Joint Replacement Materials: What's in an Implant?

More than a million people in the U.S. each year experience relief from pain and improved function as a result of total joint replacement surgery.  This life-altering procedure is regarded as one of the most valued developments in the history of surgery.  Currently Hip Replacement is the single most reliable operative procedure developed by modern medicine followed closely by second place winner, Knee Replacement surgery.  Although the procedure has been made routine in major centers, ongoing clinical research, design improvements, and improved wear resistance of the man-made materials used in the implants remain an evolving science.

WHY ARE MATERIALS IMPORTANT?

A “bearing surface” is a mechanical engineering term that describes the area of contact between two objects. Nature’s joint bearing surface is articular cartilage. When healthy articular cartilage rubs against itself in a healthy joint, very little friction is produced and virtually no wear particles are made.  Articular cartilage has no nerve endings so the natural rubbing of a joint is painless.  Articular cartilage gets all its nutrients from the fluid in a joint and has no blood supply.  This makes articular cartilage vulnerable to damage over one’s lifetime.  Some of this damage cannot be repaired and in arthritis, loss of the articular cartilage covering in a joint ensues. When arthritis develops in a joint, the articular cartilage is initially worn and eventually lost.  The result is bone rubbing on bone. Bone is a particularly poor bearing surface.  Not only is bone on bone a high friction bearing surface, it is also a very painful one.

Joint replacement components bearing surfaces are made of a variety of materials. These materials include highly cross-linked polyethylene (plastic), cobalt-chromium alloy (metal), and ceramic. Rubbing between these material couplings naturally produces some wear particles.  Man made materials still produce more friction than natural joint surfaces, and therefore experience a greater wear rate. This can impact the longevity of the implants. In addition, the wear particles produced can cause tissue inflammation surrounding the joint. We as joint designers and surgeons want to minimize both the production of wear particles and any inflammation they may stimulate around the replaced joint.  Optimal joint replacement implants are made from materials which produce very low friction and produce very little wear particles. This combination will help to maximize the longevity of an implant. The average age of joint replacement patients is getting younger.  The reasons are many.  A more active patient population is one, longer life-spans is another, and unfortunately, increasing obesity is a third.

IMPLANT MATERIAL OPTIONS AND COMBINATIONS

All materials used in joint replacement implants are very safe and only rarely cause localized or systemic reactions in the body.  Friction in an artificial joint results in the release of miniscule particles, hundreds of times smaller than a grain of sand, into the area surrounding the joint cavity.  This may occasionally result in an inflammatory response that can trick the body into reacting in an inflammatory manner and resorbing bone around the implant.  This is called osteolysis and, when extensive, can cause discomfort, loosening of the implant, and the need for revision surgery.  The best way to combat the development of this problem and increase the joint replacement implant life is by improving the wear resistance of the bearing materials in the man-made implants.

As new materials are developed and introduced, projecting long term effectiveness is a challenge.  Prior to use in joint replacement surgery, implant “wear” is tested in laboratory simulators that to the best of their ability, mimic the activity of the joint being tested. Testing in a lab outside of the body is an inexact science.  Although we can measure the size and volume of the wear particles produced, the biological reaction to them is absent.  Once safety is established with a simulator, the best way to determine the effectiveness of the implant surfaces is to implant them in patients and follow the performance carefully over years.  Prospective, randomized clinical studies tell us the performance in people!  Nothing is as reliable to tell us what works better and what does not as data from these studies.  The downside is it takes years to get the information.  National databanks called “Joint Replacement Registries” can both increase the reliability of this data and give us answers sooner because of the large volumes involved.

WHAT IMPLANT MATERIAL IS BEST?

Your surgeon will carefully evaluate your individual situation and goals to determine the material or combination of materials that is best for your joint replacement surgery.  Age, level of activity, gender, weight, deformity, and many other variables, are all factors when choosing the implant that is best for you.

MATERIALS

·         Metal-on-Metal Bearings

These materials were the first to be used in total hip replacements in the 1960’s, but less than optimal design and the development of newer materials over time resulted in these falling out of favor until recent times.  Improved manufacturing processes, recognition of the importance of a smooth surface finish, the recognized strength of cobalt-chromium alloys, and an improved understanding of the physical and chemical behaviors of metals have led to resurgence in the use of metal-on-metal materials for hip replacement.

Concerns remain about the wear particles that are generated by metal-on-metal implants and the hypersensitivity to the wear particles that may result.  Metal-on-metal implants are not used in patients with poorly functioning kidneys (the means for excretion of the particles) or in women of child-bearing age.  These implants are mostly reserved for young, active, and healthy male patients.

·         Metal-on-Polyethylene Bearings

These are the most commonly used combinations with the longest term clinical data available.  Polyethylene is a type of high durability plastic.  Although these are subject to the principles of wear as well, the results are locally less profound than metal-on-metal bearings and there is a long and successful track record for hip and knee replacements using these materials.  Ongoing research and development with these materials continue to result in implant wear improvements.  Various methods of sterilization of the polyethylene have resulted in altering the molecules in a way that strengthens them and decreases wear (a process called cross linking).  This combination of materials is commonly used in both hip and knee implants.  Dramatic improvement of polyethylene wear resistance in the last ten years is generally accepted as the biggest advance in joint replacement technology of the last 30 years.

·         Ceramic Bearings

Ceramic surfaces have been shown to be more wear resistant due to their ability to be polished to a very smooth finish and remain resistant to scratching.  Because of their brittleness, however, they are more susceptible to fracture than other materials.  Patients with highly active lifestyles who are subject to high impact activities are not the best candidates for ceramic implants.

Ceramic materials can be combined with polyethylene or ceramic itself and are most commonly found in hip implants.  A small number of patients with ceramic-on-ceramic hip implants experience an annoying “squeaking” sound during normal movement and activity.

The future evolution of joint replacement materials promises to bring newer materials with hopes of longer implant life and less wear characteristics.  One example of new technology advancement is a hybrid metal-ceramic material.  Through special manufacturing conditions, the surface of the metal material can be converted to ceramic which results in an implant with the favorable characteristics of both metal and ceramic.  More discoveries of this nature are undoubtedly on the horizon.  Long term clinical studies are essential to prove that any improvements in materials actually lead to longer lasting implant life with improved outcomes.

CONCLUSION

Joint replacement surgery is currently a highly successful procedure resulting in exceptional outcomes.  There is no combination of materials that works best in all joints for all patients.  Your surgeon will determine the implant materials that are are optimal for your individual situation. 

For more information about this topic, watch YNN, Ch. 10 news on Saturday, July 16^th to hear Dr. Brett Greenky discuss implant materials used in hip and knee replacement surgery.