Developmental dysplasia of the hip
(DDH) is a deformity of the acetabulum of a variable degree. This
is indicative of delayed modeling, which is referred to as
"immaturity". It is associated with a femoral head which is shallow
in location (subluxed) or frankly displaced out of the acetabulum
(dislocated). "Instability" indicates joint laxity which allows a
non-displaced femoral head to become subluxed or dislocated when
under stress and which can be provoked by the examiner. Instability
is in general associated with acetabular immaturity, but not
Proper growth and development of the
acetabulum is due to the presence of the femoral head in adequate
contact and without stress within it. In other words, the
acetabulum needs the femoral head for its development. DDH is the
result of the disruption of this relationship. It is estimated that
in 98% of the cases it results from a late intrauterine event of a
previously well formed hip due to persistent forces. In 2% of cases
it results from an early intrauterine event secondary to a
congenital neuromuscular disorder. The term used for the latter
type of hip is "teratologic". In a small number of cases DDH occurs
later, beyond the neonatal period, despite initial normal clinical,
and sometimes normal ultrasound, examinations.
The hip develops from a single block of
mesoderm, and at the beginning of the fetal period (8 weeks since
conception) all the structures of the joint are already in place
At birth, the acetabulum has a smaller bony component and a larger
and the femoral head coverage by the acetabulum is less than at any
other time during the fetal or postnatal periods.
This situation results in a 6-week postnatal period in which the
acetabulum is highly susceptible to modeling; it is slightly less
susceptible from 6 to 12 weeks and very slightly susceptible after
16 weeks. If the femoral head is in a normal position within the
acetabulum the end result will be a normal hip. This is the key
period for treatment of DDH. If the femoral head is in an abnormal
relationship and this is not corrected, the end result will be a
permanently dysplastic hip. Early diagnosis is, therefore, pivotal.
However, it is important to know that with minor ultrasound
findings, 78% of the hips will spontaneously become normal by the
fourth week and 90% by the ninth week.
Therefore, a conservative attitude in the interpretation of the
ultrasound findings is warranted.
The consideration of some facts should help
us understand, in a particular case, the possibilities of a
successful ultrasound examination and a positive diagnosis.
Patient's age at the time of the examination
The first problem encountered in examination
of the infant hip is technical and related to visualization of the
structures. The femoral head begins ossifying between the second
and eighth month of life in males, and earlier in females (figure
2). On ultrasound, visualization of the neovascularity of the
future ossification center begins weeks before the radiological
findings. As the size of the ossification center progressively
enlarges it will, at some point, obscure the deeper acetabulum,
making ultrasound examination impossible. The age of occurrence of
this event is variable, mostly occurring in the second semester of
life. If the patient is in that age range we might attempt the
ultrasound examination first and move on to a radiograph in case of
failure. Some authors are more dogmatic and recommend ultrasound
only up to 4 months of age, with radiography thereafter.
The second problem is deciding the timing of
the ultrasound examination on the basis of the clinical history and
results of physical examination. A newborn with an abnormal
physical examination will benefit from the ultrasound examination
if performed promptly within the first 2 weeks of life. Newborns
with a risk factor by history but with a normal physical
examination can wait for 4 to 6 weeks. In this way, the normal
instability of the hip of the first weeks of life will be avoided.
Similarly, many of the morphologically immature hips will progress
towards normality, given time.
Race, environment, and season of the year
Articles from different countries report a
different incidence of DDH in all of its forms. For example,
Australia reports a 1% incidence, Netherlands 3.7%, Poland 3.9%,
Israel 5.9%, Austria 6.57%, and Norway 16.9%. The reported average
of hip dislocation is 0.6 per thousand newborns, whereas for
indigenous North Americans and Lapps it can be as high as 25 to 50
per thousand. On the other hand, Chinese and Black Africans have a
negligible incidence, but American Blacks are affected. The
difference is most likely genetic, and Blacks have been found to
have deeper acetabula at birth. Additionally, environmental factors
cannot be ignored. It is interesting to note
that in the group with a high incidence, mothers traditionally
swaddle their infants with the hips in extension, or strap them to
a cradle board; both result in unphysiological hyperextension of
the hips. On the contrary, in the groups with a low incidence of
the problem, mothers carry their infants against their waist with
the children's legs in flexion and abduction, which is more
On the same venue, in Japan after the institution of a national
program to discourage the swaddling of infants, the incidence of
DDH dropped from 3.5 to 0.2%.
