Attachment of Dissimilar Metals in Orthopedic Implants

Metals as implant materials are used commonly in orthopedics and trauma surgery. Implants mainly used in recent days are pins, screws, plates, or total joint replacements. Common materials used in these implants include stainless steel, titanium alloys, and cobalt-chromium alloys. Better mechanical properties, biocompatibility, and corrosion resistance of biomaterials like stainless steel and titanium alloys are a great advantage for them to be commonly used in orthopedics.

What Does Attachment of Dissimilar Metals Mean in Orthopedics?

Two different types of metals when used in the same environment are called combining of different metals or attachment of dissimilar metals. Eg. titanium alloy femoral stem in combination with cobalt alloy or stainless steel prosthesis head.

Can Dissimilar Metals Be Combined in Orthopedics?

The common rule is that dissimilar metals should not be combined in a biologically active environment. Combining dissimilar metals may result in the corrosion process of the metals. Combining stainless steel alloys with titanium or cobalt-chromium alloys must be avoided for implants that are in contact with each other. However, attachment of dissimilar metals may improve joint stabilization in some cases.

What Are the Applications of Stainless Steel and Titanium Alloys in Orthopedic Implants?

Stainless steel (SS) alloys are commonly used in orthopedics and the introduction of stainless steel in orthopedics heralded a revolutionary development in fixation and implants. The advantages of stainless steel include:

• Cost-effectiveness.

• Has better torsional properties.

• More prone to bending and contouring.

• Stainless steel screws are easier to handle and better at preventing over-twisting.

Titanium (Ti) is widely used in orthopedic surgeries due to its biocompatibility and mechanical properties. When compared to other alloys, titanium has lower toxicity. When compared with stainless steel Ti has the following advantages:

• Ti is more corrosion-resistant than SS.

• Ti has a high tolerance for fatigue and stress loading.

• The titanium surface is rough and enables direct osteointegration.

What Happens When Dissimilar Metals Are Combined in Orthopedic Implants?

When two dissimilar metals are combined in orthopedic implants a process called corrosion may occur. Corrosion may be in the form of an electrochemical or mechanical process.

• Mechanical Corrosion is due to the continuous load in certain areas caused by mechanical forces.

• Electrochemical Corrosion is of three types:

  • Crevice corrosion occurs in focal areas where there is high electrolyte concentration and less oxygen. It is common in screws and plates.

  • Pitting corrosion is due to damage in the protective coating of the implants.

  • Galvanic corrosion when two different metals are in contact with each other in the presence of an electrolyte, in this case, the body fluids act as an electrolyte. The two metals can create a small electrical current, with the more reactive metal (the anode) corroding faster than the less reactive metal (the cathode). This can cause the more reactive metal to break down and release metal ions into the surrounding tissue, which can cause pain, inflammation, and even implant failure. Galvanic corrosion is a common concern while dissimilar metals are attached.

What Are the Complications Due to Combining Different Metals?

The main reason for metal implants to fail is corrosion and mechanical degradation. Complications that arise due to corrosion may lead to adverse effects in the local tissue and damage to the implants.

• Loosening of Implants - The corrosion process can result in the release of metal ions. These metal ions weaken the metallic properties of the implant resulting in aseptic loosening of the implant and destabilization.

• Periprosthetic Bone Loss - Metal ions from the corrosion products may cause the binding of proteins to complexes which activate the immune system. This can cause activation of osteoclasts and result in bone loss surrounding the implants.

• Metallosis - Metallosis denotes the abnormal accumulation of metal debris in the periprosthetic soft tissue and the nearby bones. This may lead to tissue damage and bone loss.

• Systemic Complications - The metal ions released from the implants combine with the proteins forming complexes. These complexes may be transported to various organs through blood flow and may cause serious complications. Metal ions may cause toxicity in various body systems.

• Local Tissue Reactions and Allergies - Metallic ions from corrosion may cause reactions like urticaria and dermatitis in the local tissues around the implants. In some cases, pseudotumors (soft tissue masses) may be seen near the prosthetic surfaces due to the inflammatory reaction caused by the metallic debris. Nickel ions in stainless steel alloys may also cause allergies in some people.

• Pain and Swelling - The accumulation of metallic debris may trigger the inflammatory cells which results in pain and swelling in the area surrounding the implants.

• Infection - Local inflammation and other adverse reactions may increase the chances of late infections in the implant site.

What Can Be Done to Reduce the Effects of Using Dissimilar Metals?

The corrosive effects of combining dissimilar metals are higher in joint replacements than in fracture stabilization implants. Removal of the implants is advised after the bone heals in case of fracture. Some things which can be done to prevent corrosion while using a combination of dissimilar metals include the following.

• Using Materials With Similar Electrochemical Properties: When selecting materials for implants, doctors aim to use materials with similar electrochemical properties to reduce galvanic corrosion. Eg. cobalt-chromium alloys and titanium alloys can be used together since they have similar electrochemical properties.

• Coatings: Coating metal with another material like hydroxyapatite or diamond-like carbon can help in the isolation of dissimilar metals and prevent them from coming into contact.

• Surface Treatments: Surface treatments like passivation (chemical treatment of metal surfaces) or electropolishing can be done to improve the corrosion resistance of the implant.

• Avoiding Direct Contact Between Dissimilar Metals: This can be done by using plastic or insulating materials to separate them.

• Regular Monitoring and Maintenance: Even with preventive measures, corrosion can still occur. Therefore regular monitoring and maintenance of the implants are necessary to ensure they remain stable and function correctly.

Conclusion

The attachment of dissimilar metals in orthopedic prostheses and implants can increase the risk of corrosion and implant failure. However, the risks may be minimized through careful selection of implant material and designs. It is important for orthopedic surgeons to carefully consider the potential risks and benefits of dissimilar metal attachments when choosing implant materials for their patients.