Dr. Gonzalo Delgado P.
Revista Chilena de Radiología. Vol. 19 Nº 3, año 2013; 134-139.
Articular cartilage: Evaluation by magnetic resonance
Dr. Gonzalo Delgado P.
Radiologist. Radiology Department, Clinica Alemana de Santiago.
Medical Faculty Clinica Alemana – Universidad de Desarrollo. Santiago, Chile.
Abstract: This article reviews the radiographic evaluation of articular cartilage lesions with emphasis on
its magnetic resonance imaging study, we will discuss the usefulness of conventional sequences and
advanced MRI studies which allow detection of incipient intrasubstance chondral lesions, prior to the
ulceration of its surface.
Keywords: Articular cartilage, Magnetic resonance.
Resumen: El presente artículo revisa la evaluación imagenologica de las lesiones del cartílago articular
con énfasis en su estudio por resonancia magnética, discutiendo la utilidad de las secuencias convencionales y los estudios avanzados de RM que permiten detectar lesiones condrales incipientes intrasustancia, previo a la ulceración de su superficie.
Palabras clave: Cartílago articular, Resonancia magnética.
Delgado G. Cartílago articular: Evaluación por resonancia magnética. Rev Chil 2013; 19(3): 134-139.
Correspondence: Dr. Gonzalo Delgado P. / [email protected]
Article previously published at http://www.alemana.cl/contactocientifico/1/ediciones_anteriores.html
2012; 2(5). Reproduction authorized by both editors.
Articular cartilage lesions are common in various
joints and their multifactorial etiologies, including
traumatic causes, inflammatory and infectious
(septic arthritis) arthropathies and degenerative
causes. Lesions of degenerative origin are the most
frequent, being an important public health problem
due to the high economic and social costs that represent the direct or indirect spending in relation to
treatment and absence from work. It is estimated
that approximately 75% of the population over 75
years of age have osteoarthritis (1).
Articular cartilage or hyaline cartilage is of vital
importance in diarthrosis type joints (joints with a
wide range of movement) and its main function is
to distribute and transmit forces over joint surfaces, cushioning loads and providing an adequate
sliding surface between the articular surfaces. As
a main feature, the hyaline cartilage is an avascular
tissue (nourished from the synovial fluid), it has no
innervation and no capacity to regenerate with the
same tissue, only a limited reparative capacity with
fibrocartilage, which is less resistant.
Articular cartilage consists of (Figure 1):
A. Water (65-80 %): it is present in greater quantity
in the superficial portions of the cartilage and
its content increases with the aging process
and with degenerative changes.
B. Collagen (10-20%): the predominant collagen
is type II (95%), corresponding to the support
matrix of the cartilage and provides resistance to tension forces. Collagen is the primary
component in dehydrated cartilage.
C. Proteoglycans (10-15%): are produced by chondrocytes, glycosaminoglycans (GAG) being its
subunits. They provide resistance to compression forces and have elastic resistance.
D. Chondrocytes (5%): are the only cells found in
the cartilage and are responsible for producing
proteoglycans, collagen, proteins and some
enzymes.
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Figure 1. Diagram showing the various components of
articular cartilage.
In the articular cartilage different zones are
identified depending on the depth and collagen fiber
orientation (Figure 2). Thus, a surface or superficial
zone that covers about 10 to 20% of the thickness of
cartilage is recognized where the collagen fibers are
arranged parallel to the cartilage surface. The transitional zone corresponds to 40 - 60%, where collagen
fibers are randomly arranged. In the deep or radial
zone, corresponding to approximately 30%, the collagen fibers are arranged perpendicular to the surface
and it is the zone where the interlaced framework of
collagen fibrils is more compact. Finally, the calcified
sheet (zone), corresponding to the zone where the
cartilage is fused to the articular cortical bone.
Figure 2. Diagram showing the different zones of the
articular cartilage, depending on the orientation of the
collagen fibers.
The aging processes and degeneration of the
articular cartilage are associated with loss of reproductive capacity of the chondrocytes and the reduction
of proteoglycans. Cartilage becomes more rigid and
its water content increases. Together these alterations
determine loss of the characteristics and functions
of the cartilage, making it less resistant and prone to
injury from fissures and ulcerations.
It is possible to find various classifications of
chondral lesions, not only from an imagenological
but also an arthroscopical viewpoint. Therefore
communication with clinicians is important, especially
orthopedists, as radiologists must use classifications
that are known and used by clinicians who interact
with us and if necessary adjust the basic arthroscopic
classifications to our imaging reports. One of the key
classifications most used by clinicians dedicated to
the issue of chondral lesions, is the classification by
the ICRS (International Cartilage Repair Society)
(Table I). This arthroscopic classification is easy to
compare to our imaging studies, as it based primarily
on the depth of the lesion.
