Modern dentistry has amassed some impressive triumphs, among them a dramatic reduction in cavities and longer life for natural teeth. Nevertheless, the design and fabrication of replacement teeth remain dependent upon a method introduced 75 years ago.

Like the patchwork of metal fillings that used to decorate children's smiles, however, the hand-sculpting of dental crowns could become a rarity if new computer systems prove able to do the job better, faster and cheaper.

The computer method is based on CAD-CAM -- computer-aided design and computer-aided manufacture; it is not unlike the computer systems used by the automotive industry to shape car bodies.

Dental CAD-CAM uses a computerized camera to take pictures of the tooth needing repair, a second computer to custom-design the prosthesis and a computer-driven milling machine to chisel the delicate contours. A system in the final stages of testing at the University of Southern California's School of Dentistry can accomplish all that in one appointment, according to its inventor, Dr. Francois Duret, research professor and co-director of the school's new division of dental imaging.

Two other systems are in stages of development in the United States: One is being marketed by Siemens Corp. and the other is in the design stage at the University of Minnesota. In Europe and Scandinavia, the technology already has been embraced by dentists in private practice.

The conventional method -- as anyone who has gagged through the procedure knows -- requires two or three long sessions at the dentist's office. For the dentist, making dental crowns, bridges and partial restorations called inlays or on-lays is a laborious process involving molds, wax models, casting and delicate finish work to hand-shape the replacement tooth.

The computer could change all that.

"I've been in dentistry 20 years and I tell you, this is the most exciting thing I've ever been involved with," said Dr. Milton Essig, an investigator at the University of Alabama working with Siemens's CAD-CAM. Patients with on-lays and inlays designed and milled by the computer system have been followed for 15 months with excellent results, Essig said.

The American Dental Association, however, takes a more conservative view.

"There is a tendency to get all excited about things prematurely," said Dr. Charles Schoenfeld, the association's assistant secretary for scientific affairs and staff member to the association's council on dental materials, instruments and equipment. "I think the bottom line is that, yes, it appears to have promise, but it remains to be documented."

In the United States, only the Siemens system has received marketing approval from the U.S. Food and Drug Administration, and its capability is limited to partial tooth restorations.

Duret's system is more ambitious than Siemens's. It is designed to do crowns and bridges as well as partial restorations and is under review by the FDA for those expanded applications.

Duret, 42, is confident of CAD-CAM's future and is nothing if not tenacious.

It has taken him nearly 20 years to bring the project to USC from his garage workbench in a small town in the French Alps. Along the way, he was jeered by French colleagues who thought his concept was closer to science fiction than to reality. The University of Lyon, where he was an instructor at the dental school, ordered him in 1973 to abandon the project and focus his research efforts on more promising areas.

Duret continued to work on the software secretly, however, a duplicity that lasted seven years, he said, until faculty elders discovered it and fired him.

"They thought I was crazy," he said, grinning.

Duret's system is scheduled for field testing this summer in about 20 Southern California dental offices and laboratories, according to Jean-Claude Haas, a French engineer who helped Duret design the CAD-CAM software. Haas is president of a Los Angeles-based company, Hennson Technologies Inc., created exclusively to produce and market the system.

A third system, more automated than Duret's, represents more than a decade of work by Dr. Dianne Rekow of the University of Minnesota dental school. Funding problems have slowed her work, Rekow said, but she hopes to have a prototype in several test sites by the end of the year.

All of the systems use optics, electronic design and automated milling technologies to produce a custom-fitted tooth restoration in one session with the patient.

Haas's marketing plan initially calls for leasing the CAD-CAM units to dentists or laboratories for $4,000 a month. A dentist would have to do a minimum of three crowns a day to justify the cost, Haas said.