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Experimental Studies |
1 From the Departments of Pharmacology (A.W., P.T.A.O., N.C.G., P.A.N., A.G.B., P.M.G., D.H.J.L.) and Clinical Rheumatology (D.H.J.L.), UMR 7561, CNRS-Université Henri Poincaré, Nancy I, Physiopathologie et Pharmacologie Articulaires, Faculté de Médecine, Avenue de la Forêt de Haye, BP 184, F 54505 Vandoeuvre Les Nancy, France; and the Department of Medical Magnetic Resonance Research (U2R2M), ESA CNRS 8081, Orsay, Paris, France (J.P.B.R., P.D.G., G.M.G.). Received May 5, 2000; revision requested June 17; revision received August 7; accepted September 6. Supported by grants from Projet Hospitalier de Recherche Clinique (1994 and 1998), the "Pole Européen de Santé," and Groupement de Recherches CNRS 2237. Address correspondence to P.A.N. (e-mail: netter@medecine.uhp-nancy.fr).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Patellae from four groups of rats aged 4 weeks, 8 weeks, 4 months, and more than 6 months ("old rats") were studied ex vivo with an 8.5-T microimager. T2 values were calculated on transverse rat patellar sections and displayed with a color scale (the T2 map) on a pixel-by-pixel basis. Biochemical and histologic studies were performed to evaluate the influence of proteoglycans and collagen contents on T2 values of the patellar cartilage.
RESULTS: On the T2 map, the maturation process until 10 weeks was characterized by a decrease in T2 values and in cartilage thickness. The biochemical data revealed a global decrease in proteoglycans and a progressive global increase in collagen content, whereas the histologic study revealed subtle zonal variation in matrix constituents with depth. As aging progressed, the T2 values were low, without important variations, whereas the global cartilage thickness decreased. The cartilage matrix became globally more fibrotic, especially in the deepest zone. Biochemical analysis revealed that collagen content was more determinant of MR signal intensity than was proteoglycans content during maturation and aging.
CONCLUSION: T2 mapping allows characterization of variations in cartilage matrix constituents and thickness.
Index terms: Aging, 45.91 • Animals • Cartilage, MR, 45.91 • Knee, ligaments, menisci, and cartilage, 45.91 • Knee, MR, 45.121411 • Magnetic resonance (MR), tissue characterization, 45.121411 • Patella, 453.91
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Ongoing research requires noninvasive techniques to identify early cartilage damage that is still reversible. These techniques are used in an attempt to characterize the process of cartilage degeneration and evaluate the effectiveness of chondroprotective agents in repairing damaged cartilage. In recent years, magnetic resonance (MR) imaging evolved into a valuable method for evaluating cartilage damage (5). Previous MR imaging studies (6,7) focused on techniques to assess the morphology of articular cartilage (ulcerations, thinning) and to measure the thickness and volume of cartilage. New methods that use short echo times have enabled the clinician to obtain signal intensity from the short T2 components of cartilage and from the deepest zones (8,9). These sequences allow the characterization of specific areas of cartilage defects and help unmask collagen fibers with more sensitivity and specificity, but they are not yet used in clinical practice. Different experimental approaches have demonstrated that MR imaging can be used to characterize the biochemical content of cartilage (10). Thus, magnetization transfer, which is mainly dependent on collagen integrity, showed great promise as a tool for detecting early cartilage abnormalities (11,12).
The use of ionic contrast agents provides another feasible way to quantify the decrease in proteoglycans in cartilage. The loss of proteoglycans can lead to a loss of the fixed negative charge density, which can be mapped either positively or negatively according to the contrast agents used (13,14). The last approach is represented by the T2 relaxation time of articular cartilage, which may vary with cartilage depth and may be affected by cartilage lesions such as edema or fibrosis (15–20). These T2 variations could be related to the known distribution of water content in cartilage, to the distribution of proteoglycans content with cartilage depth, or to the collagen network organization in different zones of cartilage (21,22).
Since MR imaging clinical studies are relatively limited, the contribution of experimental models is necessary. These models may enable the researcher to evaluate cartilage damage during active disease, evaluate the action of potential chondroprotective agents, and allow the establishment of histologic and biochemical correlations with cartilage signal intensity at MR imaging (23–25).
