Fifteen years ago lab studies revealed that metal-on-metal hip replacements had lower overall wear compared to metal-on-plastic hips. Engineers measured the volume of wear particles produced over simulated 10, 20 and 30-year spans of hip wear in the laboratory. The smallest volume of wear was produced by ceramic balls rubbing against ceramic sockets. Only slightly more wear was produced by metal balls rubbing against metal sockets. The traditional coupling of metal balls against plastic sockets produced far more wear than the other two. As a result, engineers believed that ceramic-on-ceramic and metal-on-metal wear surfaces in hip replacement would exhibit superior long-term performance in patients. That’s not the way things turned out.
Even though the wear in metal-on-metal hips was far less in volume than metal-on-plastic hips, it turns out many more particles of wear are produced in the metal-on-metal situation. Particles of wear can produce a biological reaction around a hip implant that can gradually lead to failure. Since all hip couplings produce wear of some sort, the goal is to reduce any long-term side effects of this wear material.
Despite what scientists initially thought, it is not the volume of wear that is most important, but rather the number of particles. Metal-on-metal hips can still perform beautifully, but only in very specific situations. The initial enthusiasm created by the lab data about low-wear volume in metal-on-metal hips has been tempered by clinical experience inferior to metal-on-plastic hips. Registry data from Australia, Great Britain and Sweden have shown much higher early failure rates with metal-on-metal hips compared to the traditional metal-on-plastic. This is especially true for people of small stature and women.
Encouraged by the initial optimistic lab data, the world’s manufacturers of hip implants all produced some metal-on-metal hips. These became initially popular in countries other than the United States. In the United States there use was short-lived over a 5-8 year period. Most metal-on-metal hips are no longer available for clinical use. Although their long term performance can still be stellar, their durability is dependant on much more stringent criteria than ceramic-on-ceramic or metal-on-plastic hips.
The most popular, and still available, metal-on-metal hip arthroplasty is called the Birmingham Hip Resurfacing. This implant was developed in Birmingham, England and is still available worldwide. Its unique application for hip arthritis is most appropriate for young men. This implant still has a higher early failure rate than a traditional hip replacement, but appears to be unique in that, although it is a metal-on-metal hip, it has remained relatively immune from the wear material issues. Candidates for this implant are almost exclusively men whose goal is to return to higher impact activities such as jogging and other recreational sports. The implant has the advantage of a lower rate of dislocation and improved range-of-motion; however, it carries the risk of metal-on-metal articulation. Registry data from Great Britain, Sweden and Australia still show higher early complication rates when compared to traditional hip replacement. There is no long-term data that suggests these implants are more durable than a traditional hip replacement.
In summary, metal-on-metal hips are not appropriate for most patients. And the overall clinical experience has been inferior to traditional hip replacement. Under certain circumstances, the Birmingham Hip Resurfacing operation may be a unique exception. If you are considering a total joint replacement, talk with your doctor about your specific circumstances and the best solution to get you back in motion.