The knee joint joins the thigh with the leg and consists of two articulations: one between the femur and tibia, and one between the femur and patella. The largest joint in the body, the knee is a mobile trocho-ginglymus (a pivotal hinge joint), which permits flexion and extension as well as a slight medial and lateral rotation. Since the knee supports nearly the entire weight of the body, it is vulnerable to both acute injury and the development of osteoarthritis.


Lateral and posterior aspects of the right knee

Bones

There are four bones joined at the knee joint, although no one bone touches all three of the other bones.

  • The femur from above and the tibia from below articulate to form a joint that approximates a hinge.
  • The patella, a sesamoid bone within the quadriceps tendon, articulates with the femur to create a joint that is similar to a pulley mechanism.
  • The tibia articulates with the second shin bone, the fibula, in a joint that allows a small degree of rotation.

The ends of the femur and tibia are covered with articular cartilage, as is the surface of the patella. In between the condyles is a space called the intracondylar notch, which houses the cruciate ligaments. The femur does not articulate with the fibula. The distal ends of the femur, the femoral condyles, can be thought of as two wheels that roll and glide along the relatively flat surface of the tibia. As the knee flexes, the femur rolls posteriorly. As the knee is straightened, the lateral plateau of the tibia reaches full extension before the medial side does; thus the tibia rotates externally on the femur (with the medial plateau continuing to extend, whereas the lateral plateau is motionless). These last few degrees of motion (the so-called “screw-home mechanism“) allow the knee to lock in full extension. For this reason an inability to fully extend (termed a flexion contracture) can lead to an abnormal gait.

The function of the patella is to hold the quadriceps anteriorly, thus increasing the so-called moment arm of extension. One can think of knee extension as a rotational motion around the center of the knee, as seen on the lateral view. This torque can be increased by either pulling harder or by increasing the distance between the line of pull and the axis of rotation. Increasing the distance between the line of pull of the quadriceps and the center of the knee is the goal of the patella. In fact it is able to increase the torque by as much as 30%. The price of that leverage is that the patella is subject to high joint reaction forces – several times body weight, in fact.

Ligaments and Soft Tissues

The knee, unlike the hip joint, does not have much bony congruity to provide stability, and unlike the shoulder, it does not rely on muscles to hold it in place. Rather, it uses ligaments to secure the joint. The main ligaments are the anterior cruciate ligament and the posterior cruciate ligament (ACL and PCL) and the medial collateral ligament and the lateral collateral ligament (MCL and LCL).

  • The MCL is a long and broad ligament, going from the femur to the tibial metaphysis. It prevents the joint from opening when hit from the outside. Since most people are slightly knock-kneed (a so-called “valgus” deformity), the MCL prevents the joint from gapping during simple weight bearing too.
  • The LCL is much smaller (reflecting, perhaps, the lower likelihood of being hit from the inside) and attaches not to the lateral tibia, but to the fibula. It is the fibular attachments to the tibia, in turn, which complete the link, stabilizing the lateral side of the knee.
  • The cruciate ligaments are found in between the condyles, in the intra-condylar notch. The ACL resists anterior subluxation of the tibia and the PCL resists posterior motion. The orientation of these ligaments are primarily vertical. This permits the knee to move through a wide arc of motion. In fact, you can think of the tibia as suspended from the femur by the cruciates, and that the ‘hinge’ motion of the knee is the swinging that such suspension allows.

When the knee is flexed, the patella is stabilized by bone congruity: it sits firmly in the “V” of the femoral trochlea. At full extension, there is no bony contact and stability is achieved by the medial retinaculum and the lateral retinaculum (ligaments that hold the patella to the femur) and balanced tension of the muscles of the quadriceps. If the pull of the quadriceps is excessively lateral (as may be the case with a knock-knee, valgus deformity) the patella may be unstable. At the minimum, this lateral pull can lead to imperfect mating between the articular surfaces, with greater contact on the lateral facet of the patella. This increased focal contact will increase pressure and, in turn, can lead to cartilage breakdown and pain.

The intra-articular space of the knee is bounded by a thin capsule. This space is lined with synovium, which produces lubricating and nourishing fluid. The capsule holds this fluid around the knee and prevents it from dissipating in the soft tissues. The extent of the pouch is at least a few inches superior to the patella, allowing a nice space for injecting or aspirating the knee (arthrocentesis), if needed.

There are also two menisci (singular “meniscus”, Greek for “little moon”) in each knee. Each are crescent shaped cartilages that rest on the tibial plateaus. They are progressively thicker towards the periphery. A sagittal or coronal slice (as seen on an MRI, for example) looks like a wedge. The purpose of the meniscus is threefold:

  • The meniscus is a cushion, functioning as a shock absorber.
  • The meniscus creates a greater contact area between the femur and tibia, allowing the force of weight bearing to be dissipated across a larger space. (An increase in the contact area decreases pressure, as pressure is defined as force per unit area.)
  • The meniscus helps stabilize the knee, much in the way a brick behind the back tire prevents the car from rolling when you change the front tire.

