DOI:10.2214/AJR.07.2051
AJR 2008; 190:449-458
© American Roentgen Ray Society
The Posterolateral Corner of the Knee
Emily N. Vinson1,
Nancy M. Major and
Clyde A. Helms
1 All authors: Department of Radiology, Duke University Medical Center, Erwin
Rd., Box 3808, Durham, NC 27710.
Received February 14, 2007;
accepted after revision July 7, 2007.
Address correspondence to E. N. Vinson
(vinso003{at}mc.duke.edu).
Abstract
OBJECTIVE. The purpose of this article is to review the clinical
importance and MRI appearances of injuries to the posterolateral corner of the
knee.
CONCLUSION. Injuries to the posterolateral corner structures of the
knee can cause significant disability due to instability, cartilage
degeneration, and cruciate graft failure. Becoming familiar with the anatomy
of this region can improve one's ability to detect subtle abnormalities and
can perhaps lead to improvements in diagnosing and understanding injuries to
this area.
Keywords: knee instability • MRI • musculoskeletal imaging • sports medicine • trauma
Introduction
The use of MRI in the evaluation of knee trauma and instability has become
a mainstay of clinical practice in great part because of its high accuracy in
the identification of meniscal and ligamentous abnormalities. The increasing
recognition of the importance of the diagnosis of posterolateral corner
injuries—and the subsequent repair of such injuries—among our
orthopedic surgery colleagues has driven those of us involved in imaging to
attempt to detect these injuries preoperatively with MRI. This article reviews
the clinical importance of posterolateral corner instability, including the
often controversial management of these cases and the normal and abnormal
appearances of the various structures of the posterolateral corner as
visualized on MRI.
Posterolateral Corner Instability: Clinical Features and Management
Rotatory instability of the posterolateral corner of the knee is a complex
and difficult clinical entity in terms of both diagnosis and treatment. The
stability of the posterolateral corner of the knee is provided by capsular and
noncapsular structures that function as static and dynamic stabilizers
[1], including the fibular
collateral ligament, the popliteus muscle and tendon including its fibular
insertion (popliteofibular ligament), and the lateral and posterolateral
capsule. Injuries to this region that result in posterolateral rotatory
instability can be but are uncommonly isolated; usually these injuries are
associated with concurrent ligamentous injuries elsewhere in the knee
[2–5].
In particular, posterolateral corner injuries are frequently seen in
combination with posterior cruciate ligament (PCL) injuries. In one study of
85 patients with acute PCL injuries, 53 (62%) were also diagnosed with
posterolateral corner tears by clinical examination, arthroscopy, or MRI (or a
combination thereof) [3]. Given
this significant association, when MRI reveals evidence of an acute PCL
injury, particular attention should also be paid to the posterolateral
structures of the knee.
The mechanism of injury is thought to be either a direct blow to the
anteromedial proximal tibia, directed posterolaterally, with the knee near
full extension [6], which is
thought to be most common, or a noncontact, external rotation hyperextension
injury [1]. This area is also
often injured in complete knee dislocations, resulting from a combination of
varus force with hyperextension (in an anterior rotatory dislocation) or a
combination of varus force with a posteriorly directed blow to the proximal
tibia with the knee flexed ("dashboard injury," in a posterior
rotatory dislocation) [4,
7]. A high degree of suspicion
is necessary to make the diagnosis of posterolateral corner instability, and
failure to recognize these often elusive injuries can have consequences of
chronic instability, a predisposition to cruciate graft failure, or both as
discussed later in this article.
When posterolateral corner injury is suspected, testing for increased varus
and external rotation should be performed at various degrees of flexion and
compared with the contralateral knee
[8]. The dial test, or
posterolateral rotation test, which assesses for increased external rotation
of the tibia relative to the femur with the knee flexed to 30°, is one of
the standard tests to assess and follow posterolateral rotatory instability
[9].
High-grade posterolateral corner injuries are usually associated with
rupture of one or both cruciate ligaments. Importantly, failure to address
instability of the posterolateral corner structures increases forces at
anterior cruciate ligament (ACL) and PCL graft sites and may ultimately
predispose to failure of the cruciate reconstruction
[10–12].
Untreated posterolateral corner injuries contribute to ACL graft failure by
allowing significantly higher forces to stress the graft with varus loading at
varying degrees of flexion than occurs with intact posterolateral corner
structures [12].
