Growth and Development

The hip joint begins to develop at about the 7th week of gestation, when a cleft appears in the mesenchyme of the primitive limb bud. These precartilaginous cells differentiate into a fully formed cartilaginous femoral head and acetabulum by the 11th week of gestation (Chapter 6.1). At birth, the neonatal acetabulum is completely composed of cartilage, with a thin rim of fibrocartilage called the labrum.

The very cellular hyaline cartilage of the acetabulum is continuous with the triradiate cartilages, which divide and interconnect the three osseous components of the pelvis (the ilium, ischium, and pubis). The concave shape of the hip joint is determined by the presence of a spherical femoral head.

Several factors determine acetabular depth, including interstitial growth within the acetabular cartilage, appositional growth under the perichondrium, and growth of adjacent bones (the ilium, ischium, and pubis). In the neonate, the entire proximal femur is a cartilaginous structure, which includes the femoral head and the greater and lesser trochanters. The three main growth areas are the physeal plate, the growth plate of the greater trochanter, and the femoral neck isthmus. Between the 4th and 7th mo of life, the proximal femoral ossification center (in the center of the femoral head) appears. This ossification center continues to enlarge, along with its cartilaginous anlage, until adult life, when only a thin layer of articular cartilage remains. During this period of growth, the thickness of the cartilage surrounding this bony nucleus gradually decreases, as does the thickness of the acetabular cartilage. The growth of the proximal femur is affected by muscle pull, the forces transmitted across the hip joint with weight bearing, normal joint nutrition, circulation, and muscle tone. Alterations in these factors can cause profound changes in development of the proximal femur.

Vascular Supply

The blood supply to the capital femoral epiphysis is complex and changes with growth of the proximal femur. The proximal femur receives its arterial supply from intraosseous (primarily the medial femoral circumflex artery) and extraosseous vessels (Fig. 670-1). The retinacular vessels (extraosseous) lie on the surface of the femoral neck but are intracapsular because they enter the epiphysis from the periphery. This makes the blood supply vulnerable to damage from septic arthritis, trauma, thrombosis, and other vascular insults. Interruption of this tenuous blood supply can lead to avascular necrosis of the femoral head and permanent deformity of the hip.

Figure 670-1 Diagrammatic illustration of vascular anatomy of the proximal femur.

670.1 Developmental Dysplasia of the Hip

Wudbhav N. Sankar, B. David Horn, Lawrence Wells, John P. Dormans

Developmental dysplasia of the hip (DDH) refers to a spectrum of pathology in the development of the immature hip joint. The original term for the condition, congenital dislocation of the hip, was replaced by the current name to more accurately reflect the variable presentation of the disorder and to encompass mild dysplasias as well as frank dislocations.

Classification

Acetabular dysplasia refers to abnormal morphology and development of the acetabulum. Hip subluxation is defined as partial contact between the femoral head and acetabulum, whereas dislocation refers to a hip with no contact between the articulating surfaces of the hip. DDH is classified into two major groups: typical and teratologic. Typical DDH occurs in otherwise normal patients or those without defined syndromes or genetic conditions. Teratologic hip dislocations usually have identifiable causes such as arthrogyposis or a genetic syndrome and occur before birth.

Etiology and Risk Factors

Although the exact etiology remains unknown, the final common pathway in the development of DDH is increased laxity of the hip capsule, which fails to maintain a stable femoroacetabular articulation. This increased laxity is probably the result of a combination of hormonal, mechanical, and genetic factors. A positive family history for DDH is found in 12-33% of affected patients. DDH is more common among female patients (80%). This is thought to be due to the greater susceptibility of female fetuses to maternal hormones such as relaxin, which increases ligamentous laxity. Although only 2-3% of all babies are born in breech presentation, the rate is 16-25% for patients with DDH.

