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1 Department of Radiology, Orthopedic University Hospital Balgrist, Forchstr.
340, CH-8008 Züurich, Switzerland.
2 Department of Orthopedic Surgery, Orthopedic University Hospital Balgrist,
CH-8008 Zürich, Switzerland.
Received September 3, 2002;
accepted after revision October 25, 2002.
Address correspondence to M. Zanetti.
Abstract
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MATERIALS AND METHODS. MR images of 102 asymptomatic knees were evaluated with regard to the prevalence of meniscal cysts, ganglionic cysts, synovial cysts of the popliteal space, fluid-filled bursae, and other fluid collections. The MR examinations were performed in patients (mean age, 42.8 years; age range, 18–73 years) with clinically suspected meniscal lesions in the contralateral knee. The craniocaudal, anteroposterior, and mediolateral diameters of detectable abnormal fluid collections were measured.
RESULTS. Medial meniscal cysts (mean size [craniocaudal x anteroposterior x mediolateral], 9 x 6 x 13 mm) were found in four asymptomatic knees. Neither lateral meniscal cysts nor ganglionic cysts of the cruciate ligaments were identified. Twenty-six synovial cysts of the popliteal space (Baker's cyst)—consisting of 11 gastrocnemius portions (mean size, 19 x 8 x 10 mm) and 15 semimembranosus portions (mean size, 20 x 7 x 9 mm)—were found in 19 knees. Twenty-four (92%) of these cysts had a maximal diameter of 30 mm or less. Fluid-filled bursae were found in 49 knees. The deep infrapatellar bursa was most commonly involved (42 knees; mean size, 6 x 3 x 5 mm). Fluid-filled anserine bursae (mean size, 27 x 12 x 10 mm) were detected in five knees.
CONCLUSION. Meniscal cysts may be present in asymptomatic knees, at least on the medial side. Synovial cysts of the popliteal space can be found in approximately one fifth of asymptomatic knees. Their maximal diameter is usually smaller than 30 mm.
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The prevalence and size of meniscal cysts, ganglionic cysts, synovial cysts of the popliteal space, and fluid-filled bursae in symptomatic knees have been described in previous publications [1, 6, 7, 8]. However, little is known about the prevalence of these fluid collections in asymptomatic knees. Knowing the prevalence and size of such abnormalities in asymptomatic knees may be useful in order to avoid overtreatment.
The purposes of this investigation were to evaluate the prevalence of meniscal cysts, ganglionic cysts, synovial cysts of the popliteal space, fluid-filled bursae, and other fluid collections on MR images of asymptomatic knees and determine the size of these fluid collections.
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Patient recruitment was consecutive. Forty additional patients were excluded because the MR examination of the contralateral knee could not be performed immediately because of scheduling problems. Two patients refused to participate in the study. The final study population consisted of 60 men and 42 women (mean age, 42.8 years; age range, 18–73 years). The period of data acquisition was from February 2001 to March 2002. All patients gave written informed consent. The study was approved by the local institutional review board.
MR Imaging Protocol
MR imaging was performed either on a 1.0-T scanner (Expert; Siemens Medical
Systems, Erlangen, Germany) in 66 patients or on a 1.5-T scanner (Symphony;
Siemens Medical Systems) in 36 patients. In both MR units a circular-polarized
send–receive extremity coil was used.
The following MR sequences were performed with the 1.0-T unit: sagittal proton weighted (TR/TE, 3800/16) and T2-weighted turbo spin-echo (3800/98) sequences; coronal T1-weighted spin-echo sequence (608/20); and coronal T2-weighted turbo spin-echo sequence with fat suppression (4500/96). For the sagittal proton weighted and T2-weighted turbo spin-echo sequences, the following parameters were used: section thickness, 3 mm; field of view, 156 x 250 mm; and matrix, 170 x 512. For the coronal T1-weighted spin-echo sequence, the section thickness was 4 mm; field of view, 140 x 160 mm; and matrix, 224 x 512. For the coronal T2-weighted turbo spin-echo sequence with fat suppression, the section thickness was 4 mm; field of view, 135 x 180 mm; and matrix, 210 x 512. The acquisition times for each sequence ranged from 2 min 18 sec to 4 min 35 sec.