Unrecognized and untreated posterolateral corner instability is possibly
the most common identifiable cause of ACL reconstruction failure
[2,
13]. Likewise, biomechanical
studies of PCL grafts have shown that PCL reconstruction grafts are rendered
ineffective and may become overloaded in the setting of deficiency of the
posterolateral corner structures due to increases in the in situ forces in the
graft during loading [11]. In
addition, better functional outcomes are achieved with repair of the PCL and
the posterolateral corner structures than with repair of the PCL alone
[14]. Given the relatively
high incidence of concomitant injuries to the posterolateral structures in the
setting of ACL and PCL injuries, unrecognized posterolateral corner injuries
are a major cause of eventual graft failure when only the cruciate injury is
recognized and addressed [11,
12].
Regardless of the status of the cruciate ligaments, failure to address
high-grade posterolateral instability may lead to the development of
significant osteoarthritis. One study evaluating the outcomes of nonoperative
treatment for grade II and grade III lateral ligament sprains in patients
without clinical evidence of complete tears of either cruciate ligament found
a high incidence—50%—of posttraumatic osteoarthritis in the
patients with grade III sprains at 8-year follow-up. The patients with grade
II sprains responded well to nonoperative treatment; although residual laxity
remained present at follow-up, no patients in this group suffered from
posttraumatic osteoarthritis. The authors postulated that with grade III
injuries, the degree of ligamentous laxity and static instability was too
great to be compensated for by the dynamic stabilizing function of the knee
muscles and tendons [15].
Because of the potential debilitating consequences of an unrecognized
injury to the posterolateral corner structures, attention with regard to the
diagnosis and management of these injuries has increased. However, there has
been much debate in the orthopedics literature regarding the optimal methods
of management of posterolateral corner injuries. Injuries to the
posterolateral ligamentous structures are often classified as grade I, II, or
III sprains, corresponding to minimal, partial, or complete tearing,
respectively. Grade III injuries are usually associated with markedly abnormal
joint motion and are the most clinically relevant from a surgical
standpoint.
Clinically, a numeric scale is often qualitatively used to describe the
degree of ligamentous instability as 1+ (mild), 2+ (moderate), or 3+ (severe).
A similar quantitative scale, depicting the degree of abnormal joint opening
with stress on clinical examination, is also used, with 3+ describing an
opening of > 10 mm with a soft or no appreciable end point
[8]. Patients with evidence of
injury but without significant pathologic laxity or functional limitations,
corresponding to grade I and II injuries, are often treated with
rehabilitation and observation
[5]. In patients with laxity,
corresponding to grade III injuries, most authors advocate either repair or
reconstruction of the posterolateral corner in the setting of acute or chronic
posterolateral instability
[8].
There is little clinical data regarding the long-term outcomes of surgical
treatment of posterolateral corner injuries, and there is no agreement about
the best surgical technique to use
[16]. In general, operative
procedures can be categorized as primary repair, reconstruction, and
advancement [5,
8]. Operative treatment in the
acute setting is thought to have a greater chance of successful outcome than
surgery for chronic injuries
[6,
8]. In particular, direct
primary anatomic repair of the injured structures is most easily accomplished
within 3 weeks of the inciting injury and when the tissues are of good quality
[5,
8]. Evaluation and treatment
proceed from deep to superficial structures, with repair of each torn
structure by sutures, suture anchors, or sutures via drill holes through bone
[1,
8].
Reconstruction of the posterolateral structures is usually performed in the
acute setting when the quality of the tissues does not allow primary repair or
in symptomatic patients with chronic posterolateral rotatory instability
[2]. Many authors favor
reconstruction over primary repair even in the acute setting because of
improved outcomes, and some think that primary repair of the fibular
collateral ligament is indicated only in patients with bone avulsions amenable
to internal fixation [17,
18].
Reconstruction techniques vary widely. Numerous reconstruction techniques
have been described in the medical literature, using a variety of allografts
and autografts (often from patellar, Achilles, hamstring, biceps femoris, and
tibialis tendon donor sites) to reinforce or replace the fibular collateral
ligament, posterolateral capsule, popliteus tendon, or popliteofibular
ligament with either anatomic or nonanatomic anchor sites
[2,
8,
16–21].
