1. Criteria for 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)

3. Nomenclature of Meniscal Tears
Shapes. There are 3 basic shapes of meniscal tears: longitudinal, horizontal and radial. Complex tears are a combination of these basic shapes.

4. Displaced Tears Bucket-handle tear = displaced longitudinal tear
Displaced Tears Bucket-handle tear = displaced longitudinal tear. Flap tear = displaced horizontal tear. Parrot beak = displaced radial tear.

5. Grades of tears

6. Longitudinal, horizontal and radial tears
Longitudinal 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.
Longitudinal tear (2) Bucket handle tear is a displaced longitudinal tear.
Longitudinal tear (3) Flipped meniscus is a form of bucket handle tear.

8. Horizontal 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.
There are 3 criteria for the diagnosis of a meniscal cyst:
Horizontal tear.
Fluid accumulation with bright signal on T2.
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.

10. Radial 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.

12. MRI grading system for meniscal signal intensity
Grade 1:
small focal area of hyperintensity, no extension to the articular surface
Grade 2:
linear areas of hyperintensity, no extension to the articular surface
2a: linear abnormal hyperintensity with no extension to the articular surface
2b: abnormal hyperintensity reaches the articular surface on a single image
2c: globular wedge-shaped abnormal hyperintensity with no extension to the articular surface
Some Grade 2 abnormal meniscal signals ware found to be associated with a meniscal tear on arthroscopy. Therefore, they were subdivided into 2a, 2b, and 2c. Dillon et al. said that 50% of patients with grade 2c had meniscal tears on arthroscopy.
Grade 3:
abnormal hyperintensity extends to at least one articular surface (superior or inferior), and is referred as a definite meniscal tear

13. Ligamentous lesions
ACL
PCL
COLLATERAL
RETINACULAR

14. Cruciate ligaments
Cruciate ligaments (also cruciform ligaments) are pairs of ligaments arranged like a letter X.
They occur in several joints of the body, such as the knee.
The crossed ligaments stabilize the joint while allowing a very large range of motion.
The cruciate ligaments of the knee are
The anterior cruciate ligament (ACL)
The posterior cruciate ligament (PCL).
These ligaments are two strong, rounded bands that extend from the head of the tibia to the intercondyloid notch of the femur.
The ACL is lateral and the PCL is medial. They cross each other like the limbs of an X. They are named for their insertion into the tibia

15. Anterior cruciate ligament (ACL) tears
They are the most common knee ligament injury encountered in radiology practice. Pathology The ACL is the most commonly disrupted ligament of the knee, especially in athletes who participate in sports that involve rapid starting, stopping, and pivoting (e.g. soccer, basketball, tennis, netball and snow skiing).

16. MRI for Anterior cruciate ligament
Imaging of ACL tears should be divided into primary and secondary signs.
Primary signs
Are those that pertain to the ligament itself. Secondary signs are those which are closely related to ACL injuries. It includes:-
Swelling
Increased signal on T2 or PD FS
Fiber discontinuity
Change in the expected course of ACL: ACL angle that is less steep than Blumensaat's line: when drawing a line in the course of a normal ACL on the sagittal image the angle should be as steep or steeper than the intercondylar roof, so the apex is pointing posteriorly. If the ACL is less steep than the intercondylar roof (i.e. the apex of the angle points anteriorly) means that ACL is completely torn and collapsed
ACL tears typically occur in the middle portion of the ligament and appear as discontinuity of the ligament or abnormal contour.
The signal of the ACL can be more hyperintense on T2. If the angle is still normal and there is a hyperintense signal, a partial rupture is more likely than a complete rupture.

17. Secondary signs
Secondary signs include:
bone contusion in lateral femoral condyle and posterolateral tibial plateau 7
>7 mm of anterior tibial translation also known as anterior tibial translocation sign or anterior drawer sign

18. Posterior cruciate ligament tears
It account for ~10% (range 2-23%) of all knee injuries 2.
Clinical presentation
Many patients will be asymptomatic and their clinical examination is unremarkable. Sports injuries and car accidents are equally responsible for these injuries.
Pathology
Three mechanisms of injury have been proposed 2:
posterior tibial displacement in a flexed knee
hyperextension
rotation with an abduction or adduction force

19. Cruciate ligaments

22. Anterior cruciate ligament