This document discusses several normal anatomical structures and imaging artifacts that can mimic meniscal tears on MRI. The anterior transverse ligament, meniscofemoral ligaments, oblique menisco-meniscal ligaments, and genicular arteries can appear as increased signal near the meniscus on MRI. Imaging artifacts like magic angle phenomenon, partial volume averaging, and truncation errors can also create the appearance of meniscal tears when none exist. Conditions like meniscal contusions, chondrocalcinosis, meniscal ossification and intra-articular gas can further complicate the diagnosis. Differentiating these mimickers from true tears requires correlation of imaging findings with arthroscopy.

1. Diagnostic pitfalls mimicking meniscal tear
and post-operative meniscal changes.
Dr/ ABD ALLAH NAZEER. MD.

2. NORMAL ANATOMIC STRUCTURES
Normal anatomic structures that lie in close proximity to the meniscus
and show similar MR signal intensity to the meniscus are common
mimickers of meniscal pathology. The anterior transverse ligament attaches
the anterior margin of lateral meniscus to anterior horn of medial meniscus.
The anterior transverse ligament is
clearly demonstrated on coronal
PD-W fat sat (a) and T1-W images
(b) and on axial PD-W fat sat (c)
and T1-W images (d) as it courses
from the anterior margin of lateral
meniscus to the anterior horn of
medial meniscus.

3. On sagittal images, a linear band of increased signal is present
between the anterior horn of lateral meniscus and the anterior
transverse ligament, occasionally simulating an oblique meniscal tear.
A linear band of increased signal intensity (arrowhead) is noticed on a sagittal PD-
W fat sat image (j), between the anterior horn of lateral meniscus and the
anterior transverse ligament (arrow), simulating an oblique meniscal tear. The
course of the ligament is clearly demonstrated on adjacent sagittal PD-W fat sat
images (ej). Osgood-Schlatter disease is also present with patellar tendon
thickening, deep infrapatellar bursitis and tibial tubercle fragmentation.

4. By the same mechanism, increased linear MR signal in the medial aspect of the posterior horn of
lateral meniscus at the attachment site of the meniscofemoral ligament can mimic the
appearance of a vertical meniscal tear. The meniscofemoral ligament originates from the lateral
meniscus posteriorly and inserts on the medial aspect of medial femoral condyle. The ligament is
composed of 2 separate branches, the ligament of Humphry and the ligament of Wrisberg.
A linear band of increased signal intensity (arrowheads) is noticed on sagittal PDW fat
sat images (e, f), between the posterior horn of lateral meniscus and the ligament of
Humphry, simulating a vertical meniscal tear. The anterior meniscofemoral ligament
(arrows) is clearly demonstrated on adjacent sagittal PD-W fat sat images (a-f).

5. The anterior meniscofemoral ligament (arrows) is depicted on
adjacent coronal PD-W fat sat images (g, h) as it courses from the
medial aspect of medial femoral condyle to the medial aspect of the
posterior horn of lateral meniscus. A horizontal tear of the posterior
horn of medial meniscus (arrowheads) is also noticed.

6. The ligament of Humphry (arrows) is demonstrated on adjacent axial PD-W
fat sat images (i-l) as it courses from the medial aspect of medial femoral
condyle to the medial aspect of the posterior horn of lateral meniscus.

7. A linear band of increased signal intensity (arrowhead) is noticed on a sagittal PD-W fat
sat image (e), between the posterior horn of lateral meniscus and the ligament of
Wrisberg, simulating a vertical meniscal tear. The posterior meniscofemoral ligament
(arrows) is clearly demonstrated on adjacent sagittal PD-W fat sat images (ae).

8. The ligament of Wrisberg (arrows) is depicted on adjacent coronal PD-W fat sat
images (f, g) as it courses from the medial aspect of medial femoral condyle to
the medial aspect of the posterior horn of lateral meniscus. A horizontal tear of
the posterior horn of medial meniscus (arrowheads) is also noticed.

