is a Resident and
was a Musculoskeletal Fellow in the Department of Radiology and
Radiological Science, Medical University of South Carolina. Dr.
Hobbs is now in private practice in Augusta, GA.
is a Professor of Radiology and Orthopedics and the Director of
the HollingsCancer Center Breast Imaging Program;
is an Assistant Professor of Orthopaedic Surgery,
is a Professor of Radiology and the Director of Musculoskeletal
Radiology, Medical University of South Carolina, Charleston,
Injuries of the knee are common. At our institution, the Medical
University of South Carolina, trauma and sports-related activities
are the most frequent causes of knee injuries. Secondary to their
role in maintaining stability, the ligaments of the knee are
commonly involved in these injuries. To prevent long-term sequelae,
early diagnosis and treatment-whether conservative or surgical-are
key in planning management of these injuries. Because of its
excellent soft-tissue contrast, magnetic resonance imaging (MRI)
has proven very useful for identifying these important structures.
In the immediate postinjury period, clinical assessment of the knee
is unreliable, which accentuates the importance of MRI as a
This article reviews the MRI appearance of the knee ligaments in
their normal and injured states.
Basic imaging principles
Because of the biochemical composition of ligaments, the tightly
bound hydrogen molecules are unavailable to participate in the
magnetic moment of MRI. Therefore, under normal circumstances,
ligaments show low signal intensity on all pulse sequences. Injury
allows loosely bound hydrogen atoms as well as
infiltrating edema and hemorrhage to produce signal on
the various pulse sequences used to evaluate these structures.
MRI protocols for the knee vary by magnet and interpreter
preference. Protocols should include sequences obtained in the
axial, coronal, and sagittal planes, with at least one
fluid-sensitive sequence. In general, a high
field-strength magnet and a dedicated knee or extremity
coil is preferred, but adequate evaluation can be attained with
mid- and low-field magnets. Patients are imaged supine
with the knee in slight external rotation both for better
visualization of the anterior cruciate ligament (ACL) and patient
comfort. The authors do not use intravenous contrast unless
evaluating for neoplasm or infection. Intra-articular contrast is
used primarily in the patient who has had prior surgical meniscal
In general, the collateral ligaments are best evaluated in the
coronal plane, and the cruciate ligaments and extensor mechanism
are best evaluated in the sagittal plane. The coronal plane is also
an important projection for the cruciate ligaments. However,
visualization of all structures in all three imaging planes is
necessary for a complete evaluation, which helps to avoid
The anterior cruciate ligament
The ACL extends in an inferior, anterior, and medial direction
from its origin on the inner surface of the posterior, lateral
femoral condyle to its insertion on the anterior tibial plateau
anterior to the tibial spines between the attachments of the medial
and lateral menisci and beneath the transverse ligament.
It consists of 2 distinct bands—the anteromedial and posterolateral
bundles—according to their distal attachment's relationship to the
tibial spine. These bands function to resist anterior displacement
of the tibia and hyperextension, respectively. Also, because of
these two separate components, the normal ACL is taut throughout
the full range of knee motion.
Further, the posterolateral bundle provides an element of
The normal ACL is a low-signal-intensity band that roughly
parallels the intercondylar roof (Blumensaat’s line). Normal
interspersed fat and connective tissue give the ACL a striated
appearance that should not be mistaken for pathology. Usually, even
with optimal positioning, the ACL is visualized on ≥2 contiguous
sagittal images rather than on a single image (Figures 1 and
The ACL is commonly injured from excessive valgus stress, also
called the “pivot-shift” mechanism. The classic example of this is
the clipping type of injury seen in American football. MRI signs of
ACL injury are a pseudomass in the normal location of the ACL,
frank discontinuity of the ligament, wavy or irregular course, or
avulsion at either the femoral origin or tibial insertion. In the
authors’ experience, the mid-substance “pseudomass” appearance
resulting from edema and hemorrhage is the most common
finding of an acute ACL tear. Occasionally, the
“pseudomass” appearance can be caused by partial volume averaging
on the sagittal image. Apparent discontinuity on sagittal images
can also lead to a misdiagnosis of an ACL tear. Correlation with
axial and coronal images is imperative to help radiologists avoid
these imaging pitfalls.
Two examples of complete ACL tears are shown in Figure 3.
