- U S C U L O S K E L E T A L) M A G I N Gs0 I C T O R I A L% S S AY
Barker et al.
Sonographic Anatomy of Posterolateral Knee
Musculoskeletal Imaging
Pictorial Essay
Normal Sonographic Anatomy
of the Posterolateral Corner
of the Knee
Robert P. Barker 1
Justin C. Lee
Jeremiah C. Healy
Barker RP, Lee JC, Healy JC
/"*%#4)6% The posterolateral corner of the knee comprises a group of structures that
are important to knee stability. MRI is currently the standard imaging technique, but visualization of individual structures is often incomplete. Sonography allows rapid real-time assessment of these superficial structures, but knowledge of the anatomy is essential to allow accurate examination.
#/.#,53)/. We present an illustrated review of the sonographic anatomy of the
posterolateral corner of the knee with MRI correlation.
T
Keywords: anatomy, MRI, posterolateral knee,
sonography, ultrasound
DOI:10.2214/AJR.07.3743
Received January 29, 2008; accepted after revision
July 2, 2008.
1
All authors: Department of Radiology, Chelsea &
Westminster Hospital, 369 Fulham Rd., London
SW10 9NH, United Kingdom. Address correspondence
to R. P. Barker (rpbarker@hotmail.com).
CME
This article is available for CME credit.
See www.arrs.org for more information.
AJR 2009; 192:73–79
0361–803X/09/1921–73
© American Roentgen Ray Society
AJR:192, January 2009
he posterolateral corner of the
knee is a complex group of structures that together form a functional musculotendinous–ligamentous unit that acts as a dynamic and
static stabilizer against abnormal varus and
posterolateral translational movements [1].
MRI is routinely used to image the posterolateral corner of the knee. However, visualization of some components is often incomplete using routine orthogonal [2–4] and
coronal oblique [5] acquisitions because of
the complex anatomy crossing planes and the
loss of signal due to magic angle artifact [6].
MRI is also limited by the need to scan with
the knee in extension. Sonography has great
potential to visualize the superficial structures of the posterolateral corner of the knee
and also has the advantages over MRI of
speed, safety, the ability to examine the knee
dynamically, and the ability to provide comparative examination of the contralateral
limb. A previous cadaveric study showed
sonography can identify many of the individual structures of the posterolateral corner
of the knee [7], but no studies have reported
the appearance in living subjects. An understanding of the normal anatomy is crucial to
enable accurate examination and is aided by
correlating the sonographic and MR appearances, as in this illustrated article on the posterolateral corner of the knee.
The images were acquired in healthy volunteers without any history of trauma or arthritis, and the components are presented in
our suggested order of examination. All
sonograms were acquired using a 12.5-MHz
linear transducer (Logiq 9, GE Healthcare),
and the MR images were obtained with a 1.5T scanner (Magnetom Avanto, Siemens Europe). The key structures are illustrated in
Figure 1.
Popliteus
The popliteus muscle is the main dynamic
lateral stabilizer of the knee. It arises from
the posteromedial aspect of the tibia and
curves superolaterally where the tendon
passes under the arcuate ligament and lateral
collateral ligament to insert in the popliteal
groove of the lateral femoral condyle. At
sonography, the tendon is identified in the
popliteal groove, and the transducer is then
moved inferiorly following the oblique
course of the muscle (Figs. 2 and 3).
Popliteofibular Ligament
The popliteofibular ligament is the main
static stabilizer of external knee rotation, especially during flexion when it remains
taught, whereas the lateral collateral ligament becomes lax [8]. At sonography with
the knee flexed, the ligament it is seen as a
linear hypoechoic structure extending from
the lateral aspect of the popliteus near the
musculotendinous junction to the medial aspect of the fibular apex (Fig. 4).
Lateral Collateral Ligament
The lateral collateral ligament prevents
varus angulation and limits internal rotation
of the knee. It has a femoral attachment at a
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point posterior to the tip of the lateral condyle and directly anterior to the origin of the
lateral head of the gastrocnemius. Distally, it
forms the conjoint tendon with the biceps
femoris to insert onto the fibular head. At
sonography, with the knee extended and
some varus angulation, the linear ligament
with a compact fibrillar pattern is identified
by scanning over the popliteal groove, where
it is seen passing superficially to the popliteus muscle and then following it distally to
the fibular apex (Fig. 5).
