The normal anatomy of the knee joint is a complex interplay of bones, cartilage, ligaments, tendons, and muscles that work together to provide stability and mobility.
Bony Structures
The knee joint is primarily formed by the articulation of three bones:
- Femur (Thighbone): The distal (lower) end of the femur forms the upper portion of the knee joint. The femoral condyles (medial and lateral) are rounded projections that articulate with the tibia.
- Tibia (Shinbone): The proximal (upper) end of the tibia forms the lower portion of the knee joint. The tibial plateau, the relatively flat surface at the top of the tibia, articulates with the femoral condyles.
- Patella (Kneecap): The patella is a sesamoid bone (a bone embedded in a tendon) that sits in front of the knee joint. It glides within a groove on the femur called the trochlear groove and improves the leverage of the quadriceps muscle.
- Fibula: The fibula does not participate in the knee joint itself, but runs parallel to the tibia and provides attachment sites for muscles and ligaments that contribute to knee stability.
Cartilage
Two types of cartilage are essential for smooth knee function:
- Articular Cartilage: This smooth, slippery tissue covers the ends of the femur and tibia and the underside of the patella. It allows the bones to glide easily against each other with minimal friction.
- Menisci: These are crescent-shaped pads of fibrocartilage located between the femur and tibia. The knee has two menisci: the medial meniscus (on the inner side of the knee) and the lateral meniscus (on the outer side of the knee). They act as shock absorbers, distributing weight evenly and enhancing joint stability.
Ligaments
Ligaments are strong, fibrous tissues that connect bones to each other and provide stability to the knee joint. The major ligaments of the knee include:
- Anterior Cruciate Ligament (ACL): Located in the center of the knee, the ACL prevents the tibia from sliding forward on the femur.
- Posterior Cruciate Ligament (PCL): Also located in the center of the knee, the PCL prevents the tibia from sliding backward on the femur.
- Medial Collateral Ligament (MCL): Located on the inner side of the knee, the MCL provides stability against forces that push the knee inward (valgus forces).
- Lateral Collateral Ligament (LCL): Located on the outer side of the knee, the LCL provides stability against forces that push the knee outward (varus forces).
Tendons
Tendons are tough, fibrous cords that connect muscles to bones. Important tendons around the knee include:
- Quadriceps Tendon: Connects the quadriceps muscles (located on the front of the thigh) to the patella.
- Patellar Tendon: Connects the patella to the tibial tuberosity (a bony prominence on the front of the tibia). This tendon is actually a ligament since it connects two bones (patella and tibia), but it is commonly referred to as a tendon.
- Hamstring Tendons: Connect the hamstring muscles (located on the back of the thigh) to the tibia and fibula.
Muscles
The muscles surrounding the knee joint are responsible for its movement and stability. Key muscle groups include:
- Quadriceps: Located on the front of the thigh, the quadriceps muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius) extend (straighten) the knee.
- Hamstrings: Located on the back of the thigh, the hamstring muscles (biceps femoris, semitendinosus, and semimembranosus) flex (bend) the knee.
- Gastrocnemius: One of the major calf muscles, it crosses the knee joint posteriorly and assists with knee flexion.
- Popliteus: A small muscle located at the back of the knee that helps with knee flexion and rotation.
Joint Capsule and Synovial Fluid
The knee joint is enclosed within a fibrous capsule that provides stability and contains synovial fluid. Synovial fluid lubricates the joint, reducing friction and providing nutrients to the cartilage.
In summary, the normal anatomy of the knee joint is a complex and coordinated system of bones, cartilage, ligaments, tendons, and muscles that enable a wide range of movements while maintaining stability.
