radiographic signs, or risk factors of knee osteoarthritis in either knee. T2 of the deep and superficial femorotibial cartilages was determined in 16 femorotibial subregions, using a multi-echo spin-echo (MESE) MRI sequence. Significant subregional variation in femorotibial cartilage T2 was observed for the superficial and for the deep (both p<0.001) cartilage layer (Friedman test). Yet, layer- and region-specific femorotibial T2 did not differ between men and women, or between healthy adults below and above the median age (54 years). In conclusion, this first study to report subregional (layer-specific) compositional variation of femorotibial cartilage T2 in healthy adults identifies significant differences in both superficial and deep cartilage T2 between femorotibial subregions. However, no relevant sex- or age-dependence of cartilage T2 was observed between age 45-78 years. The findings suggest that a common, non-sex-specific set of layer-and region-specific T2 reference values can be used to identify compositional pathology in joint disease for this age group.

T2* mapping for articular cartilage assessment: principles, current applications, and future prospects

With advances in joint preservation surgery that are intended to alter the course of osteoarthritis by early intervention, accurate and reliable assessment of the cartilage status is critical. Biochemically sensitive MRI techniques can add robust biomarkers for disease onset and progression, and therefore, could be meaningful assessment tools for the diagnosis and follow-up of cartilage abnormalities. T2* mapping could be a good alternative because it would combine the benefits of biochemical cartilage evaluation with remarkable features including short imaging time and the ability of high-resolution three-dimensional cartilage evaluation-without the need for contrast media administration or special hardware. Several in vitro and in vivo studies, which have elaborated on the potential of cartilage T2* assessment in various cartilage disease patterns and grades of degeneration, have been reported. However, much remains to be understood and certain unresolved questions have become apparent with these studies that are crucial to the further application of this technique. This review summarizes the principles of the technique and current applications of T2* mapping for articular cartilage assessment. Limitations of recent studies are discussed and the potential implications for patient care are presented.

Angle-sensitive MRI for quantitative analysis of fiber-network deformations in compressed cartilage

PURPOSE

To demonstrate the feasibility of a novel experimental method to quantitatively analyze fiber-network deformation in compressed cartilage by angle-sensitive magnetic resonance imaging (MRI) of cartilage.

METHODS

Three knee cartilage samples of an adult sheep were imaged in a high-resolution MRI scanner at 7 T. Main fiber orientation and its "offset" from the direction perpendicular to the bone-cartilage boundary were derived from MR images taken at different orientations with respect to B0. Bending of the collagen fibers was determined from weight-bearing MRI with the load (up to 1.0 MPa) applied over the whole sample surface. A "fascicle" model of the cartilage ultrastructure was assumed to analyze characteristic intensity variations in T2-weighted images under load.

RESULTS

T2-weighted MR images showed a strong variation of the signal intensities with sample orientation. In the T2-weighted weight-bearing series, regions of high signal intensity underwent shifts from the lateral to the central parts in all three cartilage samples. The bending of the collagen fibers was determined to be 27.2°, 35.4°, and 40.0° per MPa, respectively.

CONCLUSION

Assuming a "fascicle" model, the presented MRI method provides quantitative measures of structural adjustments in compressed cartilage. Our preliminary analysis suggests that cartilage fiber deformation includes both bending and crimping.

MR imaging of articular cartilage physiology

The newer magnetic resonance (MR) imaging methods can give insights into the initiation, progression, and eventual treatment of osteoarthritis. Sodium imaging is specific for changes in proteoglycan (PG) content without the need for an exogenous contrast agent. T1ρ imaging is sensitive to early PG depletion. Delayed gadolinium-enhanced MR imaging has high resolution and sensitivity. T2 mapping is straightforward and is sensitive to changes in collagen and water content. Ultrashort echo time MR imaging examines the osteochondral junction. Magnetization transfer provides improved contrast between cartilage and fluid. Diffusion-weighted imaging may be a valuable tool in postoperative imaging.

