Just like families with multiple generations (great-grandparents, grandparents, parents, children), objects like hip replacements have generations. The first group of hip implants designed in the 1960s are now referred to as first-generation products. Eventually, they gave way to second generation implants in the 1970s and 80s.
Better technology and improved materials led to the switch from cobalt-chromium-molybdenum (CoCrMo) for the bearings used to create motion to cast alloy in the second generation. By the third generation of hip implants (1990s), materials changed again to high- and low-carbon alloys.
In this report, orthopedic surgeons compare the 10-year results when using metal-on-metal bearing versus ceramic-on-ceramic. They also compare the survival rates of these two types of alternative bearings with the more traditional metal-on-polyethylene (plastic) bearings. Each type of bearing has its own advantages and disadvantages.
For example, ceramic materials are at risk for fracture. Metal bearings cannot fracture but they are more likely to loosen requiring a second (revision) surgery. Metal bearings can release tiny particles of metal into the joint, which does not happen with ceramics. Well, to clarify that last point a bit, ceramic debris is possible — it’s just much less than with metal bearings. And the body does not seem to react to ceramic wear particles like it does to the less biocompatible metal debris.
Complications associated with different bearing couples were also reported on in this article. Blood clots to the lungs or in the legs (that could travel to the lungs, heart, or brain) were the biggest concern. Most of the blood clots occurred in patients receiving the metal-on-polyethylene (MoP) implant. But this may not have as much to do with the implant materials as it does with the (older) age of the patients receiving this type of implant.
Other complications were broken down into two main groups: surgical complications (e.g., nerve damage, infection, dislocation, implant fracture) and medical complications (e.g., pneumonia, heart attack, urinary tract infection). Medical complications occurred two and a half times more often than surgical complications.
The need for revision surgery was different among the three types of bearings. Loosening and dislocation were reported in the metal-on-metal group. Fracture of the implant was the main reason for revision of implants with ceramic bearings. Dislocation and infection were reported in a smaller number of patients with ceramic bearings. Metal-on-polyethylene had the least number of revisions due to loosening, infection, or bone fracture around the implant after a fall.
What are some patient recommendations that can be made from these findings? As mentioned, metal-on-metal bearings are more likely to loosen and wear out compared with bearings made of ceramic materials. Therefore metal bearings with polyethylene liners are recommended for older, less active individuals who are going to want this surgery to be their last.
The best bearing surface for younger, active patients will have to be decided on a case by case basis. The surgeon and patient should consider all the pros and cons of the different types of bearings (metal-on-metal, metal-on-polypropylene, ceramic-on-ceramic). The choice of bearings is made according to age, activity level, bone density, and relative risks and advantages of metal-on-metal versus ceramic-on-ceramic bearings. Potential complications must also be examined and every effort made to prevent any anticipated problems.