In the setting of less severe chronic injury, if the posterolateral structures
are lax but identifiable and have sufficient intact collagenous tissue,
proximal advancement of the posterolateral complex may be performed to remove
slackness and restore stability instead of the more complex graft
reconstruction. In this procedure, the proximal attachments of the
posterolateral complex, including the fibular collateral ligament and
popliteus tendon, are excised from the femur and the complex is advanced
proximally and reattached to the femur
[19]. Some patients with varus
alignment due to chronic posterolateral instability require proximal valgus
tibial osteotomy before addressing the ligamentous abnormalities
[21]. Further long-term
clinical outcomes studies are necessary to determine the efficacy of the
variety of procedures currently performed to restore posterolateral stability.
Regardless of the surgical technique used, when necessary, concomitant
cruciate ligament reconstruction should be performed before or during the
posterolateral corner repair or reconstruction
[2], although some authors
prefer a two-stage procedure in cases of bicruciate injuries, with the PCL and
posterolateral corner reconstructions performed before the ACL reconstruction
[16].
Because MRI is commonly performed in the setting of knee injury,
radiologists familiar with the normal and abnormal appearances of the
posterolateral corner structures on MRI can suggest the diagnosis of
posterolateral corner injury when present, leading to improvements in
treatment and functional outcomes for patients in whom the injury was not
clinically suspected—in particular, in those patients with concomitant
ACL or PCL injuries requiring reconstruction. As we discussed earlier, the
most surgically relevant of these injuries are thought to be the grade III
injuries, roughly corresponding to those patients with 3+ ligamentous laxity
[8].
Although there are no precise criteria for the MRI findings required to
diagnose clinically relevant posterolateral instability, or grade III
injuries, visualization of complete tears involving two or more structures of
the posterolateral corner on MRI—in particular, the popliteus
musculotendinous unit, the fibular collateral ligament, or the posterior
lateral joint capsule—should strongly suggest posterolateral corner
injury, especially in the setting of cruciate ligament tears, and should
prompt close clinical evaluation for posterolateral rotatory instability.
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Fig. 1 —Normal fibular collateral ligament in 21-year-old woman.
Sagittal fat-suppressed fast spin-echo T2-weighted image (TR/TE, 4,000/49)
shows normal appearance of fibular collateral ligament (arrow).
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Fig. 2A —Normal fibular collateral ligament in 21-year-old woman.
Consecutive coronal fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament
(arrows).
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Fig. 2B —Normal fibular collateral ligament in 21-year-old woman.
Consecutive coronal fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament
(arrows).
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Fig. 2C —Normal fibular collateral ligament in 21-year-old woman.
Consecutive coronal fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament
(arrows).
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Other imaging signs described to occur with posterolateral corner injuries
include fractures of the fibular styloid process and anterior medial tibial
plateau, contusions of the anterior medial femoral condyle, and lack of
significant joint effusion
[22,
23]. Available data suggest
that injury or partial tear involving only one of these structures, even in
the setting of cruciate ligament tear, is not sufficient to diagnose 3+
posterolateral rotatory instability
[22]. Given that there is
evidence that surgery—in particular primary anatomic repair—is
more successful when performed within 1–4 weeks of the inciting injury
[8,
18,
23], it is important to
perform knee MRI on an urgent basis in patients suspected of having acute
posterolateral corner injury and to notify the orthopedic surgeon as soon as
possible when this diagnosis is suspected.
Posterolateral Corner Anatomy on MRI: Normal and Abnormal
There is great variability of the posterolateral aspect of the knee with
regard to both the individual structures present and their contributions to
the stability of the joint
[24]. In general terms, three
layers of structures comprise the lateral aspect of the knee, not all of which
are visible on MRI. The superficial layer consists of the iliotibial band and
the biceps femoris tendon. The middle layer consists of the quadriceps
retinaculum, patellofemoral ligaments, and patellomeniscal ligament. The
deepest layer consists of the lateral joint capsule along with its attachment
to the edge of the lateral meniscus, the coronary ligament, and the fibular
collateral ligament, which is encompassed by the capsule. The fabellofibular
and arcuate ligaments, which are variable in terms of size and contributions
to stability, are also encompassed by the capsule in this layer
[24]. In addition to these
structures, some authors describe a "deep" complex consisting of
the popliteal muscle–tendon unit, including the popliteofibular
ligament, the arcuate ligament, and the posterolateral joint capsule
[25,
26].