Viscoelastic and Biomechanical Properties of Osteochondral Tissue Constructs Generated From Graded Polycaprolactone and Beta-Tricalcium Phosphate Composites

The complex micro-/nanostructure of native cartilage-to-bone insertion exhibits gradations in extracellular matrix components, leading to variations in the viscoelastic and biomechanical properties along its thickness to allow for smooth transition of loads under physiological movements. Engineering a realistic tissue for osteochondral interface would, therefore, depend on the ability to develop scaffolds with properly graded physical and chemical properties to facilitate the mimicry of the complex elegance of native tissue. In this study, polycaprolactone nanofiber scaffolds with spatially controlled concentrations of β-tricalcium phosphate nanoparticles were fabricated using twin-screw extrusion-electrospinning process and seeded with MC3T3-E1 cells to form osteochondral tissue constructs. The objective of the study was to evaluate the linear viscoelastic and compressive properties of the native bovine osteochondral tissue and the tissue constructs formed in terms of their small-amplitude oscillatory shear, unconfined compression, and stress relaxation behavior. The native tissue, engineered tissue constructs, and unseeded scaffolds exhibited linear viscoelastic behavior for strain amplitudes less than 0.1%. Both native tissue and engineered tissue constructs demonstrated qualitatively similar gel-like behavior as determined using linear viscoelastic material functions. The normal stresses in compression determined at 10% strain for the unseeded scaffold, the tissue constructs cultured for four weeks, and the native tissue were 0.87±0.08 kPa, 3.59±0.34 kPa, and 210.80±8.93 kPa, respectively. Viscoelastic and biomechanical properties of the engineered tissue constructs were observed to increase with culture time reflecting the development of a tissuelike structure. These experimental findings suggest that viscoelastic material functions of the tissue constructs can provide valuable inputs for the stages of in vitro tissue development.

Evaluation of age-related changes in the structure of the equine tarsometatarsal osteochondral unit

OBJECTIVE

To investigate effects of age on thickness and morphologic characteristics of hyaline cartilage, calcified cartilage, total cartilage, and subchondral bone (SCB) in the equine tarsometatarsal joint.

SAMPLE POPULATION

23 tarsal joints from cadavers of 23 ponies (11 days to 25 years old); ponies were limited to pasture exercise and euthanatized for reasons not related to this study. Procedures-Tarsi were allocated into several age groups (11 days old [n = 3], 6 to 9 months old [4], 2 to 3 years old [3], 6 to 8 years old [4], 11 to 17 years old [6], and 20 to 25 years old [3]). Histologic examination and histomorphometric measurement of hyaline cartilage, calcified cartilage, total cartilage, and SCB were performed at medial and lateral sites.

RESULTS

A significant decrease was detected in thickness of hyaline cartilage and total cartilage with increasing age, but there was a significant increase in thickness of calcified cartilage and SCB with increasing age. Differences in chondrocyte and collagen fiber arrangement, tidemark, and osteochondral junction morphology were evident among age groups.

CONCLUSIONS AND CLINICAL RELEVANCE

These findings suggested that the various tissues of the osteochondral unit change in different ways with age. The response of each tissue may be related to relative response of the tissues to strains induced by pasture exercise but could have an influence on how the overall properties of the osteochondral unit change with age. The findings may also be suggestive of changes that develop prior to the onset of osteoarthritis.

Averaged and depth-dependent anisotropy of articular cartilage by microscopic imaging

OBJECTIVES

To identify the common connections among the averaged and depth-dependent anisotropic properties of articular cartilage by performing a meta-analysis of several published multidisciplinary imaging results. The imaging techniques involved include microscopic magnetic resonance imaging (microMRI), polarized light microscopy (PLM), Fourier-transform infrared imaging (FTIRI), and transmission electron microscopy (TEM).

METHODS

Several physical properties of cartilage are incorporated in this meta-analysis. These tissue properties include T(2) anisotropy from microMRI, angle and retardance from PLM, infrared anisotropy from FTIRI, and image morphology from TEM. Because the specimens in these studies all came from the same type of canine humeral joints, it is possible to correlate these multidisciplinary tissue properties using a common platform.

RESULTS

An ellipse model was used to identify the connections among these tissue properties in terms of the anisotropy of articular cartilage, in each histological zone as well as for the entire noncalcified tissue. It was found that many aspects of these tissue properties can be interpreted beyond their usual meanings as measured, based on 3 features of an ellipse: the concentration, the orientation, and the anisotropy.

CONCLUSIONS

The ellipse model is a useful graphical concept in cartilage imaging since it helps to bring together the measured physical/morphological/chemical quantities in these imaging tools and the anisotropic structure of articular cartilage. Two possible mechanisms for the angular transition of collagen fibrils in cartilage are discussed.

