This document provides information on knee radiological anatomy, including descriptions of: 1. Normal anatomy of the medial and lateral menisci, including their shapes, sizes, positions, and attachments. 2. Criteria for diagnosing meniscal tears based on abnormal shape and high signal intensity contacting the meniscal surface. Common types of meniscal tears are described. 3. Normal anatomy and criteria for diagnosing tears of the anterior cruciate ligament, posterior cruciate ligament, and medial collateral ligament. Associated injuries like bone bruises and Segond fractures are mentioned.

1. ISLAH RAOOF
KNEE-RADIOLOGICAL
ANATOMY

2. Medial meniscus
Normal Meniscal Anatomy
• Both horns are triangular in shape and have very sharp points.
• The posterior horn is always larger than the anterior horn (figure).
• If this is not the case, then the shape is abnormal, which can be a sign of a meniscal tear or
a partial meniscectomy.
• The posterior root is immediately anterior to the posterior cruciate ligament.
• If it is missing on the sagittal images, then there is a meniscal root tear (figure).
• The anterior horn has an insertion on the tibia and a second portion that travels from medial
to lateral to connect to the anterior horn of the lateral meniscus (intermeniscal or transverse
ligament).

3. Medial meniscus
Normal Meniscal Anatomy

4. Lateral meniscus
Normal Meniscal Anatomy
• On sagittal images the posterior horn is higher in position than the anterior horn.
• Both horns are about the same size.
• The lateral meniscus posteriorly comes up high over the tibial spine to insert near the
posterior cruciate ligament.
• This upward position of the posterior horn may be the reason for the higher signal
intensity of the posterior horn in all planes due to magic angle effect.

7. Criteria for tears
Meniscal Tears
• The two most important criteria for meniscal tears are an abnormal shape of the meniscus and high signal
intensity unequivocally contacting the surface on PD images.
• It is a misunderstanding that menisci should be homogeneously low in signal intensity on proton-density
images.
• The meniscus does not have to be black.
• Only when the high signal unequivocally reaches the surface of the meniscus you can make the diagnosis
of a tear.
• If there is doubt whether the high signal touches the surface, look at all the adjacent images.
• If there is still doubt, then do not diagnose a tear.
• If you have a questionmark in your head, say "meniscus is normal". (figure)

11. Longitudinal tears
Meniscal Tears
• Longitudinal tears parallel the long axis of the meniscus dividing the meniscus into an inner and outer part.
• Therefore, the distance between the tear and the outer margin of the meniscus is always the same (figure).
• The tear never touches the inner margin.
• Longitudinal tears follow the collagen bundles that parallel the contour of the meniscus.
• If a longitudinal tear has other components (horizontal or radial), then it is a complex tear violating the collagen bundles.
• This requires a higher energy trauma.
Bucket handle tear
• is a displaced longitudinal tear.
• On coronal images bucket handle tears are easier to recognize.
• Normally there are only two structures in the intercondylar fossa: the anterior and posterior cruciate ligament.
• Any other structure in the intercondylar fossa is abnormal and a displaced meniscal fragment is the most likely possibility.

15. Longitudinal tears
Meniscal Tears
• Flipped meniscus is a form of bucket handle tear.
• There is a capsular detachment or peripheral tear of the meniscus, usually the posterior
horn.
• The posterior horn flips over onto the anterior horn.

17. Horizontal tears
Meniscal Tears
• Horizontal tears divide the meniscus in a top and bottom part (pita bread).
• If horizontal tears go all the way from the apex to the outer margin of the meniscus,
they may result in the formation of a meniscal cyst.
• The synovial fluid runs peripherally through the horizontal tear and accumulates within
the meniscus and finally result in a cyst.
• The connection with the joint space is often lost, so they will not fill with contrast on MR-
arthrography.
• The synovial fluid is absorbed and is replaced by a gelatinous substance.

19. Horizontal tears
Meniscal Tears
• There are 3 criteria for the diagnosis of a meniscal cyst:
1. Horizontal tear.
2. Fluid accumulation with bright signal on T2.
3. Flat lining against the periphery of the meniscus.
• The diagnosis of a meniscal cyst is important to the surgeon because it takes one
operation on the outside of the knee to remove the cyst and another operation on the
inside for the meniscus.

