Dr. Ali, Dr. Dass, and Dr. Shah are in the Department of Radiology, and Dr. Sewards is in the Department of Orthopedic Surgery, Temple University Hospital, Philadelphia, PA.
meniscus is a crescent-shaped fibrocartilaginous structure that partly
divides a joint cavity, unlike articular discs, which completely
separate the cavity. Menisci are present in the knees and the
acromioclavicular, sternoclavicular, and temporomandibular joints. A
small meniscus is also seen in the wrist joint. This article focuses on
the menisci of the knees.
Menisci ensure normal function of the
joint, and they also protect the hyaline cartilage. The main functions
of the menisci can be summarized as providing:
- Stability. The ligaments are the most important stabilizers,
with the menisci assuming a secondary function. This secondary function
is enhanced with intact ligaments.
- Lubrication and nutrition. The knee menisci act as spacers
between the femur and the tibia. By doing so, they prevent friction and
contact between the distal femur and tibia and allow for the diffusion
of the normal joint fluid and its nutrients into the articular
cartilage. Maintenance of the integrity of the articular cartilage is
critical to preventing the development of post-traumatic or degenerative
- Shock absorption The menisci lower the stress applied to the
articular cartilage; thereby, they have a role in preventing the
development of degenerative arthritis.
Clark and Ogden studied the natural development of the menisci in the
normal knee. The shape of the meniscus is formed at the eighth week of
gestation, about the time when the knee joint is fully formed.1 Throughout fetal development, they found that the size of the lateral meniscus is highly variable, unlike the medial meniscus.
congenital anomalies affect the lateral meniscus, most commonly a
discoid meniscus, although discoid medial menisci can occur much less
We will review the common meniscal variants, which
include hypoplastic menisci, absent menisci, anomalous insertion of the
medial meniscus, discoid lateral meniscus, including the Wrisberg
variant, and discoid medial meniscus.
Congenital meniscal hypoplasia
are reported cases of complete absence of the medial meniscus as
described in thrombocytopenia absent radius syndrome (TAR syndrome).2,3 Bilateral hypoplasia of the medial meniscus has also been reported.4
measurements of the posterior horn of the medial meniscus may vary, but
the posterior horn is usually much larger than the anterior horn (the
posterior horn usually measures 12 mm to 16 mm in the sagittal plane in
an adult), and approximately twice the size of the anterior horn on
sagittal magnetic resonance (MR) images. Imaging characteristics of the
posterior horn of the medial meniscus include a triangular hypointense
structure on sagittal images on T1, proton density, and fat-saturated
FSE T2-weighted images, with a slab-like appearance on coronal images.
the example shown (Figures 1 and 2), the entire medial meniscus is
noted to be diminutive, with the posterior horn measuring 7 mm to 8 mm.
No meniscal tear is seen, but the root attachment was also noted to be
diminutive (1 mm) with no increased signal to suggest root attachment
tear. In the previously reported cases, as well as in this case, the
hypoplastic meniscus was not the cause of the patient’s pain, suggesting
that this rare condition is also clinically asymptomatic. The symptoms
in this case were attributed to an anterior cruciate ligament tear
(Figure 1). Bilateral hypoplasia of the medial meniscus has also been
Congenital absence of the meniscus
Congenital absence of the meniscus is extremely rare and has been documented in TAR syndrome and in isolated case reports.2,3
The congenitally absent meniscus appears to influence the development
of the distal femur and proximal tibia, and in the case report of
bilaterally absent menisci reported by Tolo et al,3 the
proximal medial tibia was convex and the distal medial femoral condyle
was saddle shaped. That reported case was also associated with
congenital absence of the cruciate ligaments. When the cruciate
ligaments are absent, most commonly the anterior cruciate ligament (ACL)
is affected. These findings are also frequently associated with genu
varus deformity (Figure 3).
Anomalous insertion of the meniscus
insertion of the medial meniscus (AIMM) has been described, and it is
typically into the anterior cruciate ligament. The anomalous insertion
runs from the anterior horn of the medial meniscus to either the ACL or
the intercondylar notch, most commonly to the mid ACL, and less commonly
to the base of the ACL or the intercondylar notch. The insertion site
of the AIMM into the ACL is classified as Type 1 (inferior third), Type 2
(middle third), or Type 3 (superior third; intercondylar notch) (Figure
4). The prevalence of a medial discoid meniscus in patients with AIMM
is much greater than in a discoid lateral meniscus, and the prevalence
of the transverse ligament is comparable to the general population.5
of the anterior horn of the medial meniscus, an inferior patella plica,
and ACL tears can be mistaken for AIMM, but carefully tracing the
ligament will help to exclude these conditions.5 In the first
instance, tears of the lateral aspect of the anterior horn of the
medial meniscus are extremely uncommon and should not be a diagnostic
problem in practice. Also, the inferior patella plica inserts on the
patella or Hoffa’s fat pad, and should be fairly easily differentiated
from AIMM. A tear of the ACL should also, in practice, not be a
diagnostic dilemma, as the AIMM band will be seen to extend to the
medial meniscus, and not be confined to the ACL as seen in an ACL tear.
