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(Radiology. 2000;217:193-200.)
© RSNA, 2000

Musculoskeletal Imaging

Characterization of the "Red Zone" of Knee Meniscus: MR Imaging and Histologic Correlation1

Olivier Hauger, MD, Lawrence R. Frank, PhD, Robert D. Boutin, MD, Nittaya Lektrakul, MD, Christine B. Chung, MD, Parviz Haghighi, MD and Donald Resnick, MD

1 From the Departments of Radiology (O.H., L.R.F., R.D.B., N.L., C.B.C., D.R.) and Pathology (P.H.), University of California, San Diego, Veterans Affairs Medical Center, 3350 La Jolla Village Dr, San Diego, CA 92161. Received August 10, 1999; revision requested September 20; revision received December 13; accepted January 11, 2000. Supported by Veterans Administration grant SA-360. Address correspondence to D.R. (e-mail: dresnick@ucsd.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
PURPOSE: To determine the extent and vascularity of knee menisci with conventional and gadolinium-enhanced magnetic resonance (MR) imaging in cadaveric specimens, with histologic findings as the reference standard, and to investigate signal intensity changes in menisci and perimeniscal soft tissues in symptomatic patients.

MATERIALS AND METHODS: Radial dimensions and enhancement patterns of menisci were recorded and compared in (a) 12 cadaveric menisci examined with conventional and gadolinium-enhanced intermediate-weighted and fat-suppressed T1-weighted spin-echo MR imaging, high-spatial-resolution T1-weighted and fast low-angle shot MR imaging, and gross anatomic and histologic specimens and (b) 18 patients examined with conventional and gadolinium-enhanced fat-suppressed T1-weighted spin-echo MR imaging.

RESULTS: No differences in radial measurements of the meniscus were found for different MR techniques (P = .551). Despite the presence of vessels in the peripheral 10%–15% of the menisci, no enhancement of menisci was detected in specimens or patients. Perimeniscal soft-tissue enhancement adjacent to the posterior horn was greater than that adjacent to the anterior horn (P < .05), and enhancement of the lateral meniscal body was greater than that of the medial meniscal body (P < .05).

CONCLUSION: The wedge-shaped low-signal-intensity structure seen on MR images represents the entire meniscus. Intravenous injection of contrast material does not appear to be useful for differentiation of the vascularized from the nonvascularized zone of the meniscus.

Index terms: Gadolinium • Knee, injuries, 452.4852 • Knee, ligaments, menisci, and cartilage • Knee, MR, 452.121411, 452.121415, 452.12143 • Specimens, MR, 452.121411, 452.121412, 452.121415, 452.12143


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Meniscal tears in the knee are frequent (1) and are a common indication for arthroscopic knee surgery. The choice of treatment and the ultimate prognosis associated with meniscal tears are influenced by a number of factors, including the orientation, extent, and location of the tear (2–4). The location of a meniscal tear is of paramount importance because tears in the vascular portion of the meniscus, termed the "red zone," are far more likely to heal than tears in the avascular portion, or "white zone," of the meniscus (5–8). The vascular and avascular zones of the meniscus can be differentiated histologically, but the demarcation between these zones cannot be determined with direct inspection of the meniscus at the time of surgery.

Magnetic resonance (MR) imaging allows diagnosis of a meniscal tear with sensitivity and specificity that generally exceed 90% (9–12). For the diagnosis of meniscocapsular separation, however, MR imaging reportedly has low sensitivity (13) and a poor positive predictive value (14). Recent investigations have suggested that injuries located at the periphery of the meniscus may be characterized incorrectly with MR imaging because of a fallacy in the traditional teaching that the entire meniscus is a low-signal-intensity structure (14,15), and that the more peripheral area of higher signal intensity corresponds to the vascularized red zone (15).

