AJR 2004; 183:9-15
© American Roentgen Ray Society
Meniscal Tears: Role of Axial MRI Alone and in Combination with Other Imaging Planes
Nefise Cagla Tarhan1,2,
Christine B. Chung1,
Aurea Valeria Rosa Mohana-Borges1,
Tudor Hughes1 and
Donald Resnick1
1 Department of Radiology, VA Medical Center, University of California San
Diego, 3350 La Jolla Village Dr., San Diego, CA 92161.
2 Present address: Department of Radiology, Baskent University, Faculty of
Medicine, Ankara, Turkey.
Received September 30, 2003;
accepted after revision February 2, 2004.
Address correspondence to C. B. Chung.
Abstract
OBJECTIVE. The purpose of this study was to determine the
reliability of standard axial MR images alone in the diagnosis of meniscal
tears of the knee and in combination with other imaging planes.
MATERIALS AND METHODS. Sixty-two patients (55 men, seven women; age
range, 23–68 years) with a prior MRI examination who underwent
arthroscopic surgery of the knee during a 1-year period were included in the
study group. Images were independently reviewed for identification of meniscal
tears by two musculoskeletal radiologists blinded to arthroscopic findings.
Sequences for meniscal evaluation included axial fat-saturated fast spin-echo
proton density, coronal fat-saturated fast spin-echo proton density, and
sagittal fast spin-echo proton density with 4- to 5-mm slice thicknesses.
Imaging groups for evaluation were axial, coronal, sagittal, axial and
sagittal, axial and coronal, and coronal and sagittal. Observers reported a
confidence level for the presence or absence of meniscal tear in all imaging
groups based on a 5-point scale. Statistical analysis considered medial and
lateral menisci separately.
RESULTS. Forty patients had medial meniscal tears, and 16 had
lateral meniscal tears at arthroscopy. For medial and lateral meniscal tears,
the accuracy (79% and 71%, respectively) of imaging in the axial plane was
comparable to other imaging groups but the mean confidence levels (2.82 and
3.00, respectively) were low. In one patient, the axial plane alone correctly
showed that no tear was present. No statistically significant difference was
observed between imaging plane groups of both menisci in the diagnosis of
meniscal tears (p > 0.05). The axial plane increased the accuracy
of sagittal and coronal planes of lateral meniscus when combined.
CONCLUSION. In standard knee MRI examinations, the axial imaging
plane may be valuable for the detection and characterization of meniscal
tears.
Introduction
MRI is the preferred diagnostic method for evaluation of internal
derangements of the knee. The suspicion of meniscal tear constitutes one of
the leading indications for MRI examination of this articulation
[1]. Imaging in the sagittal
and coronal planes has been proven to be valuable in the MRI diagnosis of
meniscal tears
[2–4].
The use of the axial plane for meniscal evaluation has been investigated to a
lesser degree but has been reported to be helpful in meniscal assessment,
particularly with respect to characterization of tears
[5–7].
No previous study has addressed the accuracy of the axial plane for the
detection of meniscal disease. The aim of this study was to determine the
value of standard axial MR images—alone and in combination with other
imaging planes—in the diagnosis of meniscal tears of the knee.
Materials and Methods
After approval from the investigational review board at our institution had
been obtained, the medical records of all patients who had an MRI examination
of the knee over a 14-month period were retrospectively reviewed to identify a
subpopulation of patients who underwent arthroscopy. This review yielded 62
patients (55 men and seven women; age range, 23–68 years; mean age, 46.8
years). The time interval between the MRI examination and arthroscopy ranged
between 2 weeks and 16 months (mean, 20.8 weeks).
MRI Protocol
All MRI examinations were performed with a 1.5-T unit (Signa, General
Electric Medical Systems). Images were obtained using a dedicated knee coil.
