Allergic reactions come in a variety of types, based on the body's response to the allergic material.

• The most common human allergic reaction is a "Type I" reaction triggered by immunoglobulin E (IgE), one of the antibodies produced by the body’s immune system. This reaction is seen with seasonal and food allergies, asthma, and latex allergies.
• Most human allergies to metals are delayed "Type IV" reactions and are related not to antibodies, but to a specific type of reaction involving white blood cells called T cells. This type of reaction can cause local skin irritation, or dermatitis, around the area where an implant is located, along with swelling or persistent pain. It can appear as redness or a rash, with or without itching.

Human allergies to metals are extremely rare and are a diagnosis of exclusion — meaning, all other potential causes of the patient's symptoms have been ruled out.

Proving that these delayed Type IV reactions are directly related to metal implant device failure can be very difficult, especially since these reactions are generally uncommon and, as mentioned above, are rare complications of shoulder, hip, or knee replacement implant devices.

Laboratory Tests

Orthopaedic surgeons always want to rule out infection as a cause of the patient's symptoms. For that reason, they may order blood work to evaluate:

• White blood cell counts
• Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), inflammatory markers that show whether there is inflammation in the body

If the surgeon still suspects a metal allergy, they may request additional testing from a physician who specializes in allergy & immunology.

Obtaining blood tests for cobalt and chromium metal ion levels is an effective way to determine the blood levels of these metals. Normal values are typically less than 1 part per billion (or ng/mL), and concern arises if the levels are found to be higher than this normal baseline value.

Skin Patch Tests

Skin patch testing. 

Courtesy, Getty Images

Allergists may use a "skin patch test" to better identify whether a patient is allergic to a variety of materials, including cobalt, chromium, or nickel, all of which are found in the cobalt-chrome metal alloys used in joint replacement. Few patients are allergic to titanium, which is perhaps the most commonly used metal for hip replacement implants.

However, while skin patch testing is an effective way to test for and confirm allergies on the skin, it may not be as reliable for predicting metal implant reactions.

One study where metal devices were implanted in patients with known positive metal skin patch testing showed no dermatologic or joint-related symptoms at an average of 6.3 years after surgery. One possible reason is that the skin has its own exclusive white cells — and those cells process antigens differently than the white cells found in deep tissues.

Lymphocyte Transformation Test (LTT)

Another test for delayed allergy is the lymphocyte transformation test (LTT). This test involves taking a blood sample from the patient and isolating one type of white blood cell. The collected white cells are then put into an incubator with an allergen. If a patient is allergic to that allergen, some of their white cells will react, indicating a Type IV hypersensitivity and confirming the allergy.

Unfortunately, the LTT test has similarly failed to show any ability to predict either post-surgery pain or device failure with metal implants.

As a result of the limitations of skin patch testing and LTT testing, no single test is widely accepted or recommended for diagnosing metal implant hypersensitivity.

Importantly, patients with a known, pre-existing allergy to cobalt, chromium, or nickel should notify their surgeons before their joint replacement, so the surgeon can select and plan for the use of alternative implant materials.

This includes telling your surgeon if you have had reactions to metals from earrings, necklaces, bracelets, or rings in the past, which can indicate a sensitivity to metal alloys. Most patients with skin allergies to costume jewelry do not react to pure metals such as silver, gold, or platinum.

Biologic response to metals remained an uncommon topic for orthopedic research on device failure until the widespread use of metal-on-metal hip replacements.

The first uses of metal-on-metal hip replacements came with the introduction of the McKee-Farrar and Ring hip replacement implants in the United Kingdom in the 1960s. The bearing — ball and socket — materials in the first era of hip replacements had a very high failure rate, so, surgeons developed metal-on-metal bearing surfaces, which were felt to be more durable.

Ultimately, with the adoption of conventional polyethylene plastic socket components for total hip replacement, the most commonly used bearing became a metal cobalt-chrome ball and a polyethylene socket. This became known as metal-on-poly and was the most widely used bearing for the next four decades.

However, the metal-on-poly bearings had their limitations. Wear and failure of the socket would typically occur within 10 to 15 years, causing bone cysts (osteolysis) and loosening of the implants. As a result, in the early 2000s, metal-on-metal implants came back into use as a popular option made by implant manufacturers.

