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1 Department of Radiology, Veterans Administration Medical Center, 3350 La Jolla
Village Dr., San Diego, CA 92161.
2 Present address: Department of Radiology, Trillium Health Centre,100 Queensway
W., Mississauga, Ontario, L5B 1B8 Canada.
3 Present address: Institute of Anatomy, University of Vienna, Waehringerstr.
13, A-1090 Vienna, Austria.
4 Department of Radiology, Rush-Copley Medical Center, 2000 Ogden Ave., Aurora,
IL 60504.
Received June 13, 2002;
accepted after revision September 19, 2002.
Address correspondence to M. Munshi.
Abstract
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MATERIALS AND METHODS. We assessed the posterolateral corner of the knee during dissection of one gross anatomic specimen. MR imaging and MR arthrography were performed in seven additional knee specimens. T1-weighted spin-echo MR images were obtained in the standard imaging planes as well as in the coronal oblique plane. The specimens underwent T1-weighted spin-echo MR imaging after administration of intraarticular contrast material and were sectioned into planes corresponding to those of the MR images.
RESULTS. At anatomic dissection, the following posterolateral structures were identified: the arcuate ligament (medial and lateral limbs), fabellofibular ligament, popliteofibular ligament, popliteus tendon and its two posterior attachments to the lateral meniscus, fibular collateral ligament, direct and anterior arms of the tendon of the long head of the biceps femoris muscle, and direct and anterior arms of the tendon of the short head of the biceps femoris muscle. Correlation of MR imaging and anatomic findings showed that the popliteofibular ligament and oblique popliteal ligament were found in 57% and 100% of specimens, respectively. At least one of the two limbs of the arcuate ligament was identified in 71% of specimens. The fabellofibular ligament was not identified on MR images in any of the specimens. The anteroinferior and posterosuperior popliteomeniscal fascicles were identified in all specimens.
CONCLUSION. The posterolateral corner of the knee comprises complex and variable anatomic structures. Recognition of these variations is important in the assessment of MR images of the knee.
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During the previous decade, many studies have evaluated the fine anatomic details of the posterolateral structures of the knee, whereas other studies have evaluated the MR imaging appearance of many of these structures [3, 4, 5, 6, 7, 8]. Several important muscles, tendons, and ligaments have been identified. These include the arcuate ligament, fabellofibular ligament, popliteus muscle and tendon, popliteofibular ligament, popliteomeniscal fascicles, oblique popliteal ligament (Winslow's ligament), fibular collateral ligament, and tendons of the long and short heads of the biceps femoris muscle. The terminology used by various investigators for many of these structures is not uniform. For example, the popliteofibular ligament has been called the short external lateral ligament [9], popliteofibular fascicles [10], fibular origin of the popliteus muscle, and popliteus muscle with origin from the fibular head [11]. There is also variability in anatomy of the posterolateral structures of the knee. In part, because of the varying terminology and anatomy, the posterolateral corner is often overlooked on MR imaging. An understanding of this variability—as well as of those elements of the posterolateral corner that are always present—is essential to MR imaging interpretation.
Using cadaveric specimens, we performed an anatomic dissection followed by MR imaging, MR arthrography, and anatomic correlation of the structures in the posterolateral corner of the knee to provide a more complete analysis of the MR imaging appearance of these structures and to further our general knowledge of the anatomy of the posterolateral corner of the knee.
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We administered an intraarticular injection of a 30-to 35-mL mixture of gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany) and saline (1:200 dilution) to the specimens. We then obtained coronal, sagittal oblique, axial, and coronal oblique T1-weighted spin-echo MR arthrographic images (600/22; section thickness, 2 mm; intersection space, 0.5 mm; number of signals acquired, 2; field of view, 10 x 10 cm; matrix, 512 x 256). Each of the specimens was immediately frozen at –40°C (32°F) for at least 24 hr and then cut into 3-mm-thick sections with a band saw along one of the imaging planes. Two specimens were sectioned in each of the standard imaging planes (axial, coronal, and sagittal). One specimen was sectioned in the coronal oblique plane.
Unenhanced MR images and MR arthrograms were analyzed by a musculoskeletal radiologist who determined the presence and appearance of the following structures: the biceps femoris muscle and tendon, fibular collateral ligament, fabellofibular ligament, arcuate ligament, popliteofibular ligament, popliteomeniscal fascicles, and oblique popliteal ligament. This same radiologist correlated these imaging findings with those evident on gross inspection of the cadaveric sections.
