Editorial Feature

Wear and Friction in Medical Grade Metals and Alloys

Tribology is the science of wear, friction and lubrication which has been utilized in biomedical applications.  Medical grade metals and alloys can provide internal support in the body and are employed as replacements for biological tissue. Common biomedical applications include use as replacement joints and dental roots, the formation of stents for the treatment of heart conditions and orthopedic fixation devices for repairing bone.  

Implant materials are generally comprised of surgical stainless steel, cobalt alloys or titanium alloys. The triblological events that occur at joint articulations in the body can produce corrosion in metal biomaterials because the implant metals are exposed to wear and friction which is enhanced by the effects of corrosive body fluids. The service period of metallic biomedical devices is therefore dependant on its abrasion and wear resistance.

Wear in Medical Grade Metals and Alloys

Wear and corrosion provide the primary reasons for implant failure. They can cause aseptic loosening where the bond between metal implant and bone fails and active resorption of the bone matrix through osteolysis. Furthermore, wear and friction of implants can produce metal ion release risking toxicity to cells. The different metallic alloys utilized for medical implants vary in terms of mechanical properties and wear resistance.

Surgical stainless steel has been found to produce pitting and stress corrosion when employed as an implant material. Cobalt alloys provide the highest wear resistance and are often used as a material for fabricating artificial hip joints which are frequently subjected to wear.  Both stainless steel and cobalt alloys display disadvantages such as a higher modulus of elasticity than bone, causing stress shielding in the form of reduced bone density.

Moreover, both stainless steel and cobalt alloys contain nickel and other elements that may lead to an allergic reaction.  Titanium alloys are preferentially employed in biomedicine because of their proven high biocompatibility; however, they exhibit poor friction and wear properties due to a low work hardening coefficient and low protection from forces provided by the tribo-oxides that occur at the alloy surface.

Techniques for Reducing Wear and Friction in Medical Grade Metals and Alloys

Current artificial hip and knee implants must be replaced every 10-15 years because of implant failure caused by wear-related aseptic loosening and osteolysis. A potential solution to the problem of wear and friction to artificial joint implants is the production of a wear-resistant coating. By providing a smoother surface for joint articulation, less wear will occur increasing the service period of metallic biomedical devices. Tribological properties such as the friction coefficient and elastic strain are improved through surface coatings. Studies have shown tungsten carbide coatings on titanium alloy implants via plasma spraying harden the surface of the titanium alloy and reduce the amount of wear.

Other techniques for reducing wear and friction in medical grade metals and alloys include ion implantation. This is where ions are embedded into the surface of the implant material to modify the physical and chemical properties of the outer surface. A potential application of carbon ions to cobalt alloys by plasma source ion implementation has been tested.

The results found that the modified metal surface had lower friction coefficients and increased resistance to damage. Therefore, the treatment may increase the service period of medical implants by preventing the adhesion of wear debris. Further studies are necessary to thoroughly test the biocompatibility of materials utilized for both surface coatings and ion implantation before they are applied to artificial implants.

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