A person who loses his meniscus (tearing it irreparably, for example) may feel more pressure in his knee and be more apt to sense instability too.

Menisci

The knee has two cartilage structures, the medial and lateral menisus, that are situated between the femur and the tibia. Each meniscus is separated into three parts – the anterior horn, the body, and the posterior horn. These structures act as “shock absorbers” and allow for a more effective articulation between the rounded femoral condyles and the flat tibial plateau. The menisci are primarily composed of type I collagen, with a majority of the fibers oriented in the longitudinal or circumferential direction. Occasionally, there are radial tie fibers that exist throughout the meniscus. These circumferential fibers allow for the menisci to distribute the compressive forces of weight bearing into tensile hoop stresses along the length of the fibers.


Head of right tibia seen from above, showing menisci and attachments of ligaments

The medial meniscus is semi-circular, or “C”, shaped and its posterior horn is larger than the anterior horn. The posterior attachment of the medial meniscus is always bony and attaches onto the tibia just anterior to the insertion of the posterior cruciate ligament. The anterior horn attachment is more variable. The anterior horn may have a bony attachment to the tibia, a soft tissue attachment to the anterior horn of the lateral meniscus via the transverse intermeniscal ligament, or both. The medial meniscus is adherent to the capsule throughout its entire length. At the midpoint of the meniscus (body), it is attached to the femur via the fibers of the deep medial collateral ligament. The body is also attached to the tibia via the coronary ligaments. At the posteromedial aspect of the knee, fibers of the semimembranosus muscle attach to the medial meniscus.

The lateral mensicus is more circular in shape and covers a larger portion of the articular surface of the underlying tibia. The anterior horn always has a bony attachment to the tibia. The posterior horn is attached via a bony attachment to the tibia posterior to the lateral tibial eminence, just lateral and anterior to the attachment of the posterior horn of the medial meniscus. In the Wrisberg variant knee, the attachment of the posterior horn of the lateral meniscus attaches to the femur via the posterior meniscofemoral ligament or the ligament of Wrisberg. The capsular attachment to the lateral meniscus is not as continuous as it is on the medial side posterolaterally at the level of the popliteus hiatus. There is a groove at this hiatus allowing the passage of the popliteus tendon. Some fibers of the tendon may insert onto the peripheral and superior margin of the lateral meniscus. At the same level, the arcuate ligament (component of the posterolateral corner) is formed at the level of the popliteus hiatus and is situated between the lateral meniscus and the fibula.

Muscles

Knee extension is powered by the quadriceps (signaled, primarily, by the L4 nerve root in the femoral nerve). The quadriceps, as the name implies, is composed of four muscles: the three vastus muscles (vastus lateralis, vastus medialis, and vastus intermedius), and the rectus femoris. Flexion is powered by the hamstrings, also with four parts: the long head and short head of the biceps femoris, which attach laterally, on the fibula, and the medial group, the semitendinosus and the semimembranosus, which attach on the tibia. The gastrocnemeus, which spans the knee posteriorly, attaching to the femur, also powers knee flexion. The muscles of the knee can help stabilize the joint, especially when the cruciates are injured: the quadriceps has a slight anterior pull and the hamstrings and gastroc pull posteriorly.

Although the biceps attaches to the fibular head, and the semitendonosis attaches to the anterior tibia, in terms of flexion function, the hamstrings behave as if all were attached to the posterior tibia. The actual attachments of the tendons becomes important when they inflamed or injured; the physician examining them must remain cognizant of their true attachments.

There is one tendon crossing the knee that cannot be categorized easily: the iliotibial band. This is the tendon of the tensor fascia lata muscle, and receives some fibers from the gluteus maximus. The band courses down the side of the leg, and attaches on Gerdy’s tubercle on the anterior-lateral aspect of the tibia. When the knee is flexed beyond 30 degrees, the path of the band is behind the axis of the knee, and thus flexes it. When the knee is flexed less than 30 degrees, however, the band passes anterior to the axis and serves as an extensor.

Nerves and Blood Vessels

Nerve and blood vessels are important constituents of the knee, but for the most part, they are transients, on the way to their important tasks in the leg and foot. The nerves and vessels live in back of the knee, in the diamond shaped popliteal fossa. The popliteal artery is an extension of the femoral artery, which changes its name as it dives posteriorly through the adductor canal. The popliteal artery gives off the geniculate arteries and continues to the leg as the anterior and posterior tibial arteries. The posterior tibial artery splits and gives off the peroneal artery. The popliteal artery is at risk for damage during a knee dislocation, as it is fairly well tethered within the fossa.

The nerves also pass through the popliteal fossa. At some point in the mid thigh, the sciatic nerve splits into separate tibial nerve and peroneal nerve branches. These enter the fossa distinctly, with the peroneal laterally placed. The tibial nerve remains posterior in the calf throughout. The peroneal wraps around the fibular neck (where it is subject to injury during fracture or dislocation). This nerve then splits into the deep peroneal nerve, which supplies the anterior muscles of the leg, and the superficial peroneal nerve which supplies the muscles of the lateral compartment, the peroneus longus and peroneus brevis

Attachments:


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