Knee

Genu

Definition

The knee (or knee joint) is the area between the thigh and the leg. It’s a modified hinge synovial joint allowing flexion/extension and some rotation when flexed.

Bony articulations:

The knee joint consists of three articulations, all enclosed within a single joint cavity:

  • Two articulations between femur and tibial condyles.

  • An articulation between patella and lower end of the shaft of the femur (patellofemoral joint), where the patella glides up and down on the lower end of the femoral shaft.

    Fibula does NOT contribute; articular surfaces are covered with hyaline cartilage.

Fibrous capsule:

  • The knee joint is surrounded by a fibrous capsule with an opening at the back for the tendon of popliteus to exit the joint cavity.

  • The joint is mechanically weak due to the ‘poor fit’ of the articulating surfaces. Stability is maintained by (A) muscles, primarily the quadriceps femoris muscle, and (B) ligaments.

Ligaments of the knee joint:

  • Two collateral ligaments include lateral (a.k.a. fibular) collateral ligament and medial (a.k.a. tibial) collateral ligament. These prevent adduction (lateral collateral ligament) and abduction (medial collateral ligament). Amongst the two, the tibial collateral ligament is weaker. Moreover, it has an attachment to the medial meniscus and is therefore more prone to be involved in certain sports injuries. Damage to the tibial collateral ligament can be tested by passively abducting the leg at the knee joint.

  • The anterior and posterior cruciate ligaments, lie within the joint cavity and cross one another. The cruciate ligaments are attached to the intercondylar area of tibia and to the femoral condyles. When the knee is flexed and the ligaments are loose, the femur has a tendency to roll off the tibia except for the cruciate ligaments which prevent this from happening.

    The anterior cruciate ligament (ACL) is the weaker of the two cruciate ligaments. It prevents the movement of the femur backward on the tibia. The ACL can be torn by hyperextension of the knee. The ‘Anterior drawer test’ is used to check for stability of the ACL. When the knee joint is flexed at right angle, the tibia cannot normally be pulled anteriorly (like pulling out a drawer) because it is held in place by the ACL.

    The posterior cruciate ligament (PCL) prevents the movement of the femur forward on the tibia. For instance, it stabilizes the knee while walking down a hill. The PCL can be damaged in motor collisions, as a dashboard injury: when the proximal end of the tibia hits the dashboard pushing the tibia posteriorly. The ‘Posterior drawer test’ is used to test stability of the PCL. When the knee joint is flexed at right angle, the tibia cannot normally be pushed posteriorly because it is held in place by the PCL.

  • Menisci: The medial and lateral menisci improve the fit of the joint surfaces. Both menisci are attached at their anterior and posterior ends to the intercondylar area of the tibia. Unlike the lateral meniscus, the medial meniscus is also attached to the capsule of the joint and to the tibial collateral ligament, hence, less mobile and more likely to be injured.

Bursae of the knee joint:

Several bursae around the knee prevent friction-related ‘wear and tear’.

  • Some communicate with the knee joint including the suprapatellar bursa (between the quadriceps femoris muscle/tendon and femur).

  • Others do not communicate with the joint cavity including the:

    -prepatellar bursa (between skin and patella). Friction can result in inflammation (housemaid’s knee) and swelling in knee. Common in people who work on knees without knee pads, e.g. tilers.

    -infrapatellar bursa (between skin and tibia). Friction can results in inflammation (clergyman’s knee) and swelling over proximal tibia.

Movement:

The knee joint undergoes flexion, extension, and rotation. In normal walking, knee extension ends with the femur rotation medially, locking the joint. Popliteus muscle unlocks knee joint for flexion by pulling the lateral femoral condyle, causing lateral rotation.

Note: Posterior knee has a diamond-shaped region, called the popliteal fossa. Details: link.

Text written by Muhammad A. Javaid, MD, PhD © 2023 IMAIOS.

  • Drake, R.L., Vogl, A.W. and Mitchell, A.W.M. (2009). ‘Chapter 6: Lower Limb’ in Gray’s anatomy for Students. (2nd ed.) Philadelphia PA 19103-2899: Elsevier, pp.575-585.


Comparative anatomy in animals

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