Table I. ICRS (International Cartilage Repair
Society) classification of chondral lesions.
0
1A
1B
2
3A
Normal
Superficial fibrillation or softening
Superficial fissures and lacerations
Defects <50%
Defects >50% without extending down to the
calcified layer
3B Defects >50% extending down to the calcified
layer
4A Total defects involving subchondral plate
4B Total defects with deep involvement of
subchondral plate
Magnetic resonance imaging (MRI) is the method
of choice for evaluating articular cartilage lesions as
it is non-invasive, has high contrast resolution and
multi-plane capability.
MRI performance in the detection of chondral
lesions will depend on the equipment used, high field
1.5 or 3 Tesla resonators being necessary for the evaluation of articular cartilage lesions. It is important to
use appropriate protocols and sequences to identify
subtle lesions (Figure 3). The sensitivity of the MRI
is directly proportional to the magnitude in terms of
the involved chondral surface and the depth of the
lesion (2). Furthermore, thicker cartilage such as that
of the knee, is easier to evaluate than the cartilage of
smaller joints. It is important, always and in all articular
MRI studies, to conduct a directed search for chondral
lesions, as it is usual when reviewing retrospectively
and comparing with surgical findings, to see lesions
that were not apparent during the first reading. The
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chondral lesion characteristics, especially when focal
and unique, which we must specify in the MRI report,
are summarized in Table II.
3a
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Much has been published regarding the most
useful sequences in the evaluation of articular cartilage. Sequences with good contrast between the
cartilage and fluid, and with good contrast between
cartilage and subchondral bone, are appropriate for
the evaluation of chondral pathology. Sequences that
best meet these contrast conditions are specifically
fast spin echo proton density enhanced (FSE PD)
with fat suppression and T1 gradient sequences
(T1 SPGR) with fat saturation. In FSE PD with fat
saturation, cartilage is seen with intermediate signal
intensity, the fluid with high signal and subchondral
bone with low signal (Figure 4). In T1 SPGR with fatsuppression, the cartilage is seen with high signal,
and the fluid and subchondral bone have low signal
intensity (Figure 5). This last sequence performed
with 3D technique, allows thin slicing.
3b
Figure 3. High-resolution coronal T2 slice image using
1.5 Tesla unit, shows small superficial lesions (arrows) in
cartilage of the medial compartment of the knee. Sagittal
T2 slice image on 3 Tesla unit shows thin traumatic lesion
of the tibial plateau cartilage (arrow).
Figure 4. Proton density enhanced axial slice with fat
suppression, showing the patella cartilage. The cartilage
has an intermediate signal in contrast to the articular fluid
high signal.
Table II. Important characteristics to report in a
focal chondral lesion.
1. Surface area measuring AP and transversal
area.
2. Lesion depth (percentage of thickness of
involved cartilage).
3. Location on the articular surface (involvement
of weight-bearing zone).
4. Subchondral bone alterations (edema, cysts).
5. Intra-articular chondral or osteochondral
bodies.
Figure 5. Gradient sequential T1-weighted SPGR axial
image showing the patellar cartilage and femoral trochlea.
Cartilage signal is high in contrast to the articular fluid with
a low signal.
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T2 sequences have good contrast between cartilage (low signal) and fluid (high signal), however,
the contrast between cartilage and subchondral
cortical bone is not suitable, since both have a low
signal. Although this sequence is not very sensitive to subtle changes, it is important in traumatic
chondral lesions.
There is considerable consensus that the most
useful sequences are FSE PD with fat suppression
and T1 SPGR with fat saturation (3). These sequences
have good contrast and are sensitive to pathological
changes of the articular cartilage, which makes them
the best sequences. As mentioned previously, it is
important to conduct a directed search for cartilage
lesions, independent of the protocol used for the study.
Chondral lesions of traumatic origin are typically
solitary and with well defined contours. They often
involve the whole cartilage thickness (Figura 6) and
can be associated with intra-articular chondral or
osteochondral bodies, which can cause joint locking.
Lesions of degenerative origin begin with intrasubstance biochemical alterations, followed by fibrillation,
fissures, full-thickness ulcerations and finally total
loss of the complete cartilage thickness. Usually the
degenerative lesions have irregular contours and often
involve more than one area of the articular surfaces
with lesions of variable thickness. Depending on
the stage of involvement, secondary osteoarthritic
changes such as marginal osteophytes, cysts and
alterations of the subchondral bone signal can be
observed (Figure 7).