Specific components of the posterolateral corner that can be identified on
MRI, albeit with some variability, are the biceps femoris tendon, the fibular
collateral ligament, the popliteus musculotendinous complex including the
popliteofibular ligament, the fabellofibular ligament, and the arcuate
ligament. In general, these normally low-signal-intensity structures are
defined as injured or sprained when there is thickening and intermediate
signal intensity within the structure on fat-suppressed fast spin-echo
T2-weighted images and as torn when the structure is discontinuous with a
visible gap. Some researchers support the use of a coronal oblique plane of
imaging to improve visualization of some of the finer, obliquely oriented
structures of the posterolateral corner, including the popliteofibular,
arcuate, and fabellofibular ligaments
[27], although this has not
become routine. Recognition of bone marrow changes in the fibular head,
including the so-called "arcuate" fracture that may also be seen
on radiographs is also helpful in diagnosing posterolateral corner injury.
Although posterolateral corner injuries are usually associated with
accompanying injuries to one or both cruciate ligaments, the normal and
abnormal appearances of the ACL and PCL are not reviewed here.
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Fig. 3A —Normal fibular collateral ligament in 21-year-old woman.
Consecutive axial fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament (white
arrows, A and B). Distally, fibular collateral ligament
often joins with biceps femoris tendon (black arrows, A and
B) to form conjoined structure that inserts on lateral aspect of
fibular head (white arrows, C and D).
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Fig. 3B —Normal fibular collateral ligament in 21-year-old woman.
Consecutive axial fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament (white
arrows, A and B). Distally, fibular collateral ligament
often joins with biceps femoris tendon (black arrows, A and
B) to form conjoined structure that inserts on lateral aspect of
fibular head (white arrows, C and D).
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Fig. 3C —Normal fibular collateral ligament in 21-year-old woman.
Consecutive axial fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament (white
arrows, A and B). Distally, fibular collateral ligament
often joins with biceps femoris tendon (black arrows, A and
B) to form conjoined structure that inserts on lateral aspect of
fibular head (white arrows, C and D).
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Fig. 3D —Normal fibular collateral ligament in 21-year-old woman.
Consecutive axial fat-suppressed fast spin-echo T2-weighted images (TR/TE,
4,000/49) show normal appearance of fibular collateral ligament (white
arrows, A and B). Distally, fibular collateral ligament
often joins with biceps femoris tendon (black arrows, A and
B) to form conjoined structure that inserts on lateral aspect of
fibular head (white arrows, C and D).
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The Fibular Collateral Ligament and Biceps Femoris Tendon
The fibular collateral, or true lateral collateral, ligament originates
from a small bone depression just posterior to the lateral femoral epicondyle
and just anterior to the femoral attachment of the lateral head of the
gastrocnemius tendon and extends distally and posteriorly over an oblique
course to insert on the lateral aspect of the fibular head, anterior and
distal to the tip of the fibular styloid process
[28–30].
It is visualized on axial, sagittal, and coronal imaging planes as a
low-signal-intensity structure extending from the lateral aspect of the distal
femur to the proximal fibula (Figs.
1,
2A,
2B,
2C,
3A,
3B,
3C,
3D). Just before its insertion,
the fibular collateral ligament often joins the distal biceps femoris tendon
to form a conjoined structure
[31–33].
Fibular collateral ligament abnormalities are commonly seen in
posterolateral corner injuries and are well depicted by MRI
[28,
34]. Injuries to the fibular
collateral ligament are best visualized on coronal and axial T2-weighted
images and include soft-tissue avulsion off the femoral attachment
(Fig. 4), periligamentous
edema, complete or partial-thickness intrasubstance tears, and soft-tissue or
bone avulsion from the fibular head
[28,
31,
34,
35].
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Fig. 4 —Proximal tear of fibular collateral ligament in 20-year-old
man. Coronal fat-suppressed fast spin-echo T2-weighted image (TR/TE, 3,217/73)
depicts proximal avulsion of fibular collateral ligament (arrows)
from its femoral origin.
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Fig. 5 —Normal biceps femoris tendon insertion in 19-year-old woman.
Coronal fat-suppressed fast spin-echo T2-weighted image (TR/TE, 3,500/50)
shows normal appearance of distal biceps femoris tendon (arrows).
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Fig. 6 —Biceps femoris tendon with tear at insertion in 30-year-old
man. Coronal fat-suppressed T2-weighted image (TR/TE, 4,000/70) shows avulsion
of distal biceps femoris tendon from fibular head (arrow).