Three imaging sequences (one for each plane) from the standard knee imaging
protocol were selected for this study and included axial fat-saturated fast
spin-echo proton density (TR range/TE range, 2,650–2,717/12–54),
coronal fat-saturated fast spin-echo proton density
(3,000–3,400/15–65), and sagittal spin-echo proton density (TR
range/TE, 2,117–2,367/30). Other imaging parameters included the
following: number of excitations, 2; matrix, 256 x 192 (axial and
coronal) and 256 x 128 (sagittal); and field of view, 160 units (axial)
and 140 units (coronal and sagittal). The sequences were obtained with a 4-mm
slice thickness in 43 patients and with a 5-mm slice thickness in 19 patients
because it was a retrospective review.
Imaging Analysis
The imaging sequences were separated into six evaluation groups as follows:
I, axial; II, coronal; III, sagittal; IV, axial and sagittal; V, axial and
coronal; and VI, coronal and sagittal. To control bias associated with case
recognition, we randomly distributed the studies within each group according
to sequence, and interpretation sessions were separated by at least 1 day.
Image groups were independently reviewed by two musculoskeletal radiologists
with different levels of expertise (observer 1, 1 year 6 months of fellowship
training; and observer 2, 4 years of experience as a musculoskeletal
radiologist). Reviewers were blinded to arthroscopic findings.
The data acquired were evaluated for the presence or absence of meniscal
tear. Meniscal tears were characterized as increased signal intensity on MRI
extending to the articular surface of the meniscus or abnormal morphology, or
both [1]. Observers were
requested to determine the confidence level for the diagnosis of meniscal tear
in all image groups based on a 5-point scale: 1, completely uncertain of tear;
2, small likelihood of tear; 3, equivocal for tear; 4, probable tear; and 5,
very certain of tear. A similar 5-point scale was used for the diagnosis of no
tear. Medial and lateral menisci were evaluated as separate interpretations.
All discordant results were reviewed by consensus. MRI findings were then
compared with those of arthroscopy. MRI reports of all patients were also
reviewed by a radiologist not involved in imaging interpretation, and other
associated findings were evaluated. Findings recorded were ligamentous tears;
osteoarthritic changes, including osteochondral defects and osseous
abnormalities; and the presence or absence of meniscal cysts, intraarticular
bodies, and joint effusion.
Statistical Analysis
Sensitivity, specificity, and accuracy for the detection of meniscal tears
were calculated for each observer separately and for interpretations in
consensus. Statistical analysis was performed with the McNemar test to
interpret the differences in diagnosis between the six imaging groups. Mean
confidence levels for every group of interpretation were calculated for all
observers in consensus, and the difference between the confidence levels was
analyzed by the Wilcoxon's signed rank test. Interobserver agreement for
detection of meniscal tears for each plane group was calculated using kappa
values. Kappa values were interpreted as follows: 0, poor; 0.01–0.20,
slight; 0.21–0.40, fair; 0.41–0.60, moderate; 0.61–0.80,
high; and 0.81–1.00, almost perfect.
Results
At arthroscopy, 40 medial menisci and 16 lateral menisci were diagnosed as
torn. The first observer correctly diagnosed 27 of 40 medial meniscal and four
of 16 lateral meniscal tears, and the second observer correctly diagnosed 32
of 40 medial meniscal and seven of 16 lateral meniscal tears using the axial
plane alone (Figs. 1A,
1B and
2A,
2B). In one patient with no
tear of the lateral meniscus, presence of no tear was correctly diagnosed on
the basis of the axial plane. The findings of all other plane groups were
false-positive. In 16 patients, on the basis of the axial plane alone the
presence or absence of tear of the menisci was correctly diagnosed, whereas
false-positive or false-negative findings based on other plane groups were
recorded. In 14 of these patients, the lateral meniscus was involved; and in
two, the medial meniscus was involved. The tears that were most often
diagnosed with the axial plane alone were radial or complex tears of the
posterior horn.
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Fig. 1A. —66-year-old man with radial tear of lateral meniscus. Axial
fat-saturated fast spin-echo proton density–weighted image (TR/TE,
2,717/54; slice thickness, 4 mm) of right knee shows well-defined linear
increased intensity (arrow) in body of lateral meniscus starting from
free edge.