Secondary signs of an ACL tear include anterior tibial
translation and abnormal curvature of the posterior cruciate
ligament (PCL), which are related findings, with the
latter secondary to the former. However, buckling or increased
curvature of the PCL may also be seen with hyperextension of the
knee in the setting of a normal ACL. “Kissing contusions,” a
commonly seen secondary sign of ACL injury caused by the previously
mentioned pivot-shift mechanism of injury, occur on the posterior
aspect of the tibial plateau and the mid to anterior aspect of the
femoral condyle (Figure 4). Medial and lateral meniscal tears
usually involving the posterior horns, and medial collateral
ligament (MCL) sprains and tears are also commonly associated
findings. The Segond fracture, an avulsion fracture of
the lateral joint capsule at its insertion onto the lateral tibial
plateau, is associated with an ACL tear in >90% of cases when it
is present (Figure 5).
When any of these findings are seen at MRI, a careful
assessment of the ACL in all three imaging planes is essential.
In skeletally immature individuals, the injury pattern is
somewhat different, with tibial spine avulsions and partial ACL
This pattern is most likely secondary to the greater ability of
bone to deform under stress in the immature skeleton and the lack
of osseous fusion of the tibial spine prior to physeal closure. As
the skeleton matures, the patterns of ACL injury approach that seen
Partial tears of the ACL can be hard to appreciate on MRI. Focal
or diffuse signal alteration within an intact ligament, abnormal
thickening or thinning of the ligament with abnormal
intra-substance signal, or abnormal angulation of the ligament all
can represent a partial tear (Figure 6). The importance and
treatment of partial ACL tears is still debated, but nonetheless,
the diagnosis should be sought and reported when seen
Recently, evidence suggests that patients with isolated ACL
bundle tears, either anteromedial or posterolateral,
benefit from single bundle repairs.
Additionally, complete tears may benefit from so-called
double-bundle reconstruction, especially in light of the additional
rotational stability provided by an intact posterolateral bundle.
Isolated tears of the posterolateral bundle are difficult
to appreciate using standard arthroscopic ports.
On MRI, identification of the individual bundles is
complicated by the oblique course of the ACL on all imaging planes.
Careful inspection of the ACL in all projections may allow
identification of an isolated bundle abnormality and help
direct the orthopedic surgeon to the area at arthroscopy. As more
orthopedic surgeons are performing these operations,
identification of isolated bundle tears can be of great
service to the patient (Figure 7).
The appearance of chronic ACL tears is highly variable. The
fibrosis secondary to healing of the ligament results in
signal characteristics similar to that of a normal ligament. The
most specific findings of a chronic tear are an
abnormal course or angulation of the ACL without other
abnormalities normally associated with an acute tear.
In some cases of complete tears, the ACL will settle on top of the
posterior cruciate ligament and, over time, will adhere by
fibrosis to this ligament (Figure 8). The edema seen
around and within an acutely torn ACL will be absent in a chronic
Posterior cruciate ligament
The PCL extends in an inferior, posterior, and lateral direction
from its origin on the inner surface of the anterior aspect of the
medial femoral condyle to its insertion on the far posterior aspect
of the tibial plateau. Like the ACL, the PCL is also composed of 2
bundles: the anterolateral and posteromedial bundles. The
significance of these bundles in terms of reconstruction
is less clear than for the ACL.
The PCL functions to resist posterior translation of the tibia with
respect to the femur, and portions of this structure are taut
throughout the entire range of motion of the knee. The normal PCL
lacks the striations of the ACL and can often be seen in its
entirety on a single sagittal image (Figure 9). The meniscofemoral
ligaments are intimately associated with the PCL as they pass from
the posterior horn of the lateral meniscus to the medial femoral
condyle. On coronal imaging, this structure can be mistaken for
abnormal thickening of the PCL, but correlation with sagittal
images can clarify this finding.
The PCL is less frequently injured than the ACL, and such
injuries require a greater force. Therefore, there is often
associated injury to other structures of the knee (usually the ACL
and MCL) when PCL injuries are encountered. Mechanisms of injury to
the PCL include hyperflexion, hyperextension, and
dislocation. Posterior cruciate ligament tears are often
difficult to diagnose clinically in the acute setting and
can be difficult to evaluate at arthroscopy secondary to
its far posterior location. Therefore, MRI is critical in the
diagnosis of this entity.
Signs of injury include frank disruption of the ligament,
diffuse midsubstance widening with increased signal intensity on
T1- and T2-weighted images, or an avulsion of either its femoral
origin or tibial insertion.
Partial tears are recognized by abnormal signal intensity within an
intact ligament. Of note, if the PCL appears higher in signal
intensity than the ACL on any imaging sequence, it is considered
abnormal. See Figures 10 and 11 for an example of a partial and a
Medial collateral ligament
The MCL arises from the medial femoral condyle approximately 5
cm above the joint line and extends to insert on the medial tibia
approximately 6 to 7 cm below the joint line posterior to the
insertion of the pes anserinus.
