This article in this supplement to Applied Radiology describes the MR evaluation of internal derangement of the knee (tears of the menisci and the cruciate and collateral ligaments), osteochondral abnormalities (chondromalacia, osteoarthritis, and osteochondral defects), synovial cysts, and bone bruises.
is a Professor of Radiology and the Chairman of the Department of
Radiology, University of California San Diego Medical Center, San
This article has been adapted from a newsletter publication:
Bradley WG. Knee pain.
Applied Imaging: Applications in MRI.
2001;2(1):1-4. Material reprinted with permission of Anderson
Although plain radiographic films have traditionally been the
first diagnostic imaging study performed in the evaluation of the
painful knee, today they are useful only for evaluating joint space
narrowing, alignment, and major trauma. Over the past 20 years,
magnetic resonance imaging (MRI) has become the premier, first-line
imaging study that should be performed in the evaluation of the
painful knee. This article in this supplement to
describes the MR evaluation of internal derangement of the knee
(tears of the menisci and the cruciate and collateral ligaments),
osteochondral abnormalities (chondromalacia, osteoarthritis, and
osteochondral defects), synovial cysts, and bone bruises.
In order to evaluate internal derangement adequately, the knee
must be scanned in the axial, coronal, and sagittal planes using
thin sections (3-mm thick) with a combination of T1- and
T2-weighted techniques as well as at least one 5-mm thick short tau
inversion recovery (STIR) or T2-weighted image with fat
The anterior and posterior horns of the medial and lateral
menisci appear as wedge-shaped, low-intensity structures pointing
toward each other on sagittal and coronal views.
High signal within the meniscus, extending to the superior or
inferior articular surface (Figures 1 and 2), constitutes a
meniscal tear. This is best seen on images with narrowed "meniscal"
windows (Figure 1). A caveat: In patients who have had prior knee
surgery, it can be difficult to distinguish an old scar from an
acute tear without injecting gadolinium into the joint space.
Internal meniscal signal that does not extend to the articular
surfaces or that extends only to the base of the "wedge" does not
constitute a tear. In children, this represents the normal vascular
pedicle of the meniscus; in adults, it represents myxoid
degeneration, ie, normal aging.
Horizontal-oblique tears are considered the results of myxoid
degeneration and are called "degenerative" tears. Those oriented in
the vertical direction, which involve both the superior and
inferior articular surfaces, are considered "traumatic" tears.
Tears do not need to be seen in both the sagittal and coronal
planes, as they are only visualized when they are perpendicular to
the plane of the section. In fact, tears running oblique to both
the coronal and sagittal planes may well be visualized only by a
notch on the articular surface. This "notch sign"
has served us very well over the last decade in diagnosing subtle
meniscal tears (Figure 3). Truncation of the tip of the meniscus
represents a "parrot-beak" tear (ie, fraying of the free margin of
Occasionally, a large piece of meniscal fibrocartilage will
become detached from its capsular attachment and flip back-to-front
or centrally into the intercondylar notch. Clinically, these
patients present with a locked knee. These "bucket-handle" or
are recognized on the basis of two findings: An absence of the
normal meniscus in its expected position and abnormal
meniscal-intensity material in an unexpected location (Figure 4).
Bucket-handle tears flipping from posterior to anterior appear as 2
arrowheads pointing posteriorly in the position of the anterior
horn of the meniscus, with nothing at all in the expected position
of the posterior horn. This pattern is more common for
bucket-handle tears of the lateral meniscus. Tears flipping from
the periphery into the intracondylar region of the knee appear as
an extra structure adjacent to the cruciate ligaments, with an
absence of meniscal signal in the expected position of the
posterior horn. Typically, this pattern is seen with bucket-handle
tears of the medial meniscus. When the displaced meniscal fragment
lies parallel to the posterior cruciate ligament (PCL), it may
produce the "double PCL sign."
The anterior cruciate ligament (ACL) may be seen on sagittal
images (Figure 5); however, it is always seen as an upside-down "V"
on coronal images (Figure 6). The ACL is lateral to the rounded
dark PCL. (If the coronal plane does not contain the fibula to
indicate the lateral aspect, the adductor tubercle on the medial
aspect of the femur [Figure 6] can usually be seen.) The ACL is
most commonly injured by valgus stress to the knee while the leg is
in external rotation, eg, a "clipping" injury in football or
getting hit laterally coming down from a layup in basketball.
