AJR 2005; 184:200-204
© American Roentgen Ray Society
The "Floating" Meniscus: MRI in Knee Trauma and Implications for Surgery
Ravi S. Bikkina1,
Charles A. Tujo2,
Albert B. Schraner2 and
Nancy M. Major1
1 Department of Radiology, Duke University Medical Center, Erwin Rd., PO Box
3808, Durham, NC 27710.
2 Department of Diagnostic Imaging, David Grant Medical Center, 101 Bodin Cir.,
Travis Air Force Base, CA 94535.
Received February 26, 2004;
accepted after revision May 13, 2004.
The views expressed in this article are those of the authors and do not
reflect the official position of the U.S. government, the Department of
Defense, or the Department of the Air Force.
Address correspondence to N. M. Major.
Abstract
OBJECTIVE. We describe 21 cases involving meniscal injury in which
the meniscus appears free-floating on MRI of the knee. In these cases, the
meniscus is completely surrounded by fluid. Correlation with surgical reports
shows that the "floating" meniscus corresponds to a meniscal
avulsion or detachment from the tibial plateau with an associated disruption
of the meniscotibial coronary ligaments, which attach the meniscus to the
tibia, allowing fluid to encompass the meniscus. A floating meniscus on MRI
may represent a new specific finding for an uncommon form of meniscal injury
known as meniscal avulsion.
CONCLUSION. The presence of a floating meniscus on MRI is a result
of significant trauma to the knee leading to meniscal avulsion and is often
associated with significant ligamentous injury. Alerting the surgeon to the
presence of a meniscal avulsion facilitates appropriate surgical planning with
meniscal reattachment to the tibial plateau.
Introduction
MRI has become an excellent means of evaluating internal derangements of
the knee. The accuracy of MRI for detection of meniscal injury is particularly
high, reportedly up to 95%
[1–3].
We describe 21 cases of an avulsed or "floating" meniscus, in
which the meniscus is surrounded by fluid and which is indicative of tearing
or disruption of the coronary ligaments identified on MRI of the knee. To our
knowledge, the finding of a floating meniscus has not been previously
described in the MRI literature. It is usually seen as a sequela of
high-impact injury or trauma. It has been mentioned in arthrography
literature.
Materials and Methods
We present a total of 21 cases of a floating meniscus. We retrospectively
evaluated 4,096 MR images of the knee obtained from July 1998 through
September 2003, with 17 cases showing a floating meniscus. Four other cases
were prospectively identified and confirmed at surgery. Our examinations were
correlated with surgical reports. We define a "floating" meniscus
to be equivalent to a meniscal avulsion. A floating meniscus was diagnosed if
fluid signal of greater than 3 mm in thickness in the long-axis dimension on
either sagittal or coronal images completely surrounded either the anterior or
posterior horn of the meniscus. Fluid signal extending between the meniscus
and the tibial plateau was also considered diagnostic of a meniscal avulsion
or floating meniscus. All imaging was performed on a 1.5-T magnet (Signa, GE
Healthcare). In all cases, imaging parameters were as follows: fast spin-echo
sagittal T2- and proton density–weighted sequences, axial and coronal
fast spin-echo fat-saturated T2-weighted sequences, and coronal spin-echo
T1-weighted sequences with a 16-cm field of view, 4-mm slice thickness, 256
x 192 matrix, and 2 excitations.
Results
Twenty-one patients with a floating or avulsed meniscus were diagnosed on
MRI of the knee. An avulsed meniscus was best identified on coronal and
sagittal T2-weighted sequences. Of the 21 patients, 14 were men and seven were
women. The age range of the patients was 18–51 years with an average age
of 30.5 years. Review of patient histories revealed that 19 of 21 patients
experienced severe acute trauma, including complete knee dislocation in nine
patients. The other two patients had pain related to their sports. We could
not pinpoint an exact time or mechanism of injury for those two patients.
Associated ligamentous injuries were common and included 11 anterior cruciate
ligament tears, nine posterior cruciate ligament tears, six medial collateral
ligament tears, and four lateral collateral ligament tears. All nine patients
with knee dislocations sustained osseous contusions of varying degrees. The
lateral meniscus appeared floating in 14 patients, and the medial meniscus, in
seven patients (Figs. 1A,
1B,
1C,
2A, and
2B). Only three of the avulsed
menisci had additional tears at surgery, all involving the lateral meniscus.
Two of these tears were reported as horizontal cleavage tears, and the other
one was reported as a radial tear of the body. Of these three torn and avulsed
lateral menisci, two of the tears were prospectively identified on MRI.
Eighteen patients proceeded to surgery after MRI evaluation. Review of the
surgical reports confirmed the presence of a meniscal avulsion from the tibia
or floating meniscus in 15 of these 18 patients. Three patients diagnosed with
floating menisci did not have arthroscopic surgery to confirm findings but
instead were treated conservatively.
