What Is the Knee?

For Young Learners

Structure and Location

Your knee sits halfway down your leg, between your hip and your ankle. It is where two long bones meet. The thigh bone comes down from the hip. The shin bone comes up from the ankle. The kneecap sits at the front like a small shield.

People often call it a hinge, like the hinge on a door. That is nearly right and not quite. A door hinge only swings. Your knee swings and twists a little at the same time, which is why it can do so much more than a door.

Function

The knee has two jobs that pull against each other. It has to move, so you can run, climb and sit down. It also has to stay steady, so your leg does not fold when you land.

Bone shape alone cannot do both. The rounded end of the thigh bone sitting on the flat top of the shin bone is not a stable arrangement. What holds it together is everything around it: tough cords called ligaments, rubbery pads, muscles and a slippery lining. The joint is a team, not a single part.

For Patients

Structure and Location

The knee is a synovial joint, which means it has a lubricated cavity lined with a membrane. Three bones meet here: the femur, which is the thigh bone, the tibia, which is the shin bone, and the patella, which is the kneecap. The fibula, the slim bone beside the shin, sits alongside and takes part indirectly.

There are really two joints working together. The main one lies between the femur and the tibia. A second lies between the patella and the groove at the front of the femur. Problems in one can produce symptoms that feel like they come from the other, which is one reason knee pain can be hard to place.

Function

The knee carries your body weight and lets you move. Standing still, it has to hold you up without effort. Walking, it bends and straightens roughly a thousand times a mile. Rising from a chair or climbing stairs, it takes forces several times your own weight.

It also absorbs shock. Each step sends a jolt up the leg, and the structures in the knee spread that force over a wider area so no single spot takes all of it.

Clinical Relevance

Because so many parts work together, trouble in any one of them tends to show up in the same few ways: pain, swelling, stiffness, catching, or a feeling that the knee might give way. The symptom tells you something is wrong. It does not tell you which part.

That is why a clinician asks what you were doing when it started, what makes it worse, and what the knee does now, before examining it or arranging a scan.

For Medical Students

Structure and Location

The knee is the largest synovial joint in the body. It comprises two articulations within a single capsule: the tibiofemoral joint between the femoral condyles and the tibial plateau, and the patellofemoral joint between the patella and the trochlear groove of the distal femur. The proximal tibiofibular joint lies adjacent and has its own capsule.

It is classified as a modified hinge, or bicondylar, joint. The articular surfaces are markedly incongruent: convex femoral condyles articulate with a near flat tibial plateau, so bony architecture contributes little to stability.

Function

Motion is predominantly flexion and extension in the sagittal plane, accompanied by axial rotation that increases with flexion, together with small amounts of translation and abduction and adduction. Rotation is possible because the articular surfaces are incongruent and the restraints permit it.

The joint transmits load while allowing that movement. Stability is therefore conferred almost entirely by soft tissue: the cruciate and collateral ligaments as static restraints, the menisci deepening the tibial surface, the capsule and its thickenings, and the surrounding musculature acting dynamically.

Clinical Relevance

The dependence on soft tissue rather than bone explains the injury pattern. Forces that would dislocate a congruent joint instead tear a restraint, and the knee tolerates considerable structural loss before it becomes unusable, which is why significant injury can present with modest early symptoms.

The single capsule shared by both articulations means that an effusion from any source distends the whole joint, and that pathology at one articulation produces symptoms attributed to the other. The individual structures are taken in turn across the rest of this chapter.