MR studies performed shortly after the injury will fail to reveal
the separate fascicles of the ACL, which are replaced by an
ill-defined mass of tissue that is bright on T2-weighted images
(reflecting edema). MR imaging performed much later in a
chronically "ACL-defi-cient" knee also fails to show the
well-defined fibers of the ACL (Figure 7), but lacks the T2
hyperintensity of an acute tear.
With the medial knee joint distraction resulting from valgus
stress, the medial collateral ligament (MCL) as well as the medial
meniscus may also be torn (this is known as O'Donoghue's triad).
Acute injuries of the MCL are best evaluated in the coronal plane,
revealing discontinuity of the normally black line (Figure 6) that
runs along the medial aspect of the knee. With acute injuries,
there is also a linear fluid collection (which is bright on a
T2-weighted image) superficial to the torn MCL (Figure 8). This is
usually associated with reticulation of the overlying subcutaneous
tissues, indicating soft tissue swelling. Chronic MCL tears result
in thickening without associated fluid.
In addition to the medial distraction resulting from valgus
stress during an ACL injury, there is lateral impaction. This may
lead to bone marrow contusions of both the femoral and tibial
surfaces of the lateral compartment (Figures 8 and 9). At the time
of the MR examination, the leg is in a neutral position (rather
than being externally rotated as it was at the time of ACL injury);
therefore, the "kissing contusions" are not directly opposite each
Bone marrow contusions (also called "bone bruises" and
"trabecular fractures") cannot be seen on plain films and can be a
major source of pain.
They should be suspected in the patient with an acutely injured
knee without a meniscal or ligamentous injury in the absence of
plain-film findings of fracture. Because their water content is
higher than that of adjacent fatty bone marrow, these contusions
are best detected on fat-suppressed T2-weighted images.
At high field, we use fat-saturated T2-weighted fast spin-echo
images and, at any field strength, STIR images.
In a study of 73 consecutive patients with suspected
sports-related internal derangement of the knee, fat-saturated
T2-weighted fast spin-echo imaging detected ≥1 bone bruises in 30%
of patients in whom lesions were not seen on conventional MRI.
In evaluating bone marrow contusions, it is important to
determine if they extend to the articular surface, as this is
usually an indication that the athlete must rest for 30 days.
Continued activity in this setting could convert a bone bruise with
intact cartilage (type I) (Figure 9) to one in which the cartilage
becomes disrupted (type II) (Figure 10A), leading to early
Bone bruises can be seen whenever abnormal forces are applied to
normal tissue or essentially normal forces are applied to weakened
tissue, eg, osteoporotic bones in middle-aged "weekend warriors."
Thus, milder versions of the same forces that lead to tibial
plateau fractures will lead to bone marrow contusions, resulting in
normal plain films and high signal on STIR or fat-saturated
T2-weighted fast spin-echo images.
An interesting pattern of bone marrow contusion can be seen with
traumatic dislocation of the patella. After the patella dislocates
laterally when the knee is in hyperextension, the quadriceps
contracts, jamming the medial patellar facet against the lateral
femoral condyle, resulting in bone marrow edema on both surfaces
(Figure 10B). Usually, this is also associated with disruption of
the black line of the medial retinaculum, which, acutely, may have
associated bleeding or edema.
Such injuries are seen in football and basketball players who come
down on a hyperextended knee.
Bone marrow contusions or fractures associated with cortical
disruption can leak marrow into the joint space, leading to a
lipohemarthrosis. Since sagittal images are acquired with the
patient supine, fat rises to the top, joint fluid is in the middle,
and intact red cells are at the bottom (Figure 11).
Lesions of the PCL and the lateral collateral ligament are much
less common than their anterior counterparts. The PCL is recognized
as a dark C-shaped structure on sagittal images and is visualized
as a dark, rounded structure seen en face on coronal images (Figure
The coronal images are best for visualizing partial PCL tears
(which show increased signal in a normally black structure), while
the sagittal view may better illustrate complete disruption (Figure
The earliest of the osteochondral abnormalities is simple
cartilaginous thinning, as seen in chondromalacia patellae
and osteoarthritis. Although cartilage can be evaluated on routine
MR sequences, we tend to use a higher resolution (512 × 512 matrix)
thin-slice (1.5-mm sections) gradient-echo technique when subtle
abnormalities of cartilage are being investigated.