9. The medial and lateral oblique menisco-meniscal ligaments have a
reported incidence of 1% to 4% and are an uncommon source of diagnostic
difficulty. These ligaments extend obliquely from the anterior horn of one
meniscus to the posterior horn of the opposite meniscus.
Sagittal T1-W (a) and PD-W fat sat images (b) through the intercondylar notch at
the level of PCL demonstrate the medial oblique menisco-meniscal ligament as a
thin linear structure of low signal intensity (arrows) mimicking a displaced
meniscal fragment. A Baker's cyst is also noticed (arrowheads). Coronal PD-W fat
sat image (c) shows the ligament as it passes through the intercondylar notch.

10. Bucket-handle meniscal tear of medial meniscus. The thin linear structure of low
signal intensity (arrow) which courses under the PCL (arrowheads) on a sagittal T2-W
image (a) represents a displaced meniscal fragment ("double PCL sign"). The
meniscal fragment (arrow) is also demonstrated as it passes through the
intercondylar notch under the PCL (arrowheads) on a coronal PD-W fat sat image (b).

11. The bursa of the popliteus tendon, which lies close to the posterolateral meniscus,
may also be mistaken for a meniscal tear. Fluid within the bursa appears as high-
signal intensity on T2-weighted MR images and gives the appearance of a vertical
or slightly diagonal tear in the posterior horn of lateral meniscus.
Fluid within the bursa (arrowhead) appears as high-signal intensity on a sagittal
PD-W fat sat image (e) and gives the appearance of a vertical or slightly diagonal
tear in the posterior horn of lateral meniscus. The popliteus tendon and bursa
(arrows) are clearly demonstrated on adjacent sagittal PD-W fat sat images (a-e).

12. Fluid within the bursa (arrowhead) appears as high-signal intensity on a
coronal PD-W fat sat image (f) and resembles a vertical or slightly diagonal
tear in the posterior horn of lateral meniscus. The popliteus tendon and bursa
(arrows) are demonstrated on adjacent coronal PD-W fat sat images (f, g).

13. The lateral inferior genicular artery arises from the popliteal artery at the level of the tibiofemoral
joint and courses laterally to the anterior aspect of the knee where it and other arteries compose the
genicular anastomosis. Unlike the superior genicular arteries and the inferior medial genicular artery,
the lateral inferior genicular artery is closely applied to the meniscus as it wraps around the knee,
lying in a periarticular fat pad between the meniscus and lateral collateral ligament (LCL).
Adjacent sagittal T1-W images through the lateral meniscus (a-d) demonstrate the course of the lateral
inferior genicular artery (arrows). A narrow separation (arrowhead) between the anterior horn of lateral
meniscus and the lateral inferior genicular artery (c) can sometimes be mistaken for a meniscal tear.

14. Meniscal flounce is an uncommon variant that can simulate meniscal pathology. It
occurs in the absence of a tear and is a single symmetric fold along the free edge of the
meniscus. It is a normal finding that is said to be present with ligamentous laxity,
although it is not necessarily indicative of a tear in the ligament. The appearance is like
that of a carpet that has a wrinkled edge and, in fact, presumably has a similar cause-
that is, sliding of the tibia on the femur because of ligamentous laxity with resultant
folding or buckling of the inner edge of the meniscus. It has no known significance.
Meniscal flounce of medial meniscus (arrows) which results in a
slight wavy S-shaped appearance along the free edge of the
meniscus on adjacent sagittal PD-W fat sat images (a, b).

15. A speckled appearance of the anterior horn of lateral meniscus is a frequent
finding that has been explained by fibers of the anterior cruciate ligament
inserting into the meniscus. It can be seen on one or two of the most medial
sagittal images. The appearance can be mistaken for a torn lateral meniscus.
A speckled appearance (white arrows) of the anterior horn of lateral meniscus
on adjacent sagittal PD-W fat sat images (a, b), which is caused by the
insertion of the fibers of anterior cruciate ligament (black arrow) into the
meniscus (c). The finding can easily be mistaken for a torn lateral meniscus.