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Short head of biceps femoris muscle.—The short head of the biceps femoris muscle was also found to have two tendinous components. The direct arm inserted into the fibular head anteriorly relative to the fibular styloid process and medially relative to the attachment site of the anterior arm of the tendon of the long head of biceps femoris muscle (Fig. 2). The anterior tendinous arm passed medially relative to the fibular collateral ligament and inserted into the superolateral edge of the lateral tibial condyle. It continued anteriorly as far as 1 cm posterior to the Gerdy's tubercle and almost reached the posterior border of the tibial tuberosity.
Fibular collateral ligament.—The fibular collateral ligament was attached proximally to the distal femur just proximal and posterior relative to the lateral epicondyle. It extended distally to insert into the upper facet of the fibular head. Its insertion into the proximal portion of the fibula was anterior and lateral relative to those of the fabellofibular ligament and arcuate ligament (Fig. 2). A bursa was located superficial relative to the distal portion of the fibular collateral ligament and deep relative to the anterior arm of the tendon of the long head of biceps femoris muscle (Fig. 1C).
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Fabellofibular ligament.—The fabellofibular ligament was attached proximally to the fabella (in the dissected specimen) and inserted distally into the lateral basal aspect of the apex of the fibular head (fibular styloid process), just anterolateral relative to the fibular insertion of the popliteofibular ligament. The fabellofibular ligament was found to course just posterior relative to the lateral limb of the arcuate ligament (Figs. 1D and 1F).
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Arcuate ligament.—The arcuate ligament was Y-shaped and consisted of medial (arcuate) and lateral (upright) limbs. Its base was attached distally near the base of the apex of the fibular head (fibular styloid process), just deep relative to the fabellofibular ligament (Fig. 1B). The lateral limb coursed straight upward and extended proximally along the lateral knee capsule to reach the lateral femoral condyle. The medial limb crossed over the posterior surface of the popliteal tendon and was attached proximally to the posterior knee capsule, merging with the fibers of the oblique popliteal ligament (Fig. 1D). Thus, it formed the bowed roof of the popliteal hiatus.
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Popliteus tendon (popliteofibular ligament and popliteomeniscal fascicles).—The popliteus tendon inserted proximally in the lateral femoral condyle into a sulcus inferior and deep relative to the fibular collateral ligament. It extended distally in a posteromedial direction deep relative to the fabellofibular and arcuate ligaments, which, in turn, formed the arcuate roof of the popliteal hiatus. The tendon was attached to the posterior horn of the lateral meniscus via the superior (Fig. 1E) and inferior (Fig. 1C) popliteomeniscal fascicles. Both structures were identified in the dissected specimen: the superior popliteomeniscal fascicle connected the superomedial aspect of the popliteus tendon with the posterolateral aspect of the lateral meniscus, creating a portion of the roof of the popliteal hiatus. The inferior popliteomeniscal fascicle extended from the anterior edge of the popliteus tendon to the lateral edge of the lateral meniscus, creating the floor of the popliteal hiatus. Just distal to these fascicles, the attachment site of the popliteofibular ligament was found (Fig. 1F). This ligament arose from the popliteus muscle at the lateral aspect of its musculotendinous junction and inserted into the upper facet of the apex of the fibular head, just medial and posterior relative to the insertions of the arcuate and fabellofibular ligaments (Fig. 2).
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Oblique popliteal ligament.—The oblique popliteal ligament was attached laterally and proximally to the margin of the intercondylar fossa and posterior surface of the lateral femoral condyle. Its fibers merged with those of the arcuate ligament. Distally, the oblique popliteal ligament was attached to the posterior margin of the medial condyle of the tibia. Its superficial fibers arose from the tendon of the semimembranosus muscle (Fig. 1D).
MR Imaging with Anatomic Correlation
On MR imaging supplemented with anatomic correlation, the fibular
collateral ligament and tendon of the biceps femoris muscle were well
visualized in all specimens. The fibular collateral ligament was seen as a
thick low-signal-intensity structure that extended from the distal femur just
proximal and posterior relative to the lateral epicondyle to the fibular head
(Fig. 3). In 71% (5/7) of the
specimens, the direct arm of the short head of the biceps femoris muscle, the
direct arm of the long head of the biceps femoris muscle, and the anterior arm
of the long head of the biceps femoris muscle could be separated into
individual components near their fibular attachments. This separation was most
apparent on the axial MR images (Fig.
4).
