Knee Anatomy and Knee Stabalization

The knee is essentially a simple hinge joint. Like all joints in the body the knee joint has five essential elements: bones, articular cartilage, stabilizing ligaments, muscles, and a capsule.

In addition, the knee has both a medial and lateral meniscus, two wedge-shaped discs that help stabilize the knee and provide cushion.

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The bones that make up the knee are the thigh bone (femur), the shin bone (tibia), the skinny bone of the leg (fibula) and the kneecap (patella). The femur and the tibia are the main articulating bones of the knee, and they provide the flexion and extension that make up knee motion. The end of the femur has two condyles: rounded “runners” that articulate with the top of the tibia, more specifically the tibial plateau. The medial condyle is larger, bearing more weight than the lateral condyle, and tends to wear out sooner, particularly in men. The patella is the bone through which the thigh muscles (quadriceps) exert their force while walking or running. The underside of the patella, like the ends of the femur and the tibia, is coated with articular cartilage.

Amazing Articular Cartilage

The patellar slides within the groove at the end of the femur called the trochlea. This articulation between the patella and the trochlea is often dysfunctional, especially in women who suffer from pain in the front of the knee, particularly when walking down stairs. Articular cartilage is the white substance that covers the ends of the bones. It provides cushion and is incredibly slippery. When healthy, cartilage provides painless, smooth motion. Cartilage is comprised of collagen and proteoglycan fibers, both of which combine to trap water. Normal function of the cartilage depends upon integrity of the layers. The most superficial layer (the layer closest to the joint) has collagen fibers parallel to the surface. The deeper layers of the cartilage have collagen and proteoglycan fibers perpendicular to the surface of the bone, which anchors the cartilage to the bone. Please see diagram below.

Conceptually, the cartllage coating acts as sponge. Cartilage traps water when not loaded (i.e. bearing weight). Once pressure is applied to the cartilage surface, the synovial fluid rushes out of the “sponge” providing a “boundary lubrication” that essentially turns the cartilage layers into a water-on-water interface. By the way, the “water” in the joint is no regular water. It is synovial fluid, and in fact derives its name from Latin, meaning “with egg”, since its appearance and feel are similar to the white of a freshly cracked egg. To really capture how slippery cartilage is, consider its “coefficient of friction” This mathematical concept is a measure of how slippery two objects are in relation to each other. Examine this table (The coefficient of friction is the ratio of friction over normal force and is represented by the Greek letter mu, μ.):

Healthy cartilage is even 4 times slippery than Teflon on Teflon! Over time, cartilage wears from top to bottom, which dramatically alters its function. As the cartilage thins, it becomes much less slippery, and as it loses its ability to trap water, the cushioning properties decrease substantially.

Stabalizing Factors of the Knee

There are four main stabilizing ligaments in the knee: the Anterior Cruciate Ligament (ACL), the Posterior Cruciate Ligament (PCL), the Medial Collateral Ligament (MCL), and the Lateral Collateral Ligament (LCL). The Cruciate ligaments (ACL and PCL) cross in the middle of the joint (hence their names), and provide the main anterior and posterior (forward and back) stabilization of the knee. The Cruciate ligaments also provide rotational stability of the knee, keeping the knee structurally sound while twisting. When the ACL is torn, the knee is subjected to abnormal motions, thus subjecting the other structures of the knee to increased stress or injury. See the dedicated page on ACL anatomy and function. The side ligaments, the collaterals, provide side-to-side stability to the knee. The MCL is commonly injured in conjunction with ACL tears. In cases of isolated MCL tears, non-operative treatment is the standard of care. The LCL rarely tears by itself, more often tearing with concomitant PCL tears and capsule tears to the “posterorlateral corner” of the knee. The PCL is injured much less commonly than the ACL. As mentioned above, it often tears in combination with other ligaments in high-energy accidents. It is possible to sustain an isolated PCL injury, and conservative (non-operative) care often yields high function of the knee. Similar to the ACL, the PCL has a highly complex structure, with two “bands”, or ligaments within a ligament. These bands have different roles depending on the degree of flexion of the knee. When reconstructing the PCL these bands are re-constituted to achieve normal motion and stability of the knee. When facing surgery, detailed discussions regarding knee reconstruction options are warranted given the complex nature of surgical techniques.