Ceramics have helped solve problems with hip replacements. Implants now last much longer than 10 years. Of the 950,000 hip replacements performed every year, about 250,000 are ceramic hip implants. The first problem-solving step was to replace the metal ball with transformation-toughened zirconium oxide, one of the new high-strength, hightoughness ceramics discussed in Chapter 5. Studies in Europe, Japan, and the United States in the 1980s showed that a ceramic ball was more body-compatible than metals and provided a harder, smoother surface that decreased wear. Another step was to cut grooves in the metal stem and coat them with a bioactive ceramic. This ceramic stimulated bone and tissue growth into the grooves and reduced the chance of the stem coming loose from the leg bone. Some new stems aren't even made of metal; instead, they're made of a composite of carbon fibers in a polymer matrix.