20. Radial tears
Meniscal Tears
• Radial tears are perpendicular to the long axis of the meniscus.
• They violate the collagen bundles that parallel the long axis of the meniscus.
• These are high energy tears. They start at the inner margin and go either partial or all
the way through the meniscus dividing the meniscus into a front and a back piece.
• Radial tears are difficult to recognize. You have to combine the findings on sagittal and
coronal images to make the diagnosis.

24. Radial tears
Meniscal Tears
• The following combination of findings is diagnostic:
• In one plane: triangle missing the tip and in the other plane: a disrupted bow tie.
• Small radial tears are difficult to diagnose.
• Sometimes the only sign is a disrupted bow tie.
• If you image a complete radial tear directly along the length of the tear you will see an
absent or empty meniscus.
• These complete radial tears open up and give the impression that there is a part missing.
• However you will not find a displaced meniscal fragment. It is simply separation of the
meniscal parts.

27. Meniscal root tear
Meniscal Tears
• A meniscal root tear is a radial tear located at the meniscal root.
• Normally when you image the posterior cruciate ligament on sagittal images you should
see a considerable portion of the posterior horn of the meniscus on that image or the
image adjacent to it.
• If this is not the case it is an absent or empty meniscus-sign, indicating a radial tear.

28. Post-operative meniscus
• Post-operative menisci are harder to evaluate because the two most important criteria,
i.e. abnormal signal and abnormal shape, do not apply.
• Abnormal signal is no longer a reliable sign of a tear, because if there has been a
suture repair, this will heal with scar tissue, which also has high signal on PD-images
(figure).
• Although an uncommon finding, if there is also high signal on T2-weighted images, then
you can make the diagnosis of a tear, as this is the result of synovial fluid leaking into a
meniscal tear.
• This however is an uncommon finding.

30. Post-operative meniscus
• Abnormal shape can be the result of partial meniscectomy.
• So you need to know what procedure was performed during arthroscopy.
• Only when comparison is made with prior postoperative images, can you determine, if
an abnormal shape is a new finding indicative of a new tear.
• Sometimes differentiation between normal post-op findings and a re-tear is not possible
on conventional MR-images.
• In these cases, MR-arthrography with 40cc diluted Gadolinium helps to make the
distinction because even small amounts of Gadolinium that leak into a tear are readily
visible on fat saturated T1 images.

33. Post-operative meniscus
• The case on the left shows a meniscus with an abnormal shape aswell as abnormal
signal touching the surface on PD but not on T2W-images.
• This patient had a prior partial meniscectomy and a suture repair.
• On the basis of these imaging findings, it is impossible to tell if this is a tear or a normal
postoperative finding.
• This patient had another operation for ACL reconstruction.
• The surgeon looked at the meniscus and the meniscus was found to be normal i.e. no
tear.

34. Post-operative meniscus
• This patient had a suture repair for meniscal tear.
• There was a new injury.
• On the new MR, it is impossible to determine if the old tear had healed.
• However a new tear is seen, so this case is easy.
• On an MR-arthrogram, there was very high signal intensity in the new tear comparable
with the synovial fluid, but only moderate signal intensity at the healed old tear.
• So comparison with the old films was diagnostic for the new tear, while the arthrogram
showed that the old tear has healed.

35. Post-operative meniscus
• This patient also had a suture repair for meniscal tear.
• After a new injury, the PD-images show high signal unequivocally reaching the surface
of the meniscus (seen on the original films, but not clearly seen on the compressed
image on the left).
• On this image, it is not possible to tell if the tear has healed.
• So an MR-arthrogram was performed which showed that the tear has healed.

