DOI:10.2214/AJR.04.1785
AJR 2006; 187:384-387
© American Roentgen Ray Society
MRI Appearance of Wrisberg Variant of Discoid Lateral Meniscus
Kush Singh1,
Clyde A. Helms1,
M. Todd Jacobs1 and
Laurence D. Higgins2
1 Department of Radiology, Duke University Medical Center, Box 3808, Durham, NC
27710.
2 Department of Orthopaedic Surgery, Duke University Medical Center, Durham,
NC.
Received November 17, 2004;
accepted after revision April 27, 2005.
Address correspondence to K. Singh
(kushsingh{at}hotmail.com).
Abstract
OBJECTIVE. A discoid lateral meniscus is a common normal variant,
occurring in approximately 3-5% of the population. A subgroup of the discoid
lateral meniscus known as the Wrisberg variant occurs when there is no
posterior attachment of the lateral meniscus. Instead, Wrisberg's ligament
becomes the only lateral meniscus posterior stabilizer, with a symptomatic
hypermobile meniscus. Although it is commonly described in the orthopedics
literature, to our knowledge the Wrisberg variant of the discoid lateral
meniscus is not mentioned in the radiology literature. This article describes
the MRI appearance of this important yet often unrecognized process.
CONCLUSION. The Wrisberg variant of the discoid lateral meniscus is
a rare condition with a highly suggestive clinical history. The
musculoskeletal radiologist should consider this diagnosis whenever a discoid
lateral meniscus is identified because making this elusive diagnosis may prove
immensely useful for the orthopedic surgeon.
Keywords: bone • knee • meniscus • MRI • musculoskeletal imaging • orthopedic surgery • Wrisberg variant discoid lateral meniscus
Introduction
Adisk-shaped or discoid meniscus is a common normal variant that is seen in
0.8-3.0% of the population and that is manifested by a wider than normal
meniscal body
[1-4].
The discoid shape of the meniscus results in greater coverage of the tibia and
is usually associated with increased thickness of the meniscus that may lead
to abnormal shearing forces across the knee joint
[5].
Three subgroups of discoid menisci were described by Watanabe and Takeda
[6] in 1974: complete,
incomplete, and Wrisberg variant. Complete and incomplete discoid menisci vary
in their degree of tibial plateau coverage. The least common subtype, the
Wrisberg variant, lacks the normal posterior coronary ligament and capsular
attachments. Instead, the posterior horn of the lateral meniscus is mobile,
occasionally subluxing into the joint
[7].
The prevalence of the Wrisberg variant of discoid lateral meniscus varies
depending on the series reported, with Neuschwander et al.
[1] describing an
arthroscopically proven prevalence as low as 0.2%. In our experience, the
prevalence of the Wrisberg variant of discoid lateral meniscus in the
population is much lower than in most reports; this article presents the only
two cases encountered over a period of several years.
The diagnosis of a discoid meniscus is strongly suggested by the patient's
history, with symptoms that include knee pain, snapping, and locking in a
child or adolescent. The Wrisberg variant is commonly described by patients as
snapping of the posterior horn across the femoral condyle during flexion or
extension that results in a snapping sensation. Despite the classic clinical
history, MRI or arthroscopy is nevertheless required for a definitive
diagnosis of the Wrisberg variant. However, the decision to operate is
ultimately based on the presence of symptoms.
Although it is commonly described in the orthopedics literature, to our
knowledge the Wrisberg variant of the discoid lateral meniscus is not
mentioned in the radiology literature. This article describes the MRI
appearance of this important yet often unrecognized process.
Materials and Methods
Two patients with arthroscopically proven Wrisberg variant of a discoid
lateral meniscus were identified by an orthopedic surgeon at our institution.
The MR images from these two patients were retrospectively reviewed. The
diagnosis of Wrisberg variant of a discoid lateral meniscus was prospectively
made before arthroscopy on one of the two patients. The other patient was
noted to have findings of the Wrisberg variant on retrospective review of the
preoperative MR images.
