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Your hip is the joint where your femur (thigh bone) meets your pelvis (hip bone). There are two main parts: a ball at the end of the femur, which fits in a socket in the pelvis. Your hip is known as a ball-and-socket joint. This is because you have a ball at the end of your femur, and it fits into a socket in your pelvis. This makes your hips very stable and allows for a wide range of motion. When they are healthy, it takes great force to hurt them. However, playing sports, running, overuse, or falling can sometimes lead to hip injuries such as:

• Strains
• Bursitis
• Dislocations
• Fractures (broken bones)

Certain diseases also lead to hip injuries or problems. Osteoarthritis can cause pain and limited motion. Osteoporosis of the hip causes weak bones that break easily. Both of these are common in older people.

Another problem is hip dysplasia, where the ball at the end of the femur is loose in the hip socket. It can cause hip dislocation. Babies who have hip dysplasia are usually born with it, but sometimes they develop it later.

Treatment for hip disorders may include rest, medicines, physical therapy, or surgery, including hip replacement.

Your knee joint is made up of bone, cartilage, ligaments and fluid. Muscles and tendons help the knee joint move. When any of these structures is hurt or diseased, you have knee problems. Knee problems can cause pain and difficulty walking.

Knee problems are very common, and they occur in people of all ages. Knee problems can interfere with many things, from participation in sports to simply getting up from a chair and walking. This can have a big impact on your life.

The most common disease affecting the knee is osteoarthritis. The cartilage in the knee gradually wears away, causing pain and swelling.

Injuries to ligaments and tendons also cause knee problems. A common injury is to the anterior cruciate ligament (ACL). You usually injure your ACL by a sudden twisting motion. ACL and other knee injuries are common sports injuries.

Treatment of knee problems depends on the cause. In some cases your doctor may recommend knee replacement.

NIH: National Institute of Arthritis and Musculoskeletal and Skin Diseases