36. Ligaments
• The ACL has interesting anatomy. It is an intra-articular structure, but it is extra-synovial.
• The synovium folds over the ligament. So at arthroscopy they look through the synovium.
• Sometimes when there is a tear ,the synovium layer is intact and only a hemorrhagic ACL is
seen.
• The ACL is composed of 3-5 layers of fibers. Between the fibers there can be fat or
synovium or sometimes a little bit of fluid. This explains why the ACL is not black on PD-
images.
• Do not look at the ACL on PD-images because this may give a false impression of
pathology.
• Only look at the ACL on T2W-images and even on these images the ACL does not have to
be entirely black.
Anterior Cruciate Ligament

45. Ligaments
• Criteria for the normal ACL are:
1. Fiber-orientation as steep or steeper than the intercondylar roof.
2. Fibers all the way from the tibia to the femur
• So on MR the primary signs of a tear are: discontinuity on T2, abnormal orientation or
non-visualisation.
• Many secondary signs of tears have been described, but these are not helpfull, since
we have to rely on direct visualisation of the ligament.
• Only bone bruises can be a helpful secondary sign.
Anterior Cruciate Ligament

46. Ligaments
• Notice that on coronal and axial images fibers of the ACL are right next to the bone of
the intercondylar notch (arrows).
• There should never be any fluid between these ACL-fibers and the bone of the lateral
condyle ('empty notch sign').
• Also notice that the PCL is also composed of many fibers.
• The case on the left shows a ligament that's too flat and we see disrupted fibers so
there is abnormal orientation and discontinuity.
• Based on these images we cannot differentiate between complete tear, high grade
partial tear or partial tear.
Anterior Cruciate Ligament

47. Ligaments
• MRI does not accurately differentiate between partial or complete ACL tear.
• But yes we can differentiate between high grade or low grade injury.
• A high grade injury is 'not able to see 50% of the fibers'.
• So if the othopaedic surgeons operate on a high grade injury, they will either find a
totally torn ACL or a high grade partial tear, that needs to be repaired.
• On the other hand if most of the fibers appear to be intact on MR indicating a low grade
ACL tear, they will find an intact or partially torn ACL, that is stable and doesn't need
any treatment.
Anterior Cruciate Ligament

48. Ligaments
• Bone bruises appear in a very typical location indicating the dislocation, that was the
cause of the ACL-tear.
• On X-rays an important indirect sign of an ACL-tear is a Segond fracture.
• Difficult to see on MR, but much more easy to see on radiographs.
• A Segond fracture is an avulsion fracture at the attachment of the lateral collateral band
due to internal rotation and varus stress.
• In 75-100% there will also be a tear of the ACL.
Anterior Cruciate Ligament

49. Ligaments
The unhappy triad or O'Donoghues syndrome is a different combination of injuries.
• The unhappy triad injury commonly occurs in contact sports such as football when the
knee is hit from the outside.
• This causes an injury to three knee structures:
A. ACL tear
B. MCL tear (medial collateral ligament)
C. Medial meniscal tear
Anterior Cruciate Ligament

50. Ligaments
• Case on the left shows a torn ACL.
• Fibers have an abnormal orientation (too flat).
• Yet it is difficult to see if these are attached to the femur.
• The acute angulation in the ligament is due to fact that the ACL and PCL have scarred
together (see below).
• Sometimes it is easier to see whether these fibers are attached in the coronal plane.
• Against the interior part of the lateral condyle there never should be fluid.
• If this is the case it is called the 'empty notch sign' indicating that the ACL is torn from it's
attachment to the femur.
Anterior Cruciate Ligament

51. Ligaments
• Also in the axial plane there should be ligament next to the condyle.
• At a lower level we see the torn ACL attached to the posterior cruciate ligament.
• They have scarred together.
• This is a very common appearance of a chronic ACL tear.
• This scarring leads to the acute angulation of the ligament.
Anterior Cruciate Ligament

52. Ligaments
• Even though the ACL is connected to the PCL it is not strong enough and still needs reconstruction.
• Case on the left shows a non-visualisation of the ACL on a PD-image. But the lesson is 'do not look
at ligaments on a PD-image'.
• If you want to judge the ACL-ligament look at the T2W-images.
• The T2W-images show fibers going all the way from the tibia to the femur with a normal orientation.
So the ACL is intact.
• This is a case of mucoid degeneration. Normally between the ACL-fibers there can be synovium or
fat.
• In normal aging that can change into gelatinous material.
• This has no effect on the strenght of the ACL.
Anterior Cruciate Ligament

