The term congenital dislocation of the hip dates back to the time of Hippocrates. This condition, also known as hip dysplasia or developmental dysplasia of the hip (DDH), has been diagnosed and treated for several hundred years. Most notably, Ortolani, an Italian pediatrician in the early 1900s, evaluated, diagnosed, and began treating hip dysplasia. Galeazzi later reviewed more than 12,000 cases of DDH and reported the association between apparent shortening of the flexed femur and hip dislocation. Since then, significant progress has been made in the evaluation and treatment of DDH. [1, 2, 3, 4, 5]
The definition of DDH is not universally agreed upon. Typically, the term DDH is used in referring to patients who are born with dislocation or instability of the hip, which may then result in hip dysplasia. More broadly, DDH may be defined simply as abnormal growth of the hip. Abnormal development of the hip includes the osseous structures, such as the acetabulum and the proximal femur, as well as the labrum, capsule, and other soft tissues. This condition may occur at any time, from conception to skeletal maturity. The author prefers to use the term hip dysplasia, considering it both simpler and more accurate. Internationally, this disorder is still referred to as congenital dislocation of the hip.
More specific terms are often used to describe the condition more precisely; these are defined as follows:
Subluxation – Incomplete contact between the articular surfaces of the femoral head and acetabulum
Dislocation – Complete loss of contact between the articular surface of the femoral head and acetabulum
Instability – Ability to subluxate or dislocate the hip with passive manipulation
Teratologic dislocation – Antenatal dislocation of the hip
Early diagnosis is the most crucial aspect of the treatment of children with DDH. The use of ultrasonography and other diagnostic imaging modalities and the implementation of improved educational programs will most likely decrease the number of children with DDH who are diagnosed late. Newer, less invasive surgical techniques (eg, endoscopic techniques and image-guided surgery) are being developed in an effort to decrease the morbidity of surgery and to ease recovery.
The normal growth of the acetabulum depends on normal epiphyseal growth of the triradiate cartilage and on the three ossification centers located within the acetabular portion of the pubis (os acetabulum), ilium (acetabular epiphysis), and ischium. Additionally, normal growth of the acetabulum depends on normal interstitial appositional growth within the acetabulum. The presence of the spherical femoral head within the acetabulum is critical for stimulating normal development of the acetabulum.
The anatomy of the dislocated hip, especially after several months, often includes formation of a ridge called the neolimbus. Closed reduction is often unsuccessful at a later date, secondary to various obstacles to reduction. These include adductor and psoas tendon contraction, ligamentous teres, a transverse acetabular ligament, and pulvinar and capsular constriction. With long-standing dislocations, interposition of the labrum can also interfere with reduction.
DDH involves abnormal growth of the hip. Ligamentous laxity is also believed to be associated with hip dysplasia, though this association is less clear. DDH is not part of the classic description of disorders that are associated with significant ligamentous laxity, such as Ehlers-Danlos syndrome or Marfan syndrome.
Children often have ligamentous laxity at birth, yet their hips are not usually unstable; in fact, it takes a great deal of effort to dislocate a child's hip. Therefore, more than just ligamentous laxity may be required to result in DDH. At birth, white children tend to have a shallow acetabulum. [6, 7] ; this may provide a susceptible period in which abnormal positioning or a brief period of ligamentous laxity may result in hip instability. However, this characteristic is not as true for children of African descent, who have a lower rate of DDH. 
The etiology of hip dysplasia is not clear, but this condition does appear to be related to a number of different factors.  One such factor is racial background: In Native Americans and Laplanders, the prevalence of hip dysplasia is much higher (nearly 25-50 cases per 1000 persons) than in other races, and the prevalence is very low in southern Chinese and black populations. [10, 11, 12, 8] An underlying genetic disposition also appears to exist, in that the frequency of hip dysplasia is 10 times higher in children whose parents had DDH than in those whose parents did not. 
Other factors possibly related to DDH include intrauterine positioning and sex, and some of these are interrelated. Female sex, being the first-born child, and breech positioning are all associated with an increased prevalence of DDH. An estimated 80% of persons with DDH are female,  and the rate of breech positioning in children with DDH is approximately 20% (compared with 2-4% in the general population). [15, 16] The prevalence of DDH in females born in breech position has been estimated to be as high as 1 case in 15 persons in some studies. 
Other musculoskeletal disorders of intrauterine malpositioning or crowding, such as metatarsus adductus and torticollis, have been reported to be associated with DDH. [18, 19] Oligohydramnios is also reported to be associated with an increased prevalence of DDH.  The left hip is more commonly associated with DDH than the right hip, possibly because of the common intrauterine position of the left hip against the mother's sacrum, which forces it into an adducted position.  Children in cultures in which the mother swaddles the baby, forcing the infant's hips to be adducted, also have a higher rate of hip dysplasia. 
Hip dysplasia can be associated with underlying neuromuscular disorders, such as cerebral palsy, myelomeningocele, arthrogryposis, and Larsen syndrome, though such cases are not usually considered DDH.
The overall frequency of DDH is usually reported as approximately 1 case per 1000 individuals, though Barlow believed that the incidence of hip instability during newborn examinations was as high as 1 case per 60 newborns.  According to Barlow's study, more than 60% of newborns with hip instability became stable by age 1 week, and 88% became stable by age 2 months, leaving only 12% (of the 1 in 60 newborns, or 0.2% overall) with residual hip instability.
Overall, the prognosis for children treated for hip dysplasia is very good, especially if the dysplasia is managed with closed treatment. If closed treatment is unsuccessful and open reduction is needed, the outcome may be less favorable  , although the short-term outcome appears to be satisfactory. If secondary procedures are needed to obtain reduction, then the overall outcome is significantly worse.
Some authors believe that patients with bilateral hip dysplasia have a poorer prognosis because of frequent delays in diagnosis and greater treatment requirements. [24, 25] In a study comparing the outcomes of walking-age children with bilateral hip dislocations who underwent open reduction and pelvic osteotomy with or without femoral osteotomy with those of walking-age children with unilateral dislocated hips who underwent the same set of procedures, the radiographic outcomes were similar. 
In this study, the rate of osteonecrosis was higher in the bilateral group, but this difference was explained by older age at surgery and a greater degree of hip dislocation before surgery.  The authors concluded that the clinical outcomes after surgery of the children with bilateral hip dislocations were worse mainly because of asymmetric outcomes.