With advancing osteoarthritis, the articular cartilage is
completely denuded and subchondral sclerosis can be seen as low
intensity on all sequences. With continued bone-on-bone irritation,
fibrovascular reaction can lead to bone marrow edema, which appears
bright on STIR or fat-saturated T2-weighted fast spin-echo
Osteochondral defects arise from varying degrees of traumatic
and ischemic insults to the articular surface of the knee (Figure
12). Early osteochondral defects appear as localized areas of
rounded bone marrow edema without frank fluid collection.
With advancing disease, a ring of frank fluid can be seen
surrounding the osteochondral defect, although the overlying
articular cartilage remains intact. Subsequently, the articular
cartilage also becomes disrupted and, eventually, the fragment
separates, becoming "free" within the joint space, leaving a
Cystic disease of the knee
Synovial cysts are very common in the knee given the large
number of synovium-lined surfaces. The most common is a Baker's
cyst (Figure 13), which generally results from a knee effusion that
forces synovium through the space between the tendons of the medial
head of the gastrocnemius and the semimembranosus.
Baker's cysts may persist following resolution of the knee effusion
and are a cause of medial joint line tenderness.
Another, less common cause of medial joint line tenderness is
which is inflammation of the synovium of the pes anserinus (the
confluence of tendons from the sartorius, gracilis, and
Synovial cysts can be seen in conjunction with torn menisci,
more commonly involving the lateral than the medial meniscus. These
cysts appear to result from a ball valve mechanism that allows
fluid to flow from the torn meniscus into the cyst but not in the
Intramedullary abnormalities of the knee
Excluding bone bruises, most intra-medullary abnormalities of
the knee can be seen on plain films. Tumors (eg, osteosarcomas and
metastases) are rare and result in focal destruction of bone with
an associated mass. Bone infarcts appear as metaphyseal rings of
hyperintensity on T2-weighted images, corresponding to the
peripheral ring of calcification noted on plain films. Enchondromas
(which are usually asymptomatic) have a spiculated appearance on
T2-weighted images, corresponding to the "popcorn" calcification
noted on plain film. Frank fractures appear as linear areas of
hyperintense fluid on T2-weighted images, with surrounding less
intense areas of bone marrow edema. Frequently, cortical disruption
can be seen on the tomographic, thin MR slices, but cannot be
appreciated on projectional plain films.
Over the past 2 decades, MRI has become the preferred imaging
technique for the evaluation of the painful knee following a sports
injury. It can detect soft tissue abnormalities (meniscal and
cruciate/collateral ligament tears) and fractures that cannot be
detected by plain film. These findings are critical for the
therapeutic decisions to be made by the orthopedic surgeon.
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Peyman Borghei, MD,
Research Fellow of Musculoskeletal Radiology;
Maryam Golshan Momeni, MD,
Research Fellow of Musculoskeletal Radiology; and
Jamshid Tehranzadeh, MD,
Professor of Radiology and Orthopaedics, the Departments of
Radiological Sciences and Pathology, University of California
(Irvine), Irvine, CA.
A 19-year-old male kickboxer presented with a 3-week history of
bilateral foot pain. Physical examination revealed general
tenderness of both feet. His pain worsened while walking and was
aggravated by kicking. He had taken nonprescription analgesics to
alleviate his pain. Magnetic resonance imaging (MRI) of the foot
was performed for further evaluation (Figures 1 through 6) and
established the definite diagnosis.
MR images show diffuse bone marrow edema of multiple tarsal
bones, of both distal tibias, and of the right fibula. There are
lines of low signal intensity on the T1-weighted images (T1WI) and
surrounding diffuse marrow edema on the selective partial inversion
recovery (SPIR) images in both calcanei, tali, and navicular bones.
These are compatible with fatigue fractures (Figures 1 through 4).
In addition, fatigue fractures of the left cuboid (Figure 5) and
bone contusions of the right lateral and middle cuneiforms are seen
on the axial MR image (Figure 6).
