What parts of the knee are involved?
The knee joint is formed where the femur (lower end of the thighbone) connects with the tibia (upper end of the main lower leg bone). On the front of the joint is the patella (kneecap). The patella is what is called a sesamoid bone that is a part of the extensor mechanism of the knee joint. The extensor mechanism connects the large muscles of the thigh to the tibia such that contracting the thigh muscles pulls on the tibia and allows us to straighten the knee. The parts of the extensor mechanism include the thigh muscles, the quadriceps tendon, the patella and the patellar tendon.
The knee joint is surrounded by a water-tight pocket called the joint capsule.
This capsule is formed by the knee ligaments, connective tissue, and synovial tissue. When the joint capsule is filled with sterile saline and is distended, the surgeon can insert the arthroscope into the pocket that is formed, turn on the lights and the camera, and see inside the knee joint as if looking into an aquarium. The surgeon can see nearly everything that is inside the knee joint including: (1) the joint surfaces of the tibia, femur and patella, (2) the two menisci, (3) the two cruciate ligaments, and (4) the synovial lining of the joint.
There is one meniscus on each side of the knee joint. The C-shaped medial meniscus is on the inside part of the knee, closest to your other knee (medial means closer to the middle of the body.) The U-shaped lateral meniscus is on the outer half of the knee joint (lateral means further out from the center of the body.)
The menisci (plural for meniscus) protect the articular cartilage on the surfaces of the thighbone (femur) and the shinbone (tibia) and help to create a deeper joint surface, which aids in joint stability. Articular cartilage is the smooth, slippery material that covers the ends of the bones that make up the knee joint, as well as most other joints. The articular cartilage allows the joint surfaces to slide against one another without damage to either surface.
Ligaments are tough bands of tissue that connect the ends of bones together and help to keep the bones together, creating a stable joint. The anterior cruciate ligament (ACL) is located in the center of the knee joint where it runs from the backside of the femur, or thighbone’s joint surface to connect to the front of the tibia, or shinbone’s joint surface.
The ACL runs through a special notch in the femur called the intercondylar notch and attaches to an area of the tibia called the tibial spine.
The ACL is the main controller of how far forward the tibia moves under the femur. This is called anterior translation of the tibia. If the tibia moves too far, the ACL can rupture. The ACL is also the first ligament that becomes tight when the knee is straightened. If the knee is forced past this point, or hyperextended, the ACL can also be torn.
The posterior cruciate ligament (PCL) is located near the back of the knee joint. It also attaches to the femur’s joint surface and the tibia’s joint surface, but crosses behind the ACL such that the two ligaments (the ACL and PCL) form somewhat of an X inside the joint (the term ‘cruciate’ means shaped like a cross.)
The PCL is the primary stabilizer of the knee and the main controller of how far backward the tibia moves under the femur. This motion is called posterior translation of the tibia. If the tibia moves too far back, the PCL can rupture.