The purpose of this study was to confirm or refute this concept regarding MR imaging of this crucial portion of the meniscus. Specifically, our objectives were two-fold: (a) to determine the extent and vascularity of the meniscus with conventional and contrast material–enhanced MR imaging, with histologic findings as the reference standard in cadaveric specimens and (b) and to investigate the signal intensity changes in the meniscus and perimeniscal soft tissues with conventional and contrast-enhanced MR imaging in symptomatic patients.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Cadaveric Study
The knees from eight adult cadavers were radiographed (frontal and lateral projections) to determine the presence of osteoarticular disease. Two knees were excluded from further study owing to the presence of moderate or severe osteoarthrosis, as determined in consensus by two musculoskeletal radiologists (O.H., R.D.B.). The remaining six knees (12 menisci) from the cadavers of four men and two women aged 57–78 years (mean, 70 years) at the time of death were evaluated with conventional and contrast-enhanced MR imaging, anatomic inspection, high-spatial-resolution MR imaging, and histologic assessment, and the results were evaluated with statistical analyses.

Conventional and contrast-enhanced MR imaging.—Initially, MR imaging was performed by using intermediate-weighted spin-echo (repetition time msec/echo time msec, 2,000/14) and fat-suppressed T1-weighted spin-echo (600/14) sequences in the sagittal and coronal planes. The imaging parameters were as follows: section thickness, 3 mm with no intersection gap; field of view, 8 x 8 cm in the sagittal plane and 10 x 10 cm in the coronal plane; matrix, 256 x 192; and four signals acquired. MR imaging was performed with a 1.5-T system (Signa; GE Medical Systems, Milwaukee, Wis) with a 5-inch-diameter (12.7-cm) surface coil.

The wedge-shaped structure of low signal intensity that is known to represent at least part of the meniscus (16,17) was measured. Measurements of the radial dimensions, from the free edge to the peripheral border, of the 12 menisci in the six knees were performed electronically at an MR workstation (Advantage Windows version 2.0; GE Medical Systems) by two musculoskeletal radiologists (O.H., N.L.) working independently. The radial dimension of each meniscus was measured in the sagittal plane on two contiguous sections through the midportions (as determined by counting the number of image sections in which the meniscus was visible) of the anterior and posterior horns. The radial dimensions of the body segments also were measured on two contiguous sections in the coronal plane. With these six measurements obtained for each of the 12 menisci, a total of 72 radial measurements were initially recorded. In addition, these MR images were evaluated for the presence of meniscal tears by using widely accepted diagnostic criteria (ie, grade 3 signal intensity, abnormal meniscal morphology, or both) (18–20).

After the conventional MR images were evaluated, intraarterial injection of contrast material was performed according to the technique used by Danzig et al (21). First, a cannula was inserted into the superficial femoral artery, 10 cm proximal to the joint line, and perfusion with a heparin-saline solution was performed to remove blood clots from the vascular tree. Then, the distal portion of the popliteal artery was occluded just proximal to its trifurcation, and a solution consisting of 1 mL gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany) and 250 mL of saline solution was injected in the antegrade direction by using manual pressure. After injection of the contrast material, MR imaging was performed with the same parameters as for conventional imaging. MR imaging began within 2 minutes following the injection and was completed within 30 minutes.

The radial dimensions of the wedge-shaped structure of low signal intensity again were measured by the same two musculoskeletal radiologists according to the technique already described. These 72 measurements obtained after contrast material administration were recorded and later compared with the 72 measurements obtained before contrast material administration. Subsequently, for each meniscus, the same reviewers together directly compared the MR images obtained before and after injection of contrast material by viewing the two sets of images of each knee simultaneously and recording the presence of contrast enhancement in the wedge-shaped structure and in the adjacent soft tissues. A five-point scale was used for this assessment: 0, no enhancement; 1, minimal enhancement; 2, mild enhancement; 3, moderate enhancement; and 4, marked enhancement. If a meniscal tear was present, the presence of contrast enhancement in and around the meniscal tear was recorded by using the same five-point scale.

