CORR Insights®: Is Hip Medial Ultrasound More Accurate Than Radiography for Determining the Status of Hip Reduction in Children Treated With a Spica Cast? A Retrospective Diagnostic Accuracy Study

Where Are We Now?

Ultrasonography (US) is a well-established and useful method for evaluating developmental dysplasia of the hip (DDH) in neonates. US machines are easily accessible in many facilities, including community hospitals, and some have suggested that US will become the medical device of choice for orthopaedic surgeons who care for neonates with DDH [2].

It’s easy to see why. Although other modalities (including MRI and arthrography) have some advantages in terms of confirming the position of the femoral head after reduction in neonates with DDH, US is very appealing because it does not involve ionizing radiation, does not require anesthesia, and is relatively inexpensive. Several helpful algorithms have been published to guide when and how to use US in this context, including those by Graf [4], Terjesen [11], Harcke and Kumar [5], Suzuki et al. [9], and others [1, 2, 6]. However, using US requires expertise, and gaining such expertise requires surmounting a learning curve; one needs to know how to select instruments, perform the scanning procedures, and interpret the results. This takes time, coaching, and practice.

Moreover, after reduction of a neonate’s dislocated hip, a spica cast may be applied; this makes using US challenging [6]. The current study in Clinical Orthopaedics and Related Research® [12] seeks to solve this problem. The authors used a medial approach through the inguinal area, which is conveniently accessible through the spica opening at the groin. The study found that a 10-mm triradiate cartilage–femoral distance (TFD) cutoff could identify patients who do not have hip dislocation, with both high specificity and a high negative predictive value. This technique allows for a static image during immobilization of a hip with a spica cast, and the imaging protocol can be performed with the patient in the supine position as an adjunct procedure, without anesthesia and without the need to modify the hip spica cast during the follow-up period. The results of the study also demonstrated a comparable and high negative predictive value of a conventional radiograph compared with those of a US image.

Based on these discoveries, surgeons should use US as the first line of imaging to assist a reduction procedure in order to treat DDH. For surgeons who do not have direct access to US, a standard hip radiograph might be useful as an alternative confirmatory test, and US could be performed in an equivocal case [1, 8].

Where Do We Need To Go?

The major challenges of US include interobserver and intraobserver disagreement [2]. Several techniques have been described for studying static and dynamic images [1, 6], but they have drawbacks. For example, a hockey-stick US probe may be required as recommended in the current study [12], but this adds a cost that may limit its application in resource-challenged areas.

In addition to technique, instrument standardization that allows US to be used effectively in all children who are younger than 1 year and consistent, standardized training remain unresolved issues [1, 6].

The 10-mm TFD used in the current study [12] is derived from a previous report [10]. However, this cutoff point needs to be validated. The 10-mm TFD showed high interobserver and intraobserver agreement in a recent report [13], but the TFD parameter might change based on the reduction technique. An arbitrary value (3 mm) was added to the classic TFD (7 mm) for the thickness of the acetabular soft tissue component to arrive at this 10-mm cutoff. However, intra-articular and extra-articular obstacles to the closed reduction maneuver may be removed during the open reduction procedure. This will eventually affect the TFD; thus, the 10-mm cutoff point used after a closed or open reduction procedure might not apply equally to patients who have undergone closed reductions and those who have undergone open reductions.

How Do We Get There?

A comparison among the different US approaches (anterior, medial, and transgluteal) during DDH treatment is necessary. Global standardization of instruments, positions, and approaches will improve interobserver and intraobserver agreement among pediatric musculoskeletal US experts [2, 6]. For novice surgeons, competency-based training with a pediatric hip simulator may be helpful to improve reliability and accuracy [7]. The proposed TFD used in the current study [12] needs to be confirmed in a larger-scale study. The best cutoff point needs to be elucidated; to achieve this, a receiver operating characteristic curve could be used. The medial approach reported herein is very promising.

For future research, 3D US may provide more detail about the quality of hip reduction and could be useful for demonstrating acetabular development during the follow-up period in patients with a hip spica cast, but this technique would require taking images from various angles. Although it might be difficult to obtain 3D images using medial US alone, the technique is worth exploring. Perhaps a combination of US approaches would be helpful, but the hip spica cast might require an adjustment.

In addition, contrast-enhanced US is an exciting area that is being developed, and it should be specifically explored in neonates with DDH [3]. The contrast-enhanced technique might be useful to assess femoral head vascularity during hip spica casting, and it may help detect a diminished blood supply in children with DDH at an earlier timepoint.