Metal-on-metal implant use surged among surgeons looking to reduce both the risk of plastic failures and the risk of hip dislocations. The smaller heads on the metal-on-plastic implants were more likely to pop out of the socket, or dislocate. The metal-on-metal hip components were available in larger head sizes than typical metal-on-plastic bearings of that era.

An example of a hip implant that has dislocated. 

Image courtesy of Stuart J. Fischer, MD, FAAOS

More than 1 million of these metal-on-metal devices were implanted worldwide. But concerns were raised when registry data showed that revision rates — the need to redo the procedure due to implant failure — were 2 to 3 times higher than the revision rates for metal-on-plastic total hip replacement.

The culprit behind these high failure rates is metal debris, in the form of metal ions, resulting from implant wear. This metal debris can initiate a strong type IV immune response that leads to a growth of soft tissue and swelling around the hip called a pseudotumor. When this happens, it is known as an "adverse local tissue reaction."

Because of concerns about metal-on-metal hips, a number of metal-on-metal hip devices were recalled. In 2011, the U.S. Food and Drug Administration (FDA) issued a requirement that all metal-on-metal hips currently marketed in the U.S. either be subject to post-market surveillance studies or be removed from the market altogether.

As a result of this action by the FDA, almost all metal-on-metal implants were withdrawn from the market. Only one metal-on-metal implant, used for hip resurfacing arthroplasty, remained on the hip implant marketplace.

(Left) In this X-ray of a hip resurfacing taken from the front, a metal cap now covers the femoral head and a metal socket is in place. Most total hip replacement procedures (Right) no longer involve metal-on-metal bearings.

Since metal-on-metal total hips are no longer marketed in the U.S. the issues with metal-on-metal devices have become much less common. Patients who may still have a metal-on-metal total hip replacement or resurfacing implant are advised to visit their surgeon for routine follow-up every 3 to 5 years for examination, X-rays, and metal ion blood testing to ensure they are not having any signs or symptoms of metal-on-metal implant failure or a bad reaction to metal implant debris.

For patients who may have a failing or failed metal-on-metal total hip implant, it is important to know that the cobalt and chromium metal ions can cross into the bloodstream and cause body-wide effects, including but not limited to:

• Sensory changes (e.g., unusual reactions to sights, sounds, smells, tastes, and textures)
• Tingling of the hands or feet
• Ringing in the ears (tinnitus)
• A metallic taste in the mouth

The FDA has a page for patients with information about metal-on-metal implants, including possible metal ion-related complications. Use this site as a reference if you are concerned about your metal-on-metal implant. And talk to your doctor right away if you are experiencing any of the above symptoms.

Certain patients may have an increased risk of device wear or adverse local tissue reaction with metal-on-metal implants and should follow-up with their surgeon more frequently, including:

• Patients with bilateral implants (hip replacements on both the right and left sides)
• Patients with resurfacing systems with small femoral heads (device sizes less than or equal to 44 mm)
• Female patients
• Patients receiving high doses of corticosteroids
• Patients with evidence of renal insufficiency (kidney problems)
• Patients with suppressed immune systems
• Patients with device components that are not in the ideal positions
• Patients with suspected metal sensitivity (e.g. cobalt, chromium, nickel)
• Patients who are severely overweight
• Patients with high levels of physical activity

• For the most part, it appears that pre-existing metal allergies are an extremely rare cause of metal device failure, and establishing a cause-and-effect relationship remains difficult. Adverse (bad) reactions to metals remain a diagnosis of exclusion after ruling out other possible issues, such as infection or loosening.
• Post-operative metal reactions have been largely limited to metal-on-metal devices, especially metal-on-metal total hip replacements, where large volumes of metal wear could cause a distinctive adverse response — both locally (around the joint) and systemically (body-wide).
• Most implant manufacturers today avoid metal-on-metal bearings when designing implants.
• Measurement of serum metal ion levels and metal artifact reduction sequence (MARS) MRI are the best methods for detecting systemic (body-wide) reactions to metal debris. MARS MRI is a special type of MRI that can detect a pseudotumor.
• Patients with at-risk metal-on-metal devices who have few or mild symptoms can generally be followed with annual doctor’s visits, X-rays, and blood tests to measure serum metal ion levels.