Anatomic inspection.—Each cadaveric specimen was then frozen, and 3-mm-thick sections were obtained with a band saw in either a coronal (n = 3) or a sagittal (n = 3) plane to match the MR imaging planes. The sections were collected with attention paid to preservation of all capsular attachments and synovial tissue adjacent to the menisci. This process yielded 12 anterior horns (six medial, six lateral), 12 posterior horns (six medial, six lateral), and 12 body segments (six medial, six lateral), for a total of 36 gross anatomic specimens. The radial dimensions of these sections of menisci were measured at x5 magnification from their inner apex to their peripheral border by the same two musculoskeletal radiologists working independently.

High-spatial-resolution MR imaging.—High-spatial-resolution MR imaging subsequently was performed on the 36 gross anatomic samples by using a local gradient coil and a specialized radio-frequency coil designed for small samples, both of which were built at our department (22,23). The sections were imaged by using fast low-angle shot (FLASH) gradient-echo (100/9, 30° flip angle, 16 signals acquired) and T1-weighted spin-echo (400/9, eight signals acquired) sequences performed in the coronal plane. The spatial resolution parameters were as follows: section thickness, 117 µm; field of view, 3 x 3 cm; and matrix, 256 x 256. All sections were imaged perpendicular to the main magnetic field. In addition, five samples also were imaged parallel to the magnetic field (ie, with 90° rotation) to determine if the angular orientation of the samples relative to the magnetic field influenced the meniscal signal intensity. The radial dimensions of the menisci were measured on FLASH and T1-weighted MR images at the MR workstation by the same two observers using the method detailed earlier (Fig 1).



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  		Figure 1. High-spatial-resolution FLASH MR image (100/9, 30° flip angle) of a cadaveric medial meniscus shows measurement of the radial dimension of the anterior horn from its free edge (straight arrow) to its peripheral border (curved arrow).

 
Histologic analysis.—Immediately after MR imaging of the meniscal sections, the samples were suspended in a 10% formalin solution for histologic analysis. Specimens were embedded in paraffin and sectioned further into 5-µm-thick slices. Histologic sections of the menisci were stained with hematoxylin-eosin (n = 36), and alternate sections (one-half of the samples, n = 18) also were stained immunohistochemically with the avidin-biotin immunoperoxidase technique for factor VIII and CD34, which are markers for endothelium in blood vessel walls.

Histologic sections were analyzed at light microscopy (magnification, x4 to x100) in consensus by a musculoskeletal radiologist (O.H.) and an orthopedic pathologist (P.H.), who were blinded to the results of MR imaging. The distance from the apex of the meniscus to the free edge of the dense fibrocartilage was measured for each section by using the calibrated scale on the light microscope. The examiners also recorded in consensus the presence of vessels within the fibrocartilage, as well as the histologic appearance of the soft tissues located between the peripheral border between the fibrocartilage and the joint capsule.

Statistical analyses.—Agreement between the two raters’ results was evaluated with the Pearson correlation coefficient. Assessment of measurements performed on the different portions of the menisci was conducted by using analysis of variance for repeated-measures designs with planned comparison.

Clinical Study
Eighteen patients (17 men, one woman; age range, 27–80 years; mean age, 48 years) with a clinical history of possible meniscal tear underwent both conventional and contrast-enhanced MR imaging. Investigational review board approval was obtained prior to the study, and MR imaging was performed with the informed consent of patients. The patients were selected in a prospective and consecutive fashion.

In addition to the standard knee MR imaging protocol, coronal and sagittal fat-suppressed T1-weighted spin-echo (600/14) or fast spin-echo (666/17 [repetition time msec/effective echo time msec]) images were obtained before and after intravenous injection of 0.1 mmol/kg gadopentetate dimeglumine. After the injection of contrast material (accomplished within 20 seconds), MR imaging began within 2 minutes and was completed within 12 minutes. The MR unit and surface coil were the same as those used for MR imaging of the cadaveric knees. Analysis of the MR images was performed by the same two musculoskeletal radiologists, with measurements performed in the same manner as in the cadaveric study.

