单选题 There is nothing more possible than a new hip or knee that can put the spring back in your step. Patients receiving joint implants （移植） often are able to resume many of the physical activities they love, even those as vigorous as tennis and hiking. No wonder, then, that joint replacement is growing in popularity. 

In the United States in 2007, surgeons performed about 806,000 hip and knee implants （the joints most commonly replaced）， double the number of performed a decade earlier. Though these procedures have become routine, they are not failure free. "

Implants must sometimes be replaced," said Dr. Henrik Malchau, an orthopedic surgeon （矫正外科医生） at Massachusetts General Hospital in Boston. A study published in 2007 found that 7 percent of hips implanted in Medicare patients had to be replaced within seven and a half years. "

The percentage may sound low, but the finding suggests that thousands of hip patients eventually require a second operation," said Dr. Malchau. Those patients must endure additional recoveries, often painful, and increased medical expenses. 

The failure rate should be lower, many experts agree. Sweden, for instance, has a failure rate estimated to be a third of that in the United States. Sweden also has a national joint replacement registry, a database of information from which surgeons can learn how and why certain procedures go wrong. A registry also helps surgeons learn quickly whether a specific type of implant is particularly problematic. "Even country that has developed a registry has been able to reduce failure rates significantly," said Dr. Daniel Berry, chief of orthopedic surgery at the Mayo Clinic in Rochester, Minn. 

A newly formed American Joint Replacement Registry will begin gathering data from hospitals in the next 12 to 18 months. It's good news for those who are considering replacing a knee or hip. 

People who need a new knee or hip would possibly feel __ about data gathering in the U.S.