Musculoskeletal Imaging

What Imaging Should You Order for Acute Injury in a Symptomatic Hip Arthroplasty Patient?

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What Imaging Should You Order for Acute Injury in a Symptomatic Hip Arthroplasty Patient?

A 78-year-old woman with a left total hip arthroplasty (THA) from five years ago presents to the emergency department after a ground-level fall. She has acute, weight-bearing pain in her operative hip. Initial anteroposterior (AP) and lateral radiographs are obtained, but they are equivocal for a periprosthetic fracture due to overlying hardware and subtle lucencies. The orthopedic surgeon on call needs a definitive answer to guide management—observation versus operative intervention. You are faced with the decision of which advanced imaging study to order next. This article provides a clinical workflow for this specific scenario, grounded in the American College of Radiology (ACR) Appropriateness Criteria. For a patient with a symptomatic hip arthroplasty after acute injury whose radiographs are inconclusive, the ACR designates CT hip without IV contrast as Usually Appropriate.

Who Fits This Clinical Scenario?

This guidance is for a well-defined patient population: an individual with a total hip arthroplasty who presents with new or acutely worsened symptoms directly following a specific injury or trauma. The crucial prerequisite is that initial radiographs have already been performed and are non-diagnostic, equivocal, or negative despite high clinical suspicion for an acute process like a fracture.

This workflow specifically applies when your primary concern is an acute mechanical or structural complication. It is essential to distinguish this from other common post-arthroplasty presentations that follow different diagnostic pathways:

  • Exclusion 1: Suspected Infection. If the patient presents with chronic, worsening pain, fever, erythema, or elevated inflammatory markers (ESR, CRP) without a clear traumatic event, the workup shifts. This patient fits the “symptomatic hip arthroplasty patient, infection not excluded” scenario, which has a different set of imaging recommendations.
  • Exclusion 2: Chronic Aseptic Loosening. For a patient with gradually progressive, activity-related pain suggesting aseptic loosening or polyethylene wear, the workup is non-emergent. This falls under the “symptomatic hip arthroplasty patient, infection excluded” variant.
  • Exclusion 3: Metal-on-Metal or Trunnionosis Concerns. Patients with specific metal-on-metal implants or symptoms suggesting adverse local tissue reaction (ALTR) or pseudotumor require a dedicated workup, often involving MRI with specialized metal artifact reduction sequences.

This article is exclusively for the post-trauma setting where the key question is identifying an occult fracture or acute component failure.

What Diagnoses Are You Working Up in This Scenario?

After an acute injury in a patient with a THA, the differential diagnosis is focused on urgent structural problems. The choice of imaging is driven by the need to definitively rule in or rule out these conditions to guide immediate management.

Periprosthetic Fracture This is the most common and clinically significant diagnosis to consider. A fracture around the implant can occur in the femur or, less commonly, the acetabulum. These can be subtle and easily missed on plain radiographs, especially non-displaced or hairline fractures obscured by the prosthesis. The Vancouver classification system, used to guide treatment, relies on the fracture’s location, implant stability, and bone stock, all of which are best delineated with cross-sectional imaging.

Prosthetic Dislocation or Subluxation While often obvious on radiographs, subtle subluxation or dislocation that has spontaneously reduced may not be. Advanced imaging can confirm the concentric reduction of the femoral head within the acetabular component and identify any associated bony injury, such as a fracture of the greater trochanter or acetabular rim, that could predispose to recurrent instability.

Acute Component Failure Though less common from a single traumatic event, acute fracture of the implant itself (e.g., femoral stem) or dissociation of a modular component can occur. Radiographs may be suggestive, but CT provides superior detail to confirm the integrity of the hardware.

Soft Tissue Injury or Hematoma Significant trauma can cause an acute hematoma or injury to the surrounding abductor muscles or other soft tissues. While CT is not the primary modality for soft tissue evaluation, a large, high-density fluid collection representing a hematoma can be identified and may be the source of the patient’s pain if no fracture is present.

Why Is CT Hip without IV Contrast the Recommended Study for This Presentation?

The ACR rates CT hip without IV contrast as Usually Appropriate because it directly and effectively addresses the primary clinical question—the presence of an occult periprosthetic fracture—with high diagnostic confidence.

The rationale is multi-faceted:

  • Superior Bone Detail: CT offers excellent spatial resolution and contrast for evaluating cortical and trabecular bone. It can easily depict non-displaced fracture lines, assess comminution, and characterize the relationship of the fracture to the implant, which is critical for surgical planning.
  • Metal Artifact Reduction (MAR): Modern CT scanners employ sophisticated MAR algorithms. When ordering, it is crucial to specify this technique. MAR protocols use specialized software to correct for the beam-hardening artifacts (streaking) caused by the high-density metal prosthesis, dramatically improving visualization of the adjacent bone-cement and bone-prosthesis interfaces.
  • Speed and Accessibility: CT is widely available, rapid to perform, and well-tolerated by patients in acute pain, making it ideal in an urgent or emergency setting.
  • Radiation and Contrast Tradeoffs: The study is performed without intravenous contrast, eliminating any risk related to contrast allergies or nephrotoxicity. The radiation dose is moderate (ACR RRL: ☢☢☢ 1-10 mSv), a justifiable exposure given the high clinical stakes of missing a periprosthetic fracture.

Why are other studies rated lower for this specific scenario?