Both before and after contrast material administration, the two radiologists recorded two measurements of the radial dimensions of each major portion the meniscus (anterior horn, posterior horn, and body) and rendered a diagnosis with regard to a meniscal tear. In addition, the radiologists assessed the enhancement of tissues by directly comparing the MR images obtained before with those obtained after contrast material administration for findings of increased signal intensity (a) in the wedge-shaped structure traditionally regarded as the meniscus, (b) in the adjacent soft tissues, and (c) in and around any meniscal tear.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Cadaveric Study
Conventional and contrast-enhanced MR imaging.—The results of intraarterial contrast material administration are shown in Table 1. The wedge-shaped structure of low signal intensity generally considered to be the meniscus demonstrated no enhancement, not even in the peripheral portion. However, contrast enhancement was observed to varying degrees in the soft tissues peripheral to the low-signal-intensity meniscus in all specimens. Contrast enhancement was significantly greater at the periphery of the posterior horn than at the periphery of the anterior horn (P < .05) and at the periphery of the body of the lateral meniscus than at the periphery of the body of the medial meniscus (P < .05) (Figs 2, 3).


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  		TABLE 1. Gadolinium Enhancement in the Meniscus and Perimeniscal Soft Tissues in Cadaveric Knees
 


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  		Figure 2a. Sagittal fat-suppressed T1-weighted spin-echo MR images (600/14) obtained (a) before and (b) after intraarterial injection of a gadolinium-containing contrast material into a cadaveric knee specimen. Soft-tissue enhancement is substantially greater peripheral to the posterior horn (curved arrow) of the meniscus than that peripheral to the anterior horn (straight white arrow). No enhancement is observed in the periphery of the meniscus itself (black arrow).

 


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  		Figure 2b. Sagittal fat-suppressed T1-weighted spin-echo MR images (600/14) obtained (a) before and (b) after intraarterial injection of a gadolinium-containing contrast material into a cadaveric knee specimen. Soft-tissue enhancement is substantially greater peripheral to the posterior horn (curved arrow) of the meniscus than that peripheral to the anterior horn (straight white arrow). No enhancement is observed in the periphery of the meniscus itself (black arrow).

 


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  		Figure 3a. Coronal fat-suppressed T1-weighted spin-echo MR images (600/14) obtained (a) before and (b) after intraarterial injection of a gadolinium-containing contrast material into a cadaveric knee specimen. Soft-tissue enhancement is substantially greater peripheral to the body of the lateral meniscus (curved arrow) than the enhancement in the body of the medial meniscus (straight arrow). No enhancement is observed in the periphery of the meniscus itself (arrowhead).

 


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  		Figure 3b. Coronal fat-suppressed T1-weighted spin-echo MR images (600/14) obtained (a) before and (b) after intraarterial injection of a gadolinium-containing contrast material into a cadaveric knee specimen. Soft-tissue enhancement is substantially greater peripheral to the body of the lateral meniscus (curved arrow) than the enhancement in the body of the medial meniscus (straight arrow). No enhancement is observed in the periphery of the meniscus itself (arrowhead).

 
The measurements of the radial dimensions of the low-signal-intensity wedge-shaped structure are shown in Table 2. There was no significant difference in the radial dimensions of the low-signal-intensity wedge-shaped structure between MR images obtained before and those obtained after administration of contrast material (P = .574).


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  		TABLE 2. Mean Radial Dimensions of Cadaveric Knee Menisci
 
Anatomic inspection.—Gross anatomic inspection revealed two distinctive-appearing zones in each meniscus (Fig 4). At the periphery of the meniscus, each sample exhibited a thin, vertically-oriented, reddish band of tissue with a mean radial dimension of 1.1 mm (range, 0.8–1.6 mm). More centrally, the remainder of the meniscus had a pale yellow coloration; the mean radial dimension of this portion of the meniscus was 13.4 mm (range, 10.1–18.5 mm). All menisci contained linear strands of branching fibers that originated at the periphery of the meniscus and became progressively smaller and more numerous as they coursed centrally. Interposed between the meniscus and the joint capsule, connective tissue containing small-caliber blood vessels and adipose tissue were observed.