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Fig. 1B. —66-year-old man with radial tear of lateral meniscus. Coronal
fat-saturated fast spin-echo proton density–weighted image (3,750/60;
slice thickness, 4 mm) shows tear as vertical defect in body of lateral
meniscus (arrow).
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Fig. 2A. —62-year-old man with bucket-handle tear of medial meniscus.
Axial fat-saturated fast spin-echo proton density–weighted image (TR/TE,
2,717/54; slice thickness, 4 mm) of right knee reveals thinning and
irregularity of medial meniscus body with bucket-handle component
(arrows) extending from anterior to posterior.
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Fig. 2B. —62-year-old man with bucket-handle tear of medial meniscus.
Bucket-handle tear (arrow) is shown on coronal fat-saturated fast
spin-echo proton density–weighted image (3,267/60; slice thickness, 4
mm).
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All findings of sensitivity, specificity, and accuracy are summarized in
Table 1 for each imaging group
and for each observer. In the evaluation of the medial meniscus, the highest
sensitivity (95%) was observed in imaging group VI (coronal and sagittal
planes). Sensitivity in the axial plane (80%) was comparable to other imaging
plane groups. Specificity was increased with group V (axial and coronal
planes) when compared with group I (axial plane) and group III (coronal
plane). Highest accuracy (92%) was obtained in group III (coronal plane) and
group V (sagittal and coronal planes). Accuracy in the axial plane alone (79%)
was comparable to that of other imaging plane groups. The axial plane when
combined with other imaging planes did not result in a significant change in
overall accuracy.
In the evaluation of the lateral meniscus, highest sensitivities were found
in group II (sagittal plane), group III (coronal plane), and group IV (axial
and sagittal planes) (81%). Sensitivity was lower (37.5%) in the axial plane
when compared with the other planes. Specificity was also increased in group V
(axial and coronal planes) when compared with group I (axial plane) or with
group III (coronal plane) for the lateral meniscus. The highest accuracy was
observed in group III (coronal plane) and in group IV (axial and sagittal
planes) (90% and 94%, respectively). Accuracy in the axial plane alone for the
lateral meniscal tears was also comparable to other imaging plane groups
(71%). The axial plane when combined with other planes increased the accuracy
of the sagittal plane for the first observer and the accuracy of the coronal
plane for the second observer. Group IV (axial and sagittal planes) had the
highest accuracy (94%) of any plane group with regard to consensus
interpretation, including group VI (coronal and sagittal planes) (85%).
When diagnosis of meniscal tears was compared between the plane groups, no
statistically significant difference was observed in assessment of the medial
or lateral meniscus (p > 0.05).
The mean confidence levels of each imaging group are summarized for both
menisci in Table 2. The axial
plane had the lowest mean confidence levels for both the medial and lateral
menisci. These levels proved significantly lower for this imaging plane than
all other imaging plane groups (p < 0.001). Group IV (axial and
sagittal planes) had significantly higher mean confidence levels compared with
group V (axial and coronal planes) and group III (coronal plane) for the
second observer with regard to the lateral meniscus (p <
0.05).
The kappa values are given in Table
3 for the assessment of both menisci. Interobserver agreement was
high for the axial plane with regard to the medial meniscus (
= 0.64)
and moderate for the axial plane with regard to the lateral meniscus (
= 0.41). The lowest kappa value was obtained for group III (coronal plane)
with regard to the lateral meniscus ( = 0.35, fair) and highest for
group VI (sagittal and coronal planes) with regard to the medial meniscus
( = 0.93, almost perfect).
Associated MRI findings that were identified included osteoarthritic
changes (n = 39), anterior cruciate ligament tear (n = 12),
medial collateral ligament tear (n = 5), posterior cruciate ligament
tear (n = 1), patellar tendon rupture (n = 1), popliteus
muscle injury (n = 1), semimembranosus tendon avulsion (n =
1), joint effusion (large, n = 10; small, n = 5), meniscal
cyst (n = 5), intraarticular bodies (n = 5), and
osteochondritis dissecans (n = 1). In three patients, either motion
or metallic artifact was present leading to degradation of the MRI study.