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The arcuate ligament was seen as a thin low-signal-intensity structure attached distally to the fibular styloid process. The lateral limb coursed proximally just anterior relative to the fabellofibular ligament along the lateral capsule before attaching to the lateral femoral condyle (Figs. 5A and 5B). The lateral limb of the arcuate ligament was seen in 57% (4/7) of the specimens overall. It was found in 50% (2/4) of the specimens in which a fabella was present and in 67% (2/3) of the specimens in which a fabella was absent. The medial limb coursed obliquely, posterior relative to the popliteus tendon, attaching to the posterior knee capsule (Figs. 5C and 5D). The medial limb of the arcuate ligament was seen in 57% (4/7) of the specimens overall. It was seen in 75% (3/4) of the specimens in which a fabella was present and in 33% (1/3) of the specimens in which a fabella was absent. At least one limb of the arcuate ligament was seen in 71% (5/7) of the specimens. The inferior lateral genicular artery passed anterior relative to the arcuate ligament in all specimens.
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Regardless of the presence or absence of a fabella, the fabellofibular ligament could not be identified on inspection of any of the cross-sectional specimen sections or on the related MR images.
In all seven specimens that were imaged, the tendon of the lateral head of the gastrocnemius muscle was seen proximally at the supracondylar tubercle of the distal aspect of the femur. It extended distally intimal relative to the posterior joint capsule and posterolateral relative to the fibular styloid process. If present, a cartilaginous or bony fabella was embedded into this tendon and was optimally visualized on sagittal MR images.
The popliteus tendon and its posterior attachments to the lateral meniscus (the anteriorly located inferior and the posteriorly located superior popliteomeniscal fascicles) were identified in all seven specimens and on the corresponding MR images. The popliteomeniscal fascicles were most clearly seen on sagittal images (Fig. 5A, 5B, 5C, 5D). The popliteofibular ligament was visualized in 57% (4/7) of the specimens. It appeared as a low-signal-intensity structure connecting the popliteus muscle to the upper facet of the apex of the fibular head (Fig. 6A, 6B). It was optimally visualized on the coronal oblique images. The oblique popliteal ligament was identified in all seven specimens and was seen most clearly on the axial MR images.
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The tendinous components of both the long and short heads of the biceps femoris muscle each consist of anterior and direct arms. As described by Sneath [12], the direct arm of the tendons of both the long and short heads and the anterior arm of the long head insert into the fibular head. These arms were identified as separate structures on MR images in 71% (5/7) of the specimens in our study.
The fibular collateral ligament, also called the lateral collateral ligament, has a femoral attachment just proximal and posterior relative to the lateral epicondyle. It inserts distally into the lateral aspect on the fibular head anteriorly and laterally relative to the insertions of the fabellofibular ligament and arcuate ligaments. It is easily identified on standard MR images. The fibular collateral ligament–biceps femoris bursa is located superficially relative to the distal aspect of the fibular collateral ligament and deep relative to the anterior arm of the long head of the biceps femoris muscle. LaPrade and Hamilton [13] identified this bursa in all 50 of their dissected specimens. They described the bursa as consistently displaying a J shape around the anterior and anteromedial portions of the fibular collateral ligament. LaPrade and Hamilton also reported that this bursa had a mean width of 8.4 mm and a mean length of 18 mm. In our study, although this bursa was present on inspection of the dissected specimen, it was not seen on MR imaging with anatomic correlation.
Although the fabellofibular ligament was well visualized at dissection of the initial specimen, it could not be identified either on MR imaging or at inspection of the sectioned specimens. When the ligament was present, it was usually found just posterior relative to the lateral limb of the arcuate ligament. A prominent tendon of the lateral head of the gastrocnemius muscle, which courses through the fabella, may easily be mistaken for the fabellofibular ligament. However, the lateral head of the gastrocnemius muscle, in contrast to the fabellofibular ligament, courses posterior relative to the fibular styloid process.
The arcuate ligament is Y-shaped and consists of medial (arcuate) and lateral (upright) limbs. It is attached distally to the fibular styloid process. The lateral limb extends proximally along the lateral knee capsule to reach the lateral femoral condyle. The medial limb is superficial relative to the popliteus tendon and attaches proximally to the posterior knee capsule, merging with the fibers of the oblique popliteal ligament. Either the medial or lateral limb was identified in 71% (5/7) of the specimens in our study. Yu et al. [8] identified the arcuate ligament in standard coronal MR images in 10% of patients. However, in their study, use of coronal oblique images resulted in visualization of the arcuate ligament in 46% of patients.