16. A discoid lateral meniscus is a normal variant seen in about 3% of knees. An uncommon
variant of a discoid lateral meniscus is a Wrisberg's variant, in which the posterior horn is
not attached to the capsule and is, therefore, mobile enough to move freely and sublux
into the joint, causing pain and, occasionally, locking. The MR imaging appearance is a
discoid lateral meniscus with no posterior horn attachment or a free-floating posterior
horn. Unlike the incidental discoid meniscus, which should be asymptomatic unless torn,
a Wrisberg's variant can be a source of pain and require surgery. It is most commonly
found in children, although it can be seen in patients at any age.
Peripheral tear-meniscocapsular detachment (arrows) of the posterior
horn of medial meniscus on sagittal (a) and coronal PD-W fat sat images
(b). Bone contusions of the posterior tibial epiphysis are also noticed.

17. The most significant characteristic in the appearance of pediatric meniscus is the presence of
intrasubstance signal that represents perforating vessels within the meniscus. This signal has been
described in 82% of menisci in children and is considered to be a normal finding that should not be
misinterpreted as mucinous or myxoid degeneration like in adults. The MR criteria for establishing
the diagnosis of a meniscal tear are the same in children and adults. The 2 most important criteria
are intrameniscal signal that extends to the surface and abnormal meniscal morphology .
The presence of intrasubstance signal in the posterior horns of medial
(arrows) and lateral (arrowheads) meniscus represents perforating vessels
within the menisci. This signal is considered a normal finding and should not
be misinterpreted as mucinous or myxoid degeneration like in adults.

18. MRI ARTIFACTS
MRI artifacts created by the pulse sequences used can result in
image degradation and errors in diagnosis.
The magic angle phenomenon occurs on short TE images in
fibers that are orientated 55° relative to the static magnetic field.
This MRI artifact is a cause of increased signal in the medial
segment of the posterior horn of lateral meniscus. Imaging the
knee joint in slight abduction can alter the orientation of the
fibers in the posterolateral meniscus and eliminate this artifact.
Partial volume averaging is a type of artifact that occurs when 3-
dimensional volumetric data is used to create a 2-dimensional image. The
attachment of the capsule produces a concavity at the outer margin of the
meniscus that is filled with periarticular fat and neurovascular structures.
When the signal from a normal dark meniscus is averaged with the bright,
fatty tissue adjacent to the outer concave margin of the meniscus, high-
signal intensity linear artifacts that may simulate a tear are created
through the periphery of the meniscus on sagittal images.

19. Partial volume averaging produces linear artifacts mimicking horizontal
tears (arrows) in the anterior and posterior horns and in the body of lateral
meniscus on adjacent sagittal PD-W fat sat (a-c) and T1-W images (d-f).

20. Motion artifact can be particularly problematic in MRI because of the long image
acquisition times. If a structure moves to different positions during image acquisition,
the image appears blurred or double exposed. Intrameniscal signal resembling a tear
may result. If motion artifact is present on an MRI scan and a meniscal tear is
suspected by imaging criteria, the sequence should be repeated without motion.
Motion artifact mimicking a tear (arrow) in medial meniscus on a coronal
PD-W fat sat image (a). The meniscus appears normal on a coronal T1-W
image at the same level (b), which is obtained without motion.

21. Wrisberg rip and pseudotear of posterior horn of lateral meniscus.
Truncation error is a type of MRI artifact that occurs at high-contrast boundaries, such
as between the articular cartilage and the menisci. Truncation artifact is a result of
inherent errors in the Fourier transformation method of image reconstruction. It
appears as a series of alternating parallel bands of bright and dark signal. When a line
of high-signal intensity projects over a low-signal meniscus, a pseudo tear is created.

22. Wrisberg rip for a
longitudinal tear in
the posterior horn
of the lateral
meniscus that
extends laterally
from the Wrisberg
ligament
attachment. They
feel that the tear is
the result of
traction from the
ligament of
Wrisberg in cases
of anterior cruciate
ligament tears.

23. A meniscal contusion occurs when the meniscus gets trapped between the tibia and the
femur during a traumatic event-usually involving ACL tears. Increased signal in the
periphery of the meniscus can resemble a tear; however, the signal intensity of a contusion
is indistinct and amorphous rather than sharp and discrete. An adjacent bone contusion
should alert one to the possible presence of a contusion rather than a meniscal tear .
Meniscal contusion with amorphous and indistinct signal in the periphery of the
posterior horn of medial meniscus (arrows) on adjacent sagittal (a-c) and coronal
PD-W fat sat images (d-f) after an acute traumatic event with bone contusions,
complete ACL and partial medial collateral ligament (MCL) tear. Follow-up MRI
5 months after initial imaging revealed resolution of abnormal meniscal signal.

