Identification of isolated articular cartilage injuries with magnetic resonance MR imaging prior to arthroscopy is im-portant because articular cartilage injuries can clinically mimic me
Trang 1Interest in cartilage imaging has
increased recently for many
rea-sons As the mean age of the
popu-lation has risen, the incidence of
os-teoarthritis has increased Articular
cartilage abnormalities are common,
with nearly 75% of persons over age
75 years having osteoarthritis.1 The
advent of arthroscopy has brought a
greater demand for accurate
preop-erative evaluation Identification of
isolated articular cartilage injuries
with magnetic resonance (MR)
imaging prior to arthroscopy is
im-portant because articular cartilage
injuries can clinically mimic
menis-cal tears.2 In addition,
prearthro-scopic evaluation of articular
carti-lage allows better prediction of
prognosis for planned interventions
because of the association of
articu-lar cartilage defects with a less
satis-factory clinical outcome.3
The most important reason for
the increased interest in accurate
imaging evaluation of articular car-tilage is the development of carti-lage replacement therapies Detec-tion of articular cartilage defects is necessary to identify patients for whom such therapies are appropri-ate Magnetic resonance imaging allows surgeons to evaluate treat-ment options on the basis of knowl-edge of the size and location of artic-ular cartilage derangements before arthroscopy or surgery Further-more, MR imaging offers the poten-tial for follow-up of patients in trials
of these new cartilage replacement therapies A noninvasive alternative for articular cartilage evaluation is important because these patients are often unwilling to undergo
follow-up arthroscopy to determine success
of treatment
The ability to visualize articular cartilage with MR imaging has advanced with the development of new sequences, receiver coils, and
gradient technology, which have improved image quality, spatial re-solution, and speed of imaging.4
These improvements have resulted
in the ability to use MR imaging to detect moderate- and high-grade articular cartilage abnormalities with a high degree of accuracy
Cartilage Structure and Function
The structure of hyaline cartilage is critical to its function Understand-ing this structure helps explain the imaging appearance of normal and abnormal cartilage and has been essential to the development of new imaging techniques
Normal articular cartilage is composed of hyaline cartilage Chondrocytes account for 1% of
Dr McCauley is Associate Professor of Diagnostic Radiology and Chief of MRI, Yale University School of Medicine, New Haven, Conn Dr Disler is Associate Clinical Pro-fessor of Radiology, Virginia Commonwealth University, Richmond, and is in private prac-tice with Commonwealth Radiology, Richmond Reprint requests: Dr McCauley, Diagnostic Radiology, Yale University School of Medicine, Box 208042, 333 Cedar Street, New Haven, CT 06520-8042.
Copyright 2001 by the American Academy of Orthopaedic Surgeons.
Abstract
Recently developed magnetic resonance (MR) imaging techniques allow
accu-rate detection of modeaccu-rate- and high-grade articular cartilage defects There has
been increased interest in MR imaging of articular cartilage in part because it is
useful in identifying patients who may benefit from new articular cartilage
replacement therapies, including chondrocyte transplantation, improved
tech-niques for osteochondral transplantation, chondroprotective agents, and
carti-lage growth stimulation factors The modality also has the potential to play an
important role in the follow-up of patients during and after treatment.
Detection of articular cartilage defects is beneficial for patients undergoing
arthroscopy for other injuries, such as meniscal tears, because the presence of
articular cartilage injury worsens prognosis and may modify therapy options.
