The DePuy ASR Metal-on-Metal Hip Implant: A Scientific Analysis of its Problems

The last ten years have seen surgical replacement of hip joints with an artificial prosthesis increase significantly. The main reason for this surge in hip replacement surgeries is our aging population. With people living longer, more active lives, degenerating hip joints can put a halt to that activity. From 1997 through 2004, the number of hip replacement surgeries increased nearly 50% and if the trend continues it is estimated that the annual number of hip replacements in the United States will reach 600,000 by 2015.

Factors in Hip Replacement Surgery

Primary reasons for patients undergoing hip replacement surgery include chronic pain, impairment of daily functions and severe arthritis in the hip joint. The most common type of arthritis leading to total hip replacement is osteoarthritis and is generally seen with aging or trauma to the hip joint. In some instances necrosis of the hip can be present, usually caused by a fracture, alcoholism, lupus, or taking steroid drugs such as prednisone. Patients may experience progressively worsening chronic pain which prohibits them from walking, climbing stairs or even getting up from a sitting position.

Types of Hip Implants

There are many different types of hip replacement implant including the metal-on metal, metal-on-polyethylene and metal-on-ceramic as well as the implants which are 100% ceramic or polyethylene. Other than the materials the implant is constructed with, the only other major difference is in how the implant is affixed—it can be cemented to the bone or attached in such a manner that it grows into the patient’s remaining bone. The idea behind the cobalt and chromium metal-on-metal hip implant was to create an implant which would not have to be re-done for fifteen to twenty years—unlike the ceramic and polyethylene hip implants which last approximately 8-12 years.

While metal-on-metal hip implants were actually used as far back as 1955, they were not FDA approved until 1999. Crude attempts at implanting stainless steel devices in the hip joint actually first occurred in 1937 using stainless steel components fitted to the bone with bolts and screws.  According to an article by John M. Restaino titled The Science, Medicine, Epidemiology and Liability Analysis of the DePuy ASR Metal-on-Metal Prosthetic Implants, between October 1, 2005 and December 31, 2006 approximately 35% of the hip replacement surgeries implemented a metal-on-metal implant.

Chromium and Cobalt Toxicity from Hip Implants

While the high carbon chromium and cobalt hip replacement joint does exhibit considerably less wear, the tradeoff comes in the form of tiny shards of metal ions which shear off from the hip implant during periods of normal activity. Patients who have undergone hip replacement surgery with the cobalt and chromium implant have legitimate concerns regarding the elevated levels of metal ions in the bloodstream and urine. Often, these microscopic metal pieces will lodge into the tissues surrounding the implant causing painful joint inflammation and even necrosis of the tissue. It is believed that the inflammatory and necrotic changes seen in the tissues of those with a metal-on-metal implant can be linked to either a hypersensitive reaction by the implant recipient or cytotoxicity which means the heavy metals are toxic to the body’s cells. Because the particles of cobalt and chromium are so incredibly tiny, they can be spread widely throughout the body killing cells or causing damage to DNA.

Necrosis and Necrobiosis from Metal Implants

One study investigated the level of tissue erosion due to these metal particles in the McKee-Farrar prostheses which were implanted in the early 1970’s as compared to the more recent metal-on-metal hip devices which had been implanted for less than two and a half years. There were various levels and kinds of tissue reactions to the cobalt and chromium particles in both types of implants. One reaction seen was necrobiosis which is similar in nature to necrosis or death of tissues however necrobiosis is considered the natural death of cells or tissues while necrosis involves the death of body tissues or cells due to injury, lack of blood flow, radiation, chemicals or toxins. Cobalt and chromium ions fall under the category of toxins and for unknown reasons some bodies will respond more negatively to the heavy metals than others. Both the older models of the cobalt and chromium hip implants as well as the newer models leeched significant amounts of heavy metals into the bloodstream and tissues surrounding the hip joint.

Acceptable Levels of Cobalt and Chromium

While both cobalt and chromium are used required for normal biological functioning, high concentrations are known to be toxic and can interfere with normal biological functioning of the body even altering the body’s DNA. Slightly elevated levels of these heavy metals are considered “normal” among those who have received a metal-on-metal implant but higher levels can be quite dangerous. While reports from DePuy claim that any concentration up to 7.0 mg/L are not cause for concern, the Mayo Clinic states that chromium blood serum concentrations greater than 1 mg/L and cobalt levels greater than 5.0 mg/L should be considered toxic.  It is not uncommon for patients with ASR hip implants to have cobalt levels as high as 20 mg/L. Patients who have been exposed to excessive levels of cobalt can have levels as high as one hundred times the norm and levels of 220 mg/L were found in a deceased patient who had a worn metal-on-metal hip implant.

Cobalt Toxicity Symptoms

Persons with higher levels of cobalt and chromium than those considered safe might actually experience no overt symptoms in the beginning however the long-term damage can be significant. Cobalt can damage multiple organs and is well known for its damaging properties for those working in certain industrial applications. Cobalt is generally used in manufacturing magnets and platinum jewelry, petroleum, tires, sterilization of medical supplies and foods, and in the pigments which color certain ceramics.

Deficiencies of cobalt can lead to anemia (very rare) however the health effects from cobalt particles entering the bloodstream from a metal implant are grave. Severe neurological and cardiovascular symptoms have been noted in those with high levels of cobalt in their bloodstream. Other symptoms of cobalt toxicity include gastrointestinal disorders, nerve damage, thyroid issues, renal problems, skin disorders and hemorrhages. Research shows that those patients with the highest risk of cobaltism are those with malposition of the implant and those with chronic hip pain.

