The knee joint is a complex synovial joint that plays a crucial role in lower limb movement and stability. In this article, we will explore the anatomy, ligaments, bursae, and movements of the knee joint, as well as some important clinical considerations.
Anatomy of the Knee Joint
The knee joint consists of two condylar joints between the femur and tibia and a saddle joint between the patella and femur. The joint cavity is divided into upper and lower compartments by the menisci, which enhance joint stability and distribute forces during movement.
The main articular surface of the knee joint is between the femoral and tibial condyles. Additionally, there is a separate patella joint between the patella and the patellar surface of the femur. Notably, the fibula does not directly contribute to the knee joint.
Capsule and Synovial Membrane
The knee joint is surrounded by a capsule that attaches beyond the margin of the articular surface of the tibia and femur. Anteriorly, the capsule is deficient and replaced by the patella and the quadriceps femoris muscle. The capsule is strengthened by various structures, including the patella retinacula, quadratus femoris, iliotibial tract, sartorius, semimembranosus, and oblique popliteal ligament.
The synovial membrane lines the non-articular surfaces of the joint cavity and covers the deep surface of the infra patella fat pad. Cruciate ligaments are not covered by the synovial membrane. Additionally, there is an infrapatellar fat pad deep to the ligamentum patellae.
Relations and Ligaments
The knee joint has various relations and ligaments that contribute to its stability:
- Anterior: Prepatellar, subpatellar, and infrapatellar bursae, as well as the patellar plexus.
- Posterior: Muscles forming the borders of the popliteal fossa and its contents.
- Medial: Semitendinosus, tendon of Sartorius and gracilis, and the great saphenous vein with the saphenous nerve.
- Lateral: Biceps femoris, common peroneal nerve, and the tendon of origin of the popliteus.
The knee joint also has extracapsular ligaments, including the tibial collateral ligament (which blends with the capsule and medial meniscus) and the fibular collateral ligament (which is separated from the capsule and lateral meniscus). These ligaments are embraced by the tendon of the biceps femoris
Clinical Point: Collateral ligaments
Taut in full extension, so liable to injury in this position.
Medial C.L- Damaged in violent abduction
Lateral C.L- Damaged in violent adduction.
Bursae
There are 12 bursae associated with the knee joint. The capsule of the joint communicates with various bursae, including the supra patellar bursa above the lower femoral shaft and quadriceps, and the bursa under the medial head of the gastrocnemius. There are also bursae that communicate with the bursa under the semimembranosus muscle.
Movements of the Knee Joint
The knee joint allows for several movements facilitated by different muscle groups:
- Flexion: Hamstrings, Sartorius, gracilis, popliteus, gastrocnemius, and plantaris muscles contribute to knee flexion.
- Extension: Quadriceps femoris and tensor fascia lata muscles are responsible for knee extension.
- Medial Rotation: The popliteus muscle performs medial rotation.
- Lateral Rotation: The piriformis, obturator internus, gemelli muscles contribute to lateral rotation.
Flexion and extension primarily occur between the femoral condyles and the menisci, while rotation takes place between the menisci and the tibial condyles.
Locking Mechanism
The locking mechanism of the knee joint enables it to remain in full extension during standing without much muscular effort. This mechanism occurs during the last stage of extension through medial rotation of the femur. The articular surface of the lateral condyle is fully utilized, while the medial condyle is partially used. The lateral condyle acts as an axis, and the medial condyle rotates backward around this axis. When the knee is locked, it becomes completely rigid, and all the ligaments of the joint are taut. To unlock the knee, a reversal of medial rotation is required, which is accomplished by lateral rotation of the femur with the assistance of the popliteus muscle.
Menisci
The knee joint contains two crescent-shaped fibro-cartilaginous discs called menisci. These menisci are intra-capsular and intra-synovial, and they deepen the articular surface of the tibial condyles. The peripheral part of the menisci is thick and convex, firmly attached to the joint capsule, while the inner border is thin and concave, allowing some mobility. The upper surface of the menisci is concave and articulates with the femur, while the lower surface is flat and rests on the tibial condyles. The menisci serve multiple functions, including shock absorption, increased mobility, softening of movements, proprioception, and division of the joint cavity.
The medial meniscus is semicircular and adheres peripherally to the tibial collateral ligament. Due to its greater fixity, it is more prone to damage compared to the lateral meniscus. On the other hand, the lateral meniscus is circular and separated from the fibular collateral ligament by the joint capsule and popliteal tendon. It is controlled by the pulling action of the popliteal tendon, making it less susceptible to injury. Additionally, there are two meniscofemoral ligaments that join the posterior end of the lateral meniscus to the femur.
Clinical Point: Cartilages (Menisci)
Only damaged in flexed position. Medial—In abduction. Lateral—In adduction.
*Medial meniscus is more vulnerable to damage than lateral meniscus.
Cruciate Ligaments
The knee joint is supported by two strong fibrous connections known as the cruciate ligaments, which determine anteroposterior stability.
- Anterior Cruciate Ligament (ACL): It arises from the anterior part of the intercondylar space and the posterior part of the medial surface of the lateral condyle of the femur. The ACL runs upwards, backwards, and laterally, and it is taut during extension. Its primary function is to prevent forward displacement of the tibia on the femur.
- Posterior Cruciate Ligament (PCL): It arises from the posterior part of the intercondylar space and the anterior part of the lateral surface of the medial condyle of the femur. The PCL runs upwards, forwards, and medially. It is taut during flexion and prevents backward displacement of the femur on the tibia.
Clinical Point: Cruciate ligaments
Anterior Cruciate—in hyperextension & twisting
Posterior Cruciate—in hyperflexion
Both can be damaged in violent adduction or abduction
Stability of the Knee Joint
The knee joint's stability is maintained by both ligaments and surrounding structures. The cruciate ligaments play a crucial role in providing anteroposterior stability, while the collateral ligaments contribute to side-to-side stability. The capsule of the knee joint is strengthened by the iliotibial tract, which plays an important role in maintaining stability. Despite ligamentous damage, the knee joint can still function efficiently due to the presence of the powerful quadriceps femoris muscle.
The patella, also known as the kneecap, is an important component of the knee joint's stability. It is stabilized by the forward prominence of the lateral condyle of the femur, the tension exerted medially by the pull of the patellar ligament, and the lowest fibers of the vastus medialis muscle.
Understanding the anatomy and function of the knee joint is crucial for healthcare professionals in diagnosing and managing knee-related conditions. The knee joint's stability and mobility allow for various movements and activities, making it a vital joint for lower limb function.
