Metal-on-metal hip implants can shed metal particles into the body, causing metallosis or metal poisoning. Symptoms include bone and tissue death, implant failure and severe pain.
Metal poisoning occurs when toxic levels of metal build up in the body. This can cause damage to tissue, bone and the nervous system. People can be exposed to high levels of metal through their environment and diet. For example, metals like arsenic, mercury, lead and chromium have many industrial and agricultural uses and find their way into a variety of products including paint and pesticides. They may also contaminate water supplies, and one of the biggest dietary sources that may contribute to mercury poisoning is fish.
Unapproved or counterfeit medications and natural remedies may also contain aluminum, mercury, lead or even uranium. The U.S. Food and Drug Administration (FDA) warns against using drugs obtained from questionable online pharmacies. These metals may cause stomach upset, kidney toxicity and multi-organ toxicity.
Medical devices made from metal can also cause metal poisoning. Metallosis is a type of metal poisoning that can occur as a side effect from certain joint replacement devices, such as metal-on-metal (MoM) hip implants or some knee implants. It can lead to several complications, including: implant failure, tissue and bone death, and organ damage that may require revision surgery to treat.
MoM hip implants like the DePuy ASR typically use a metal ball and cup, and some like the Stryker Rejuvenate and ABG II use a metal neck and stem. All of these components are made of cobalt and chromium. Both total hip replacement and hip resurfacing systems can cause metallosis, though the rate is higher with resurfacing components.
Cobalt, Chromium & Hip Implants
Naturally, the body stores a certain level of cobalt and chromium for healthy cellular function. But, when metal implant components rub together, they release cobalt and chromium ions into the surrounding tissues and into the bloodstream and can create toxic levels in the body, leading to metallosis.
When excessive metal builds up near the joint, the lymph nodes and surrounding joint fluid attempt to “clean” the body by absorbing the ions and transporting them to the kidneys, where they are removed from the body in the urine. However, sometimes the body is overwhelmed by the amount of metal, and this can cause a number of complications.
Because cobalt-chromium (CoCr) implants are the most corrosion-resistant, doctors began using them widely in the 1950s. The first published reports of metallosis surfaced in the 1970s. Hip patients presented with pain and instability in the joint less than five years after hip replacement surgery – some as early as nine months after receiving the implant. Doctors found the tissue surrounding the implant was stained green or grey and surrounded with a paste-like material. Joint fluid tests revealed high levels of cobalt.
Decades-old studies revealed that these prostheses released cobalt and chromium ions into tissues, joint fluid, blood and urine, but it wasn’t until recently that the problems became more widespread because of an increase in MoM implant use.
The studies did not deter manufacturers from making these implants, and in the early 2000s MoM total hip replacement and hip resurfacing implants grew in popularity. Manufacturers claimed that the newer metal-on-metal hips were designed with larger articulation surfaces to cut down on dislocation, and that metal implants lasted longer. Studies showed they are more durable, but these new prostheses also have a larger surface, creating more metal debris, and have a higher failure rate. Premature failure from metallosis requires revision surgery to correct.
Those injured by these implants filed lawsuits against manufacturers including DePuy, Stryker, Smith & Nephew and Biomet for failing to warn them about metallosis and for the defective metal-on-metal design.
Symptoms of Metallosis
Because each person can react differently to high levels of cobalt, the symptoms of metallosis may vary. One of the most common is tissue or bone death – called necrosis.
With tissue or bone necrosis, the tissue around the implant turns gray or black from being exposed to the metal debris flaking off the implant. This creates pain and instability, which can lead to spontaneous dislocation, bone loss and fracture.
On occasion, non-cancerous pockets of fluid called pseudotumors will form around the affected tissue. The fluid from these pockets or effusions contains gray, rust or cloudy yellow fluid. In addition to tissue reactions around the joint, high levels of cobalt in the blood can also cause other symptoms.
Metallosis also affects the nervous system, skin and other organs, and can lead to the following symptoms:
- Cardiomyopathy (heart problems), including heart failure
- Visual impairment that may lead to blindness
- Cognitive impairment
- Nerve problems
- Thyroid problems
- Auditory impairment that may lead to deafness
- Skin rashes
- Noise coming from the hip
- Implant loosening
One orthopaedic doctor, Stephen S. Tower, wrote about two cases of metallosis in the 2010 State of Alaska Epidemiology Bulletin. Both patients were fit, healthy 49-year-old men who received MoM implants.
|Within a year of receiving the implants, both men suffered from:|
|Skin rashes||Poor memory and mental fogginess|
Despite normal kidney function, both men had high levels of cobalt in the blood that led to these symptoms. Both patients had revision surgery, which revealed tissue necrosis around the implants.
