Meniscus

Olivier Hauger, MD
Lawrence R. Frank, PhD
Robert D. Boutin, MD
Nittaya Lektrakul, MD
Christine B. Chung, MD
Parviz Haghighi, MD
Donald Resnick, MD
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
Radiology 2000; 217:193–200
Abbreviation:
FLASH ⫽fast low angle shot
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 Cal-
ifornia, San Diego, Veterans Affairs Med-
ical Center, 3350 La Jolla Village Dr, San
Diego, CA 92161. Received August 10,
1999; revision requested September 20;
revision received December 13; ac-
cepted January 11, 2000. Supported by
Veterans Administration grant SA-360.
Address correspondence to D.R. (e-
mail: [email protected]).
©
RSNA, 2000
Author contributions:
Guarantors of integrity of entire study,
O.H., L.R.F., R.D.B., D.R.; study con-
cepts, 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 stud-
ies, O.H., L.R.F., N.L., P.H.; data acqui-
sition, 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 re-
view, O.H., R.D.B., D.R.
Characterization of the “Red
Zone” of Knee Meniscus: MR
Imaging and Histologic
Correlation
1
PURPOSE: To determine the extent and vascularity of knee menisci with conven-
tional 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 im-
ages represents the entire meniscus. Intravenous injection of contrast material does
not appear to be useful for differentiation of the vascularized from the nonvascu-
larized zone of the meniscus.
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 demar-
cation 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 struc-
ture (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
193

material–enhanced MR imaging, with his-
tologic 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
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 osteoar-
throsis, as determined in consensus by
two musculoskeletal radiologists (O.H.,
R.D.B.). The remaining six knees (12 me-
nisci) 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 histo-
logic assessment, and the results were eval-
uated with statistical analyses.
Conventional and contrast-enhanced MR
imaging.—Initially, MR imaging was per-
formed 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 imag-
ing parameters were as follows: section
thickness, 3 mm with no intersection
gap; field of view, 8 ⫻8 cm in the sagittal
plane and 10 ⫻10 cm in the coronal
plane; matrix, 256 ⫻192; and four sig-
nals acquired. MR imaging was per-
formed 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 sig-
nal intensity that is known to represent
at least part of the meniscus (16,17) was
measured. Measurements of the radial di-
mensions, from the free edge to the pe-
ripheral border, of the 12 menisci in the
six knees were performed electronically
at an MR workstation (Advantage Win-
dows version 2.0; GE Medical Systems) by
two musculoskeletal radiologists (O.H.,
N.L.) working independently. The radial
dimension of each meniscus was mea-
sured in the sagittal plane on two contig-
uous 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 re-
corded. In addition, these MR images were
evaluated for the presence of meniscal
tears by using widely accepted diagnostic
criteria (ie, grade 3 signal intensity, ab-
normal meniscal morphology, or both)
(18–20).
After the conventional MR images
were evaluated, intraarterial injection of
contrast material was performed accord-
ing 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 hep-
arin-saline solution was performed to re-
move 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 (Mag-
nevist; 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 conven-
tional 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 measure-
ments obtained before contrast material
administration. Subsequently, for each
meniscus, the same reviewers together di-
rectly compared the MR images obtained
before and after injection of contrast ma-
terial 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 en-
hancement; 1, minimal enhancement; 2,
mild enhancement; 3, moderate enhance-
ment; 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 col-
lected with attention paid to preserva-
tion of all capsular attachments and sy-
novial 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 speci-
mens. The radial dimensions of these sec-
tions of menisci were measured at ⫻5
magnification from their inner apex to
their peripheral border by the same two
musculoskeletal radiologists working in-
dependently.
High-spatial-resolution MR imaging.—
High-spatial-resolution MR imaging sub-
sequently was performed on the 36 gross
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 bor-
der (curved arrow).
TABLE 1
Gadolinium Enhancement in the
Meniscus and Perimeniscal Soft
Tissues in Cadaveric Knees
Meniscus and
Meniscal
Segment Meniscus Soft Tissues
Medial
Anterior horn 0 (0) 0.9 (0–2)
Body 0 (0) 0.2 (0–1)
Posterior horn 0 (0) 1.8 (1–3)
Lateral 0 (0)
Anterior horn 0 (0) 1.3 (1–3)
Body 0 (0) 2.7 (2–4)
Posterior horn 0 (0) 1.9 (1–3)
Note.—Data are mean ratings determined
with a five-point scale for degree of enhance-
ment: 0, none; 1, minimal; 2, mild; 3, mod-
erate; 4, marked. Numbers in parentheses are
the range.
194 䡠Radiology 䡠October 2000 Hauger et al