J Am Acad Orthop Surg 2001;9:2-8
Magnetic Resonance Imaging of Articular Cartilage of the Knee
Thomas R McCauley, MD, and David G Disler, MD
Trang 2hyaline cartilage volume; a
hydro-philic matrix, which is 80% water,
constitutes the remaining 99% of
cartilage volume The matrix serves
three major functions: providing a
nearly frictionless surface,
distrib-uting forces to underlying
sub-chondral bone with little
deforma-tion, and transporting nutrients to
the chondrocytes.5 After water, the
two largest constituents of the
hya-line cartilage matrix are collagen
(which makes up 60% of the dry
weight of cartilage) and
proteogly-can aggregates (30% of the dry
weight).5 Collagen provides the
structural framework, tensional
stability, and covering surface of
cartilage Proteoglycan aggregates
are extremely large
macromole-cules that contain many hydroxyl
and negatively charged moieties
These attract water and cations,
thereby creating osmotic, ionic, and
Donnan forces that result in a
swell-ing pressure in the collagen
frame-work, which resists compression.6
There is a highly ordered
struc-ture to the collagen in cartilage,
which is critical to its
biomechani-cal function This structure can be
divided into four zones, or laminae,
on the basis of the collagen
orien-tation seen microscopically.7 The
most superficial portion of the
car-tilage is the tangential zone, which
contains collagen fibers oriented
parallel to the articular surface
The second, or transitional, zone
contains fibers oriented oblique to
the cartilage surface In the third,
or radial, zone, the fibers are
ori-ented perpendicular to the cartilage
surface and are thicker than in the
more superficial zones The fourth
zone, the zone of calcified cartilage,
is present at the interface of the
car-tilage with the underlying bone
The arcadelike configuration of the
collagen fibers provides even
dis-tribution of forces to underlying
bone and resists shearing forces
The swelling pressure created by
the proteoglycans provides the
main resistance to compressive loads.6 The biomechanical proper-ties of cartilage are lost when there
is damage to this highly ordered structure
Cartilage Damage and Repair
Osteoarthritis and trauma are the most common causes of cartilage damage Inflammatory arthritis is less common
There are three stages of osteo-arthritis.8 In the first stage, there is disruption of the collagen frame-work with softening associated with decreased proteoglycan con-tent and increased water concon-tent
In the second stage, there is repair with proliferation of chondrocytes and increased anabolic activity
Thickening of cartilage may occur
in this stage; however, the thickened cartilage has abnormal mechanical properties In the third stage, the repair mechanisms can no longer be sustained, and decreased cellular proliferation and anabolic activity
of the chondrocytes occurs, result-ing in articular cartilage loss, fibril-lation, erosion, and cracking.8
When articular cartilage defects form due to osteoarthritis or trauma, there may be repair; however, nor-mal hyaline cartilage does not re-generate Repair generally does not occur when there are partial-thickness cartilage defects, as the repair response is usually initiated only with damage extending to subchondral bone, as occurs in full-thickness defects.6 Full-thickness defects initiate repair by filling with fibrin clot and inflammatory cells, which release growth factors and other proteins that stimulate repair Unfortunately, fibrocarti-lage usually only partially fills the defects in the articular cartilage surface The fibrocartilage does not have the same mechanical proper-ties as normal hyaline cartilage
because of abnormal collagen structure and because of produc-tion of smaller chain lengths and smaller amounts of proteoglycan aggregates, which decreases the attraction for water The fibrocarti-lage usually begins to degenerate within a year after formation be-cause of its abnormal biomechanical properties.6
Pain is not directly caused by cartilage damage because articular cartilage is aneural Cartilage ab-normalities and associated bone abnormalities likely cause forces that act on the subchondral bone, joint capsule, menisci, and other supporting structures of the joint, resulting in pain.9
MR Imaging of Articular Cartilage
