Knee Joint

The knee is the joint that connects your thigh to your lower leg. It’s the biggest joint in your body.

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. They help you walk, run and jump.

Visit a healthcare provider if you’re experiencing knee pain. They’ll diagnose what’s causing it and suggest treatments to help you get back to your usual routine and activities.

What do knee joints do?

Your knees have several important jobs, including:

• Moving your legs
• Supporting your body when you stand and move
• Stabilizing you and helping keep your balance

Where is the knee joint located?

The knee is the joint in the middle of your leg. It’s where your thigh bone (femur) meets your shin bone (tibia).

Which type of joint is the knee?

There are lots of joints in your body, and healthcare providers group them together in many ways. Joints are usually classified based on:

• Their composition: What they’re made of
• Their function: How they move

The knee is a synovial joint. Synovial joints have the most freedom to move. They’re made of a space in one bone that another bone fits into. Slippery hyaline cartilage covers the ends of bones that make up a synovial joint. A synovial membrane lines the space between the bones. This protects and lubricates your joint. This extra cushioning helps synovial joints move with as little friction as possible.

Functionally, the knee is a hinge joint. Think about the hinges that hold a door in place. They have a few parts that don’t move, but other pieces move a specific distance to open and close. Hinge joints like your knee follow the same basic rule. They open and close in one direction.

What is the structure of the knee joint?

You knee is made of:

• Bones
• Cartilage
• Ligaments
• Muscles
• Nerves

Bones in the knee

Three bones make up the knee joint:

• Thigh bone (femur)
• Shin bone (tibia)
• Kneecap (patella)

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: 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: 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:

• Collateral ligaments: 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.
• Cruciate ligaments: 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:

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

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

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

Nerves in the knee

Nerves are like cables that carry electrical signals from your brain to the rest of your body. They help you feel sensations and move your muscles. Nerves in your knee include the:

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

What are the common conditions and disorders that affect knee joints?

Anything that damages your bones or connective tissue can affect your knees, including:

• Arthritis
• Bursitis
• Tendinitis
• Osteoporosis

Sports injuries are some of the most common knee injuries:

• Ligament tears
• Meniscus tears
• Sprains
• Broken or dislocated bones
• Patellofemoral pain syndrome (PFPS)

Signs/symptoms something isn’t working right

Many knee issues cause symptoms like:

• Pain, especially when moving
• Swelling
• Skin discoloration around your knee
• A feeling of warmth or heat
• A grinding feeling
• A popping noise or feeling