The knee is not a simple hinge but a rolling, sliding spiral
Babies are born without bony kneecaps. The patella starts as cartilage and only turns to bone somewhere between three and five years of age, slowly because it is the largest sesamoid bone in the body. It anchors the biggest joint we have, which bends like a hinge yet also rolls, glides and twists.
The knee is really two joints sharing one capsule: thigh bone against shin bone, and thigh bone against kneecap. Anatomists call it a modified hinge because, besides bending and straightening, it allows a little rotation inward and outward. The trick lies in the rounded ends of the femur, called condyles. Their curve tightens toward the back, so the pivot point shifts along a spiral as the leg bends. That lets the bone roll and slide at once, while keeping the side ligaments slack enough for the twist.
Two kinds of cartilage cushion the surfaces. Glassy hyaline cartilage coats the gliding faces, its collagen fibres rising from the bone in arches and lying flat at the surface to resist abrasion. It contains no blood vessels and is fed by diffusion from the joint fluid and the marrow beneath. Lose either supply and it degenerates. It barely repairs itself, and what grows back is mostly tougher, lower-grade fibrous cartilage, which cracks again in time.
Between the bones sit two crescent pads, the menisci, which only partly divide the joint and can shift across the top of the tibia. The inner one is larger, flatter and uneven in thickness; the outer one is smaller, almost a full circle and more uniform, and is loosely held because a tendon grooves it away from the capsule.
Ligaments keep all this in line. Inside the joint, the two cruciates cross: the anterior one stops the shin sliding forward and is often torn by twisting, while the posterior one blocks backward slip and is rarely hurt without a direct blow. On the sides, the collateral ligaments resist the knee being bent open from either direction. It is a big, busy joint carrying the body through running and jumping, which is also why it is so prone to wear.
Source: Knee