Biceps Femoris
The biceps femoris muscle and tendon act
as a strong dynamic stabilizer. Distally the
tendon inserts onto the anterolateral fibular
apex by forming the conjoint tendon with the
lateral collateral ligament. Proximally, the
tendon blends with the hypoechoic muscle
fibers (Fig. 6).
Lateral Head of the Gastrocnemius
The main function of gastrocnemius muscle is plantar flexion, but the lateral head
also acts as a dynamic posterolateral stabilizer. The tendon courses posterior to the
fibular styloid and attaches to the lateral
femoral condyle. At sonography, the hyperechoic fibrillar tendon is identified by its insertion immediately posterior to the lateral
collateral ligament and, when present, the
enclosed fabella. The tendon can then be
traced inferiorly to the hypoechoic muscle
belly (Fig. 7).
Fabellofibular Ligament
The fabella is a sesamoid bone in the lateral head of gastrocnemius tendon. It has a
prevalence of approximately 10–30% [9]
and, if present, is bilateral in 60–80% of
cases [10]. The ligament extends from the
fabella to the fibular styloid process and acts
as a static stabilizer. An inverse relationship
exists between the size of the fabellofibular
and the arcuate ligaments [9]. At sonography, the fabella is easily seen as a small,
rounded hyperechoic structure with posterior
acoustic shadowing in the gastrocnemius
tendon. The linear hypoechoic fabellofibular
ligament is identified passing to the lateral
fibular apex (Fig. 8).
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Arcuate Ligament
The arcuate is not a true ligament but a
condensation of popliteus tendon fibers forming a Y-shaped structure in which the medial
(oblique) and lateral (upright) limbs share an
origin on the fibular apex and act as a static
stabilizer. Proximally it blends with the
oblique popliteal ligament and ultimately
with the femur. At sonography, in knees with
no fabella, the lateral limb is visualized distally as a thin linear structure extending from
the fibular apex deep in relation to the lateral
head of the gastrocnemius muscle before tapering out proximally (Fig. 9). The broad
and flat medial limb cannot be discerned.
Lateral Geniculate Artery
The lateral inferior geniculate artery
serves no structural function but provides a
useful anatomic landmark. It arises from the
popliteal artery and passes inferolaterally
around the knee deep in relation to the lateral
head of the gastrocnemius, the lateral collateral ligament, and the biceps femoris tendon,
but superficial in relation to the popliteofibular ligament (Fig. 10).
Conclusion
We have presented an illustrated review of
the normal sonographic anatomy of the posterolateral corner of the knee, with MRI correlation. Biomechanical studies have shown
that the key stabilizing structures are the
popliteus tendon, the popliteofibular ligament, and the lateral collateral ligament [11,
12]. The ability of sonography to reveal these
and the other components that are inconsistently seen on MRI may be useful in the assessment of this region. The ability to examine
these structures dynamically also potentially
allows sonography to complement MRI in
showing posterolateral corner injuries, which
is important because clinical assessment can
be unreliable [13], and failure to identify an
injury before a cruciate ligament is repaired
is associated with an increased risk of graft
failure [14].
References
1. Sudasna S, Harnsiriwattanagit K. The ligamentous
structures of the posterolateral aspect of the knee.
Bull Hosp Jt Dis Orthop Inst 1990; 50:35–40
2. Theodorou DJ, Theodorou SJ, Fithian DC, Paxton
L, Garelick DH, Resnick D. Posterolateral complex
knee injuries: magnetic resonance imaging with
surgical correlation. Acta Radiol 2005; 46:297–305
3. Lee J, Papakonstantinou O, Brookenthal KR,
Trudell D, Resnick DL. Arcuate sign of posterolateral knee injuries: anatomic, radiographic, and
MR imaging data related to patterns of injury.
Skeletal Radiol 2003; 32:619–627
4. Munshi M, Pretterklieber ML, Kwak S, Antonio
GE, Trudell DJ, Resnick D. MR imaging, MR arthrography, and specimen correlation of the posterolateral corner of the knee: an anatomic study.
AJR 2003; 180:1095–1101
5. Yu JS, Salonen DC, Hodler J, Haghighi P, Trudell
D, Resnick D. Posterolateral aspect of the knee:
improved MR imaging with a coronal oblique
technique. Radiology 1996; 198:199–204
6. Rajeswaran G, Lee J, Healy J. MRI of the popliteofibular ligament: isotropic 3D WE-DESS versus coronal oblique fat-suppressed T2W MRI.
