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DOI: 10.1148/radiol.2412051750
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(Radiology 2006;241:407-414.)
© RSNA, 2006

Experimental Studies

Cartilage T2 Assessment: Differentiation of Normal Hyaline Cartilage and Reparative Tissue after Arthroscopic Cartilage Repair in Equine Subjects1

Lawrence M. White, MD, Marshall S. Sussman, PhD, Mark Hurtig, DVM, MVSc, Linda Probyn, MD, George Tomlinson, PhD and Rita Kandel, MD

1 From the Department of Medical Imaging, Mount Sinai Hospital and the University Health Network, University of Toronto, 600 University Ave, Toronto, ON, Canada M5G 1X5 (L.M.W., M.S.S., L.P.); Comparative Orthopaedic Research Laboratory, Department of Clinical Studies, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada (M.H.); Department of Medicine and Medical Imaging, University of Toronto, Toronto, Ontario, Canada (G.T.); and Department of Pathology and Laboratory Medicine, Mount Sinai Hospital, University of Toronto, Ontario, Canada (R.K.). From the 2005 RSNA Annual Meeting. Received October 29, 2005; revision requested December 22; revision received January 9, 2006; final version accepted February 6. Supported by a research grant from the Canadian Arthritis Network and by the Ontario Ministry of Agriculture Equine Research Program. Address correspondence to L.M.W. (e-mail: lwhite{at}mtsinai.on.ca).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Purpose: To prospectively assess T2 mapping characteristics of normal articular cartilage and of cartilage at sites of arthroscopic repair, including comparison with histologic results and collagen organization assessed at polarized light microscopy (PLM).

Materials and Methods: Study protocol was compliant with the Canadian Council on Animal Care Guidelines and approved by the institutional animal care committee. Arthroscopic osteochondral autograft transplantation (OAT) and microfracture arthroplasty (MFx) were performed in knees of 10 equine subjects (seven female, three male; age range, 3–5 years). A site of arthroscopically normal cartilage was documented in each joint as a control site. Joints were harvested at 12 (n = 5) and 24 (n = 5) weeks postoperatively and were imaged at 1.5-T magnetic resonance (MR) with a 10-echo sagittal fast spin-echo acquisition. T2 maps of each site (21 OAT harvest, 10 MFx, 12 OAT plug, and 10 control sites) were calculated with linear least-squares curve fitting. Cartilage T2 maps were qualitatively graded as "organized" (normal transition of low-to-high T2 signal from deep to superficial cartilage zones) or "disorganized." Quantitative mean T2 values were calculated for deep, middle, and superficial cartilage at each location. Results were compared with histologic and PLM assessments by using {kappa} analysis.

Results: T2 maps were qualitatively graded as organized at 20 of 53 sites and as disorganized at 33 sites. Perfect agreement was seen between organized T2 and histologic findings of hyaline cartilage and between disorganized T2 and histologic findings of fibrous reparative tissue ({kappa} = 1.0). Strong agreement was seen between organized T2 and normal PLM findings and between disorganized T2 and abnormal PLM findings ({kappa} = .92). Quantitative assessment of the deep, middle, and superficial cartilage, respectively, showed mean T2 values of 53.3, 58.6, and 54.9 msec at reparative fibrous tissue sites and 40.7, 53.6, and 61.6 msec at hyaline cartilage sites. A significant trend of increasing T2 values (from deep to superficial) was found in hyaline cartilage (P < .01). Fibrous tissue sites had no significant change with depth (P > .59).

Conclusion: Qualitative and quantitative T2 mapping helped differentiate hyaline cartilage from reparative fibrocartilage after cartilage repair at 1.5-T MR imaging.