anatomic samples by using a local gradi-
ent 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 us-
ing 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 spa-
tial resolution parameters were as fol-
lows: section thickness, 117 ␮m; field of
view, 3 ⫻3 cm; and matrix, 256 ⫻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 de-
tailed earlier (Fig 1).
Histologic analysis.—Immediately after
MR imaging of the meniscal sections, the
samples were suspended in a 10% forma-
lin solution for histologic analysis. Spec-
imens were embedded in paraffin and
sectioned further into 5-␮m-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, ⫻4to
⫻100) in consensus by a musculoskeletal
radiologist (O.H.) and an orthopedic pa-
thologist (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 exam-
iners also recorded in consensus the pres-
ence of vessels within the fibrocartilage,
as well as the histologic appearance of
the soft tissues located between the pe-
ripheral border between the fibrocarti-
lage and the joint capsule.
Statistical analyses.—Agreement between
the two raters’ results was evaluated with
the Pearson correlation coefficient. Assess-
ment of measurements performed on the
different portions of the menisci was con-
ducted by using analysis of variance for
repeated-measures designs with planned
comparison.
Clinical Study
Eighteen patients (17 men, one wom-
an; age range, 27–80 years; mean age, 48
years) with a clinical history of possible
meniscal tear underwent both conven-
tional and contrast-enhanced MR imag-
ing. Investigational review board ap-
proval was obtained prior to the study,
and MR imaging was performed with the
informed consent of patients. The pa-
tients were selected in a prospective and
consecutive fashion.
In addition to the standard knee MR im-
aging 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 intrave-
nous injection of 0.1 mmol/kg gado-
pentetate dimeglumine. After the injec-
tion of contrast material (accomplished
within 20 seconds), MR imaging began
within 2 minutes and was completed
within 12 minutes. The MR unit and sur-
face coil were the same as those used for
MR imaging of the cadaveric knees. Anal-
ysis of the MR images was performed by
the same two musculoskeletal radiolo-
gists, with measurements performed in the
same manner as in the cadaveric study.
Both before and after contrast mate-
rial administration, the two radiologists
recorded two measurements of the ra-
dial dimensions of each major portion
the meniscus (anterior horn, posterior
horn, and body) and rendered a diag-
nosis with regard to a meniscal tear.
In addition, the radiologists assessed the
enhancement of tissues by directly com-
paring the MR images obtained before with
those obtained after contrast material ad-
ministration for findings of increased sig-
nal 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
Cadaveric Study
Conventional and contrast-enhanced MR
imaging.—The results of intraarterial con-
Figure 2. 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).
Figure 3. Coronal fat-suppressed T1-weighted spin-echo MR images (600/14) obtained (a) be-
fore 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).
Volume 217 䡠Number 1 Knee Meniscus: MR Imaging and Histologic Correlation 䡠195