Discoid lateral meniscus
discoid lateral meniscus is a relatively uncommon developmental variant
of the meniscus. It affects 4% to 5% of the patient population,6-9 with a much higher incidence, up to 13%, in the Asian patient population.10 It is the most common meniscal variant in children.11
Discoid lateral meniscus was originally believed to result from an
incomplete breakdown of the central meniscus, but this is now disputed,
as at no time in development does the meniscus have a discoid
appearance.12 It is now believed that the knee develops from a
mesenchymal mass that differentiates into the tibia, femur, and
intra-articular structures at 8 weeks’ gestation. This mesenchymal
tissue only persists at the edges, where differentiation into the
menisci occurs. It is believed that discoid
menisci develop from this mesenchymal tissue in a site where this tissue
does not normally occur.13
The condition is typically asymptomatic and, therefore, is infrequently diagnosed.14
The most frequent symptom is pain that usually begins with a minor
trauma; however, other symptoms include clicking, snapping, and locking
during movement, and less commonly joint-line tenderness, reduced
mobility, and a “giving-way” sensation.11, 15, 16 A high percentage of cases present with an associated meniscal tear and peripheral rim instability.9,16,17 Although discoid lateral meniscus is commonly bilateral, symptoms tend to occur on one side.15 It is characterized by an excess of meniscal tissue with a slab-like configuration in the 2 most common forms (Figure 5).
There are 3 main types, according to the Watanabe classification:18
- Type 1: A complete slab of meniscal tissue with complete tibial coverage.
- Type 2: An incomplete slab of meniscal tissue with 80% coverage of the lateral tibial plateau.
3: The Wrisberg variant, where the meniscus may have a normal
morphology but lacks its posterior attachments; ie, the meniscotibial
ligament and meniscal fascicles.
- Type 1 is most common, and type
3 is least common. Monllau et al in 1998 proposed adding a fourth type,
the rare ring-shaped meniscus, to the classification.
Most patients are asymptomatic, but injury to the meniscus can
occur with minor trauma. The Wrisberg variant may present with a
snapping knee due to hypermobility. The most commonly practiced
treatment for stable complete or incomplete types of discoid lateral
meniscus is partial meniscal excision, leaving a 6- to 7-mm peripheral
rim circumferentially, anteriorly, and posteriorly,19 which
is in fact reducing the volume of the meniscus and restoring a “normal”
morphology. Radiographs are usually not diagnostic, but they may show a
high fibula head and a widened lateral joint space.20 Several
MR criteria are used to make the diagnosis. These include looking for a
slab-like configuration on sagittal MR images, with > 3 “bowties”
seen on standard 4- to 5-mm slices.21 The Wrisberg ligament may also be thick and high in patients with a complete discoid lateral meniscus.22 Other criteria used to diagnose lateral discoid meniscus include the following:
- > 20% ratio of meniscus to tibia on the coronal image;
- Minimum diameter 14-15 mm on a midcoronal image;
for the ratio of the sum of the width of the anterior and posterior
horns to the meniscal diameter on a sagittal slice that shows a maximum
Wrisberg variant discoid lateral meniscus
Wrisberg variant, the morphology of the meniscus may be normal, but the
posterior fascicles and meniscotibial ligament are absent and a high
signal fluid cleft interposed between the posterior horn and the capsule
may simulate a peripheral tear (Figure 6).23 The only
attachment of the posterior horn is the Wrisberg meniscofemoral
ligament, and the posterior horn may translate or rotate due to
hypermobility. The meniscus may also become hypertrophic.
Discoid medial meniscus
Discoid medial menisci are much less common than discoid lateral menisci,24 and they may be bilateral. When bilateral, they are usually symmetric. The reported prevalence is 0.06% to 0.3%.25
Problems encountered in a discoid medial meniscus are the same as a
discoid lateral meniscus, including a propensity for tears to occur and
with mechanical features of clicking and locking.
show cupping of the medial tibial plateau, proximal medial tibial physis
collapse and widening of the medial joint space (Figure 7). There are
no specific MR criteria for classifying discoid medial menisci, and the
MR criteria for discoid lateral menisci are used for discoid medial
menisci (Figure 8). A Wrisberg type variant has not been documented in
the medial meniscus.
Associated anomalies in a discoid medial
meniscus are not uncommon; they include an anomalous insertion of the
anterior horn of the medial meniscus into the anterior cruciate ligament
(as previously described), meniscal cyst,26 discoid lateral meniscus in the same knee (Figure 9),25 and pathologic medial patella plica.27
variants of the meniscus are relatively uncommon and are frequently
asymptomatic, although there is a greater propensity for discoid menisci
to tear. However, recognizing these variants is important, as they can
be misinterpreted for more significant pathology on MRI. The most common
of these meniscal variants is the discoid lateral meniscus, and the
least common is complete congenital absence of the menisci.
