Knee: Anatomy: Menisci

General points:
3-5 mm in height
Width: medical meniscus (MM): anterior horn - 6 mm, posterior - 12 mm; lateral meniscus (LM): 10 mm throughout
Attached to the condylar surface of the tibia
Superior surface - concave; Inferior surface - flat
Periphery - convex and thick, attached to the inside of the joint capsule, considered non-articular, hence usually not involved in degenerations and tears
Towards the intercondylar notch- thin, tapers, free edge
Ligaments - meniscofemoral, meniscotibial, coronary, transverse
Divided into anterior horn, posterior horn and body
Relatively avascular, except for the peripheral 10-25%, supplied by perimeniscal capillary plexus, from lateral and medial geniculate arteries. Posterior horn of LM is relatively vascular, hence allowing complex repairs

MR appearance:
Uniform low signal on T1, T2, T2*, GRE, STIR due to lack of mobile protons
Cross section: triangular, outer curve being convex and apex pointing intercondylar notch
Sagittal: peripherally bow-tie appearance, centrally opposiing triangular shapes by anterior and posterior horns

Collagen bundles form 2 distinct zones: circumferential (peripheral third- resist longitudinal loading) and transverse (divided in to superior and inferior leaves by middle perforating collagen bundle) (middle perforating bundle corresponds to grade 2 meniscal changes).
Radial fibres provide structural rigidity and resistance to splitting.
Secondary vertical collagen fibres may be present within the transverse zone.
Peripheral to circumferntial zone is the meniscocapsular junction.

Medial meniscus (MM):
More open C shape with wide posterior horn, larger than the LM.
Less mobile, because of attachement to deep layer of MCL and capsule peripherally (more rigid than LM), hence more prone for injury
Anterior horn - attached to tibial intercondylar fossa, anterior to the attachment of ACL and transverse ligament
Posterior horn - attached to tibial posterior intercondylar fossa, between attachments of PCL and lateral meniscus
Periphery - attached entirely to joint capsule and deep layer of MCL
On sagittal MR, posteriomedial tibial articular cartilage should be covered by posterior horn of MMUncocvering of this cartilage by 5 mm or more is suggestive of peripheral detachment.
A small bursa seperates posterior horn from joint capsule

Lateral meniscus (LM):
Tight C shape, hence more circular than MM and relatively symmetrical in width in anterior to posterior direction
Relatively mobile
Covers 2/3rd of tibial articular surface
Anterior horn - attached between tibial intercondylar eminence and anterior attachment of ACL Posterior horn - attached between tibital intercondylar eminence and posterior horn of MM.
Posterior horn - attached to PCL through ligament of Wrisberg (posterior to PCL) and Humphrey (anterior to PCL), which are branches of meniscofemroal ligament and attach posterior horn to medial femoral condyle
Posterior horn - forms superior and inferior fascicles, best seen on T2. The inferior fascilce extends more laterally than superior fascile. These fasciles are close to popliteus tendon sheath and utmost care is needed to diagnose tears and differentiate from pseuotears due to popliteus sheath
Peripheral - loose attachment to capsule
No direct attahment to LCL (cf. MM attahced to MCL)
Accomodates poplitius tendon posteriorly. Popliteus exits through popliteus recess and hiatus of 1cm

Transverse ligament:
Connects anterior horns of menisci
Absent in 40%. Seen completely in 15% of MR examinations.
Attached to anterolateral to the central rhombdoid attachment of the lateral meniscus.
Surrounded by Hoffa's fat
Best seen on transverse images as uniform low signal band connecting anterior horns
Can simulate oblique tear adjacent to anterior horn of LM on sagittal images and is created by fat between anterior horn and transverse ligament

Meniscofemoral ligaments:
Direct attachment to LM
Insets to medial femoral condyle
Ligament of Humpry: Anterior to PCL, 1/3rd of anatomical specimen. Best seen on sagittal images, on coronal too
Ligament of Wrisberg: Posterior to PCL, larger than Humpry, 2/3rd of anatomical specimen. Best seen on coronal images posteriorly
Both seen on MRI in 1/3rd of cases. Both are seen onlyn in 3%

Meniscocapsular ligaments:
Thickened parts of joint capsule
Medial capsular ligament is thicked medial third of the joint capsule and is divided into meniscofemoral and meniscotibial (coronary ligament) components
Coronary ligament attaches psoterior horn of MM to tibia and is prone for injury
A bursa is seen betweem meniscocapsular ligament and MCL

Meniscal flounce:
Wavy appearance along the free edge of the meniscus
Seen especially when the knee is in 10 degree flexion, resolves when the knee is extended completely
Seen in 0.2-6%
May be seen in vulgus injury

Meniscal ossicles:
Seen in 0.15%
Developmental or post traumatic
Typically found in the posterior horn of the MM
A/W tears

1. Stoller DW. MRI in orthopaedics & sports medicine; Second edition; 1997. Chapter 7: the knee. Lippincott Williams & Wilkins.
2. Michael G. Fox. MR Imaging of the Meniscus: Review, Current Trends, and Clinical Implications. RCNA Volume 45, Issue 6, Pages 931-1068 (November 2007)