Metal-on-Metal Hip & Tribocorrosion Evaluation & Treatment

The levels of cobalt and chromium ions in the blood are measured because high levels of these ions can be an indicator of excessive wear of the metal bearing surface. The metal ion levels are usually low in well functioning MoM hip implants. The higher the level of these metal ions, the more concern we have. We take very special precautions in measuring the level of these metal ions, because the technique is very complex.

In measuring trace metals with concentrations in the part-per-billion range, the risk of contamination is a major technical challenge. At the Mass General Center for Metal-on-Metal Hip & Tribocorrosion Evaluation & Treatment, we adhere to stringent protocols which are required from specimen collection to sample analysis.

A special MRI examination with the capacity to reduce the artifacts caused by the presence of the MOM THR will be performed. These tests are far superior to standard x-rays in assessing whether the wear of a MOM THR has begun to affect the tissues around the hip joint. Musculoskeletal Radiology at Mass General performs MRI using specific protocols designed to reduce artifact from metal implants. These techniques allow evaluation of the soft tissues surrounding a MoM hip replacement and assesses for any abnormalities such as abnormal fluid collection or solid lesions.

If the implants are not quite placed properly, there may be increased wear of the metal implants. In order to more accurately determine the orientation of the MoM hip implants in certain patients, three-dimensional Computed Tomography (3D-CT) of the patients’ pelvis and lower limbs are obtained using a high-resolution CT scanner to see if the positioning of the MOM THR might be contributing to excessive wear of the prosthesis. We use scanning protocol which lowers the effective radiation exposure by reducing the volume of the patient imaged.

In selected patients, a research study involving a specialized motion analysis will be performed. Because the most serious problems of MOM THR come from wear of the metal parts and because the most serious wear occurs at the edge of the metal parts creating a chisel-like effect, this major wear is called “edge wear”. When edge-loading occurs in the metal implants, this leads to accelerated metal wear and increased metal ion levels in the blood.

Dr. Young-Min Kwon, one of the Directors of the Center, in conjunction with colleagues at the University of Oxford, England, developed a test of walking, sitting and climbing stairs that may detect and measure edge wear. For the patient, this test involves just two non-invasive examinations. The first is the CT mentioned above and the second is the “gait study” (frequently used in professional athletes to analysis motion) of walking, sitting and climbing stairs while being videotaped. This research study may be useful in determining:

1. If edge wear is occurring
2. How often it occurs
3. What actions or motions cause it

All MoM implants retrieved at the time of corrective/revision surgery are carefully and thoroughly analyzed.

i) Serum Metal Ion Analysis
In measuring trace metals with concentrations in the part-per-billion range, the risk of contamination is a major technical challenge. At the Mass General Center for Metal-on-Metal Hip & Tribocorrosion Evaluation & Treatment, we adhere to stringent protocols which are required from specimen collection to sample analysis. The venepuncture is performed with needles, syringes, and tubes that have been verified to be free of trace metal contamination at the part per billion level. Blood specimens are stored and processed in plastic vessels verified to be contamination-free by appropriate testing.

We measure metal ion levels utilizing Inductively Coupled-Plasma Mass Spectroscopy (ICP-MS), which improves the detection limits. This is important because cobalt and chromium are normally present in body fluids and tissues in trace amounts that may be at below or slightly higher than the detection limits. The ICP-MS technique allows multi-element determinations, which improves throughput and reduces the amount of blood sample required.

ii) Metal Artifact Reduction Magnetic Resonance Imaging (MARS MRI)
Utilization of metal artifact reduction techniques allows for high resolution imaging around metal implants. Musculoskeletal Radiology at Mass General performs MRI using specific protocols designed to reduce artifact from metal implants. These techniques allow evaluation of the soft tissues surrounding a MoM hip replacement and assesses for any abnormalities such as abnormal fluid collection or solid lesions.

