Developmental dysplasia of the hip (DDH) is one of the most common congenital disorders affecting the musculoskeletal system. DDH denotes a wide spectrum of abnormalities ranging from mild acetabular dysplasia, to subluxation, to complete irreducible high-riding dislocation of the hip. Defining the incidence of DDH is challenging and controversial because of variations in the terminology, such as clicking, instability, and dislocation, previously described in the literature. The development of ultrasonographic techniques for screening and diagnosis contributed to the confusion about actual incidence in the literature. For instance, one previous study reported an incidence of 55.1/1000 detected by ultrasound screening.¹ However, the authors suggested the true incidence of DDH was 5/1000 patients, which corresponds to those that remained abnormal and required treatment.

The etiology of DDH remains unknown, although it is most likely multifactorial, including genetic and environmental intrauterine factors. The most universally recognized factors include female gender, breech position at delivery, and history of a first-degree relative affected by DDH. Other reported factors include improper swaddling, oligohydramnios, deformities of the foot, congenital torticollis, first-born birth order, and large birth weight.

The pathophysiology of DDH has been well described, including an abnormality of the posterosuperior acetabular cartilaginous rim that allows the femoral head to subluxate or dislocate. In high-riding untreated dislocation, anatomic obstacles to a concentric reduction include the fibrofatty pulvinar, hypertrophy of the ligamentum teres, a thickened transverse acetabular ligament, an inverted labrum, and an interposed iliopsoas tendon. Concentric reduction of a dislocated hip early in life allows for remodeling of the acetabulum and femur. On the other hand, secondary changes, such as flattening of the acetabular roof, thickening of the medial acetabular wall, and formation of a false acetabulum will develop if a dislocated hip is left untreated.

Classically, examination of the hip includes the Barlow and Ortolani maneuvers performed on one hip at a time. The Barlow² test is a provocative maneuver to test for hip instability. It was originally described by Le Damany but popularized by Barlow to identify a hip that is reduced but dislocatable. The proximal femur is held between the thumb placed in the groin and the middle finger placed in the greater trochanter. The hip is adducted, and a gentle posteriorly directed force is applied. In a positive test, subluxation or complete dislocation of the hip is noted. It has been our preference to avoid the use of the provocative Barlow maneuver. We favor gently ranging the hip in a flexed position from abduction to adduction without applying an axial pressure to provoke instability. We do not see any benefit in dislocating a reduced hip by a provocative test. Barlow found that 88% of unstable hips in his series eventually became stable without treatment. The Ortolani test³ assesses whether a dislocated hip can be reduced (Figure 1.1). It is performed by abducting the flexed hip with an anteriorly directed force applied at the level of the greater trochanter. If the hip reduces, then a physical sensation can be felt, which is considered a positive Ortolani test. It is important to note that negative Ortolani tests do not guarantee a normal hip. In fact, an irreducible dislocated hip may feel stable during the test. However, in fixed dislocated hips, the maximum degree of abduction may be limited. In babies older than 3 to 4 months, both the Barlow and Ortolani tests are typically absent, and restricted hip abduction becomes a more reliable indication for further investigation with imaging. In patients with unilateral hip dislocation, the Galeazzi sign is typically present. The affected limb feels shorter when flexing both the hips and the knees to approximately 90° and comparing the relative levels of the knees. The Klisic test⁴ is helpful to identify bilateral dislocation
of the hips. The test is performed by placing the index finger on the anterior superior iliac spine and the middle finger on the greater trochanter. A line between these two points should intersect the umbilicus if the hip is anatomically normal, but it is inferior to the umbilicus if the hip is dislocated.

Ultrasonographic evaluation of the hip is the best imaging modality to assess the hip morphology and dynamic stability during the first 6 months of life. Interpreting hip ultrasonography depends on the quality of the image acquisition, which can be influenced by experience of the examiner that can lead to high variability. Several methods for morphologic and dynamic ultrasonographic examination of the hip have been described.⁵,⁶,⁷,⁸,⁹,¹⁰,¹¹,¹²,¹³,¹⁴ Ultrasonography has been used for screening programs resulting in lowering the rate of late diagnosis and surgical interventions. However, application of ultrasonography is limited by patient’s age. Once the ossific nucleus of the femoral head ossifies, ultrasonography becomes less reliable and an anteroposterior (AP) pelvic radiograph may be more helpful to evaluate the hip morphology. Our preference is to use ultrasonography up to 6 months of age and radiographs thereafter.

When performing ultrasound for evaluation of hip dysplasia, it is critical that the examination of the hip be performed with experienced sonographers, using a high-frequency linear transducer that penetrates to the level of at least the triradiate cartilage. The unossified femoral head and the unossified portion of the acetabular physis and triradiate cartilage appear as hypoechoic structures (Figure 1.3), whereas the ossified pelvic bones and femoral neck are hyperechoic and are useful anatomic landmarks. Various imaging methodologies have been described in the literature and include both static and dynamic maneuvers, which are described in more detail later.

The most widely used method of hip ultrasonography was described by Graf.⁵,⁶ According to Graf, ultra-sonography is performed with a linear probe with the baby in lateral decubitus using a pillow for positioning and a lever-arm structure to hold the probe. The hip is in neutral position with about 30° to 55° of flexion. The standard coronal plane should include the lower portion of the osseous ilium, which resembles a straight line and produces strong posterior acoustic shadowing (the iliac line), the entire depth of the acetabular fossa (to the triradiate cartilage), and the acetabular labrum. Graf recommended measurement of the α angle to assess the osseous acetabular roof and the β angle to assess the cartilaginous acetabular roof. The α angle corresponds to the osseous coverage of the femoral head by the acetabular socket, and the smaller the angle, the more severe the degree of dysplasia. The lower limit of normal value for the α angle has been established at 60°.⁵ These angles are combined with the morphologic description of the hip for the proposed classification system for ultrasonographic imaging of the hip⁵,⁶ (Table 1.1; Figure 1.2). Careful imaging acquisition is required to ensure that the standard coronal plane is obtained. The Graf technique has been criticized for its relatively low accuracy and interobserver and intraobserver variability.¹⁵,¹⁶ Small changes in probe rotation may change the diagnostic criteria by one or two levels. Notably, the upper limit of agreement of interscan variability has been reported to range from 8° up to 19°.