On completion of this chapter, you should be able to:
Discuss anatomy of the neonatal hip
Describe normal movements of the hip
Describe sonographic evaluation of the neonatal hip, including technique and protocol
Describe the normal sonographic appearance of the neonatal hip
Describe the sonographic evaluation of the neonatal hip for developmental displacement of the hip
Define the Barlow and Ortolani maneuvers
Differentiate between subluxation of the hip and dislocation of the hip
Sonographic evaluation of the neonatal hip allows for a dynamic view of the soft tissues and cartilaginous structures, with the added advantages of a low cost and lack of ionizing radiation, contrast media, and sedation. It plays an important role in the diagnosis and management of developmental displacement of the hip (DDH) in the infant. It can also easily detect fluid within the joint space, providing clinicians a quick way to assess for joint effusions or septic arthritis in the child with a painful hip.
Until around 4 to 6 months of age when the femoral head ossifies, sonography is optimal for evaluating DDH in the neonatal/infant hip compared with other imaging modalities. Although magnetic imaging can provide excellent anatomic detail of the hip anatomy in the young infant, it is not dynamic, requires a long scanning period, is expensive, and requires sedation. Computed tomography requires gonadal ionizing radiation and sedation and is also nondynamic, but may be helpful in infants confined to a cast. Radiography, although it was originally used to diagnose DDH in young infants, is not reliable in detecting the soft cartilaginous structures of the hip that are not yet ossified in this population.
This chapter aims to provide readers with the necessary anatomy, sonographic findings, and techniques to better assess for hip pathology frequently performed in the pediatric setting. Although techniques vary by pathology, this chapter is separated into developmental hip displacement, joint effusions of the hip, and other hip conditions and differential diagnoses. A large majority of attention is given to developmental displacement (dysplasia/dislocation) of the hip.
Normal anatomy and sonographic findings
The anatomy assessed in the pediatric hip consists of the bony pelvic girdle, superior portions of the femur, hip joint (including the acetabulum and acetabular labrum), and supporting ligaments and muscles.
Bones and cartilage
The sacroiliac joints unite the two pelvic bones, or hipbones, with the sacral part of the vertebral column. The pubic symphysis is where these two hipbones unite with each other anteriorly. The pelvic bones, also referred to as the coxal or innominate bones, consist of the fusion of three separate bones: the ilium, ischium, and pubis. Together they form the pelvic girdle. The triradiate cartilage connects these three bones and is made of their three distinct growth plates, or physes, that do not ossify until adulthood, then becoming part of the acetabulum.
The bone of the upper thigh is the femur, which is surrounded by muscles, ligaments, and tendons. The upper part of the femur, the head, articulates with the hipbone to make the hip joint ( Figure 28-1 ).
The three pelvic bones and the shaft (or diaphysis) of the femur are ossified at birth, and sonographically appear hyperechoic, casting an acoustic shadow. The triradiate cartilage is a useful landmark, lying posterior to the femoral head, and appears hypoechoic in the neonate. The greater and less trochanters, as well as the neck and head of the femur, are also cartilaginous and well seen with sonography. The greater trochanter sits superolateral, whereas the lesser trochanter projects from the posteromedial proximal femoral shaft. The neck of the femur tapers medial, superior, and anterior toward the femoral head.
The head appears as a large hypoechoic circle and may ossify as early as 4 weeks, but typically between 3 and 8 months. Ossification begins centrally, with girls often showing earlier ossification than boys. Sonography can be performed until the femoral head ossifies. Once the femoral head is completely ossified, it is difficult to obtain adequate sonographic images because of beam artifact interference ( Figure 28-2 ).
The articulation of the rounded head of the femur with the cup-shaped acetabulum of the pelvic bone forms the ball-and-socket hip joint ( Figure 28-3 ). The hip joint is not directly palpable because it is surrounded and protected by muscles of the upper thigh. The greater trochanter of the femur forms a palpable knob at the side of the region.
The acetabulum is horseshoe shaped and has a smaller articular surface than the femur’s articular surface. The acetabular labrum is a rim of fibrocartilage that surrounds the acetabulum. It also forms an extension of the acetabular roof. The acetabular labrum narrows the acetabulum and increases its depth, functioning to stabilize and support the femoral head articulation within the acetabulum.