The following MR sequences were performed with the 1.5-T scanner: sagittal proton weighted (3610/14) and T2-weighted turbo spin-echo (3610/95) sequences; coronal T1-weighted spin-echo sequence (450/14); and a coronal short tau inversion recovery (STIR) sequence (5550/35; inversion time, 160 msec). For the sagittal proton weighted and T2-weighted turbo spin-echo sequences, the following parameters were used: section thickness, 3 mm; field of view, 143 x 180 mm; and matrix, 204 x 512. For the coronal T1-weighted spin-echo sequence, the section thickness was 3 mm; field of view, 138 x 170 mm; and matrix, 208 x 512. For the coronal STIR sequence, the section thickness was 3 mm; field of view, 135 x 170 mm; matrix, 203 x 512. The acquisition times for the sequences ranged from 2 min 57 sec to 4 min 38 sec.
Image Analysis
All 102 MR examinations were analyzed by two musculoskeletal radiologists
with 5 and 10 years of experience in this subspecialty. The reviewers
evaluated the MR examinations in consensus with regard to the presence or
absence of meniscal cysts; ganglionic cysts; synovial cysts of the popliteal
space; fluid-filled bursae; and other fluid collections including those from
an indeterminate origin, which were referred to as the miscellaneous group.
The most probable origin of the fluid collection was determined. A distinction
was made between meniscal cysts (medial or lateral, with or without meniscal
tear) and ganglionic cysts of the cruciate ligament (anterior or posterior,
with or without cruciate ligament tear). Synovial cysts of the popliteal space
were categorized according to location: a gastrocnemius portion (dorsal
extrusion located medial to the medial head of the gastrocnemius muscle and
lateral to the insertion of the semimembranosus tendon) and a semimembranosus
portion (medial to the insertion of the semimembranosus tendon)
[6,
9]. Four types of bursae were
distinguished: deep infrapatellar bursa (between the patellar tendon and tibia
[6]), superficial infrapatellar
bursa (between subcutaneous tissue and the patellar tendon
[6]), medial and lateral
gastrocnemius bursae (between the joint capsule and the medial or lateral head
of the gastrocnemius muscle [7,
10,
11]), and anserine bursa
(which separates the pes anserine tendons consisting of the distal sartorius,
gracilis, and semitendinosus tendons from the subjacent distal portion of the
tibial collateral ligament and the bony surface of the medial tibial condyle
[12,
13]). The miscellaneous group
consisted of fluid collections relating to the proximal tibiofibular joint,
the periosteum, the iliotibial tract, the popliteal tendon, the medial
collateral ligament, and the transverse meniscal ligament and of fluid
collections of indeterminate origin.
Measurements of the abnormalities marked on hard copies during the consensus review were made to the nearest millimeter by the first author. On the sections with the largest amount of fluid, the diameters in craniocaudal, anteroposterior, and mediolateral directions were determined. Fluid collections with a diameter of less than 1 mm were not evaluated.
Statistical Analysis
Descriptive statistics were used, and prevalences
(Table 1) were calculated.
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Synovial cysts of the popliteal space were found in 19 of the 102 asymptomatic knees. Seven of the 19 synovial cysts of the popliteal space consisted of two portions: a gastrocnemius portion and a semimembranosus portion. Twenty-four of the 26 synovial cysts of the popliteal space portions had a maximal size of 30 mm or less. Eleven gastrocnemius portions had a mean size of 19 x 8 x 10 mm, and 15 semimembranosus cysts had a mean size of 20 x 7 x 9 mm (Figs. 1C and 1D). The mean age of the patients with synovial cysts of the popliteal space was 43 years (range, 23–64 years).
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Fluid-filled bursae (n = 58) were the most frequent findings (49 knees), with nine patients having two bursae. The deep infrapatellar bursa (Fig. 2) was the most commonly seen fluid-filled bursa and was found in 42 knees. It represented the smallest of all fluid collections, with a mean size of 6 x 3 x 5 mm. In 98% (41/42) of the knees with deep infrapatellar bursae, the bursae had a maximal diameter of 14 mm or less. Only one fluid-filled superficial infrapatellar bursa (24 x 4 x 13 mm) was detected (Fig. 3).
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Ten fluid-filled gastrocnemius bursae (mean size, 16 x 6 x 11 mm) were seen. Ninety percent (9/10) of the fluid-filled gastrocnemius bursae had a maximal diameter of 20 mm or smaller. Eight of these 10 bursae presented as a fluid collection anterior to the medial gastrocnemius muscle (Fig. 4), and two were anterior to the lateral gastrocnemius muscle.
