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 that 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.

Diagnosis
There are two general kinds of knee problems:

• Mechanical 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 that result from certain rheumatic diseases, such as rheumatoid arthritis and systemic lupus erythematosus, can damage the knee.

Doctors use several methods to diagnose knee problems, including taking a thorough medical history, performing a physical examination, and using diagnostic tests.

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. These might include:

• 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 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.

Preventing Knee Problems
Some knee problems, such as those resulting from an accident, can't be foreseen or prevented. However, a person can prevent many knee problems by following these suggestions:

• Before exercising or participating in sports, warm up by walking or riding a stationary bicycle, then do stretches. Stretching the muscles in the front of the thigh (quadriceps) and back of the thigh (hamstrings) reduces tension on the tendons and relieves pressure on the knee during activity.
• Strengthen the leg muscles by doing specific exercises (for example, by walking up stairs or hills, or by riding a stationary bicycle). A supervised workout with weights is another way to strengthen the leg muscles that support the knee.
• Avoid sudden changes in the intensity of exercise. Increase the force or duration of activity gradually.
• Wear shoes that both fit properly and are in good condition to help maintain balance and leg alignment when walking or running. Knee problems can be caused by flat feet or overpronated feet (feet that roll inward). People can often reduce some of these problems by wearing special shoe inserts (orthotics). Maintain a healthy weight to reduce stress on the knee. Obesity increases the risk of degenerative conditions such as osteoarthritis of the knee.

Exercises
Three types of exercise are best for people with arthritis:

• Range-of-motion exercises help maintain normal joint movement and relieve stiffness. This type of exercise helps maintain or increase flexibility.
• Strengthening exercises help keep or increase muscle strength. Strong muscles help support and protect joints affected by arthritis.
• Aerobic or endurance exercises improve function of the heart and circulation and help control weight. Weight control can be important to people who have arthritis because extra weight puts pressure on many joints. Some studies show that aerobic exercise can reduce inflammation in some joints.