Zimmer Holdings recently announced it was suspending sales of its metal hip socket replacement called the Durom cup until it trained doctors how best to implant it. The company said a “low” percentage of the 13,000 patients who got the socket would need replacements, but some doctors fear the number could reach into the hundreds. If those patients lived in other countries where artificial joints were tracked by national databases, including Australia and Britain, many might have been spared that risk. But the United States lacks such a joint registry national database that tracks how patients with artificial hips and knees fare.