Jeffrey M.

Silverman, MD Jerrohd
#{149} H. Mink, MD Andrew
#{149} L Deutsch, MD

Discoid Menisci of the Knee:

MR Imaging Appearance’

Discoid menisci of the knee are not A discoid meniscus is an abnormal- field of view, one excitation, and a dedi-
uncommon, and the criteria for ar- ly tall and elongated meniscus cated transmit-receive extremity coil (GE
thrographic diagnosis and the chini- that may be symmetrically or asym- Medical Systems) were used. All patients
cal symptoms are well kflown. Al- metrically increased in size. Young were examined in the supine position
(1) first described a discoid lateral with the leg in full extension and exter-
though enlarged menisci have been
nally rotated iO#{176}-15#{176}.
recognized at magnetic resonance meniscus in 1889, and Cave and Sta-
Since the transverse diameter of the
(MR) imaging, there are no criteria ples (2) reported the first discoid me- midbody of a normal meniscus averages
for the MR imaging diagnosis. The dial meniscus in 1941. The preva- 11.6 mm (12), only two contiguous 5-mm-
authors describe 29 discoid menisci lence of discoid lateral meniscus thick sagittal sections should show the
imaged by means of MR A discoid (1.5%-15.5%) (3-6) is greater than black “bow-tie” appearance; the midzone
meniscus was said to be present if that of discoid medial meniscus height should taper from a maximum of
three or more 5-mm-thick contigu- (0.1%-0.3%) (4,6,7). Multiple descrip- 4-5 mm at its periphery to the paper-thin
ous sagittal images demonstrated tions of the arthrographic appearance free edge (Fig 1). Hence, a discoid menis-
cus was said to be present when three or
continuity of the meniscus between of discoid menisci are present in the
more contiguous sagittal sections (5-mm
the anterior and posterior horns. literature (8-10). In addition, the as-
thick) demonstrated continuity of the me-
High-resolution coronal images al- sociated clinical symptoms have been niscus between the anterior and posterior
lowed more graphic depiction of the elucidated (8,11). As magnetic reso- horns.
abnormally wide meniscus. In ap- nance (MR) imaging of the knee is We chose Hall’s classification scheme
proximately one-third of the cases being used more often, the criteria (8) for describing both the discoid medial
in which coronal images were ob- for diagnosis of this entity with MR meniscus and discoid lateral meniscus;
tamed, the measurable height dif- imaging need to be established. menisci were classified as slab, biconcave,
ference between the discoid and the Herein, we define MR imaging cri- wedge, asymmetric anterior, forme fruste,
opposite meniscus was greater than teria for the detection of discoid me- or grossly torn.
The width of the meniscus was mea-
or equal to 2 mm. Arthroscopic cor- nisci and present a series of 29 (27
sured on coronal images at the midpor-
relation (obtained in 10 cases) re- lateral, two medial) presumed cases
tion of the meniscus body. We also used
vealed that six cases of discoid me- as determined with MR imaging cri- coronal images to measure the height of
niscus were diagnosed correctly teria. the medial and lateral menisci at the mid-
with MR imaging, although one portion of the periphery of the mensicus.
meniscus was considered discoid at We defined a meniscus height differen-
MR imaging but was not considered MATERIALS AND METHODS tial of 2 mm or greater as abnormally tall.
discoid at arthroscopy. Of three dis- Seven of the 29 cases could not be eval-
Between August 1985 and July 1988 we
coid menisci seen to be torn at ar- uated for an associated medial meniscal
performed more than 4,000 MR imaging
throscopy, two were seen to be torn tear. One patient had undergone medial
examinations of the knee with a 1.5-T
meniscectomy, and images were insuffi-
at MR imaging. imager (Signa; GE Medical Systems, Mil-
waukee). Except for an 11-year-old boy
cient in four. The remaining two patients
had discoid medial meniscus by MR im-
and a 9-year-old girl,the patients in this
Index terms: Knee, abnormalities, 4524.1495, aging criteria.
study were men (n 20) and women
= (n
4525.1495 Knee, MR studies, 452.1214
#{149} We attempted to ascertain the surgical
7) ranging in age from 26 to 82 years. All
diagnosis for all cases. Ten patients un-
patients were presumed to have discoid
Radiology 1989; 173:351-354 derwent subsequent knee surgery. We
menisci based on our MR imaging crite-
were unable to obtain surgical follow-up
na. Almost all of the MR imaging studies,
in nine cases (ie, no obtainable operative
including those of the children, were per-
report from surgery or arthroscopy). Ten
formed to rule out meniscal tear or inter-
nal derangement of the knee.
other patients never underwent subse-
quent knee surgery.
The routine knee study consisted of in-
All images were reviewed by all three
termediate-weighted and T2-weighted
authors.
(repetition time msec/echo time msec
1 From the Department of Diagnostic Radiol-
2,000/20, 80) sagittal sequences with con-
ogy, Cedars-Sinai Medical Center, 8700 Beverly tiguous 5-mm-thick sections. After March RESULTS
Blvd, Los Angeles, CA 90048. From the 1988
RSNA annual meeting. Received December 30
1987, a Ti-weighted (600/20) coronal se-
ries was then obtained with interleaved The number of sagittal sections on
1988; revision requested February 9, 1989; final
5-mm-thick sections; 10 of our cases were which the anterior and posterior
revision received June 27; accepted July 3. Ad-
dress reprint requests to J.M.S. before March 1987, so no coronal images horns connected varied from three to
#{176}RSNA,1989 are available. A 256 X 128 matrix, 16-cm five in 28 of our 29 cases. Specifically,

