Evaluation of Prosthetic Implant Degradation
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Published: December 2011Print Record of Viewing
Over a million artificial hips are implanted annually. Recently, a small number of adverse reactions to metal debris have occurred, resulting in significant pain and need for intervention. This problem is related to the use of metal-on-metal implants. Dr. Sierra and Dr. Moyer discuss when a patient should be evaluated for excessive implant wear, the simple serum test that can rapidly identify when a metal-on-metal implant is wearing excessively, and the need for proper specimen collection for a successful evaluation.
Presenter: Rafael J. Sierra, MD - Consultant in Orthopedic Surgery, Division of Orthopedics
Presenter: Thomas P. Moyer, PhD - Consultant in Laboratory Medicine and Pathology, Division of Clinical Biochemistry & Immunology
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Welcome to Mayo Medical Laboratories' Hot Topics. These presentations provide short discussions of current topics and may be helpful to you in your practice.
Our presenters for this program are from Mayo Clinic in Rochester, Minnesota. Dr. Rafael J. Sierra is Associate Professor of Orthopedics and Consultant in the Division of Orthopedic Surgery and Dr. Thomas Moyer is Professor of Laboratory Medicine and Consultant in the Division of Clinical Biochemistry and Immunology. Dr. Sierra will discuss the clinical aspects of metal-on-metal hip implants and Dr. Moyer will explain the role of serum chromium and cobalt in evaluating prosthetic implant degradation
Thank you, Sharon, for that introduction.
Joint replacement is a successful treatment for advanced joint disease, and implantation of orthopedic prosthetic devices contributes to quality of life. Following the initial launch of metal-on-polyethylene bearings, improved bearing surfaces made of ceramic-on-ceramic and metal-on-metal came into the marketplace to enhance the durability for younger and more active individuals. These newer devices were expected to stand up better to constant wear. Metal-on-metal hip replacements have the advantage of increased toughness, decreased wear, and reduced risk of dislocation because of the larger bearing surface.
More than 1 million total hip replacements are surgically inserted worldwide each year, with patients benefitting by improved mobility and quality of life. However, implant-specific local and systemic adverse effects related to sensitivity to the metal or due to the wear of the metal surfaces themselves, called adverse reaction to metal debris, or ARMD, affect a small number of implant recipients. Serum chromium and cobalt concentrations correlate with ARMD.
Patients experiencing joint pain after a metal-on-metal hip replacement should be evaluated appropriately. The majority of failures associated with metal-on-metal implants are not necessarily due to the bearing surface itself, and therefore an evaluation should be carried out to rule out implant loosening and infection. Pain is a characteristic symptom. Patients can experience groin or buttock pain, and in select patients with pseudotumors, a fluid-filled collection can be present around the lateral aspect of the hip. Flexion around the hip from activities such as going up stairs or sliding into a car are particularly painful and may point towards iliopsoas irritation or impingement, which is commonly seen with these implants. Patients with pain should undergo evaluation with an anteroposterior pelvis and a lateral radiograph and should get screening blood work including a complete blood cell count with differential, sedimentation rate, and C-reactive protein level as a baseline to rule out infection. If these tests are elevated, the hip should be aspirated. Once infection is ruled out and adverse reaction to metal debris is suspected, then cobalt and chromium serum concentrations and an ultrasound or MRI of the hip with metal suppression is performed to rule out the presence of pseudotumor. The term most accepted today for this phenomenon around metal-on-metal hip arthroplasty is ARMD or adverse reaction to metal debris.
Issues of Concern
Metal implants wear due to continuous motion at the metal-on-metal surface resulting in release of micro-particles into the surrounding tissues. These micro-particles can corrode, resulting in release of metal ions into the systemic circulation. Hip implants are made from several different alloys. The acetabular cup and femoral stem are frequently made of titanium with a porous coating to facilitate bone in-growth. The acetabular cup and implant head in metal-on-metal implants are made of cobalt-chromium alloys. There are also variations available where the acetabular cup allows for placement of an insert made out of polyethylene and the head is made of cobalt-chromium; these implant devices are not the subject of this presentation.
