DDH is one of the most common limb deformities in the pediatric population and refers to a spectrum of hip conditions that occur before and after birth when the femoral head and acetabulum have abnormalities in their development and/or anatomical relationship. It can be a very mild form of acetabular dysplasia or a hip pathology that leads to severe loss of joint function in adulthood. It was originally thought to be due to primary acetabular dysplasia and laxity of the joint ligaments, hence the term congenital dislocation of the hip (CDH). The incidence rate is about 0.1%~1.5‰. Girls are more common, accounting for 80%~90%. Starting from the 16th week of pregnancy, the development of the fetal acetabulum obviously lags behind the development of the femoral head, and the length of the round ligament grows much faster than the depth of the acetabulum, and the depth of the acetabulum is relatively shallow at the time of the baby’s birth, which makes the mobility of the hip joint increase, which is favorable for the delivery of the fetus, and is not conducive to the stability of the hip joint. The disease is often combined with joint ligamentous laxity, congenital clubfoot, congenital diastasis and other systemic malformations. DDH is divided into three types:1. Acetabular dysplasia, also known as unstable hip. The incidence of newborn babies is about 0.1%~3.5%.X-ray is often characterized by increased acetabular index, and gradually stabilized with growth and development. Most of the cases can be cured spontaneously by using the hip joint external table, and a few of them have persistent acetabular dysplasia, which becomes symptomatic when they grow up and requires surgical treatment. 2. Hip subluxation, the femoral head and acetabulum are poorly developed, the femoral head is shifted outward and upward, but not completely detached from the acetabulum, retaining part of the articular surface contact. x-ray can see that the femoral head is outwardly shifted, the acetabular index increases to more than 30°, and the femoral head can still be felt in front of the groin. Hip subluxation is not necessarily the result of acetabular dysplasia, nor is it an inevitable transitional stage of hip dislocation, which can persist for a long time and become an independent type. 3. Hip dislocation, this type is the most common, the femoral head has obviously detached from the acetabulum, most of the outward and upward displacement, the original articular surface without contact. The labrum is embedded in the joint, the acetabulum is isolated from the femoral head, and the femoral head is unable to enter the acetabulum. With age, there are many secondary changes that make treatment more difficult. There are three degrees of femoral head dislocation (femoral head at the same level as the acetabulum; femoral head at the upper outer rim of the acetabulum; femoral head at the level of the iliac wing). Clinical manifestations and meticulous clinical examination in the neonatal period and in young infants (within 6 months of birth) are particularly important. This includes Ortolani’s sign and Barlow’s sign. Asymmetry of the hip stripe, elevated or multiple stripes on the affected side, and shortening of the entire lower extremity in a mild externally rotated position can be used as a screening test in the newborn period, but further testing is needed to confirm the diagnosis, such as ultrasound or radiographs of the hip joint. In older children, lameness while walking is the only chief complaint on presentation. In unilateral dislocation, the symptoms are obvious, and in bilateral dislocation, the gait is wobbly and shows a “duck walk”. Treatment:Conservative treatment, children under 18 months of age. The Pavlik Retractor is the treatment of choice for DDH in newborns and small infants under 6 months of age. Suitable for under 6 months, acetabular dysplasia, subluxation and total dislocation with positive Ortolani sign. The course of treatment is usually 3 to 6 months. Traction reset for children under 6 months of age with degree III dislocation and severe contracture of the adductor muscles. The femoral head is gradually reset to avoid ischemic necrosis. Manual repositioning, suitable for infants and children from 6 to 18 months old. Under general anesthesia the adductor muscle is cut, gentle manipulation is performed to reset the femoral head, and cast immobilization is used. It is maintained for at least 3 months. Surgical treatment, which consists of two aspects, one is incision and reset, removing the soft tissue structures that prevent reset, and realizing centric reset of the femoral head. The second is correction of the deformity of the acetabulum and proximal femur by pelvic or femoral osteotomy. Iliac osteotomy, acetabuloplasty, free acetabular osteotomy, Salter iliac osteotomy, Pemberton acetabuloplasty, and reconstructive pelvic osteotomy are the most commonly performed. Ultrasonography of the hip in small infants reference standard