© RSNA, 2006


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Several techniques are used to surgically treat focal cartilage lesions, including marrow stimulation and autologous or allogeneic osteochondral transplantation. The most widely used marrow stimulation techniques include abrasion arthroplasty, subchondral drilling, and microfracture (1–3). Local stimulation techniques rely on bleeding from the penetration of the subchondral bone to form a fibrin clot containing pluripotent mesenchymal progenitor cells, which can differentiate and form fibrocartilaginous repair tissue. In contrast, osteochondral transplantation techniques involve resurfacing a cartilage or osteochondral defect with a graft composed of subchondral bone and overlying hyaline articular cartilage (4–11). Conventional magnetic resonance (MR) imaging after cartilage repair procedures has been shown to be sensitive to morphologic alterations at the repair site (12–14) but insensitive to articular surface tissue cover composition.

Quantitative T2 MR mapping of articular cartilage is a noninvasive imaging technique that has the potential to characterize hyaline articular cartilage and repair tissue. Normal articular hyaline cartilage illustrates a predictable spatial variation in T2 relaxation time with depth at MR imaging, with an increase in T2 values from the subchondral bone to the articular surface correlating to macroscopic collagen organization and orientation seen in normal articular cartilage (15–31). Alteration in this orderly transition in T2 values within cartilage has been shown to correlate to changes in water content and changes in collagen structure and organization associated with hyaline articular cartilage degradation (31–36).

We hypothesized that T2 mapping performed at 1.5-T field strength could be used as an indicator of normal collagen organization in normal hyaline articular cartilage and would be sensitive to alterations in collagen structure that result from the ingrowth of reparative fibrocartilage and/or fibrous tissue. Thus, the purpose of our study was to prospectively assess T2 mapping characteristics of normal articular cartilage and of cartilage at sites of arthroscopic repair, including comparison with histologic findings and collagen organization assessed at polarized light microscopy (PLM).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
Our study protocol was compliant with the Canadian Council on Animal Care Guidelines (37) and was approved by the institutional animal care committee of the University of Guelph.

Study Animals
Equine subjects studied in our investigation were former race horses no longer fit for racing. The animals were assessed by the Veterinary College at the University of Guelph as not suitable for other activities, and they were to be sold for slaughter. Equine joints have been recommended as biologic models for comparative cartilage repair research by the Cell, Tissue, and Gene Therapy Advisory Committee of the Department of Health and Human Services of the U.S. Food and Drug Administration (38). A total of 10 equine subjects (seven female, three male; age range, 3–5 years) were studied as part of our investigation. All subjects were skeletally mature, with closed physes at the time of investigation.

Procedures
All surgical procedures were performed by a subspecialized veterinary surgeon (M.H.) with 22 years of experience in equine arthroscopic surgery. Initial diagnostic arthroscopic evaluation was performed in one stifle (knee) joint of each animal, at which time pre-existing articular cartilage disease was excluded. In each joint, three experimental sites were created including osteochondral autograft transplantation (OAT) harvest sites (ie, donor sites), OAT plug sites (ie, the transplanted osteochondral grafts), and microfracture arthroplasty (MFx) sites of chondral defects. OAT harvest sites (donor sites) were created by harvesting 6.5-mm diameter cylindrical OAT plugs (two plugs each in nine stifle joints and three in one stifle joint) from the lateral femoral trochlear ridge, creating defects that were left unfilled. Grafts were harvested by using atraumatic techniques that maximized the viability of graft cartilage (39,40) and were temporarily placed in sterile Ringer solution. Osteochondral grafts (one each in eight stifle joints and two each in two stifle joints) were then transplanted (ie, OAT plug site) by using a 6.5-mm instrument set (MosaicPlasty; Acufex, Smith, and Nephew, Andover, NJ) at a reproducible anatomic location in the central weight-bearing aspect of the medial femoral condyle 2 cm posterior to the sulcus terminalis. Emphasis was put on transplantation of these grafts congruent with the surrounding cartilage surface. The third surgical site was an 8.5 x 1.5-mm full-thickness cartilage defect created by using a curette in each experimental joint (n = 10) at a reproducible anatomic location in the distal third of the medial femoral trochlear surface. MFx was then performed at each defect site by using a 30° arthroscopic awl to perforate the subchondral bone plate at 3–4-mm intervals, inducing foci of hemorrhage.