Metal-on-Metal Wear-Induced Tissue Necrosis
The junction of the acetabular cup and femoral head is exposed to tremendous pressure during motion; this joint bears the total body weight. In the case of metal-on-metal wear, these surfaces can deteriorate, generating micro-particles of metal. These metal particles can become integrated into the soft tissue surrounding the implant, leading to necrosis. These metal particles also undergo corrosion, allowing metal ions to enter and circulate in blood serum. Now, Dr. Thomas Moyer, will continue with our discussion.
Several key points need to be made at this stage in this discussion. As compared to serum from humans with no implants, all patients with metal-on-metal implants have modestly elevated serum chromium and cobalt concentrations. In most patients with metal-on-metal implants, serum-chromium and cobalt are approximately 10 times elevated as compared to serum concentrations found in unexposed individuals. Significantly elevated serum chromium and cobalt concentrations correlate with implant wear. Clinically important implant wear is indicated when serum chromium exceeds 15 ng/mL and cobalt exceeds 10 ng/mL.
Effect of Metallosis
An important work published by DeSmet in the Journal of Bone and Joint Surgery in 2008 presented findings from a large population of hip implant patients, comparing these patients with signs and symptoms of metallosis to those without metallosis. DeSmet reported that the median serum chromium and cobalt concentrations in patients with metallosis are approximately 5 times the median value observed in patients without metallosis.
Results from the DeSmet study show complete differentiation of patients with metallosis compared to those without metallosis based on serum chromium or cobalt concentration. Joint synovial fluid concentration shows less discriminating capability in differentiating patients with metallosis and without metallosis; joint synovial fluid evaluation is not a good discrimanator for implant wear. Joint fluid represents the total metal implant environment, whereas the serum reflects implant wear-related micro-particles that have undergone corrosive dissolution.
These findings lead us to some initial conclusions: First, compared to controls without implants, chromium and cobalt concentrations are elevated in blood, serum, and synovial fluids collected from ALL patients with metal-on-metal orthopedic implants. Second, serum chromium and cobalt concentrations increase with time after implant, reaching a steady state in about 3 years. And third, patients with implants showing significant metal-on-metal wear have significantly higher serum chromium and cobalt concentrations than those patients without implant wear. This relationship is not always true when evaluating chromium and cobalt concentration in synovial fluids.
Our recommendations regarding interpretation of chromium and cobalt concentrations in various biological tissues are shown here. As noted previously, patients with functioning implants with no pain will have increased serum chromium and cobalt concentrations as compared to individuals with no implants. Patients with deteriorated implants typically have serum chromium in excess of 15 ng/mL and cobalt in excess of 10 ng/mL. The difference between synovial concentrations in patients with functioning implants compared to deteriorated implants is not as significant. We recommend serum chromium and cobalt tests for evaluation of these patients, rather than synovial fluid evaluation for chromium and cobalt.
It is important to point out at this time that some laboratories are offering tests for blood concentrations of chromium and cobalt for orthopedic evaluation. With 1 exception, all studies published in the peer-reviewed literature document an association between increased serum concentrations of chromium and cobalt and implant wear. One study by Liu, et al published in Tohoku Journal of Experimental Medicine in 1998 evaluated blood, but only correlated elevated blood chromium and cobalt with implant loosening. No peer-reviewed publications demonstrate that blood analysis correlates with implant wear. Despite this, the FDA has recommended blood analysis. Because CLIA rules require that all clinical laboratory tests be clinically validated, my interpretation is that the FDA implied evaluation of blood serum, since that specimen has been clinically validated.
Are These Concentrations Toxic?