The purpose of our study was to investigate the usefulness of T2 mapping in characterizing the evolution of cartilage matrix content and thickness during the maturation and aging process in the rat patella. The rat knee is a common experimental model of joint disease, and the femoropatellar joint is generally easy to explore with MR imaging, which allows comparisons among MR imaging and histologic and biochemical examination results.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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months, 4 months, 6 months, and more than 6 months ("old rats"). For MR imaging, an additional population of 12 rats was examined; three animals of each of the following groups: 4 weeks, 8 weeks, 4 months, and old rats. Rats were anesthetized and killed by means of cervical dislocation, and their knees were dissected to perform histologic and biochemical examinations.
Histologic Processing
For each of the 10 groups, six of 16 patellae were randomly selected, dissected, and fixed in 12% neutral buffered formalin solution. Patellae were decalcified for 7 days in ethylenediaminetetraacetic acid, or EDTA, and were then dehydrated and embedded in paraffin. The central region of each patella was serially sectioned (5 µm) in the longitudinal plane with a microtome. The sections were stained with hematoxylin-eosin to assess cellularity, with toluidine blue to assess proteoglycan, and with picrosirius red to assess collagen. To examine the collagen network organization, picrosirius red sections were analyzed with a polarizing microscope. For thickness measurement, sections stained with hematoxylin-eosin were digitized with a color digital video camera (WV-CL350; Panasonic, Osaka, Japan), recorded in tagged image file format, or TIFF, on a workstation (Indy; Silicon Graphics, Mountain View, Calif), and transferred to a personal computer (Macintosh 9600; Apple Computer, Cupertino, Calif). As for MR imaging, the cartilage thickness was defined as the mean value of 10 measurements performed from the superficial to the calcified zone, perpendicular to the cartilage surface, with a stage micrometer and imaging software.
Biochemical Analysis
The contents of sulfated glycosaminoglycan and collagen were determined in the 10 groups defined. For each group, 10 of 16 patellae were randomly selected, dissected, and decalcified for 18 hours in 5% formic acid. After decalcification, cartilage was stripped from the underlying bone and weighed.
Quantification of sulfated glycosaminoglycans.—For each group, five of 10 cartilage specimens were randomly selected and digested at 60°C for 2 hours in 210 µL of 20 mmol/L sodium phosphate buffer containing 60 µg of papain (Sigma Aldrich, Saint Quentin Fallavier, France). The reaction was stopped with 220 mmol/L iodoacetic acid. The volume was made up to 1 mL by the addition of 50 mmol/L Tris HCl (pH 8.0).
The assay was calibrated by using reagent blanks and standards containing 5–100-µg chondroitin sulfate in the same solvent as the one used for the samples. The metachromatic reaction of dimethylmethylene blue was measured at 525 nm with a spectrometer (MR5000; Dynatech, Guyancourt, France) connected to a computer (486SX; Packard Bell, Paris, France) for data analysis. The results were expressed as chondroitin sulfate equivalents per milligram of cartilage (26).
Quantification of hydroxyproline in cartilage.—For each group, five of 10 cartilage specimens were randomly selected, weighed, and used to determine the hydroxyproline concentration, which reflects the tissue collagen content. Each cartilage specimen was hydrolyzed in 6N HCl at 130°C for 3 hours in a small sealed Pyrex test tube. The tubes were opened and neutralized with 6N NaOH solution (pH 5–6), and 200 µL of hydrolysate was taken from each tube. Hydroxyproline oxidation was initiated by adding 500 µL of chloramine T. The tube contents were mixed and incubated for 20 minutes at room temperature. Finally, 500 µL of aldehyde-perchloric acid reagent was added, mixed, and incubated for 20 minutes at 70°C. The assay was calibrated by using reagent blanks and standards containing 0.5–8.0 µg of hydroxyproline in the same solvent as the one used for the samples. The absorbance was determined with a spectrophotometer (UV-1601; Shimadzu, Duisburg, Germany) at 550 nm, connected to a personal computer (486SX; Packard Bell) for data analysis. The results were expressed as hydroxyproline equivalent per milligram of cartilage (27).
