Transient Synovitis (sometimes called Toxic Synovitis) of the Hip
This is the most common form of temporary hip joint inflammation in children. It can develop suddenly between three and nine years of age and then disappear after a short time (4-6 weeks), with no serious lasting effects. The most common cause is a virus, so it is frequently seen after an upper respiratory infection.
Treatment is rest and anti-inflammatory medication (such as ibuprofen), which may make the symptoms go away quicker.
In most cases, children have no further problems and return to all of their activities.
Most children will recover completely with no serious lasting effects.