Imaging of the Hip

Ahmet Tuncay Turgut, Elif Ergun, Pinar Koşar, and Vikram S. Dogra

Based on its wide availability, noninvasiveness, no need for patient preparation, portability, and the absence of ionizing radiation, ultrasound is the primary imaging method for hip disorders. This is especially true for pediatric patients, whose motion or lack of cooperation presents a contraindication for other imaging modalities, and for infants because of the inability to image cartilage by radiography at this age. The management of some clinical problems can usually be performed by the imaging data provided by ultrasound alone. Moreover, the relatively lower cost of the ultrasound equipment compared with other cross-sectional methods like computed tomography (CT) or magnetic resonance imaging (MRI) makes this method an essential tool for hip evaluation for cases in which close follow-up or an extensive screening may be required.

Clinical Indications

The main clinical indication for an ultrasound evaluation of the hip is the investigation of developmental dysplasia of the hip (DDH) or the follow-up of the pediatric cases with DDH. Another indication is the evaluation of the cases with hip pain, mostly having a joint effusion. The technique is also used as an adjunct to other alternative imaging modalities for the diagnosis of Legg–Calve–Perthes disease, usually presenting with the complaint of hip pain. Moreover, it is useful for the diagnosis of proximal femoral focal deficiency. It can also be used to guide percutaneous aspiration of the articular and/or extraarticular fluid collections.

Pathologies

Developmental Dysplasia of the Hip

DDH includes a wide spectrum of pathologic conditions, from the stable acetabular dysplasia to irreducible dislocations. Since Hippocrates, the term congenital dysplasia was used to describe hip dysplasia and displacement in infants because it was thought to be of congenital origin. Since the 1980s, the developmental nature of the disease has been clearly understood as some infants with a normal hip examination at birth developed dysplasia of the hip during the first months after birth. Acetabular dysplasia and ligamentous laxity are the key factors in the pathophysiology of DDH.

Pathophysiology of Developmental Dysplasia of the Hip

• Acetabular dysplasia and the ligamentous laxity are the key features.

• Adequate development of the hip joint necessitates a balanced interaction of the acetabulum and femur.

• Dysplastic acetabulum causes incomplete support for the femoral head and allows its movement freely in and out of the socket resulting in hip instability.

• Acetabular development deteriorates in the lax hip where the femur moves freely in and out of the socket, resulting in acetabular dysplasia.

• DDH is three times more common on the left side due to the left occiput anterior positioning of the nonbreech fetus, in which the left hip lies against the mother’s spine and hip abduction is limited.

• There are two types of dislocation: typical dislocation (98%) and teratologic dislocation (2%) with all parts of the hip joint being abnormal.

To prevent long-term complications of the disease, an early and correct diagnosis as well as an effective treatment strategy is very important. Ultrasound is an excellent diagnostic tool for the evaluation of infants with hip dysplasia; both clinically and radiographically undetected abnormalities can be easily demonstrated by ultrasound examination alone. Graf, an Austrian pediatric orthopedic surgeon, first introduced the use of ultrasound in the diagnosis of pediatric hip dysplasia in 1980. For 30 years, it has been used to examine the hips of infants in clinical practice.

Incidence

There is no gold standard for the diagnosis of DDH in the newborn period. Both physical examination and various imaging studies, such as radiography and ultrasound, have false-positive and false-negative results. Although MRI and arthrography demonstrate the precise anatomic details of the hip, these techniques are not suitable as screening methods; instead, they are used for problem solving in selected cases. Moreover, various factors such as the age of the child at the time of the examination, genetic and racial factors, diagnostic criteria and methods, influence the incidence of DDH. Therefore, it is not possible to give a true incidence of DDH: its incidence can only be estimated. In the newborn period, the reported incidence of DDH varies from 1 to 5%. The incidence of frank dislocations is 1 to 1.5 per 1000 newborns.

Normal Sonographic Anatomy of the Hip

Knowledge of the sonographic hip anatomy is crucial for an accurate diagnosis of DDH (Fig. 7.1).

Fig. 7.1 Standard coronal plane. (A) Presence of the three landmarks, which are the lower limb of osilium (circle), midportion of the acetabular roof accompanied by straight iliac wing parallel to the horizontal plane (arrow), and labrum (open arrow) should be checked. (B) Anatomic structures to be identified adjunct to the ultrasound landmarks are the chondroosseous junction (arrows), joint capsule (oblique open arrow), head of the femur (h), hyaline cartilage roof (*), bony acetabular roof (straight open arrow), bony rim (white dot), and triradiate cartilage (double arrow) are shown.

Sonographic Anatomy

• The ilium, ischium, and pubis converge to form the acetabulum.

• At the junction of these three bones a cartilaginous structure called the triradiate cartilage is situated, which is the most central and the deepest part of the acetabulum. It appears hypoechoic on ultrasound and allows through transmission of the sound beam.

• Acetabulum has a bony and cartilaginous portion in the newborn.

• The cartilaginous portion has a triangular hyaline cartilage roof appearing hypoechoic on ultrasound and fibrocartilaginous acetabular labrum with triangular hyperechoic ultrasound appearance.

• The base of the hyaline cartilage roof triangle is in contact with the femoral head and the apex is located superiorly on the iliac wing.

• In the well-developed bony acetabulum, the base of cartilaginous triangle is narrow; in the immature bony acetabulum, it is broad.

