Menisci

For Young Learners

Structure and Location

The thigh bone ends in two rounded knuckles. The shin bone is almost flat on top. A round thing resting on a flat thing touches in only a tiny spot, and a tiny spot means a lot of pressure.

The menisci solve that problem. They are two rubbery pads shaped like the letter C, and they sit on top of the shin bone like a pair of shallow dishes. The rounded end of the thigh bone settles into them. There is one on the inner side of the knee and one on the outer side.

Each pad is thick at its outer edge and thin at its inner edge, so it works like a wedge that stops the thigh bone sliding off the side.

Function

Think of a drawing pin pushed into a wall. All the force goes through one sharp point. Now think of the same push spread through your whole palm. Same force, far less pressure on any one spot.

That is what the menisci do. They spread the squeeze from a small patch to a much wider one, which protects the smooth white cartilage underneath. They also help the knee feel steady when you turn, and they help the slippery joint fluid reach the surfaces that need it.

They are not bolted down. They shift as your knee bends, which keeps the contact area wide through the whole movement. A twist with the foot planted can tear one, and a torn pad spreads load less well than an intact one.

For Patients

Structure and Location

The menisci are two wedges of fibrocartilage inside the knee joint. Fibrocartilage is tougher and more rope like than the smooth cartilage that coats the ends of the bones, which suits it to being squeezed and stretched.

One sits on the inner side of the knee and is called the medial meniscus. It is the larger of the two and more open in shape, closer to a letter C. The other sits on the outer side and is called the lateral meniscus. It is smaller and more circular.

The difference matters. The medial meniscus is firmly attached to the capsule and to the ligament on the inner side of the knee, so it moves very little. The lateral meniscus is more loosely held and slides further as the knee bends.

Function

The menisci share load between the thigh bone and the shin bone, spreading force across a wider surface so the cartilage underneath is protected. They add stability by deepening an otherwise shallow surface. They also help spread joint fluid, which keeps the joint slippery and feeds the cartilage.

Clinical Relevance

Blood reaches only the outer part of each meniscus. The rim has a supply, the middle has a partial one, and the inner portion has almost none. Clinicians describe these as the red zone, the red-white zone and the white zone. Healing follows the same pattern, which is why a tear at the rim behaves differently from a tear near the centre.

Tears happen in two broad ways. A younger knee usually tears on a twist with the foot planted. An older knee may tear gradually, sometimes with no single moment that caused it.

Losing meniscal tissue raises the pressure on the cartilage underneath, which is why surgeons preserve it wherever the tear allows. The choice between treatments is covered on the surgery pages.

A scan can show a tear in a knee that does not hurt, so the examination and the history carry as much weight as the image.

For Medical Students

Structure and Location

Each meniscus is a semilunar fibrocartilaginous structure interposed between the femoral condyle and the tibial plateau, triangular in cross section with a thick peripheral border and a thin free inner edge. The superior surface is concave and articulates with the femoral condyle. The inferior surface is flat and rests on the tibial plateau.

The medial meniscus is the larger and more open of the two and covers a smaller proportion of its tibial plateau. The lateral is more nearly circular and covers a greater proportion. Each is anchored to the tibia at an anterior and a posterior root.

The ligaments and capsular attachments that tether each meniscus, and the vessels that supply them, belong to the Anatomy Academy.

Function

Collagen is arranged predominantly in circumferential bundles running parallel to the periphery, with radial tie fibres crossing them. Axial load applied by the femoral condyle tends to extrude the meniscus outwards. The circumferential bundles resist that extrusion in tension, which is termed hoop stress, and the roots anchor the ends so the tension can develop.

The result is load distribution: contact area increases and peak contact stress on the articular cartilage falls. The menisci also deepen the tibial articular surface and so contribute secondarily to joint stability, and their movement assists distribution of synovial fluid across the articular surfaces.

Both menisci translate posteriorly during flexion, the lateral considerably further than the medial, which reflects the difference in how firmly each is tethered.

Clinical Relevance

Vascular penetration is limited to the peripheral portion, producing the described red, red-white and white zones. The inner zone is nourished by diffusion from synovial fluid, and healing capacity follows the same gradient. This single fact explains most of how meniscal injury behaves.

Composition is largely water, with type one collagen dominating the dry weight. Proteoglycan content is low relative to articular cartilage, consistent with a tissue loaded principally in tension rather than compression.

Loss of meniscal tissue reduces contact area and raises peak contact stress, which is the mechanical basis for preferring preservation where a tear allows it.

Magnetic resonance imaging is the standard investigation for suspected meniscal injury. Signal grading and the choice between treatment options are covered in the imaging and surgery sections.