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The five fluid-filled anserine bursae had the largest mean size of all fluid collections: 27 x 12 x 10 mm (Figs. 5A, 5B). Their maximal diameters varied considerably, ranging from 14 to 39 mm. The mean age of patients with fluid in the bursae was 46 years (range, 22–73 years).
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Six fluid collections originating from the proximal tibiofibular joint (mean size, 15 x 7 x 9 mm) were seen (Figs. 6A, 6B). There were two fluid collections that probably originated from the periosteum, one from the iliotibial tract, one from the popliteal tendon sheath, and two from the transverse meniscal ligament. A single fluid collection was found deep in relation to the medial collateral ligament.
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Two fluid collections had an indeterminate origin: one was located between semimembranosus muscle and the dorsomedial femur condyle, and the other was anterior to the popliteal vessels. The mean age of the patients in the miscellaneous group was 49 years (range, 19–73 years).
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Ganglionic and meniscal cysts are benign cystic masses that are lined by flat spindleshaped cells. These cysts have no synovial lining and are characterized by myxoid contents [5, 6, 12, 14].
Lateral meniscal cysts have been described as occurring from three to 10 times more frequently than medial meniscal cysts [15]. However, other investigators found a more balanced incidence of lateral and medial cysts or a higher incidence of medial cysts [16, 17, 18]. In our investigation of asymptomatic knees, no lateral meniscal cyst was found.
In all of the knees with meniscal cysts in our study (n = 4), an underlying meniscal tear or degeneration was shown on MR imaging. This finding supports the current concept that meniscal cysts are associated with meniscal tears. According to one theory about the pathogenesis of meniscal cysts, synovial fluid enters the tear from the joint space and accumulates at the meniscocapsular junction. The synovial fluid is then resorbed slowly, leaving thick jellylike material [6]. Other theories suggest that congenital or traumatic displacement of synovial cells into the meniscus produces mucin that then results in the formation of a cyst [19, 20]. Some investigators believe that chronic infection, contusion, or hemorrhage may result in meniscal cyst formation [21, 22] or that mucopolysaccharides produced by mesenchymal cells may progressively accumulate to form this type of cyst [23].
Campbell et al. [18] reviewed 2572 MR imaging reports of knee examinations for the presence of meniscal tears and cysts. These investigators found a prevalence of 4% (109/2572) of meniscal cysts. They reported that medial meniscal cysts had a mean size of 19 mm and that the cysts ranged from 3 to 80 mm. The mean size (mediolateral diameter) of the medial meniscal cysts in our study was 13 mm, the cysts ranged from 8 to 16 mm, and the prevalence of these cysts was 4% (4/102). Clinically relevant meniscal cysts could be slightly larger than those in asymptomatic volunteers, but the prevalences seem to be almost the same. However, differences between our results of 102 asymptomatic knees and those from studies that included more than 1000 symptomatic patients [18, 24] (2572 patients and 1685 patients, respectively) should not be overestimated. The difference of numbers between our population and the symptomatic populations is substantial. On the other hand, we believe that the prevalence for symptomatic populations would not differ considerably from asymptomatic populations if a larger asymptomatic population were analyzed.
The absence of a ganglionic cyst of the anterior cruciate ligament in our
study population is probably explained by the low prevalence of this rare
finding in symptomatic patients. In a retrospective review, Kim et al.
[24] found ganglionic cysts of
the cruciate ligament in only 15 (
1%) of 1685 consecutive knee MR
examinations [24].
Baker's cyst probably results from the extrusion of joint fluid into the gastrocnemiosemimembranosus bursa through a weak portion of the posteromedial joint capsule of the knee. Whether these fluid collections represent a bursa or a recess is controversial [6]. The gastrocnemiosemimembranosus bursa or recess is situated between the medial head of the gastrocnemius muscle and the semimembranosus muscle [25]. The gastrocnemiosemimembranosus bursa is a composite of two bursae: the gastrocnemius and semimembranosus [14]. A central septum may be present, resulting in separate semimembranosus and gastrocnemius portions of the bursa [6]. If joint fluid extends in the gastrocnemiosemimembranosus bursa or recess, the synovial cyst of the popliteal space may be communicating with both portions of the bursa, or if a septum is present, the cyst may be communicating with an isolated portion of the bursa. In our study, communicating portions were found in seven knees, whereas isolated synovial cysts of the popliteal space were found in 12 knees. There is not a uniform description of the precise location of the synovial cyst of the popliteal space [8, 25, 26, 27]. Communication between the gastrocnemiosemimembranosus bursa and the knee joint is required for a synovial cyst of the popliteal space to form. Communications between the gastrocnemiosemimembranosus bursa and the knee joint are present in approximately half of all asymptomatic persons [28]. This finding could explain the high prevalence (19% [19/102]) of synovial cysts of the popliteal space in asymptomatic knees in our study.
