Concerns with metal-on-metal hip implants
Additional Risks Associated With Metal-on-Metal
Hip replacement failure can be an unpleasant experience no matter what your artificial hip is made from. Metal-on-metal hip implants carry with them additional risks on top of those generally associated with hip replacement surgery. Scientists, doctors, and manufacturers have all known of the additional risks metal-on-metal hips carry since the 1970s. A 1975 study revealed that in active patients, the friction produced by the metal ball rubbing against metal socket caused small particles of metal to flake off into the bloodstream. The study also showed that metal particles in the bloodstream caused pain, metal poisoning (metallosis), and soft tissue death/decay (necrosis) in patients with metal-on-metal wear.
The United States was late to the party on making public the concerns with metal-on-metal hip implants. In April 2010 the United Kingdom Medicines and Healthcare products Regulatory Agency (MHRA) issued a medical device alert directing patients to follow up with their surgeons for blood tests and imaging. It wasn’t until February 2011, that the FDA issued their initial public safety communication about metal-on-metal hip devices.
Other safety communications soon followed both in the United States and abroad
On May 6, 2011, the Food and Drug Administration (FDA) issued a postmarket surveillance study monitoring adverse events associated with metal-on-metal hips after they had been “approved”.
In May 2012, Health Canada issued their own communication to orthopaedic surgeons and patients regarding the dangers associated with metal-on-metal hips and necessary precautions. Australia followed suit in September of 2012.
On January 17, 2013 the FDA issued a public health communication about the potential dangers of hip replacements consisting of a metal ball and socket.
What are the injuries associated with metal-on-metal implants?
Metallosis, Bone and Tissue Damage, and Pseudotumors
Metal Poisoning (Metallosis)
One way in which hip replacement failure occurs is through metallosis. Because metal-on-metal hip implants are made of metal materials, metallic ions and particles, often from the cobalt-chromium components, can cause major medical issues for the patient. The shedding of cobalt and chromium in the bloodstream is relatively harmless to some, but extremely harmful to others. Even if you have metal poisoning (metallosis), it is not always painful, so you may not immediately feel it or know that it is happening. This is why it is important to ask your doctor to test your blood for cobalt and chromium levels if you have a metal-on-metal hip implant.
Damage of the Bone and Soft Tissue
Metal particles contaminate the tissue and bone surrounding the hip implant. In some individuals, their body has a negative reaction to the metal particles known as adverse local tissue reaction (ALTR). When this happens the bone and soft tissue become damaged leading to other problems like implant loosening, device failure, and the need for a revision surgery. In a revision surgery, the surgeon replaces the existing hip implant with a new one. Revision surgery is a gruesome process where the device is literally chiseled and hammered out of the surrounding bone and soft tissue. Often surgeons open the patient’s hip to find that the surrounding soft tissue and/or bone has died and turned black due to necrosis. Necrosis is defined as the premature death of cells in an organ or tissue due to disease, injury, or poor blood supply to the organ. One can only imagine the physical therapy and rehabilitation it would take to ever stand or walk again after needing to replace not only the hip joint, but also the bone and tissue surrounding it.
When soft tissue surrounding the hip implant reacts adversely legions called pseudotumors can begin to form, but they are not always noticeable to the patient. A study on the prevalence of pseudotumors in metal-on-metal hip recipients published in The Journal of Arthroplasty showed that, “68.6% [of patients evaluated] developed pseudotumor with 60.9% of the asymptomatic group developing pseudotumor.” The study also showed that elevated cobalt levels in the blood stream were associated with larger pseudotumors.
How are metal-on-metal hips failing?
Taper Lock Failure
The advantage of modular metal-on-metal hip implants is that they are customizable. Because there are a variety of neck sizes and angles, the Surgeon has the flexibility to implant a neck that is the best fit for the patient. The disadvantage of modular metal-on-metal hip implants is that there is now one more place for metal to rub against metal, leading to corrosion of the metal components. Corrosion of this neck/stem juncture causes further release of metallic particles, negative reactions with the tissue, and often ultimately requires revision surgery. A May 2016 study in the Journal of Arthroplasty recently concluded that the high prevalence of severe taper corrosion may be related to elevated cobalt and chromium levels in the patient before revision surgery takes place.
Dissociation of the hip implant occurs when the components fall apart or disconnect. This type of complication occurs most in hip implants with large modular femoral heads. A large femoral head can place a large amount of stress on the taper-neck junction where the ball connects to the neck of the femoral stem. As a result, fretting and corrosion of the taper become more likely due to the design of the implant. If there is too much fretting and corrosion between the head and taper, the system falls apart, setting the patient up for a less than pleasant experience.
Acetabular Component Failure
An artificial acetabular system includes the cup, sometimes liner and ball of the artificial hip joint. The acetabular components fail for similar reasons as the taper lock. When adjacent pieces of the acetabular system are metal, the metals can rub together with movement, resulting in metal wear, corrosion, and metallosis. This metal on metal wear can damage the tissue and muscle around the hip device, causing metal to be released into the body, including the bloodstream, and can cause loosening, or device repositioning, severe pain and often a required removal surgery. This can also lead to a painful dislocation of the hip from its socket which is an experience most people would not wish on their worst enemy.