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Knee pain may be the only sign of a hip problem
DEAR DR.PAUL: Our child complained of knee pain and was limping. To our surprise the doctor said that our son had an inflammation of the hip. How could this be as he did not complain of any hip pain?
PEDIATRICIAN DR.PAUL Answers: Your question is logical, but one of the first things we teach young doctors training to become pediatricians or family doctors is exactly what you observed. Hip pain often radiates or transmits to the knee area, and so we routinely examine the hip region as well as the knee, when a child complains of knee pain.
One of the most frequent causes of a limp in a child (aside from injury) is a hip problem. Clues that lead us to suspect a hip problem include isolated knee pain, hip pain or a limp without pain. The causes of hip problems range from what is called "Transient Synovitis" which goes away on its own, to a severe bacterial infection of the hip joint (septic arthritis), which is considered a pediatric emergency. The possibility of this infection is why hip problems or symptoms are taken very seriously.
When we see a child with a suspected hip problem of recent onset, we want to know if there is a history of trauma, fever or signs of an infection like a recent or ongoing cold. In general, a child with a history of fever and pain warrants further tests, usually done in the emergency room setting. Fortunately, most children have no fever, do not look very sick and show no abnormality in the blood tests or hip X-rays. In this case, the more serious bacterial hip infection is eliminated and the diagnosis of "Transient Synovitis" is made. Fortunately, this is much more common than a severe hip infection or other hip problems and goes away on its own.
The term "Synovitis" indicates inflammation of the lining of the joint. The inflammation typically occurs after a viral illness such as a cold. How exactly this happens is not understood, but some speculate that this may be a reaction of the hip joint to the virus causing the recent upper respiratory tract infection.
Another cause of hip problems in children that is not an infection, but can be serious, is Leg Perthes Disease (also know as "Avascular Necrosis of the Hip"). Typically, this presents with either new onset of hip and/or knee pain, but can present with repeated episodes as well.
There usually is no evidence of infection or inflammation. A special test called a bone scan may reveal an abnormality (like an erosion) of the tip or head of the femur, the long thigh bone that fits into the hip joint-socket. The treatment of Leg Perthes disease is usually long term, and may require specific leg casts under the observation of a Pediatric Orthopedic surgeon.