AJR 2001; 177:221-227
© American Roentgen Ray Society
Synovial Plicae in the Knee
Carol A. Boles1 and
David F. Martin2
1
Department of Radiology, Wake Forest University Baptist Medical Center,
Medical Center Blvd., Winston-Salem, NC 27157-1088.
2
Department of Orthopaedic Surgery, Wake Forest University Baptist Medical
Center, Winston-Salem, NC 27157-1088.
Received September 21, 2000;
accepted after revision January 9, 2001.
Address correspondence to C. A. Boles.
Introduction
Synovial plicae are common folds of synovium that may be clinically
important [1]. Inflammation may
cause pain, and chronic inflammation may lead to thickening and fibrosis
[2]. A large plica,
particularly if thickened, may impinge on and cause chondromalacia of adjacent
cartilage [1,
2]. However, the clinical
importance of plicae is controversial because their presence and size do not
necessarily correlate with symptoms with which they may be associated
[1]. Knowledge of the typical
location and MR appearance of plicae aids physicians in evaluating MR images
of the knee.
Embryology
Understanding of the embryologic development of the knee has changed. It
had long been held that the knee is formed as three separate compartments and
that plicae are remnants of those compartments
[3]. That concept explains the
presence of superior and inferior plicae, but it does not explain the
formation of the medial plica or other plicae within the joint. A study
performed by Ogata and Uhthoff
[4] suggests that there is no
normal stage of three separate compartments and that plicae develop as the
joint space forms.
At approximately 7 weeks of fetal development, mesenchymal condensations
for future menisci and cruciates are present. Cavitations develop in the
interzone tissue, which will form the patellofemoral joint space as the
cavitations coalesce. At 8 weeks of development, similar femoromeniscal
cavitations appear. Meniscotibial cavitations are found at approximately 9
weeks of development. At 10 weeks, fetal specimens have numerous independent
small, inconsistent cavitations that coalesce. Once fetal development has
progressed beyond 10 weeks, the knee joint consists of a single cavity with
synovial lining. Mesenchymal tissue in the suprapatellar, mediopatellar, and
infrapatellar regions, which could be considered plicae, was found in 33-50%
of fetal specimens in a study
[4].
Anatomy
Three plicae are commonly encountered at arthroscopy: the superior, medial,
and inferior plicae (Figs.
1,2A,2B,3,4,5,6A,6B,7A,7B,8A,8B,8C,8D).
The size and location of plicae are extremely variable
[1,
2], as are the names by which
they are identified. In the literature, nomenclature has been inconsistent,
which has led to some confusion regarding the frequency and clinical
importance of plicae [1,
5]. For example, the superior
plica is also known as the suprapatellar plica, plica synovialis
suprapatellaris, superomedial plica, and medial suprapatellar plica. It has
also been called the superomedial plica, a term referring to the frequent
location of this synovial fold along the medial side of the suprapatellar
pouch.
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Fig. 1. —Illustration of common plica in knee shows anterior view with
knee slightly flexed and cut to reveal internal structures, displaying
superior, medial, and inferior plicae. Lateral plica illustrated is probably
lateral transverse arcuate fold, which is more common than lateral plica (see
Figs. 11A and
11B). (Reprinted with
permission from [14])
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Fig. 7A. —14-year-old previously asymptomatic boy with trauma to knee.
Sequential axial fat-suppressed, T2-weighted MR images (TR/effective TE,
5500/84) reveal large medial plica extending toward midline of trochlear
groove (solid arrows). Subcutaneous hematoma is seen laterally
(open arrow).
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Fig. 7B. —14-year-old previously asymptomatic boy with trauma to knee.
Sagittal fat-suppressed proton density—weighted image (TR/TE, 2266/37)
of medial joint shows prominent vertically oriented medial plica
(arrow).
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Fig. 8A. —Arthroscopic images depicting Sakakibara's four types of
medial plica. Type A (arrow) is thin elevation of synovium under
medial retinaculum. Patient is 40-year-old man with chondromalacia.
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Fig. 8B. —Arthroscopic images depicting Sakakibara's four types of
medial plica. 69-year-old man with osteoarthritis. Type B is narrow pleat
(arrows) that does not impinge on medial condyle. Uninflamed ridge is
seen amid synovial hypertrophy in patient with osteoarthritis.
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Fig. 8C. —Arthroscopic images depicting Sakakibara's four types of
medial plica. 58-year-old man with medial meniscus tear. Type C is larger
structure (asterisk) that partially covers medial femoral condyle.
Note inflammation of plica and frayed cartilage of adjacent medial femoral
condyle (arrowheads).
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Fig. 8D. —Arthroscopic images depicting Sakakibara's four types of
medial plica. Type D is fenestrated type C or bandlike plica
(asterisk). Note generalized synovial hypertrophy. Data about patient
were not recorded.
