Knee menisci

The knee menisci are fibrocartilaginous structures that sit within the knee joint, deepening the tibiofemoral articulation. Their main role is shock absorption, improve stability of the knee joint, and load transmission. They also play an important role in synovial fluid dynamic circulation and also considered by some to be involved in proprioception. 

Gross anatomy

There are two fibrocartilaginous menisci in the knee joint: a medial meniscus within the medial tibiofemoral compartmentand a lateral meniscus within the lateral tibiofemoral compartment. Each meniscus is C-shaped in the transverse plane and consists of the following connected parts:

• anterior root attachment
• anterior horn
• body - located centrally
• posterior horn
• posterior root attachment

In cross-section, they have a triangular shape, being thicker peripherally and thinning to a free-edge centrally. 

Medial versus lateral meniscus

The medial and lateral menisci differ in size and shape as follows:

• medial meniscus
  • C-shape - open and wide, almost semi-circular
  • larger than lateral meniscus
  • posterior horn substantially larger than anterior horn
• lateral meniscus
  • C-shape - tight and closed off more like an incomplete circle, i.e. anterior and posterior tibial roots are very close to each other
  • smaller than medial meniscus
  • anterior and posterior horns same size

Surfaces

The meniscal surfaces are named according to a simple nomenclature which is particularly helpful when describing meniscal tears. 

• superior articular surface - also known as femoral surface
• inferior articular surface - also known as tibial surface
• inner free edge
• periphery 

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
  • lateralmedial 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 
  • attached to joint capsule except anterior horn and posterior most portion of 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 meniscofemoral 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: the floor of popliteal hiatus
    • superior and inferior meniscopopliteal ligaments at body-posterior horn junction
    • superior meniscopopliteal ligament at posterior horn: the roof of popliteal hiatus

Blood supply

The menisci are vascularised only via the periphery and the root attachments and therefore the inner portions are avascular. This has important implications on the healing and surgical management of meniscal tears.  

• red zone: outer one-third
  • supply from the peripheral meniscal plexus, in turn formed from the medial, lateral and middle genicular arteries ^6,8
• white zone: inner two-thirds
  • no vascular supply; diffusion dependent ⁶
• red-white zone: transition between outer third and inner two-thirds
  • reflecting the reality that the loss of perfusion is gradual and also variable from site to site, patient to patient

Innervation

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

Variant anatomy

• discoid meniscus ^4,5
• congenital hypoplasia or absence ⁴
• anomalous insertion of the medial meniscus (AIMM) (2%)
  • anterior root onto the ACL or intercondylar notch ⁴
  • anterior margin of the tibia in the midline ⁵
  • no attachment: stabilised by the transverse meniscal ligament ⁵
• speckled anterior horn of lateral meniscus
• meniscal flounce
• meniscal ossicle (rare; incidence <0.2%) ⁵

Related pathology

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