The MRI is important in evaluating surgically
treated chondral lesions. In this sense the two most
frequently used surgical techniques are microfracture
and autologous osteochondral grafts. Microfracture
involves drilling in the area of the chondral lesion,
which induces bleeding of the subchondral bone
with the formation of undifferentiated bone marrow
mesenchymal cell clusters that will produce the reparative fibrocartilage, which is of lower resistance than
hyaline cartilage. Autologous osteochondral grafts
are used mainly in chondral lesions on weight-bearing
surfaces of the knee, which consists in taking an
osteochondral fragment from a non-weight-bearing
area (usually femoral trochlea) to be placed in the
area of the original chondral lesion. This procedure,
although more demanding technically, has the advantage of repairing the lesion with hyaline cartilage.
MRI advanced studies
Special methods of MRI have been developed
for the evaluation of articular cartilage. The majority
of these are studies that remain mainly in research,
without important clinical application, the exception
being the T2 mapping study, which has been developed mainly in daily practice and will be reviewed
in more detail.
6a
6b
Figure 6. Sagittal slice images of the external Tibiofemoral
compartment of the knee, PD enhanced with fat saturation
(a) and T2 (b). A full thickness focal chondral lesion of
traumatic origin can be seen.
Figure 7. Axial PD enhanced fat saturated image, showing
advanced degenerative chondropathy of the patella.
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Dr. Gonzalo Delgado P.
The articular cartilage T2 mapping, is a quantitative method for evaluating the internal structure of the
cartilage. With this technique it is possible to measure
the T2 relaxation time of the cartilage.
T2 relaxation times in normal cartilage are lower
in the deeper layers where interlacing of cartilage
fibers is more compact and there is less amount of
water. T2 relaxation times increase toward the more
superficial areas of cartilage.
This method is based mainly on the fact that
degenerative changes produce disorganization of
the collagen matrix, which becomes more relaxed
allowing higher content of H2O protons which are also
freer, producing an increase in T2 relaxation values,
over normal levels.
With the right program, at the work station it is
possible to measure the T2 relaxation time in milliseconds, placing the area of interest where deemed
necessary, allowing objective measurement of alterations. Furthermore this can be represented morphologically in color image, using a predefined scale to
make it visually detectable (Figure 8). As this is not
a morphological study, but more so quantitative, its
main use is to detect initial intrasubstance alterations
of the cartilage, before ulcerations or fissures on
its surface (Figure 9). This capability enables this
method to evaluate the evolution of emerging chondral alterations after pharmacological treatments or
others. Of the more advances techniques for cartilage
evaluation, this is the one which is being most used
in clinical practice (4-6).
Delayed gadolinium reinforcement
This method involves intravenously injecting
ionic gadolinium, which has negative charges, and
undertaking active movements and exercising the
articulation under study to allow the contrast to pass
into the synovial fluid. This method allows evaluation
of the proteoglycans concentration in the articular
cartilage (7).
This study is based on the negative charges that
the glycosaminoglycans have, which are subunits of
the proteoglycans. It is known that the degenerative
and aging processes of articular cartilage decreases
the amount of proteoglycans. If there is a normal
amount of glycosaminoglycans (negative charge),
the contrast (negative charge) will be repelled and
will not diffuse into the cartilage. When the amount of
glycosaminoglycans is reduced, it allows the contrast
to penetrate and permeate the cartilage in the affected
areas. This increased uptake can be represented in
color image.
Among other advanced study methods and used
mainly in research more so than in clinical application,
we can mention specific techniques such as short
echo time projection reconstruction and cartilage
spectroscopy.
Revista Chilena de Radiología. Vol. 19 Nº 3, año 2013; 134-139.
Figure 8. T2 color scale map of normal patella cartilage,
where the deepest part is colored red, indicating low levels
of T2 relaxation times, the central part is colored yellow and
the green colored more superficial zone indicates higher
levels of T2.
9a
9b
Figure 9. T2 color map in two separate patients, showing
altered areas (arrows) with increased T2 levels in the articular
cartilage thickness.
Summary
The articular cartilage is a highly resistant tissue.
However, lesions are frequent and MRI is the method
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of choice for image evaluation of these. Conventional
sequences are useful for this, although some other
specialized MRI techniques exist that can allow a
more objective, quantitative evaluation of the emerging degenerative chondral alterations. Several of
these special techniques are still in the process of
development and investigation and have not yet been
applied to clinical practice. An exception to this is the
T2 mapping of articular cartilage, which it is being
used more routinely.
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