There is a higher incidence of DDH in Central Europe and in the
South American Andean countries.
A higher winter incidence of DDH, in the
range of 1.5% as compared to that of summer (1%), has been
Cases of families with several members
involved by DDH are well known. There is a reported familial
incidence in 20% of patients.
The risk of DDH for a female fetus in breech
presentation with a maternal history of the disease is 1 in 15; the
risk for those with a positive maternal history but a presentation
other than breech is 1 in 25.
A newborn with a sibling affected by the dysplasia carries a risk
of 6%, 12% if one parent is affected, and 36% if both a sibling and
a parent are.
DDH is from 4 to 8 times more frequent in
females than in males.
This is thought to be possibly related to an increased level of
circulating estrogen observed in affected newborns, to which
females are more sensitive. Estrogenic action results in the
blockage of maturation of collagen, thus affecting the development
of the acetabulum.
However, elevated levels of estrogens were not confirmed by other
researchers. Similarly, the pregnancy hormone relaxin has been
found in variable levels in newborns with DDH, representing another
possible causative factor.
Intrauterine mechanical restriction: way of
The development of a fetus requires enough
room to move freely without inordinate tension applied. This is as
valid for the proper development of the extremities as for the
lungs. When ample space is not available, abnormalities such as DDH
can develop. For example, oligohydramnios severely restricts motion
and places stress on the extremities, which are placed in
hyperflexion and/or hyperextension for prolonged periods of time
Additionally, first pregnancies find
unstretched maternal abdominal and uterine walls which also may
limit free movement. Sixty percent of patients with DDH occur in
primiparous mothers. Breech presentation results in increased
tension and hyperflexion of the hips due to their location within
the inelastic maternal pelvis and the impossibility of free active
motion. This is observed in 30 to 50% of patients with DDH, whereas
only 2 to 4% of deliveries are in the breech presentation. Most
fetuses in breech presentation lie with the left lower extremity on
the maternal spine which forces its adduction and limits its
motion. This is thought to be responsible for the increased
incidence of left hip involvement that is seen in 80% of these
Bilateral involvement is described in 25%.
High birth weight also is associated with an increased incidence of
Twin fetuses have a similar incidence of DDH to that of singletons.
The mode of delivery, vaginal or by Cesarean section, does not seem
to affect the likelihood of DDH.
Musculoskeletal and other abnormalities
DDH has an association with other postural
and non-postural musculoskeletal abnormalities. It is found in 2%
of patients with club foot and metatarsus varus and in 20% of those
Also, it has been seen with scoliosis, head and facial deformities,
generalized joint laxity, spina bifida, sacral agenesis,
myelodysplasia, arthrogryposis multiplex, and cardiac and renal
abnormalities (due to secondary oligohydramnios), as well as other
We might receive a referral for ultrasound on
the basis of abnormal physical findings. On observation, an infant
with a dislocated hip might show a shortened lower limb and
redundant skin folds in the thigh due to an apparent excess of
skin. Both findings are secondary to an upward displacement of the
dislocated femoral head. These findings are more likely to be seen
in older infants, not in the newborn.
By performing different maneuvers we may find
a limitation of abduction on the affected side as compared to the
opposite side. The Barlow's maneuver, performed with hip and knee
flexion, attempts to dislocate a well placed femoral head (figure
4). With the limb in adduction, a gentle posterior push, like a
piston, is made. When positive (subluxable or dislocatable hips),
we refer to the hip as "unstable". It should be noted that a mild
displacement of a few millimeters is normal in the first 2 weeks of
life due to circulating humoral factors. Clinically, instability is
found in 1 to 3% of all newborns.
The Ortolani's maneuver attempts
to reduce a dislocated hip (figure 5).
It is performed with flexion of the hip and knee. While the
extremity is being abducted, an anterior and sustained push to the
thigh is applied from the posterior aspect, attempting to relocate
the posteriorly dislocated femoral head. On the basis of the
we refer to the dislocated hip as "reducible" or "non-reducible".
It should be noted that the sensitivity of the clinical Ortolani's
and Barlow's maneuvers has been reported by several authors,
ranging from 10 to 34.4%, and their specificity has been found to
be from 84.3 to 99%.
False negative results of the Ortolani's maneuver can occur in
newborns with extreme capsular laxity in which the dislocated
femoral head can be fully abducted without actually reducing it. It
also can occur in older infants with a longstanding dislocation in
which the hip muscles become contracted and shortened, trapping the
femoral head outside the acetabulum.