The accuracy of articular cartilage assessment with MR imaging has greatly improved with the recent development of imaging sequences designed specifically for hyaline cartilage The two most widely used imaging techniques are the T1-weighted fat-suppressed three-dimensional spoiled gradient-echo technique and the T2-weighted fast spin-echo technique Cartilage is well visualized with these tech-niques due to the differences in T1 and T2 between articular cartilage and fluid Cartilage is higher in sig-nal intensity than fluid on T1-weighted images and is lower in signal intensity than fluid on T2-weighted images
Magnetic resonance arthrogra-phy with injection of contrast mate-rial into the joint is not generally necessary for articular cartilage evaluation The accuracy of MR arthrography has not been found to
be higher than that of imaging techniques that do not entail con-trast injection.10,11 However, MR arthrography is useful in a subset
of patients for whom assessment of
Trang 3cartilage integrity over
osteochon-dral defects12 or identification of
loose bodies is necessary.13
The fat-suppressed
three-dimen-sional spoiled gradient-echo
se-quence provides high accuracy,
with a sensitivity of 86%, specificity
of 97%, and accuracy of 91% for
detection of cartilage lesions in the
knee (data are for detection of
carti-lage lesions excluding softening
without cartilage loss)2(Figs 1–3)
T2-weighted fast spin-echo
tech-niques, both without and with fat
suppression, have recently been
shown to result in similarly high
accuracy, with a sensitivity of 87%,
specificity of 94%, and accuracy of
92%14,15(Fig 2) As with other MR
techniques, accuracy is highest in
the patellofemoral joint, likely due
to the thickness of the patellar
car-tilage.2 In addition, high-grade
ab-normalities with thinning or focal
defects in cartilage are detected with
greater accuracy than low-grade
cartilage abnormalities, in which
there is little or no loss of cartilage
thickness.2
The two imaging techniques
have different advantages and
dis-advantages The T2-weighted fast
spin-echo technique is less
suscep-tible to metal artifacts, which can be
an advantage when imaging patients
after surgery The fat-suppressed
three-dimensional spoiled
gradient-echo sequence provides thinner
sec-tions, which has been found
advan-tageous in identifying morphologic
defects T2-weighted sequences can
better visualize signal abnormalities
within cartilage and thus may allow
detection of lower grades of
carti-lage abnormality, especially in the
patellar cartilage (Fig 4) These two
techniques for detection of defects
have not yet been directly
com-pared Neither has the ability of
these techniques to accurately
mea-sure the area of defects, which can
influence selection of cartilage
re-placement therapies, been
investi-gated
Both the fat-suppressed three-dimensional spoiled gradient-echo technique and the T2-weighted fast spin-echo technique have been val-idated in patients at 1.5 T.2,14,15
Lower accuracies would be expected
at lower field strengths because
of the lower signal-to-noise ratio
available at those field strengths,16
along with decreased reliability or unavailability of fat suppression
No direct comparison has been per-formed at different field strengths; however, in a recent study,17 the accuracy of evaluation of cadaveric patellar articular cartilage at 0.2 T
Figure 1 Full-thickness traumatic articular cartilage defect in the knee of a 14-year-old soccer player seen on fat-suppressed three-dimensional spoiled gradient-echo images Cartilage appears as high signal intensity; fluid and other tissues appear as low signal
intensity A, Sagittal image (repetition time [TR] = 60 msec; echo time [TE] = 5 msec)
shows articular cartilage defect in the lateral femoral condyle at the trochlear groove (solid arrow) Note low-signal lamina due to truncation artifact in adjacent normal cartilage
(open arrow) B, Sagittal image (TR/TE = 60/5) obtained lateral to A shows cartilage frag-ment in suprapatellar recess (arrow) C, Surface rendering of the defect from an anterior perspective, created from the three-dimensional image set D, Arthroscopic image of
trochlear groove as seen from below confirms the presence of the articular cartilage defect
seen with MR imaging (Part A reproduced with permission from Disler DG, McCauley
TR, Wirth CR, Fuchs MD: Detection of knee hyaline cartilage defects using fat-suppressed three-dimensional spoiled gradient-echo MR imaging: Comparison with standard MR
imaging and correlation with arthroscopy AJR Am J Roentgenol 1995;165:377-382 Parts C
and D reproduced with permission from McCauley TR, Disler DG: MR imaging of
articu-lar cartilage Radiology 1998;209:629-640.)
