Ultrasound Evaluation of Pediatric Orthopaedic Patients

Jody Litrenta, MD; Karim Masrouha, MD; Amy Wasterlain, MD; Pablo Castaneda, MD

Disclosures

J Am Acad Orthop Surg. 2020;28(16):e696-e705. 

In This Article

Developmental Dysplasia of the Hip

Developmental dysplasia of the hip (DDH) exists as a spectrum of anomalies of the developing hip, from a dysplastic acetabulum to dislocation.[7–9] Physical findings such as a Barlow or an Ortolani positive hip, or a Galeazzi sign suggest hip instability or frank dislocation. Even for the experienced examiner, the physical examination of the hip in infants is challenging; instability or even dislocation can be overlooked, particularly in uncooperative infants. In addition, stable hip dysplasia is clinically silent.

Any diagnostic test has to be subjected to the questions of sensitivity, specificity, safety, and accessibility; as such, we can look at physical examination, radiographs, and ultrasonography for detecting the spectrum of pathology that makes up hip dysplasia. For hip dislocation in children younger than the age of 4 months, the sensitivity of physical examination alone is only 37%, which improves to 66% with radiographs and to 89% with ultrasonography.[10] For hip instability, the sensitivity of physical examination is lower. Because instability is dynamic, physical examination is a better predictor than radiographs, but sensitivity remains low. Ultrasonography captures dynamic images and therefore maintains its sensitivity for both instability and stable dysplasia.

Because hip dysplasia is a common cause of early osteoarthritis and total hip replacement,[11] improving our diagnostic tools may be the best way to reduce this. Currently, risk factors such as a first-born female, family history of DDH, and breech position select infants for ultrasonography screening.[8] Rather than considering the use of ultrasonography a screening tool, we suggest that ultrasonography be considered an integral portion of an enhanced physical examination of the hip. Our American Academy of Orthopaedic Surgeons guidelines assert that there is moderate evidence to support ultrasonography screening for infants with known risk factors, but in addition, note that there is also moderate evidence to support not implementing universal screening. Although the broad application of hip ultrasonography does have benefits, we acknowledge that it also has drawbacks, mainly in cost-effectiveness.[12] On the contrary, there are several European countries that have implemented universal screening and demonstrated reduction in both the number and severity of interventions done for DDH.[13] In our opinion, there are many more ultrasonography devices available in the clinical setting. Educating clinicians to incorporate this technique into routine clinical practice will ultimately prove cost-saving to the lifetime burden of hip dysplasia.

Another final interesting technique to consider, particularly in regions with limited resources, is the use of acoustics.[14,15] Although variations in this technique have been described, the principle is that sound transmission is measured between the patella and the pelvis and that simple instrumentations such as a stethescope and a tuning fork can be used for this purpose. Using this method, dysplastic hips have lower sound transmission compared with the normal side, providing a simple screening tool which is reliable in infants and young children.[14,15]

To perform the ultrasonography, the infant is placed in a supine position so that images in both the transverse and coronal planes of the hip can be viewed.[16–18] The setup is simple. The baby should be comfortable, in a quiet, warm, and relatively dark room. The baby is placed on their own blanket, in front of the monitor, uncovering only the necessary parts to avoid stress (Figure 2).

Figure 2.

Photograph demonstrating the setup to perform an infant hip ultrasonography.

The first step is to determine whether the hip is reduced or dislocated, which can be determined by placing the transducer parallel to the long axis of the femur and evaluating the relationship of the femoral head in the acetabulum. This produces a transverse image, which can be thought of as cross-sectional imaging of the hip with the infant lying on its side (Figure 3). If the femoral head is seated next to acetabulum, regardless of whether it may be dysplastic, that femoral head is reduced.

Figure 3.

Figure demonstrating transverse ultrasonography of the hip with anatomical landmarks. The bony landmarks are labeled as follows: femoral head, ischium, pubis, and sacrum. The posterior labrum is represented by * and the gluteus medius by ×.