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The long and short heads of the biceps femoris tendon typically join above
the knee and course distally to insert predominantly onto the fibular head.
Although both the long and short heads of the biceps femoris tendon have
multiple tendinous and fascial components, not all of these components are
consistently visible as separate structures on MRI
[32,
36]. The direct and anterior
tendinous arms of the long head of the biceps femoris attach to the anterior
and posterolateral aspects of the fibular head, and the direct arm of the
short head of the biceps femoris tendon attaches to the more anteromedial
aspect of the fibular head, with the anterior arm of the short head attaching
along the superolateral edge of the lateral tibia
[36].
On MRI, the insertions of the direct arms of the short and long heads are
often seen as a single low-signal-intensity structure on coronal T2-weighted
images [32]
(Fig. 5), and as mentioned
earlier, the biceps femoris tendon is often joined by the fibular collateral
ligament just above their insertions to form a conjoined insertion
[32,
33,
35]. Injuries to the biceps
femoris tendon are often seen in conjunction with posterolateral corner
injuries; include myotendinous junction tears above the level of the knee and
soft-tissue or bone avulsion from the fibular head
[28,
32]
(Fig. 6); and are best shown
on coronal and axial MR images.
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Fig. 7A —Normal popliteus tendon and muscle in 15-year-old girl. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 3,750/46) show normal
appearance of popliteus tendon (arrows) and muscle belly (black
arrowheads, D) and relationship between popliteus tendon and
fibular collateral ligament (white arrowheads, A and
B).
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Fig. 7B —Normal popliteus tendon and muscle in 15-year-old girl. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 3,750/46) show normal
appearance of popliteus tendon (arrows) and muscle belly (black
arrowheads, D) and relationship between popliteus tendon and
fibular collateral ligament (white arrowheads, A and
B).
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Fig. 7C —Normal popliteus tendon and muscle in 15-year-old girl. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 3,750/46) show normal
appearance of popliteus tendon (arrows) and muscle belly (black
arrowheads, D) and relationship between popliteus tendon and
fibular collateral ligament (white arrowheads, A and
B).
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Fig. 7D —Normal popliteus tendon and muscle in 15-year-old girl. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 3,750/46) show normal
appearance of popliteus tendon (arrows) and muscle belly (black
arrowheads, D) and relationship between popliteus tendon and
fibular collateral ligament (white arrowheads, A and
B).
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The Popliteus Musculotendinous Complex and Popliteofibular Ligament
The popliteus muscle is a major dynamic stabilizer of the lateral knee and
arises from the posterior medial proximal tibia, extending superiorly and
laterally to form a tendon that continues into the joint through the popliteal
hiatus, deep in relation to the fabellofibular and arcuate ligaments
[33,
35–37].
The popliteus tendon has a major insertion at the anterior aspect of the
popliteal sulcus of the lateral femoral condyle, anterior and inferior to the
femoral origin of the fibular collateral ligament
[29]. The popliteus tendon
also sends fibers to insert on the posterior horn of the lateral meniscus
(anteroinferior, posterosuperior, and posteroinferior popliteomeniscal
fascicles) that form a strong attachment to the posterior horn lateral
meniscus around the popliteal hiatus
[28,
37] and prevent the lateral
meniscus from excessive forward displacement during knee extension
[31].
The popliteus tendon and muscle are best seen on axial and coronal images
as low- and intermediate-signal-intensity structures, respectively (Figs.
7A,
7B,
7C,
7D and
8A,
8B,
8C,
8D). Although avulsions at the
femoral insertion may occur, injuries of the popliteus muscle and tendon
usually involve the muscle belly or musculotendinous junction
[37]. Because this area is a
challenge for the arthroscopist to view, the radiologist plays a key role in
making this diagnosis. Partial tears of the musculotendinous junction appear
as amorphous increased signal intensity within the tendon and muscle.
Disruption of fibers, enlargement of the muscle belly, or both may be present
[33] (Figs.
9A and
9B).
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Fig. 8A —Normal popliteus tendon and muscle in 15-year-old girl.
Coronal fast spin-echo T2-weighted images (TR/TE, 3,950/49) depict normal
popliteus tendon (arrows) and popliteus muscle belly
(arrowheads, D).
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Fig. 8B —Normal popliteus tendon and muscle in 15-year-old girl.
Coronal fast spin-echo T2-weighted images (TR/TE, 3,950/49) depict normal
popliteus tendon (arrows) and popliteus muscle belly
(arrowheads, D).