351
a. b. c.

d. e. f.

g. h. i.
Figure 1. (a-c) Diagrammatic representation of how MR study of the lateral meniscus from the periphery to the notch was performed.
(d-f) Corresponding MR images (2,000/20; 5-mm-thick sections) of a normal lateral meniscus. (g-i) Corresponding sagittal MR images of a
surgically proved discoid lateral meniscus in a 35-year-old man (2,000/20; 5-mm-thick sections). The normal lateral meniscus tapers rapidly
from the periphery to the free edge (arrows in d-f), but the discoid meniscus demonstrates continuity between the anterior and posterior
horns on all three images (arrows in g-i).

13 discoid menisci were demonstrat-

ed on three contiguous sagittal sec-
tions, 12 on four contiguous sagittal
sections, and three on five contigu-
ous sagittal sections. We were unable f t Ii
to retrieve the sagittal images for one Figure 2. Slab type discoid lateral menis- Figure 3. Slab type discoid medial menis-
case, but an obvious discoid lateral cus (surgically proved) in a 36-year-old man. ctis in a 40-year-old man. MR imaging was
meniscus was present on the coronal There is an associated medial meniscal tear performed to rule out medial meniscus tear;
images. (large arrow). This coronal image (2,000/20; surgery was not performed on this man’s
Of the 27 patients demonstrating 5-mm-thick section) is located at the mid- knee. Coronal image (600/20; 5-mm-thick
zone of the lateral meniscus and shows the section) depicts this presumed discoid me-
presumed discoid lateral meniscus
meniscus extending almost to the intercon- niscus (arrows) to best advantage.
according to these MR imaging crite- dylar notch (small arrows).
na, 10 underwent subsequent knee
surgery. Results of surgery con- ferent in size from adult menisci (12),
firmed a discoid lateral meniscus in but the size of menisci in children
nine of the patients; one lateral me- 1 1-year-old boy who had a discoid has not been critically evaluated.
niscus that was interpreted as a medial meniscus
by MR imaging cri- The height of the presumed dis-
wedge-type discoid meniscus at MR teria was 1 1 mm wide and 5 mm tall coid lateral menisci ranged from 3 to
imaging was thought not to be dis- at the midzone; these dimensions are 9 mm on MR images; no operative re-
coid at arthroscopy. within the normal limits of size for ports mentioned meniscal heights.
At MR imaging, the width of the an adult lateral meniscus. The coro- Six of our cases demonstrated a
discoid meniscus in those cases that nal and sagittal images of the normal height differential of greater than or
could be evaluated ranged from 15 to medial meniscus in the 9-year-old equal to 2 mm; however, 13 cases
36 mm. Sixteen of the 19 cases in girl with a discoid lateral meniscus demonstrated no significant height
which coronal images were obtained by MR imaging criteria (no surgery differential.
had meniscal widths greater than or performed) are not available. Thus, Utilizing Hall’s classification (8),
equal to 20 mm. in one older child the normal lateral we classified eight menisci as slab
The normal lateral meniscus in the meniscus was not significantly dif- type, (Fig 2), 13 as wedge type, one as

