Magnetic resonance (MR) imaging of the knee is a useful modality for assessing internal derangements of the knee. Recently, imaging of articular cartilage has been studied intensely owing to advances in surgical therapy for focal articular cartilage disease. MR imaging developments have shown clear advantages over other modalities in noninvasively detecting and grading articular cartilage defects. This report will focus on MR imaging techniques, pitfalls and limitations in detecting cartilage lesions, current therapy, and the future of articular cartilage imaging.
is currently a fourth-year Radiology Resident at Brigham and
Women's Hospital/Harvard Medical School in Boston, MA. He graduated
summa cum laude from McGill University, Montreal, Canada, with a
BSc in Biochemistry in 1994 and received his MD and Master of
Surgery from McGill University in 1998. Dr. Chow will remain at
Brigham and Women's Hospital to begin a fellowship in
Musculoskeletal Radiology in July 2003.
Magnetic resonance (MR) imaging of the knee is a useful
modality for assessing internal derangements of the knee.
Recently, imaging of articular cartilage has been studied
intensely owing to advances in surgical therapy for focal
articular cartilage disease. MR imaging developments have shown
clear advantages over other modalities in noninvasively detecting
and grading articular cartilage defects. This report will focus
on MR imaging techniques, pitfalls and limitations in detecting
cartilage lesions, current therapy, and the future of articular
In recent years, intense interest in articular cartilage imaging
has been generated by several advances in the surgical treatment of
focal articular cartilage injury. In the United States, there is a
high prevalence of articular cartilage disease; >75% of the
population >65 years of age have evidence of osteo-arthritis and
1.6% of the entire population have symptomatic osteoarthritis of
In a retrospective review of all arthroscopies performed at a
single institution during a 4-year period, >60% of 31,516 knee
arthroscopies showed articular cartilage damage, and 37% of
patients <40 years of age showed isolated articular cartilage
injury without ligamentous or meniscal pathology.
It is generally believed that, over time, these articular cartilage
defects eventually lead to osteoarthritis, the incidence of which
is extremely common in the United States and increases with age.
Osteoarthritis costs the U.S. economy nearly $65 billion per year
in direct expenses, lost wages, and lost productivity.
After cardiovascular disease, osteoarthritis is the second leading
cause of chronic disability in America.
This has created a necessity for accurate noninvasive detection and
evaluation of articular cartilage disease and monitoring of
therapy. A complete evaluation is needed to select the criteria
that will best determine which cartilage defects are amenable to
treatment and which patients will benefit from surgical treatment.
The clinical symptoms associated with articular cartilage
defects are variable and range from almost no pain to chronic pain,
joint effusions, and immobility. For cartilage evaluation, physical
examination and history are inaccurate and radiography is an
indirect and insensitive modality. While arthroscopy is the current
standard method for assessing articular cartilage integrity,
certain disadvantages to this method are evident. Surveillance by
arthroscopy is a minimally invasive procedure but requires trocar
insertion and anesthesia. Arthroscopy is also limited to the
evaluation of surface integrity and cannot accurately assess
In addition, arthroscopy gives the orthopedic surgeon insufficient
preoperative information about what to expect from the surgery.
This hinders their ability to counsel patients regarding treatment
options and rehabilitation.
Imaging plays an important role in noninvasively assessing
patients who are suitable for surgical treatment. Magnetic
resonance (MR) imaging plays an integral and complementary role in
noninvasively assessing the integrity of articular cartilage
beneath the surface articulations and within subchondral bone. This
discussion of MR imaging of articular cartilage will focus mainly
on the knee because research has been concentrated in this
Modalities for imaging articular cartilage
Radiography is an indirect method of assessing cartilage by
measuring joint narrowing.
It is inaccurate and insensitive for pure articular cartilage
disease, but may be revealing in osteochondral injuries, such as
Ultrasound has been used in assessing ligaments and other
soft-tissue structures in the knee; however, it is unable to
delineate cartilage abnormalities clearly.
Computed tomographic arthrography has also been used in assessing
internal derangements of the knee, but is more invasive than
conventional MR imaging. Optical coherence tomography--a
high-resolution micron scale imaging method that measures the
intensity of back-reflected infrared light--is being researched
intensely but has not been clinically proven.