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  		Figure 4. Gross anatomic 3-mm-thick section of the posterior horn of a medial meniscus shows two distinctive zones: a peripheral reddish band (small straight arrow) and a central yellow portion (large straight arrow). Branching fibers (arrowhead) in the periphery of the meniscus also are visible. Fatty connective tissue (curved arrow) can be seen between the meniscus and the joint capsule.

 
High-spatial-resolution MR imaging.—In all cases, the menisci appeared as wedge-shaped structures of low signal intensity with a peripheral border that was clearly demarcated from the surrounding soft tissues of higher signal intensity. High-spatial-resolution MR images revealed a branching network with intermediate signal intensity that arborized into progressively smaller and more numerous bands within the outer portion of the meniscal parenchyma (Fig 5). The appearance and measurements of the menisci did not differ according to pulse sequence (FLASH or T1-weighted spin-echo) or orientation of the meniscus within the MR unit (perpendicular or parallel to the main magnetic field).



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  		Figure 5. High-spatial-resolution FLASH MR image (100/9, 30° flip angle) of the medial meniscus of a cadaveric knee specimen (same specimen as in Fig 4) shows a branching network of intermediate-signal-intensity fibers (arrow) that arborize within the outer portion of the meniscus.

 
Histologic analysis.—At light microscopy in all specimens, two zones of tissue were visualized in the fibrocartilage. With both hematoxylin-eosin and immunohistochemical vascular staining, an outer segment of fibrocartilaginous tissue was observed; this segment contained small-caliber blood vessels that penetrated the outer 1.2–3.1 mm (mean, 2.5 mm) of the meniscus, which thus corresponded to 10%–15% (mean, 13%) of the total width of the meniscus (Fig 6). The extent of vascularization was similar in both the medial and the lateral menisci and was more pronounced in the meniscal horns than in the bodies. A second zone of fibrocartilage, which comprised the inner 85%–90% of this structure, contained no blood vessels. In both zones of the fibrocartilage, mucinous and myxoid degeneration were observed in each specimen. Light microscopy also revealed a branching pattern of collagen fibers in the outer half of each meniscus. These fibers emanated from the periphery of the meniscus and became progressively smaller and more numerous; correlation with high-spatial-resolution MR imaging findings revealed that these fibers exhibited intermediate signal intensity on T1-weighted and FLASH MR images.



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  		Figure 6. Photomicrograph shows that the outer portion of the meniscus (m) contains delicate fibrovascular septa (s). Peripheral to the meniscus, perimeniscal soft tissues containing blood vessels (arrows), adipose tissue, and irregular collagen fibers (f) are visible. (Hematoxylin-eosin stain; objective magnification, x2.)

 
With regard to the zone between the outer border of the fibrocartilage and the joint capsule, histologic analysis revealed collagenous connective tissue containing a variable amount of fat and numerous small blood vessels in all sections (Fig 7), except at the periphery of the medial meniscus body that was contiguous with the deep fibers of the medial collateral ligament.



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  		Figure 7. Photomicrograph of the posterior horn of a cadaveric medial meniscus shows blood vessels (arrows) in the outer portion of the meniscus (m), as well as in the soft tissue peripheral to the meniscus. Small spaces within the meniscus represent tissue preparation artifact. (Factor VIII-related antigen immunoperoxidase and hematoxylin-eosin stains; objective magnification, x2.)

 
Statistical analyses.—The radial dimensions of the fibrocartilage as measured on conventional and contrast-enhanced MR images, at anatomic inspection, on high-spatial-resolution MR images, and at histologic examination showed no significant differences (P = .551). The coefficients (r values) for interrater agreement ranged from 0.819 to 0.915 (P < .05), which were indicative of a high level of agreement. Subsequent analyses were, therefore, based on the mean of the values from the two independent raters (see Table 2).