352 . Radiology November 1989

biconcave type, two as asymmetric lateral joint widening, abnormally cus show a complete meniscus in all
anterior type, and five as grossly shaped lateral malleolus, and an en- sections through the knee. An asym-
torn. No discoid menisci of the forme larged inferior lateral geniculate ar- metric discoid meniscus with an en-
fruste type were present. tery (3,9,17,18). We saw none of these larged body may have a wide menis-
Two of the 29 presumed discoid associated lesions in our series. cal body on coronal images but nor-
menisci in our series were presumed The snapping-knee syndrome is a mal anterior and posterior horns on
discoid medial menisci by MR imag- typical manifestation of discoid later- sagittal images (13). Thus we empha-
ing criteria (no knee operation was al menisci (7,8,11). When a patient size the need for high-resolution cor-
performed on these patients) (Fig 3). flexes or extends his or her knee, a onal images.
By MR imaging criteria (13), 13 pa- snapping sound is heard; this finding Of the three torn discoid lateral
tients had an associated medial me- is associated with knee joint pain or menisci with surgical follow-up, two
niscal tear, whereas nine did not. Of lateral joint line tenderness. Never- were noted to be torn at arthroscopy.
these 13 patients, seven underwent theless, this clinical syndrome occurs However, one discoid lateral menis-
surgery, and all seven had surgical in only a minority of such cases cus that was classified as not torn at
confirmation of a medial meniscal (8,15). In addition, a discoid medial MR imaging was reported to be torn
tear. meniscus usually is manifested as a at arthroscopy. The images were re-
Of the five patients with discoid medial meniscal tear without specific viewed again, and no definite tear
lateral menisci that were thought to findings to suggest a discoid menis- was found. The discrepancy may be
be torn on the basis of MR images, cus. It is probable that the snapping- explained by an interval tear of the
two patients did not undergo subse- knee syndrome primarily occurs with meniscus (1 month passed between
quent surgery. In one patient there only Wrisberg-ligament-type discoid the time of MR imaging and arthros-
was no mention of a discoid menis- lateral menisci (13). copy), a false-negative MR image, an
cus in the operative report; in the The diagnosis of a discoid menis- iatrogenic tear created during ar-
other two, menisci were confirmed to cus with MR imaging depends on de- throscopy, or error in arthroscopic
be torn at arthroscopy. Four discoid piction of an abnormally large me- diagnosis. Of these, the most likely
menisci that were observed to be in- niscus; however, differentiation be- explanations are an interval tear or a
tact at arthroscopy were also consid- tween a true discoid meniscus and a false-negative MR imaging study. In
ered not torn at MR imaging. In one slightly large meniscus may be diffi- addition, one wedge-shaped discoid
case of an intact discoid lateral me- cult (13,19). This may be further com- lateral meniscu.s diagnosed by means
niscus diagnosed at MR imaging, the phicated by the incomplete documen- of MR imaging was reported not to
operative report contained no com- tation, to date, of age-related changes be discoid at arthroscopy; again, this
ment regarding the lateral meniscus. in the size and shape of menisci with diagnostic disagreement may be sec-
One discoid lateral meniscus that was growth, maturation, and aging. The ondary to a false-positive MR imag-
not seen to be torn at MR imaging misshapen meniscus is best seen in ing result, a false-negative arthrosco-
was reported to be torn at arthros- the middle zone of the MR image. pic result, or, more likely, differences
copy. Normally only two 5-mm-thick con- in terminology. Specifically, the ra-
tiguous sagittal sections intersect the diologic diagnosis of discoid menis-
midportion of the lateral meniscus, cus depends on demonstration of an
DISCUSSION
as the average meniscal transverse di- abnormally large meniscus, and, as
MR imaging is rapidly becoming ameter is 11.6 mm (12). Nevertheless, previously noted, differentiation be-
the modality of choice for imaging it is conceivable to observe three con- tween a true discoid meniscus and a
the musculoskeletal system, especial- secutive sagittal images from a me- large meniscus may be difficult
ly the knee. Not only are the menisci niscus that is at the upper limits of (13,19). The classic slab type menis-
well seen, but the surrounding soft normal in size (eg, 12 mm) secondary cus is readily diagnosed by means of
tissues, bursae, cartilages, and bones to volume averaging. We consider a MR imaging, arthrography, and ar-
are exquisitely depicted as well. In discoid meniscus to be present when throscopy. Indeed, this type of me-
spite of the superb depiction of me- three or more contiguous sagittal sec- niscus often extends into the inter-
niscal abnormalities, it was not until tions that are 5 mm thick demon- condylar notch (Fig 2).
1987 that the detection of discoid me- strate a continuous band of meniscus We observed many cases with an
nisci with MR imaging was reported from the front to the back of the pla- associated medial meniscal tear, as
by Reicher et al (14). teau. Further, as discoid menisci may might be expected, since the usual
Multiple classifications of discoid have increased superoinferior reason for performing MR imaging
menisci exist, including those by height, an abnormally thickened was to rule out meniscal tear. How-
Smillie (6,9), Watanabe et al (15), and bow-tie appearance should suggest ever, our series did not demonstrate a
Hall (8). Watanabe et al developed an discoid meniscus (13). Because the convincing positive or negative asso-
orthopedic classification of three height of the normal meniscus rapid- ciation of discoid lateral meniscus
types. Complete and incomplete ly tapers from the periphery to the with medial meniscal tears. Similar-
types indicate the degree of interpo- center, the presence of two adjacent ly, a significant height differential
sition between the femoral condyle peripheral 5-mm-thick sagittal sec- between the discoid lateral meniscus
and tibial plateau; both of these types tions demonstrating equal or nearly and the medial meniscus or between
have an intact posterolateral menis- equal meniscal height probably mdi- the two discoid medial menisci and
cotibial ligament (10,15,16). cates a discoid meniscus. The anterior accompanying lateral meniscus was
Many musculoskeletal abnormali- and posterior horns of the lateral me- not evident in a statistically signifi-
ties are associated with discoid lateral niscus normally are equal in height; cant number of cases. However, this
meniscus, including high fibular an asymmetric discoid lateral menis- is a small series, and it is possible that
head, fibular muscular defects, hypo- cus may have an abnormally large a large series may show both an asso-
plasia of the lateral femoral condyle, anterior or posterior horn. ciation between discoid lateral
hypoplasia of the lateral tibial spine, Coronal images of a discoid menis- menisci and medial meniscal tears

Volume 173 Number

#{149} 2 Radiology 353
#{149}
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354 Radiology
#{149} November 1989