Skeletal Radiol 2007; 36:1141–1146
7. Sekiya J, Jacobson J, Wojtys E. Sonographic imaging of the posterolateral structures of the knee:
findings in human cadavers. Arthroscopy 2002;
18:872–881
8. Sugita T, Amis AA. Anatomic and biomechanical
study of the lateral collateral and popliteofibular
ligaments. Am J Sports Med 2001; 29:466–472
9. Watanabe Y, Moriya H, Takahashi K, et al. Functional anatomy of the posterolateral structures of
the knee. Arthroscopy 1993; 9:57–62
10. Houghton-Allen BW. In the case of the fabella a
comparison view of the other knee is unlikely to
be helpful. Australas Radiol 2001; 45:318–319
11. Shahane SA, Ibbotson C, Strachan R, Bickerstaff
DR. The popliteofibular ligament: an anatomical
study of the posterolateral corner of the knee. J
Bone Joint Surg Br 1999; 81:636–642
12. Gollehon DL, Torzilli PA, Warren RF. The role of
the posterolateral and cruciate ligaments in the
stability of the human knee: a biomechanical
study. J Bone Joint Surg Am 1987; 69:233–242
13. Veltri DM, Deng XH, Torzilli PA, Maynard MJ,
Warren RF. The role of the popliteofibular ligament in stability of the human knee: a biomechanical study. Am J Sports Med 1995; 23:436–443
14. Harner CD, Vogrin TM, Höher J, Ma CB, Woo
SL. Biomechanical analysis of a posterior cruciate
ligament reconstruction: deficiency of the posterolateral structures as a cause of graft failure. Am J
Sports Med 2000; 28:32–39
AJR:192, January 2009
Sonographic Anatomy of Posterolateral Knee
A
Fig. 1—Schematic drawing of major components
of posterolateral corner of knee: lateral collateral
ligament (1), lateral head of gastrocnemius muscle
(2), fabella in lateral head of gastrocnemius tendon
(3), fabellofibular ligament (4), popliteofibular
ligament (5), popliteus tendon (6), biceps femoris
tendon (7), iliotibial tract (8), conjoint tendon (9), and
arcuate ligament (10).
Fig. 3—Popliteus tendon in 30-year-old healthy
woman.
A and B, Coronal sonogram (A) and coronal fatsaturated proton density–weighted MR image (B)
show popliteus tendon in sulcus (arrowheads) of
lateral femoral condyle (Fm) and lateral collateral
ligament superficial to tendon (arrows). Distally,
lateral collateral ligament attaches to fibular apex
(Fb, B). Box in B indicates position of corresponding
ultrasound image A.
AJR:192, January 2009
B
Fig. 2—Popliteus muscle and tendon in 32-year-old healthy man.
A and B, Photograph of knee (A) and extended-field-of-view sonogram (B) show starting position of transducer
in coronal plane to identify popliteus tendon in sulcus of lateral femoral condyle. Probe is then moved
posteromedially (arrow, A) following oblique course of the muscle (arrowheads, B). This may be easier to
perform when patient is lying prone. Box in A indicates position of ultrasound transducer to obtain image B.
A
B
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A
B
C
Fig. 4—Popliteofibular ligament in 32-year-old healthy man.
A, Photograph of lateral knee shows probe position required to image popliteofibular ligament. Knee is flexed and musculotendinous junction of popliteus is identified
(dashed box). While heel of probe is kept in this position, toe of probe is turned to fibular apex (solid box).
B and C, Sonogram (B) and coronal fat-saturated proton density–weighted MR image (C) show popliteofibular ligament (arrowheads) extending from popliteus muscle
(straight arrow, C) to fibular apex (Fb). Proximally, popliteus tendon is seen in femoral sulcus (asterisk). Anisotropy artifact or tendon calcification is seen (curved arrow,
B), causing apparent hypoechogenicity on sonography. Box in C indicates position of corresponding ultrasound image B.
A
B
C
Fig. 5—Lateral collateral ligament in 32-year-old healthy man.
A, Photograph of lateral knee shows probe position for imaging lateral collateral ligament. Knee is extended with varus stress applied, and popliteus tendon is first
identified in coronal plane by scanning over palpable lateral femoral condyle. Lateral collateral ligament is seen passing superficially to femoral condyle proximally to its
femoral attachment and distally to fibular apex via echogenic, fibrillar conjoint tendon. Box in A indicates position of ultrasound transducer to obtain image B.
