voluntary habitual dislocation of the hip ( hdh ) in children is a rare entity ( 1 ) . few cases of habitual dislocation of the hip ( hdh ) as a cause of snapping hip have been reported in the literature ( 2 ) . various causative factors have been described for hdh and it has been classified as recurrent , voluntary and habitual dislocations with different prognosis and treatment regarding to its etiology ( 3 ) . generally , it has good prognosis and most of the cases which are not associated with congenital or genetic disorder like trisomy can be treated by conservative management ( 4 ) . we report a case of hdh in a 9 year - old girl who was suffering from frequent snapping hip at night after separation of her parents . a 9 year - old girl has been referred to our clinic because of popping in her right hip for 9 months . her mother stated that the popping sound was so frequent and loud that was disturbing to the sleep of the family during the night for the last couple of months . her mother had a normal pregnancy with vaginal delivery without any problem . according to the records , postnatal physical examination by pediatrician in hospital was normal . according to the family history she did not have any problem until 9 months ago . although the patient was able to produce popping voluntarily during the day but the chief complaint was the frequent and loud popping sound at night after falling asleep . in physical examination there was no ligamentous laxity and the hip joints were painless , normal in range of motion and stable in all directions . she positioned the right hip up to 90 degrees flexion and 15 - 20 degrees adduction , after doing a slight internal rotation we heard a sudden loud popping sound and hip was displaced in posterior direction . pelvic and right hip radiographs were normal ( fig . 1 , 2 ) . an anteroposterior radiograph after dislocation maneuver revealed hip dislocation with wide space between the femoral head and the acetabulum ( fig 3 ) . we discussed the problem and its nature with parents and referred the family to the psychological consultation . after 3 months the popping disappeared and did not recurred during 15 months of follow up period . anteroposterior pelvic radiograph lateral right hip radiograph right hip radiograph during a dislocation episode which reveals  vacuum phenomenon snapping or popping hip as described in the literature usually is a benign and common problem . it can be produced by moving of muscles or tendons on bony prominences and synovial fold . this entity can occur by rare condition named as habitual dislocation of the hip ( hdh ) ( 4 ) . ahmadi et al reported a 9 year - old girl with hdh and to overcome the confusing nomenclature of the literature on this disorder they proposed a simple classification for hdh and related disorders . they classified these disorders in 3 groups : 1- recurrent dislocation ( posttraumatic , nonvoluntary ) ; 2- voluntary dislocation ( nonhabitual , associated with ligamentous laxity or paralytic disease ) ; 3- habitual dislocation ( not associated with significant ligamentous laxity ) . the management for this disorder will be oriented according to the proposed classification ( 3 ) . right hip involves more frequently than left side and the direction of the dislocation more commonly is posterior ( 5 ) . as mentioned , our patient was female with the right hip involvement and these were in accordance with previously reported cases in the literature . sonographic examination has also been used to describe the so called  vacuum phenomenon  which is a characteristic hyperechoic interface between the acetabular fossa and the dislocating femoral head ( 6 ) . song et al reported 8 cases of hdh , their clinical courses and treatment . describing their experiences and after reviewing the literature they stated that the treatment should be conservative in the first stage which it can be observing the patient and psychiatric consultation . surgical procedures to stabilize the hip are indicated after failure of all conservative treatments ( 2 ) . hdh has also been reported in down syndrome . in patients with this syndrome , morphological abnormalities in acetabulum and posterior acetabular wall hypoplasia imagama et al described a case of a 12-year - old girl with down syndrome presented with hdh which was accompanied with acetabular wall deficiency . knight et al reported 9 children with down syndrome who underwent surgical treatment for habitual dislocation of their hips ( 8) . in this report they recommended femoral varus derotation osteotomy for the treatment of habitual subluxation and dislocation of the pediatric hip in trisomy 21 ( 8) . recommendations for treatment of hdh almost always started with conservative treatments ( 4 ) . in the absence of morphologic and musculoskeletal disorders , conservative treatment and observation with or without psychological consultation will result in fading the popping and dislocation ( 9 ) . in our case we agree with current literature that the prognosis and nature of this entity is good and the treatment in most of cases will be conservative . hdh is a rare entity but is should be considered in differential diagnosis and approaching to a popping and snapping hip .
habitual dislocation of the hip ( hdh ) in children is a rare entity and can be a causative factor for popping or snapping hip which is a common problem in children with good prognosis . 
 we report a case of hdh in a 9 year old girl who was suffering from frequent snapping hip at night , its course and treatment process