A 20 x 20-mm site of arthroscopically normal cartilage was additionally documented in each joint, along the medial femoral condyle at a location 15 mm immediately posterior to the OAT plug site, as a normal control site.

Postoperatively, all animals were confined to a stall for 7 days and were walked twice daily. The animals were thereafter allowed access to an outdoor facility with no restriction to activity. Subjects were euthanized at 12 weeks (n = 5) and 24 weeks (n = 5) postoperatively, and knee joints were harvested en bloc for subsequent imaging.

MR Imaging and T2 Analysis
Each knee joint was imaged within 3 hours of harvest with a 1.5-T MR imaging system (Signa; GE Healthcare, Milwaukee, Wis) by using a multichannel head coil (MRI Devices, Waukesha, Wis). In all cases, a 10-echo multiecho spin-echo pulse sequence (26) was used to acquire the data: 2500/8.5–86.5 (repetition time msec/echo time msec); field of view, 14 x 14 cm; matrix, 256 x 256; section thickness, 3 mm with no intersection gap; and four signals acquired.

Regions of interest (ROIs) 8–10 mm in length, outlining tissue cover overlying the central sagittal section of each evaluation site (OAT harvest, n = 21; MFx, n = 10; OAT plug, n = 12; control, n = 10), were manually traced by one author (L.M.W., with 12 years of experience in musculoskeletal MR imaging). T2 maps of each ROI were calculated on a pixel-by-pixel basis by means of a linear least-squares monoexponential curve fit by using custom software written with Matlab (Matlab, Natick, Mass). Since magnitude images were used for the T2 fit, any data that were below five times the noise level were excluded from the T2 calculations to avoid biasing the fit with non-Gaussian noise (41). Note that this exclusion criterion may cause a spatial variation in the accuracy of the T2 map, because shorter T2 data will tend to have more points excluded, and thus a less-accurate T2 fit (42).

Qualitative analysis of gray-scale representation of the T2 maps obtained at each ROI was performed independently at separate sittings by two readers (L.M.W., L.P.). Readers were presented with the T2 map gray-scale image data, with annotation of deep and superficial (articular) aspects of each ROI. The remainder of the image data outside of the ROI were cropped from view. Readers were able to change the window and level of the gray-scale T2 maps for qualitative evaluation. Readers graded each evaluation site T2 map as illustrating either an "organized" T2 pattern, defined as a normal transition or gradient of low-to-high T2 values from deep to superficial aspects of the ROI, or a "disorganized" T2 pattern without illustration of a normal transition or gradient of low-to-high T2 values from deep-to-superficial zones of the ROI. Discrepant assessment results were re-evaluated at a third sitting by both readers, with agreement by consensus.

Quantitative T2 assessment of each ROI was also performed. For this evaluation, each ROI was divided into equal one-third sections (deep, middle, and superficial) through the depth of the articular tissue cover, from the subchondral to the articular aspect of the ROI. Quantitative mean T2 values were calculated for the deep, middle, and superficial sections of each ROI (43).

Histologic Evaluation
After imaging, each joint was dissected and surgical sites were grossly inspected by one author (R.K.). Each evaluation site (OAT harvest, n = 21; MFx, n = 10; OAT plug, n = 12; control, n = 10) was excised as an approximately 25 x 25-mm block. Each specimen was then sectioned through the middle of the site in the true sagittal plane that corresponded as closely as possible to the sagittal plane used for MR imaging. Specimens were fixed in 10% buffered formalin and then decalcified in 10% formic acid. Tissue was subsequently paraffin-embedded, and 5-µm slices were cut. Hemotoxylin-eosin–stained slices of each site were assessed for the presence or absence of hyaline articular cartilage or fibrous reparative tissue. Additional slices were stained with picrosirius red to assess collagen distribution by means of PLM. Each site was graded as illustrating an organized PLM pattern, defined as a normal transition of birefringence from the deep to superficial aspects of the cartilage cover if present (21), or a disorganized PLM pattern, without the normal transition birefringence from deep to superficial zones of the tissue overlying the bone. All slices were evaluated by two observers, with agreement by consensus; one of these observers (R.K.) was a coauthor of the investigation, with 18 years of subspecialty experience in bone and soft-tissue pathology.

