The knee's physiology centers around its function as a modified hinge joint, providing both stability and mobility necessary for locomotion and various daily activities.
Knee Joint Mechanics
The knee joint is a complex structure that allows for movement in multiple planes, deviating from a simple hinge joint. This complexity arises from the incongruence between the articulating surfaces of the femur and tibia.
- Degrees of Freedom: The knee exhibits six degrees of freedom, essential for its diverse range of motion.
- Translation: Three translational movements are possible:
- Anterior-Posterior (forward and backward)
- Medial-Lateral (side to side)
- Inferior-Superior (compression/distraction)
- Rotation: Three rotational movements occur:
- Flexion-Extension (bending and straightening)
- Internal-External Rotation (twisting inward and outward)
- Adduction-Abduction (sideways movement - minimal)
- Translation: Three translational movements are possible:
Ligaments: Ensuring Stability
Ligaments are crucial for maintaining knee stability and preventing excessive or unwanted movements.
- Cruciate Ligaments: The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) control anterior-posterior translation and rotational stability. The ACL prevents the tibia from sliding too far forward relative to the femur, while the PCL prevents the tibia from sliding too far backward.
- Collateral Ligaments: The medial collateral ligament (MCL) and lateral collateral ligament (LCL) provide stability against valgus (inward) and varus (outward) stresses, respectively.
Menisci: Shock Absorption and Load Distribution
The medial and lateral menisci are fibrocartilaginous structures that sit between the femur and tibia. They serve several important functions:
- Shock Absorption: The menisci absorb impact forces, protecting the articular cartilage.
- Load Distribution: They increase the contact area between the femur and tibia, distributing weight more evenly and reducing stress on the bones.
- Joint Stability: The menisci contribute to joint stability, particularly during rotation.
Muscles: Powering Movement
Numerous muscles surrounding the knee joint are responsible for its movement and stability.
- Flexion: Hamstrings (biceps femoris, semitendinosus, semimembranosus) and gastrocnemius.
- Extension: Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius).
- Rotation: Popliteus, sartorius, gracilis, and tensor fasciae latae contribute to internal and external rotation.
Articular Cartilage: Smooth Articulation
Articular cartilage covers the ends of the femur and tibia, providing a smooth, low-friction surface for joint movement. Damage to this cartilage can lead to osteoarthritis.
Neurovascular Supply
- Nerves: The knee is innervated by branches of the femoral, obturator, tibial, and common peroneal nerves. These nerves provide sensory feedback and motor control.
- Blood Vessels: The genicular arteries (superior medial, superior lateral, inferior medial, inferior lateral, and middle) supply blood to the knee joint.
Factors Affecting Knee Physiology
Several factors can impact the normal physiology of the knee, including:
- Age: Cartilage degradation and ligament laxity can occur with age.
- Activity Level: High-impact activities can increase the risk of injury.
- Weight: Excess weight places increased stress on the knee joint.
- Injury: Trauma to ligaments, menisci, or cartilage can disrupt normal function.
- Disease: Conditions like arthritis can damage the joint.
In summary, the knee's physiology is a complex interplay of bone structure, ligamentous support, meniscal cushioning, muscular control, and neurovascular supply, enabling a wide range of movements while maintaining stability.