Trang 4was lower than that obtained in
evaluation of patellar cartilage with
1.5-T magnets.2 In addition to
high-quality equipment, the appropriate
pulse sequences and imaging
pa-rameters must be used (Table 1) In
our experience, radiologists are
better able to identify articular
car-tilage injuries with increased
expe-rience, including feedback based
on arthroscopic findings from
re-ferring orthopaedic surgeons
A number of factors may
influ-ence the appearance of articular
cartilage at MR imaging Articular
cartilage has uniform high signal
intensity on fat-suppressed
three-dimensional spoiled gradient-echo
images2; however, artifactual
low-signal laminae may be visualized in
the center of cartilage due to
trun-cation artifact18 (Fig 1) This
arti-fact occurs due to undersampling of
signal from small objects with high
contrast The location and
appear-ance of truncation artifact is
pre-dictable Truncation artifact can be
decreased by increasing the
in-plane resolution; however,
increas-ing resolution typically increases
imaging time This artifact is usually
easily recognized and does not
im-pede visualization of cartilage fects; it can even be helpful in de-termining the depth of the defects
High-resolution T2-weighted MR imaging can demonstrate a nonarti-factual laminar signal-intensity pat-tern in cartilage, predominantly due
to the laminar structure of the colla-gen fiber orientation.19,20 The size and signal intensity of the laminae can vary with changes in imaging variables and with changes in
orien-tation of the cartilage with respect to the magnetic field The latter varia-tion is due to the anisotropy of the collagen fibers in the various layers
of the cartilage.19 Experienced read-ers can recognize normal variation
in the laminar appearance and there-fore are not hindered in the detec-tion of cartilage damage
Clinical Importance of Cartilage Imaging
The ability to accurately evaluate articular cartilage with MR imaging can provide more complete infor-mation with which to make thera-peutic decisions Articular cartilage injury in the knee is common; in one study,2it was visualized on MR images of 32 (67%) of 48 patients who subsequently underwent ar-throscopy of the knee In that study, two thirds of the patients with artic-ular cartilage defects had concur-rent meniscal tears or ligament injuries; however, one third had isolated articular cartilage injuries Detection of articular cartilage defects with MR imaging can ex-plain symptoms in patients with isolated articular cartilage injuries that might otherwise have eluded
Figure 2 Images depicting a near-full-thickness articular cartilage defect in the medial
femoral condyle in a 28-year-old man with chronic knee pain No other abnormality was
found in the knee at arthroscopy A, Sagittal fat-suppressed three-dimensional spoiled
gradient-echo image (TR/TE = 40/6) shows a defect (arrow) containing fluid, which
appears as low signal intensity B, Coronal T2-weighted fast spin-echo image (TR/TE =
4,000/96) shows the same defect (arrow) containing fluid, which appears as high signal
intensity.
Figure 3 Sagittal fat-suppressed three-dimensional spoiled gradient-echo images (TR/TE
= 40/6) of a 17-year-old girl 1 year after osteochondral transplantation to repair a femoral
articular cartilage defect A, Image obtained at the site of osteochondral plug placement (arrow) shows slight depression of the articular surface B, Image obtained at the donor
site along the lateral margin of the intercondylar notch depicts filling with intermediate-signal-intensity tissue, likely representing repair tissue in the osteochondral defect (arrow).
Trang 5detection Identification of articular
cartilage injury with MR imaging in
patients with intact menisci is
espe-cially useful because symptoms due
to isolated cartilage defects often
mimic those due to meniscal tears.2
Identification of cartilage damage is
important in patients with
associ-ated injuries because the presence
of defects can worsen the prognosis
after arthroscopic surgery.3
Iden-tification of defects also facilitates
preoperative planning for articular
cartilage replacement therapies
Future Developments
Currently available techniques allow
detection of morphologic defects in
articular cartilage with high accuracy
However, low-grade injuries with
internal cartilage damage without
morphologic change are not
accurate-ly visualized.21 A number of MR
techniques for detection of cartilage
damage at early stages are being
developed First, Brossmann et al22
reported that a technique utilizing
ultra-short echo times resulted in
100% sensitivity and specificity for
detection of cartilage defects with
sta-tistically significant higher accu-racy than that obtained with a fat-suppressed three-dimensional spoiled gradient-echo technique in a study of
10 human cadaveric patellae The authors hypothesized that the high sensitivity of this technique was due
to signal changes related to disorgani-zation of collagen fibers Another
group has used short-echo-time ac-quisitions to obtain proton spectra in articular cartilage, which has the potential to provide more detailed analysis of biochemical information.23
Second, imaging techniques are being developed that use magneti-zation transfer contrast Magnetiza-tion transfer contrast is dependent predominantly on collagen integrity
in cartilage.24 Unfortunately, these techniques have not yet been found
to be superior to other routinely available MR imaging techniques Third, ionic gadolinium contrast material is being used for detection
of early biochemical changes with cartilage degeneration.25 The con-trast medium is introduced into the joint by either direct or intravenous injection In normal cartilage, the negative charges of proteoglycan aggregates exclude the negatively charged gadolinium chelate Be-cause proteoglycans are lost early
in cartilage degeneration, increased amounts of the negatively charged gadolinium can gain entry into de-generating cartilage, with resulting signal enhancement A study of cadaveric patellar cartilage found
Figure 4 Images of a patellar cartilage abnormality due to osteoarthritis in a 46-year-old
man A, Fat-suppressed three-dimensional spoiled gradient-echo image (TR/TE = 40/6)
shows cartilage abnormality as decreased signal intensity in the normally
high-signal-intensity cartilage with associated surface irregularity (arrows) B, Abnormality is more
clearly seen on T2-weighted axial image (TR/TE = 2,000/80) of patella, where it is depicted
as increased internal signal within the normally low-signal-intensity cartilage (arrows).
