Menisci of the knee

Each meniscus is thick at its periphery and thin at its free inner edge, triangular in cross-section, deepening the shallow tibial surfaces for the rounded femoral condyles. Their horns attach to the intercondylar areas of the tibia; the coronary ligaments bind their outer margins to the tibial condyles and the transverse ligament of the knee joins their anterior horns. Circumferential collagen fibres convert vertical load into hoop stress, so the menisci transmit ≈50–70% of the load across the knee (more in flexion); removal greatly increases contact stresses and leads to osteoarthritis.

The medial meniscus is C-shaped and broader posteriorly; its periphery is firmly attached to the joint capsule and the deep fibres of the tibial collateral ligament, so it is less mobile and more frequently torn. The lateral meniscus is smaller and more O-shaped (almost circular); it is separated from the fibular collateral ligament by the popliteus tendon, which attaches to it and pulls it backward during flexion, and it is linked to the medial femoral condyle by the meniscofemoral ligaments (of Humphrey and Wrisberg). Being more mobile, it is torn less often in isolation.

Only the outer ≈10–30% (the red zone) is vascularised, by perimeniscal branches of the genicular arteries; the inner white zone is avascular and nourished by synovial fluid — so peripheral tears can be repaired, whereas inner tears usually cannot heal.