AJR 2000; 174:161-164
© American Roentgen Ray Society
Inferiorly Displaced Flap Tears of the Medial Meniscus
MR Appearance and Clinical Significance
Lynn K. Lecas1,
Clyde A. Helms1,
Frank J. Kosarek1 and
William E. Garret2
1
Department of Radiology, Duke University Medical Center, Erwin Rd., Box 3808,
Durham, NC 27710.
2
Department of Orthopedic Surgery, University of North Carolina, Manning Dr.
Chapel Hill, NC 27514.
Received April 12, 1999;
accepted after revision June 18, 1999.
Address correspondence to L. K. Lecas.
Abstract
OBJECTIVE. We describe the MR imaging features of medial meniscus
flap tears in which the fragment becomes located inferomedial to the tibial
plateau and deep in relation to the medial collateral ligament.
CONCLUSION. Inferior flap tears of the medial meniscus can be
inconspicuous and overlooked by both radiologists and orthopedic surgeons.
Inferiorly displaced meniscal fragments may escape detection during
arthroscopic surgery unless the fragment is sought with a probing hook.
Recognition of this meniscal abnormality on MR imaging is important for
preoperative planning.
Introduction
Evaluation of meniscal injury accounts for most requests for MR imaging of
the knee at most institutions. MR imaging is reliable in the detection of
meniscal tears and identification of meniscal fragmentation and displacement
[1,
2,
3,
4].
Displaced meniscal fragments are often clinically significant lesions
requiring surgical intervention and, therefore, are important to identify. One
type of displaced fragment is the inferior flap tear
(Fig. 1). A meniscal flap tear
or displaced flap is the result of a short-segment, horizontal meniscal tear
with either superior or inferior displacement of a meniscal fragment
[5]. Usually this type of
meniscal injury involves the medial meniscus with superior displacement. In
one orthopedic study of symptomatic meniscal lesions, superior flaps were six
times more common than inferior flaps
[6]. We describe the MR imaging
appearance of inferiorly displaced flap tears, investigate their incidence,
and evaluate their significance. To our knowledge, patients with these tears
have not been described in the radiology literature. If the displaced fragment
is located inferior and medial to the tibial plateau and extends deep in
relation to the medial collateral ligament, the surface of the meniscus may
appear healthy on arthroscopy while the abnormally positioned fragment goes
unnoticed unless the meniscus is probed with a hook. Orthopedic surgeons at
our institution report that this is a common arthroscopic finding.
Materials and Methods
We retrospectively searched our musculoskeletal MR computer database for
all knee MR imaging examinations at our institution from June 1993 through
August 1998. In this period, 3686 MR examinations of the knee were found. We
identified 236 patients with free or displaced meniscal fragments. Our search
was further limited to patients in whom a meniscal fragment was located by MR
imaging or surgery inferior and medial to the tibial plateau and deep in
relation to the medial collateral ligament. The patients identified at surgery
were chosen from a review of one orthopedic surgeon's arthroscopy records. All
patients with a documented history of prior meniscal surgery were excluded.
Our patient population consisted of 11 patients with MR imaging findings of
displaced meniscal fragments inferior to a torn medial meniscus. The fragments
lay inferomedial to the tibial plateau and deep in relation to the medial
collateral ligament. Three of the 11 patient images were reexamined only after
arthroscopy showed the presence of flap tears with inferomedial
displacement.
The knee MR imaging protocol evolved over the 5-year period. The MR
examinations of our patients were performed with Signa 1.5-T systems (General
Electric Medical System, Milwaukee, WI) with a transmit-receive knee coil.
Sagittal proton density-weighted (2000/20 [TR/TE]) and sagittal, axial, and
coronal fast spin-echo T2-weighted MR images with fat saturation
(3000-5000/65-75 [TR range/TE range]) were acquired. The field of view was 16
x 16 cm and the slice thickness was 4.0 mm with a 0.4 mm interslice gap.