The symptoms of metallosis correspond to the level of metal ions in the blood and become more severe as the levels rise.
|Symptoms of Metallosis Specific to Serum Cobalt Levels from Study Results|
|Cobalt level in micrograms per liter of blood (>µg/L)||Possible symptoms|
|1 – 5||Heart and memory problems|
|> 7||Hip pain, tissue necrosis, pseudotumors|
|23||Hip pain, mental problems (concentration, memory), vertigo, some deafness|
|> 66||Blindness, hip pain, deafness, seizures, tremor, heart failure, goiter, rashes, abnormal blood pressure, depression, weakness|
Metallosis and Cancer
While a metallosis link to cancer has not been proven, the World Health Organization (WHO) International Agency for Research on Cancer lists one type of chromium — trivalent — and cobalt as potential carcinogens. After thousands of metal-on-metal implants failed, the London Implant Retrieval Centre released test results confirming that MoM implants release trivalent chromium ions.
Doctors usually diagnose metallosis by doing a blood test in patients who complain of symptoms, though the early stages of metallosis can be present without symptoms. In fact, elevated levels of cobalt ions are present in the blood, joint fluid, hair and urine of anyone with an MoM implant even if there are no symptoms.
The U.S. Food and Drug Administration (FDA) has yet to recommend a specific safe level of metal ions in the blood because there are few studies. However, the normal measurement for cobalt in joint fluid (serum) is typically less than 2 micrograms per liter (µg/L).
A 2012 report in the British Medical Journal stated that the Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area (MAK Commission) limit for cobalt in the blood is 5 µg/L. Patients with some metal-on-metal hip implants showed levels above 5 µg/L only a few months after implantation. The two severely poisoned patients from Dr. Tower’s Alaskan study had levels at 122 µg/L and 23 µg/L.
In addition to blood and joint fluid tests, imaging tests like X-rays and magnetic resonance imaging (MRI) may also prove useful in finding tissue damage before pain sets in. Dr. Hollis Potter of the Hospital for Special Surgery (HSS) in New York City advocates using an MRI to detect tissue damage early and minimize complications during revision surgery.
|According to the FDA, some patients are more at risk of suffering adverse events from metal ions, including those:|
|Who are female||With bilateral implants|
|With resurfacing systems with small femoral heads (44mm or smaller)||Who receive high doses of corticosteroids|
|With evidence of renal insufficiency||With suppressed immune systems|
|With suboptimal alignment of device components||With suspected metal sensitivity (e.g. cobalt, chromium, nickel)|
|Who are severely overweight||With high levels of physical activity|
Aside from revision surgery, there are few treatments for metallosis. There have been some studies on chelation therapy to remove excess metal from the body. Chelating agents are administered orally, intravenously or intramuscularly and bind metal ions and remove them from the body. Chelation therapy is used to treat lead, arsenic, iron and mercury poisoning.
However, excess cobalt cannot be chelated from the joint or fluid, so this type of treatment does not work well with metallosis. The only proven treatment is revision surgery, and earlier diagnosis leads to a better surgical outcome. Dr. Tower’s Alaskan patients with severe metallosis had surgery to remove the metal implants, and both patients showed improvement in their symptoms.
With severe metallosis, the amount of tissue and bone necrosis determines the outcome of the surgery. Some patients suffer fractures during revision surgery because of weak bones and extensive tissue damage, making recovery more difficult. During revision surgery, surgeons replace the metal-on-metal implant with a ceramic-on-metal or plastic-on-metal implant to minimize future problems with metal ions.
Metallosis and the FDA
The FDA refers to metallosis as “adverse local tissue reaction (ALTR)” or “adverse reaction to metal debris (ARMD).” According to the FDA, metal-on-metal implants pose more risks than other hip implants because of the metal ions that enter the tissues.
The agency does warn that these ions can cause implant loosening, pain, and illness or damage to other parts of the body. The agency recommends patients with MoM implants follow up with their doctor immediately if they have symptoms. It also recommends that patients with extreme symptoms be considered for earlier revision surgery to minimize tissue, muscle and bone damage.
Patients without symptoms are encouraged to see their doctor every one to two years. The FDA does not recommend routine blood tests to check metal levels.
The FDA has yet to determine the extent of the damage caused by metal ions. Recently, the agency ordered makers of metal-on-metal hip implants to conduct more studies on product safety and effectiveness.