Wednesday, August 29, 2012

The Science behind the DePuy ASR Metal-on-Metal Hip Implant Problems


Hip implant devices are made from several different materials including metal-on-metal, metal-on-polyethylene, metal-on-ceramic as well as those which are made entirely of ceramic or polyethylene. While each type of hip implant has its pros and cons, the metal-on-metal implant was developed as a way of sparing implant recipients the necessity of a revision surgery eight, ten or twelve years down the line. The metal-on-metal implant will typically last from fifteen to twenty years meaning those recipients in their sixties might be spared a revision surgery. As far back as 1937, very crude stainless steel implants attached with bolts and screws was tried then in 1955 a more sophisticated version of the metal-on-metal hip implant was used. The FDA did not actually approve the metal-on-metal implant until 1999 and since that time almost 17,000 problems with the all-metal hip implants have been reported to the FDA, with 12,000 of those reports coming in during 2011 alone.  

Failure Rate of the Metal-on-Metal Implant

Of the estimated 400,000 hip replacements done in the U.S. each year, nearly a third involved a metal-on-metal device prior to the recall of the ASR in 2010. While some reports say the overall problems reported with metal-on-metal implants are roughly the same as those seen in polyethylene and ceramic implants, there are very specific risks caused by the metal itself.  Other research points to compelling evidence that the metal-on-metal hip implants—aside from concerns about metal poisoning—simply fail at a higher rate. While DePuy originally placed their ASR hip implant’s failure rate at 5%, then upgraded that number to 12% when the device was recalled, independent studies show the failure rate to be a staggering 50% at six years and 80% at eight years.

Even though the ceramic and polyethylene may only have a life of 8-12 years, their failure rates are between 3-5%--a number which is considered acceptable for these devices. The metal used in the all-metal hip implants is a blend of cobalt and chromium and it has been found that during normal activity levels the ball and cup will rub against one another causing microscopic metal ions to shear off from the implant. These metal shards may become lodged in the surrounding tissues, causing the tissue to die or become severely infected or can enter into the bloodstream leading to metal toxicity and a whole host of physical problems.  Metal poisoning can be a serious health issue for those with all-metal hip implants causing minor to severe effects. In truth, the long-term ill-effects from all-metal hip implants is not yet known and research is ongoing.

Why Patients Undergo Hip Replacement Surgery

A recent article by John M. Restaino titled The Science, Medicine, Epidemiology and Liability Analysis of the DePuy ASR Metal-on-Metal Prosthetic Implants, discusses in-depth the problems with the DePuy ASR all-metal hip implant. Restaino discusses the typical reasons people decide to have a device implanted into their hip joint. Possibly the number one reason is chronic and severe pain; those people who suffer pain on a constant level are simply unable to enjoy their activities—both the “fun” ones as well as the day-to-day activities we all engage in. Our aging population has caused a surge in hip replacement surgeries as people live longer, more active lives. Degenerative hip disease or chronic pain can put an end to physical activities therefore in the past decade alone the number of hip replacement surgeries performed has more than doubled. Osteoarthritis of the hip is generally seen in aging patients or those who have suffered a trauma to the hip region. In some cases the hip tissue can begin to die, most often from a severe fracture, alcoholism, steroidal drugs such as prednisone or auto-immune diseases like lupus. Patients may begin to suffer worsening pain which will eventually prohibit them from walking, climbing stairs or even standing from a sitting position.


Infection and Inflammation are Common

Although the chromium and cobalt hip replacement joint may last longer, the potential for the patient getting metal ions in the bloodstream and in the tissue surrounding the implant are considerable. Those patients who currently have an all-metal implant have legitimate concerns regarding the safety of the implant and the elevated levels of cobalt and chromium in their bloodstream. In many cases these tiny metal shards will lodge into the tissue surrounding the hip joint, causing severe inflammation, infection or even the death of the tissue—known as necrosis. Whether these reactions are due to a hypersensitive reaction by the patient or simply the toxicity of the metal to the body’s cells is not clear, but because of the microscopic size of the particles of cobalt and chromium, they have the potential to spread widely throughout the body.

Studies on Metal-on-Metal Implants

As early as the 1970’s there were all-metal hip implants used in hip replacement patients. These prostheses were known as McKee-Farrarr and were compared all-metal implants placed in patients within the last two and a half years. In both types of implants cobalt and chromium particles were seen to cause serious tissue reactions in the patients including necrobiosis and necrosis. Necrosis is the death of tissue from lack of blood flow, radiation, chemicals, injury or other toxins. Necrobiosis is similar to necrosis however it occurs when cells or tissues die naturally. The older models of the all-metal hip implants as well as the newer models both showed significant amounts of heavy metal leeching into the tissues surrounding the hip joint as well as higher than normal levels in the bloodstream.

In another study concentrations of chromium and cobalt were measured in those patients whose all-metal implant had failed. As compared to patients with no metal implant the concentrations were extremely elevated with a reduction in white blood cells as well as the number of lymphocytes and the body’s natural killer cells. The researcher’s hypothesis was that the heavy metals of the all-metal hip implant were toxic to the lymph tissues which were closest to the device. DNA damage was also routinely seen in those whose hip implant was made from chromium and cobalt. Six cases of severe cobalt poisoning directly related to metal-on-metal hip implants were reported as well as hundreds of less severe but still quite serious side effects.

