The knee is not only the largest joint in the body, it is also central to nearly every routine activity. Just walking from one room to another involves the knee. The major structures of the knee are:

Bones
• Femur—the lower end of the thigh bone
• Tibia—the upper end of the shin bone on which the femur rotates
• Patella (knee cap)—a rectangular bone that slides in a groove on the end of the femur; it protects the knee and improves leverage to muscles

Cartilage
• Articular cartilage—the tough, plastic-in-appearance material that covers the ends of the 3 bones, cushioning them and enabling them to move smoothly; articular cartilage is a mixture of collagen and sponge-like molecules that is maintained by living cells called chondrocytes
• Meniscal cartilage—2 crescent-shaped discs positioned between the femur and tibia on the outer and inner sides of the knee; they impact, cushion, and help distribute force between the lower part of the leg and the weight of the body, as well as provide some stability

Synovial Lining
This thin, smooth membrane covers the remaining soft tissue aspects of the joint and produces a lubricating fluid to reduce friction and wear. When irritated, this tissue may produce an overabundance of fluid.

Ligaments
These strong bands of connective tissue span between the knee bones, providing stability and aiding in alignment. The main ligament groups of the knee are:

• Anterior cruciate ligament (ACL)—limits rotation and the forward movement of the tibia
• Posterior cruciate ligament (PCL)—limits backward movement of the tibia
• Medial collateral ligament (MCL)—provides stability to the inner (medial) aspect of the knee
• Lateral collateral ligament (LCL)—provides stability to the outer (lateral) aspect of the knee
• Patellar ligaments—maintain the central position of the patella in the femoral groove (sulcus)

Muscles
There are 3 groups of muscles at the knee:

• Quadriceps muscle—comprises 4 muscle bellies on the front of the thigh that work to straighten the leg from a bent position
• Hamstring muscles—run along the back of the thigh; they work to bend the leg at the knee
• Gastroc soleus complex—run from the femur in back of the knee to the Achilles tendon

Tendons
These tough cords of tissue connect muscle to bone:

• Quadriceps tendon—connects the quadriceps muscle to the patella and powers leg extension
• Patellar tendon—connects the patella to the tibia (technically, it is a ligament)
• Hamstring tendons—connect hamstrings about the knee ("cords" felt behind the knee)


In the healthy knee, all of these parts work together in harmony. But disease or injury can disrupt the knee’s normal functioning and cause pain, muscle weakness, and limited function.