Clinical Study
Clinical and radiologic features in the 18 patients (20 menisci) are summarized in Table 3. MR imaging demonstrated varying degrees of contrast enhancement in the perimeniscal soft tissues. This enhancement was highly variable, depending on the segment of the meniscus that was analyzed (Figs 8, 9). Contrast enhancement was significantly greater (P < .05) at the periphery of the posterior horns of the menisci as compared with that at the periphery of the anterior horns. Similarly, enhancement was greater (P < .05) at the periphery of the body of the lateral meniscus as compared with that at the periphery of the body of the medial meniscus. At the level of the insertion of the deep fibers of the medial collateral ligament, no enhancement was seen. However, enhancement was observed immediately anterior and posterior (mean enhancement rating, 1.4; range, 0–2) to this site. No contrast enhancement was visualized in the substance of the wedge-shaped low-signal-intensity structure that is traditionally considered to be the meniscus. The measurements of the menisci performed before and after contrast injection did not show a significant difference (P = .614).


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  		TABLE 3. Radial Dimension and Contrast-Enhancement Features in Menisci and Permeniscal Soft Tissues as Measured on Nonenhanced and Gadolinium-enhanced MR Images in 18 Patients
 


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  		Figure 8a. (a) Nonenhanced and (b) gadolinium-enhanced sagittal fat-suppressed T1-weighted spin-echo MR images (600/14) in a 67-year-old man with a clinical history of meniscal tear. Soft-tissue enhancement is substantially greater peripheral to the posterior horn (curved arrow) of the meniscus than that peripheral to the anterior horn (straight white arrow). No contrast enhancement is observed in the periphery (black arrow) of the meniscus itself.

 


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  		Figure 8b. (a) Nonenhanced and (b) gadolinium-enhanced sagittal fat-suppressed T1-weighted spin-echo MR images (600/14) in a 67-year-old man with a clinical history of meniscal tear. Soft-tissue enhancement is substantially greater peripheral to the posterior horn (curved arrow) of the meniscus than that peripheral to the anterior horn (straight white arrow). No contrast enhancement is observed in the periphery (black arrow) of the meniscus itself.

 


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  		Figure 9a. (a) Nonenhanced and (b) gadolinium-enhanced coronal fat-suppressed T1-weighted spin-echo MR images (600/14) in a 55-year-old woman with a clinical history of chronic knee pain and suspected meniscal tear. Soft-tissue enhancement is substantially greater at the periphery of the body of the lateral meniscus (curved arrow) than in the body of the medial meniscus (straight arrow). No enhancement is observed in the periphery (arrowhead) of the meniscus itself.

 


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  		Figure 9b. (a) Nonenhanced and (b) gadolinium-enhanced coronal fat-suppressed T1-weighted spin-echo MR images (600/14) in a 55-year-old woman with a clinical history of chronic knee pain and suspected meniscal tear. Soft-tissue enhancement is substantially greater at the periphery of the body of the lateral meniscus (curved arrow) than in the body of the medial meniscus (straight arrow). No enhancement is observed in the periphery (arrowhead) of the meniscus itself.

 
Nine meniscal tears were identified in nine patients. Six tears were located in the posterior horn of the medial meniscus, one in the anterior horn of the medial meniscus, one in the posterior horn of the lateral meniscus, and one in the anterior horn of the lateral meniscus. All but one of the tears was located in the inner 80% of the low-signal-intensity wedge-shaped area. One tear involved the peripheral 10% of the meniscus, and, after intravenous injection of contrast material, increased signal intensity was observed in this tear (Fig 10). None of the other meniscal tears exhibited high signal intensity. At arthroscopic surgery (performed in eight patients), the location and extent of seven meniscal tears, including the one in the red zone of the meniscus, were confirmed.



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  		Figure 10a. (a) Nonenhanced and (b) gadolinium-enhanced sagittal fat-suppressed T1-weighted fast spin-echo MR images (666/17 [effective]) in a 28-year-old man with posteromedial knee pain and a suspected meniscal tear. A peripheral meniscal tear (arrow), which shows enhancement after injection of the contrast material, is observed in the posterior horn of the medial meniscus.