• The os ilium extends superiorly from the upper part of the acetabulum to form the iliac wing.

• At birth, the femoral head, greater trochanter, and proximal portion of the femoral neck—as they are made up of hyaline cartilage and are separated from the bony shaft by the chondroosseous border—appear hypoechoic on ultrasound, whereas the chondroosseous junction is highly echogenic. The femoral shaft is also seen as a highly echogenic linear structure below the chondroosseous junction.

• The ossification center of the femoral head, which may be present at birth, is seen as an echogenic structure on ultrasound. It appears on ultrasound 3 to 4 weeks earlier than it appears on plain x-ray. The center is not round and it is not always in the center of the head, so it cannot be used to assess the position of the head sonographically. It is not possible to determine the size of the ossification center by ultrasound.

Technical Guidelines

Ultrasound’s real-time capability allows evaluation of the dynamics of the instability as well as morphology of the hip. Soon after Graf’s initial publications about the use of hip ultrasound for the diagnosis of DDH, a different technique was developed by Novick et al and Harcke et al known as the “dynamic method” it is an imaging mimic of a clinical hip examination. Moreover, other techniques such as a modified Graf technique (Rosendahl), the Morin technique, and the modified Morin technique (Terjesen) are routinely used in the diagnosis of the hip. Whatever method is used, it is apparent that ultrasound is more sensitive than a clinical examination.

Graf’s Static Technique

Graf’s static technique is based on a morphologic evaluation of the hip in a coronal neutral view. In this technique, no emphasis is given to the position of the femoral head. The patient lies in the lateral decubitus position with the leg in neutral position slightly flexed at the hip and knee. The transducer is longitudinally oriented from a lateral approach to the hip (Fig. 7.2). Graf designed a positioning device placing the infant in the desired position, but the use of this device is optional. The highest frequency linear ultrasound probe should be used to permit an optimal penetration of the soft tissues.

Fig. 7.2 Patient is in lateral decubitus position with the leg in neutral position and the transducer oriented longitudinally.

From a technical point of view, obtaining images in the same section through the hip is crucial for a correct and reproducible ultrasound examination. Graf described a standard coronal plane for this purpose.

Standard Coronal Plane

• Involves the lower limb of the bony ilium in the depth of the acetabular fossa, midposition of the acetabular roof, and the acetabular labrum (Fig. 7.1A)

• The iliac wing above the bony roof should be straight and parallel to the transducer to make sure that the sectional plane through the midposition of the acetabular roof is obtained (Fig. 7.1A).

• Anatomic structures to be identified as ultrasound landmarks are the chondroosseous junction, femoral head, synovial fold, joint capsule, acetabular structures from lateral to medial (labrum, hyaline cartilage, and bony acetabular roof), and the osseous rim (the point where concavity of the bony acetabulum changes to the convexity of ilium) (Fig. 7.1B).

After subjective evaluation of the bony acetabular modeling, the bony rim, the cartilage roof triangle acetabular inclination angle (α), and the cartilage roof angle (β) are measured.

Measurement of α and β Angles

• To define these angles, three measurement lines are drawn on the image.

• Bony roof line is tangential to the bony roof drawn from the inferior limb of os ilium laterally to the promontory (Fig. 7.3A).

• Baseline originates at the apex of the cartilaginous roof triangle and it is tangential to the lateral surface of the iliac wing (Fig. 7.3B).

• Cartilage roof line is drawn from the bony rim through the center of the acetabular labrum (Fig. 7.3C).

• α is the angle between the bony roof line and baseline and is the measurement of the maturity of the bony roof (Fig. 7.3D).

• β is the angle between the cartilage roof line and baseline and indicates the degree of cartilaginous roof coverage (Fig. 7.3D).

Graf classified the hips into five main groups.

Graf’s Classification

• Type I is mature hip (Fig. 7.4A) with α ³60 degrees. It is divided into two subgroups as type Ia and type Ib having a β angle greater than and less than 55 degrees, respectively.

• Type IIa is the physiologic immature hip (Fig. 7.4B): the α angle is between 50 and 59 degrees in an infant younger than 12 weeks of age.

• If type IIa morphology persists beyond 12 weeks, it is termed as type IIb (acetabular dysplasia): α angle is between 50 and 59 degrees.

• Type IIc is the hip in critical range (α = 43 to 49 degrees). It is divided into two subgroups: type IIc stable and type IIc unstable.

• In type IId hips, the α angle is in the same range as in the type IIc hip; however, decentering of the hip starts and β > 77 degrees in type IId hips.

• Type III and type IV hips are both decentered hips, α < 43 degrees and β > 77 degrees in both of them. Determination of the position of the cartilaginous roof is crucial for their differentiation, which is pushed cranially in type III hips (Fig. 7.4C) and caudally in type IV hips (Fig. 7.4D).

• Type III hip is further divided into two subgroups according to the echogenicity of the cartilaginous roof. In type IIIa hips it is hypoechoic, whereas in type IIIb hips the hyaline cartilage is deformed and appears hyperechoic.

Fig. 7.3 (A) Bony roof line; tangential to the bony roof and drawn from the inferior limb of os ilium (white dot) laterally. (B) Baseline; tangential to the lateral part of the iliac wing and drawn from the uppermost part of the hyaline cartilage roof caudally. (C) Cartilaginous roof line; drawn from the bony rim (white dot) through the center of the labrum. (D)

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