Developmental Dysplasia of the Hip (Congenital Dislocation of the Hip)

GENESIS

The definition of developmental dysplasia of the hip (DDH), formerly termed congenital dislocation of the hip , remains complex and controversial, making precise determination of incidence figures difficult. Fig. 10.2 depicts some of the interacting factors that relate to the genesis of DDH. When hip dislocation develops because of a genetic connective tissue dysplasia, such as Marfan syndrome, Ehlers-Danlos syndrome, Larsen syndrome, or osteogenesis imperfecta, it may not become evident until after birth and special treatment considerations may apply. Under such circumstances, use of the term congenital hip dislocation is inappropriate. Laxity of connective tissue is an important factor, and the 4:1 to 5:1 female-to-male predilection toward the occurrence of DDH is considered a consequence of the female fetus being laxer than the male. This might be because of a lack of testosterone effect in the female fetus. Testosterone in the male fetus may result in tighter muscles and tougher connective tissue, as has been demonstrated in the hip capsule of young rodents. Pelvic ligaments are also more relaxed in infant girls, possibly because of differential receptor responses to relaxing hormones in late gestation. Dunn has shown the excess of dislocation of the hip in the female to be as high as 13:1 when it is an isolated deformation; however when there is breech presentation and/or the presence of multiple deformations, the sex ratio is closer to equal. Carter and Wilkinson noted laxity of three or more joints in 7% of individuals in the general population compared with an incidence of 22% among first-degree relatives with sporadic cases of dislocation of the hip. This suggests a possible heritable tendency toward increased joint laxity in association with congenital hip dislocation. When there were multiple cases of dislocation of the hip in a family, 65% of first-degree relatives had undue joint laxity.

The same forces that tend to thrust the head of the femur out of the acetabulum also cause stretching of the joint capsule and ligamentum teres ( Fig. 11.1 ). These forces are most commonly the result of constraint in late fetal life and thus are more likely to affect firstborn offspring. Certain presentations during late gestation are particularly likely to cause dislocation of the hip, and breech presentation is notorious in this respect, with about 50% of patients with hip dislocation having been in breech presentation at birth. Among breech term births (about 3.5% of all births), the frequency of dislocation of the hip is 17%, and for breech births with extended legs in utero (frank breech), the incidence is 25%. When 224 term breech singleton infants were compared with 3107 term vertex singleton infants, DDH occurred 15 times more frequently among the infants in breech presentation (0.9%) compared with infants in vertex presentation (0.06%). Breech presentation, with the fetal buttocks located in the maternal pelvis and the hips tightly flexed against the abdomen, tends to displace the femoral heads out of the acetabulum, especially when the legs are extended and “caught” between the fetal abdomen and the maternal uterine wall.

The fact that the fetus more commonly lies with the left side toward the mother’s spine may explain why the left hip is more commonly dislocated than the right. Left-sided hip dislocation is noted four times more frequently than right-sided hip dislocation. Constraint of the fetus because of oligohydramnios deformation sequence is associated with an increased frequency of hip dislocation, partly because of the increased frequency of breech presentation with oligohydramnios. Dunn noted that hip dysplasia is more common when other postural deformations are present, particularly congenital muscular torticollis, talipes equinovarus, metatarsus adductus, and hyperextension or dislocation of the knee (usually all resulting from the same deforming posture in late gestation).

The position of the infant after birth is also important in the genesis of DDH. The hip joint may be lax at birth and usually becomes tighter after birth; hence hips rarely become dislocated after birth unless some type of DDH is present. Undue leg extension in early infancy, at a time when the femoral head is not yet perfectly round, can be a factor in the postnatal genesis of DDH. Thus the hip that has been flexed, with a relatively contracted psoas muscle in utero, is more likely to be thrust out of its socket by forced extension of the leg. The increased frequency of DDH among Mongolian infants has been attributed to their practice of swaddling young infants with their legs extended. On the other hand, the relatively low incidence of hip dislocation among the Hong Kong Chinese population has been attributed to the custom of carrying the infant on the mother’s lower back, with the infant’s hips flexed and partially abducted (which mechanically favors maintenance of the femoral head in the acetabulum). The majority of mothers in the sub-Saharan African country of Malawi back-carry their infants during the first 2–24 months of life in a position similar to that of the Pavlik harness, which results in a very low incidence of DDH in this country where no infant presented with or underwent surgical intervention for symptomatic DDH between 2002 and 2012 in a large hospital where 9842 children underwent other surgical procedures. In addition, there is established evidence indicating that swaddling, the opposite position to back-carrying, causes an increase in the incidence of DDH. In dogs, canine hip dysplasia is the most common developmental defect and the risk in very large-sized breeds is 20–50 times greater, respectively, than that in small and medium breeds, with no apparent difference evident between genders.

Therefore certain risk factors warrant close surveillance for congenital hip dislocation: family history of DDH, breech presentation, firstborn children, female gender, accompanying postural deformations (particularly congenital muscular torticollis, which also manifests a left-sided predominance), and certain postnatal positioning practices (papoose swaddling or swaddling with the hips fully extended). All gradations of dislocation, from a partially stretched dislocatable hip to a fully dislocated hip, occur with an overall frequency of about 1%–3%. Considering the full spectrum, Barlow observed that 60% of these dislocatable hips are stable in the normal location by 1 week of age and 88% are stable by 2 months of age. The remaining 12% represent frank dislocation of the hip that was evident at birth or during early infancy, suggesting a frequency of 1.2 per 1000.

Additional controversy regarding the incidence of DDH results from differences in criteria for defining a genuinely pathologic neonatal hip. Before the introduction of routine clinical screening programs for neonatal detection of DDH (c. 1920–1950), diagnostic criteria were so varied that incidence figures ranged from 0 to 200 per 1000 children. After routine clinical neonatal screening was introduced (c. 1950–1980), data on the incidence of DDH were based mainly on clinical findings that denoted neonatal hip instability, leading to some overdiagnosis and overtreatment, with an incidence of 0.41–168.6 per 1000. After ultrasonography began to be used in neonatal screening for DDH, higher sensitivity resulted in higher incidence figures (71.5–518.5 per 1000), but sonographic diagnoses did not always correlate with clinical diagnoses. In an effort to correlate these data with Barlow’s clinical findings (which suggested that 88% of unstable hips eventually become normal without treatment), Bialik and coworkers tried to correlate neonatal ultrasound screening with clinical findings and treatment to determine the true incidence of DDH. They screened 9030 neonates at 1–3 days of life; any infants found to have sonographic abnormalities were reexamined at 2–6 weeks, depending on the severity of their findings. Only hips that had not improved or had deteriorated were treated; all others were examined periodically until 12 months of age. Sonographically abnormal hips had an incidence of 55.1 per 1000 hips, whereas the true incidence of DDH requiring treatment was 5 per 1000 hips, with all others evolving into normal hips and no new instances of DDH at 12 months.