Information about Soft Tissue Imaging and Metal Ion Testing
In certain clinical situations, orthopaedic surgeons should consider additional testing including soft tissue imaging and metal ion testing to assess patients with metal-on-metal (MoM) hip implants. 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 :
- For some patients, cross-sectional imaging is required to assess and diagnose soft tissue findings 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
- Strengths: MRI offers the best visualization of soft tissue surrounding a MoM hip implant and uses non-ionizing radiation.
- Weaknesses: MRI is contraindicated for some patients with implants. 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.
- During the June 2012 Orthopaedic and Rehabilitation Devices Advisory Panel meeting, panel members recommended imaging using MRI with metal artifact reduction.
- Magnetic Resonance Imaging (MRI) with metal artifact reduction
- 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.
The FDA is working with the manufacturers of MoM hip systems to evaluate devices for safety in the MR environment and to develop MR Conditional labeling, which would help ensure safe scanning of patients with MoM hip implants. The FDA is also encouraging the development of metal artifact reduction technologies for MRI scanners to improve image quality around metal implants.
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 (e.g. cobalt and/or chromium) in their bloodstream. Depending on the clinical scenario, orthopaedic surgeons may recommend measuring metal ion levels.
Obtaining accurate and precise metal ion test results in patients can be difficult. For example, patient samples can become contaminated during the sample collection and processing. Needles and blood collection tubes (even trace element tubes) can potentially contaminate patient samples with metal ions. Serum samples may become contaminated with metal ions from the environment during harvesting especially since this would be done outside of the testing lab. There are also known interferences, including polyatomic interferences that can affect the accuracy of metal ion test results.
In addition, not all commercial labs can accurately and precisely measure trace (greater than 10 µg/L) or ultratrace (less than 10 µg/L) elements. These factors have important implications for the accuracy, reproducibility and clinical interpretation of the test results.
Who Should Be Tested?
As part of their overall clinical evaluation, orthopaedic surgeons should consider measuring and following serial measurements of metal ion levels in symptomatic patients with MoM hip implants.
- The orthopaedic surgeon should consider testing patients with MoM hip implants who develop any symptoms 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.
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.
- To minimize well known interferences and accurately measure chromium, the laboratory can use available techniques that can resolve interferences. If these techniques are used properly, accurate chromium measurements can be obtained. However, it is important that the testing laboratory carefully validate their method to make sure that it is working properly and producing accurate test results.
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 ultratrace 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.
- 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, the FDA believes there is insufficient evidence in the U.S. demonstrating a correlation between a metal ion level and the presence of localized lesions, clinical outcomes and/or the need for revision surgery.