Surgical arthroscopy is regarded as the standard method, but it is
invasive and costly. To this day, conventional MR imaging has been
the mainstay of noninvasive articular cartilage imaging.
MR imaging techniques
The advantages of MR imaging are cross-sectional multiplanar
capability, high spatial resolution, and good contrast resolution.
However, small articulations and early cartilage changes are still
challenging. Many different clinical imaging sequences are
available today. These imaging sequences rely on the differences in
contrast resolution between articular cartilage and adjacent
structures such as joint fluid, subchondral bone, and menisci, as
shown in Table 1.
The mainstay of MR pulse sequences optimized for articular
cartilage visualization are fast-spin echo (FSE) (or turbo-spin
and three-dimensional (3D) spoiled gradient-recalled echo (3D-SPGR)
(or fast low-angle shot [FLASH]).
These two pulse sequences exploit the dark cartilage/ bright fluid
and bright cartilage/dark fluid differences, respectively.
For evaluating focal cartilage defects, MR arthrography, which uses
the dark cartilage/bright fluid technique, has generated much
interest; however, it has the added advantage of joint distension.
Spin-echo (SE) imaging, routinely used in imaging of the knee,
reveals a bilaminar appearance of cartilage with a dark basal layer
and an intermediate signal upper layer. However, owing to the
similar signal intensity of subchondral bone with the basal
cartilage layer, the thickness of the cartilage is underestimated.
Although the cartilage-fluid interface is well visualized on
T2-weighted SE images, poor definition of the cartilage-subchondral
bone interface limits its use in the evaluation of articular
This report will concentrate on FSE and 3D-SPGR techniques.
New MR imaging techniques are currently under development in
order to optimize the sensitivity and specificity of detecting and
grading articular cartilage defects. These new developments include
magnetization transfer (MT), delayed gadolinium-enhanced MR imaging
of cartilage (dGEMRIC), sodium nuclear magnetic resonance (NMR)
imaging, T2 mapping, driven equilibrium Fourier transform (DEFT),
and fluctuating equilibrium magnetic resonance imaging (FEMR).
Although these imaging protocols have yet to be utilized in routine
clinical practice, the potential to understand the pathophysiology
of focal articular cartilage disease and the pathogenesis of
osteoarthritis holds great promise. These imaging techniques will
be discussed in detail later in this report.
FSE (TSE) with fat suppression
Fast-spin-echo technique generates a higher signal-to-noise
ratio and improved image contrast when compared with SE technique.
The superficial layers of cartilage appear intermediate in signal
intensity while joint fluid appears bright. With fat suppression,
the basal layer of cartilage appears slightly more hyperintense
compared with subchondral bone and bone marrow, but more
hypointense when compared with joint fluid. Marrow edema and
contusions can be detected easily by adding a chemical shift
selective fat-suppression technique. This technique can also be
used to adequately evaluate other structures within the knee,
including menisci, ligaments, and bone marrow. Thus, FSE with fat
suppression is a useful technique in routine knee imaging. Metallic
artifact is also much decreased with FSE as compared with
gradient-echo techniques. All of this is achieved with a shorter
acquisition time compared with SE, with certain investigators
advocating an effective echo time (TE) of approximately 34 msec for
the greatest sensitivity.
With collagen loss and increasing water content in degenerating
cartilage, T2 prolongation occurs and proton density increases,
resulting in higher signal intensity.
The disadvantages of FSE include decreased spatial resolution
compared with 3D-SPGR due to the two-dimensional (2D) acquisition
and blurring artifacts of short T2 structures, such as cartilage
There are technical developments being investigated for 3D-FSE, and
with shorter echo spacing available, it is possible to reduce
blurring artifacts for short TE imaging.
3D-SPGR (3D-FLASH) with fat suppression
Numerous advantages are achieved with 3D-SPGR imaging of
articular cartilage. A consistent trilaminar appearance of the
articular cartilage--bright against dark joint fluid, subchondral
bone, and bone marrow--is evident.
Greater spatial resolution and contrast-to-noise (CNR) ratio are
attained because of the 3D acquisition.