The labrum is composed of hyaline cartilage and is hypoechoic, except at its tip, which is echogenic due to its fibrous content. Sonographically, the labrum is best depicted from a coronal view and appears superolateral to the femoral head, adjacent to the ilium. Approximately two thirds of the femoral head should be covered by the labrum.
The acetabular notch is a bony deficiency at the lower part of the acetabulum through which nerves and blood vessels pass, and is covered by a band of fibrous tissue called the transverse ligament. The ligamentum teres of the femur, or rounded ligament, runs from this acetabular notch to the pit or fovea in the head of the femur. In the younger child, this ligament contains the branch of the obturator artery to supply the femoral head. The artery usually disintegrates by 7 years of age.
Supporting ligaments and muscles
The hip joint is surrounded by a tough capsule, which attaches to the intertrochanteric line of the femur and is reinforced by outside ligaments and muscles. The psoas tendon crosses the center of the hip joint just inferior to the inguinal ligament. The iliacus muscle sits lateral to the psoas tendon and, along with the psoas muscle, flexes the hip.
The most important hip ligament is the iliofemoral ligament, which passes from the anterior inferior iliac spine to each end of the intertrochanteric line. It is one of the strongest ligaments in the body and is very important for standing and maintaining correct upright balance.
The large gluteus maximus muscle overlies other muscles superior and posterior to the hip joint. The gluteus maximus is a powerful extensor of the hip. The gluteus minimus muscle is the immediate cover for the upper part of the hip joint. The gluteus medius and gluteus minimus pass from the outer surface of the hipbone to the greater trochanter. Together they act as abductors of the hip joint. Their most important function is to prevent adduction and keep the pelvis level during walking.
Movements of the hip
The movements of the hip are somewhat limited in range because of the tight fit between the femur and acetabulum and because the hipbone is immobile ( Figure 28-4 ). The following are the hip movements and their actions. Note that sonographic evaluation is primarily concerned with the abduction and adduction motions.
Flexion (bending forward) and extension (bending backward): The primary flexors of the hip are the psoas major, iliacus, and rectus femoris. Extension is limited to 20 degrees and is brought about by the hamstrings and gluteus maximus.
Adduction (moving sideways inward) and abduction (moving sideways outward): An example of hip adduction is crossing your legs when in a seated position, an action performed by the adductor group of muscles. In abduction, the gluteus medius and minimus muscles open the limbs. The more important function of these muscles, however, is to prevent adduction, which is the function they perform during walking.
Medial and lateral rotation: Medial and lateral rotation is related to the angle at which rotation occurs at the head of the femur, which is about 120 degrees angle to the shaft of the femur. When the trochanter moves forward, the femur rotates medially, and when the trochanter moves backward, the femur rotates laterally. Thus the medial rotators are the anterior fibers of gluteus medius and minimus. The lateral rotators are the small muscles at the back of the joint—piriformis, obturator internus, and quadratus femoris, with assistance from the gluteus maximus.
Developmental displacement of the hip
Developmental dysplasia/dislocation of the hip has been described as far back as Hippocrates. Formerly known as congenital hip dislocation, this misleading term referred to a spectrum of pathology that usually develops after birth. Therefore the term developmental displacement of the hip (DDH), or developmental dysplasia of the hip, is now used. DDH encompasses a spectrum of pathologies including subluxated, dysplastic, dislocatable, and dislocated hips.
Development of both sides of the neonatal hip requires the femoral head to be seated normally and congruently within the acetabulum. If the femoral head and acetabulum are not in their normal position, both sides of the hip will develop abnormally. A displacement of the hip is a relatively common abnormality, and when diagnosed in the neonatal period (up to 4 to 6 weeks of age) it is often attributed to normal laxity of the hips. Whereas 90% of mild cases diagnosed with sonography may resolve, in more severe cases DDH can cause impaired function and degenerative joint disease. Diagnosis should be made early and treatment instituted promptly.
In the newborn period, the femoral head may dislocate in a lateral and posterosuperior position relative to the acetabulum ( Figure 28-5 ). When this occurs, the femoral head can usually be reduced without deformity to the joint. However, when the dislocation is not recognized early, the muscles tighten and limit movement, which causes the acetabulum to become dysplastic because it lacks the stimulus of the femoral head. In turn, the ligamentous structures stretch and fibrofatty tissue occupies the acetabulum, making it impossible to return the femoral head into the acetabulum. This fibrofatty pulvinar may also develop in subluxation.
