Why the Material Used for Hip Replacement Implants Matters

Hip replacement surgery is becoming more common, and it is one of the most successful surgical procedures in terms of improving an individual's life and function, while also having a relatively low complication rate. When the normally smooth cartilage surface of the hip joint is worn away, even simple activities can become difficult. Hip replacement removes the worn-out hip joint and replaces it with an artificial implant.

However, that is not to say that there are no risks with the surgery, and one of your surgeon's primary goals is to avoid potential complications. One issue is that hip replacements can wear out over time.

Designs of hip replacements and the materials used to manufacture a hip replacement implant have changed, been refined, come in and out of favor, and continue to be investigated to determine the optimal design of an artificial hip. Numerous times over the past several decades, new implants and materials have come to the market with great excitement, only to find that over time they did not work as well as expected.

What the Bearing Surface Has to Do With Hip Replacement Implant Longevity

The most critical interface of a hip replacement, in terms of longevity of the implants, is the bearing surface. This is the surface of the hip replacement that allows for movement between the ball and socket of the implant.

A typical hip replacement implant recreates the ball-and-socket of the normal hip joint with an artificial ball and an artificial socket. These implanted parts come in many shapes and sizes, and what is implanted in your body may depend on:

• The size of your femur and pelvis
• Any deformities or abnormalities of your hip joint
• Prior surgery performed on your hip
• Your surgeon's preference

The ball and socket of the hip replacement implant will be the surface that moves each time your hip moves over the course of your lifetime. This bearing surface will articulate millions of times and is prone to wearing out over decades of activities. Much like the tread of your car tires can wear out as you drive hundreds and thousands of miles, the bearing surface of the hip replacement can wear out over years and decades of activity.

Sometimes hip replacement implants last for much longer or much shorter periods of time. The goal is to design an implant that will last as long as possible. Among the factors that are important to determine the longevity of the bearing surface are:

• The materials used on the bearing surfaces
• The size of the bearing surfaces

There are also other factors that can be important, but these are two factors your surgeon will consider that could impact how long your hip replacement implant will last. For this reason, more people having a hip replacement are becoming interested in the types of materials being placed inside their body.

Evolution of Hip Implants

A traditional hip replacement implant uses a metal femoral head (the ball of the implant) and conventional polyethylene, or plastic, as the liner of the acetabular component (the socket of the implant).

Historically, the femoral head was quite small, in large part because that means less surface area to rub between the ball and the socket, leading to lower wear rates of the implants. The problem with smaller femoral head implants is that they have inferior stability and a higher dislocation rate. For that reason, femoral head sizes have increased to make hip replacements more stable.

There was great excitement about 20 years ago when new implants were designed using a metal acetabular socket with a metal femoral head. These implants, called metal-on-metal hip replacements, showed exceedingly small wear rates in laboratory studies, were very stable because of the large femoral heads that could be used, and became very popular.

Unfortunately, the wear of the implants created microscopic metallic particles that have caused problems with local and systemic tissues, making metal-on-metal hip replacement implants almost unheard of today.

One of the more significant advances in hip replacement implants has been the development of newer polyethylene (plastic), which lasts longer than conventional polyethylene. A recent advancement has been with the use of a highly cross-linked polyethylene.

Cross-linked polyethylene is created by radiating and reheating the polyethylene implants, allowing the plastic to strengthen its molecular structure through a cross-linking process. Cross-linked polyethylene has been shown to have lower wear rates than conventional polyethylene.

Ceramic implants have also been investigated in an effort to find a longer-lasting implant. Ceramic is a very hard material, and hard materials do not wear out as quickly as softer metals and plastic. One rare risk of older ceramic bearing surfaces is that they can develop cracks that lead to implant fractures.

While newer ceramic implants show promise, squeaking can sometimes be heard as the hip moves. While not dangerous, squeaking can be annoying to many people with these implants. Newer ceramic has shown much fewer problems with failure, although there is also less research about the long-term results of ceramic implants, particularly the newer ceramic materials.

The Material With the Best Track Record

The current thinking among most orthopedic surgeons is that a cross-linked polyethylene socket, combined with either a ceramic or metal femoral head, has the best track record. There is also some good data to support the use of implants with both a ceramic femoral head and a ceramic socket, but there is not as much long-term clinical follow-up on these implants.

The difference between using a ceramic and a metal femoral head with a cross-linked polyethylene insert has not been shown to be significant. There are some reasons why surgeons may select ceramic femoral heads, particularly in younger patients, but the reality is that to date there has not been shown to be much of a difference.

When Newer Isn't Better

It is always tempting, both for patients and for surgeons, to be attracted to the newest implant on the market. Often these implants are promoted by orthopedic manufacturing companies as being better, and potentially lasting longer, than implants that are currently available. However, there have been instances when highly touted innovations did not perform as expected once they were tried in humans.

Innovation and development of better hip replacement implants is an ongoing process, but, unfortunately, not every step of innovation turns out to be beneficial for patients. In most situations, people should not be seeking the newest treatment available on the market. Choosing a replacement implant with an excellent track record may mean that you are not getting the newest type of implant, but it may still mean that you are getting the best implant available.

A Word From Verywell

Not everyone wants to know about the hip replacement materials that will be implanted during their surgery, but if you are, don't hesitate to have a conversation with your surgeon about why they prefer the implants they use.

The implants with the longest track record available are those made with metal femoral heads and cross-linked polyethylene acetabular sockets.

There is no doubt that hip replacement materials will continue to evolve in future years, but patients should exercise caution about newer implants that have only recently been introduced to the market. At this point, the implants with the longest available track record are metal femoral head with cross-linked polyethylene acetabular liners.

If you are interested or have concerns about the hip replacement materials that will be implanted during their surgery, don't hesitate to have conversation with your surgeon. They should be able to provide a thorough explanation for why they prefer the implants they use.