MR Imaging and T2 Mapping of Rat Patellar Cartilage
For practical considerations, we voluntarily limited the MR imaging study population to four age groups: the 4- and 8-week-old animals with immature cartilage (groups 1 and 2, respectively) and the 4-month-old rats and old rats with mature cartilage (groups 3 and 4, respectively). These groups were representative of the different steps that characterize the maturation and aging process as defined in preliminary histologic and biochemical studies.
For each of these four groups, six patellae were studied with MR imaging and then submitted to histologic study. Knees were dissected and kept frozen until MR imaging was performed. Preliminary studies permitted specification of the effects of deep freezing, freeze-drying, and rehydration of the thawed patellae on the MR signal intensities. The conditions that permitted the most reproducible results were obtained after the thawed patellae had been dissected from the knee just before exploration with MR imaging and being put into a closed atmosphere in a sample tube.
All images were obtained with an 8.5-T MR microimager (Oxford Instruments, Osney Head, Oxford, England) with identical experimental parameters. MR imaging data were acquired in the transverse plane through the middle part of the patella, previously determined with a study performed in the longitudinal plane. All the patellae were oriented in the same direction in a sample tube placed inside the coil, with the superficial collagen fibers of patellar cartilage oriented perpendicular to the magnetic field strength, or B0; this orientation limited the potential anisotropic effect which was, consequently, not evaluated. The eight spin-echo images were obtained as follows: constant repetition time of 1.5 seconds; eight different echo times—5.5, 7.5, 10.5, 12.5, 15, 20, 25, and 30 msec; four signals acquired; and 105 minutes of total acquisition time. The section thickness was 1 mm, the field of view was 4 x 4 mm, and the matrix size was 128 x 128. The spatial resolution was 31 x 31 x 1,000 µm. The mean number of pixels in the profile across the patellar cartilage varied from 13 in young rats to 9 in 4-month-old rats. During the MR imaging acquisitions, the magnet bore temperature was 25–30°C.
T2 values were calculated from the eight spin-echo images by using nonlinear least squares curve fitting on a pixel-by-pixel basis. The signal intensity, SI, for the ith jth pixel as a function of time (t), can be expressed as follows: SIij(t) = SOij · exp (-t/T2ij), where SOij is the pixel intensity at t = 0, and T2ij is the T2 time constant of pixel ij. The T2 map was displayed on a pixel-by-pixel basis (Transform; Spyglass, Savoy, Ill) for the cartilage itself but also for adjacent structures such as the subchondral and trabecular bone. The T2 map is the representation of these values, with a color scale, that was optimized with regard to the cartilage T2 values expected at 0 and 32.76 msec with an 8.5-T microimager. The superficial border of the cartilage was easily defined owing to the large difference in T2 values between air and cartilage. Conversely, both the calcified cartilage and the subchondral bone had low T2 values, which did not permit a clear delineation between these two tissues. On the T2 map, the value of 4 msec was considered to be the lower threshold value for the deepest part of the noncalcified cartilage. The global T2 value of rat patellar cartilage was calculated as the mean of pixel values between these two borders. Zonal variations in T2 values were appreciated as differences in T2 values between the two borders—the superficial and deep layers of noncalcified cartilage.
Cartilage thickness was measured on T2 images with a computer (Macintosh 9600; Apple Computer) with a public domain program (NIH IMAGE 1.54; National Institutes of Health, Bethesda, Md; available at: rsb.info.nih.gov/nih-image/. Accessed July 1, 1999). The cartilage thickness considered for each patella was defined as the mean value of 10 measurements performed at different places, from the superficial to the calcified zone of cartilage, perpendicular to the cartilage surface.
