The knee joint is the biggest joint in your body. It connects your thigh bone (femur) to your shin bone (tibia). It helps you stand, move and keep your balance. 

Like all joints, your knees are part of your skeletal system. Your knees also contain cartilage, muscles, ligaments and nerves.

Your knees help support your weight and let your legs bend and move. Almost any movement that uses your legs relies on your knees — your knees help when you're walking, running and jumping.

Bones in the knee

Three bones make up the knee joint:

Femur (thigh bone).

Tibia (shin bone).

Patella (kneecap).

The places these bones meet are called articulations or articulating surfaces. There are two articulations in your knee:

• Patellofemoral: Where your patella meets your femur.
• Tibiofemoral: Where your tibia meets your femur.

Cartilage in the knee

Cartilage is a strong, flexible connective tissue that protects your joints. It acts as a shock absorber throughout your body.

There are two types of cartilage in your knee:

Hyaline cartilage is the most common type of cartilage in your body. Some healthcare providers call it articular cartilage. It lines your joints and caps the ends of your bones. Hyaline cartilage is slippery and smooth, which helps your bones move smoothly past each other in your joints. The surfaces of your femur, tibia and patella that touch each other have a hyaline cartilage lining.

Fibrocartilage is what its name sounds like — tough cartilage made of thick fibers. It's tough enough to hold parts of your body in place and absorb impacts. The meniscus in your knee is two wedges of fibrocartilage. It cushions the space between your femur and tibia.

Ligaments in the knee

Knee ligaments are bands of tissue that connect your thigh bone to your lower leg bones.

There are two types of ligaments in your knee:

The two collateral ligaments are like straps on opposite sides of your knee. The medial collateral ligament (MCL) is on the inner side of your knee. It attaches your femur to your tibia. The lateral collateral ligament (LCL) is on the outer side of your knee. It connects your femur to your calf bone (fibula). The collateral ligaments prevent your knee from moving side to side too much.

The two cruciate ligaments are inside your knee joint. They also connect your femur to your tibia. They cross each other to create an X. The cruciate ligaments control the way your knee moves front to back. The anterior cruciate ligament (ACL) is at the front of your knee. The posterior cruciate ligament (PCL) is behind the ACL at the back of your knee.

Muscles in the knee

Muscles are soft tissue made of stretchy fibers. They tense up (flex) to pull and move parts of your body. Lots of muscles attached to your legs control your knee movements.

Muscles called flexors pull your knee in, including the:

• Articularis genus.
• Rectus femoris.
• Vastus lateralis.
• Vastus intermedius.
• Vastus medialis.

Extensors are muscles that work opposite your flexors — they let you extend your knee out. The extensors that control your knee include the:

• Biceps femoris.
• Semitendinosus.
• Semimembranosus.
• Gastrocnemius.
• Plantaris.
• Gracilis.
• Popliteus.

Nerves in the knee

Nerves are like cables that carry electrical impulses between your brain and the rest of your body. These impulses help you feel sensations and move your muscles. Nerves in your knee include your:

• Femoral nerve.
• Sciatic nerve.
• Tibial nerve.
• Peroneal nerve.