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Fig. 8C —Normal popliteus tendon and muscle in 15-year-old girl.
Coronal fast spin-echo T2-weighted images (TR/TE, 3,950/49) depict normal
popliteus tendon (arrows) and popliteus muscle belly
(arrowheads, D).
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Fig. 8D —Normal popliteus tendon and muscle in 15-year-old girl.
Coronal fast spin-echo T2-weighted images (TR/TE, 3,950/49) depict normal
popliteus tendon (arrows) and popliteus muscle belly
(arrowheads, D).
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Fig. 9A —Popliteus injury in 51-year-old man. Axial fat-suppressed
fast spin-echo T2-weighted image (TR/TE, 4,000/69) shows fluid signal adjacent
to popliteus musculotendinous junction (black arrow), consistent with
partial tear, and increased signal intensity, consistent with injury, within
visualized popliteus muscle belly (white arrow).
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Fig. 9B —Popliteus injury in 51-year-old man. Sagittal fat-suppressed
fast spin-echo T2-weighted image (4,000/50) shows fluid signal extending along
margins of popliteus muscle belly (arrows), consistent with partial
tear.
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The existence of a consistent attachment of the popliteal tendon to the
fibular head is well established and is called the popliteofibular ligament.
This structure originates near the popliteus musculotendinous junction and
courses distally and laterally to attach to the medial aspect of the fibular
styloid process and is thought to be present in most knees as an important
static stabilizer of the posterolateral corner
[10,
25,
38,
39]. The popliteofibular
ligament originates from the popliteus tendon just distal to the
popliteomeniscal fascicles and proximal to the popliteus musculotendinous
junction and extends distally to insert on the anterior downslope of the
medial aspect of the fibular styloid process, near to the tibiofibular joint
[28–30].
This ligament is a short, strong tendinous band that is as wide as or even
wider than the popliteus tendon
[39]. Despite this fact and
the fact that this ligament is present in most, if not all, knees, the
popliteofibular ligament is only variably visualized on MRI
[34,
36]. The ligament can
sometimes be seen as a small low-signal-intensity structure on coronal
(Fig. 10) and sagittal
(Fig. 11) images and can
occasionally be followed over several images in the axial imaging plane (Fig.
12A,
12B,
12C,
12D). In one study, the use of
a coronal oblique imaging plane compared with a standard coronal plane
improved visualization of this ligament from 8% to 53% of knees
[27]. Injury may be detected
as increased signal within the ligament, discontinuity, or avulsion of the
ligament off of the fibular styloid (Fig.
13) and is best visualized on coronal or coronal oblique views
[28].
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Fig. 10 —Intact popliteofibular ligament in 75-year-old woman. Coronal
fat-suppressed fast spin-echo T2-weighted image (TR/TE, 4,117/69) shows
prominent intact popliteofibular ligament (arrow) extending from
popliteus tendon to fibular styloid process.
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Fig. 11 —Intact popliteofibular ligament in 60-year-old woman.
Sagittal fat-suppressed fast spin-echo T2-weighted image (TR/TE, 3,900/50)
depicts popliteofibular ligament (arrow) inserting on fibular styloid
process (arrowhead).
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Fig. 12A —Normal popliteofibular ligament in 60-year-old woman. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 4,350/50) show intact
popliteofibular ligament (white arrows, B–D) and its
relationship to popliteus tendon (white arrowheads), fibular
collateral ligament (black arrows, A and B), and biceps
femoris tendon (black arrowheads).
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Fig. 12B —Normal popliteofibular ligament in 60-year-old woman. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 4,350/50) show intact
popliteofibular ligament (white arrows, B–D) and its
relationship to popliteus tendon (white arrowheads), fibular
collateral ligament (black arrows, A and B), and biceps
femoris tendon (black arrowheads).
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Fig. 12C —Normal popliteofibular ligament in 60-year-old woman. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 4,350/50) show intact
popliteofibular ligament (white arrows, B–D) and its
relationship to popliteus tendon (white arrowheads), fibular
collateral ligament (black arrows, A and B), and biceps
femoris tendon (black arrowheads).
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Fig. 12D —Normal popliteofibular ligament in 60-year-old woman. Axial
fat-suppressed fast spin-echo T2-weighted images (TR/TE, 4,350/50) show intact
popliteofibular ligament (white arrows, B–D) and its
relationship to popliteus tendon (white arrowheads), fibular
collateral ligament (black arrows, A and B), and biceps
femoris tendon (black arrowheads).