24. Grade 2C signal in the posterior horn of medial meniscus
(arrows) on adjacent sagittal PD-W (a, b) and coronal PD-W fat
sat images (c, d), confirmed as intact meniscus on arthroscopy.

25. Chondrocalcinosis is defined as a radiographically visible
calcification in the cartilage of a joint. It can occur in the hyaline
articular cartilage lining the articular surface or in the
fibrocartilage of a meniscus. Although it can occur from many
types of calcium crystals, the most commonly seen is from
calcium pyrophosphate dihydrate crystal deposition in
pseudogout, which is also known as calcium pyrophosphate
dihydrate deposition disease. When MR imaging is performed on
a meniscus with chondrocalcinosis, the T1-weighted or PD-
weighted sequence shows high signal, which can be mistaken for
a meniscal tear. Differentiating a meniscal tear from the high
signal of chondrocalcinosis can be difficult, if not impossible.
Most meniscal tears have a more linear appearance than the
globular high signal seen in chondrocalcinosis. Comparison with a
conventional radiograph of the knee will help avoid this pitfall.
However, chondrocalcinosis can also obscure a tear and result in
a false-negative report.

26. Chondrocalcinosis in medial meniscus. Radiograph of the knee (a) reveals meniscal
calcifications in the posterior horn of the meniscus (arrow). Coronal PD-W (b) and
sagittal PD-W images (c, d) show marked high signal throughout the posterior horn of
medial meniscus (arrows) that resemble a tear. No meniscal tear at the arthrography.

27. Chondrocalcinosis in lateral meniscus. Anteroposterior knee radiograph (a)
shows meniscal calcifications in lateral meniscus (arrow). Sagittal PD-W
image (b) shows high signal intensity both in the anterior and posterior horn
of lateral meniscus with extension to inferior articular surface (arrowheads),
interpreted as meniscal tears. No meniscal tear was found on arthroscopy.

28. Meniscal ossification is a rare finding of unknown origin with a prevalence of 0,15% in MR
examinations. Meniscal ossicles are frequently asymptomatic and discovered incidentally. They
mimic intra-articular loose bodies and lead to inappropriate surgical treatment. Imaging is
required to confirm the diagnosis and assess meniscal integrity. The typical appearance is that of
an intrameniscal lesion which shows central high signal on T1-weighted images, due to normal
fat bone marrow, surrounded by low signal on both T1-weighted and T2-weighted images
related to the cortical rim. The high signal of the inner portion, which is the same as viable
medullary bone, allows differentiation from loose bodies, which may present internal low signal.
However, loose bodies, if ossified, may present with an internal high signal.
Ossification in the discoid lateral meniscus
with signal intensities equal to bone.
Anteroposterior view of the left knee
showing corticated bone densities
projected in the lateral aspect of the knee.

29. Gas within the knee joint may represent a vacuum phenomenon or iatrogenic placement
during arthrography or joint aspiration. The low magnetic susceptibility of gas produces a
discrete signal void. If the gas collects adjacent to the meniscus, it can simulate a meniscal
tear, abnormal meniscal volume, or a displaced meniscal fragment .
Gas within the knee joint (arrows) produces a discrete signal void on adjacent
coronal T2 GRE (a, b) and T1-W images (c, d), as well as on sagittal T2-W images
(eg), that simulates a radial meniscal tear in medial meniscus (arrowheads).

30. Post-operative changes. In recent years, there has been increasing
demand for postoperative evaluation of the meniscus because of the
increased number of patients who undergo meniscal surgery. The most
common surgical procedures include partial meniscectomy and meniscal
repair. Recognition of the normal postoperative MRI appearance of the
meniscus and of abnormalities is essential for the accurate assessment of
symptomatic patients.
After partial meniscectomy, meniscal morphology may differ from that of a
nonoperative meniscus with its postoperative appearance depending on the amount
and anatomical location of meniscal tissue resected and the morphology of the
previous meniscal tear. On MR imaging, such postsurgical variations in meniscal shape
typically include diminution in the overall size of the meniscus or meniscal horns,
blunting of the meniscal apical margin, or variable degrees of meniscal truncation.