- Clark CR, Ogden JA. Development of the menisci of the human knee
joint: Morphologic changes and their potential role in childhood
meniscal injury. J Bone Joint Surg Am. 1983;65:538-547.
- Heron, D, Bonnard C, Moraine C,Toutain A. Agenesis of cruciate
ligaments and menisci causing severe knee dysplasia in TAR syndrome. J Med.Genet. 2001;38;e27.
- Tolo VT. Congenital absence of the menisci and cruciate ligaments of the knee: A case report. J Bone Joint Surg. 1981;63:1022-1024.
- Monllau J, Gonzalez G, Puig L, Caceres E. Bilateral hypoplasia of the medial meniscus. Knee Surg Sports Traumatol Arthrosc. 2006;14:112-113.
- Nakajima T, Nabeshima Y, Fujii H, et al. Symptomatic anomalous insertion of the medial meniscus. Arthroscopy. 2005;21:629.
- Dickhaut SC, DeLee JC. The discoid lateral-meniscus syndrome. J Bone Joint Surg Am. 1982;64:1068-1073.
- Kocher MS, Klingele K, Rassman SO. Meniscal disorders: Normal, discoid, and cysts. Orthop Clin North Am. 2003;34:329-340.
- Neuschwander DC, Drez D Jr, Finney TP. Lateral meniscal variant with absence of the posterior coronary ligament. J Bone Joint Surg Am. 1992;74:1186-1190.
- Rohren EM, Kosarek FJ, Helms CA. Discoid lateral meniscus and the frequency of meniscal tears. Skeletal Radiol. 2001;30:316-320.
- Fukuta S, Masaki K, Korai F. Prevalence of abnormal findings in magnetic resonance images of asymptomatic knees. J Orthop Sci. 2002;7:287-291.
- Kelly BT, Green DW. Discoid lateral meniscus in children. Curr Opin Pediatr. 2002;14:54-61.
- Kaplan EB. Discoid lateral meniscus of the knee joint: Nature, mechanism, and operative treatment. J Bone Joint Surg Am. 1957;39:77-87.
- Ross JA,Tough ICK, English TA. Congenital discoid cartilage. Report
of a case of discoid medial cartilage, with an embryological note. J Bone Joint Surg Br. 1958 40:262-267.
- Davidson D, Letts M, Glasgow R. Discoid meniscus in children: Treatment and outcome. Can J Surg. 2003;46:350-358.
- Connolly B, Babyn PS, Wright JG, Thorner PS. Discoid meniscus in children: Magnetic resonance imaging characteristics. Can Assoc Radiol J. 1996;47:347-354.
- Rao PS, Rao SK, Paul R. Clinical, radiologic, and arthroscopic assessment of discoid lateral meniscus. Arthroscopy. 2001;17:275-277.
- Klingele KE, Kocher MS, Hresko MT, et al. Discoid lateral meniscus: Prevalence of peripheral rim instability. J Pediatr Orthop. 2004;24:79-82
- Watanabe M, Takada S, Ikeuchi H. Atlas of Arthroscopy. Tokyo, Japan:Igaku-Shoin;1969.
- Youderian A, Chmell S, Stull MA. Discoid lateral meniscus. Applied Radiology. 2008;37:30-32.
- Kim SJ, Moon SH, Shin SJ. Radiographic knee dimensions in discoid lateral meniscus: Comparison with normal control. Arthroscopy. 2000;16:511-516.
- Samoto N, Kozuma M, Tokuhisa T, Kobayashi K. Diagnosis of discoid lateral meniscus of the knee on MR imaging. Magn Reson Imaging. 2002;20:59-64.
- Kim EY, Choi SH, Ahn JH, Kwon JW. Atypically thick and high location
of the Wrisberg ligament in patients with a complete lateral discoid
meniscus. Skeletal Radiol. 2008;37:827-833.
- Singh K, Helms CA, Jacobs MT, Higgins LD. MRI appearance of Wrisberg variant of discoid lateral meniscus. AJR Am J Roentgenol. 2006;187:384-387.
- Resnick D, Goergen TG, Kaye JJ, et al. Discoid medial meniscus. Radiology. 1976;121:575-576.
- Tachibana Y, Yamazaki Y, Ninomiya S. Discoid medial meniscus. Arthroscopy. 2003;19:E12-18.
- Kim SJ, Choi CH. Bilateral complete discoid medial menisci combined with anomalous insertion and cyst formation. Arthroscopy. 1996;12:112–115.
- Pinar H, Akseki D, Karaoglan O, et al. Bilateral discoid medial menisci: Case report. Arthroscopy. 2000;16:96-101.