Macroscopic structure of articular cartilage of the tibial plateau: influence of a characteristic matrix architecture on MRI appearance

OBJECTIVE

The purpose of our study was to describe the structural organization of the extracellular matrix of articular cartilage of the tibial plateau and its influence on MRI appearance.

MATERIALS AND METHODS

Spin-echo images of 11 resected tibial plateaus acquired at 7 T were compared with the structure of the extracellular matrix as shown by fracture sectioning the samples in the plane of imaging. Four samples were scanned at two different orientations relative to the main magnetic field (B(0)). T2 maps were acquired in two orientations on three of these four samples.

RESULTS

On the basis of the presence of reproducible regional variations in the shape of the matrix, a characteristic matrix architecture was described. The location of peak signal intensity and T2 on MRI correlated with the level at which the matrix was estimated to be aligned at approximately 55 degrees to B(0) (r = 0.91). This correlation of matrix orientation relative to B(0) with T2 and signal intensity on MRI was not altered by regional variations in the shape of the matrix or by imaging samples at two different orientations.

CONCLUSION

The structure of the extracellular matrix, through its orientation-dependent influence on T2 decay, exerts a strong influence on the MRI appearance of cartilage. At the tibial plateau, a characteristic matrix architecture is associated with an equally characteristic MRI appearance.

Imaging the physical and morphological properties of a multi-zone young articular cartilage at microscopic resolution

PURPOSE

To characterize a number of physical and morphologic properties of young articular cartilage. These properties include the anisotropy of T(2) relaxation, optical retardation, orientation of the collagen fibrils, total thickness of the tissue, number of histologic sub-zones in the tissue, width of individual sub-zones, and correlation between the depths of the local T(2) maxima and the local retardation minima.

MATERIALS AND METHODS

Microscopic magnetic resonance imaging (mciro MRI) and polarized light microscopy (PLM) were used to examine three side-by-side specimens from a humeral head of a three-month-old beagle that exhibited a unique topographic heterogeneity from three-zones centrally to multi-zone peripherally.

RESULTS

The centrally located specimen showed that the collagen fibrils across the tissue depth have a pattern of the classic three histologic sub-zones (tangential, transitional, and radial). A much more complicated multi-zone structure was found in the specimen located peripherally, with a second transitional zone and a second tangential zone located at the deep part of the tissue. We also showed that the orientation of the collagen fibrils that form the cocoon-shaped territorial matrix surrounding the clusters of chondrocytes can be imaged by our PLM technique.

CONCLUSION

The results from the young animal in this report, together with our observations from older animals, demonstrate that MRI and PLM can be used to study the epiphyseal expansion of cartilage in young animals during its growth and subsequent loss in older animals. An illustrative model for the structure of collagen fibrils in a humeral head is suggested as an extension to the classic three-zone model for young articular cartilage.

Characteristics of topographical heterogeneity of articular cartilage over the joint surface of a humeral head

OBJECTIVE

To investigate the topographical variations among the morphological and physical properties of articular cartilage over a canine humeral head.

DESIGN

Nine side-by-side specimens from a canine humeral head were used in the combined polarized light microscopy (PLM) and microscopic magnetic resonance imaging (muMRI). Sixteen images were acquired from each specimen in muMRI. Subsequently, ten to fourteen histological slices were prepared at the location of the muMRI slice from each specimen. Four optical images were acquired from each histological slice. Using these images, the images of the T(2) relaxation, the optical retardation, and the collagen-fiber orientation were constructed in two dimensions.

RESULTS

Along the medial/lateral direction of the humeral head, we have observed a number of topographical variations. These include the total thickness of the cartilage tissue, the thickness of the individual histological zones, the depth of the minimum optical retardation in the tissue, and the depth of the maximum T(2) relaxation in the tissue. We found that the depth of the minimum retardation from PLM occurs at the geometric middle point of the transitional zone in histology, and that the depth of the maximum T(2) relaxation from muMRI is closely correlated with the minimum retardation depth in PLM. In addition, although the thickness of the total tissue increases approximately by the same amount on both the lateral and medial sides of the joint, a slight asymmetry exists between the measurements from the medial and lateral halves of the humerus.

CONCLUSIONS

Cartilage from different sites over a single joint could have different properties, possibly the consequence of the variation in load-bearing status. Because of these topographical variations, one must keep in mind that a different sampling site within a relatively small area of the same joint may significantly influence the results of the study.