Musculoskeletal Imaging

What Is the Best Imaging Study for Suspected Infection Around Surgical Hardware?

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What Is the Best Imaging Study for Suspected Infection Around Surgical Hardware?

A 58-year-old patient presents to your clinic six weeks after an open reduction and internal fixation (ORIF) of a tibial plateau fracture. They complain of increasing pain, warmth, and erythema over the surgical plate. Inflammatory markers are elevated. You obtain initial radiographs, which show the hardware is in a stable position, but there are some subtle, nonspecific soft tissue changes and perhaps a hint of periosteal reaction. The clinical picture is highly suspicious for a hardware-associated infection, but the plain films are inconclusive. What is the most appropriate next imaging study to confirm or exclude osteomyelitis and guide management?

This clinical decision point is critical for preventing chronic infection and hardware failure. According to the American College of Radiology (ACR) Appropriateness Criteria, for a patient with suspected osteomyelitis or soft tissue infection around extra-articular hardware where initial radiographs are equivocal, MRI of the area of interest without and with IV contrast is rated as Usually Appropriate. This article details the clinical workflow for this specific scenario.

Who Fits This Clinical Scenario for Hardware-Associated Infection?

This guidance is tailored for a specific patient population. Correctly identifying if your patient fits this scenario is the first step to ordering the right test and avoiding diagnostic delays.

Inclusion Criteria:

  • The patient has implanted extra-articular surgical hardware, such as plates, screws, or intramedullary nails used for fracture fixation.
  • There is a clinical suspicion of infection, based on signs like localized pain, erythema, swelling, warmth, wound drainage, or systemic markers like elevated C-reactive protein (CRP) or erythrocyte sedimentation rate (ESR).
  • Initial radiographs have already been performed and are either normal or show nonspecific findings that do not confirm or rule out infection.

Exclusion Criteria (These patients follow a different diagnostic path):

  • Patients with intra-articular hardware: This workflow does not apply to suspected infection of joint replacements (e.g., total knee or hip arthroplasty). That presentation falls under a distinct ACR scenario for Suspected septic arthritis with arthroplasty.
  • Patients without any hardware: If the suspicion for osteomyelitis exists in native bone without any surgical implants, the imaging considerations differ slightly. This is covered in the ACR variant for Suspected osteomyelitis in a patient with normal or suggestive initial radiographs.
  • Diabetic foot infections: This is a complex and distinct clinical entity with its own dedicated guidelines and is explicitly excluded from this topic.

What Diagnoses Are You Working Up Around Surgical Hardware?

When evaluating a patient with suspected infection around orthopedic hardware, the differential diagnosis extends beyond a simple “infection yes/no” question. The imaging study must help differentiate several possibilities, each with different management implications.

Periprosthetic Osteomyelitis This is the primary and most serious concern. It involves an infection of the bone directly adjacent to the surgical implant. The hardware can act as a nidus for biofilm formation, making the infection difficult to eradicate without surgical intervention. Imaging must be sensitive enough to detect early bone marrow changes before significant cortical destruction occurs.

Soft Tissue Infection or Abscess The infection may be confined to the overlying soft tissues without involving the bone. This can range from superficial cellulitis to a deep, organized, and drainable fluid collection (abscess) or a more diffuse, non-liquefied inflammatory process (phlegmon). Differentiating these is crucial, as a discrete abscess often requires drainage.

Sinus Tract Formation A chronic hardware-associated infection can lead to the formation of a sinus tract—an epithelialized channel that extends from the deep infection source (the hardware or bone) to the skin surface. These tracts are a definitive sign of deep infection and must be identified and delineated for surgical planning.

Aseptic Loosening or Hardware Failure Not all pain around hardware is infectious. Mechanical causes, such as aseptic (non-infectious) loosening of screws or hardware failure, can produce similar clinical symptoms of pain and swelling. Imaging plays a key role in distinguishing inflammatory changes of infection from the mechanical signs of loosening, like lucency around screws without other signs of infection.

Why Is MRI Without and With Contrast the Recommended Study for Suspected Hardware Infection?

The ACR designates MRI of the area of interest without and with IV contrast as Usually Appropriate because of its superior ability to evaluate both bone and soft tissue, providing the most comprehensive diagnostic picture in this challenging scenario.

The primary advantage of MRI is its exceptional soft tissue contrast and high sensitivity for detecting bone marrow edema, the earliest sign of osteomyelitis. It can clearly delineate the extent of soft tissue inflammation, identify fluid collections, and distinguish a phlegmon from a well-defined, rim-enhancing abscess that requires drainage. The administration of gadolinium-based intravenous contrast is key to this differentiation and is also invaluable for identifying and mapping sinus tracts.

A common concern with MRI around metal is susceptibility artifact. However, modern MRI scanners and specialized software protocols significantly mitigate this issue. It is crucial to specify the presence of hardware when ordering the study so the radiology department can employ metal artifact reduction sequences (MARS) or similar techniques (e.g., SEMAC). These advanced sequences improve image quality near the implant, allowing for a confident diagnosis.

Why are other studies rated lower for this specific scenario?