The higher spatial resolution enables multiplanar reformatting,
which allows for additional manipulation and volumetric
quantification of cartilage volume using commercially available
Curved surfaces of articular cartilage, such as the femoral
trochlea and patellar cartilage, can also be evaluated more
accurately with standard and user-specified planes. One
disadvantage of this technique is greater susceptibility to
metallic artifacts, limiting its use in postoperative knees. In
addition, evaluation of other important knee structures, such as
ligaments and menisci, are not assessed adequately; therefore, this
sequence is used in addition to other routine imaging sequences of
the knee. Furthermore, 3D-SPGR also has a longer acquisition time
than FSE techniques.
The concept of MR arthrography parallels conventional
arthrography in that distension of the joint with a contrast agent
helps to visualize and delineate surface abnormalities more
precisely. With MR arthrography, enhancement of joint fluid can be
achieved with either direct injection of a dilute solution of
gadolinium into the joint (intra-articular [IA] or direct MR
arthrography), or by intravenous ([IV] or indirect MR arthrography)
injection of contrast material with subsequent diffusion of the
contrast into the joint.
This contrast diffusion out of the blood pool and into the
extracellular space is achieved with active exercising of the joint
for approximately 10 to 20 minutes before imaging for the greatest
enhancement of joint fluid. This technique has been used to
greatest advantage in evaluating the shoulder joint for rotator
cuff and glenoid labrum tears.
Subsequently, it has also been shown to be an excellent method for
evaluating articular cartilage.
By revealing enhanced joint fluid beneath an osteochondral lesion,
it is currently the most useful technique in determining stability
of an osteochondral fragment.
It has also been shown to have high specificity and accuracy in the
detection and grading of patellar cartilage defects when compared
with conventional SE and 3D-SPGR imaging.
Some of the disadvantages of direct (IA) MR arthrography are
that it is more invasive and more time-consuming than indirect (IV)
MR arthrography. Also, direct MR arthrography requires a
radiologist, assisted by a techologist or a nurse, to inject the
diluted gadolinium solution into the joint under fluoroscopy, using
a sterile technique. Total time required for the technique depends
on familiarity with conventional arthrography techniques, but is
usually approximately 20 to 30 minutes. Imaging time is the same
for both direct and indirect MR arthrography. With indirect MR
arthrography, there is no direct control of the degree of joint
Grading and detection of cartilage defects
There are many grading schemes for focal articular cartilage
defects. The modified Outerbridge classification, adopted from its
original use in grading chondromalacia patella, is based on
morphological appearance at arthro-scopy and uses a 5-point system
: grade 0 represents normal cartilage; grade 1 lesions display
softening of the cartilage; grade 2 lesions show shallow ulceration
or blister-like swelling; grade 3 lesions show partial-thickness
fibrillation or deep ulceration not extending to bone (Figure 1);
and grade 4 lesions are ulcerations with exposure of the
subchondral bone (Figure 2). Another grading scheme, proposed by
Bauer and Jackson,
categorizes articular cartilage injuries according to arthroscopic
patterns of cartilage fracture. MR imaging grading schemes
correspondingly follow this system: grade 1 lesions show signal
abnormality within the cartilage; grade 2 lesions have surface
irregularity; grade 3 lesions show partial thickness loss
(<50%); and grade 4 lesions show full-thickness cartilage loss
to subchondral bone (Figure 3).
Detection of articular cartilage lesions on MR imaging relies on
signal intensity changes and contour abnormalities. Fat-suppressed
3D-SPGR images are useful for assessing surface undulations or
irregularities, whereas FSE images are useful for detecting
abnormal signal intensity and contour changes if a joint effusion
is present. MR arthrography can show both contour abnormalities and
signal changes within the substance of the articular cartilage.
However, MR imaging is relatively insensitive to grade 1 lesions.
With the use of FSE, 3D-SPGR, or MR arthrography, the sensitivity
rate for detecting lesions improved as the grade of cartilage
lesions increased: the sensitivity rates ranged from 32% for grade
1, 72% for grade 2, 94% for grade 3, and 100% for grade 4.
