A Guide for Patients with Metal-on-Metal Total Hip Replacements
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.
Clinical Consultations 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:
A. Analysis of the Metal Ion Levels in the Blood 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.
B. Special MRI Testing 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.
C. Computed Tomography (CT) Testing 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.
D. Special Hip Motion Testing (Research) 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:
- If edge wear is occurring
- How often it occurs
- What actions or motions cause it
E. Analysis of Any MoM Hip Devices Removed at Reoperations All MoM implants retrieved at the time of corrective/revision surgery are carefully and thoroughly analyzed.
Summary 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.