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Fig. 1A. —28-year-old man after motor vehicle crash with associated
knee dislocation. Coronal fast spin-echo fat-saturated T2-weighted image shows
large quantity of fluid extending between lateral meniscus and tibial plateau
(arrow), representing "floating" meniscus, which was
confirmed surgically. Note high signal within anterior cruciate ligament
representing tear and contusion of medial femoral condyle. Also note high
signal in chondral defect.
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Fig. 1B. —28-year-old man after motor vehicle crash with associated
knee dislocation. Sagittal fast spin-echo fat-saturated T2-weighted image
shows large quantity of fluid surrounding lateral meniscus representing
meniscal avulsion from tibial plateau. Note associated elevation of floating
anterior horn of meniscus.
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Fig. 1C. —28-year-old man after motor vehicle crash with associated
knee dislocation. Intraoperative photograph shows forceps grasping floating
lateral meniscus that has been avulsed from tibial plateau (arrow).
Note gap between meniscus and tibia.
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Fig. 2A. —29-year-old man after acute knee dislocation. Coronal fast
spin-echo fat-saturated T2-weighted image shows relatively small quantity of
fluid extending between tibial plateau and lateral meniscus (arrow)
suggestive of "floating" meniscus. Note absence of fluid beneath
normal medial meniscus and contusions of lateral femoral condyle and lateral
tibial plateau.
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Fig. 2B. —29-year-old man after acute knee dislocation. Sagittal fast
spin-echo fat-saturated T2-weighted image confirms meniscal avulsion, with
fluid completely surrounding anterior horn of lateral meniscus and extending
beneath posterior horn. This floating meniscus was repaired surgically.
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Discussion
The menisci are fibrocartilaginous structures that attach to the tibia and
accommodate the femur. They serve to transmit axial and torsional forces
across the joint, cushion mechanical loading, limit comprehensive
displacement, distribute synovium, increase the surface area for femoral
condylar motion, and prevent synovial impingement
[4]. Meniscocapsular ligaments,
including meniscofemoral and meniscotibial
(Fig. 3) components, attach the
menisci to the posterior femur and tibial plateau, respectively. The
meniscotibial ligaments are short confluent ligamentous bands that attach
peripherally to the body of the meniscus and serve to stabilize and maintain
the meniscus in the appropriate position on the tibial plateau. The
meniscotibial or coronary ligaments further form a portion of the third or
deepest layer of the lateral joint capsule
[5]. Together the meniscotibial
and meniscofemoral ligaments also compose the medial capsular ligament, which
represents a portion of the medial joint capsule. The meniscotibial ligament
fibers are, however, difficult to separate from the adjacent capsule and
collateral ligament fibers. The meniscotibial or coronary ligaments attach to
the tibia several millimeters inferior to the articular cartilage and
occasionally result in a small synovial recess. Although this recess may be
seen on MRI, the meniscotibial ligaments themselves are rarely separately
identified [6].
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Fig. 3. —Schematic diagram shows attachment of medial meniscus to
adjacent tibial plateau by normal meniscotibial ligament and relationship of
meniscotibial ligament to adjacent medial collateral ligament (MCL).
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In the setting of acute trauma, however, the meniscotibial ligaments may
become disrupted, and subsequently, the meniscus is avulsed from the tibial
plateau. Meniscal avulsion is a phenomenon well known to the orthopedic
surgeon but has received little attention in the radiology literature. Most
surgeons believe that a meniscus that has been avulsed or detached from the
tibial plateau should be reattached if possible. The meniscus is typically
sutured back into its anatomic position
(Fig. 1C).
Normally, the tibial plateau articular cartilage should be completely
covered by the posterior horn of the menisci. If fluid is found insinuating
beneath the meniscus, particularly the posterior horn, a detachment should be
considered [7]. Fast spin-echo
T2-weighted sagittal and coronal images best show the relationship of the
menisci to the tibial plateau and therefore are optimal for identification of
meniscal avulsions sustained as a result of disruptions of the meniscotibial
ligaments.
In the orthopedic literature, "meniscal detachment" is
described as displacement of the meniscus 5 mm or more from the tibial plateau
in association with uncovering of the tibial plateau cartilage or fluid
interposition between the peripheral edge of the meniscus and the tibial
plateau. Furthermore, a complete peripheral detachment of the posterior horn
of the medial meniscus has been described on arthroscopy as a free-floating
meniscus (Figs. 4A and
4B) and is often associated
with a medial collateral ligament tear
[8]. El-Khoury et al.