Published data have shown sensitivity and specificity for the
detection of cartilage lesions to range from 81% to 100% for
correlation within 1 arthroscopic grade.
Pitfalls and limitations in MR articular cartilage
Pitfalls in detecting chondral lesions on MR imaging include a
focal bright signal in articular cartilage at 55º relative to the
main magnetic field. This is due to the highly organized structure
of collagen fibers in cartilage causing the T2 relaxation
anisotropy--the so-called "magic-angle effect."
An attempt to reduce this artifact can be made by increasing the TE
(increasing T2 weighing). Chemical shift artifacts can also arise
at fat-cartilage interfaces and can lead to spatial misregistration
in the frequency direction; however, this can be eliminated by fat
Truncation artifacts develop from the undersampling of signal
arising from small objects with high contrast. This occurs with
3D-SPGR, producing a false laminar appearance of cartilage.
Finally, metallic susceptibility artifact arises in the
postoperative knee; gradient-echo imaging is much more sensitive to
this artifact than is the FSE technique, which limits the ability
of 3D-SPGR in this context.
The spatial resolution and image contrast of current imaging
sequences somewhat limit the determination of cartilage defects.
When thin fissures and flap tears are present, adequate image
contrast and spatial resolution are necessary to identify a thin
cleft at the articular surface (Figure 4). Assessment of curved
articular surfaces also presents difficulty owing to partial volume
averaging effects and may lead to overestimation or underestimation
of defect size and severity. Differentiation of full-thickness from
partial-thickness defects requires absolute image contrast between
the deep zones of cartilage and subchondral bone. Despite these
limitations, MR imaging is reasonably successful in the accuracy of
grading cartilage lesions. Various studies have shown >85%
sensitivity and >90% specificity for moderate-to-severe
cartilage lesions (grades 2 to 4).
Given the current limitation of spatial resolution and partial
volume averaging, the spatial resolution of lesion dimensions has
been reported to be accurate to 0.5 mm in vivo.
These spatial and contrast resolution requirements emphasize the
need for high-field MR imaging systems with dedicated coils to
improve the signal-to-noise ratio.
Clinical challenges and surgical therapy
Currently, there is little consensus as to which cartilage
defects can be managed conservatively versus lesions that are
amenable to surgical therapy. Classification of articular defects
into degenerative, inflammatory, and posttraumatic etiologies helps
define the patient population and type of chondral defects or
injury that will benefit most from surgical treatment.
Degenerative chondrosis or advanced chondromalacia changes are
typically multifocal defects that have shallow angles and
indistinct margins, usually occurring along the posterior condyles.
Meanwhile, inflammatory arthritis usually shows diffuse and uniform
articular cartilage thinning without focal defects. This is
presumably due to the effect of inflammatory pannus and catabolic
enzymes within the joint fluid.
Acute chondral/osteochondral injury characteristically shows an
acute angle of the lesion wall with sharp borders occurring at
weight-bearing surfaces. Pure chondral fractures occur more
frequently in the skeletally mature patient because of shearing
forces propagating along the tidemark, a natural cleavage plane
between the nonmineralized cartilage layers from the deepest
calcified layer of cartilage and the subchondral bone plate.
Osteochondral fractures generally occur in the skeletally immature
patient because tidemark has not been defined, thus allowing
injuring forces to penetrate the calcified cartilage and
subchondral bone plate.
It has been suggested that approximately 78% of acute
chondral/osteochondral injuries are associated with focal
subchondral edema at the site of injury, and that these lesions are
associated with treatable cartilage defects.
Some investigators have proposed an algorithm for symptomatic
full-thickness articular cartilage defects with surgical repair as
first-line treatment (Figure 5).
Medical or conservative therapies have been ineffective at
preserving joint function and stopping progressive degeneration
that may lead to osteoarthritis in this clinical situation.
Articular cartilage, or hyaline cartilage, is histomorphologically
composed of a well-organized network of type II collagen fibers and
interspersed chondrocytes in a hydrated gel matrix that contains
proteoglycans and hyaluronate.
