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Center for Hip & Knee Replacement
The Mass General Center for Metal-on-Metal Total Hip Replacement provides comprehensive clinical care for patients by our award-winning team of surgeons.
The Mass General Center for Metal-on-Metal Total Hip Replacement provides state-of-the-art comprehensive care to patients with metal-on-metal total joint replacements and conducts cutting-edge research. Our physicians and researchers have transformed basic discoveries into clinical practice and provide critical evaluation for and specific advice for metal-on-metal implants.
Because some patients with Metal-on-Metal (MoM) hip replacements develop unexplained pain, and because a few develop abnormal tissue reactions to the wear of the implants and require a reoperation, we established a Center that patients could rely on for comprehensive clinical evaluation.
In the majority of cases, conventional pre-operative tests (plain radiographs and standard blood tests) do not reveal any abnormality. Thus, the ordinary follow up evaluation may be of limited value. The mission of Mass General Center for Metal-on-Metal Total Hip Replacement is to provide excellence in clinical care to MoM patients by integrating various specialized tests (analysis of blood samples for metal ions, and imaging studies) in order to establish if an adverse reaction to MoM THR implants is present and to formulate the best way to treat these problems.
In conjunction with the detailed assessment by one of our expert hip surgeons, a number of special tests will be done in addition to the usual standard x-rays. Based on reviews of these tests, a treatment plan will be formulated which is tailored to each individual patient. The treatment recommendations may range between further close monitoring and reoperation surgery. The special tests include the followings:
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 Total Hip Replacement, 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 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.
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:
All MoM implants retrieved at the time of corrective/revision surgery are carefully and thoroughly analyzed.
Although the vast majority of patients with metal-on-metal hip replacement do well, some patients with MoM THR are having trouble with excessive wear, often requiring complex reoperations. If surgery becomes necessary, finding out early is important. Standard clinical examination and standard x-rays may not detect the problems early. Specialized tests, as outlined in this Guide to Patients, can detect the problem and provide important advice for each patient.
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:
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.
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:
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:
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.
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.
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).
The Mass General Center for Metal-on-Metal Total Hip Replacement is committed to improving the treatment and care of patients with metal-on-metal implant devices. The Center’s surgeon-scientists engage in translational research, transforming their laboratory discoveries into clinical practice. The Center is sharply focused on fulfilling the research needs of the patients with MoM hip implants.
The Mass General Center for Metal-on-Metal Total Hip Replacement strives to expand its foundation of knowledge and the technical tools with which rapid advances can be made in understanding the mechanisms of MoM THR device failures.
Additional current research studies at the Center include:
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