Knee menisci

The knee menisci are fibrocartilaginous structures that sit within the knee joint, deepening the tibiofemoral articulation. They function to improve stability, shock absorption and load transmission of the knee.

Gross anatomy

There are two knee menisci in each joint: medialand lateral. The menisci are described as having a central body with anterior and posterior horns. In cross-section, they have a triangular (bow-tie) shape, being thicker peripherally and thinning to a free-edge centrally. Each meniscus has a differing shape, size and attachments:

• medial meniscus
  • C-shape or semilunar 
  • larger
  • more open and wider; semi-circular in shape
• lateral meniscus
  • semicircular-like an incomplete circle, i.e. anterior and posterior tibial roots are very close to each other
  • smaller

Attachments

• medial meniscus
  • the anterior horn of the medial meniscus attaches immediately anterior to the tibial attachment of the anterior cruciate ligament (ACL) in the intercondylar area
  • lateral aspect attached to the deep (third) layer of the medial collateral ligament
  • posterior horn is attached to the posterior intercondylar area of the tibial plateau, between PCL insertion posteriorly and posterior root attachment of lateral meniscus, anteriorly
• lateral meniscus
  • the anterior horn of the lateral meniscus attaches immediately lateral to the tibial attachment of the ACL on the intercondylar area
  • no attachment to thelateral collateral ligament except anterior horn and the posterior most portion of the posterior horn; this is due to the passage of the intra-articular portion of the popliteus tendon 
  • posterior horn of the lateral meniscus attaches to the posterior intercondylar area of the tibial plateau anterior to the medial meniscus and posterior to ACL 
• other attachments
  • meniscofemoral ligament: posterior horn of the lateral meniscus to the lateral aspect of the medial femoral condyle
    • posterior meniscofemoral ligament of Wrisberg: more common 
    • anterior menisciofemoral ligament of Humphrey 
  • transverse meniscal (or genual) ligament (of Winslow) attaches the anterior horns to each other
  • oblique menisco-meniscal ligament
  • joint capsule peripherally except the lateral meniscus at the site of the popliteus tendon hiatus
  • small ligaments/retinaculia from LM to the capsule:
    • inferior meniscopopliteal ligament at the body: floor the floor of popliteal hiatus
    • superior and inferior meniscuopopliteal ligaments at body-posterior horn junction
    • superior meinscupopliteal ligament at posterior horn: the roof of popliteal hiatus

Blood supply

• outer one-third: supply from the peripheral meniscal plexus, in turn formed from the medial, lateral and middle genicular arteries ^{6, 8}
• inner two-thirds: no vascular supply; diffusion dependent ⁶

Innervation

• posterior articular branch of the posterior tibial nerve and terminal obturator and femoral nerve branches ⁸

Variant anatomy

• discoid meniscus ^4-5
• congenital hypoplasia or absence ⁴
• aberrant insertion of the anterior horn of the medial menisci (2%)
  • into the ACL or intercondylar notch ⁴
  • anterior margin of the tibia in the midline ⁵
  • no attachment: stabilised by the transverse meniscal ligament ⁵
• meniscal ossicle (rare; incidence <0.2%) ⁵

Related pathology

• meniscal tear
  • absent bow tie sign
  • double PCL sign 
• meniscal flounce
• meniscal cyst