Arthroscopic criteria in general for the Wrisberg variant of discoid
lateral meniscus include hypermobility of the menisci shown both by probing
and flexion and extension during arthroscopy and by the absence of a
fascicular attachment to the capsule as well as the absence of the normal
coronary ligament attachment to the tibia.
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Fig. 1A —15-year-old boy with knee pain. Sagittal proton
density-weighted image (TR/TE, 2,000/20; slice thickness, 4 mm; interslice
gap, 0.4 mm; field of view, 14-16 cm; matrix, 256 x 192) of lateral
discoid meniscus shows no normal meniscocapsular attachments
(arrow).
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Fig. 1B —15-year-old boy with knee pain. Sagittal proton
density-weighted image (using same parameters as for A) through
posterior horn of lateral meniscus adjacent to A shows another body
segment, consistent with discoid meniscus. Again note lack of normal
meniscocapsular attachments (arrow).
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Fig. 1C —15-year-old boy with knee pain. Arthroscopic image shows
posterior horn of lateral meniscus (M). Surgical probe is seen pulling on
Wrisberg's ligament (W) and displacing hypermobile posterior horn of lateral
meniscus off tibial plateau.
|
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Each discoid lateral meniscus was inspected on the MR images for capsular
attachments and attachment to the tibia by the coronary ligaments. The lateral
meniscus was deemed a Wrisberg variant if no capsular attachment by the
meniscal fascicles could be identified. The meniscus was deemed unattached to
the tibia by coronary ligaments if it could be seen displaced off the tibia.
Although the meniscopopliteal fascicles vary in healthy individuals, in the
setting of a discoid lateral meniscus this finding defines a Wrisberg
variant.
All imaging was conducted on a 1.5-T closed magnet MR scanner (Signa, GE
Healthcare). Imaging protocols included spin-echo fat-saturation proton
density-weighted sagittal images (TR/TE, 2,000/20; slice thickness, 4 mm;
interslice gap, 0.4 mm; field of view, 14-16 cm; matrix, 256 x 192) and
fast spin-echo T2-weighted images with fat-saturation (4,000/75; slice
thickness, 4 mm; interslice gap, 0.4 mm; field of view, 14-16 cm; matrix, 256
x 192) in all imaging planes.
Results
Findings of Wrisberg variant discoid lateral meniscus were identified in
both patients. The MRI findings of the Wrisberg variant encompass those of a
discoid lateral meniscus, which include a specific measurement of the
left-to-right width of the meniscus at the center of the body greater than 15
mm on a coronal slice (reference average, 12.8 mm)
[3,
4]. Alternatively, a discoid
lateral meniscus shows more than two meniscal body segments on consecutive
sagittal images of the knee.
Absence of the normal fascicles attaching the posterior horn of the lateral
meniscus to the joint capsule is also seen (Figs.
1A,
1B, and
1C). As a result, a high T2
signal is interposed between the lateral meniscus and the joint capsule,
simulating a peripheral tear or injury to the meniscal fascicles. In addition,
the hypermobile posterior horn may abnormally sublux anteriorly within the
joint (Figs. 2A and
2B).
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Fig. 2A —62-year-old woman with lateral knee pain and joint line
tenderness. Sagittal fast spin-echo T2-weighted image with fat satuation
(TR/TE, 4,000/75; slice thickness, 4 mm; interslice gap, 0.4 mm; field of
view, 14-16 cm; matrix, 256 x 192) shows vertically oriented posterior
horn of lateral meniscus (arrow), indicating absence of attachment by
meniscotibial (coronary) ligaments.
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Fig. 2B —62-year-old woman with lateral knee pain and joint line
tenderness. Coronal fast spin-echo T2-weighted image with fat satuation (same
parameters as for A) shows popliteal tendon (arrow) as it
passes between posterior horn of meniscus and joint capsule. Note absence of
meniscal struts or fascicles that should normally attach meniscus to
capsule.
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Discussion
Although the discoid meniscus is a common normal variant, the least common
subtype of the discoid meniscus, the Wrisberg variant, is rarely encountered.
