Developmental dysplasia of the hip: Epidemiology and pathogenesis

INTRODUCTION — Developmental dysplasia of the hip (DDH) describes a spectrum of conditions related to the development of the hip in infants and young children. It encompasses abnormal development of the acetabulum and proximal femur and mechanical instability of the hip joint.

Newborns often have physiologic laxity of the hip and immaturity of the acetabulum during the first few weeks of life. In most cases, the laxity resolves, and the acetabulum proceeds to develop normally. With assessment of risk factors, serial physical examination of the hips, and appropriate use of imaging, most children with pathologic hips can be correctly diagnosed and treated without long-term sequelae. (See "Developmental dysplasia of the hip: Treatment and outcome".)

Typical DDH, which generally occurs in otherwise healthy infants, will be the focus of this topic review. Hip dysplasia and instability also occur in association with other conditions. Teratologic hip dysplasia occurs in association with various syndromes (eg, Ehlers-Danlos, Down syndrome, arthrogryposis), and neuromuscular hip dysplasia occurs when there is weakness and/or spasticity in some or all of the hip muscle groups (eg, in spina bifida or cerebral palsy). The diagnosis and management of teratologic and neuromuscular hip dysplasia differ from the diagnosis and management of hip dysplasia in otherwise healthy infants.

The epidemiology and pathogenesis of DDH in otherwise healthy children will be reviewed here. The clinical features, diagnosis, treatment, and outcome are discussed separately. (See "Developmental dysplasia of the hip: Clinical features and diagnosis" and "Developmental dysplasia of the hip: Treatment and outcome".)

TERMINOLOGY — The terminology for developmental problems of the hip has changed over time. "Developmental dysplasia of the hip" (DDH) is the preferred term [1]. "Developmental" is preferred to "congenital," since the condition is not always present or identifiable at birth and develops during early childhood [2-4]. "Dysplasia" is preferred to "dislocation" because it includes a broader spectrum of changes in the shape and position of the femoral head and acetabulum. "Congenital hip dislocation" is an outdated term that was used at a time when most children were diagnosed after they began walking and the dislocation was assumed to be congenital.

Specific terms describing the position, stability, and shape of the hip are defined below:

●Dislocation – There is a complete loss of contact between the femoral head and the acetabulum.

●Subluxation – The femoral head is partially outside of the acetabulum, but remains in contact.

●Dislocatable – The femoral head is reduced (ie, within the acetabulum) at rest, but can dislocate in other positions or with examination maneuvers. This is a hip with instability.

●Subluxatable/subluxable – The femoral head is reduced at rest, but can be partially dislocated or subluxated with examination maneuvers. This is a hip with mild instability or laxity.

●Reducible – The hip is dislocated at rest, but the femoral head can be positioned into the acetabulum with manipulation (generally flexion and abduction).

●Dysplasia – Abnormality of the shape of the hip joint (usually shallowness of the acetabulum, involving the superior and anterior margins).

EPIDEMIOLOGY — Estimates of the incidence of DDH are quite variable and depend upon the means of detection, the age of the child, and the diagnostic criteria. It is estimated that dislocatable hips and hips with severe or persistent dysplasia occur in 3 to 5 per 1000 children [5-7]. Historically, the incidence of DDH with dislocation is 1 to 2 per 1000 children [8,9]. Mild hip instability is more common in newborns, with reported incidence as high as 40 percent [5]. However, mild instability and/or mild dysplasia in the newborn period often resolve without treatment [5,8,10]. Infants with mild instability and/or mild dysplasia in the newborn period should not be included in estimates of incidence. Their inclusion results in overestimation.

In a prospective study, 9030 infants (18,060 hips) were routinely screened for DDH by physical examination and ultrasonography at one to three days of life [5]. Sonographic abnormalities were detected in 995 hips (representing a sonographic incidence of 5.5 percent). However, on repeat examination at two to six weeks of age with no interval treatment, residual abnormalities were detected in just 90 hips (representing a true DDH incidence of 0.5 percent), which then proceeded to treatment. In other words, 90 percent of newborn hips with clinical or sonographic signs of DDH improved spontaneously before two to six weeks of age. It is our opinion that newborns with clinical or sonographic findings of mild laxity or minimal dysplasia have normal immaturity of hip development and should not be diagnosed with or treated for DDH.

