Knee Replacement and Metal Allergies

Knee replacement failure due to metal allergy or sensitivity is very rare but possible. A reaction to a knee replacement can appear months or even years after the initial surgery. Its symptoms—itching, knee pain, skin rashes, joint stiffness, and swelling—can also have other causes, which makes this difficult to diagnose.

Knee replacement metal allergies and sensitivities are more common with older, metal-on-metal joints. Prosthetic joints containing metals nickel, chromium, and cobalt are most likely to cause a reaction. Hypoallergenic knee replacement materials are available.

People with known metal allergies or sensitivities should tell their surgeon prior to knee replacement surgery. 

This article discusses knee replacement metal allergies and sensitivities. It explores the causes and symptoms of a reaction and what to do if you suspect one.

Reactions to Metal in Knee Replacements

Total knee arthroplasty (TKA) is generally a very successful surgery, and the vast majority of people who undergo it are satisfied with the results. However, around 10% to 15% are not satisfied for various reasons, including persistent pain and the loss of joint flexibility.

Some experts suspect a metal sensitivity or allergy might explain some of these post-operative complaints, particularly if the symptoms develop long after the implant is placed.

A 2021 study found a link between failed joint replacements and metal hypersensitivity. Using skin patch tests and blood work, researchers identified metal sensitivity in nearly one-third of the study participants.

Reactions to metals in knee replacements happen when the immune system overreacts or launches a defensive assault on the artificial joint and surrounding tissues, triggering local inflammation.

Although inflammation is the body's natural response to anything it considers foreign and harmful, it can cause pain and other symptoms if it occurs in the absence of a genuine threat.

Metal Allergy vs. Sensitivity

Metal allergy and metal sensitivity are often used interchangeably in everyday conversations. Throughout this article, we do the same.

However, while both are in the "allergy family," there is an important difference worth clarifying.

Most reactions to knee replacements are hypersensitive reactions. These are not classic allergic responses in that they do not have the potential to cause body-wide symptoms including anaphylaxis, a severe and potentially life-threatening allergic reaction.

Instead, metal hypersensitivities typically present as contact dermatitis. The skin (or other tissue) that comes in contact with the offending metal becomes red, itchy, and irritated. This can cause mild to significant local discomfort.

Metal hypersensitivity can begin at any time, even if you have never had a reaction before. The immune response associated with sensitization occurs from prolonged exposure.

Metal allergies and sensitivities can be assessed using skin patch testing. A blood test known as a nickel lymphocyte proliferation test is used to identify nickel sensitivity.

People with suspected metal allergies or sensitivities should undergo testing prior to knee replacement surgery. However, pre-surgical allergy testing cannot completely rule out a potential reaction.

Metal Allergy/Sensitivity Symptoms

Symptoms of an allergy or sensitivity to metal in a knee replacement include:

• Chronic (persistent) knee pain
• Chronic or intermittent knee redness, warmth, and swelling
• Knee stiffness and reduced range of motion
• Itchy, scaly skin patches around the affected joint

Symptoms of a metal allergic reaction do not typically appear for months or years after a knee replacement.

Why Is a Reaction to Knee Replacement Delayed?

Metal sensitivity is not an uncommon condition. Around 15% of the general population is known to have nickel sensitivity, while 2% or more have reactions to cobalt or chromium.

With prosthetic knees, stainless steel used to build the appliance contains around 10% nickel. Cobalt and chromium may be found not only in the appliance but also in bone cements used to set the prosthetic.

Metal allergies are referred to as delayed-type hypersensitivity (DHT). The delayed hypersensitivity occurs due to the natural corrosion of metals over time.

Any metal that comes into contact with body tissues will corrode to some degree. This corrosion causes the formation of metal ions that ultimately trigger the inflammatory immune response.

Because of this, the symptoms of DHT can get progressively worse as more and more metal ions are released due to corrosion.

Diagnosis

The diagnosis of DHT can be difficult, in part because a hypersensitive reaction on the skin doesn't always cause a hypersensitive reaction on internal tissues, or vice versa. So, skin testing for nickel, cobalt, and chromium hypersensitivity has limited value in diagnosing DHT.

DHT is typically diagnosed by excluding all other causes of post-operative knee pain, such as:

• Wear, damage, or loosening of prosthetic joint components (which can be detected with an X-ray or MRI)
• Bacterial infection and joint sepsis (which can be detected with blood tests and joint aspiration)
• Autoimmune conditions like gout or rheumatoid arthritis (which can also be detected with blood tests and joint aspiration)

It is only after these causes are excluded that metal sensitivity is explored. Joint aspiration (in which fluids are extracted from the joint compartment) can be used to obtain and evaluate the composition of immune cells in the knee fluid sample.

Based on the level and proportion of immune cells in the sample—including leukocytes, eosinophils, and basophils—the healthcare provider can diagnose DHT with a relatively high degree of confidence.

Treatment Options

The challenge with metal hypersensitivity is that the diagnosis is not always 100% certain. This is why it is important to exclude all other causes of joint inflammation and pain before exploring the next step in treatment: revision surgery.

If surgery is needed, the implant would be removed and replaced with a special implant made of ceramic or titanium.

If you have a known hypersensitivity to metal and are scheduled to undergo a total knee replacement, it is not clear if starting with a ceramic or titanium appliance is the best option. It requires a case-by-case evaluation weighing the potential benefits and risks.