Statistical Evaluation
Differences in T2 measurements (global T2 values and superficial and deep T2 values), thickness measurements, and histologic and biochemical data were analyzed by means of nonparametric tests. The Kruskal-Wallis test was used in the 10 groups of rats to look for significant differences in proteoglycans content and collagen content. The Kruskal-Wallis test was also used to compare the global, superficial, and deep T2 values among the four groups considered for the MR imaging study. For each variable, statistical significance tests and multiple comparison procedures were performed with a level of significance of 
less than .05. A paired comparison of the global, superficial, and deep T2 values was performed. As a result of Bonferroni correction, the level of significance of each comparison was set at .0167.
Correlations between the MR imaging and histologic thickness measurements, global T2 values and matrix constituents, and proteoglycan and collagen contents were examined by means of the Spearman rank correlation test. In all cases, the associations were considered significant for P less than .05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Besides the organization of global cartilage, the histologic sections also showed subtle variations in matrix composition with depth. The immature cartilage was poorly stained with picrosirius red, which corresponded to collagen constituents, and superficial and transitional zones were more intensively stained than the hypertrophic zone (Fig 1, C). Conversely, this immature cartilage was markedly stained with toluidine blue, which corresponded to proteoglycan constituents, without spatial variations between the different zones (Fig 1, A). However, significant decrease in proteoglycan staining was observed with maturation. This fact was confirmed with biochemical data with a proteoglycan content that significantly decreased from 147.21 µg/mg ± 1.31 of cartilage in 4-week-old rats to 96.60 µg/mg ± 1.48 of cartilage in 8-week-old rats (P < .001) (Fig 2, A). Compared with the proteoglycan content, the collagen content increased slightly from 50.78 µg/mg ± 0.62 of cartilage to 58.56 µg/mg ± 0.48 of cartilage in 4-week-old and 8-week-old rats, respectively (P < .001) (Fig 2, B).
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The superficial layer appeared similar to that of immature cartilage, with ellipsoidal cells and dense orange collagen fibers parallel to the articular surface at polarizing light microscopy. The transitional zone in mature rats occupied only 8% of cartilage thickness versus 35% in immature rats. At polarizing light microscopy, this layer demonstrated no birefringence, since the collagen fibers were arching in several directions (Fig 1, F).
The radial zone was the largest, representing 68% of the cartilage thickness. In this zone, most of the cells were assembled in clusters with three to four cells each that were organized in columns oriented perpendicular to the articular surface. At polarizing light microscopy, these cells were surrounded by thick and green collagen fibers that were perpendicular to the articular surface. As in immature cartilage, the hypertrophic zone was formed by only one row of hypertrophic cells surrounded by a thin vertical organization of collagen fibers. From the age of 6 months, a calcified cartilage zone, constituting 6%–8% of the total cartilage thickness, took the place of the hypertrophic zone. This zone anchors the extensive network of collagen fibrils in the subchondral bone. We did not observe any osteoarthritic lesions in the histologic sections in old rats.
Regarding the matrix composition, a global increase in collagen staining was observed in mature groups compared with that in the immature ones (Fig 1, D). Thick bundles oriented perpendicular to the articular surface and surrounding the clusters were observed in a radial zone different from the one observed in immature cartilage. The cartilage matrix was weakly stained with toluidine blue (proteoglycan), without spatial variations from the superficial to the deepest zones of patellar cartilage (Fig 1, B). Biochemical analysis revealed a biphasic profile of proteoglycan content, which progressively decreased to 52.07 µg/mg ± 0.98 in 4-month-old rats before it slightly increased to 71.63 µg/mg ± 0.44 in old rats (Fig 2, A). Conversely, we noted a slight increase in collagen content until the age of 4 months, and then this content remained unchanged until the end of the study (Fig 2, B).
As a whole, the range of variation in matrix content was less important in mature than in immature cartilage. However, in both groups, the variation in matrix constituents was characterized by an inverse correlation between collagen and proteoglycan contents (r = -0.85; P < .01, Spearman rank test).