Now let's move on to a key question: Are these chromium and cobalt concentrations toxic? Let's first explore information about chromium. Chromium is toxic when it is present in the chemical species known as chromium 6. Inhalation of chromium 6 vapors leads to pulmonary epithelial cancer. Chromium 6 is a by-product of electrolysis and is of concern in the chrome electroplating industry. When chromium 6 becomes exposed to tissues, it is instantly converted to chromium 3, by cellular reducing enzymes. Reduction of chromium 6 to chromium 3 releases electrons that are destructive to surrounding tissues, initiating the carcinogenic process. A key point to note is that chromium 3 is not toxic, and that only chromium 3 is released from orthopedic implants. There are a number of internet blogs and websites relying on the experience with chromium 6 exposure in the electroplating industry to make comments on the toxicity of chromium from hip implant deterioration. It is inappropriate to make that comparison, because chromium 6 is not released during implant wear.
Cobalt is an essential element. It is integral to Vitamin B12 and required for carbonic anhydrase activity. Cobalt toxicity, known as cobaltism, can occur after ingestion of large amounts of cobalt. Symptoms associated with cobaltism include myocardial damage, renal failure, hypothyroidism, and interstitial lung disease develops if cobalt is inhaled. There are only a few case reports suggesting any deleterious effect from orthopedic implant wear and high cobalt serum concentrations. The case reports point out nonspecific neurologic symptoms such as dyspnea, fatigue, headache, vertigo, and decreased cognition. These symptoms could be seen in most any patient experiencing continuous pain.
Perhaps the most important factor in getting this testing right is specimen collection. Biological specimens for chromium and cobalt evaluation are very easily contaminated. Look around the room where you sit to observe all of the chrome plated furniture and room accoutrements — chromium and cobalt are ubiquitous in our environment. Dust present in normal room air contains chromium at concentrations 1,000 times or more compared to the chromium concentration in a blood serum sample. If the sample is exposed to room air after collection, it may very well become contaminated. Most routine blood collection devices are contaminated with chromium and cobalt. Any evacuated blood collection tube with a red, yellow, or pale blue rubber stopper, or a syringe with a black rubber plunger seal, or a latex catheter, contain high concentrations of chromium and cobalt. Vacutainer manufacturers have created special evacuated blood collection tubes designed for trace metal analysis; these vacutainers have a royal-blue rubber stopper and are labeled "Metal-Free." Also, decontaminate the venipuncture site with an alcohol swab. Avoid the use of Betadine swabs as these contain significant concentrations of chromium and cobalt.
Metal-Free Blood Tubes
There are only 2 manufacturers making devices for sample collection that are suitable for chromium and cobalt analysis. The Monoject royal blue top metal-free tubes for serum or blood collection are approved by the Food and Drug Administration for chromium and cobalt analysis. If specimens require collection using a syringe, the only syringe approved by the Food and Drug Administration for use in human subjects that is metal free is the NORM-JECT syringe by HSW.
In summary, serum chromium and cobalt are elevated in all patients with metal-on-metal orthopedic implants as compared to unexposed individuals. Serum concentrations increase with implant duration, even in patients with no significant implant wear. The degree of elevation of serum chromium and cobalt correlates with metal-on-metal orthopedic implant deterioration. Elevated serum chromium and cobalt in the absence of symptoms such as joint pain or unsteady gait do not, independently, indicate implant wear. And, proper specimen collection is essential for successful evaluation.
I have listed here a few pertinent references for your review if you wish to dig deeper into this topic.
Thank you very much for your attention.
I will be pleased to respond to inquiries submitted to the email address noted on this slide. But please be aware that this is a laboratory-oriented presentation. I will not be able to comment on whether you have ARMD, nor will I be able to offer you the name of an orthopedist for your prosthesis evaluation. If you are having joint pain, you should see the surgeon who performed your implantation. Thank you.
Disclosure (Dr. Sierra)
I have significant financial relationships with companies and these companies make metal-on-metal hip replacements. I am a consultant for Biomet and our department obtains research support from DePuy, Stryker, and Zimmer.
Disclosure (Dr. Moyer)
I have no significant financial relationships nor intellectual property to disclose. At the end of this presentation I made recommendations to use several FDA-approved products. I have no relationship with the companies that make these products, nor do I know anybody employed by these companies.