Statistical Analysis
Interobserver agreement in the qualitative assessment of the T2 maps was performed by means of {kappa} value calculation. A {kappa} value analysis was also performed to assess agreement between qualitative grading of an organized T2 pattern and histologic findings of hyaline articular cartilage and between a disorganized T2 pattern and histologic findings of fibrous reparative tissue or a lack of normal hyaline articular cartilage tissue. Similar {kappa} value evaluation was used for the assessment of agreement between qualitative grading of an organized T2 pattern and PLM findings of an organized birefringence pattern and between qualitative grading of a disorganized T2 pattern and PLM findings of a disorganized birefringence pattern.

Formal analysis of the trend in quantitative T2 measurements across cartilage depth (deep, middle, and superficial sections) of normal control, OAT plug, OAT harvest, and MFx sites needed to account for multiple T2 measurements at each evaluation site depth. To accomplish this analysis, a random effects linear regression model (44) was used (R, version 2.2, 2005; R Foundation for Statistical Computing, Vienna, Austria). This model was used to assess whether there was a trend of increasing T2 values from deep to middle to superficial sections and whether this trend differed by site (control, OAT plug, OAT harvest, and MFx sites). A Student t test was used to assess for differences between joints harvested at 12 weeks postoperatively and those harvested at 24 weeks postoperatively for quantitative mean T2 values of deep, middle, and superficial sections of control, OAT plug, OAT harvest, and MFx sites. All analyses used P < .05 as a threshold to indicate a statistically significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 
A total of 53 sites were available for evaluation (OAT harvest, n = 21; MFx, n = 10; OAT plug, n = 12; control, n = 10). Qualitative analysis of T2 maps was performed for tissue cover overlying all 53 evaluation sites. Two OAT plug sites illustrated complete denudation of articulating surface tissue histologically and were thus excluded from quantitative analysis, leaving a total of 51 sites for quantitative evaluation of mean T2 values of cartilage cover.

Qualitative Assessment
Qualitative T2 map analysis showed near perfect agreement ({kappa} = 0.97) between readers. T2 maps were quantitatively assessed as organized at 20 of 53 sites (10 of 10 control sites and 10 of 12 OAT plug sites) (Figs 1 and 2) and as disorganized at 33 of 53 sites (two of 12 OAT plug, 21 of 21 OAT harvest, and 10 of 10 MFx sites) (Figs 3 and 4). Perfect agreement ({kappa} = 1.0) was found between qualitative assessment of an organized T2 map pattern and histologic findings of hyaline articular cartilage (Fig 5) and between a disorganized T2 map pattern and histologic findings of fibrous reparative tissue (21 of 21 OAT harvest sites and 10 of 10 MFx sites) (Fig 6) or complete denudation of articular tissue cover (two of 12 OAT plug sites) (Table 1). Similarly, strong agreement ({kappa} = 0.92) was seen between an organized T2 pattern and an organized pattern of birefringence at PLM assessment (Fig 7) and between a disorganized T2 pattern and a disorganized pattern of birefringence at PLM assessment (Fig 8) (Table 2). Two of 12 OAT plug sites that showed an organized T2 mapping pattern at qualitative assessment illustrated a disorganized pattern of birefringence at PLM. Histologic assessment of these two OAT plug sites showed irregularly thinned but intact hyaline articular cartilage with a layer of fibrous tissue overlying the articular surface of the OAT plug transplant.


Figure 1
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  		Figure 1: Sagittal intermediate-weighted (2500/39.7) MR image of the weight-bearing aspect of the medial femoral condyle. T2 map of a control articular cartilage site, 1.5-cm posterior to an OAT plug graft (arrow), is superimposed on the sagittal MR image. An organized cartilage T2 pattern is shown at the control site ROI, with a transition of low-to-high T2 values seen from deep to superficial aspects of the cartilage thickness.