MR Imaging Study
The T2 relaxation time of patellar cartilage is a parameter reflecting inherent differences in water mobility secondary to the surrounding macromolecular matrix environment (collagen and proteoglycan content). The T2 map was the result of eight spin-echo sequences and characterizes the spatial distribution of T2 values (Fig 3). The T2 map permitted the differentiation of cartilage from other tissues. Although the trabecular bone was clearly identified as a structure with high intensity related to a high fat content (Fig 4), the cartilage appeared as a thin structure that had moderate intensity on color-scale-coded images and that was surrounded by two structures with low intensity, which corresponded to the air and subchondral bone. Whatever the age, a zonal variation was present, with a high T2 value in the superficial layer and a lower T2 value in the deepest layer, with a global range of 3 msec, except for group 2, which displayed a 1.5-msec difference (Fig 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Rat Patellar Cartilage: Mature and Immature Tissues
Maturation is a complex process for which several factors have to be taken into account: the organization of the collagen network, cell density, thickness, and matrix content. The organization of the collagen network was clearly defined by Benninghoff (28) in an early work in which adult cartilage was examined with polarizing light microscopy. In rat histologic sections, we observed this pattern only in animals at least 10 weeks old. Contrary to this pattern, immature cartilage was characterized by a thick transitional zone with a horizontal alveolar organization and by a thick hypertrophic zone composed of two or three rows of cells. These changes in the organization of the cartilage constitute the progressive process of maturation, which seems to end abruptly between 8 and 10 weeks.
We observed a significant decrease in cartilage thickness with age, which can be attributed to the fact that cartilage was partly replaced by bone, especially during maturation. Such morphologic and histochemical aging changes in patellar cartilage of the rat have been also described by Schiefke et al (29). However, in the study by Schiefke et al, matrix content was not evaluated. We observed that during the process of maturation there was a decrease in proteoglycan content without zonal variation with depth on histologic sections.
After this decrease in proteoglycan, the mature cartilage presented only slight variations in proteoglycan content, without zonal variation with depth on histologic sections. Unlike the proteoglycan content, the collagen content was initially low in immature cartilage, without zonal variation in histologic sections. This collagen content progressively increased during the maturation process and reached a plateau in the old rats. A slight zonal variation in collagen was observed in this mature cartilage, with an increase in collagen staining in the superficial and deepest zones. This finding has already been reported by Hwang et al (30).
MR Imaging and Cartilage
Compared with conventional radiography, MR imaging can depict a laminar appearance on T2-weighted images, which potentially is related to variations in matrix content or in histologic organization of this tissue (20,22,31,32). Owing to the limitations of visual analysis of T2-weighted images, we developed the T2 mapping technique with an 8.5-T microimager, which enabled us to obtain quantitative information on the global and zonal T2 values of cartilage.
Global T2 Values
The T2 relaxation time of articular cartilage characterizes the interactions of cartilage fluid (protons) with the solid matrix (collagen and proteoglycan). In this tissue, there are several distinct "pools" of water molecules: the molecules associated with the collagen fibrils, the molecules hydrogen-bonded to proteoglycan by means of electrostatic attraction, and free water molecules. Only this last pool is responsible for the cartilage signal intensity. We measured T2 values of articular cartilage between 12.07 msec in 4-week-old rats and 7.92 msec in old rats. Our values were similar to those already reported in studies (17,33,34) performed with high-field-strength imagers; values from 29 to 11 msec were obtained in the white-tailed deer with a 7.5-T imager; a value of 9 msec, in rabbit knee with a 2.35-T imager; and values of 20–40 msec, in guinea pig cartilage with a 2.35-T imager. With a 1.5-T imager, the T2 values of human and bovine cartilage were higher, ranging between 32 and 90 msec (16,18,20). These differences in T2 values depend on field strength as reported by Duewell et al (35), who noted a 15%–20% decrease in T2 values between a 1.5-T imager and a 4-T imager, and may also depend on species variability (31–34).
Regarding the evolution of values with time, we noted a decrease in T2 values with maturation. This fact was not reported in the single study available by Tyler et al (34), who noted an increase in T2 values in guinea pig cartilage from 23.7 msec ± 0.8 in 5–20-week-old animals to 39.2 msec ± 9.3 in animals more than 20 weeks old (34).
