Information about Soft Tissue Imaging and Metal Ion Testing
Diagnostic testing (such as soft tissue imaging and metal ion testing) may provide important clinical information for assessing the risks of metal on metal (MoM) hip implants in patients with MoM implants. While the results of these diagnostic tests can be indicative of adverse tissue reactions, they cannot be relied upon as the only sign that a hip is failing. Please see the General Recommendations for Orthopaedic Surgeons AFTER Metal-on-Metal Hip Replacement Surgery for recommendations on which patients may require additional diagnostic testing.
When conducting soft tissue imaging or metal ion testing in MoM patients, please consider the following:
Soft Tissue Imaging
- For some patients, cross-sectional imaging is required to assess and diagnose potential changes in the soft tissues surrounding an implant.
- The benefits and risks of using different types of diagnostic imaging procedures should be considered when determining the most appropriate imaging modality for each patient.
- MoM hip implants can create metal artifacts (distortions of the image due to the presence of metal in the imaging field) in some imaging modalities.
- The most utilized methods of imaging soft tissue surrounding MoM hip implants include:
- Magnetic Resonance Imaging (MRI) with metal artifact reduction sequences (MARS)
- Strengths: MRI uses non-ionizing radiation and offers the best visualization of soft tissues surrounding a MoM hip implant and detection of potential ALTR.
- Weaknesses: Most MoM hip implants in the U.S. have not been evaluated for safety in an MR environment so the likelihood of adverse events, such as heating of the tissue near the implant, may not be known.
- Computed Tomography Scan (CT)
- Strength: CT offers the best visualization of implant positioning and bony tissue
- Weaknesses: CT uses ionizing radiation and provides lower soft tissue visualization. Image artifacts from the implant may distort the image.
- Strengths: Ultrasound allows soft tissue visualization without metal artifacts and uses non-ionizing radiation.
- Weaknesses: Image quality in ultrasound is very operator dependent. Ultrasound provides a lower resolution soft tissue image than MRI and has a limited depth penetration.
- Magnetic Resonance Imaging (MRI) with metal artifact reduction sequences (MARS)
- If it is determined that MRI imaging of a MoM hip implant patient is appropriate, the FDA recommends that the orthopaedic surgeon:
- Consult with the radiologist to evaluate the benefits and risks of utilizing MRI with metal artifact reduction;
- Use the available device-specific labeling to help determine appropriate scan sequences; and
- Inform the MRI site that the patient has a MoM hip implant.
Metal Ion Testing
As described in "Concerns about Metal-on-Metal Hip Implants," some patients with a metal-on-metal (MoM) hip implant may have elevated metal ion levels in their bloodstream. These elevated metal ions are predominately cobalt and , chromium, but can also include other metals such as molybdenum and titanium. Depending on the clinical scenario, orthopaedic surgeons may recommend measuring metal ion levels.
Who Should Be Tested?
As part of their overall clinical evaluation, orthopaedic surgeons should consider measurements of metal ion levels in symptomatic patients with MoM hip implants. Serial measurements of metal levels in conjunction with clinical dynamics can help optimizing individual patient management.
- The orthopaedic surgeon should consider testing patients with MoM hip implants who develop any symptoms (hyperlink to the section where the general risks are described) that may indicate that their device may not be functioning properly.
- At the current time, there is insufficient evidence to recommend metal ion testing in patients with MoM hip implants that have none of the signs or symptoms described above and the orthopaedic surgeon feels the hip is functioning properly and has normal radiographs.
What Should Be Tested?
- Measure cobalt using an inductively coupled plasma mass spectrometer (ICP-MS), equipped with either a collision cell or dynamic reaction cell technology, or use a sector field inductively coupled plasma mass spectrometer (SF-ICP-MS). The FDA recommends testing cobalt in EDTA anti-coagulated whole blood using a validated method that meets the Clinical Laboratory Improvement Amendments (CLIA) validation criteria. The method should accurately and precisely measure concentrations as low as 1 µg/L cobalt.
- If chromium is measured, use an ICP-MS, equipped with either a collision cell or dynamic reaction cell technology, or use SF-ICP-MS (in medium or high resolution mode) to resolve potential polyatomic interferences. The FDA recommends testing chromium in EDTA anti-coagulated whole blood using a validated method that meets the CLIA validation criteria. The method should accurately and precisely measure concentrations as low as 1 µg/L chromium.
Selecting a Testing Lab:
- Ensure the lab is CLIA-certified and is using a validated method that meets the CLIA validation criteria for EDTA anti-coagulated whole blood;
- Ensure the lab has an ongoing quality assurance and quality control program for the measurand in EDTA anti-coagulated whole blood;
- Ensure the lab provides collection kits (including all components necessary for sample collection and processing) that are free of significant metal contamination and fit for purpose (i.e., measuring trace and ultra-trace cobalt and chromium). The lab is responsible for validating that all components within the collection kit are not contaminated;
- Ensure the lab participates in a Centers for Medicare and Medicaid Services (CMS) approved trace element proficiency testing (PT) program (e.g. New York State Department of Health, Wadsworth Center) for that particular measurand. A lab's results for that measurand should be within the defined acceptance criteria of the PT program.
- Ensure that the lab has performed an appropriate study, or references appropriate data, to define valid reference range values.
Interpreting Test Results:
- The metal ion concentration values should be considered in addition to the overall clinical scenario including symptoms, physical findings, and other diagnostic results when determining further actions.
- When assessing metal ion concentration values, it is important to determine if the patient has any other reasons to have metal ion concentration values above reference range values including:
- Other implanted metal hardware
- Occupational exposure
- Renal insufficiency
- Dietary supplements
- Metal ion concentration values should be interpreted in conjunction with blood urea nitrogen (BUN) and creatinine to concurrently evaluate the patient's renal function.
- Test results should be interpreted considering the measurement error of the method and the lab's reference range. The measurement error can be estimated by reviewing the lab's proficiency testing results and the imprecision of the method.
- Ideally, follow test results serially in a patient, ensuring that the same sample type, measurement method and the same laboratory are utilized for each measurement. If a different laboratory was utilized, the ordering physician needs to consider the test results in light of potentially differing laboratory methods that may not have values that are interchangeable. The ordering physician should interpret the test results considering the total error of the method(s) to determine if metal ion levels are changing.
International regulatory agencies developed specific follow-up recommendations for patients with metal-on-metal (MoM) hip implants, including recommendations for blood tests including specific metal ion threshold values. As noted earlier, there are differences between the usage and availability of metal-on-metal hip implants in the U.S. and those outside the U.S., as well as differences in laboratory practices. For this reason, these recommendations may not be appropriate for patients in the U.S. As noted above, several factors can impact the accuracy, reproducibility, and clinical interpretation of the test results.
At the current time, there is not enough evidence in the U.S. demonstrating a correlation between a metal ion level in isolation and the presence of localized lesions, clinical outcomes and/or the need for revision surgery.