Chromium Toxicity Symptoms

Like cobalt, chromium is an essential nutrient and is actually required by the body in order to maintain a normal glucose tolerance. The human diet generally supplies a sufficient level of chromium for most all human beings. Chromium toxicity can lead to DNA mutation, reproductive system problems, impaired liver functions and even the potential of an elevated cancer risk. Excess amounts of chromium can also adversely affect the respiratory system and lead to severe inflammation surrounding the hip implant. Toxicity from excess levels of cobalt or chromium can take years to fully manifest and every person exposed to higher-than-normal levels of these heavy metals will experience toxicity in a different manner.

Studies on Metal Levels in Failed Implants

The concentrations of chromium and cobalt were measured in those with failed metal-on-metal implants; when compared to patients with no implant the concentrations of cobalt and chromium were significantly elevated. White blood cells were reduced as were the number of lymphocytes and natural killer cells.  It was hypothesized by the researchers that the release of chromium ions from metal-on-metal hip implants could be toxic to the lymph tissues which were relatively close to the implant. It was also found that DNA damage or destruction occurred in those with cobalt and chromium metal implants and both metals are classified as human carcinogens. At least six reports of cobaltism—severe cobalt poisoning—related to the metal-on-metal hip implant have been reported as well as scores of reports of less severe—yet still serious—side effects from the heavy metals.

Corrosion Levels Tied to Implant Head Size

As understanding of how head radius, clearance and surface finish affected the lubrication and wear of hip implants increased, the appeal of the metal-on-metal implant’s resistance to wear caused a surge in popularity. Further, the reduced risk of dislocation due to the larger head size made the metal-on-metal implants an attractive option, particularly among younger, more active patients. The rate of corrosion among metal-on-metal hip implants appears to be tied directly to the overall surface area and it is theorized that if lubrication conditions are less than ideal the release of harmful metal ions may increase.

Metallosis from Metal-on-Metal Hip Implants

Metallosis, according to Restaino’s article is the “macroscopic staining of the soft tissues…associated with abnormal wear.” Metalosis can also be tied to the accumulation of the cobalt and chromium ions which shear off the ball and head and cause fibrosis and necrosis in the surrounding tissues. In some cases a mass—known as a pseudotumor—occurred in those with metal-on-metal hip implants. These tumors generally cause symptoms and can be cystic, solid or a combination. The overall incidence of these pseudotumors among those with metal hip implants is approximately 3.4% at 7 years following the initial implant surgery.

Among the patients who were required to undergo revision surgery due to unexplained pain, significant numbers of joint effusions and granulomas were noted. Bone impingement was seen in approximately 18% of the patients, femoral failure in 14%, loosening of the socket in 4.5%, necrosis with an associated fracture in 4.5%, cup malposition in 4.5% and post-op pain in 4.5%. The remaining 50% of patients suffered adverse reactions to the metal debris.

Results of Various Studies

A study done in 1996 compared serum cobalt levels in patients with metal-on-metal hip implants as compared to those with ceramic on polyethylene implants. Those with the all-metal implant exhibited detectable serum cobalt levels after one year while those with the ceramic implants exhibited only microscopic levels of cobalt in the bloodstream. In 2006, the levels of chromium, cobalt, titanium and aluminum were measured in two groups of patients—one group with an all-metal implant, the other with a ceramic-on-ceramic hip implant which implemented a titanium/aluminum alloy in the stem.

The results of this study showed a “highly significant release of chromium and cobalt in patients with metal-on-metal bearings compared with the ceramic-on-ceramic bearings and the control group. In fact, 76% of the patients in the all-metal hip implant group had extremely high chromium levels. In a separate five-year study reported on in 2011 and comprised of 173 patients with all-metal hip implants, an analysis of the cobalt and chromium levels was done in relation to the position of the implant. The levels of toxic metals were found to be significantly different when analyzed by femoral head size—the metal concentrations were much higher in the implants which utilized a smaller head size.

Signs of Cobalt Poisoning

Excessive levels of cobalt in the bloodstream can lead to visual impairment and even blindness, cognitive and neurological impairments including memory loss, hearing difficulties or deafness, convulsions, chronic headaches, dizziness and loss of balance, hypothyroidism, skin disorders and rashes and peripheral neuropathy. Even in the absence of symptoms, all recipients of a metal-on-metal hip implant should have their serum cobalt levels measured yearly. Levels over 5 mg/L but less than 20 mg/L are considered to be toxic and require that the serum cobalt levels be re-tested every six months. At this level visual and auditory disturbances may be present and should be assessed. Cardiovascular tests such as an echocardiogram and thyroid TSH levels should also be tested. Levels of cobalt above 20 mg/L should lead the patient to consider revision surgery as well as receiving a follow-up metal test every three months.

Conclusions

Restaino’s article concludes that while some lab studies have demonstrated significant reduction in wear rates for metal-on-metal implants, the benefits in terms of reducing the necessity of revision surgery has not been conclusively demonstrated. The hip implant differs from other static metal implants in that the ball and joint continuously wear against one another, meaning metal ions are released into the surrounding tissue and bloodstream. Excess levels of cobalt in the bloodstream can interfere with a variety of DNA processes, including inhibiting DNA repair enzymes. It seems clear that the recall of the DePuy ASR hip replacement device was warranted and that further research must be done regarding the long-term effects of metal-on-metal hip implants. Those recipients of the DePuy metal-on-metal hip implant should talk to their physician regarding the future of their health and to a knowledgeable personal injury attorney regarding potential compensation for the failure of the DePuy ASR hip implant.