53. Ligaments
• Another case of ACL Mucoid degeneration.
• Often this is associated with cyst-formation in the bone.
• You could call it ganglion cyst, but you could also call it normal because it has no clinical meaning.
• This is part of normal aging.
• Case on the left shows cyst seperate from ACL unlike mucoid degenaration.
• This is a ganglion cyst. Probably also a form of degeneration.
• The difference with Mucoid degeneration is that these cysts can be symptomatic.
• Sometimes these cysts will drained under ultrasound guidance.
• Be sure to use a very large needle, because it is very thick material.
Anterior Cruciate Ligament

54. Ligaments
• We use the same criteria for all the other ligaments in the body.
• The case on the left shows a high grade PCL tear.
Posterior Cruciate Ligament

55. Ligaments
• The superficial medial collateral ligament (MCL) extends from the medial epicondyle to insert not just near the
joint but 7 cm below the joint space.
• At that point there are three landmarks: the inferomedial geniculate artery and paired veins (figure).
• The deep part of the MCL, even when it is normal, you may not be able to see.
• It is closely applied to the medial meniscus and the superficial MCL.
• The case on the left shows a Grade I sprain of the medial collateral ligament.
• The case on the left shows a Grade II sprain of the medial collateral ligament
• The case on the left shows a superficial MCL that is torn from it's attachment on the tibia.
• Remember it should be attached 7 cm below the joint line.
• Deep MCL is also torn the ligament is absent.
Medial collateral ligament

58. Posterolateral Corner injury
• Posterolateral corner contains seven or eight structures.
• Only three of them are important to us because they are visible on MR and because the
surgeon might want to fix them.
• These structures are:
1. Fibular collateral ligament
2. Biceps femoris muscle and tendon.
3. Popliteal tendon
Normal anatomy

61. Posterolateral Corner injury
• The fibular collateral ligament together with the tendon of the biceps femoris form the letter V
on sagittal images.
• They inserts on the fibulahead as the conjoined tendon.
• On the left a football player, who was hit in the front part of the knee.
• The image on the far left shows a bone bruise anteromedially.
• So you suspect ligamentous injury on the contralateral side, which is the posterolateral corner.
• The next image shows a normal popliteus tendon but biceps femoris tendon is not attached to
the fibula.
• On the left more images of the same patient located more anteriorly.
Normal anatomy

62. Posterolateral Corner injury
• The fibular collateral ligament has a normal proximal attachment but is not attached to the
fibula.
• On a sagittal plane there is a gap between biceps femoris tendon and collateral ligament on
one side and the fibular head on the other.
• These findings indicate a conjoined tendon rupture.
• On the left PD-fatsat images after severe injury.
• There are bone bruises and many ligaments are ruptured.
• There is a posterolateral corner injury with proximal rupture of the fibular collateral ligament.
• There is also a rupture of the popliteus tendon because it is not attached proximally.
Normal anatomy

63. Cysts, Bursae and Recesses
• There are about 12 named bursae and recesses in the knee.
• Some very common and others uncommon.
• These are synovial lined structures.
• The most common recess is the popliteal or Baker's cyst.
• The origin is between the semimembranosus and gastrocnemius tendon.

65. Cysts, Bursae and Recesses
• On the left the typical imaging findings of prepatellar bursitis.
Prepatellar bursitis

66. • An uncommon form of bursitis is the deep infrapatellar bursitis.
• Sometimes associated with Osgood-Schlatter.
• These bursae are all named by the structures next to them.
• So a bursitis of the bursa between the deep MCL and the superficial MCL is called a
medial collateral ligament bursitis.
Cysts, Bursae and Recesses
Deep infrapatellar bursitis

68. • Adventitial bursae are bursae, that are formed in places where normally there is no
bursa>
• The bursa is formed due to abnormal friction.
Cysts, Bursae and Recesses
Adventitial bursae