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Fig. 11A. —34-year-old woman with transverse arcuate fold. Axial
fat-suppressed T2-weighted MR image (TR/effective, TE, 5500/84) obtained at
level of intercondylar notch reveals curvilinear band in lateral gutter
(arrow). Transverse arcuate fold is more sagittally oriented on axial
plane, whereas true lateral plica is more anterior and has oblique coronal
orientation (see Fig.
10A,10B,10C).
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Fig. 11D. —34-year-old woman with transverse arcuate fold. Coronal
T1-weighted MR image (TR/TE, 433/15) reveals soft tissue between medial
meniscus and cartilage of medial femoral condyle (arrow).
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The structure of the superior plica varies from a complete, intact septum
to a septum with a porta to a small crescentic fold. It is typically located 2
cm superior to the patella, posterior to the quadriceps tendon. This plica
should not be confused with the medial plica, which is less common but more
frequently symptomatic.
Ranging in size from a small ridge to a shelf or well-formed cord, medial
plicae arise on the medial wall of the synovial pouch or under the medial
retinaculum and travel in a coronal plane parallel to the medial edge of the
patella. If large, the free margin of the medial plica can impinge over the
medial facet of the trochlea. Medial plicae insert distally on the
infrapatellar fat pad. The medial plica has also been called plica synovialis;
(medio)patellaris shelf; medial shelf; medial intraarticular band; medial
patellar plica; Iino's band; synovial chorda; medial pleat; intraarticular
medial band, ledge, wedge, or cleat; plica alaris; alar ligament; plica alaris
elongata; semilunar fold; mediopatellar pseudomeniscus; and patellar
meniscus.
Some of the names applied to the medial plicae also refer to a different
structure, the alar fold, which is a longitudinal fold of synovium just medial
to the patella. An inconsistently present structure, the alar fold
(Fig. 9) is wide and very
soft. Triangular at the cross-section, it is thought to be the folding of
synovium in the relaxed position of retinaculum. The alar fold has also been
called the plica alaris elongata, plica alaris, alar ligament, semi-lunar
fold, mediopatellar pseudomeniscus, and patellar meniscus.
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Fig. 9. —39-year-old man. Relatively T1-weighted MR image (TR/TE,
1000/16) reveals alar fold (thick arrow) anterior to medial plica
(thin arrow). Alar fold may not be seen at arthroscopy but is usually
found on axial MR images. Thin minor synovial fold is noted just anterior to
medial plica (arrowhead). Alar fold may also be found lateral to
patella, which is typically closer to patella than is medial fold.
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The inferior plica is located in the intercondylar notch, just anterior to
the anterior cruciate ligament. Proximally, the inferior plica inserts on the
very anterior part of the notch, whereas distally it attaches to the fat
pad.
As with the other plicae, the inferior plica has been given more than one
name. It may also be called the ligamentum mucosum, plica synovialis
patellaris, plica synovialis patellae, infrapatellar plica, infrapatellar
fold, infrapatellar septum, and septum mucosum.
Plicae may occur in other areas in the knee. The rarely seen lateral plica
(Fig.
10A,10B,10C)
is oriented in an oblique coronal plane because it originates from the lateral
wall at a level superior to the popiteal hiatus and extends to its attachment
in the infrapatellar fat pad.
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Fig. 10A. —40-year-old man with rare lateral plica. Axial short tau
inversion recovery (STIR) image (TR/effective TE, 3950/ 36) reveals bandlike
low signal intensity in lateral joint at level of patella (arrow).
Note prominent medial plica (arrowhead).
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Fig. 10B. —40-year-old man with rare lateral plica. Sagittal STIR image
(4466/39) shows obliquecoronal orientation of lateral plica (arrow).
Compare with transversely oriented, more posterior arcuate fold in Fig.
11A,11B,11C,11D.
Lateral plicae may limit arthroscopic visualization of lateral gutter.
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Other plicae are less consistent in location. Transverse arcuate folds are
variable, transversely oriented synovial folds or pleats found in the medial
or lateral gutter. Medially, a variable number of small synovial folds occur
in the inferior gutter. Frequently, a prominent transverse arcuate fold is
present in the lateral gutter of the knee joint. Transverse arcuate folds
(Figs. 11A and
11B) may be found at
arthroscopy or imaging. In addition, the anteromedial fringe (Fig.
11C and
11D), which is a synovial fold
that may cover the anterior horn of medial meniscus, is rare but can produce
symptoms if there is impingement of the fringe between the medial femoral
condyle and the medial meniscus
[1]. The anteromedial fringe
(Figs. 11C and
11D), which is a synovial fold
that may cover the anterior horn of the medial meniscus, is rare but can
produce symptoms if there is impingement of the fringe between the medial
femoral condyle and the medial meniscus
[1].
Frequency of Plicae
Plicae are common. In a study of 200 dissections, only 10% of cadavers had
no plicae [1]. The inferior
plica is very common, being found in up to 65% of cadavers and in more than
85% of patients undergoing arthroscopy
[1,
5]. Also common is the superior
plica, which is found in 55% of cadavers and, in one study, 91% of 500
patients undergoing arthroscopy
[1,
5]. Of those cadavers with
superior plicae, 63% have superior plicae that are bilateral and symmetric.
