At the beginning of the 21st century, metal-on-highly cross-linked polyethelene bearings were introduced, along with metal-on-metal bearings and ceramic-on-ceramic bearings. All of these “newer” bearings were introduced to try and combat wear associated with metal-on-conventional polyethylene, which in some patients would lead to significant bone loss around total hip replacement implants. Several mid-term studies revealed promising results utilizing metal-on-metal bearings in both hip replacement and hip resurfacing in younger, more active, individuals. However, over the last 3 to 4 years, studies have been published that revealed failures of metal-on-metal hips associated with an adverse tissue reaction felt to be associated with elevated levels of metal ion around the implants. These “adverse tissue reactions” can manifest as a small fluid collection, a cystic mass, or a solid mass referred to as a pseudotumor. This is a mass which is neither malignant nor infectious, can be small or large, and occurs in the soft tissue around hip replacement components. These adverse tissue reactions appear to be related to metal-on-metal bearing wear and may be an allergic reaction to low levels of wear debris or a toxic reaction to elevated levels of wear debris. Most likely, a combination of factors may be involved in these reactions. At present, there is no consensus as to what is the cause and why they occur.
Why does this happen?
All bearings utilized in hip replacements lead to wear between moving parts. This wear leads to particles in the surrounding soft tissue of the joint. The body reacts to different particles in a variety of ways. We know from experience in the 1980s and 1990s that polyethylene particles can lead to bone destruction by activation of cells that try to vacuum up the polyethylene particles. These cells release chemicals that lead to destruction of bone around the hip implants. Metal-on-metal particles, which are quite a bit smaller and in some cases dissolve in the fluid, are absorbed by cells and in some cases lead to a toxic reaction killing the cells. Therefore, the reaction of the cell to the particle is variable and varies from one patient to the next. The amount of wear of bearings used in total hip replacements is influenced by implant design and component placement. We know that smaller cup sizes may lead to greater wear or vertical cups may also lead to increased wear due to an edge-loading phenomenon where the edge of the socket articulates against the ball of the hip decreasing the contact area, decreasing the amount of coverage of the ball by the socket, all of which leads to less lubrication between the bearing surfaces and increased wear of the parts.
A central question in metal on metal bearings is why do some patients develop an adverse tissue reaction while others do not. It would appear that the level of wear and, therefore, wear debris influences this reaction. The patient’s response to wear debris is highly variable. Some patients with very low wear can develop a significant soft tissue reaction, while in other patients significant amounts of wear can lead to no reaction whatsoever. It, therefore, appears this is a combination of inflammatory and possibly allergic-type reaction to the debris.
What are the risks of different bearings?
It is important to realize that the overwhelming majority of hip replacements are highly successful. To date, metal-on-metal hip replacements still have greater than 90% success rate at followup available in 2012. Because of concerns about soft tissue reactions to metal debris, the number of metal-on-metal bearings implanted in the U.S. has dropped significantly. There has also been quite a bit of hyperbole in the press regarding patients who have had an adverse tissue reaction. While the complication in any one patient can be devastating, it is important to realize that the majority of patients continue to function quite well. Each bearing surface has its own set of potential complications and time will tell which bearing proves to be the most successful.
What should I do going forward?
If you have a hip replacement, you should follow up with your surgeon on a routine basis, usually at the first and second year at a minimum, then per the surgeon’s protocol thereafter. If you have a metal-on-metal hip, you should follow up with your surgeon on a yearly basis to check in to see if there is any change in the status of your hip. If you have symptoms, the most common of which are pain in and around the hip, a thorough history, physical exam, x-ray, and if appropriate, blood studies, will be ordered. If there is concern about an adverse tissue reaction to a metal-on-metal bearing, your surgeon may decide to measure your blood cobalt and chromium levels and/or check a specialized imaging study, such as an MRI or ultrasound, looking for soft tissue reaction around the implant. Like with any drug or implant, careful monitoring over time will help us better determine what is the appropriate use for specific devices.
William P. Barrett, MD