69. • A common place for abnormal friction is between the iliotibial band and the lateral
condyle in speedwalkers, bicyclists and sometimes runners.
• When a bursa is formed in this location it is called the 'Iliotibial Band Friction syndrome'.
• On the left a speedwalker with lateral knee pain.
• Between iliotibial band and the lateral condyle there should be fat, but in this case it is
missing.
• Same patient. On axial images fluid within a bursa is seen between the iliotibial tract
and the underlying femur.
Cysts, Bursae and Recesses
Iliotibial Band Friction syndrome

72. • Sometimes fluid in this location has to be differentiated from joint fluid.
• You have to look at all the images.
• In this case the joint fluid stops at the red arrows.
• Remember that not everything that's bright on a T2W-image is fluid.
• You have to be suspicious, if there is something, that looks like a fluid collection, but it
is not in a location, where there normally is a bursa, cyst or recess.
• Give Gadolineum to differentiate cystic from solid.
Cysts, Bursae and Recesses
Iliotibial Band Friction syndrome

74. • The extensor mechanism of the knee is composed of the quadriceps muscle and
tendon, the patella and the patellar tendon.
• The quadriceps tendon is made of four tendons but comes in three layers on sagittal
images.
• It has a broad attachment all the way from the front of the patella almost to the back.
• The tendons of the quadriceps aswell as the patellar tendon are homogeneous in signal
but don't have to be black on PD-images.
• They have a sharp posterior demarcation.
• There should be no focal thickness.
Quadriceps and Patellar tendon
Normal Extensor mechanism

76. • The case on the left shows an abnormal quadriceps attachment.
• There is only one layer and the attachment does not go from the front of the patella to
nearly the back.
• In such a case extra images higher up have to be made after repositioning of the coil to
see what's going on up there.
• The missing part of the torn quadriceps tendon is retracted.
• The deep part is still intact.
Quadriceps and Patellar tendon
Quadriceps tendon tear

78. • Same patient, axial images.
• The torn quadriceps tendon is very thick indicating tendinopathy.
• Normal tendons do not tear, so always look for signs of pre-existing tendinopathy.
• Anywhere in the body, if you see a tendon that looks torn, but there is no pre-existing
tendinopathy, think hard, if you really have the right diagnosis.
• An image below this level shows normal vastus intermedius muscle and tendon.
• Another example of a partially torn quadriceps tendon.
• If there is no continuity between the patella and the quadriceps tendons it is a complete
tear.
Quadriceps and Patellar tendon
Quadriceps tendon tear

80. • Jumper's knee is a spectrum from tendinopathy to tear.
• Just the same as with the quadriceps tendon or any other tendon the MR shows a
spectrum from eccentric tendon thickening, indistinct posterior border, increased signal
on T2W-images and finally fiber disruption.
Quadriceps and Patellar tendon
Jumper's Knee

81. • Patient on the left is a professional ballet dancer with pain underneath the knee cap.
• Patellar tendon proximally is too thick. Posterior border is indistinct. In patella a little bit
of edema ( or bone bruise). If left untreated could end up like…
• Partial patellar tendon tear. Image on the right of a different patient.
Quadriceps and Patellar tendon
Patellar tendinopathy

82. • Images on the left show no continuity between fibers and patella. The tendon is
thickened.
Quadriceps and Patellar tendon
Complete Patellar tendon tear

83. • In children we have a different situation. They don't develop tendinopathy.
• The case on the left shows images of a girl who had pain beneath the patella after
doing gymnastics.
• Although the X-ray is normal there accually is a fracture through the cartilage part of the
lower pole.
• On MR it looks just like the jumper's knee above. Only on coronal images the dark
fractureline within the bright cartilage is visible.
• Usually these fractures are sutured.
• When these lesions are unrecognized they heal with ossification just below the patella.
Quadriceps and Patellar tendon
Patellar sleeve avulsion.

