Pediatric Imaging

What Is the Right First Imaging Study for Suspected DDH After 6 Months of Age?

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What Is the Right First Imaging Study for Suspected DDH After 6 Months of Age?

A 9-month-old infant presents for a well-child visit. During the examination, you note asymmetric thigh creases and limited abduction of the left hip. The Barlow and Ortolani maneuvers are difficult to assess in an infant of this age, but the clinical picture raises concern for underlying hip instability. You need to decide on the most appropriate initial imaging study to evaluate for Developmental Dysplasia of the Hip (DDH). This article provides a detailed workflow for this specific clinical scenario, guiding you through the rationale for the recommended imaging, the differential diagnosis, and the next steps based on the results. For a child older than 6 months with suspected DDH, the American College of Radiology (ACR) Appropriateness Criteria rate a pelvis radiograph as Usually Appropriate for initial evaluation.

Who Fits This Clinical Scenario for Suspected DDH?

This guidance is specifically for the initial imaging workup of a child, older than 6 months of age, in whom there is a new clinical concern for Developmental Dysplasia of the Hip. This concern typically arises from physical exam findings such as limited hip abduction (the most reliable sign in this age group), apparent limb length discrepancy (a positive Galeazzi sign), or asymmetric inguinal or gluteal skin folds.

It is critical to distinguish this scenario from others that require a different imaging approach:

  • Infants younger than 6 months: For infants between 4 and 6 months, and especially those younger than 4 months, hip ultrasound is the preferred initial imaging modality. The femoral head is primarily cartilaginous in this younger age group, making it poorly visualized on radiographs but perfectly suited for sonographic evaluation. Applying this radiographic workflow to a 3-month-old would be inappropriate.
  • Surveillance of known DDH: This workflow does not apply to children already diagnosed with DDH who are undergoing nonoperative surveillance imaging, for example, while being treated in a Pavlik harness or abduction brace. That represents a separate clinical question focused on monitoring treatment response.
  • Acute trauma or suspected infection: If the clinical presentation suggests an acute injury, septic arthritis, or osteomyelitis (e.g., fever, refusal to bear weight, focal tenderness, warmth, or erythema), the imaging workup and differential diagnosis are entirely different and may involve other modalities or laboratory tests.

What Diagnoses Are You Working Up in an Older Infant with Hip Concerns?

While DDH is the primary consideration, the initial radiograph helps evaluate for several conditions that can present with similar signs of abnormal hip development or function in this age group.

Developmental Dysplasia of the Hip (DDH)
This is the most common and critical diagnosis to establish. DDH is not a single entity but a spectrum of abnormalities affecting the developing hip joint, ranging from shallow acetabular development (dysplasia) to partial displacement (subluxation) or complete displacement (dislocation) of the femoral head from the acetabulum. Early and accurate diagnosis is crucial, as untreated DDH can lead to premature osteoarthritis, chronic pain, and significant functional disability in adolescence and adulthood.

Congenital Coxa Vara
A less common but important differential is coxa vara, a developmental condition characterized by a decreased neck-shaft angle of the femur. This can lead to a leg length discrepancy and a waddling or Trendelenburg gait once the child begins to walk. While often diagnosed later, its developmental nature places it on the differential for an abnormal hip exam in an older infant. Radiographs are diagnostic, clearly demonstrating the abnormal femoral neck angle.

Proximal Femoral Focal Deficiency (PFFD)
This is a rare, non-hereditary congenital anomaly involving improper development of the proximal femur and, in many cases, the acetabulum. The severity can range from mild femoral shortening and hypoplasia to complete absence of the femoral head and acetabulum. While often more clinically apparent than subtle DDH, milder forms can be on the differential. Radiography is the first step in classifying the severity of PFFD.

Neuromuscular Hip Dysplasia
In children with known or suspected neuromuscular disorders such as cerebral palsy or spina bifida, muscular imbalance across the hip joint can lead to progressive hip subluxation and dislocation. The underlying pathophysiology is different from idiopathic DDH, but the radiographic findings of hip instability can be similar. The initial imaging evaluation follows the same principles.

Why Is a Pelvis Radiograph the Recommended Study for DDH After 6 Months?

The ACR designates a pelvis radiograph as Usually Appropriate for this scenario because it provides a reliable and standardized method for assessing the bony anatomy of the hip once the femoral head has begun to ossify.

The key rationale is the development of the capital femoral epiphysis (the ossific nucleus of the femoral head), which typically appears between 4 and 6 months of age. Once this structure is ossified, it becomes visible on an X-ray, allowing for objective measurements of hip development and alignment. Radiologists and orthopedic surgeons use established lines and angles on an AP pelvis radiograph to diagnose and classify DDH, including:

  • Hilgenreiner’s Line: A horizontal line drawn through the top of both triradiate cartilages.
  • Perkin’s Line: A vertical line drawn downward from the lateral edge of the acetabulum on each side, perpendicular to Hilgenreiner’s line. A normal femoral head ossific nucleus should be in the inferomedial quadrant.
  • Acetabular Index: The angle of the acetabular roof relative to Hilgenreiner’s line, which indicates the steepness (dysplasia) of the socket.
  • Shenton’s Line: A continuous arc formed by the medial border of the femoral neck and the inferior border of the superior pubic ramus. Disruption of this line suggests hip subluxation or dislocation.