Results of our combined study of MR imaging and biochemical data underline the narrow interaction between the matrix constituents and the pool of free water protons that are solely responsible for the T2 values. In immature cartilage, most of these free water protons are located in the spaces between the numerous proteoglycans macromolecules entrapped within the collagen network, which is reflected in the high T2 values observed. In mature cartilage, the combined action of the decrease in proteoglycan and the increase in collagen limits the space available for free water, and, consequently, the T2 values are low. Results of correlation study between biochemical and MR imaging data suggest that the increase in collagen would be more determinant than the decrease in proteoglycan. Thus, T2 mapping can be used to characterize the evolution of cartilage matrix content during the maturation and aging process in the rat patella.
T2 Zonal Variation
We observed zonal variation in T2 values in the different age groups. This observation is consistent with findings in the literature, since investigators in similar studies (15–17,20,21,36) reported such variations in human, deer, bovine, canine, and porcine cartilage. The results of these studies demonstrated that T2 zonal variation may depend on the anisotropic arrangement of the collagen fibers in the cartilage with respect to the magnetic field. This anisotropism leads to an increase in T2 values when the fibers approach a 55° angle with respect to the magnetic field owing to the "magic angle phenomenon" (21,22). Our results are consistent with these hypotheses, since we noted a variation in the anisotropic arrangement of the collagen network in different zones in both immature and mature cartilage. This anisotropic arrangement was observed mainly in the transitional zone of rat patellar cartilage.
Results of some studies (17) suggested that the orientation of the collagen fibers does not explain the T2 zonal variation and that the zonal variations in matrix constituents can influence T2 relaxation time. We observed both a progressive increase in collagen content and the existence of thick collagen bundles in the radial zone in mature rats, which may be responsible for the decrease in T2 values in the deep layer. This phenomenon may explain the increase in interactions between the cartilage fluid (protons) and the solid matrix (collagen and proteoglycan). Since we did not observe zonal variation in toluidine blue sections, the proteoglycan content does not seem to be a major constituent in the T2 signal intensity. However, since its structure varies with the cartilage depth, with small proteoglycans such as decorine and biglycan in the superficial zone and large proteoglycans such as agrecans in the radial zone, this constituent could influence the proportion of free water and thus T2 values (3).
Cartilage Thickness Measurements
We noted a decrease in thickness in rat patellar cartilage with age, as was shown in many MR imaging studies (29) performed in human and animal cartilage. We noted that this decrease was more marked during the maturation process, whereas stabilization was observed in mature animals. In young animals, cartilage thickness measured with MR imaging was in good agreement with that measured in histologic sections. This correlation seemed to be lower in animals with mature cartilage in which thickness measurements at MR imaging were underestimated in comparison with histologic measurements.
T2 Mapping and Clinical Studies
At the time this article was written, two experimental studies have been performed in human patellar cartilage in which the T2 profile and cartilage depth were similar. With a 3-T imager, Dardzinski et al (16) showed a zonal variation in T2 values in patellar cartilage in seven young asymptomatic volunteers. They demonstrated a monotonic increase in the T2 values from 32 msec ± 1 in the deep radial zone to 67 msec ± 2 in the superficial layer. This finding was also reported by Frank et al (37) with a clinical imager; they performed a study in normal and pathologic patellae obtained from cadaveric specimens. However, neither histologic nor biochemical analysis results were available for correlation with the T2 maps.
Practical applications: This T2 mapping technique could provide information on the physiopathologic mechanisms of the chondral lesions observed in osteoarthritic and arthritic diseases. Judging from the ability with the T2 map to localize and quantitatively assess the modifications of the internal structure such as fibrosis or edema before any morphologic lesion appears, we suggest that this method enables the characterization of early stages of disease. MR imaging allows a noninvasive and longitudinal evaluation of chondroprotective agents and may constitute an attractive approach to follow up patients after the incorporation of biomaterials, such as chondrocyte grafts and biopolymers, in cartilage defects.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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) and deep (
) T2 values differ by 3 msec for groups 1, 3, and 4, whereas this difference is only 1.5 msec for group 2. However, because of the limited number of samples, we were unable to determine whether these differences were significant. With the maturation and aging process, there is a progressive and significant decrease in T2 values for the superficial (P < .05) and deepest layers (P = .01), respectively. Bars represent the mean.
