Types of Materials

Generally, the most common materials used in orthopaedic implants are metals and a type of plastic called polyethylene.  These two material types are combined in most joint implants, that is, one component is made from metal, and one from polyethylene.  When properly designed and implanted, the two components can rub together smoothly while minimizing wear. 

While some pure metals have excellent characteristics for use as implants, most metal implants are made from a mixture of two or more metals.   These mixed metals are called alloys.  By combining metals, a new material can be created that has a good balance of the desired characteristics.  The most common metal alloys used in orthopaedic implants are stainless steel, cobalt-chromium alloys, and titanium alloys.       

Stainless Steel

Stainless steel is most often used in implants that are intended to help repair fractures, such as bone plates, bone screws, pins, and rods.   Stainless steel is made mostly of iron, with other metals such as chromium or molybdenum added to make it more resistant to corrosion.  There are many different types of stainless steel.  The types of stainless steel used in orthopaedic implants are designed to resist the normal chemicals found in the human body.

Cobalt-chromium Alloys

These alloys are used in a variety of joint replacement implants, as well as some fracture repair implants.  While cobalt-chromium alloys contain mostly cobalt and chromium, they also include other metals, such as molybdenum, to increase their strength and corrosion resistance.

Titanium Alloys

Titanium alloys are the most flexible of all orthopaedic alloys.  They are also lighter weight than most other orthopaedic alloys.  Consisting mostly of titanium, they also contain varying degrees of other metals, such as aluminum and vanadium.

Commercially Pure (CP) Titanium

CP titanium may also be used in some implants.  It is used, for example, to make fiber metal, which is a layer of metal fibers bonded to the surface of an implant to allow the bone to grow into the implant, or cement to flow into the implant, for a better grip.

Tantalum

Tantalum is a pure metal with excellent physical and biological characteristics.  It is flexible, corrosion resistant, and biocompatible.

Ultra High Molecular Weight Polyethylene (UHMWPE)

UHMWPE is a type of plastic commonly used on the surface of one implant that is designed to come in contact with another implant, as in a joint replacement.  Although other types of polyethylene have many familiar household uses, the polyethylene used in orthopaedic implants is a much higher grade.  In fact, a special type of medical-grade polyethylene was developed specifically for use in orthopaedic implants.

Polyethylene is very durable when it comes into contact with other materials.  When a metal implant moves on a polyethylene surface, as it does in most joint replacements, the contact is very smooth and the amount of wear is usually minimal.

Polyethylene can be made even more resistance to wear.  This can be accomplished through a process called crosslinking, which creates stronger bonds between the molecular chains that make up the polyethylene.  The appropriate amount of crosslinking depends on the type of implant.  For example, the surface of a hip implant may require a different degree of crosslinking than the surface of a knee implant.

Ceramics

Ceramic materials are usually made by pressing and heating certain metal oxides (typically aluminum oxide and zirconium oxide) until they become hard and dense.  These ceramic materials are strong, resistant to wear, and biocompatible.  They are used mostly to make implant articulating surfaces that do not require flexibility, as in the surfaces of a hip joint.

Composite Materials

Composite materials are made by mixing two or more separate materials without creating a chemical bond between the materials.  Metal alloys and ceramics are not considered to be composite materials because their ingredients are chemically bonded to create a new material.

On a larger scale, two layers of different materials can be combined to create a composite material with the desired characteristics.  The stem of a hip implant, for example, may consist of layers of two different materials that together provide the desired combination of strength and flexibility.

Trabecular Metal

Trabecular Metal™ material is made from tantalum over carbon.  It is strong, flexible, and biocompatible.  The structure of Trabecular Metal material is similar to the structure of bone.  It is very porous, which means it has small spaces or pores.  New tissue can grow into these pores and help hold the implant in place.

Bioabsorbable Materials

Bioabsorbable materials are designed to be absorbed by the body when their job is complete.  They are made from a biocompatible plastic that can be dissolved by normal body fluids.  Many sutures used today in all types of surgery are bioabsorbable.  These bioabsorbable materials may also be used in implants that reattach soft tissue to bone.

Silicone

Silicone is a rubbery material that is very flexible.  In orthopaedics, it is most commonly used in implants that replace the joints of the toes.

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