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Fig. 13 —Torn popliteofibular ligament in 27-year-old man. Coronal
fat-suppressed fast spin-echo T2-weighted image (TR/TE, 3,500/65) shows
avulsion of distal popliteofibular ligament (arrow) from fibular
styloid process (arrowhead).
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The Lateral Head of the Gastrocnemius Tendon and the Fabellofibular Ligament
The tendon of the lateral head gastrocnemius is located at the far lateral
aspect of the lateral gastrocnemius muscle–tendon unit, and injuries to
this structure are rare [28].
The fabella is a variably present sesamoid bone in the lateral head
gastrocnemius tendon, and the fabellofibular ligament is also variably
present, found in approximately 40% of knees in two anatomic studies
[30,
40] and identified on
approximately one third of MRI examinations in one study
[35]. This ligament is a
thickening of the distal edge of the capsular arm of the short head biceps
femoris muscle [28].
Proximally, its origin is from the lateral margin of the fabella, if a fabella
is present, or from the posterior aspect of the supracondylar process of the
femur [30]. Distally, the
fabellofibular ligament inserts on the posterior and lateral edges of the
fibular styloid process, anterolateral to the insertion of the popliteofibular
ligament [30,
36].
The fabellofibular ligament is occasionally visualized on MRI and is best
seen on coronal T2-weighted images as a low-signal structure located
posteriorly with respect to the fibular collateral ligament
[35]
(Fig. 14). MR evidence of
injury includes distal avulsion from the fibular styloid process, which can be
seen concurrently with avulsion of the direct arm of the short head of the
biceps femoris tendon, thickening, and increased signal intensity
[28]. Because of the
infrequency with which this ligament is well visualized in even noninjured
knees, it is not as useful as some of the other structures in the evaluation
for posterolateral corner injuries with MRI.
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Fig. 14 —Normal fabellofibular ligament in 43-year-old woman. Coronal
fat-suppressed fast spin-echo T2-weighted image (TR/TE, 3,450/69) depicts
normal fabellofibular ligament (arrow), which extends from fabella
(arrowhead) to fibular styloid process.
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The Arcuate Ligament
The arcuate ligament is a variably present Y-shaped structure with medial
and lateral limbs, both of which insert distally at the apex of the fibular
styloid process just anterior to the fabellofibular ligament. Several anatomic
series have reported the presence of at least one of the limbs as between
47.9% and 71% of knees [30,
36,
40,
41]. The lateral, or upright,
limb extends superiorly along the joint capsule to the lateral femoral
condyle, and the medial, or arcuate, limb extends superomedially, over the
popliteus muscle, to merge with the posterior capsule
[31,
33,
36].
The arcuate ligament is, in general, difficult to visualize on MRI
[35,
36]. However, it can be
thought of as a thickening of the posterolateral capsule, a portion of which
forms the bowed roof of the popliteal hiatus
[31,
36], and can be seen as a
low-signal structure on axial images (Fig.
15). Inspection of the posterolateral joint capsule on axial MR
images at the level of the joint line may reveal gross disruption
(Fig. 16), implying injury to
or a tear of the arcuate ligament. The medial limb occasionally can be
visualized immediately posterior to the popliteus tendon, just below the level
of the popliteal hiatus, on sagittal images
[36]. As we mentioned earlier,
some authors advocate the use of a coronal oblique imaging plane to improve
visualization of this ligament complex, with visualization using that plane in
46% of patients in one series
[27]. Some studies have noted
an association between posterolateral corner injuries and a lack of
significant joint effusion on MRI; this association is thought to be due to
the presence of disruption of the posterior lateral joint capsule
[23].
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Fig. 16 —Torn arcuate ligament in 53-year-old man. Axial
fat-suppressed fast spin-echo T2-weighted image (TR/TE, 4,000/49) shows tear
of posterolateral joint capsule (arrowheads) at level of joint, which
is consistent with arcuate ligament tear.
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Fig. 18 —Arcuate fracture in 55-year-old man. Sagittal fat-suppressed
fast spin-echo T2-weighted image (TR/TE, 4,000/75) depicts avulsion fracture
of fibular styloid process (arrow), which is also called
"arcuate" fracture.
