Hip and knee pain in children is quite common. Most often, it is an overuse syndrome (often referred to by the pediatricians incorrectly as "growing pains"). It generally occurs between the ages of 3 and 10 years, is almost always bilateral (both legs), occurs at the end of the day and in the evenings, never interferes with activities, improves when the parent massages the legs or applies a warm wrap, and resolves by about age 10.
There are other types of "overuse" syndrome such as Osgood-Schlatter's disease. This is caused by excess stress on the upper growth plate of the tibia (shin bone) in adolescents, and results in a swelling just below the knee and pain with jumping, running, and particularly kneeling. This is a benign condition most often treated with stretching and anti-inflammatory medications. It resolves when the growth plates close.
Other common causes include contusion (bruise), sprain (injury to a ligament), and fracture. These are related to specific injuries and are usually easy to diagnose based on the history, physical examination and/or radiographs (x-rays).
There are other causes, which are more worrisome. These include vascular abnormalities (Legg-Calve-Perthes disease), infections, tumors, and other causes. Perthes disease is an abnormality of the blood supply to the femoral head and occurs between the ages of 3 and 12 years. It may occur bilaterally, but if so occurs at different times. The children often present with a painless limp. On an x-ray, the femoral head looks more dense and may collapse (flatten). It is more common in boys than girls. In younger children, the prognosis is good for the hip to recover and be normal. In older children, the hip may become permanently deformed and result in arthritis. Treatment varies depending on the involvement, and ranges from observation (routine radiographs) to physical therapy to casts and surgery.
Infection may occur in the hip joint or leg. It is most often spread by the blood stream. If it happens in the hip joint, the child often will refuse to walk and will hold the hip out to the side (in and abducted position). The child will usually have a fever (but not always), and their white blood count will be elevated. These children will usually require emergent surgery and then antibiotics to prevent permanent damage to the surface of the joint.
Infection in the bone is usually more subtle, and the child may limp and have a fever but will not refuse to walk altogether. Because of this, the diagnosis is often delayed. Eventually, changes may be seen on an xray, although an MRI may show changes earlier. Treatment for bone infections (osteomyelitis) is intravenous antibiotics followed by oral antibiotics, often for 6 weeks or more. If an abscess is present in the bone, surgery is necessary.
Tumors of the bone are very rare in children. However, when they occur, they often affect the child's activities and often result in pain in the middle of the night. Generalized symptoms such as fever and weight loss may also be associated.
Tumors can be benign (don't spread to other areas) or malignant (spread and usually are life threatening). Benign tumors include fibrous dysplasia, simple bone cyst, osteoid osteoma, and non-ossifying fibroma (fibrous cortical defect). These are often picked up on an x-ray when it is taken for an injury or fracture. Malignant tumors may cause swelling, fracture of the bone, and severe pain. However, malignant tumors and infections generally don't occur bilaterally.
If your child has a limp or leg pain, consult your pediatrician. Most commonly, it will be a benign condition which requires little if any treatment. Occasionally, it may represent a real problem, so obtaining good medical evaluation is important.