Multiple tarsal bone fatigue fractures and bone contusions
Stress-related injury to bone is common in athletes. These
injuries comprise a wide spectrum ranging from bone edema to
fatigue fracture and are caused by intense and repetitive activity
without adequate periods of rest.
Some fatigue fractures have sports specificity, such as those in
the coracoid process in trap shooters,
those in the isthmus of the lumbar spine in divers,
the tibial stress injury in ballet dancers, and the metatarsal
stress in marching soldiers.
In kickboxing, throwing punches with both the upper and lower
extremities is permitted, and trauma is not confined to a specific
limb. Therefore, injury to any or every organ can be anticipated.
Gartland et al
reported that lower limb injuries were second only to head and neck
injuries in a group of Thai kickboxers. This case is a kickboxer
who complained of 3 weeks of pain in both feet and who was
diagnosed with multiple tarsal bone fatigue fractures and
contusions by MRI. A review of the literature revealed no reports
of multiple tarsal bone contusions or fatigue fractures due to a
specific sports injury. However, there are reports of bone
contusion or fracture in the talus or cuneiform of runners and
In our case, the radiographs of the feet were normal. The
conventional radiograph should always be the initial study in this
clinical setting, although demonstrable findings may be minimal or
absent, as in this case. Radiography of fatigue fractures shows a
linear area of sclerosis within the cancellous marrow cavity, which
is usually perpendicular to the long axis of the bone and is often
seen in the chronic phase of the process. MRI findings of fatigue
fractures are highly sensitive and are usually evident several
weeks before any radiographic manifestations. The sensitivity and
specificity of MRI for stress-related injuries has been reported to
be 60% to 100%.
Acute bone contusions show reticular hypointense areas on T1WI that
are hyperintense on T2WI and on fat-suppressed T2WI and are due to
microfractures of the trabecular bone and edema or hemorrhage of
the bone marrow. These bone contusions usually resolve within 8 to
12 weeks if the causal activity is stopped. Continuous stress,
however, may lead to a complete fracture and can be significant in
areas such as the femoral neck.
A condition known as "stress response" precedes the fatigue
fracture and shows edema, hyperemia, and osteoclastic activity. In
this situation, MRI shows abnormal signal intensity similar to a
bone contusion. If the stress persists and a fracture develops, MRI
will show an irregular, hypointense line within the area of edema
and hyperemia. Bone marrow abnormalities are best evaluated with
fat-suppression techniques such as short tau inversion recovery
(STIR) or selective partial inversion recovery (SPIR),
and these imaging protocols should be used in all cases of
suspected stress-related injury.
A 19-year-old kickboxer presented with a 3-week history of
bilateral feet pain. MRI showed multiple tarsal bone fatigue
fractures and diffuse bone marrow contusion. We have termed these
, since the individual developed these injuries from this
In all cases of suspected stress injury, a high index of
clinical suspicion is necessary, especially in athletes who are
involved in martial arts. In these cases, MRI should be performed
expeditiously, as radiography is often not very revealing.
Persistent foot pain related to loading activity should alert the
clinician to the possibility of stress injury or bone
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196 cases. J Orthop Sci.2003;8:273-278.
- Boyer DW Jr. Trap shooter's shoulder: Stress fracture of the
coracoid process. Case report. J Bone Joint Surg Am.1975; 57:
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spondylolisthesis in symptomatic elite athletes: Radiographic
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Injuries of the Musculoskeletal System. Basel, Switzerland:
Karger Publishing Co.; 1988.
- Gartland S, Malik MH, Lovell ME. Injury and injury rates in
Muay Thai kick boxing. Br J Sports Med.2001;35;308-313.
- Pitsis GC, Best JP, Sullivan MR. Unusual stress fractures of
the proximal phalanx of the great toe: A report of two cases. Br
J Sports Med.2004; 38:e31-e33.
- Hodler J, Steinert H, Zanetti M, et al. Radiographically
negative stress related bone injury. MR imaging versus two-phase
scintigraphy. Acta Radiol.1998;39:416-420.
- Masala S, Fiori R, Marinetti A, et al. Imaging the ankle and
foot and using magnetic resonance imaging. Int J Low Extrem
Wounds. 2003; 2:217-232.