The MCL actually consists of 2 layers separated by a small bursae
and minimal peribursal fat. The superficial MCL is the
true tibial collateral ligament. The deep layer is contiguous with
the meniscofemoral and meniscotibial ligaments of the medial
meniscus. The MCL functions as the chief valgus stabilizer of the
knee and is therefore most commonly injured with abnormal valgus
angulation of the knee.
As mentioned, the MCL is best visualized in the coronal plane,
where it is normally seen as a linear, low-signal-intensity
structure (Figure 12). Meniscal injuries are graded as 1 through 3
based on imaging findings. Adjacent edema with no signal
abnormality within the ligament is characterized as a sprain, or
grade 1 injury (Figure 13). More extensive edema with abnormal
signal intensity, thickening, or thinning of the ligament
signifies grade 2 injury, or partial tear (Figure 14),
and complete disruption of the ligament or its attachments
qualifies as a grade 3 injury (Figure 15).
Injuries of the MCL are commonly associated with medial meniscal
tears and meniscocapsular separation. Meniscocapsular separation is
defined as disruption of the normal tight attachment of
the MCL to the medial meniscus and joint capsule. It is recognized
on MRI as fluid signal interspersed between the MCL and
medial meniscus. A potential pitfall in this diagnosis is
fluid in the deep MCL bursae separating the
superficial and deep components as mentioned above. The
characteristic appearance and location of this bursa helps the
radiologist avoid this error.
Lateral collateralligamentous complex
Laterally, the knee is stabilized by a group of structures,
collectively known as the lateral collateral ligamentous complex
(LCLC), which resist varus stress and external rotation.
The most important of these structures, from anterior to posterior,
are the iliotibial band (ITB), a continuation of the tensor fascia
lata inserting on Gerdy’s tubercle on the anterolateral tibia
(Figure 16), the fibular or true lateral collateral
ligament (Figure 17), and the tendon of the biceps femoris muscle
that converges with the fibular collateral ligament to
form the conjoined tendon prior to inserting on the
fibular head (Figures 18 and 19). The popliteus tendon
also contributes to lateral stability and should be evaluated for
injury on knee MRI, but will not be discussed further in this
Because of the numerous contributors to the lateral complex,
increased force is required to cause injury in this location. MRI
findings of injury here are similar to those seen with
the MCL: surrounding soft tissue edema and hemorrhage, increased
signal intensity within these normally low-signal-intensity
structures, or frank discontinuity of the individual components.
Figure 20 shows a complete tear of the conjoined tendon.
Inflammation adjacent to the structures of the LCL can
also have clinical significance. The most common of these
conditions involves inflammation adjacent to the
iliotibial band (ITB) and distension of an adjacent bursae
associated with ITB syndrome.
The transverse meniscal ligament and the meniscofemoral
ligaments are other commonly visualized knee ligaments on MRI. The
transverse meniscal ligament is a thin, fibrous band that
connects the anterior horns of the menisci.
The ligament is occasionally misdiagnosed on sagittal images as a
tear of either of the anterior meniscal horns where it inserts on
these structures. Knowledge of this structure's normal ap pearance
and following it across the joint space on contiguous sagittal
images will help radiologists avoid this pitfall (Figure 21).
The meniscofemoral ligaments have already been briefly
mentioned in terms of their relationship to the PCL. These
structures extend from the posterior horn of the lateral meniscus
superomedially to the inner aspect of the medial femoral condyle
(Figure 22). If the ligament is anterior to the PCL, it is referred
Humphry ’s ligament
(Figure 23), and if it is posterior to the PCL, it is referred to
Wrisberg ’s ligament
(Figure 24). Rarely, the ligament will bifurcate, surrounding the
PCL and giving the appearance of both.
Some individuals do not have an identifiable
The function of the meniscofemoral ligament is to stabilize the
lateral meniscus against the pull of the popliteus muscle.
In the PCL-deficient knee, the meniscofemoral ligament
resists posterior drawer.
Again, these structures are important primarily in that knowing
their normal appearance and position can help a radiologist avoid
pitfalls in diagnosis. A prominent Humphry ’s ligament can
occasionally mimic a flipped meniscal fragment from a
bucket-handle tear or a loose body in the intercondylar notch.
Also, similar to the transverse meniscal ligament, the attachment
of the menisco femoral ligament to the lateral meniscus can
occasionally mimic a meniscal tear.
Injury to the knee ligaments is relatively common. MRI is the
best imaging technique available to identify these abnormalities
and to plan arthroscopic or open surgical repair. Knowledge of the
normal MRI appearance of the major knee ligaments and the most
common findings seen following injury to these structures
is critical for the interpreting radiologist to be a successful