DOI:10.2214/AJR.07.3743
AJR 2009; 192:73-79
© American Roentgen Ray Society
Normal Sonographic Anatomy of the Posterolateral Corner of the Knee
Robert P. Barker1,
Justin C. Lee1 and
Jeremiah C. Healy1
1 Department of Radiology, Chelsea & Westminster Hospital, 369 Fulham Rd.,
London SW10 9NH, United Kingdom.
Received January 29, 2008;
accepted after revision July 2, 2008.
CME This article is available for CME credit. See
www.arrs.org
for more information.
Address correspondence to R. P. Barker
(rpbarker{at}hotmail.com).
FOR YOUR INFORMATION
This article is available for CME credit. See
www.arrs.org
for more information.
Abstract
OBJECTIVE. 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.
CONCLUSION. We present an illustrated review of the sonographic
anatomy of the posterolateral corner of the knee with MRI correlation.
Keywords: anatomy • MRI • posterolateral knee • sonography • ultrasound
Introduction
The 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 1.5-T scanner
(Magnetom Avanto, Siemens Europe). The key structures are illustrated in
Figure 1.
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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).
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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. 2A,
2B,
3A, and
3B).
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Fig. 2A —Popliteus muscle and tendon in 32-year-old healthy man. 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.
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Fig. 2B —Popliteus muscle and tendon in 32-year-old healthy man. 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.
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Fig. 3A —Popliteus tendon in 30-year-old healthy woman. Coronal sonogram
(A) and coronal fat-saturated 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.
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Fig. 3B —Popliteus tendon in 30-year-old healthy woman. Coronal sonogram
(A) and coronal fat-saturated 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.
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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 (Figs.
4A,
4B, and
4C).
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Fig. 4A —Popliteofibular ligament in 32-year-old healthy man. 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).
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Fig. 4B —Popliteofibular ligament in 32-year-old healthy man. 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.
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Fig. 4C —Popliteofibular ligament in 32-year-old healthy man. 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.
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Lateral Collateral Ligament
The lateral collateral ligament prevents varus angulation and limits
internal rotation of the knee. It has a femoral attachment at a 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 (Figs.
5A,
5B, and
5C).
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Fig. 5A —Lateral collateral ligament in 32-year-old healthy man. 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.
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Fig. 5B —Lateral collateral ligament in 32-year-old healthy man. 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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Fig. 5C —Lateral collateral ligament in 32-year-old healthy man. 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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Fig. 6A —Biceps femoris muscle and tendon in 32-year-old healthy man.
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.
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Fig. 6B —Biceps femoris muscle and tendon in 32-year-old healthy man. 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.
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Fig. 6C —Biceps femoris muscle and tendon in 32-year-old healthy man. 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.
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Fig. 7A —Long head of gastrocnemius muscle in 32-year-old healthy man.
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.
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Fig. 7B —Long head of gastrocnemius muscle in 32-year-old healthy man.
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.
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Fig. 7C —Long head of gastrocnemius muscle in 32-year-old healthy man.
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.
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Fig. 8A —Fabellofibular ligament in 34-year-old healthy man. 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.
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Fig. 8B —Fabellofibular ligament in 34-year-old healthy man. 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.
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Fig. 8C —Fabellofibular ligament in 34-year-old healthy man. 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.
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Fig. 9A —Arcuate ligament in 32-year-old healthy man. 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.
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Fig. 9B —Arcuate ligament in 32-year-old healthy man. 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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Fig. 9C —Arcuate ligament in 32-year-old healthy man. 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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Fig. 10A —Lateral geniculate artery in 34-year-old healthy man. 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).
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Fig. 10B —Lateral geniculate artery in 34-year-old healthy man. 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).
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Fig. 10C —Lateral geniculate artery in 34-year-old healthy man. 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..
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Fig. 10D —Lateral geniculate artery in 34-year-old healthy man. 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..
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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 (Figs.
6A,
6B, and
6C).
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 (Figs.
7A,
7B, and
7C).
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
(Figs. 8A,
8B, and
8C).
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 (Figs.
9A,
9B, and
9C). 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 (Figs.
10A,
10B,
10C, and
10D).
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].
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Abstract
Introduction