Any condition that leads to a tighter intrauterine space and, consequently, less room for normal fetal motion may be associated with DDH. These conditions include oligohydramnios, large birth weight, and first pregnancy. The high rate of association of DDH with other intrauterine molding abnormalities, such as torticollis and metatarsus adductus, supports the theory that the “crowding phenomenon” has a role in the pathogenesis. The left hip is the most commonly affected hip; in the most common fetal position, this is the hip that is usually forced into adduction against the mother’s sacrum.

Epidemiology

Although most newborn screening studies suggest that some degree of hip instability can be detected in 1/100 to 1/250 babies, actual dislocated or dislocatable hips are much less common, being found in 1-1.5 of 1000 live births.

There is marked geographic and racial variation in the incidence of DDH. The reported incidence based on geography ranges from 1.7/1,000 babies in Sweden to 75/1,000 in Yugoslavia to 188.5/1,000 in a district in Manitoba, Canada. The incidence of DDH in Chinese and African newborns is almost 0%, whereas it is 1% for hip dysplasia and 0.1% for hip dislocation in white newborns. These differences may be due to environmental factors, such as child-rearing practices, rather than to genetic predisposition. African and Asian caregivers have traditionally carried babies against their bodies in a shawl so that a child’s hips are flexed, abducted, and free to move. This keeps the hips in the optimal position for stability and for dynamic molding of the developing acetabulum by the cartilaginous femoral head. Children in Native American and Eastern European cultures, which have a relatively high incidence of DDH, have historically been swaddled in confining clothes that bring their hips into extension. This position increases the tension of the psoas muscle-tendon unit and might predispose the hips to displace and eventually dislocate laterally and superiorly.

Pathoanatomy

In DDH, several secondary anatomic changes can develop that can prevent reduction. Both the fatty tissue in the depths of the socket, known as the pulvinar, and the ligamentum teres can hypertrophy, blocking reduction of the femoral head. The transverse acetabular ligament usually thickens as well, which effectively narrows the opening of the acetabulum. In addition, the shortened iliopsoas tendon becomes taut across the front of the hip, creating an hourglass shape to the hip capsule, which limits access to the acetabulum. Over time, the dislocated femoral head places pressure on the acetabular rim and labrum, causing the labrum to infold and become thick.

The shape of a normal femoral head and acetabulum depends on a concentric reduction between the two. The more time that a hip spends dislocated, the more likely that the acetabulum will develop abnormally. Without a femoral head to provide a template, the acetabulum will become progressively shallow, with an oblique acetabular roof and a thickened medial wall.

Clinical Findings

The Neonate

DDH in the neonate is asymptomatic and must be screened for by specific maneuvers. Physical examination must be carried out with the infant unclothed and placed supine in a warm, comfortable setting on a flat examination table.

The Barlow provocative maneuver assesses the potential for dislocation of a nondisplaced hip. The examiner adducts the flexed hip and gently pushes the thigh posteriorly in an effort to dislocate the femoral head (Fig. 670-2). In a positive test, the hip is felt to slide out of the acetabulum. As the examiner relaxes the proximal push, the hip can be felt to slip back into the acetabulum.

Figure 670-2 The Barlow provocative test is performed with the patient’s knees and hips flexed. A, Holding the patient’s limbs gently, with the thigh in adduction, the examiner applies a posteriorly directed force. B, This test is positive in a dislocatable hip.

The Ortolani test is the reverse of Barlow test: The examiner attempts to reduce a dislocated hip (Fig. 670-3). The examiner grasps the child’s thigh between the thumb and index finger and, with the 4th and 5th fingers, lifts the greater trochanter while simultaneously abducting the hip. When the test is positive, the femoral head will slip into the socket with a delicate clunk that is palpable but usually not audible. It should be a gentle, nonforced maneuver.

Figure 670-3 The Ortolani maneuver is the sign of the ball of the femoral head moving in and out of the acetabulum. A, The examiner holds the patient’s thigh and gently abducts the hip while lifting the greater trochanter with two fingers. B, When the test is positive, the dislocated femoral head falls back into the acetabulum with a palpable clunk as the hip is abducted.

