The Food and Drug Administration yesterday permitted a unit of Johnson & Johnson to market an artificial spinal disk in the United States as a new form of treatment for people with severe lower back pain. The disc, called the Charité (pronounced shar-ee-TAY), represents the first of a wave of such devices that are expected to become available over the next decade as an alternative to spinal fusion surgery. Spinal disk implants are projected to become a billion-dollar market for medical device companies in the next decade, although many orthopedics researchers think that artificial disks will eventually be overtaken by...