trast 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 ante-
rior horn (P⬍.05) and at the periphery
of the body of the lateral meniscus than
at the periphery of the body of the me-
dial meniscus (P⬍.05) (Figs 2, 3).
The measurements of the radial dimen-
sions 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-inten-
sity wedge-shaped structure between MR
images obtained before and those obtained
after administration of contrast material
(P⫽.574).
Anatomic inspection.—Gross anatomic
inspection revealed two distinctive-ap-
pearing zones in each meniscus (Fig 4).
At the periphery of the meniscus, each
sample exhibited a thin, vertically-ori-
ented, 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 colora-
tion; the mean radial dimension of this
portion of the meniscus was 13.4 mm
(range, 10.1–18.5 mm). All menisci con-
tained linear strands of branching fibers
that originated at the periphery of the me-
niscus and became progressively smaller
and more numerous as they coursed cen-
trally. Interposed between the meniscus
and the joint capsule, connective tissue
containing small-caliber blood vessels and
adipose tissue were observed.
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 pro-
gressively smaller and more numerous
bands within the outer portion of the
meniscal parenchyma (Fig 5). The ap-
pearance and measurements of the me-
nisci 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).
Histologic analysis.—At light micros-
copy in all specimens, two zones of tissue
were visualized in the fibrocartilage. With
both hematoxylin-eosin and immunohis-
tochemical 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 in-
ner 85%–90% of this structure, con-
tained no blood vessels. In both zones of
the fibrocartilage, mucinous and myxoid
degeneration were observed in each spec-
imen. 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 exhib-
ited intermediate signal intensity on T1-
weighted and FLASH MR images.
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 me-
niscus body that was contiguous with the
deep fibers of the medial collateral ligament.
Statistical analyses.—The radial dimen-
sions of the fibrocartilage as measured on
Figure 4. Gross anatomic 3-mm-thick sec-
tion of the posterior horn of a medial me-
niscus shows two distinctive zones: a pe-
ripheral 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.
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 net-
work of intermediate-signal-intensity fibers
(arrow) that arborize within the outer portion
of the meniscus.
TABLE 2
Mean Radial Dimensions of Cadaveric Knee Menisci
Meniscus and
Meniscal
Segment
MR Imaging Measurement (mm) Gross
Anatomic
Measurement
(mm)
Histologic
Measurement
(mm)Nonenhanced
Gadolinium
Enhanced
High Spatial
Resolution
Medial
Anterior horn 16.1 (15–17) 16.2 (15–17) 16.5 (16.3–17.2) 16.6 (16–17) 16.7 (16.2–17.3)
Body 10.6 (10–11) 10.5 (10–12) 11.2 (10.7–11.7) 11.2 (11–12) 11.3 (11.0–11.7)
Posterior horn 18.8 (18–20) 19.1 (18–20) 19.7 (18.3–20.8) 19.5 (18–21) 19.5 (18.0–20.6)
Lateral
Anterior horn 13.5 (13–14) 13.4 (13–14) 13.6 (12.8–14.2) 13.7 (13–14) 13.5 (12.7–14.0)
Body 11.5 (11–12) 11.7 (11–13) 12.0 (10.5–13.2) 12.2 (11–13) 12.0 (10.3–13.4)
Posterior horn 14.7 (12–19) 14.8 (12–20) 15.2 (11.2–21.3) 15.2 (11–21) 15.1 (11.0–21.2)
Note.—Numbers in parentheses are the range.
196 䡠Radiology 䡠October 2000 Hauger et al

conventional and contrast-enhanced MR
images, at anatomic inspection, on high-
spatial-resolution MR images, and at his-
tologic 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 vary-
ing degrees of contrast enhancement in
the perimeniscal soft tissues. This enhance-
ment was highly variable, depending on
the segment of the meniscus that was an-
alyzed (Figs 8, 9). Contrast enhancement
was significantly greater (P⬍.05) at the
periphery of the posterior horns of the me-
nisci as compared with that at the periph-
ery of the anterior horns. Similarly, en-
hancement was greater (P⬍.05) at the
periphery of the body of the lateral menis-
cus 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 en-
hancement was seen. However, enhance-
ment was observed immediately anterior
and posterior (mean enhancement rating,
1.4; range, 0–2) to this site. No contrast
enhancement was visualized in the sub-
stance of the wedge-shaped low-signal-in-
tensity structure that is traditionally con-
sidered to be the meniscus. The measure-
ments of the menisci performed before and
after contrast injection did not show a sig-
nificant difference (P⫽.614).
Nine meniscal tears were identified in
nine patients. Six tears were located in
the posterior horn of the medial menis-
cus, one in the anterior horn of the me-
dial 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 sig-
nal intensity. At arthroscopic surgery
(performed in eight patients), the loca-
tion and extent of seven meniscal tears,
including the one in the red zone of the
meniscus, were confirmed.
DISCUSSION
The menisci, C-shaped structures com-
posed 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 pe-
ripheral portion of the meniscus is fun-
damental for maintaining these func-
tions, 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 tech-
niques (eg, suture repair); by contrast,
tears in the white zone of the meniscus
typically are treated by means of de´bride-
ment (3,7). It also is important to identify a
tear located in the red zone because the
prognosis associated with such a tear is su-
perior 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 deter-
mine 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 an-
terior and posterior horns. We cannot
explain this discrepency. Our measure-
ments indicated that the low-signal-in-
tensity tissue seen at MR imaging repre-
sents the entire meniscus, including the
vascularized red zone. This finding con-
tradicts a that in a recent report (15) in
which the authors concluded that the
low-signal-intensity wedge-shaped struc-
ture seen on MR images corresponds to
the avascular (white) zone of the menis-
Figure 6. Photomicrograph shows that the outer portion of
the meniscus (m) contains delicate fibrovascular septa (s). Periph-
eral to the meniscus, perimeniscal soft tissues containing blood
vessels (arrows), adipose tissue, and irregular collagen fibers (f)
are visible. (Hematoxylin-eosin stain; objective magnification,
⫻2.)
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 prepara-
tion artifact. (Factor VIII–related antigen immunoperoxidase and he-
matoxylin-eosin stains; objective magnification, ⫻2.)
Volume 217 䡠Number 1 Knee Meniscus: MR Imaging and Histologic Correlation 䡠197