  • CT with IV contrast (May be appropriate): While CT is excellent for visualizing cortical bone detail, sequestra (dead bone fragments), and periosteal reaction, it is far less sensitive than MRI for detecting early bone marrow edema. Its soft tissue resolution is also inferior, making it harder to characterize abscesses and phlegmon. Metal artifact can also be substantial on CT, though often less distorting than on older MRI sequences.
  • Nuclear Medicine Scans (e.g., Labeled WBC scan, May be appropriate): These studies are highly sensitive for inflammation and infection and can be useful when MRI is contraindicated or when metal artifact is too severe for a diagnostic MRI. However, their spatial resolution is poor, making it difficult to precisely localize the infection to bone versus soft tissue. Furthermore, they involve a significant radiation dose (ACR RRL ☢☢☢☢, 10-30 mSv for adults) and are often more complex and time-consuming to perform.

MRI provides the best balance of high diagnostic accuracy for the entire differential with no ionizing radiation (ACR RRL=O).

What’s Next After MRI? Downstream Workflow for Hardware-Associated Infection

The MRI results directly inform the next steps in patient management, creating a clear decision tree for the clinical team.

  • If the MRI is positive for osteomyelitis or a deep abscess: This finding typically necessitates a consultation with orthopedic surgery. Management often involves surgical debridement, hardware removal or exchange (if bone healing allows), and a prolonged course of targeted antibiotic therapy. An interventional radiology or surgical biopsy may be performed to obtain cultures to guide antibiotic choice.
  • If the MRI shows only soft tissue infection (cellulitis/phlegmon) without bone involvement: The patient may be managed with intravenous or oral antibiotics alone. A deep phlegmon without a drainable collection still requires close monitoring, as it can evolve into an abscess.
  • If the MRI is negative for infection and suggests an alternative diagnosis (e.g., aseptic loosening, seroma): The workup shifts away from infection. For suspected aseptic loosening, the focus becomes surgical planning for hardware revision. A simple seroma may be observed or aspirated if symptomatic.
  • If the MRI is indeterminate: This is most often due to severe, non-correctable metal artifact. In this situation, a nuclear medicine study, such as a WBC scan and sulfur colloid scan (May be appropriate), becomes the logical next step to increase diagnostic specificity for infection.

Pitfalls to Avoid (and When to Get Help)

Navigating the workup for hardware-associated infection requires careful attention to detail to avoid common diagnostic errors.

  • Forgetting to specify hardware: Always inform the radiology department about the presence and type of metal hardware when ordering an MRI. This is essential for them to select the appropriate metal artifact reduction sequences.
  • Relying solely on non-contrast MRI: While a non-contrast MRI (Usually Appropriate) can show marrow edema, it is significantly limited in its ability to characterize soft tissue collections and identify sinus tracts. Omitting contrast can lead to a missed abscess.
  • Misinterpreting postoperative changes: Normal postoperative inflammation can mimic infection in the early weeks. Correlate imaging findings closely with the time since surgery and the patient’s clinical trajectory.
  • Delaying imaging: If clinical suspicion is high, proceeding to advanced imaging promptly is crucial. Delay can allow an infection to become more established, leading to greater bone destruction and a more complex surgical course.

If MRI results are equivocal or conflict with a strong clinical suspicion of infection, escalate by consulting with both an orthopedic surgeon and a musculoskeletal radiologist to determine the best next step, which may include a nuclear medicine study or image-guided aspiration for culture.

Related ACR Topics and Tools

For a comprehensive understanding of imaging for musculoskeletal infections and to explore related tools, the following resources are available:

Frequently Asked Questions

My patient has a pacemaker. Can they still get an MRI to evaluate for hardware infection?

A pacemaker or other implanted electronic device is a relative contraindication to MRI. The decision depends on whether the device is MRI-conditional and requires careful coordination with cardiology and radiology to ensure patient safety protocols are followed. If a safe MRI is not possible, a nuclear medicine study like a labeled WBC scan is the best alternative.

Why is image-guided aspiration rated ‘Usually not appropriate’ as the next imaging study?

Image-guided aspiration is a diagnostic and therapeutic procedure, not a primary imaging modality for this scenario. The ACR rates it ‘Usually not appropriate’ as the *next imaging study* because cross-sectional imaging (like MRI or CT) should be performed first to identify the extent of disease, locate the optimal target for aspiration (e.g., a fluid collection), and plan a safe needle trajectory. Aspiration is often the step that follows a positive MRI, but it doesn’t replace it.

How soon after surgery can an MRI reliably detect infection versus normal postoperative inflammation?

This is a challenging distinction. In the first 4-6 weeks post-surgery, bone marrow edema and soft tissue enhancement are expected findings and can be difficult to differentiate from infection. However, specific findings like a well-defined fluid collection with a thick, enhancing rim (abscess), a clear sinus tract, or progressive, intense marrow edema are highly suggestive of infection even in the early postoperative period. Clinical correlation is paramount.

Is there a role for ultrasound in this scenario?

Ultrasound is rated as ‘May be appropriate’ by the ACR. Its primary role is to evaluate for superficial soft tissue fluid collections, such as an abscess or seroma, that may be amenable to aspiration. It is excellent for guiding this procedure. However, ultrasound cannot visualize bone marrow to assess for osteomyelitis and its view of deep structures can be obscured by the hardware. It is a useful adjunct but does not replace MRI for a comprehensive evaluation.

Should I order a 3-phase bone scan instead of an MRI?

A standalone 3-phase bone scan is rated ‘Usually not appropriate’ for this scenario. While highly sensitive for bone turnover, it is not specific for infection. Increased uptake is seen in fracture healing, aseptic loosening, and other inflammatory conditions. To improve specificity for infection, a bone scan is typically combined with a labeled white blood cell (WBC) scan. However, given the lack of radiation and superior anatomical detail, MRI remains the preferred initial advanced imaging test when feasible.

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