Comparison to Alternative Studies

  • US hips: Ultrasound is rated as Usually not appropriate in this age group. The progressive ossification of the femoral head, which makes radiography effective, simultaneously degrades the quality of ultrasound. The ossified bone creates an acoustic shadow that prevents visualization of the deeper joint structures, rendering the study unreliable for diagnostic purposes in children older than 6 months.

Radiation and Practical Considerations
The radiation exposure from a single AP pelvis radiograph is very low. The ACR notes the pediatric relative radiation level is ☢☢, corresponding to an effective dose of 0.03-0.3 mSv. This is a small fraction of the average annual background radiation. No contrast material is required for this examination.

When ordering, it is crucial to request a high-quality, well-positioned AP pelvis view. The patient must be positioned symmetrically without pelvic rotation, as even minor obliquity can distort the radiographic lines and angles, potentially leading to a misdiagnosis.

What’s Next After a Pelvis Radiograph? Downstream Workflow

The results of the pelvis radiograph will guide the immediate next steps, which almost always involve consultation with a pediatric orthopedic surgeon.

  • Positive for DDH: If the radiograph confirms acetabular dysplasia, hip subluxation, or dislocation, the patient requires prompt referral to pediatric orthopedics. Treatment in this age group is more complex than in younger infants and may involve a period of closed reduction and spica casting or, in some cases, open surgical reduction. Further advanced imaging, such as an MRI or CT scan, may be ordered by the specialist to better define the anatomy and guide surgical planning, particularly to assess for obstacles to a stable reduction.
  • Negative or Normal Study: If the radiograph is unequivocally normal and the physical exam findings are mild or equivocal, clinical follow-up is a reasonable next step. The provider can re-evaluate the child in a few months to ensure no new clinical signs have developed. If a high degree of clinical suspicion persists despite a normal radiograph, a discussion with a pediatric radiologist or orthopedic surgeon may be warranted to review the images and decide if any further evaluation is needed.
  • Indeterminate or Borderline Findings: In cases where the radiographic findings are borderline for dysplasia, management can be nuanced. This is another critical point for referral to pediatric orthopedics. The specialist will integrate the imaging findings with the physical exam and patient risk factors to decide between continued observation with repeat imaging in 3-6 months or initiating treatment.

Pitfalls to Avoid (and When to Get Help)

  • Ordering the wrong study: The most common pitfall is ordering a hip ultrasound in a child over 6 months old. Remember that ossification makes ultrasound unreliable in this age group; radiography is the correct initial test.
  • Accepting a poor-quality image: A rotated or tilted pelvis radiograph is non-diagnostic. If you receive a report stating the study is limited by patient positioning, insist on a repeat study. Key treatment decisions depend on accurate measurements.
  • Delaying referral: A diagnosis of DDH in a child older than 6 months is a significant finding. Treatment becomes more invasive and less predictable with increasing age. Do not delay referral to a pediatric orthopedic surgeon.
  • Dismissing clinical signs: Do not let a “normal” radiology report on a poorly positioned film override strong clinical suspicion. If the exam is clearly abnormal, escalate your concern by speaking directly with the radiologist or referring to orthopedics for a second opinion.

Related ACR Topics and Tools

This article focuses on a single clinical scenario. For a comprehensive overview of all DDH-related scenarios, from newborns to older children, please consult our parent guide. For additional resources on imaging selection, protocols, and radiation safety, the following tools are available.

Frequently Asked Questions

Why can’t I just order a hip ultrasound on a 7-month-old if it has no radiation?

While ultrasound is radiation-free, it becomes diagnostically unreliable in infants older than about 6 months. The femoral head begins to ossify (turn to bone), which blocks the ultrasound waves and prevents a clear view of the hip joint. A radiograph (X-ray) is required to see the ossified structures and make accurate measurements.

What if the radiograph is reported as normal but I am still clinically concerned?

Persistent clinical concern despite a normal initial radiograph warrants further action. First, ensure the radiograph was well-positioned, as pelvic rotation can mask subtle dysplasia. If the technique is good, referral to a pediatric orthopedic surgeon is the appropriate next step. They can provide an expert physical exam and may opt for surveillance with repeat imaging in 3-6 months or decide the exam is reassuring.

Is an MRI or CT scan ever used for initial DDH diagnosis in this age group?

No, MRI and CT are not used for the initial diagnosis of DDH in an infant. A pelvis radiograph is the standard. However, if the radiograph confirms DDH and surgical intervention is being planned, the orthopedic surgeon may order a post-reduction CT or MRI to confirm the hip is properly seated in the socket and to identify any soft tissue (like an inverted labrum) that might be blocking a stable reduction.

What specific view should I order for a pelvis radiograph for DDH?

You should order a single anteroposterior (AP) view of the pelvis. It is critical that the technologist obtains a well-centered, non-rotated image with the infant’s legs in a neutral position. Additional views, like a frog-leg lateral, are sometimes obtained but the AP view is the most important for making the standard diagnostic measurements for DDH.

Does a family history of DDH change the imaging recommendation for a 9-month-old with a normal exam?

In an infant older than 6 months, the physical exam is the primary driver for ordering initial imaging. While a family history is a significant risk factor, most screening guidelines focus on performing a good physical exam at every well-child check. If the hip exam is unequivocally normal in a 9-month-old, imaging is not typically required based on risk factors alone. This contrasts with newborns, where risk factors often trigger a screening ultrasound.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 26, 2026