[9] described an arthrographic
floating meniscus in which the meniscus floats above the tibial plateau
without separating entirely from the capsule, indicating a tear of the
meniscotibial coronary ligament. Although disruption of the meniscotibial
ligaments results in a floating meniscus, the meniscofemoral ligaments
typically remain intact in the presence of meniscal avulsion. We suggest that
a floating meniscus can be identified on MRI, and its presence should be
carefully evaluated in cases involving severe knee trauma with dislocation or
multiligamentous injury or both. The presence of fluid completely surrounding
the entirety of one or both horns of either meniscus, best seen on T2-weighted
images, should alert the radiologist to the possibility of a meniscal
detachment.
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Fig. 4A. —29-year-old man after clipping injury without dislocation
sustained while playing football. On coronal fast spin-echo fat-saturated
T2-weighted image, far posterior aspect has large quantity of fluid extending
between tibial plateau and medial meniscus. No other injuries were identified
on MRI or at surgery.
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Fig. 4B. —29-year-old man after clipping injury without dislocation
sustained while playing football. Sagittal fast spin-echo non-fat-saturated
T2-weighted image shows subtle small quantity of fluid extending beneath
posterior horn and body of medial meniscus.
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In our experience, a floating meniscus typically remains intact without
evidence of a tear within the substance of the meniscus as depicted on images
of three patients having meniscal tears seen at surgery. The relative sparing
of the avulsed menisci from tearing probably relates to a different mechanism
of stress loading, with the bulk of the forces leading to shearing of the
meniscotibial ligaments, rather than injury to the meniscus itself.
In 15 patients, the suspected meniscal avulsion was confirmed surgically,
and the detachment was appropriately repaired. However, four patients with
suspected floating menisci did not have meniscal avulsion specifically
mentioned in the surgical reports. One of the four possible false-positive
findings was in a patient with dislocation and tears of the anterior and
posterior cruciate ligaments and both the medial and lateral collateral
ligaments, with significant widening of the lateral joint space. In this
instance, the extent of the patient's injuries may have lessened attention to
a readily evident meniscal avulsion identified on MRI (Figs.
5A and
5B). Another patient with knee
dislocation had popliteal thrombosis and secondary above-the-knee amputation,
thus obviating repair of an avulsed meniscus. In the remaining two patients,
repair of an avulsed meniscus may have been performed, although it was not
specifically mentioned in the surgical reports. Another explanation may
involve the degree of attachment of the meniscus to the tibial plateau by the
meniscotibial ligaments, such that a small quantity of fluid insinuating
beneath the meniscus could represent a normal variation. In our opinion,
however, fluid signal greater than 2–3 mm in thickness in the
craniocaudal dimensions beneath the meniscus should strongly suggest a tear of
the meniscotibial ligaments, particularly in an acute setting.
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Fig. 5A. —51-year-old man after motorcycle crash with associated knee
dislocation. Coronal fast spin-echo fat-saturated T2-weighted image shows
significant quantity of fluid extending between tibial plateau and
"floating" lateral meniscus. Note complete lateral and medial
collateral ligament tears and marked widening of lateral joint space.
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Fig. 5B. —51-year-old man after motorcycle crash with associated knee
dislocation. Sagittal fast spin-echo non-fat-saturated T2-weighted image shows
large quantity of fluid extending beneath avulsed lateral meniscus and tibial
plateau, with associated superior migration of floating anterior horn.
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An entity that can mimic a floating meniscus is a rare variant of a discoid
lateral meniscus, the Wrisberg's ligament type. The Wrisberg's ligament type
has no attachment to the tibial plateau posteriorly
[8]. This form of discoid
meniscus has a single attachment only, the lateral meniscofemoral ligament or
Wrisberg's ligament. The anterior horn of this discoid meniscus is
appropriately attached to the tibial plateau, although the meniscus itself may
sublux significantly with flexion and extension. These variants should also be
repaired. This lateral meniscal variant may mimic an avulsion and should be
considered if the meniscus appears discoid on MRI. The patient's history
should also help distinguish this variant from a true floating meniscus, with
meniscal avulsion suggested in the setting of acute severe trauma.
Arthroscopic knee surgery results in routine evaluation of the integrity of
the meniscotibial ligaments. Although the meniscotibial ligaments can be
difficult to visualize directly on MRI, the sequela of significant injury to
these ligaments can be visualized as a floating meniscus. Alerting the
orthopedic surgeon to the possibility of a floating meniscus preoperatively
allows estimation of tourniquet time and preparation for appropriate surgical
repair. Also, because arthroscopy requires instillation of saline into the
joint, an avulsed, but otherwise normal, meniscus could be relocated onto the
tibial plateau intraoperatively, thus making detection more challenging.
Knowledge of the MRI appearance of the floating meniscus should therefore
become an important part of our diagnostic armamentarium, particularly in the
setting of severe acute knee trauma.
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Abstract
Introduction