Incidence of developmental displacement of the hip.
The incidence of hip dislocation is difficult to define as there is no gold standard test, but it is estimated between 1.5 and 20 cases per 1000 live births, and milder forms of displacement, such as subluxation, are more common. When the diagnosis is based on diagnostic sonographic findings it may be as high as 40 to 60 cases per 1000 population. However, epidemiologic and demographic data vary widely, ranging from 0.06 per 1000 live births among Africans, up to 76.1 per 1000 among some Native Americans.
Multiple risk factors may contribute to the condition. Approximately 12% to 16% of all newborns have one or more of these factors and are at a higher risk of developing DDH. The most salient factors include breech presentation in both pregnancy and at delivery, which increases the risk nearly 4 times over the general population. Females are affected 2.5 times more frequently than males, although some population estimates are reported as high as 4 to 6 times. The left hip is affected 64% of the time and occurs unilaterally 64% of the time. It is more common among firstborn children and those with a family history of DDH.
The condition also affects certain races, such as Caucasians and certain North American tribes, more than those of African or Asian decent. Some suspect these differences may be due to swaddling practices. Swaddling has been strongly associated with DDH; DDH has a more prevalent diagnosis in winter months as well. Other risk factors may include maternal hypertension, fetal growth restriction, oligohydramnios, premature rupture of membranes, prolonged gestation, increased birth weight, Potter’s syndrome, and neonatal intensive care. Congenital muscular torticollis and congenital foot deformities are also associated factors. Prematurity, however, has not been shown to be a predisposing risk factor for DDH.
Causes of developmental displacement of the hip.
Although the exact etiology of DDH is unknown, the primary root is thought to be increased laxity within the joint capsule, causing a gradual migration of the femoral head away from the acetabulum. Hormonal, mechanical, and genetic factors are thought to play a role.
The hormonal theory suggests that the sex hormones act on the laxity of the connective hip joint tissue. The maternal hormonal effect of estrogen, which increases muscle laxity late in pregnancy (aiding childbirth), is thought to account for increased risk among female neonates, as this effect is reduced by the male sex hormones. Other sex hormones affecting laxity include progesterone and estrogen. Progesterone increases the collagen content in the joint capsule, likely facilitating hip dislocation, whereas estrogen has the opposite effects.
As for the mechanical causes of DDH, swaddling, oligohydramnios, breech presentation, and the primigravid uterus are considered risk factors because each limits the mobility of the hip in its own way. Improper swaddling may keep the hips in an adducted position if swaddled too tight and straight; instead, the legs should be able to bend up and out of the hips. Oligohydramnios limits mobility because there is less than a normal amount of amniotic fluid present for the fetus to move freely within the amniotic sac. In breech presentation, the fetus’s hip rests against the maternal sacrum and is usually flexed, which limits movement. This usually affects the left hip. The frank breech presentation of the fetus is the highest risk because the hips are maximally flexed and the knees are extended. The primigravid uterus is smaller than the multigravida uterus and is more confining, limiting mobility.
Genetic factors are currently being investigated in DDH, and findings from family and twin heritability suggest a strong genetic predisposition at onset (although less so with regard to progression or severity). It has been reported that there is a 5% chance that a child will be affected if a sibling has DDH and a 36% chance if one sibling and one parent are affected. There is a 12% chance that an affected individual will have a child with DDH.
Causes of dislocation of the hip.
Neonatal hip dislocation is often not present at birth and can be acquired, teratogenic, or developmental. Acquired causes of hip dislocation can be traumatic or nontraumatic (i.e., neuromuscular diseases). Teratogenic dislocations occur in utero and are associated with neuromuscular disorders.
A careful physical examination remains the universal screening for DDH and is therefore critical to the diagnosis. DDH detection, however, may vary widely between novice clinicians and experienced pediatric orthopedists. On visual inspection, the dislocated hip shows asymmetric skin folds and shortening of the affected thigh. The knee is lower in position on the affected side when the patient is supine and the knees are flexed, known as the Galeazzi sign ( Figure 28-6 ).