 


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  		Figure 10b. (a) Nonenhanced and (b) gadolinium-enhanced sagittal fat-suppressed T1-weighted fast spin-echo MR images (666/17 [effective]) in a 28-year-old man with posteromedial knee pain and a suspected meniscal tear. A peripheral meniscal tear (arrow), which shows enhancement after injection of the contrast material, is observed in the posterior horn of the medial meniscus.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
The menisci, C-shaped structures composed of fibrocartilage, contribute to knee joint congruity and help to absorb and distribute forces that are applied to the knee (7,24). Blood supply to the peripheral portion of the meniscus is fundamental for maintaining these functions, and this supply is a crucial factor in determining the treatment and prognosis of meniscal tears (6,7,24). Tears in the red zone of the meniscus may be treated with a variety of meniscus-preserving techniques (eg, suture repair); by contrast, tears in the white zone of the meniscus typically are treated by means of débridement (3,7). It also is important to identify a tear located in the red zone because the prognosis associated with such a tear is superior to that associated with tears in the white zone, regardless of whether white-zone tears are treated surgically (4–6).

Cadaveric Study
The first goal of our study was to determine the extent and vascularity of the meniscus as observed at MR imaging, with histologic findings as the reference standard. To our knowledge, there are few data in the literature regarding the extent of the menisci. Ferrer-Roca and Vilata (25) reported radial measurements similar to ours for the bodies of both menisci but smaller than ours for the anterior and posterior horns. We cannot explain this discrepency. Our measurements indicated that the low-signal-intensity tissue seen at MR imaging represents the entire meniscus, including the vascularized red zone. This finding contradicts a that in a recent report (15) in which the authors concluded that the low-signal-intensity wedge-shaped structure seen on MR images corresponds to the avascular (white) zone of the meniscus, whereas the high-signal-intensity zone peripheral to the wedge-shaped area represents the vascularized (red) zone. In all specimens, our histologic analysis showed irregular tightly packed connective tissue peripheral to the fibrocartilage that contains fat and highly vascularized tissue; this tissue exhibited none of the histologic criteria of a meniscus.

In our study, high-spatial-resolution MR imaging allowed further understanding of the signal intensity of the meniscus. Linear regions of intermediate to high signal intensity in a meniscus have been recognized in adults as occurring as a result of meniscal tears, myxoid degenerative tissue in elderly patients (26), and the "magic-angle" phenomenon (27). Our findings indicate that another potential cause of linear intermediate signal intensity in the meniscus is the presence of radially oriented collagen "tie" fibers that can be seen on both conventional and high-spatial-resolution MR images (Figs 11–13) (28). Those collagen fibers have an important functional role because they act to prevent splitting apart of the concentric collagen fibers when hoop stress is applied to the meniscus (29,30).



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  		Figure 11. Diagram shows the orientation of collagen fibers in the lateral meniscus in a coronal section. The majority of fibers are oriented concentrically (curved arrow) and course approximately parallel to the inner and outer margins of the meniscus in the axial plane. The radial "tie" collagen fibers (straight arrow) are oriented perpendicular to most other meniscal fibers and become progressively smaller as they course from the periphery toward the inner edge of the meniscus. (Adapted, with permission, from reference 28.)

 


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  		Figure 12. Sagittal intermediate-weighted spin-echo MR image (2,000/14) of the medial meniscus from a cadaveric knee specimen. A branching network of intermediate-signal-intensity fibers (arrow), corresponding to radial "tie" collagen fibers, arborizes within the outer portion of the posterior horn of the meniscus.

 


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  		Figure 13. High-spatial-resolution FLASH MR image (100/9, 30° flip angle) of the medial meniscus from a cadaveric knee specimen (almost identical level as that in Fig 12). Large radial "tie" collagen fibers (arrow) of intermediate signal intensity extend centrally from the periphery and arborize within the outer portion of the meniscus.