iii) Three-Dimensional Computed Tomography (3D-CT)
Suboptimal MoM implant orientation is associated with high metal implant wear. In order to more accurately evaluate MoM hip implant orientation (measured usually using plain radiographs) three-dimensional Computed Tomography (3D-CT) reconstruction scans of the patients’ pelvis and lower limbs are obtained using a high-resolution CT scanner using a metal artifact reduction sequence. CT scanning protocol is performed in accordance with ‘Low Dose’ CT Scan Protocol. The protocol lowers the effective radiation dose by reducing the volume of the patient imaged. Instead of continuous scanning from pelvis to lower limbs, the protocol recommends scanning a set of sequential regional areas. Thus, the low-dose protocol minimizes radiation exposure without compromising image quality.

i) Accurate 3-D Determination of Component Positioning during Standing
The first is a standing 3-D radiographic evaluation at low radiation using EOS technology that provides unique information critical to evaluating edge wear:

1. Determination of the position of the acetabular component in the position of load bearing (i.e. upright). Conventional x-rays and CT are taken recumbent
2. Accurate assessment of the femoral anteversion from the 3-D image

Because component position while walking, sitting and climbing stairs is critical to edge loading, and because this information cannot be obtained any other way, knowing the position of the acetabular component while the patient is upright can be essential. Moreover, these accurate determinations of component positioning can then be used in the unique ability to actually detect edge loading in vivo.

ii) Detection of Edge-Loading In Vivo (Research)
The occurrence, frequency, magnitude of edge-loading in MoM patients during functional activities may be quantified by the unique process of integrating three-dimensional motion analysis system performed with the 3-D CT reconstruction. This research study may provide valuable information to identify patients who are at risk of accelerated wear of the metal implants.

The integration of three-dimensional motion analysis system performed with CT reconstruction may be useful in order to:

• Identify edge loading in vivo
• Quantify the frequency of edge loading
• Quantify the force of the edge loading
• Determine what activities (walking, rising from a chair or climbing stairs) are causing the problem

This research study has the potential to provide valuable information to identify patients who are at risk of accelerated wear of the metal implants. These data may be helpful in telling a patient whether he/she is edge loading during functional activities. We may also be able to use the data to advise whether it is common or rare or which motion to reduce or avoid to decrease edge loading.

iii) Wear Analysis of Explanted MoM Hip Devices
MoM implants retrieved at the time of revision surgery are analyzed. Roundness machine measurement is a well established technique that has been used to measure wear in MoM implants.

All Patients:

• Clinical Evaluations and Review of All Prior X-rays
• Current Plain X-rays
• Exclude other etiology of symptoms such as infection, component loosening, fracture, osteonecrosis, psoas impingement and referred pain
• Perform Blood Metal Ion Levels & MARS MRI
  • Normal test results with No symptoms: Follow-up in 12 months
  • Normal test results with Mild symptoms: Follow-up in 6 months and repeat the tests
  • Normal test results with Severe/Worsening symptoms: Consider Revision surgery
  • Equivocal test results with No symptoms: Follow-up in 6 months and repeat the tests
  • Abnormal test results: Consider Revision surgery
• If there are signs of adverse soft tissue reactions or elevated ion levels, the treatment plan will depend on the magnitude of the soft tissue reactions and the height of the ion levels.

The decision regarding when to advise revision surgery remains primarily clinical. It needs to be made based on patient symptoms and the test results. At the Mass General Center for Metal-on-Metal Hip & Tribocorrosion Evaluation & Treatment, each decision for revision surgery is reviewed by second opinion from another experienced hip revision orthopaedic surgeon in order to provide the best possible treatment to these patients.

Revision surgery may be complex, often requiring extensive debridement and reconstruction. It is important to perform a thorough debridement of the abnormal tissue. Cross-sectional imaging studies (MRI) helps to elucidate intra-pelvic extension pre-operatively. When indicated, revision surgery is performed early to minimize progressive soft tissue damage. For revision surgery, we provide both the skills necessary to reconstruct the hip and the research skills to critically analyze the retrieved components to advance the ongoing analysis of why the failure occurs and to improve the outcome of THR.