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Fig. 19 —Fibular head edema in 34-year-old woman. Coronal
fat-suppressed fast spin-echo T2-weighted image (TR/TE, 4,000/70) shows edema
in fibular head (arrowheads). There is increased signal, consistent
with injury, in distal biceps femoris tendon (arrow).
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The "Arcuate" Fracture
The popliteofibular, fabellofibular, and arcuate ligaments attach distally
to the fibular styloid process. An avulsion fracture of this styloid process,
the so-called "arcuate" sign, often indicates injury to one or
more of these ligaments and thus to the posterolateral corner. This fracture
may be seen on anteroposterior or lateral knee radiographs as a small
displaced bone fragment [42,
43]
(Fig. 17). On MRI, the
fracture fragment may be more difficult to identify
(Fig. 18). The term
"arcuate" sign has also been applied to larger avulsion fractures
of the fibular head in the region of the conjoined structure insertion
[43,
44]. Even in cases without an
evident fracture, MRI may reveal edema either localized to the fibular styloid
process, suggesting injury to the popliteofibular, fabellofibular, or arcuate
ligament, or more diffuse edema in the lateral aspect of the fibular head,
suggesting injury to the fibular collateral ligament, biceps femoris tendon,
or both [43]
(Fig. 19).
Anterior Medial Tibial Margin Fracture
Fractures of the peripheral anteromedial tibial plateau have also been
described to occur in the setting of posterolateral corner injury. One study
detected this relatively uncommon fracture in six of 16 knees with clinical
and MRI evidence of posterolateral corner injury
[22]. In two of these cases,
the fracture resulted from hyperextension with forced varus angulation, and in
the remaining four, the fracture resulted from a direct blow to the
anteromedial tibia with the knee flexed
[22]. Cohen et al.
[45] suggested a mechanism of
varus rotation and posterior tibial translation that results in disruption of
both the posterolateral corner and PCL and allows the anterior medial femoral
condyle to impinge on the anteromedial tibial plateau, resulting in fracture
of the anteromedial tibial rim. Given its association with posterolateral
corner injuries, an anteromedial tibial plateau fracture seen on either
radiography or MRI should prompt close clinical and imaging evaluations of the
posterolateral corner structures.
Bone Marrow Contusion
MRI fluid-sensitive sequences are superb in detecting bone marrow
contusions. In one series of six patients with acute posterolateral knee
injuries, a characteristic bone marrow contusion on the anteromedial femoral
condyle was seen in the five patients with complete posterolateral complex
disruption [23]. This
contusion is best seen as increased signal intensity in the subchondral bone
marrow of the weight-bearing surface of the anterior aspect of the medial
femoral condyle on sagittal T2-weighted images and is thought to be due to a
hyperextension varus type of injury
[23,
32]
(Fig. 20).
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Fig. 20 —Medial femoral condyle contusion in 48-year-old man. Sagittal
fat-suppressed fast spin-echo T2-weighted image (TR/TE, 3,750/69) shows
increased signal intensity within bone marrow of anterior aspect of medial
femoral condyle (arrowheads), consistent with hyperextension varus
contusion.
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Conclusion
Being aware of the normal and abnormal MRI appearances of the structures of
the posterolateral corner of the knee and of the patterns of injury often seen
in patients with posterolateral corner rotatory instability will help
radiologists suggest the diagnosis of posterolateral corner injury even when
not clinically suspected. Although we may not be able to accurately define
when instability exists with imaging alone, available data indicate that tears
of two or more of the posterolateral structures—most importantly, the
fibular collateral ligament, the popliteus musculotendinous unit including the
popliteofibular ligament, and the posterolateral joint capsule—suggest
the diagnosis of a high-grade posterolateral corner injury and should direct
the orthopedic surgeon to carefully examine for posterolateral corner rotatory
instability in the pre- or perioperative setting because urgent repair or
reconstruction is associated with better functional outcomes. This diagnosis
is especially important in the setting of combined injuries because
unrecognized and unaddressed posterolateral corner injuries may contribute
significantly to ACL and PCL graft failure. Unrecognized and untreated
high-grade posterolateral corner injuries have also been shown to lead to
significant posttraumatic osteoarthritis in the affected knee
[8,
15]. Conversely, injury to
only one of the posterolateral structures supports the presence of a grade I
or II injury, which, depending on the presence of associated injuries, can
often be successfully treated nonsurgically.
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Abstract
Introduction
