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Knee Problems
What Do the Knees Do? How Do They Work?
The knees provide stable support for the body and allow the legs to bend and straighten. Both flexibility and stability are needed for standing and for motions like walking, running, crouching, jumping, and turning.
Several kinds of supporting and moving parts, including bones, cartilage, muscles, ligaments, and tendons, help the knees do their job. Any of these parts can be involved in pain or dysfunction.
What Causes Knee Problems?
Knee problems commonly occur in young people and adults.
There are two general kinds of knee problems: mechanical and inflammatory.
Mechanical Knee Problems
Some knee problems result from injury, such as a direct blow or sudden movements that strain the knee beyond its normal range of movement. Other problems, such as osteoarthritis in the knee, result from wear and tear on its parts.
Inflammatory Knee Problems
Inflammation that occurs in certain rheumatic diseases, such as rheumatoid arthritis and systemic lupus erythematosus, can damage the knee.
Joint Basics
The point at which two or more bones are connected is called a joint. In all joints, the bones are kept from grinding against each other by padding called cartilage. Bones are joined to bones by strong, elastic bands of tissue called ligaments. Tendons are tough cords of tissue that connect muscle to bone. Muscles work in opposing pairs to bend and straighten joints. While muscles are not technically part of a joint, they're important because strong muscles help support and protect joints.
What Are the Parts of the Knee?
Like any joint, the knee is composed of bones and cartilage, ligaments, tendons, and muscles (see the diagram).
Bones and Cartilage
The knee joint is the junction of three bones: the femur (thigh bone or upper leg bone), the tibia (shin bone or larger bone of the lower leg), and the patella (knee cap). The patella is 2 to 3 inches wide and 3 to 4 inches long. It sits over the other bones at the front of the knee joint and slides when the leg moves. It protects the knee and gives leverage to muscles.
The ends of the three bones in the knee joint are covered with articular cartilage, a tough, elastic material that helps absorb shock and allows the knee joint to move smoothly. Separating the bones of the knee are pads of connective tissue. One pad is called a meniscus (muh-NISS-kus). The plural is menisci (muh-NISS-sky). The menisci are divided into two crescent-shaped discs positioned between the tibia and femur on the outer and inner sides of each knee. The two menisci in each knee act as shock absorbers, cushioning the lower part of the leg from the weight of the rest of the body as well as enhancing stability.
Muscles
There are two groups of muscles at the knee. The quadriceps muscle comprises four muscles on the front of the thigh that work to straighten the leg from a bent position. The hamstring muscles, which bend the leg at the knee, run along the back of the thigh from the hip to just below the knee. Keeping these muscles strong with exercises such as walking up stairs or riding a stationary bicycle helps support and protect the knee.
Tendons and Ligaments
The quadriceps tendon connects the quadriceps muscle to the patella and provides the power to extend the leg. Four ligaments connect the femur and tibia and give the joint strength and stability:
• The medial collateral ligament (MCL) provides stability to the inner (medial) part of the knee.
• The lateral collateral ligament (LCL) provides stability to the outer (lateral) part of the knee.
• The anterior cruciate ligament (ACL), in the center of the knee, limits rotation and the forward movement of the tibia.
• The posterior cruciate ligament (PCL), also in the center of the knee, limits backward movement of the tibia.
Other ligaments are part of the knee capsule, which is a protective, fiber-like structure that wraps around the knee joint. Inside the capsule, the joint is lined with a thin, soft tissue called synovium.
How Are Knee Problems Diagnosed?
Doctors use several methods to diagnose knee problems.
Medical history--The patient tells the doctor details about symptoms and about any injury, condition, or general health problem that might be causing the pain.
Physical examination--The doctor bends, straightens, rotates (turns), or presses on the knee to feel for injury and discover the limits of movement and the location of pain. The patient may be asked to stand, walk, or squat to help the doctor assess the knee's function.
Diagnostic tests--The doctor uses one or more tests to determine the nature of a knee problem.
• X ray (radiography)--An x-ray beam is passed through the knee to produce a two-dimensional picture of the bones.
• Computerized axial tomography (CAT) scan--X rays lasting a fraction of a second are passed through the knee at different angles, detected by a scanner, and analyzed by a computer. This produces a series of clear cross-sectional images ("slices") of the knee tissues on a computer screen. CAT scan images show soft tissues such as ligaments or muscles more clearly than conventional x rays. The computer can combine individual images to give a three-dimensional view of the knee.
• Bone scan (radionuclide scanning)--A very small amount of radioactive material is injected into the patient's bloodstream and detected by a scanner. This test detects blood flow to the bone and cell activity within the bone and can show abnormalities in these processes that may aid diagnosis.
• Magnetic resonance imaging (MRI)--Energy from a powerful magnet (rather than x rays) stimulates knee tissue to produce signals that are detected by a scanner and analyzed by a computer. This creates a series of cross-sectional images of a specific part of the knee. An MRI is particularly useful for detecting soft tissue damage or disease. Like a CAT scan, a computer is used to produce three-dimensional views of the knee during MRI.
• Arthroscopy--The doctor manipulates a small, lighted optic tube (arthroscope) that has been inserted into the joint through a small incision in the knee. Images of the inside of the knee joint are projected onto a television screen. While the arthroscope is inside the knee joint, removal of loose pieces of bone or cartilage or the repair of torn ligaments and menisci is also possible.
• Biopsy--The doctor removes tissue to examine under a microscope.
