Information for Physicians Wishing to Refer Patients to MoM.THR Center
Although the vast majority of patients with metal-on-metal hip replacement do well, a growing number of patients with MOM THR have required revision surgery. Over 93,000 MOM THR have been recalled by an implant manufacturer. Over 643,000 MOM THR have been done in North America.
The severity of these reactions can, in their extreme forms, be far worse than the adverse reactions to other forms of total hip replacements. In addition to destruction of bone (periprosthetic osteolysis), the severity may extend to destruction of large segments of muscles or the sciatic nerve. It has been reported that the revision operations in these extreme examples (which have been described as “pseudotumors”), are distinctly inferior to the ordinary revision operations for multiple causes of other sorts, including periprosthetic osteolysis from polyethylene.
In Great Britain, a medical alert has been issued calling attention to this problem and recommending further investigations for all patients who have metal-on-metal implants who become symptomatic or whose metal ion levels reach above 7 parts/billion. A national implant retrieval center has been established in London, England to which a vast majority of the implants removed at reoperations are being sent.
In the United States, the FDA has released a statement of concern and the American Academy of Orthopaedic Surgeons (AAOS) has issued follow up statements concerning both the British and the FDA alerts. FDA has informed all implant manufacturer of MoM THR who market implants in the USA that they are now required to survey all their MoM THR patients.
Because the adverse tissue reactions to this wear can be severe and because they may be clinically silent until late in the course, it is important to detect the adverse reactions as early as possible. And yet, conventional clinical examinations and conventional X-ray examinations are not helpful diagnosing these reactions in most cases.
Therefore, we have established the Center for Metal-on-Metal Total Hip Replacement at the MGH. The mission of MGH Center for Metal-on-Metal Total Hip Replacement is to provide excellence in clinical care to MoM patients by integrating various specialized tests in order to establish if an adverse reaction to MoM THR implants is present and to formulate the best way to treat these problems.
Our standard evaluation of a MoM THR patient begins with: clinical assessment; standard x-rays; assessment of all prior x-rays; measurement of blood ion levels of chromium and cobalt using the specialized techniques required; and metal artifact reducing MRI of the hip.
In addition to these tests, in selected patients, we perform additional tests:
- Accurate determination of Component Positioning during standing
- Detection of Edge Loading In Vivo (Research)
A. MoM THR Center Evaluation of MoM Patients
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 MGH Center for Metal-on-Metal Total Hip Replacement, 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 MGH 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.
B. Specialized Functional Tests in Selected Patients
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:
- 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
- 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.
C. Current MGH Center for MoM THR Clinical Evaluation Guidelines 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.
D. Revision Surgery 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 MGH Center for Metal-on-Metal Total Hip Replacement, 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.
Standardized Reports All referring surgeons/physicians will be provided with a full report of the specialized analysis, including blood test, imaging studies, and wear analysis (whereupon revision surgeries have been performed).