The Graf method is perhaps the most widely used US screening technique. If the well-established techniques for examination, interpretation, and measurement are meticulously followed, it is easy to manage newborn hip problems via this method [5].

To perform hip US, the hip joints must be evaluated in the standard coronal plane with a linear array probe [8]. Before evaluation of the hip joint, it is essential to identify the chondroosseous junction. This is because the echo of the chondro-osseous junction is an important landmark used to identify the femoral neck and other anatomical landmarks: the femoral head, iliac bone, lower limb of the ilium, acetabular bony roof, cartilaginous acetabular roof, acetabular labrum, joint capsule, and synovial fold (Fig. 1) [8].

The coronal view can be obtained with the hip in either the physiologic neutral position (15°-20° flexion) or the flexed position. The ultrasound transducer is then placed in the anatomic coronal plane (Fig. 2A). Next, the transducer is moved backwards and forwards from the basic position to identify the round structure of the hip joint. If the superior edge of the transducer is rotated posteriorly by 10° to 15° into an oblique coronal plane, the ilium will appear to be straight (Fig. 3A, C). If a sonogram contains a straight iliac wing contour, triradiate cartilage, and an apparent acetabular labrum, this indicates that it has a standard plane (Fig. 2B) [8,9]. However, in dislocated hips, lateral and posterior displacement of the femoral head prevents visualization of the femoral head and the center of the acetabulum in the standard plane. Therefore, if the displaced femoral head is followed, the ultrasound plane is no longer in the standard plane. The direction of displacement of the femoral head means that it is usually the posterior sectional plane that is visible (Fig. 3A, D) [8].

The American College of Radiology also recommends that a standard ultrasound examination be performed in the transverse view with the hip flexed at 90° (Fig. 4A). The femoral shaft is seen anteriorly, terminating in the femoral head, which rests on the ischium (Fig. 4B). To identify a dislocatable hip, the transducer is placed in a posterolateral position during the Ortolani and Barlow maneuvers. If the relationship between the posterior acetabulum and the femoral head changes with gentle stress, the hip is unstable [9].

Three anatomic landmarks (the iliac line, triradiate cartilage, and labrum) are used to measure the α and β angles. Meticulous measurement of the α angle is necessary because false-positive findings can occur in the anterior or posterior sectional planes (Fig. 3B, D). The α angle should not be determined as the average angle, but rather as the largest angle [2].

The current gold-standard treatment for DDH in newborns to infants aged 6 months remains the Pavlik harness. The Pavlik harness provides the best results, with a success rate of 85%-95%, and thus should be started as early as possible to achieve the best outcome. The purpose of the harness is to maintain the hip in a flexed and abducted position in order to bring the femoral head as close to the acetabular ring as possible [1].

Between 6 months and 2 years of age, closed reduction and casting is attempted under general anesthesia to maintain the femoral head in the proper position without damaging it. Arthrography is a useful tool to evaluate the success of the reduction. The cast is applied to the reduced hip for a period of approximately 12 weeks. If closed reduction cannot be accomplished at this stage, open reduction may be necessary [1].

Open reduction is usually required above the age of 2 years, mostly by femoral osteotomy to relieve pressure over the femoral head and to reshape the acetabulum. The patient is usually immobilized in a spica cast for 6-12 weeks [1].

Transient synovitis of the hip is the most common cause of acute hip pain and limping. It typically occurs in children aged 3-8 years with localized pain in the groin region, anterior thigh, or medial knee [10]. Because the imaging results are similar to those of other causes of acute hip pain in children, the diagnosis is essentially made by exclusion. The main differential diagnoses are septic arthritis [11] and juvenile rheumatoid arthritis [10].

Hip US must be performed in the neutral position (hip extended with slight external rotation) using an anterior approach in the parasagittal plane along the long axis of the femoral neck. It may be useful to compare the resulting image with that of the contralateral normal hip (Fig. 8B). On US examination, joint effusion can be seen between the anterior and posterior layers of the anterior joint capsule (Fig. 8A) [10,12]. In children, the distance between the outer margin of the hip capsule and the surface of the femoral neck should not be greater than 5 mm, or more than 2 mm thicker than the contralateral normal side [13].

Septic arthritis typically affects the hip, knee, shoulder, elbow, and ankle. Staphylococcus aureus is the most common causative organism. In neonates, group B streptococci and coliform bacteria were previously common causative organisms. In children aged from 3 months to 5 years, Haemophilus influenzae is an important cause, but the incidence has declined considerably due to the use of vaccinations. US of septic arthritis shows hypoechoic joint fluid with or without echogenic debris in patients with clinical signs of joint infection. US can be used for the guided aspiration of joint fluid for the early diagnosis and treatment of septic arthritis [13,14]. Additionally, power Doppler may show increased capsular vascularity [15].

Spinal US is performed in both the longitudinal and transverse planes using a linear 5-12 MHz transducer (Fig. 9A, B). The vertebral level is usually determined by counting up from the L5-S1 junction. Generally, the infant is examined in the prone position [7,16].

There are two main indications for lumbar spinal US in infants. Spinal US is used to evaluate midline cutaneous malformations of the back, such as dimples, as well as hemangiomatous or hairy lesions associated with dysraphic anomalies of the spinal cord. Spinal US is also used to determine the reason for the failure of a lumbar puncture and the location from which cerebrospinal fluid (CSF) may be tapped [17]. Low-risk lesions include simple midline dimples <5 mm in diameter, located within 2.5 cm of the anus, without other cutaneous stigmata. High-risk lesions include atypical dimples >5 mm in diameter, a location >2.5 cm above the anus, hairy patches, hemangiomas, skin tags, and cutis aplasia [6,7,18].

The longitudinal scan shows a hypoechoic tubular spinal cord with an echogenic central echo complex (Fig. 10A), representing the central canal. On histologic analysis, the central canal corresponds to the border between the myelinated ventral white commissure and the central portion of the anterior median fissure. In healthy newborns, the central canal is not filled with CSF but is instead overgrown with glial fibrils [6].

The caudal end of the spinal cord corresponds to the conus medullaris, which continues into the filum terminale (Fig. 10A). In healthy newborns, the tip of the conus medullaris is no lower than the top of L3 in a term infant (Video clip 1), or the bottom of L3 in a preterm infant [19]. The filum terminale appears as a thin cord-like echogenic structure, forming a parallel line extending from the conus medullaris. The filum terminale is surrounded by the nerve roots of the cauda equina, and it is sometimes difficult to differentiate the filum terminale from the nerve roots [6].

An axial scan of the spinal cord shows a hypoechoic spinal cord with an echogenic central echo complex and paired dorsal and ventral nerve roots within the anechoic subarachnoid space (Fig. 10B) [6].

Several common normal variants that may be confused with certain disorders on lumbar spine US are discussed below, including transient dilatation of the central canal, ventriculus terminalis, prominent filum terminale, filar cysts, cauda equina pseudomasses, and pseudosinus tract [6].

Congenital spinal dysraphisms can be classified into two categories on the basis of the absence or presence of a soft-tissue mass and skin covering. Those without a mass include spinal lipoma, tethered cord, diastematomyelia, and anterior sacral meningocele. Lipomyelomeningocele and myelocystocele are skin-covered soft-tissue masses. Myelomeningocele and myelocele show a mass located on the back but are without a skin covering [6,20].