Dr. Craven Kurz, 55, Beverly Hills orthodontist who invented "invisible" braces. The Canadian-born Kurz studied at the University of Saskatchewan and McGill University in Montreal and New York's Columbia University. He established his practice in Beverly Hills and taught for five years at UCLA. In the 1970s, Kurz became concerned about his patients--actors, television announcers and even Playboy bunnies--who needed to straighten their teeth but could not endanger their jobs with a "tin grin" of metal braces. So he invented the Kurz Lingual Appliance, which came to be known as "invisible" braces and revolutionized adult orthodontics. The braces take both their name and nickname from their position on the tongue-side ("tongue" is lingua in Latin) of the teeth. Kurz obtained a patent, along with 24 other patents on orthodontic appliances. In 1986 he was founding president of the American Lingual Orthodontic Assn. Kurz wrote and lectured widely in North America, Europe and Asia. On Monday in Marina del Rey of unknown causes.