 

Figure 2
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  		Figure 2: Sagittal intermediate-weighted (2500/39.7) MR image of the weight-bearing aspect of the medial femoral condyle. T2 map of cartilage cover overlying an OAT plug site is superimposed on the sagittal MR image. An organized T2 pattern is demonstrated at the plug site ROI, with a normal gradient of low-to-high T2 values from deep to superficial aspects of the cartilage cover thickness.

 

Figure 3
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  		Figure 3: Sagittal intermediate-weighted (2500/39.7) MR image of the lateral femoral trochlear articular ridge. T2 map of cartilage cover at an OAT harvest site is superimposed on the sagittal MR image. A disorganized T2 pattern is seen at the harvest site ROI, with lack of a normal transition of low-to-high T2 values from deep to superficial articular aspects of the ROI.

 

Figure 4
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  		Figure 4: Sagittal intermediate-weighted (2500/39.7) MR image of the inferior trochlear articular surface of the medial femoral condyle. T2 map of cartilage cover overlying a site of prior MFx (MFx site) is superimposed on the sagittal MR image. Low signal intensity is seen of the subchondral aspect of the microfracture site (arrows). A disorganized T2 pattern of cartilage cover at the MFx site ROI is seen, with a lack of normal transition of low-to-high T2 values seen from deep to superficial articular aspects of the ROI.

 

Figure 5
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  		Figure 5: Photomicrograph of an OAT plug (arrow) shows intact organized articular cartilage (C) and underlying bone (B). There is no fusion to the adjacent cartilage (*). Cartilage flow (lateral extension of cartilage observed at margins of cartilage defects as a result of weight-bearing) is seen at the edges of the plug (double arrows). Arrowhead indicates a processing artifact. (Hematoxylin-eosin stain; original magnification, x25.)

 

Figure 6
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  		Figure 6: Photomicrograph of joint surface after MFx. The articular cartilage is completely eroded, and the articulating surface is covered by fibrous tissue (arrow). The underlying bone (B) is undergoing resorption and remodeling. (Hematoxylin-eosin stain; original magnification, x63.)

 

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  		Table 1. Qualitative T2 Assessments Relative to Histologic Findings in Experimental Evaluation Sites

 

Figure 7
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  		Figure 7: PLM image of an OAT plug with intact organized articular cartilage (C) and underlying bone (B). There is an organized pattern of birefringence, with a normal gradient from the deep to the superficial aspect of the cartilage. Arrowhead indicates a processing artifact. (Picrosirius red stain, dark-field image; original magnification, x63.)

 

Figure 8
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  		Figure 8: PLM image of joint surface after MFx. The fibrous tissue (arrow) overlying the bone (B) shows a disorganized pattern of collagen birefringence, and there is a lack of a cartilage-type gradient in light transmission from deep-to-superficial zones of this tissue. (Picrosirius red stain, dark-field image; original magnification, x63.)

 

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  		Table 2. Qualitative T2 Assessments Relative to Qualitative PLM Assessment of Experimental Evaluation Sites

 
Quantitative Assessment
Quantitative T2 assessment showed mean T2 values of 53.3 (deep cartilage), 58.6 (middle cartilage), and 54.9 msec (superficial cartilage) for cartilage cover at sites of reparative fibrous tissue and mean T2 values of 40.7 (deep cartilage), 53.6 (middle cartilage), and 61.6 msec (superficial cartilage) for cartilage cover at sites of histologically documented hyaline articular cartilage (Table 3). A statistically significant trend (P < .01) for increasing mean T2 values from deep to middle to superficial aspect of cartilage cover was found in regions of histologically confirmed articular cartilage. In contrast, sites where reparative tissue covered the articulating surface showed no significant change (P > .59) in mean T2 values over depth. The trend in the control and OAT plug sites was significantly larger than the trend at the other two types of sites (P < .01).