Our experience over a period of several years at a large academic institution
includes only two cases of arthroscopically proven Wrisberg variant of a
discoid lateral meniscus with associated MRI of the affected knee.
MRI in the setting of knee pain in a child has become a viable alternative
to invasive arthroscopy, serving as a road map for operative conditions and
preventing unnecessary surgery in nonoperative conditions. Although they are
uncommon, the findings of the Wrisberg variant of a discoid lateral meniscus
should be sought whenever a discoid meniscus is identified on MRI of the knee.
The implications of a Wrisberg variant being identified by the radiologist are
important. The decision to operate, the surgical approach, and the technique
may all be altered by a preoperative diagnosis of a Wrisberg variant of the
discoid lateral meniscus.
The treatment of a discoid meniscus depends on its type and whether it is
associated with a tear. If a discoid meniscus is discovered with no evidence
of a tear, then its presence should be considered incidental and it should be
left intact. If a tear is associated with a complete or incomplete discoid
meniscus, then partial meniscectomy should be performed as a saucerization
technique. The goal should be to resect enough tissue to result in a
well-contoured 6-mm stable rim
[8,
9]. The traditional treatment
for a Wrisberg's ligament variant has been total meniscectomy. More recently,
techniques have been developed to reduce the meniscus and repair it by
providing a posterior attachment
[10].
The two patients in our study presented in quite different ways. The first
patient, who had a more classic presentation, was a 15-year-old boy with no
history of trauma who presented with lateral knee pain and an audible click
with flexion. The second patient was a 62-year-old woman, a much older age
than is typical for presentation. However, the patient had been experiencing
symptoms for several years and had undergone MRI in which the diagnosis of
Wrisberg variant of discoid lateral meniscus was missed. This patient
presented with lateral knee pain, joint line tenderness, and a positive
McMurray's test. MRI performed 4 years before subsequent positive arthroscopy
showed a discoid lateral meniscus with "high T2 signal/irregularity of
posterior horn lateral meniscus, suspicious for tear." No surgery was
performed at that time.
The MRI appearance of a Wrisberg variant discoid lateral meniscus is quite
specific. A Wrisberg variant consists of a discoid lateral meniscus with
absent normal fascicles and coronary ligaments that would normally attach the
posterior horn of the lateral meniscus to the joint capsule and tibia. This
deficiency is unique to the Wrisberg variant. Although fascicles of the
lateral meniscus are normally seen, the coronary ligaments are usually not
seen even in healthy patients and thus were not included in our MR criteria
for a Wrisberg variant. On MRI, this deficiency of normal fascicles and
coronary ligaments is manifested as high T2 signal interposed between the
lateral meniscus and the joint capsule and simulating a peripheral tear or
injury to the fascicles. Because the posterior horn is no longer attached to
the capsule, the lateral meniscus may also sublux anteriorly. This finding was
identified in the second patient reported in this article.
This study has several limitations. Because of the rarity of this
condition, only two patients with both MRI and corroborative arthroscopy are
described. A larger patient volume would help to confirm the MRI findings of
Wrisberg variant of a discoid lateral meniscus presented in this article. In
addition, only those two patients identified at arthroscopy by a single
orthopedic surgeon at our institution were used in this article. Arthroscopy
reports were not reviewed over the time period studied. Another limitation is
that the body, posterior horn, and root attachments were not considered
individually. Also, this was a retrospective study and has the inherent
limitations of such a study.
Alternative MRI techniques may be used for further evaluation of affected
patients, including MR arthrography and stress views (partial flexion). Each
of these techniques may hold promise in making this infrequent condition more
apparent to both radiologists and orthopedists.
In summary, the Wrisberg variant of a discoid lateral meniscus is a rare
condition with a highly suggestive clinical history. The musculoskeletal
radiologist should consider this diagnosis whenever a discoid lateral meniscus
is identified because making this elusive diagnosis may prove immensely useful
for the orthopedic surgeon.
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Abstract
Introduction