  • MRI hip without IV contrast is rated May be appropriate. While it avoids ionizing radiation (ACR RRL: O 0 mSv), it has significant limitations in this context. Metal implants cause substantial magnetic susceptibility artifacts on MRI, which can obscure the very periprosthetic bone you need to evaluate. Even with advanced MAR sequences (like MAVRIC or SEMAC), the artifact can be severe enough to render the study non-diagnostic for an occult fracture. MRI’s strength is in evaluating soft tissues, which is not the primary concern after acute trauma.
  • Bone scan with SPECT or SPECT/CT is rated Usually not appropriate. A bone scan is highly sensitive but lacks specificity. It will show increased radiotracer uptake in the setting of a fracture, but also with loosening, infection, or even normal post-surgical remodeling. It cannot provide the precise anatomical detail of the fracture pattern needed for orthopedic planning. Its radiation dose (ACR RRL: ☢☢☢ 1-10 mSv) is comparable to CT without offering the same diagnostic clarity for this indication.

What’s Next After CT Hip without IV Contrast? Downstream Workflow

The results of the CT scan create a clear decision tree for patient management. The primary goal is to provide the orthopedic team with actionable information.

  • If the CT is POSITIVE for a periprosthetic fracture: The next step is an immediate orthopedic surgery consultation. The CT images will be used to classify the fracture (e.g., using the Vancouver classification) and determine the management plan, which may range from non-operative treatment with protected weight-bearing to open reduction and internal fixation (ORIF) or complex revision arthroplasty.
  • If the CT is NEGATIVE for fracture or other acute pathology: If the patient’s symptoms are mild and there is no other explanation for their pain, they can often be managed conservatively with analgesia and activity modification. The negative CT provides strong reassurance that no urgent surgical intervention is needed. The patient should follow up with their orthopedic surgeon as an outpatient.
  • If the CT is NEGATIVE but clinical suspicion remains high or symptoms persist: If the patient has persistent mechanical symptoms, instability, or severe pain despite a negative CT, the differential may shift toward a soft tissue or ligamentous injury. In this situation, you might consider the May be appropriate study, MRI hip without IV contrast, specifically to evaluate the abductor tendons, joint capsule, and surrounding musculature. This moves the workup toward the “Hip arthroplasty patient with trochanteric pain” or a similar soft-tissue-focused scenario.

Pitfalls to Avoid (and When to Get Help)

Navigating this clinical scenario requires attention to a few key details to ensure an accurate and timely diagnosis.

  • Pitfall 1: Not specifying Metal Artifact Reduction (MAR). Ordering a “CT Hip” without specifying MAR may result in a non-diagnostic study due to severe streaking artifact. Always include “with metal artifact reduction protocol” in the order.
  • Pitfall 2: Overlooking the acetabulum. While femoral periprosthetic fractures are more common, be sure the imaging field of view and review of the images adequately cover the acetabular component and surrounding bone.
  • Pitfall 3: Mistaking chronic changes for acute findings. Chronic, stable radiolucent lines or pedestal formation can be present. Correlating with prior imaging and the acute history is essential to avoid misinterpreting a chronic, non-traumatic finding as an acute fracture.

If a patient presents with signs of gross instability, a limb-length discrepancy, or any neurovascular deficit (e.g., foot drop, diminished pulses), this constitutes an orthopedic emergency. Escalate immediately with a direct call to the on-call orthopedic surgeon, as management may need to precede any advanced imaging.

Related ACR Topics and Tools

For further exploration of imaging guidelines and related clinical scenarios, the following resources are available:

Frequently Asked Questions

Why not order an MRI to avoid the radiation dose from a CT scan?

While MRI avoids ionizing radiation, it is rated ‘May be appropriate’ rather than ‘Usually Appropriate’ for this scenario due to significant metal artifact. The metal prosthesis distorts the magnetic field, which can obscure the adjacent bone and make it impossible to see a subtle fracture. CT with metal artifact reduction (MAR) provides superior bone detail, directly answering the primary clinical question about a potential fracture.

Does the recommendation for a non-contrast CT change if my patient has chronic kidney disease?

No, it does not. The recommended study is a CT hip *without* intravenous contrast. Because no IV contrast is administered, the patient’s renal function is not a factor in the decision, making this a safe choice for patients with any stage of kidney disease.

Is it necessary to specifically request ‘metal artifact reduction’ when ordering the CT?

Yes, absolutely. This is a critical part of the order. Standard CT protocols are not optimized to scan around large metal implants and will produce severe artifacts that make the images uninterpretable. Specifying ‘metal artifact reduction’ or ‘MAR protocol’ ensures the radiology department uses the appropriate software and techniques to generate a diagnostic-quality study.

What if the initial X-ray clearly shows a displaced periprosthetic fracture?

This clinical workflow is for the scenario where initial radiographs are negative or equivocal. If radiographs clearly demonstrate a fracture, the patient should have an immediate orthopedic consultation. A CT scan may still be ordered, but its purpose shifts from diagnosis to pre-operative planning to help the surgeon visualize the fracture pattern in three dimensions.

Is there any role for ultrasound in evaluating this patient?

For this specific scenario of suspected fracture after acute trauma, the ACR rates ultrasound as ‘Usually not appropriate.’ Ultrasound cannot penetrate bone or visualize the prosthesis-bone interface. Its use is limited to evaluating for superficial fluid collections (hematomas) or for a different clinical scenario, such as suspected trochanteric bursitis.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 30, 2026