In 31 patients with erroneous interpretation of meniscal disease,
osteoarthritic changes were noted (Fig.
3A,
3B,
3C,
3D), including secondary
osteoarthritis in one patient with osteochondritis dissecans. In nine of 12
patients with anterior cruciate ligament tears, "meniscal disease"
was misinterpreted. In six of these patients, false-positive interpretation of
a lateral meniscus tear was made (Fig.
4A,
4B). Incorrect diagnosis
occurred in four patients with variable amounts of joint effusion and in three
patients with tendon injuries. In four patients who had only fraying of the
lateral meniscus noted at arthroscopy, false-positive interpretations were
rendered by both observers. In one patient with a small medial meniscal tear
that was stable arthroscopically, both observers rendered false-negative
interpretations.
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Fig. 3A. —54-year-old man with osteoarthritis and chondromalacia with
misinterpretation of axial images for lateral and medial menisci. Sequential
axial fat-saturated fast spin-echo proton density–weighted images
(TR/TE, 2,700/54; slice thickness, 4 mm) of right knee show medial and lateral
menisci as intact.
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Fig. 3B. —54-year-old man with osteoarthritis and chondromalacia with
misinterpretation of axial images for lateral and medial menisci. Sequential
axial fat-saturated fast spin-echo proton density–weighted images
(TR/TE, 2,700/54; slice thickness, 4 mm) of right knee show medial and lateral
menisci as intact.
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Fig. 3C. —54-year-old man with osteoarthritis and chondromalacia with
misinterpretation of axial images for lateral and medial menisci. Coronal
fat-saturated fast spin-echo proton density–weighted image (3,767/60;
slice thickness, 4 mm) shows tear (arrow) of lateral meniscus as
tapering and increased intensity.
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Fig. 3D. —54-year-old man with osteoarthritis and chondromalacia with
misinterpretation of axial images for lateral and medial menisci. Sagittal
spin-echo proton density–weighted image (2,167/30; slice thickness, 4
mm) shows tear (arrow) on body of medial meniscus as vertical
defect.
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Fig. 4A. —45-year-old woman with anterior cruciate ligament tear with
false-positive interpretation of lateral meniscus tear. Axial fat-saturated
fast spin-echo proton density–weighted image (TR/TE, 2,717/54; slice
thickness, 4 mm) of right knee shows linear increased signal intensity
(arrow) in anterior horn of lateral meniscus thought to be tear.
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Fig. 4B. —45-year-old woman with anterior cruciate ligament tear with
false-positive interpretation of lateral meniscus tear. Coronal fat-saturated
fast spin-echo proton density–weighted image (3,750/60; slice thickness,
4 mm) shows meniscus is intact.
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Discussion
In the evaluation of the menisci of the knee, MRI has been found to be
highly accurate for the diagnosis of meniscal tears
[1]. Many investigators have
suggested that the sagittal imaging plane is the most useful for the detection
of meniscal disease and that the coronal plane improves the overall diagnostic
accuracy
[2–4].
For most institutions, the axial fast spin-echo fat-saturated proton density
sequence is the only technique used for evaluation in the axial plane
[7]. There have been reports on
thin-section axial 3D gradient-echo sequences in the evaluation of the menisci
[5,
6]. These studies have shown
that the extent and severity of meniscal tears were better delineated compared
with coronal and sagittal images, especially when disease existed in the
periphery of the meniscus. Small radial tears of the free edge and displaced
fragments have also been reported to be better visualized in this plane
[5,
6]. These sequences, however,
are not routinely used in clinical practice, primarily related to concerns
regarding the study acquisition time. A preliminary report
[7] suggested that most of the
different types of meniscal tears have discrete appearances in axial
fat-saturated fast spin-echo proton density images that provide additional
information regarding lesion characterization. Their results, however, were
not compared with those of arthroscopy.