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  		Table 3. Quantitative T2 Measurements with P Value from Test for Trend across Cartilage Depth

 
No statistically significant differences were observed in the mean T2 values of deep, middle, and superficial sections of cartilage cover for control, OAT plug, OAT harvest, and MFx sites between joints harvested at 12 weeks and those harvested at 24 weeks postoperatively (P > 0.2).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ADVANCES IN KNOWLEDGE
 References
 
MR imaging of articular cartilage has been shown to be an accurate technique in the detection and evaluation of chondral abnormalities (45–55). MR imaging has additionally been shown to be a valuable technique in the noninvasive assessment of repair tissue following attempted cartilage repair procedures (12–14). However, despite reasonable accuracy in the detection of gross morphologic abnormalities of cartilage and cartilage repair tissue, current clinically used conventional MR cartilage imaging techniques are insensitive to intrasubstance alterations of cartilage composition.

Results of prior studies, typically performed by using high-field-strength (3–7-T) MR imaging systems, have demonstrated that normal articular cartilage demonstrates an increase in T2 values from the subchondral bone to the articular surface that has been correlated with type II collagen fiber matrix organization (aniosotropy) in these zones (15–31). In the (deep) radial zone, where the collagen fiber matrix has a preferred orientation perpendicular to the cartilage surface, the T2 value is shortest, whereas in the transitional zone, where the collagen fiber matrix has an oblique orientation, longer T2 values are demonstrated. The superficial zone is typically too thin to be observed as a distinct "zone." In contrast, reparative tissue with a lack of zonal organization of collagen throughout its depth would not be expected to demonstrate a similar organized transition of T2 values from the deep to superficial aspects of the tissue.

As a result, we sought to compare the T2 signal mapping characteristics of normal articular cartilage (control sites), cartilage overlying osteochondral autografts (OAT plug sites), and sites of markedly abnormal articular cartilage with potential repartative fibrocartilage (MFx and OAT harvest sites). This was done in an attempt to detect alterations in collagen matrix organization accompanying arthroscopic cartilage repair procedures and to compare qualitative and quantitative assessments of resultant T2 maps with the histologic and macroscopic evaluation of collagen organization of articular cartilage cover at these sites.

Our results demonstrated strong agreement between qualitative assessment of an organized T2 pattern, histologic findings of hyaline articular cartilage, and organized cartilage PLM birefringence indicative of an organization of collagen fibers. Strong agreement was also observed between a disorganized T2 pattern, histologic findings of fibrocartilage and/or fibrous reparative tissue or a lack of hyaline cartilage, and a disorganized birefringence pattern at PLM assessment.

Two OAT plug sites (two of 12) that demonstrated an organized T2 map pattern at qualitative assessment demonstrated a disorganized pattern of birefringence at PLM. Histologic assessment of these two OAT plug sites showed irregularly thinned but intact hyaline articular cartilage, with a layer of fibrous tissue overlying the articular surface of the OAT plug transplant. PLM assessment of these sites showed an organized pattern of birefringence within the deep-to-middle aspects of the cartilage that was present but a disorganized superficial layer; therefore, these sites were graded as disorganized at PLM evaluation. However, qualitative T2 evaluation of these sites showed a transition from low T2 values in the deep radial zone of the thinned but intact hyaline articular cartilage to higher T2 values through the mid-to-superficial aspects of the sites, which included the transitional layer of hyaline cartilage and the overlying overgrowth of fibrous tissue. This may be explained by the fact that quantitative mean T2 values observed at sites of reparative tissue in our investigation are in the same range as T2 values seen within the mid-to-superficial aspects of sites of normal articular cartilage (range of mean T2 values from deep to superficial cartilage of MFx and OAT harvest sites, 48.0–59.0 msec; range of mean T2 values from middle and superficial sections of control sites of normal hyaline articular cartilage and OAT plug sites, 49.4–63.2 msec). As a result, the observed T2 characteristics of the superficial overgrowth of fibrous tissue in these cases may have mimicked or paralleled the normal T2 values of the mid-to-superficial aspects of organized T2 maps of normal hyaline articular cartilage.