(Reproduced with permission from McCauley TR, Disler DG: MR imaging of articular
car-tilage Radiology 1998;209:629-640.)
Table 1 Suggested Protocols for Articular Cartilage Imaging 2,4,14,15 *
Fat-Suppressed Three-Dimensional Technique Spoiled Gradient-Echo Fast (Turbo) Spin-Echo Pulse sequence TR = 30-50 msec; TE = TR = 3,500-5,000 msec;
<10 msec (minimum TE = 30-54 msec; echo full echo); 40° flip angle train length = 8-10 Tissue contrast Fat suppression or Fat suppression
pref-water excitation erable
Acquisition matrix 160 ×256 256-512 ×256-384 Section description 1.5-mm sections, 3.5- to 4.0-mm sections;
60 locations gap = 0 to 1 mm Number of excitations 0.75 or 1 2
* Sagittal and axial planes are most useful Three-dimensional images can be reformatted
to obtain high-quality axial images.
Trang 6that use of gadolinium allowed
de-tection of loss of proteoglycans
from mechanically intact articular
cartilage, while changes in T2 in
cartilage could be used to detect
mechanical damage.26
Fourth, MR imaging of sodium
rather than hydrogen has been
investigated as a potential method
for evaluation of proteoglycan
con-tent in articular cartilage.27 Imaging
techniques that detect early
bio-chemical changes can facilitate
iden-tification of cartilage abnormalities
before morphologic abnormalities
occur, which may allow
chondro-protective interventions before loss
of the morphologic integrity of
carti-lage occurs
Another area of ongoing
devel-opment takes advantage of the
three-dimensional information
avail-able with MR imaging of articular
cartilage Surface models of articular
cartilage can be created from MR
imaging data sets (Fig 1, C)
Measure-ment of cartilage volume with MR imaging has been shown to be very accurate28 and may allow quantifi-cation of the progression of arthritis
Studies of the configuration of car-tilage surfaces may also provide information on the influence of car-tilage configuration on the progres-sion of osteoarthritis
A critical area for future devel-opment is the imaging of cartilage after treatment (Fig 3) Studies of both the normal appearance after repair and the pathologic changes that reflect complications are ongo-ing The results of these studies will likely lead to the use of MR im-aging as a noninvasive technique for following the results of articular cartilage replacement therapies
Summary
New commercially available MR imaging techniques can be used to
accurately detect moderate- and high-grade cartilage defects These techniques have been shown to be highly accurate when images are obtained with state-of-the art equipment and are interpreted by experienced musculoskeletal radi-ologists Detection of articular car-tilage defects provides useful information on which to base treat-ment selection, which is increasing
in importance because of the ad-vancements in therapies for carti-lage damage In addition, accurate serial assessment of lesions after treatment will facilitate evaluation
of these therapies In the future,
MR imaging will likely have an important role in the understand-ing and evaluation of cartilage degeneration and repair, and de-velopment of new techniques will increase our ability to accurately assess both morphologic and bio-chemical abnormalities in articular cartilage
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