Previous reports have documented variability in the prevalence of the arcuate and fabellofibular ligaments at inspection of gross anatomic specimens. Seebacher et al. [14] detected both ligaments in the dissection of 67% of specimens. The arcuate ligament alone was identified in 13% and the fabellofibular ligament alone, in 20%. The presence of a bony fabella implied that the fabellofibular ligament was also present. Conversely, the absence of a bony fabella suggested the absence of the fabellofibular ligament and a prominent arcuate ligament. A large fabella implied the absence of the arcuate ligament. The presence of a cartilaginous fabella implied that both ligaments were present. Watanabe et al. [11] classified the posterolateral structures into seven major types, depending on the prevalence of the arcuate, fabellofibular, and popliteofibular ligaments. They reported the occurrence of both the arcuate and fabellofibular ligaments in only 11% (13/115) of cadaveric specimens. The arcuate ligament occurred in the absence of the fabellofibular ligament in 37% of the specimens. The fabellofibular ligament occurred in the absence of the arcuate ligament in 40% of the specimens. In 12% of the specimens, neither of these two ligaments was present because of the dominance of the popliteofibular ligament.
The popliteofibular ligament is a bandlike structure extending from the musculotendinous junction of the popliteus muscle to the apex of the fibular head. Its fibular attachment is just medial to those of the arcuate and fabellofibular ligaments. In our study, the popliteofibular ligament was well visualized in the dissected specimen and also seen on MR imaging in 57% (4/7) of the specimens. Watanabe et al. [11] identified the popliteofibular ligament in 93% (108/115) of their cadaveric knee specimens. Maynard et al. [15] identified this structure in 100% (20/20) of their cadaveric specimens. However, the identification of this structure on MR images is more difficult. De Maeseneer et al. [3] reported visualization of the popliteofibular ligament on standard MR images in 38% of patients. Its visualization is enhanced with the use of coronal oblique MR imaging. Yu et al. [8] reported identifying this ligament in 53% of patients on oblique MR images and in only 8% on standard coronal MR images. Biomechanical studies show that the popliteofibular ligament provides static stability to the knee [16, 17]. It prevents excessive posterior translation, varus angulation, and external rotation of the knee. Biomechanical testing with a varus stress shows that the popliteofibular ligament fails after the fibular collateral ligament and before failure of the popliteus tendon [14].
Investigators have previously described two distinct ligamentous fascicles attaching the popliteus tendon to the lateral meniscus—the superior and inferior popliteomeniscal fascicles [10, 18]. In our study, both fascicles were identified in the dissected specimen and were seen in all seven specimens on MR imaging. These fascicles consist of broad low-signal-intensity structures extending from the popliteal tendon to the lateral meniscus. The superior popliteomeniscal fascicle forms the roof of the popliteal hiatus, and the inferior popliteomeniscal fascicle forms the floor of the popliteal hiatus. Johnson and DeSmet [19] identified both popliteomeniscal fascicles on sagittal MR images in 64 of 66 patients who had no evidence of injury to the lateral compartment.
Terry and LaPrade [20] described a third popliteomeniscal fascicle, the posteroinferior popliteomeniscal fascicle. This structure was joined to the aponeurotic attachment of the popliteal muscle at the posterior capsule rather than to the popliteus tendon. The posteroinferior popliteomeniscal fascicle connected to the inferior surface of the posterior horn of the lateral meniscus. This structure was absent in our dissected specimen, and its presence is debated.
The popliteomeniscal fascicles prevent motion of the lateral meniscus during knee extension [10, 21]. Injury to these structures has been reported to be clinically important. Simonian et al. [22] reported identifying on MR imaging three cases of isolated popliteomeniscal disruption without intrameniscal abnormality that resulted in knee locking. Recent data from DeSmet et al. [23] suggested that an abnormal superior popliteomeniscal fascicle is a secondary sign of a lateral meniscal tear.
A biomechanical selective-cutting study has shown that the major structures preventing posterolateral instability of the knee are the fibular collateral ligament and the popliteus tendon [24]. These structures are easily seen on MR imaging. As previously discussed, more recent studies have identified the popliteofibular ligament as an important component in preventing posterolateral instability by providing static stability [16, 17]. As a result, surgical techniques have primarily focused on restoring the integrity of the fibular collateral ligament, popliteus tendon, and popliteofibular ligament in patients with posterolateral corner injuries. As described earlier, the presence of the popliteofibular ligament is fairly consistent in cadaveric studies. However, its identification on MR imaging in our study and in those of others varied, even with inclusion of coronal oblique MR images [8]. This difficulty in visualization is clearly a limitation in the use of MR imaging in the assessment of posterolateral corner injuries.
A limitation of our study is the lack of knowledge of prior injuries of the cadaveric specimens. The presence of a prior injury could certainly account for the variability of the posterolateral structures.
In conclusion, the supporting structures in the posterolateral corner of the knee are characterized by complex and variable anatomy, and the anatomic structures in this region have been subject to inconsistent terminology. Recognition of the variations in anatomy and of the limitations of MR imaging is important in the imaging assessment of these structures.
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