The described maneuvers have been incorporated into the ultrasound
examination, improving their sensitivity and specificity.
At our institution we sometimes get referrals
because of audible high pitch "clicks" observed during the
performance of the described clinical maneuvers. In general, this
finding results from joint capsule and tendon stretching and
snapping, and has no pathological significance. Occasionally,
though, we might find a positive finding on ultrasound.
The anatomy of the infant hip is illustrated
in figures 6 and 7. Two techniques for the ultrasound examination
of this area are widely in use. Both use a lateral approach with
the infant supine or in the lateral decubitus position. The first
reported technique is based on the morphology of the acetabulum. It
was described by Graf in 1980
and consists of a single coronal image through the deepest portion
of the acetabulum (figure 8). Its key is to correctly identify this
"standard plane". For that, we have to place the iliac bone
parallel to the surface of the transducer. This is achieved by
gently sliding the transducer in the anteroposterior direction. The
final step is to obtain a sharp definition of the lower end of the
bony acetabulum. This is achieved by slightly rotating the
transducer. If these parameters are obtained correctly, we should
be in view of the entire acetabulum, including its cartilaginous
portion and labrum.
On the described image, the slope of the
acetabulum (alpha angle) is measured with respect to the iliac
line. An angle of 60 degrees or more is normal; from 50 to 60
degrees is considered physiologic before 3 months of age, but needs
to be followed for observation. Values under 50 degrees are
abnormal at any age. A second angle (beta) is drawn between the
iliac line and a line drawn from the labrum to the transition point
between the iliac bone and the bony acetabulum. This measurement is
indicative of the acetabular cartilaginous roof coverage and is
secondary in significance to the alpha angle. The smaller the angle
the less the cartilaginous coverage due to a better acetabular bony
containment of the femoral head.
Graf's classification of DDH is based on
these 2 angles. In this technique the position of the femoral head
is not considered, based on the premise that its position (normal,
subluxed, or dislocated) will be reflected by the morphology of the
evaluated acetabulum. Figures 9 and 10 are examples of a subluxed
and a dislocated hip, respectively.
The second ultrasound technique used in the
diagnosis of DDH is dynamic and was described by Harcke et al in
This technique incorporates the use of real time. The examination
is performed in the coronal, similar to Graf's technique, and axial
planes. However, two key points differentiate it from Graf's.
First, it takes into consideration the position of the femoral
head. Second, it incorporates the Barlow's maneuver in both imaging
planes in trying to demonstrate instability (subluxable,
dislocatable hips) (figure 11). Sonographically, mild instability
is found in all newborns for the first few days, with spontaneous
resolution in normal cases. When finding a dislocated hip, the
Ortolani's maneuver should be performed to check for
Different institutions use one method or the
other, or a combination of the two, as recommended at a symposium
on the subject held in Maryland in 1993. The recommended protocol
of a "dynamic standard minimum examination" asks for the patient to
be placed in a supine or lateral decubitus position. Scanning is
performed in the coronal plane with the hips extended or flexed.
Stress views and angle measurements are optional. In the axial
plane the thighs are in 90 degrees flexion, and images are obtained
with and without stress.
A third method, which is complementary to the
other two described above, is based on the measurement of the
acetabular coverage of the femoral head (figure 12). Described by
Morin et al in 1985,
this maneuver is based on the radiographic migration percentage
(MP). It is calculated by the equation (
/D) * 100. On the above described "standard plane" of the coronal
is the distance from the iliac line to the medial aspect of the
femoral head; D is the maximum diameter of the femoral head. An
acetabular coverage of the femoral head of 58% or more is normal,
and below 33% is abnormal (subluxation). Intermediate values are
indeterminate. This measurement is of no use in dislocation because
the acetabular/femoral head relationship is lost. Values are
unrelated to patient's age. It is an effective method because it is
intuitive. At a glance, we can see if more than half of the femoral
head is covered by the acetabulum, which tells us a normal
hip is present. The less the coverage the more immature the
Finally, it is important to understand the
relationship between hip morphology and instability in newborns. In
one study, out of 80 morphologically dysplastic hips, 91% were
sonographically unstable or dislocated, and out of 142
sonographically unstable hips, 49% were morphologically normal or
Therefore, treatment should be indicated on the basis of morphology