The incidence of DDH also varies by race. It is increased in the Lapp and Native American populations (25 to 50 cases per 1000 births) [11,12] and decreased in African and Asian populations [13,14]. (See 'Swaddling' below.)

Both hips are involved in as many as 37 percent of patients [15,16]. Among the unilateral cases, the left hip is affected more often than the right [16,17]. The preponderance of left-sided cases may be related to the typical left occiput anterior fetal positioning, in which the left hip is forced into adduction against the mother's sacrum [15]. (See 'Other conditions' below.)

RISK FACTORS — DDH is more common among infants with certain risk factors (eg, female sex, breech position in the third trimester, positive family history, tight lower extremity swaddling). However, with the exception of female sex, most infants who are diagnosed with DDH have no risk factors [18,19].

Female sex — The risk of DDH in females is estimated to be 1.9 percent [6]. DDH is two to three times more common in female than in male infants [17]. In a meta-analysis of risk factors for DDH that included 24 studies and >556,000 patients, the relative risk ratio for females was 2.5 (95% CI 2.1-3.1) [17].

The increased incidence in females has been attributed to a transient increase in ligamentous laxity related to increased susceptibility of female infants to the maternal hormone relaxin. However, some studies refute this hypothesis [20].

The increased incidence of DDH in females is difficult to separate from the increased risk of DDH in breech deliveries, which is also more common in females. (See 'Breech position' below.)

Breech position — Breech position during the third trimester is the greatest single risk factor for DDH [21]. The absolute risk of DDH is estimated to be as high as 12 percent in breech females and 3 percent in breech males [6,22]. In a meta-analysis of risk factors for DDH that included 15 studies (>359,300 patients), the relative risk for breech presentation was 3.8 (95% CI 2.3-6.2) [17]. It is unclear in the literature if the amount of time spent in breech position or the point during gestation at which the fetus was breech affects risk of DDH.

The risk is higher in infants with frank breech (ie, hips flexed, knees extended) than with footling breech presentation [15,23,24]. The risk of DDH appears to be similar among preterm and term breech infants, although the evidence is limited [22,25,26]. Whether successful external cephalic version affects the risk of DDH was evaluated in a cohort of 498 singleton infants with breech presentation at ≥34 weeks gestation for whom external cephalic version was attempted [27]. DDH requiring treatment was diagnosed in 35 infants. Successful external cephalic version was associated with decreased risk of DDH requiring treatment (2.8 versus 9.3 percent; multivariable odds ratio 0.29, 95% CI 0.09-0.95). However, additional studies are necessary to clarify the relationship between successful cephalic version and risk of DDH.

Recommendations for imaging in infants with breech presentation are discussed separately. (See "Developmental dysplasia of the hip: Clinical features and diagnosis", section on 'Normal examination and risk factors'.)

The increased risk of DDH is present regardless of the method of delivery [17,22]. However, decreasing the time spent in the breech position by prelabor cesarean delivery may decrease the risk of clinically significant DDH [23,24,28,29]. This was illustrated in a retrospective review that found a decreased rate of DDH among breech infants who were delivered by cesarean delivery before the onset of labor (3.7 percent versus 6.6 percent among those delivered by intrapartum cesarean delivery and 8.1 percent among those delivered vaginally) [24].

Family history — Genetic factors appear to play a role in the development of DDH [30,31]. The absolute risk of DDH in infants with a positive family history ranges from approximately 1 to 4 percent [6]. In a meta-analysis of risk factors for DDH that included four studies (>14,000 patients), the relative risk for positive family history was 1.39 (95% CI 1.23-1.57) [17]. If one of a set of twins has DDH, the risk in the other twin is greater if they are monozygotic than dizygotic (40 versus 3 percent) [21].

In a review of 589 patients with DDH, the risk of recurrence in subsequent children was 6 percent when there was one affected child, 12 percent when there was one affected parent, and 36 percent when there was an affected parent and an affected child [32].

Family members of children with DDH also appear to be at increased risk of occult acetabular dysplasia, which often develops before 30 years of age. (See "Developmental dysplasia of the hip: Treatment and outcome", section on 'Management of family members'.)

Swaddling — The incidence of DDH is increased in populations that use swaddling clothes and cradle boards [11,16,33,34]. These practices limit hip mobility and position the hip in adduction and extension, which may play a role in the development of DDH [35]. In an experimental study in rats, traditional swaddling in hip adduction and extension resulted in a greater rate of dislocation and dysplasia than no swaddling [36].