Hip instability may resolve after 4 to 6 weeks, due to waning maternal hormones. Therefore neonates with a slightly positive or inconclusive physical examination, and newborns with a risk factor for DDH, should be examined with sonography after this period to reduce false-positive results. Neonates with a grossly positive physical examination result or dislocation, however, are often seen earlier.
Two basic maneuvers are helpful in the diagnosis of DDH. The Barlow maneuver determines whether the hip can be dislocated, and the Ortolani maneuver determines whether the dislocated femoral head can be reduced back into the acetabulum.
In the Barlow maneuver ( Figure 28-7 ) the patient lies in the supine position with the hip flexed 90 degrees and adducted. Downward and outward pressure is then applied. If the hip can be dislocated, the examiner will feel the femoral head move out of the acetabulum with his or her fingers.
In the Ortolani maneuver ( Figure 28-8 ) the patient lies in the supine position. The examiner’s hand is placed around the hip to be examined, with the fingers over the femoral head. The examiner’s middle finger lies over the greater trochanter and the thumb is over the lesser trochanter. The hip is flexed 90 degrees and the thigh is abducted. Movement in the normal hip should feel smooth. In cases of DDH, a “clunk” is appreciated as the femoral head returns into the acetabulum. A “click” does not imply DDH. Each hip should be examined individually.
Sonography has been found to be more sensitive than physical examination in the detection of DDH. It has been shown to change the diagnosis in over half of cases while changing the management plan in one third of cases presenting to pediatric orthopedist surgeons for suspected DDH. Although universal screening sonography has been recommended, and is currently used in Europe, it is likely cost-prohibitive at this point. Currently, indications for neonatal hip sonography include the presence of certain risk factors for DDH (e.g., female gender with breech presentation, or a family history), an abnormal hip examination, and the need to evaluate the response to treatment. Sonography for DDH is often practical for most infants up to 6 months of age. Exceptions may include older infants if the femoral head is not yet ossified. Typically, after 4 to 6 months of age a radiograph is the preferred modality. If the patient is in a harness, the ordering physician should specify if the ultrasound is to be done in or out of the harness. Infants are often examined in the harness.
To achieve a satisfactory examination, the infant should be relaxed and as comfortable as possible. Feeding before or during the examination helps to soothe the infant. Make sure the room is warm and keep blankets close for warmth. Toys and other distractions help to quiet the infant so the examination may be performed. Parental assistance is helpful to keep the infant calm, with the mother or father near the infant’s head.
Sonography of the neonatal hip is performed with a high-frequency linear-array transducer ( Box 28-1 ). Sector or curved-array transducers will distort anatomy and are not optimal.
The infant is placed in a supine or decubitus position perpendicular to the table with the feet toward the sonographer. In a decubitus view, rolled towels, bolsters, or even specialized cradles may help keep the child on his or her side. It is wise to leave the diaper on through the examination and expose only the side of the hip being examined. Ambidextrous scanning is required for this bilateral examination; the right hip is examined with the transducer in the sonographer’s left hand and vice versa. The opposite hand is used for manipulation of the hip being examined. Scanning with the dominant hand first may be helpful. Sonographic imaging is performed from the lateral or posterolateral aspect of the hip.
Sonographic examination overview.
The sonographic examination for DDH evaluates the degree to which the femoral head is covered by the labrum, as well as the position of the femoral head in the acetabulum at rest and during motion and stress. Throughout the examination, the sonographer is able to assess the position and stability of the femoral head in addition to assessing the development of the acetabulum. The stability of the hip is determined through guided motion and the application of gentle stress. The stress maneuvers are the imaging counterparts of the clinical Barlow (adduction) and Ortolani (abduction) maneuvers.
The sonographic appearance of the femoral head location is described as normal, subluxed, or dislocated. Sonographic description of the acetabulum is assessed visually and described as normal, immature, or dysplastic. Validation of the acetabular morphology by the static sonographic alpha, beta, and femoral head measurements is optimal. Stability testing is reported as normal, lax, subluxable, dislocatable, and reducible or irreducible.
The basic hip anatomy is imaged in the coronal and transverse views. Views should be properly annotated denoting the view and position of the hips, as appropriate. Figure 28-9 provides images of a normal hip evaluation for DDH.