Anatomy

What structures are most commonly injured?

The knee joint has most of its motion in a single plane, bending and straightening from zero to about 145 degrees. This motion is allowed by the rounded shape of the end of the femur (thigh bone) and the flat shape of the upper end of the tibia (lower leg).

The surfaces of the femur and tibia that touch each other at any point of this movement are covered by smooth joint surface (tibiofemoral joint surface). The bones themselves are smooth but they are also covered with a layer of gristly hard-wearing articular cartilage. Articlular cartilage is softer than bone and does not contain the calcium salts that make bone hard. As a result this part of the joint surface does not show on standard X-rays. On X-ray, it looks as though there is a gap between the bones.

The end of the femur joins the top of the tibia to create the knee joint. Two round knobs called femoral condyles are found on the end of the femur. These condyles rest on the top surface of the tibia. This surface is called the tibial plateau. The outside half (farthest away from the other knee) is called the lateral tibial plateau, and the inside half (closest to the other knee) is called the medial tibial plateau. The patella glides through a special groove formed by the two femoral condyles called the patellofemoral groove.

The smaller bone of the lower leg, the fibula, never really enters the knee joint. It does have a small joint that connects it to the side of the tibia. This joint normally moves very little.

Either or both the tibiofemoral and the patellofemoral joint surfaces can be injured in a knee fracture. Making sure they end up as smooth as possible once the fracture heals is an important consideration.

Surrounding the knee is the joint capsule. The joint capsule is a water tight sac that is made up of ligaments and connective tissue between the ligaments. The joint capsule forms a pocket that is filled with joint fluid (also called synovial fluid). Joint fluid is necessary to lubricate the joint and nourish the articular cartilage.

Ligaments are strong bands of inelastic fibrous tissue that go from one bone to the next. The collateral ligaments limit side-to-side movement of the knee and the cruciate ligaments limit front-to-back motion between the femur and tibia.

Together, the joint capsule and ligaments stabilize the knee and keep abnormal movement from occurring. There is a small amount of rotation but almost no side-to-side or front-to-back motion in the normal knee.

The posterior capsule is tight when the knee is straight, but slack when the knee is flexed. It prevents over-straightening.

When the knee ligaments are normal the knee moves smoothly and feels stable and reliable in all positions. This stability may be lost when the ligaments are injured. If the ligaments are stretched or torn the knee feels unstable. There is often a shifting or opening sensation during knee movement.

During an injury ligaments may be stretched, torn or tear away from their attachment to the bone. When the ligament pulls away from the bone with a fragment of bone attached it creates an avulsion fracture. It is important to realize that ligaments may stretch as well. Even if the avulsion fracture heals back exactly in position the knee can still be unstable. The collateral ligaments and capsule have the capacity to heal and may even tighten up with time, however the cruciate ligaments have very limited healing capacity. If a fracture pattern suggests a ligament injury, that part of the injury may need the most attention.

The mechanism to straighten the knee against gravity is interesting. The quadriceps muscle (sometimes referred to as the quads) is the biggest and strongest muscle in the body and makes up most of the mass of the thigh. If you put your hand on the front of the thigh and straighten or tense the knee you can feel the muscle contract. Most muscles end in a tendon which passes over a joint and attaches to bone.

The quadriceps is so powerful that it would damage the knee joint if the tendon went directly over it. Instead the muscle is attached to the kneecap and the kneecap is attached to the tibia by a strong ligament (the patellar ligament) which passes from the lower tip of the kneecap to a bump of bone in front of the knee at the top of the tibia. When the quads contract the kneecap moves up on the front of the knee and pulls the tibia with it, straightening the knee.

You can feel the patellar ligament tightening and transmitting the force to the shin if you feel just below the tip of the kneecap. When the leg is relaxed this area is soft. When you straighten the knee or lift the leg off the bed the region tightens up and feels like a bar. The pressure of the extension mechanism is transmitted from patella to femur through joint surface so the bone is not damaged - the joint surface is evolved to transmit forces from bone to bone.

As the kneecap tracks in front of the knee it is held in position by ligaments that pass from the sides of the femur to the kneecap. If the kneecap dislocates one or both of these structures is disrupted. Fractures of the kneecap interrupt the mechanism to straighten the knee. The pull of the quadriceps is no longer transmitted through to the tibia so one cannot bear weight on that leg until the fracture is healed. Since most fractures of the kneecap go into the joint it is important that the two parts heal without a step, otherwise there would be a wear problem.

In summary, the anatomy of the knee is complex. Bones, ligaments, muscles and joint surface are all required for normal function. During an injury, any or all of these structures can be damaged. They must all be restored to near normal for the knee to recover full function.

Related Document: A Patient's Guide to Knee Anatomy