Infant and Pediatric Hip

Bony Pelvic Girdle (“Socket”)

The bony pelvis is a ring-like bony structure composed of two hipbones (or pelvic bones), the sacrum, and the coccyx. The sacroiliac joints unite these two hipbones with the sacral part of the vertebral column. The pubic symphysis is where these two hipbones unite with each other anteriorly. The two hipbones (also referred to as the pelvic girdles or hip girdles) serve as attachment sites, one for each side of the lower limbs, to the axial skeleton. Each pelvic girdle is made up of a single hip (or coxal) bone and, collectively, may be referred to as the innominate bones or os coxae. The pelvic girdle is ossified at birth and consists of three joined bones, individually identified as the ilium, ischium, and pubis.

Within the pelvic girdle, these bones join and form the bony acetabulum. The acetabulum makes up the hip socket and is normally a deep, cup-shaped structure. In childhood, the triradiate cartilage connects these three bones and is made of three distinct growth plates, or physes, that do not ossify until around puberty. The cartilage, once ossified, becomes part of the adult bony acetabulum, or hip socket ( Fig. 28.1 ).

Femoral Head (“Ball”)

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 ( Fig. 28.2 ). The shaft (or diaphysis) of the femur is ossified at birth and sonographically appears hyperechoic, casting an acoustic shadow.

The greater and lesser trochanters, as well as the neck and head of the femur, are cartilaginous at birth and seen well 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 femoral head appears as a large hypoechoic circle and typically ossifies between 3 and 8 months of age. Ossification begins centrally (which may be seen as early as 2 to 4 weeks of age), with girls often showing earlier ossification than boys. The sonographic value in evaluating the hip greatly decreases as the femoral head ossifies. Ossification as the infant ages, combined with structures located deeper in the body, equates to a substantial diminishment in image quality around 4 to 6 months of age. Once the femoral head is completely ossified, it is difficult to obtain adequate sonographic images because of the beam artifact interference ( Fig. 28.3 ). Radiography is the preferred modality after 4 to 6 months of age unless the acetabulum and triradiate cartilage can be adequately identified with ultrasound.

Hip Joint (“Ball-and-Socket”)

The rounded femoral head and the cup-shaped bony acetabulum form the freely movable “ball-and-socket” hip joint . 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.

Within the hip joint, the bony acetabulum has a smaller articular surface than the femur’s articular surface and is often referred to as the bony acetabular roof . The acetabulum is made deeper by a rim of cartilage that surrounds it, called the acetabular labrum . The labrum is mostly composed of hyaline cartilage and is hypoechoic, except at the lateral marginal tip, where it changes to fibrocartilage and appears echogenic and triangular. Furthermore, this lateral margin forms an extension of the acetabular roof (often referred to as the cartilaginous roof ). The acetabular labrum narrows the acetabulum and increases its depth, functioning to stabilize and support the femoral head articulation within the acetabulum. Normally, the labrum, or cartilaginous acetabulum, covers two-thirds of the femoral head. Sonographically, the labrum is best depicted from a coronal view and appears superolateral to the femoral head, adjacent to the ilium. Both the bony and cartilaginous roofs are evaluated and may be measured.

The acetabular notch is a bony deficiency at the inferior portion of the central acetabular depression and is covered by a band of fibrous tissue called the transverse acetabular ligament. The ligamentum teres of the femur runs from this acetabular notch through a foramen to the pit or fovea in the head of the femur, through which nerves and blood vessels pass. 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 ( Fig. 28.4 ).

Within the hip joint evaluation, the triradiate cartilage is a useful landmark, lying posterior to the femoral head, and appears hypoechoic in the neonate. The posterior rim of the acetabulum, part of the triradiate cartilage in the immature skeleton, is often referred to as the posterior lip .

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. This fibrous capsule covers the labrum and appears sonographically to extend from it inferiorly. 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 to keep the pelvis level while 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. 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

• Extension—bending backward

• Abduction—moving sideways outward

• Adduction—moving sideways inward

• Medial rotation

• Lateral rotation

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 when walking ( Fig. 28.5 ).

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 the 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.

Sonographic Examination Overview

The sonographic examination for DDH moves through a series of images in the coronal and transverse planes, taken at rest and with stress, allowing the sonographer to assess the position and stability of the femoral head in the hip joint.

Examination Preparation

To achieve a satisfactory examination, the infant should be relaxed and as comfortable as possible. Parental assistance is helpful to keep the infant calm. Having them stand on the opposite side of the bed allows the parent to be near the infant’s head. 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 that the examination may be performed.

Sonography of the neonatal hip is performed with a high-frequency linear-array transducer, often a 10 to 12 MHz ( Box 28.1 ). Sector or curved-array transducers will distort anatomy and are not recommended. This examination is always performed on both hips. Because ambidextrous scanning (commonly with the aid of a foot pedal) is a requisite, the sonographer often requires a good amount of experience before feeling fully competent. Scanning with the dominant hand first may be helpful. 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, while a foot pedal comes in handy to freeze and capture images without having to constantly reposition the infant.

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.

Sonographic Protocol

The basic hip anatomy may be imaged in four different views. A two-word combination labels the view by both the plane of the body (coronal or transverse) and hip position (neutral or with flexion). While institutions may vary on which ones are performed, they include (1) coronal/neutral, (2) coronal/flexion, (3) transverse/neutral, and (4) transverse/flexion. Next, dynamic stress maneuvers from the flexed transverse position are then often performed. A coronal posterior lip view with stress may also be obtained. In the setting of pathology, a 3D acquisition of the coronal views is possible. Finally, measurements for a quantifiable evaluation are obtained from the coronal view(s) and may be performed by either the sonographer or radiologist, preferably by the latter.