Articular cartilage has little regenerative or reparative capacity
when damaged; when repair tissue is formed, it is fibrocartilage
and its biomechanical properties are significantly different from
Because fibrocartilage is unable to withstand joint loading and
degenerates over time, clinical symptoms return.
This often requires reconstructive knee surgery and eventually a
Many different surgical approaches for decreasing symptoms,
restoring joint function, and preventing osteo-arthritic
degeneration have been attempted with variable success. These
include debridement and lavage, abrasion and microfracture
techniques, periosteum and perichondrium transplantation,
osteochondral autograft and allograft plug transfer (mosaicplasty),
and autologous chondrocyte implantation.
With the new advances in surgical treatment of focal cartilage
defects, MR imaging has become an important noninvasive means of
monitoring the effectiveness of treatment (Figure 6). Currently,
the accepted modality for monitoring treatment is arthroscopy with
biopsy, an invasive and expensive means of follow-up.
Future of articular cartilage imaging
The appropriate deployment and selection of newer treatment
interventions for osteoarthritis depends on the development of
better methods for the longitudinal assessment of the disease
process. New MR imaging sequences are contributing to our
understanding of the natural history of focal cartilage defects and
the progression to osteoarthritis. Although these sequences have
yet to be used in routine clinical imaging, they show great promise
in understanding the biochemical, physiologic, and functional
characteristics of articular cartilage.
Magnetization transfer (MT) imaging uses an off-resonance
radiofrequency pulse to interrogate the integrity of the collagen
matrix in cartilage, showing decreased signal in an intact collagen
Several studies have reported that MT imaging can be used to
differentiate reliably among articular cartilage, adjacent joint
fluid, and inflamed synovium.
Delayed gadolinium-enhanced MR imaging of cartilage and sodium
imaging methods can indirectly measure the glycosamino-glycan (GAG)
concentration in articular cartilage, a constituent of the
proteoglycan macromolecule that largely accounts for the
biomechanical compressive strength of cartilage.
The functional significance of GAG-depleted cartilage is the loss
of the compressive strength of articular cartilage, suggestive of
the early stages of cartilage degeneration.
T2 mapping of articular cartilage has also been investigated for
detecting early degeneration (Figure 7). Spatial distribution
measurements of T2 relaxation times can be mapped and may reveal
areas of increased water content, correlating with damage to the
Other promising pulse sequences include driven equilibrium Fourier
transform imaging and steady-state free-precession methods--such as
fluctuating equilibrium MR imaging (Figures 8 and 9). Both of these
techniques use bright synovial fluid signal while maintaining
cartilage signal based on the ratio of T1/T2 in tissue with high
spatial resolution due to the 3D-acquisition technique.
In addition to these novel pulse sequences, applications such as
quantitative image processing and analysis techniques of cartilage
thickness and volume with 3D reconstruction allow monitoring of
medical and surgical therapies.
Biomechanical analysis can also be obtained from a
computer-generated model of a subject with fiducial markers
climbing stairs, which may give insight into the "wear and tear"
aspects of degenerative chondrosis.
All of these pulse sequences and novel applications may play a
significant role in evaluating progress in new chondro-regenerative
drug therapy, such as intra-articular hyaluronate or oral
or new surgical treatments.
MR imaging, with its superior soft-tissue contrast, is the best
technique available for assessing normal articular cartilage and
cartilage lesions. With this modality, noninvasive imaging can be
performed before treatment to determine the best possible therapy
and after treatment to assess the response to therapy. With the
promise of higher field MR imaging systems and new imaging
sequences to evaluate articular cartilage with greater
signal-to-noise ratio, CNR, and spatial resolution, radiologists
will be better prepared to assess the clinical significance of
early diagnoses and treatment options.
The author wishes to thank Dr. Carl S. Winalski (Brigham and
Women's Hospital, Harvard Medical School) for his support in
reviewing the manuscript and generously providing the images and
technical assistance. Special thanks go to Drs. Philipp Lang, John
A. Carrino, and Frank J. Rybicki (Brigham and Women's Hospital,
Harvard Medical School) for reviewing the manuscript and providing
thoughtful input and assistance. Dr. Garry Gold (Stanford
University) and Dr. Timothy J. Mosher (Pennsylvania State
University) also provided additional images.