 
When we compared conventional nonenhanced MR images with conventional contrast-enhanced images, we detected no enhancement in the wedge-shaped low-signal-intensity structure that, according to our results, represents the entire meniscus. At histologic analysis, however, we found small blood vessels that penetrated the outer 10%–15% of the meniscus. Our data on the percentage of the meniscus that is vascularized are similar to the 10%–25% values reported by many previous investigators (31–33) but are less than the 33% found in one study (34). This discrepancy may be related to the presence of senescent vascular disease in the elderly population that we studied. This discrepancy also may be related to sampling error based on slight variations in measurement of the meniscus (the apex may be difficult to define precisely in some cases) or in the histologic method of detection of vessels (endothelium markers vs vascular enhancement).

Clinical Study
The second goal of this study was to investigate whether increased signal intensity can be demonstrated in the meniscus or in meniscal tears on contrast-enhanced MR images. Despite the fact that the peripheral 10%–15% of the menisci was vascularized in our cadaveric specimens, no enhancement was detected after intravenous injection of contrast material. This finding may be related to the anatomy of the vessels (which are tiny and tortuous), the quantity of contrast material injected, the delay between injection and imaging, insufficient resolution with our MR imaging techniques, or a combination of these factors. As observed in the cadaveric specimens, contrast enhancement was documented only in the perimeniscal soft tissues. With further refinements in MR techniques (eg, perfusion imaging), it may become possible to differentiate between the red and white zones by using MR imaging.

Given our findings, we expect that MR imaging would demonstrate a meniscal tear in the red zone as a region of abnormally increased signal intensity in a wedge-shaped structure of low signal intensity; tears in the red zone do not occur in an area of high signal intensity, as has been suggested (15). Intraarterial or intravenous injection of gadolinium-containing contrast material was not helpful in distinguishing between the red and white zones of the meniscus. The use of contrast material also did not appear to be helpful in the diagnosis of meniscal tears; in our clinical study, eight of nine meniscal tears did not show enhancement, although the only lesion that enhanced after contrast material injection was located in the red zone of the meniscus. However, the diagnosis of a peripheral meniscal tear in this case was already established because the signal intensity abnormality was located in the outer 10% of the meniscus.

This study has two major limitations. First, the number of observations in both the cadaveric and the patient studies was limited. However, our data showed little variability between observations, and the number of cadaveric specimens was similar to that used in previous studies (35,36). Second, it is theoretically possible that the signal intensity of the meniscus may differ between cadavers and patients. We chose to use a cadaveric model in our study, however, because this allowed us to inject contrast material intraarterially prior to MR imaging and then section the menisci for purposes of anatomic inspection, high-spatial-resolution MR imaging, and histologic analysis.

In summary, our results indicate that the wedge-shaped low-signal-intensity structure seen on MR images represents the entire meniscus. We also found that intermediate to high signal intensity in the parenchyma of the meniscus may correspond not only to a tear, myxoid degeneration, or magic-angle phenomenon, but also to radial collagen fibers. Intravenous injection of contrast material does not appear to be useful in routine MR imaging for differentiation of the vascularized from the nonvascularized zones of the meniscus; however, as an approximation, the red zone of the meniscus may be considered to be located in the peripheral 10%–15% of the meniscus.


   ACKNOWLEDGMENTS
 
The authors thank Paul L. Clopton, MS, for help in the statistical analysis and Debra Trudell, RA, for technical assistance in preparing the cadaveric specimens and acquiring the MR images.


   FOOTNOTES
 
Abbreviation: FLASH = fast low angle shot

Author contributions: Guarantors of integrity of entire study, O.H., L.R.F., R.D.B., D.R.; study concepts, D.R.; study design, O.H., L.R.F., R.D.B., D.R.; definition of intellectual content, O.H., R.D.B., D.R.; literature research, O.H.; clinical studies, O.H., N.L., R.D.B., C.B.C.; experimental studies, O.H., L.R.F., N.L., P.H.; data acquisition, O.H., L.R.F., R.D.B., N.L.; data analysis, O.H., R.D.B., D.R.; statistical analysis, P.L.C.; manuscript preparation, O.H., R.D.B.; manuscript editing and review, O.H., R.D.B., D.R.


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TOP
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 MATERIALS AND METHODS
RESULTS
DISCUSSION
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