The superior plica is a complete septum in 12% and has a porta, or opening, in
20%. The rest of the superior plicae are located medially or laterally or
both. Most plicae are small; more than 60% extend less than a third of the
width of the suprapatellar pouch
[5]. Medial plicae have been
reported as being found in 24% of cadavers, but medial plicae longer than 1 mm
have been found in up to 70% of patients undergoing arthroscopy
[1,
5]. Most medial plicae have a
depth of less than 1 cm [5].
Lateral plicae are rare, with the incidence generally accepted as less than
1%.
Medial Plica Syndrome
Affecting both men and women, medial plica syndrome is a combination of
clinical symptoms and physical findings associated with a pathologic plica
[1,
2] (Figs.
8C,
12A,12B,12C,13A,13B,14A,14B,14C).
There is inflammation and often fibrosis of the plica, which becomes
inflexible. The patient is typically young and involved in a repetitive
athletic activity. Sporting activities requiring repetitive
flexion—extension, such as rowing, swimming, cycling, and basketball,
are commonly associated with plica syndrome. Symptoms may follow trauma
although the specific incident may have occurred months to years earlier.
Medial patellar pain, usually above the joint line, is the most common
complaint. The pain is always present when the patient is active and is
exacerbated during flexion—extension, but it may be present even when
the patient is at rest. Physical findings are nonspecific—crepitation
and possibly effusion. A painful cord may be palpable: although such a finding
is pathognomonic, it is not a typical symptom
[6]. At arthroscopy, the medial
plica can be seen through a routine anterolateral portal but is better
visualized from a superolateral approach.
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Fig. 13A. —Middle-aged man with knee pain. Axial fat-suppressed
T2-weighted MR image of cord-like medial plica adjacent to medial facet of
patella (arrow). Cartilage irregularity and signal changes in
adjacent subchondral trochlea are present (arrowhead).
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Fig. 13B. —Middle-aged man with knee pain. Sagittal fat-suppressed
T2-weighted MR image of cord-like medial plica (arrow). Note signal
changes in adjacent subchondral medial femoral condyle (arrowhead).
Figure 8D is corresponding
arthroscopic image.
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Fig. 14A. —18-year-old university baseball player with medial knee pain.
Very little fluid seen in joint but sequential axial fat-suppressed gradient
echo MR images (TR/TE, 916/15; flip angle, 30°) reveal
low-signal-intensity structure adjacent to trochlear cartilage
(arrows).
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Fig. 14B. —18-year-old university baseball player with medial knee pain.
Sagittal fat-suppressed proton density—weighted MR image (TR/TE,
2100/30) shows typical extra line at medial joint (arrow). Pulsation
artifact makes extra line difficult to see.
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Fig. 14C. —18-year-old university baseball player with medial knee pain.
Arthroscopic images reveal medial plica (asterisks) as it abuts
medial femoral condyle and extends into trochlea. M = medial femoral condyle,
L = lateral femoral condyle, P = patella.
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Sakakibara [7] classified
the medial plicae that are found at arthroscopy into four types (Fig.
8A,8B,8C,8D).
The surgeon also evaluates the width, thickness, color, vascularity, tension,
and status of the free edge of the plica. The patient's knee is flexed to
determine if there is contact with the medial femoral condyle or the medial
facet of the patella. These structures may have chondromalacia caused by
repetitive contact with the plica
[2,
6]. The joint distention needed
for the arthroscopy may limit the evaluation of this impingement.
Medial meniscal tears and patellofemoral maltracking can have similar
symptoms. Other differential diagnoses include ligament strain, bursitis,
contusion, and osteochondritis dissecans
[6].
Imaging
Plicae seen on arthrograms were first described in 1945
[8]. Although the superior
plica was revealed readily and the inferior plica was also frequently seen,
arthrography proved a particularly limited technique when physicians attempted
to visualize the medial plica
[9,
10]. Special views could
improve the likelihood of visualization. In 1983, CT arthrography was found to
be able to reveal the medial plica
[11]. MR images show plicae as
low-signal-intensity bands of varying sizes found in the regions in which the
plicae are anticipated to be located
[12,
13].
Conclusion
Synovial plicae in the knee are common folds of synovium that can be seen
on knee MR images as bands of low signal intensity within the
high-signal-intensity joint fluid. They are not consistently present, but they
are normal anatomic structures found in the knee. Although not readily seen in
regions with a paucity of joint fluid (the medial plica in particular), plicae
can often be found with careful inspection of images. Their ubiquitous nature
has led to lack of reporting of readily seen plicae on routine MR
examinations. However, the medial plica may be a cause of medial knee pain
[1,
2,
6]. The correlation of symptoms
with the presence of a medial plica and the evaluation of the adjacent
cartilage may alter treatment algorithms and should be included in reports on
MR imaging of the knee.
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