[Note that metal debris can shed from the metal implants and cause elevated levels of cobalt and chromium, which can result in "metallosis", which is a serious blood infection complication of surgeries involving metal-on-metal hip replacements.]
In certain clinical situations, orthopedic surgeons should consider additional testing including soft tissue imaging and metal ion testing to assess patients with metal-on-metal (MoM) hip implants.
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 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 Orthopedic and Rehabilitation Devices Advisory Panel meeting, panel members recommended imaging using 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 orthopedic 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.
Metal Ion Testing
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, orthopedic 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, orthopedic surgeons should consider measuring and following serial measurements of metal ion levels in symptomatic patients with MoM hip implants.
- The orthopedic 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 orthopedic surgeon feels the hip is functioning properly.
Source: FDA Website
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