Quantitative analysis of mean T2 values from the deep, middle, and superficial aspects of tissue cover showed a statistically significant trend of increasing T2 values from deep to middle to superficial aspects of hyaline articular cartilage at control and OAT plug sites. In contrast, no statistically significant trend in change of mean T2 values was seen from the deep to middle to superficial aspects of tissue cover at MFx and OAT harvest sites.

Limitations of our investigation include the fact that a small number of surgical sites were evaluated. However, our study results suggest that a prospective evaluation of a larger number of subjects and surgical sites with histologic correlation is worthwhile to confirm the observed findings. Previous investigations of T2 mapping of articular cartilage have primarily been performed with research MR imaging systems that operate at high field strengths (1–25,27,29–31,33,35,36) in order to optimize imaging signal-to-noise ratio and image spatial resolution. To our knowledge, few studies have attempted to measure and correlate results of T2 maps acquired at 1.5-T MR with histologic findings and assessments of cartilage collagen matrix structure. It is possible that evaluation of cartilage repair in clinical practice may be more accurately assessed with higher-field-strength systems, allowing for decreased imaging time while preserving adequate signal-to-noise ratio and image spatial resolution for T2 evaluation. An additional acknowledged limitation of our investigation was the fact that morphology of manually traced ROIs of evaluation site tissue cover may have varied, with control sites showing a normal articular contour and normal thickness relative to the MFx or OAT harvest sites, which may have potentially illustrated thinning, thickening, or irregularity of tissue cover. While attempts were made to trace ROIs of similar size and shape for each evaluation site, minor degrees of morphologic variation between ROIs may have biased readers in the qualitative assessment of the ROI T2 maps.

Qualitative evaluation of T2 map "organization," and quantitative mean T2 values of deep, middle, and superficial cartilage of control, OAT plug, OAT harvest, and MFx sites were used in the current study for comparison with histologic and PLM evaluation. These indexes were used for assessment of T2 mapping data, because absolute quantitative T2 values of a particular voxel within any individual evaluation site may vary based on position and orientation of the articular surface and collagen macrostructure relative to the main magnetic field orientation at MR imaging (15,18,24,25,27). It is also acknowledged that articular cartilage T2 measurements may vary based on regional anatomic variations in collagen macrostructural architecture along an articular surface (24,27). We did attempt to reproduce the anatomic position of surgical and control sites for evaluation between subjects, and we attempted to position evaluation sites within the joint such that they would be aligned as closely perpendicular as possible to the main magnetic field orientation at MR imaging. We additionally recognize that the mean T2 values of the deep, middle and superficial cartilage used for quantitative T2 assessments of control, OAT plug, OAT harvest, and MFx sites are not precisely reflective of T2 values corresponding to the radial, transitional, and superficial anatomic zonal organization of hyaline articular cartilage. However, reporting of T2 values obtained from two or three zones, from the cartilage surface to the bone surface, has recently been proposed as a means of reporting cartilage T2 results (43).

Practical application: In summary, our investigation has shown that cartilage T2 mapping at 1.5-T MR imaging can be used as a noninvasive tool to study cartilage composition in an animal model after surgical cartilage repair procedures. Our results suggest that qualitative and quantitative T2 assessments can be used to accurately differentiate between hyaline articular cartilage and reparative fibrous tissue after arthroscopic cartilage repair. As such, the technique holds the promise of leading to a better understanding and measurement of cartilage repair tissue in patients undergoing cartilage repair procedures.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
References
 


   FOOTNOTES
 

Abbreviations: MFx = microfracture arthroplasty • OAT = osteochondral autograft transplantation • PLM = polarized light microscopy • ROI = region of interest

Authors stated no financial relationship to disclose.

Author contributions: Guarantor of integrity of entire study, L.M.W.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, L.M.W., M.S.S., M.H., G.T., R.K.; experimental studies, L.M.W., M.S.S., M.H., L.P., R.K.; statistical analysis, L.M.W., G.T.; and manuscript editing, all authors


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ADVANCES IN KNOWLEDGE
 References
 

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