B and C, Coronal sonogram (B) and coronal STIR MR image (C) show lateral collateral ligament (arrows) extending from lateral femoral condyle (Fm) to lateral aspect of
fibular apex (Fb). Proximally, it passes over popliteus tendon in lateral femoral sulcus (arrowhead, B). Box in C indicates position of corresponding ultrasound image B.
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AJR:192, January 2009
Sonographic Anatomy of Posterolateral Knee
Fig. 6—Biceps femoris muscle
and tendon in 32-year-old
healthy man.
A, Photograph of lateral knee
shows probe position for
imaging conjoint tendon of
biceps femoris muscle. With
patient lying on side or prone and
knee flexed against resistance,
echogenic conjoint tendon is
identified at fibular apex. Probe
is then moved proximally to
biceps muscle belly. Box in A
indicates position of ultrasound
transducer to obtain image B.
B and C, Coronal sonogram (B)
and sagittal fat-saturated proton
density–weighted MR image (C)
show biceps tendon (arrowhead)
fusing with lateral collateral
ligament (curved arrow) to form
conjoint tendon (straight arrow),
which attaches to fibular apex
(Fb). Box in C indicates position
of corresponding ultrasound
image B.
A
Fig. 7—Long head of gastrocnemius
muscle in 32-year-old healthy man.
A, Photograph of posterior knee
shows probe position for imaging
long head of gastrocnemius
muscle. With patient prone, tendon
is first identified by its insertion
immediately posterior to lateral
collateral ligament on lateral
femoral condyle and, when present,
enclosed fabella. Tendon can then
be traced inferiorly to muscle belly
in leg. Box in A indicates position
of ultrasound transducer to obtain
image B.
B and C, Sagittal oblique sonogram
(B) and sagittal T1-weighted MR
image (C) show muscle and tendon
of long head of gastrocnemius
(arrows) passing laterally to fibular
head (Fb, B) and femoral condyle
(Fm, B). Tendon of this knee contains
a fabella (arrowhead, C). Box in C
indicates position of corresponding
ultrasound image B.
AJR:192, January 2009
B
A
B
C
C
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Fig. 8—Fabellofibular ligament in
34-year-old healthy man.
A, Photograph of lateral knee
shows probe position for
imaging fabellofibular ligament.
With patient prone or lying on
side, ligament of long head of
gastrocnemius is examined,
looking for echogenic fabella.
If one is present, toe of probe
is fixed at this point and heel is
swung around to lie over fibular
apex to identify ligament. Box in
A indicates position of ultrasound
transducer to obtain image B.
B and C, Sagittal oblique sonogram
(B) and coronal T1-weighted MR
image (C) show fabellofibular
ligament (arrows) extending from
fabella (asterisk) to fibular apex
(Fb). Box in C indicates position
of corresponding ultrasound
image B.
A
B
A
C
B
C
Fig. 9—Arcuate ligament in 32-year-old healthy man.
A, Photograph of lateral knee shows probe position for imaging arcuate ligament. In knee with no fabella, knee is extended with valgus stress applied, and fibular apex is
searched looking for thin, linear hypoechoic structure extending toward lateral femoral condyle between collateral ligament and popliteofibular ligament. Upright limb of
arcuate ligament lies immediately superficial to lateral geniculate artery. Box in A indicates position of ultrasound transducer to obtain image B.
B and C, Coronal sonogram (B) and coronal fat-suppressed proton-density MR image (C) show medial or upright limb of arcuate ligament (straight arrows) arising
from fibular apex (Fb). Arcuate ligament lies between lateral collateral ligament (arrowheads) and popliteofibular ligament (curved arrow, B) just superficial to lateral
geniculate artery (circle, B). Box in C indicates position of corresponding ultrasound image B.
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AJR:192, January 2009
Sonographic Anatomy of Posterolateral Knee
Fig. 10—Lateral geniculate artery in 34-year-old
healthy man.
A and B, Coronal color Doppler sonograms show
lateral geniculate arteries (arrowheads) passing
superior to popliteofibular ligament (arrow, A) and
deep in relation to lateral collateral ligament (arrow,
B).
C and D, Coronal T1-weighted MR images show
lateral geniculate artery (white arrows) arising
from popliteal artery (arrowhead, D) and passing
inferolaterally under lateral collateral ligament (short
arrow, D). Also shown are conjoint tendon (curved
arrow, C) formed by lateral collateral ligament and
biceps tendon (black arrow, C). Box in C indicates
position of corresponding ultrasound image B..
A
C
B
D
FOR YOUR INFORMATION
This article is available for CME credit. See www.arrs.org for more information.
AJR:192, January 2009
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