The matrix size was 256 x 192 with one excitation for the proton
density-weighted sequences and two excitations for the fast spin-echo
sequences.
Results
Two hundred thirty-six patients (6.4%) of the 3686 who underwent MR imaging
of the knee had a displaced meniscal fragment. Eleven (4.7%) of the 236
patients with displaced meniscal fragments displayed the fragments
inferomedially. The study population included seven men and four women, 39-68
years old (average age, 56 years 5 months). In all 11 patients (including the
three retrospective patients), a meniscal fragment could be identified on MR
imaging between the medial collateral ligament and medial tibia and was best
seen on coronal T2-weighted MR images
(Figs. 2,
3A, and
3B). Two patients had prominent
findings of meniscal tissue in the medial gutter on sagittal MR images
(Figs. 3C and
4,
4). The displaced meniscal
fragment was isointense with and in continuity with the parent meniscus in all
patients. In our study cohort, six patients underwent arthroscopy and all six
had surgically proven medial meniscus tears. Five of these six patients had
surgical confirmation of inferomedial displacement of meniscal fragments,
although for only two of these patients were the displaced meniscal fragments
documented in the text of the surgery report. The other three patients with
surgically proven tears had intraoperative photographs and a verbal report or
just a verbal report by the attending orthopedic surgeon confirming that the
meniscal flaps were inferomedially displaced; however, the surgery reports
dictated by the surgical residents made no mention of these findings. In eight
of the 11 patients a prospective MR diagnosis of a tear of the medial meniscus
with a displaced fragment was made. The flap tear and fragment displacement of
the remaining three patients were proven surgically and with a retrospective
review of the MR images.
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Fig. 2. —44-year-old man with inferior flap tear of medial meniscus. Coronal
fast spin-echo T2-weighted MR image (3283/76 [TR/TE]) with fat saturation
shows inferomedial displacement of medial meniscus fragment (arrow)
deep in relation to medial collateral ligament and adjacent to tibial
plateau.
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Fig. 3. —65-year-old man with inferior flap tear of medial meniscus.
A, Coronal fast spin-echo T2-weighted MR image (3816/70 [TR/TE])
with fat saturation shows inferomedial displacement of medial meniscal
fragment (arrow) deep in relation to medial collateral ligament and
adjacent to tibial plateau.
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Fig. 3. —65-year-old man with inferior flap tear of medial meniscus.
B, MR image (3816/70) obtained posterior to A shows displaced
fragment (short arrow) in gutter. Note abnormal signal extending
horizontally through parent meniscus to involve superior and inferior
surfaces, representing flap tear (long, thin arrow).
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Fig. 3. —65-year-old man with inferior flap tear of medial meniscus.
C, Sagittal proton density-weighted MR image (2000/20) with fat
saturation obtained through most medial aspect of knee reveals large fragment
(arrow) of meniscus in medial gutter.
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Fig. 4. —44-year-old man with inferior flap tear of medial meniscus.
A Sagittal proton density-weighted image (2000/20 [TR/TE]) with fat
saturation through most medial aspect of knee. Large fragment (arrow)
of meniscus is in medial gutter.
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Fig. 4. —44-year-old man with inferior flap tear of medial meniscus.
B, MR image obtained more toward intercondylar notch than A
shows abnormally shaped body segment with flap (arrows) of meniscus
beneath inferior surface of parent meniscus.
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Discussion
Displaced meniscal injuries can occur in both the medial and lateral
meniscus and include flap tears, bucket-handle tears, and free fragment
displacement [5]. By far,
bucket-handle tears are the most common, occurring in about 10% of patients
[7]. Bucket-handle tears with
displacement result from a longitudinal, oblique, or vertical tear of a
meniscus that has an attached fragment displaced away from the meniscus.
Horizontal tears give rise to flap tears, which can be classified as superior
or inferior when displaced. Superior flaps arise from the superior surface of
the meniscus and inferior flaps arise from the inferior surface
[6].