A 1996 study compared cobalt levels in those patients with all-metal hip implants to those with ceramic or polyethylene implants. The patients with the all-metal implants had detectable levels of cobalt in their bloodstream a year after the hip implant while those with other types of implants exhibited negligible levels of cobalt. Another 2006 study measured the levels of cobalt, chromium, titanium and aluminum in two groups of patients—one with metal-on-metal implants the other with ceramic hip implants. Those patients with all-metal implants showed significantly increased levels of chromium and cobalt—76% of them exhibited extremely high levels.

Normal Biological Functions of Cobalt and Chromium

Both cobalt and chromium are required for normal biological body functions however high concentrations can lead to toxicity and can alter DNA. Cobalt occurs throughout our environment as a natural earth element which is present in smaller amounts in soil, plant and in our everyday diets. Cobalt is also released into the environment from burning coal oil from car, truck and airplane exhausts as well as from industrial processes which use cobalt or its compounds. Just like cobalt, chromium is an essential nutrient and is required by the human body to maintain normal glucose levels. Most people’s diets contain sufficient levels of chromium.

Chromium and Cobalt Toxicity and Adverse Health Effects

Patients with higher levels of either of these heavy metals may not be experiencing any ill-effects from cobalt and chromium leeching into their bloodstream, however high levels can be very detrimental to health. DePuy originally claimed that an amount of either metal up to 7.0 mg/L were considered “normal” and were not a cause for concern. The Mayo Clinic believes that any amount greater than 1 mg/L for chromium in the bloodstream and any amount greater than 5.0 for cobalt in the bloodstream are a significant cause for alarm. Unfortunately, patients with ASR hip implants have been found to have cobalt and chromium levels as high as 20 mg/L. In one case a deceased patient with an ASR implant was found to have levels of 220 mg/L of these heavy metals in the bloodstream.

Cobalt can cause multiple organ damage and those who work in certain industrial applications have to guard against cobalt poisoning. Cobalt is used in the manufacturing of magnets and platinum jewelry as well as in the production of tires, petroleum, some types of sterilization of food and medical supplies in ceramic coloring pigments. Neurological damage as well as cardiovascular issues have been seen in those with high levels of cobalt in their blood; other symptoms of cobalt poisoning can include gastrointestinal disorders, excessive bleeding, disorders of the skin, kidney failure nerve damage and thyroid problems. Research has indicated that all metal hip implant patients whose device was not positioned correctly had the highest risk of cobalt poisoning as well as severe and chronic hip pain. Excessive levels of cobalt can even lead to blindness, deafness and convulsions. Patients with levels above 5 mg/L but less than 20 mg/L are toxic and require frequent testing of the metal levels in the body.

Chromium toxicity can lead to the mutation of DNA as well as problems with the reproductive system. Impaired liver functioning is often seen in those with elevated levels of chromium and in some cases the respiratory system can be damaged and there is the potential of an elevated risk of cancer. Excessive chromium levels can lead to severe inflammation around the hip implant and while every person will react in a different way to exposure to heavy metals the full effects can literally take years to manifest.

Other Issues Related to the All-Metal Hip Implant

Restaino’s article details how an increased understanding of the head radius, clearance and finish of the surface of the hip implants affect the wear of the device. The lubrication is also extremely important in the metal-on-metal implant’s resistance to undue wear. Metal-on-metal hip implants offer a reduced risk of dislocation largely due to the bigger size of the head. Younger patients in particular who lead very active lives find the all metal implant an attractive option. Corrosion of the all-metal implant appears to be tied to less-than ideal lubrication conditions and under these circumstances it is likely the release of heavy metals will increase.

Metallosis Issues

Restaino details the issue of metallosis among those with metal-on-metal hip implants and calls it the “macroscopic staining of the soft tissues…associated with abnormal wear.” Many patients with all-metal hip implants develop pseudotumors or masses which cause specific symptoms and can be solid, cystic or a combination of both. The number of patients who develop pseudotumors following a metal-on-metal hip implant is approximately 3.4% at 7 years following the initial hip implant surgery. Among those patients who underwent revision surgery due largely to unexplained and chronic pain, both joint effusions and granulomas were commonly seen. In 18% of the hip replacement patients bone impingement was seen, with femoral failure in 14%, loosening of the socket in 4.5%, necrosis with associated fracture in 4.5%, cup malposition in 4.5% and post op pain in 4.5%. Fifty percent of all patients who underwent revision surgery suffered adverse reactions to the cobalt and chromium ions. 

Conclusions

The conclusion of Restaino’s article notes that while some lab studies note a reduction in the wear rates for all-metal implants the benefits as far as reducing the necessity of revision surgery are not proven. In many all-metal implants the ball and joint can rub against each other causing serious wear and releasing metal ions into the tissues and bloodstream. Excessive levels of these metal ions can cause a variety of negative health effects as well as inhibit DNA repair enzymes. Clearly the DePuy ASR hip implant recall was necessary and those patients with this implant must work closely with their doctor in order to ensure they receive frequent testing for metal toxicity.
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Author: Andrew Sullo
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Andrew Sullo

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