All about the knee

The knee is the largest joint in your body and one of the most easily injured. It is a hinge joint in the leg that connects your larger shinbone (tibia) and your smaller shinbone (fibula connects into the top of the tibia) with your thigh bone (femur). Your knee cap (patella) is a bone embedded within a tendon (sesamoid bone) that rests over a groove at the bottom of the rounded femur and the top of the flat tibia. It protects the bones and soft tissue in your knee joint and slides when your knee moves, giving leverage to your leg muscles. Tendons are tough cords of tissue that connect muscle to bone and help control movement of your joint. The upper leg muscles provide your knees with mobility (extension, flexion and rotation) and strength. The quadriceps muscles straighten your legs (located on the front of your thigh rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), and the hamstring muscles bend your knees (located on the back of your thigh semitendinosus, semimembranosus, biceps femoris).

Ligaments are strong, elastic-like tissues that connect bone to bone and provide stability and protection to your knee joint by limiting the forward and backward movement of the shin bone. There are four main ligaments in the knee joint that connect your femur and tibia:

• Medial collateral ligament (MCL) runs along the inside of the knee joint, it provides stability to the medial (inner) part of the knee.
  
• Lateral collateral ligament (LCL) runs along the outside of the knee joint, it provides stability to the lateral (outer) part of the knee.
  
• Anterior cruciate ligament (ACL) is in the center of the knee, it limits rotation and forward leg movements.
  
• Posterior cruciate ligament (PCL) is in the center of the knee, it limits backward leg movements.

A type of slick, hard, bone-like, flexible connective tissue known as articular cartilage (also called hyaline cartilage) covers the surface ends of the tibia and femur at your knee joint, reducing friction and allowing them to move easily against one another. It is generally 1/8 to 1/4 inch thick. A thick, stringy, egg-like fluid (synovial fluid) found inside the knee capsule, lubricates your knee joint and reduces friction.

In Greek, the word meniscus means "little moon". The menisci are crescent shaped wedges (similar to shock absorbers) located in your knee joint under bumpy knobs at the bottom of your thigh bone (lateral and medial condyles of the femur) and on top of the flat upper surface of your shin bone (tibia plateau). These cover approximately 2/3 of your tibia surface and are thinner on the inside and thicker on the outside. They fill the space between these bones and cushion your femur so it doesn't rub against your tibia or slide off. They are made up of a dense, tougher collagen connective tissue than articular cartilage (fibro-cartilage). When you walk, your weight shifts from one meniscus to the other, and forces on your knee can increase by up to 2 - 4 times your body weight; when you run these increase up to 6 - 8 times your body weight, and are even higher when landing from a jump.

There are 2 menisci - lateral meniscus (located on the outside of your knee) and the medial meniscus (located on the inside of your knee).

The lateral meniscus is more of an o-shape, and although it is slightly shorter in length, it covers a larger portion of the tibia surface. It is very mobile and is only attached to the tibia on the outside and back of your joint. It does have a muscular connection, where the popliteus tendon goes along the edge and breaks attachment to your joint capsule. It slides forwards and backwards, moving freely in the joint, absorbing up to 80% of the load applied to the outside of your knee. The lateral meniscus is less likely to be injured or torn by force, because it can move and change shape.

The medial meniscus is about 3.5cm in length and more of a c-shape, resembling a wedge of an orange. It is tightly attached to the tibia and joint capsule on the back and inside of your knee. It has no direct muscular connection; rather it helps the ACL and MCL stabilize your knee, absorbing up to 50% of the load applied to the inside of the knee. The medial meniscus is quite inflexible and does not move freely in the joint; therefore it is torn more frequently than the lateral meniscus.