Two types of fibres
1. Longitudinal circumferential (cylindrical) fibres
2. Radial tie fibres
Forces:
1. Compression
2. Shear
3. Tension
4. Distraction
Compression force: Cranio-caudal direction
1. Meniscal contusion: meniscus within meniscus appearance
2. Longitudianal vertical tear (vertical longitudinal tear): occurs along the cranio-caudal axis between the cylindrical bundles. Best seen as shown below on sagittal images, may be difficult on coronal and hardly appreciated on axials. Breaches both articular surfaces. If force is more, the cylindrical fibres may also get disrupted, causing longitudinal vertical oblique tear. May breach only one articular surface. Occur perpendicular to the tibial plateau.
Shear force:
1. Longitudianl horizontal tear (horizontal tear, cleavage tear): occurs along the long axis of the meniscus (along radial tie fibres), between cylindrical bundles. Seen on sagittal and coronal axis, not well seen on axials. Usually runs to apex, may not extend all way to the periphery. If more force, cylindrial fibres may be disrupted causing oblique tear, usually breaching inferior articular surface. Commonly associated with meniscal cysts. Parallel to the tibial plateau. Divides meniscus into upper and lower segments
2. Central cavitation: seen in discoid meniscus3. Radial tear (vertical radial tear): disrupts longitudinal cylindrical bundles, hence tear in the AP direction. Seen in sagittal planes as well as on coronal and axial planes. May be large gap leading to 'ghost meniscus'. Common in MM. Most common in posterior horn (80%). Signs on MR: ghost sign (high signal replacing the meniscus with ghost meniscus adjacent to it), truncated triangle (abrupt truncation of inner meniscus), cleft sign (abnormal signal in meniscus perpendicular to imaging plane), marching cleft (meniscus appears to move on successive images).
To simplify further more:
There are 3 types of tears:
1. Horizontal tear (longitudinal horizontal). If displaced - flap tear
2. Vertical tear (longitudinal vertical). If displaced - bucket handle tear
3. Radial tear. If displaced - parrot beak tear
Free edge tear:
Can be longitudinal horizontal or radial. The former appears sharper and the later is blunted
Root tears
MM: Normally on 3 mm slices, meniscus should be seen medial to the PCL, if not suspect a root tear; this can be confirmed on coronal
LM: posterior horn is not covering the medial most posterior tibial plateau on at least one coronal image
With ACL tear, LM root tear is 3 times more common than MM root tear
Meniscocapsular separation:
Common medially
Anatomy is best seen on coronal and sagittal T and PD, and pathology is best seen on FS-T2 or STIR
Displacement of the meniscus
Extension of tear into superior or inferior corner of the peripheral meniscus
Irregular outer margin of the meniscual body on coronal images
Increased distance between MM and MCL
