The Bearing Surface: Why Material Matters With Hip Replacement

This part of your hip replacement can wear out

LAWRENCE LIVERMORE NATIONAL LABORATORY/ SCIENCE PHOTO LIBRARY / Getty Images

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 possible complication is the tendency for hip replacements to 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 do not work as well as expected.

The Bearing Surface

The most critical interface of a hip replacement, in terms of longevity of the implants, is the so-called 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 much longer or much shorter. 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 this 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 that your surgeon can choose that could impact how long your hip replacement implant will last. For this reason, more people having a hip replacement are getting 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 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 10 to 20 years ago when new implants were designed using a metal acetabular socket. 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) that lasts longer than conventional polyethylene. The recent advancement has been with the use of a highly cross-linked polyethylene. Cross-linked polyethylene is created by radiating and re-heating 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. The problem with older ceramic implants is that they developed cracks that could lead to the implant failing suddenly by breaking. 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.

Current Thinking: The Best Right Now

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.

In the early 2000s, one of the major orthopedic implant manufacturers called DePuy Orthopedics, a division of Johnson & Johnson, came to the market with a metal-on-metal hip replacement implant that was specifically designed for stability and longevity. These implants were touted to solve the challenging problem of hip replacements in younger, more active individuals. In the end, these implants were recalled and taken off the market, and many of these patients ended up needing additional surgeries to remove and replace this implant.

One of the major problems with joint replacement implants is that they often come to the market with little or no clinical investigations. While patients may believe that any new implant has undergone extensive clinical testing, the reality is that most implant manufacturers use a mechanism called the 501(K) pathway to bypass the FDA's regulatory approval process. As long as the manufacturer can state that the new device is "substantially equivalent" to current devices on the market, they need not present any clinical data to be able to sell the new implant.

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. Having an excellent track record may mean that you are not getting the newest type of implant available, but it may still mean that you are getting the best implant available.

A Word From Verywell

Not every patient wants to know about the hip replacement materials that will be implanted during their surgery, but some are very interested. In addition, I can assure you that your joint replacement surgeon is interested in the materials used during your surgical procedure, so don't hesitate to start the conversation and get informed.

The best data available at this point in time find that hip replacements will last the longest when the femoral head is either made of ceramic or metal and the acetabular socket is made of either cross-linked polyethylene or ceramic. 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 in the coming years and decades hip replacement materials will continue to evolve. However, great caution should be used whenever a new implant is introduced to the market.

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