The second step is to determine stability by adducting and applying stress, simulating a Barlow test under ultrasonographic examination. Displacement of the femoral head during this maneuver represents instability. There are two methods to assess displacement sonographically. One way is to measure the distance between two set points, typically the femoral head and the triradiate cartilage. Displacement greater than 4 mm between the acetabulum and femoral head signifies instability (Figure 4). The second option is to look for the "bird-in-flight" sign, which is a line drawn along the acetabulum and along the proximal femoral metaphysis. This virtual line is akin to a Shenton line on a radiograph and should be contiguous. A broken line signifies an unstable hip (Figure 5).

Figure 4.

Figure demonstrating the transverse displacement of >4 mm of the hip signifies instability.

Figure 5.

Figure demonstrating the bird-in-flight sign, an analogous line to Shenton's line on an AP radiograph.

The third and final step is to determine hip morphology. A coronal view is constructed by rotating the transducer 90°, producing an image analogous to an AP pelvis (Figure 6). To accurately assess acetabular development, these coronal images should be captured with a perfectly flat ilium, from which measurements can be constructed. To measure acetabular depth, a line is drawn along the lateral border of the ilium. This line should intersect the femoral head, with at least 50% of the head inferior to the line, with smaller values suggesting dysplasia. A second line is then drawn along the acetabular roof to the triradiate cartilage to construct the alpha angle. The alpha angle should measure at least 60° by 4 weeks and subsequently increases with age.[19] The beta angle bisects the limbs of the alpha angle and should be no more than 55°, with increased angles representing increased severity of subluxation.

Figure 6.

Figure demonstrating the ultrasound in relation to an AP radiograph, using measurements of coverage, alpha and beta angles on a coronal image.

The obtained measurements can be evaluated based on the Graf classification system, which is divided into four main types as follows: I, normal; IIa-c, dysplastic; III, subluxation; and IV, dislocation. This classification has demonstrated prognostic values for the likelihood of normalization without treatment and the success of pavlik harness treatment.[16,20,21] Some disadvantages are that it has been modified several times, can be complicated to understand, and has low interobserver reliability.[22] Furthermore, sonographic measurements can vary widely between clinicians. A study by Kolb et al[23] demonstrated that relatively small variations in the positioning of the transducer can overestimate or underestimate the alpha angle. Many experienced clinicians and orthopaedic surgeons consider the construction of alpha angles and the use of classification systems unnecessary and find it is simplest to think of the spectrum of hip dysplasia as being one of these three following variables: dysplasia, instability, or dislocation. In clinical practice, treatment should be initiated in all three scenarios.

Once treatment in a Pavlik harness is begun, ultrasonography remains a useful tool. Acetabular development and the appearance of the ossific nuclei of the femoral head can be monitored. The length of treatment in a harness can be adapted to how long it takes for sonographic normalization. In addition, dislocated hips that do not reduce with a Pavlik harness can be easily identified.[24] By understanding how to perform an ultrasonography evaluation as an enhanced physical examination of the hip, we will improve our ability to detect hip dysplasia at an early and optimal age and obtain the best outcomes in the long term.

In addition to diagnosis and monitoring hip dysplasia treated in a pavlik harness, ultrasonography is also effective in evaluated closed reductions. Older infants, either diagnosed late or who have failed pavlik harness, are treated with closed reduction and spica casting. Either MRI or CT is typically used to verify the reduction of the femoral head within the spica cast. In many institutions, neither CT nor MRI can be performed intraoperatively. Ultrasonography offers an attractive alternative,[25] which can be quickly performed to either verify reduction (Figure 7) or demonstrate the need for recasting while the patient is in the operating room (Figure 8).

Figure 7.

Figure demonstrating ultrasonography after closed reduction and spica casting with a persistently dislocated hip.

Figure 8.

Figure demonstrating ultrasonography after closed reduction and spica casting, with a successful hip reduction.

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