85. • The patellar cartilage is the thickest in the body. It should have smooth contours.
• The most important part of the medial retinaculum is the medial patellofemoral ligament
which inserts all the way posteriorly just in front of the MCL.
• Case on the left is a female soccerplayer who twisted her knee.
• Four MR-images from caudal to cranial demonstrate all the imaging features of a patellar
dislocation with rupture of the medial patellar femoral ligament .
• The patella was dislocated and the medial facet has bumped onto the lateral condyle.
• The patella has spontaneously reduced.
• Bone bruise may be complicated by cartilage fracture.
Patellar dislocation
Normal anatomy

87. • Patellar dislocation is a common condition, but clinically often unrecognized because
the patella after the dislocation comes back in it's normal position.
• The patient comes with a swollen painfull knee which could be anything from ACL-,
MCL- or meniscal tear to a fracture.
• So the MRI-findings are important in recognizing this condition.
• Patients who have loose bodies or continuing dislocation may undergo operation with
retinaculum repair.
Patellar dislocation

89. • In adults the bone marrow is largely composed of fat.
• Normal islands of red marrow may produce confusing images.
• Red bone marrow can be pronounced in young women, cigarette smoking, high altitude,
hemoglobinopathy or for no reason at all.
• As long as the criteria on the left are fullfilled it is normal.
• Normal red marrow on the left.
• Restricted to the metaphysis and not into the epiphysis.
• Comes in islands.
• On T1 brighter than muscle.
Bone and Cartilage
Normal and abnormal bone marrow

91. • Case on the left shows abnormal bone marrow.
• On T1W the signal intensity is lower than muscle.
• On T2W-images the signal is very bright.
• The abnormal signal comes into the epiphysis.
• Another case with abnormal marrow.
• In this case the marrow is too dark on T1 and T2 due to iron deposition in the marrow
after many blood transfusions in a patient with hemosiderosis.
Bone and Cartilage
Abnormal Bone Marrow

92. • The most common marrow abnormality is Avascular Necrosis (AVN).
• Some people will say 'AVN, Osteochondrosis Dissecans and Stress fracture all look the same'.
• There is however a distinct difference.
• AVN has the following features:
• 1. Focal abnormality is subchondral and originates in the bone.
• 2. Normal cartilage (until it collapses).
• 3. Wedge shaped marrow edema due to bone infarction.
• The wedge-shaped pattern of bone marrow edema is just the same as any other infarction in the
body i.e. liver infarction or kidney-infarction.
Bone and Cartilage
Avascular Necrosis

94. • On the left a different entity, but the patient had the same symptoms.
• Acute onset of medial pain.
• There is diffuse marrow edema on T2W-image.
• On T1W-image the focal abnormality is not directly subchondral.
• The abnormality on the T1 is more inside the edema.
• On the T1W-image a dark line is visible indicating a insufficiency fracture.
• This patient will get better with no weight bearing.
• On the left another patient with knee pain after trauma.
• There is some effusion but otherwise the x-rays are normal.
• In the same patient the MRI shows an obvious tibiaplateau fracture.
• The point is that any patient who is unable to bear weight in the hip, knee or ankle with normal X-rays needs another study.
Bone and Cartilage
Insufficiency fracture

96. • The diagnosis Osteochondritis Dissecans is usually made on X-rays.
• The question for MRI is whether it is stable or unstable.
• The case on the left is unstable for two reasons:
1. - small cysts at the base of the lesion (red arrow)
2. - even more important is fluid at the base of the lesion (blue arrow)
• Notice that this layer of fluid is different from AVN where the fluid is between the cartilage and the bone.
• Not helpfull for the discussion stable versus unstable OD are
1. - bone marrow edema (could be stable or unstable)
2. - break in the osteochondral surface.
Bone and Cartilage
Osteochondritis Dissecans

98. • So the case on the left is unstable because there is fluid at the base of the lesion.
• The case on the left shows a OD with bone marrow edema and a break in the
osteochondral surface.
• But since there is no fluid we cannot tell if this is stable or unstable.
• At operation the OD was found to be stable
• In those cases where you cannot tell whether the lesion is stable or unstable MR-
arthrogram is helpfull.
• We look for Gadolineum tracking around the osteochondral lesion.
Bone and Cartilage
Osteochondritis Dissecans