A hip click is the high-pitched sensation (or sound) felt at the very end of abduction during testing for DDH with Barlow and Ortolani maneuvers. Classically, a hip click is differentiated from a hip clunk, which is felt as the hip goes in and out of joint. Hip clicks usually originate in the ligamentum teres or occasionally in the fascia lata or psoas tendon and do not indicate a significant hip abnormality.

The Infant

As the baby enters the 2nd and 3rd months of life, the soft tissues begin to tighten and the Ortolani and Barlow tests are no longer reliable. In this age group, the examiner must look for other specific physical findings including limited hip abduction, apparent shortening of the thigh, proximal location of the greater trochanter, asymmetry of the gluteal or thigh folds (Fig. 670-4), and pistoning of the hip. Limitation of abduction is the most reliable sign of a dislocated hip in this age group.

Figure 670-4 Asymmetry of thigh folds in a child with developmental dysplasia of the hip.

Shortening of the thigh, the Galeazzi sign, is best appreciated by placing both hips in 90 degrees of flexion and comparing the height of the knees, looking for asymmetry (Fig. 670-5). Asymmetry of thigh and gluteal skin folds may be present in 10% of normal infants but suggests DDH. Another helpful test is the Klisic test, in which the examiner places the 3rd finger over the greater trochanter and the index finger of the same hand on the anterior superior iliac spine. In a normal hip, an imaginary line drawn between the two fingers points to the umbilicus. In the dislocated hip, the trochanter is elevated, and the line projects halfway between the umbilicus and the pubis (Fig. 670-6).

Figure 670-5 Positive Galeazzi sign noted in a case of untreated developmental dysplasia of the hip.

Figure 670-6 Klisic test. A, In a normal hip, an imaginary line drawn down through the tip of an index finger placed on the patient’s iliac crest and the tip of the long finger placed on the patient’s greater trochanter should point to the umbilicus. B, In a dislocated hip, this line drawn through the two fingertips runs below the umbilicus because the greater trochanter is abnormally high.

The Walking Child

The walking child often presents to the physician after the family has noticed a limp, a waddling gait, or a leg-length discrepancy. The affected side appears shorter than the normal extremity, and the child toe-walks on the affected side. The Trendelenburg sign is positive in these children, and an abductor lurch is usually observed when the child walks. As in the younger child, there is limited hip abduction on the affected side and the knees are at different levels when the hips are flexed (the Galeazzi sign). Excessive lordosis, which develops secondary to altered hip mechanics, is common and is often the presenting complaint.

Diagnostic Testing

Ultrasonography

Because it is superior to radiographs for evaluating cartilaginous structures, ultrasonography is the diagnostic modality of choice for DDH before the appearance of the femoral head ossific nucleus (4-6 mo). During the early newborn period (0-4 wk), however, physical examination is preferred over ultrasonography because there is a high incidence of false-positive sonograms in this age group. In addition to elucidating the static relationship of the femur to the acetabulum, ultrasonography provides dynamic information about the stability of the hip joint. The ultrasound examination can be used to monitor acetabular development, particularly of infants in Pavlik harness treatment; this method can minimize the number of radiographs taken and might allow the clinician to detect failure of treatment earlier.

In the Graf technique, the transducer is placed over the greater trochanter, which allows visualization of the ilium, the bony acetabulum, the labrum, and the femoral epiphysis (Fig. 670-7). The angle formed by the line of the ilium and a line tangential to the boney roof of the acetabulum is termed the α angle and represents the depth of the acetabulum. Values >60 degrees are considered normal, and those <60 degrees imply acetabular dysplasia. The β angle is formed by a line drawn tangential to the labrum and the line of the ilium; this represents the cartilaginous roof of the acetabulum. A normal β angle is <55 degrees; as the femoral head subluxates, the β angle increases. Another useful test is to evaluate the position of the center of the head compared to the vertical line of the ilium. If the line of the ilium falls lateral to the center of the head, the epiphysis is considered reduced. If the line falls medial to the center of the head, the epiphysis is undercovered and is either subluxated or dislocated.