Developmental dysplasia of the hip (DDH) usually refers to dysplasia of the hip joint in infants and children, and such disorders are thought to develop gradually after birth. It is a medical term that includes joint instability and laxity. Due to differences in the timing and severity of the disorder, doctors use a range of terms, including: hip dysplasia, developmental hip dislocation, hip dislocation, developmental hip dysplasia, acetabular dysplasia, and congenital hip dislocation. The exact cause remains unknown, but it is commonly believed that hip dysplasia is developmental, which develops mainly at birth, after birth, and even during childhood. This is why hip dysplasia is usually referred to as developmental hip dysplasia. Causes include: 1. Familial There is a genetic correlation in hip dysplasia, but it is not a direct cause. The incidence of hip dysplasia increases 11-fold in cases with a family history. That is, if one child in the family has DDH, the incidence of another child is about 6% (one in seventeen), if one parent has DDH, the incidence of their child is about 12% (one in eight), and if one parent has DDH and already has a child with DDH, the chance of their second child having DDH is about 36% (one in three). 2. Intrauterine mechanical compression The positional factors of the fetus in the uterus can cause increased joint pressure and ligamentous stretching. It is generally believed that the mechanical pressure on the left hip joint is greater than that on the opposite side in normal gestational position, which is the reason why the left side of hip dysplasia is more likely to be involved. Hip instability is more prevalent in the breech position than in the normal gestational position due to mechanical stress. Other factors such as first birth, multiple births, and low amniotic fluid can cause intrauterine mechanical compression. 3, concomitant deformities Newborns with fixed foot deformities or cases of oblique neck have an increased risk of hip dysplasia, which is also mainly suggestive of insufficient space for the fetus in the uterus. 4, hormonal factors During the birth of the fetus, the mother will produce estrogen to make the mother’s ligaments relax and make the fetus easy to pass through the birth canal, some infants will be more sensitive to estrogen than other children, causing excessive laxity of the child’s ligaments, so the ligaments of girls are more lax than those of boys, and their incidence of hip dysplasia is 4-5 times higher than that of boys. 5, infant joint characteristics The hip joint of infants is softer than the adult hip joint and is easily dislocated. The acetabular fossa of infants is composed of soft and elastic cartilage, and the ligaments of infants are lax; whereas adults are composed of hard bones with little cartilage, and under the same stress, the hip joint of infants is more likely to be semi-dislocated or dislocated than that of adults. 6. Position of the infant in the first year of life Usually, the fetus’s legs are flexed and crossed in the womb, and the sudden straightening of the legs into a standing position after birth will lead to hip laxity and damage to the cartilage of the acetabular fossa. In the literature, North American Indians have a high incidence of hip dysplasia in their children due to cultural and customary differences in hip extension. In contrast, in Africa, the incidence of hip dysplasia is low in those who use a hip-forked holding style. For these reasons, swaddling with the hips straight and together should be avoided in the first few months of life; incorrect swaddling can worsen hip dysplasia. Early symptoms that parents can detect: 1. Asymmetry of skin lines In children with hip dislocation, especially in unilateral dislocation, families can usually detect asymmetry of hip or leg lines in both lower extremities, but this is not a one-to-one indicator. However, this is not a one-to-one indicator, because 25% of normal children also have dermatoglyphic asymmetry. 2. Joint popping Joint popping is often indicative of hip dysplasia, but in normal children, especially newborns, joint popping often occurs due to joint laxity. 3. Limited joint movement Families often find it difficult to change diapers in children because the legs do not fully expand. The literature reports that limited hip abduction in infants at 8 weeks of age is a sensitive indicator of DDH. 4. Pain Hip dysplasia in infants and children is often painless, which is the insidious and undetectable nature of this disease. Pain often does not appear until adolescence and early adulthood. 5. Walking sway What families can usually detect is a painless, pronounced body swaying gait in children after learning to walk, and the legs are of different lengths. If the dislocation is bilateral, it is often possible to find walking with an obvious duck-trot limp wobbly gait. Therefore, families should also pay attention to abnormalities in the morphology and movement of the growing lower extremities of the child and seek timely consultation with a pediatric orthopedic surgeon if abnormalities are found.