Accuracy for the diagnosis of meniscal tears with MRI has been studied by
different authors, and several different sensitivity, specificity, and
accuracy levels have been found
[1,
8–10].
Sensitivity and specificity ranged between 87% and 97% and 82% and 91% for the
medial meniscus and between 69% and 92% and 91% and 98% for the lateral
meniscus, respectively, in these previous studies
[1,
8,
9]. Our findings were similar
to the highest level of these ranges for both sensitivity and specificity. In
these previous studies, only coronal and sagittal planes were used together
with no consideration given to the value of the axial plane.
Our study showed high accuracy levels for the detection of meniscal disease
using the axial plane alone. Confidence levels, however, were low with regard
to the assessment of both the medial and lateral menisci. In sagittal and
coronal orientations, menisci are seen in almost 13 or 14 images, but in the
axial plane with 4- to 5-mm slice thickness, they are seen only in two or
three images. This is one potential reason for low confidence levels related
to meniscal tears associated with assessment of the axial plane. We evaluated
axial images that were characterized by a 4- to 5-mm slice thickness because
such images are used in routine MRI protocols of the knee. Although spatial
resolution decreases with 5-mm-thick sections compared with thinner slices,
signal-to-noise ratio increases and this may compensate, to some degree, for
the lower spatial resolution. It has been shown that 5-mm-thick images do not
have lower diagnostic accuracy compared with that of thinner slices used for
MRI studies of the knee
[2].
With respect to the diagnosis of meniscal tears in axial images, confidence
intervals may also reflect a relative lack of experience of the observers. As
previously mentioned, the sagittal and coronal images are generally relied on
for the diagnosis of meniscal disease. Because no statistically significant
difference was evident in the diagnosis of meniscal tears when comparing the
axial imaging plane alone and all other imaging groups, it can be postulated
that the axial plane is also valuable in the evaluation of meniscal tears.
Our observations indicate that the overall accuracy for the diagnosis of
medial meniscus tear was not increased by the addition of the axial plane to
other imaging planes. Accuracy and specificity were, however, improved by the
addition of the axial plane to other imaging planes for the lateral meniscus.
The highest accuracy was observed in group IV (axial and sagittal planes) for
assessment of the lateral meniscus, and mean confidence levels were
significantly higher for this imaging group. Most of the true-positive and
true-negative evaluations compared with other plane groups were found for the
lateral meniscus. Therefore, the axial plane was especially helpful when
interpreted with the sagittal plane in the evaluation of the lateral
meniscus.
We have found that most of the correctly diagnosed tears on the axial plane
alone were radial or complex tears of the posterior horn. These findings will
lead us to future studies of the accuracy of the characterization of meniscal
tears with the addition of the standard axial plane to other planes.
We believe that the low kappa values, particularly with regard to the
lateral meniscus, may have been related to patient selection. Most of the
patients who underwent arthroscopy had cartilage abnormalities and
osteoarthritis (n = 39), which contributed to false-positive and
false-negative evaluations when only one plane was used for assessment. We
also had a significant number of patients with tears of the anterior cruciate
ligament who had false-positive interpretations of meniscal disease. An
anterior cruciate ligament tear is especially important in patients with
false-positive diagnoses of lateral meniscal tears. Fraying and small stable
tears of the menisci were also mistaken during our evaluation. In addition, a
long interval (> 1 year) between MRI and arthroscopy in four patients may
account for this finding as a result of interval healing or interval
development of lesions or both.
This study provided accuracy levels for the assessment of both meniscal
tears in the axial plane alone and in combination with other imaging planes.
The high levels of accuracy for detection of medial meniscal disease and
increased levels of accuracy for detection of lateral meniscal disease with
the addition of axial images shown in this study suggest that the axial
imaging plane may prove helpful for the detection of meniscal tears in routine
studies of the knee.
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Abstract
Introduction