The American Academy of Pediatrics, the Pediatric Orthopaedic Society of North America, and the International Hip Dysplasia Institute recommend "hip-healthy swaddling," which consists of allowing ample room for hip and knee flexion and free movement of the legs [35,37-39]. The risk of sudden infant death with swaddling is discussed separately. (See "Sudden infant death syndrome: Risk factors and risk reduction strategies", section on 'Sleep environment'.)

Other conditions — Other conditions related to decreased fetal movement or abnormal fetal position have been associated with DDH but have not been proven to increase the risk [21,40]. These include torticollis [41-43], plagiocephaly, metatarsus adductus [44,45], clubfoot [46-49], being the firstborn infant [15,17,23], oligohydramnios, birthweight >4 kg [8,15,23], and multiple gestation pregnancies [50-53].

EMBRYOLOGY AND PATHOGENESIS — Normal hip joint development depends upon normal contact between the acetabulum and the femoral head promoting mutual induction. Abnormal development is a result of abnormal contact, which may result from multiple genetic and environmental factors (intrauterine and postnatal).

By the 11^th week of gestation, the hip joint is fully formed. The femoral head is spherical and deeply set within the acetabulum [54]. However, the femoral head grows at a faster rate than the acetabulum, so that by the end of gestation, the femoral head is less than 50 percent covered by the acetabular roof. During the final four weeks of gestation, the hip is vulnerable to mechanical forces, such as adduction, that direct the femoral head away from the central portion of the acetabulum [55]. Conditions that limit fetal mobility, including breech positioning, accentuate these mechanical forces. This results in eccentric contact between the femoral head and the acetabulum.

In the newborn period, ligamentous laxity makes the developing hip susceptible to other external mechanical forces. Positioning with the hips extended (eg, swaddling) can result in eccentric hip joint contact as the femoral head glides within or moves outside of the acetabulum [56]. If these factors persist, abnormal hip joint contact leads to structural anatomic changes. If the femoral head is not deeply seated within the acetabulum, the labrum may become everted and flattened and the ligamentum teres elongated. Abnormal ossification of the acetabulum occurs and a shallow acetabulum develops (figure 1) [57].

With time, the intra-articular structures hypertrophy, including the labrum with a thickened ridge (neolimbus), ligamentum teres, and fibrofatty tissue (pulvinar). Contractures develop in the iliopsoas and hip adductors, and the inferior capsule is pulled into the empty acetabulum, further decreasing the potential for the femoral head to reduce into the acetabulum. A false acetabulum may form where the femoral head contacts the lateral wall of the pelvis above the true acetabulum. The lack of contact between the femoral head and acetabulum inhibits further normal development of the hip joint.

With or without full dislocation, the hip can develop dysplastic changes. Most commonly, the result is a shallow acetabulum with decreased anterior and lateral coverage of the femoral head. There also can be asphericity of the femoral head, a valgus neck-shaft angle, and persistence of excess femoral anteversion.

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or email these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient education" and the keyword[s] of interest.)

●Basics topic (see "Patient education: Developmental dysplasia of the hip (The Basics)")

SUMMARY

●Developmental dysplasia of the hip (DDH) describes a spectrum of conditions related to the development of the hip in infants and young children. The spectrum includes abnormalities of stability (dislocation/dislocatability and subluxation/subluxatability) and abnormalities of shape of the femoral head and acetabulum (dysplasia) (table 1). (See 'Terminology' above.)

●The incidence of DDH depends upon the definition, the method of detection, and the age of the child at the time of examination. The incidence of true dislocation is estimated to be 1 to 2 per 1000 newborn infants. Large studies with ultrasonographic screening suggest that up to 40 percent of newborns have laxity or immaturity, but 90 percent of these improve spontaneously. (See 'Epidemiology' above.)

●Risk factors for DDH include female sex, breech position in the third trimester, positive family history, and swaddling with the hips held in extension and adduction. However, with the exception of female sex, most patients who are diagnosed with DDH have no risk factors. (See 'Risk factors' above.)

●DDH has a multifactorial pathogenesis. Ligamentous laxity predisposes the developing hip to mechanical forces that cause eccentric contact between the femoral head and the acetabulum. Abnormal contact results in abnormal development of the acetabulum and femoral head. (See 'Embryology and pathogenesis' above.)