31. Diagram of medial meniscus (a) illustrating the two types of partial meniscectomy:
circumferential (b)-where the posterior horn and anchor, and therefore the meniscal
"hoop" has been preserved and segmental (c)-where the posterior attachment has
been resected and the load transmitting function has been sacrificed.

32. Normal appearance
of a partially resected
meniscus. Adjacent
coronal PD-W fat sat
images from anterior
to posterior (a-c)
demonstrate a small
blunted body of medial
meniscus (arrow)
following circumferential
partial meniscectomy.

33. Normal appearance
of a partially resected
meniscus. Adjacent
coronal PD-W fat sat
images from anterior
to posterior (a-c)
demonstrate a resected
posterior horn of medial
meniscus (arrows)
following segmental
partial meniscectomy.

34. Diminutive morphology
of the posterior horn of
medial meniscus on
adjacent coronal PD-W
fat sat (a, b) and sagittal
PD-W images (c, d)
following prior
meniscectomy with
intrameniscal increased
signal intensity extending
to the inferior meniscal
surface (arrows), which
was interpreted as new
tear. Second-look
arthroscopic evaluation
revealed an intact
meniscus.

35. Intrameniscal increased
signal intensity (arrows)
on adjacent coronal PD-
W fat sat (a-c) and
sagittal PD-W images
(d, e) in the resected
posterior horn of medial
meniscus after partial
meniscectomy was
interpreted as new tear.
Second-look arthroscopy
revealed normal signal
postoperative changes.

36. PD and T2W images. Prior partial meniscectomy and
suture repair. At arthroscopy, there was no tear.
Patient had a suture repair. RIGHT: On new exam, there is a new tear
(yellow arrow). It is not possible to tell if the old tear has healed.
MR-arthrogram: In the new tear the signal is as bright as in the synovial
fluid (yellow arrows). In the healed tear the signal is not as bright.
PD and MR-arthrogram after suture repair
for meniscal tear: healed tear.

37. Sagittal T1-W fat-sat MR
arthrography image
following intra-articular
injection of gadolinium
demonstrates contrast
extending into a subtle
cleft (arrow) in the
posterior horn of medial
meniscus corresponding
to a peripheral vertical
tear, which was
previously missed on
conventional MRI.

38. Meniscocapsular tear and meniscocapsular recess.

39. MR image 1 year after anterior cruciate ligament reconstruction and partial
meniscectomy of medial meniscus shows diminutive body of medial
meniscus with vertical high-signal-intensity cleft through body (arrowhead),
consistent with recurrent or residual tear confirmed surgically.

40. Sagittal T2-weighted MR image obtained
without intraarticular contrast material shows
retear of repaired meniscus after meniscal
repair. Fluid can be seen in the tear (arrow).
Retear of previously repaired meniscus on sagittal
fat-suppressed T1-weighted MR arthrograms

41. Normal intrameniscal signal evolution in the site of surgery, with very good final result.

42. Normal healing sequence. Typical worsening in the first scans - intrameniscal
signal after surgery is worse then before surgery. No healing disturbance.

43. Normal healing sequence. Typical worsening in the first scans - intrameniscal
signal after surgery is worse then before surgery. Any of these scans without
sequentional imaging would be difficult to differentiate from disturbed healing.

44. Abnormal healing of degenerated meniscus.

45. Vascular variants, such as popliteal artery entrapment, cystic adventitial disease,
or deep vein thrombosis, may cause symptoms around the knee and can
potentially be overlooked on MRI of the knee unless specifically reviewed.
Cystic adventitial disease of popliteal artery. Sagittal fat-suppressed T2-
weighted MR image of knee after acute knee injury shows incidental
extensive cystic changes in relation to popliteal artery (arrowheads).

46. Thank You.