Knee Joint

What is the 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 — your knees help when you’re walking, running and jumping.

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

What does the knee joint 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 and many ways healthcare providers group them together. 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 cavity 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 — a fluid-filled sac that lubricates and protects the joint — lines the space between the bones. 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?

Your knee is made of:

• Bones.
• Cartilage.
• Ligaments.
• Muscles.
• Nerves.

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: 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:

• 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.

What are the most common issues or conditions that affect knees?

Your knees can be affected by anything that damages your bones or connective tissue, including:

• Arthritis.
• Osteoarthritis.
• Bursitis.
• Tendinitis.
• Osteoporosis.

Sports injuries are some of the most common knee injuries:

• ACL tears.
• MCL tears.
• Meniscus tears.
• Hyperextended knees.
• Sprains.
• Bone fractures.
• Dislocations.
• Patellofemoral pain syndrome (PFPS).

What are common symptoms that affect knee joints?

The condition or issue that’s affecting your knee will determine which symptoms you experience. Many knee issues cause symptoms like:

• Pain (especially when moving).
• Inflammation (swelling).
• Discoloration or redness around your knee.
• A feeling of warmth or heat.
• A grinding feeling.
• A popping noise or feeling.

Which tests are done to knees?

Visit a healthcare provider as soon as you notice changes or pain in your knees. Some of the most common tests providers use to diagnose issues include:

• X-rays.
• Ultrasounds.
• Joint aspiration.
• CT scans.
• MRIs.

A provider might use physical movements and motions to help them diagnose a torn meniscus or ligament damage in your knee, including:

• McMurray test.
• Apley test.
• Thessaly test.
• Valgus and varus stress tests.

How can I take care of my knees?

The best way to take care of your knees is to maintain good overall health, including:

• Following a diet and exercise plan that’s healthy for you.
• Visiting a healthcare provider for regular checkups and as soon as you notice any changes or symptoms in your knees.
• Wearing proper protective equipment for any activity, sport or work you’re doing.

A note from Cleveland Clinic

Your knees are your biggest joint, and they have a big job — helping you stand, move and use your legs. No matter what you’re doing, you use your knees a lot throughout your daily routine.

Because we use them for so many activities, knees are one of the most injured joints. Everyone has the occasional ache or pain, but don’t ignore symptoms in your knees. Visit a provider as soon as you notice any changes. They’ll help you understand what’s going on inside your knee joint and will find treatments that keep your knees healthy, strong and working like they should.