In a review article about synovial cysts of the popliteal space in adults, Handy [8] reported that between 5% (55/1113) and 32% (64/202) of symptomatic knees have these cysts depending on the population studied and imaging technique used. Marti-Bonmati et al. [29] found an even higher prevalence of Baker's cysts (38% [145/382]) in symptomatic knees on MR imaging. The prevalence of synovial cysts of the popliteal space is apparently similar in symptomatic and asymptomatic knees.
In a large number of the asymptomatic knees in our study, a fluid-filled deep infrapatellar bursa was present. According to the literature, this bursa is usually collapsed but may contain a small amount of synovial fluid without causing any symptoms [12, 30]. Klein [31] dissected cadaver specimens and described the bursa as being 20 mm in length and containing less than 0.5 mL of fluid. In our series, 98% (41/42) of the deep infrapatellar bursae with visible fluid had a maximal diameter of 14 mm or less. This finding is compatible with the results in the cadaveric study [31]. Vahlensieck et al. [7] found this bursa in only 19% (25/133) of their MR imaging examinations in symptomatic knees, whereas the prevalence of this bursa in our asymptomatic population was 41% (42/102). The lower prevalence of fluid accumulation in symptomatic knees that was reported by Vahlensieck et al. may be explained by differences in the cutoff measurement used for detection of these small fluid accumulations. In our study, we used a cutoff of 1 mm as the minimum diameter to describe a fluid collection. It is possible that Vahlensieck et al. used a larger minimum diameter for identifying infrapatellar bursae.
The anserine bursa had the largest mean diameters compared with the other fluidlike lesions in our investigation. However, we found only five cases and they varied in size considerably. These factors made it difficult to draw a conclusion about the size of anserine bursae in the asymptomatic knee. However, from our data we can at least conclude that not every fluid-filled anserine bursa represents bursitis.
The fluid-filled gastrocnemius bursae of our study had a maximal size of 20 mm or smaller in nine of 10 cases, assuming that this value could be a limit under which such findings on MR images are not clinically relevant. Vahlensieck et al. [7] found a fluid collection in the medial gastrocnemius bursae in 55% (73/133) of symptomatic knees, whereas in our study we detected medial fluid collections in only 8% (8/102) and lateral fluid collections in only 2% (2/102). Vahlensieck et al. mention that there is a communication with the joint in half of all cases, according to the anatomy literature. Therefore, a joint effusion may increase the size of the gastrocnemius bursae [7, 32].
A single fluid collection was found deep in relation to the medial collateral ligament. This fluid collection probably derives from the medial collateral ligament bursa. The MR imaging appearance of this bursa and the anatomic correlation were previously described by De Maeseneer et al. [33].
Stratifying the data by patient age revealed a homogeneous distribution in the different groups of fluid collections. An agerelated correlation could not be identified.
The fact that we did not have surgical and pathologic proof to verify our MR findings is a limitation to our study that is inherent to a study that includes only asymptomatic patients. The prevalences of fluid accumulations around the asymptomatic knee in our series of patients with symptoms in the contralateral knee may differ from the results of volunteers with two asymptomatic knees. However, we believe that the rigorous inclusion criteria for asymptomatic knees preclude major differences.
In conclusion, medial meniscal cysts can be found in up to 4% of asymptomatic knees. Baker's cysts are more commonly present (19%) in asymptomatic knees. However, their maximal diameter is rarely larger than 30 mm. Our data provide baseline information about the distribution and size of fluid collections around asymptomatic knees. These data may enable radiologists to draw conclusions about the clinical importance of these findings when seen in symptomatic patients. Our findings indicate that not all fluid collections found around the knee joint are clinically significant.
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