Identification of displaced meniscal tissue is important because
arthroscopy may be required to excise or reattach the fragment. Preoperative
planning may be affected by knowledge of the severity of the lesion. Although
most arthroscopies are performed with local anesthesia, many may require
general anesthesia for correct treatment, and the preoperative evaluation of
the meniscal lesion on MR imaging may play an important role in planning
treatment [8].
MR imaging has proven to be an accurate method for detecting meniscal tears
and locating displaced meniscal fragments
[1,
2,
3,
4]. The typical locations of
displaced meniscal tissue from bucket-handle tears include the intercondylar
notch, anterior and parallel to the posterior cruciate ligament, and
vertically or horizontally juxtaposed to the anterior horn
[5]. The superior part of the
joint is a common location for displaced fragments of flap tears. In addition
to these locations, the joint space medial and inferior to the torn meniscus
should be carefully examined. We have found 11 patients with meniscal
fragments in this area.
A study of the anatomy of 1000 symptomatic meniscal lesions found that 6%
of all medial meniscus fragments were inverted. Fifty-one percent of the
inverted fragments were flaps and the rest were ruptured bucket-handle
fragments [6]. Experienced
orthopedic surgeons at our institution believe that nearly all of the
inferomedially displaced fragments seen at arthroscopy from the medial
meniscus are the result of flap tears. According to the classification of
meniscal tears by Dandy [6],
the type of tear that correlates best with our MR imaging and the results of
our arthroscopy is the inferior flap tear. Therefore, we refer to them as
inferior flap tears.
Other normal low-signal-intensity structures, such as the semimembranous
tendon, should not be mistaken for displaced menisci
(Figs. 5 and
6). Recognizing the location of
displaced meniscal tissue on MR imaging and conveying this information to the
orthopedic surgeon is of paramount importance, particularly for the
arthroscopist. Inferiorly displaced or inverted fragments may escape detection
during arthroscopic evaluation by an inexperienced or unsuspecting surgeon
unless the fragment is sought with a probing hook
[6].
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Fig. 5. —60-year-old man with inferior flap tear of medial meniscus with MR
image depicting semimembranous tendon as separate structure from displaced
meniscal fragment. Axial fast spin-echo T2-weighted MR image (4666/75 [TR/TE])
with fat saturation through tibial plateau. In medial joint is large displaced
fragment (M) of meniscus. Semimembranous tendon (arrow) is seen
separate from fragment. Between two structures is joint fluid extending in
popliteal cyst (P).
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Fig. 6. —66-year-old man with inferior flap tear of medial meniscus with MR
image depicting semimembranous tendon as separate structure from displaced
meniscal fragment. Coronal fast spin-echo T2-weighted MR image (3283/76
[TR/TE]) with fat saturation. Inferomedially displaced fragment (straight
arrow) is clearly seen as separate from semimembranous tendon (curved
arrow).
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The incidence (4.7% of tears with displacement) of inferomedially displaced
flap tears in our study is somewhat misleading because eight of 11 were
retrospective diagnoses identified only in the last 18 months of the database
entries, undoubtedly because of ignorance of this finding in prior years. In
three patients, a retrospective diagnosis was made on the basis of
reevaluation of the MR imaging findings only after arthroscopy confirmed the
presence of meniscal tissue displaced in the medial gutter.
In conclusion, displaced meniscal fragments have been described in the
intercondylar notch, as a double posterior cruciate ligament, as a flipped or
stacked meniscus, and now as an inferiorly displaced fragment that extends
between the tibia and the medial collateral ligament. Identification of the
meniscal flap is important because arthroscopy may be necessary for its
removal or reattachment. When located inferomedial to the tibial plateau and
deep in relation to the medial collateral ligament, these fragments may become
an arthroscopic pitfall when the fragment is unapparent until probed with a
hook. MR imaging is a sensitive, noninvasive method of detection of meniscal
tears and their displaced fragments. MR imaging may also help in preoperative
planning and may facilitate the detection of these inferiorly displaced
fragments that might have gone unnoticed during surgery.
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Abstract
Introduction
