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The standard crown procedure involves a couple of anesthetized hours during which the dentist drills away 30 percent of a tooth, takes an impression and cements on a temporary crown. Then there is usually a two-week wait for a lab to produce the crown, and another afternoon of numbness and drilling when it is attached.

Cerec, enhanced by a new 3-D software package introduced earlier this year by Sirona, lets dentists preserve more of the original tooth structure and create a crown while you wait.

For Michele Shafer, whose home is an hour and fifteen minutes from Dr. Spector's office, that meant making only a single trip for a crown; she spent more time in her car than in the chair.

Dr. Spector began Mrs. Shafer's procedure with an injection of anesthetic through a digital syringe that slows the flow of medication when it senses pressure, thereby making the injection almost painless. He used the Cerec infrared camera to create an optical impression of the original tooth. That step replaces the green paste and uncomfortable metal trays that are typically used to make an impression.

Dr. Spector then removed the diseased tissue and took a second image, which the Cerec combined with the first to create a rough 3-D model of the crown on the screen. Using a trackball, he tweaked the design, smoothing contours and adjusting the fit. He then sent the data to the milling chamber, which produced the crown in 17 minutes.

Not only is the process more convenient than waiting for the lab, Dr. Spector said, it also produces a stronger crown. Instead of enamel, the crown is made of compressed porcelain "40 percent stronger than what a lab can press," he said. "And the material has the same properties as enamel - it wears away at the same rate, and it has the same thermal coefficient, so when you drink hot coffee it will expand just as much as the teeth around it." It leaves more of the original tooth intact because the material can be cemented directly to the tooth with no metal interlayer.

"No one loves getting a crown," said Dr. Jeffrey Katz, a San Francisco dentist. "They hate the Novocain. They hate the gooey stuff. This transforms the whole experience."

"The biggest thing for me is the control," said Dr. George Maryniuk, a prosthodontist in Brookline, Mass. "My father was a lab technician, and I have high expectations. Now I'm guaranteed to get the results I want."

But with its $100,000 price tag, the Cerec does not make sense for every practice. The math works out for Dr. Maryniuk, who was paying $5,000 a month in lab expenses, most of which Cerec eliminated, and now pays $2,000 a month to the Cerec leasing company. The machine reduces his costs for a crown, for example, to $30 from the usual $200 and saves him the expense of second appointments.

That savings will have no effect on the patient's pocketbook, because dentists depend on the margin to cover their Cerec payments and the other myriad overhead costs of a high-tech office. For instance, a digital X-ray machine costs some $18,000, compared with about $4,500 for a conventional one, but dentists don't charge any more for the service, and insurance companies don't make a distinction.

"When I get a claim form for a crown, I don't know if it was lab-processed or Cerec," said Max Anderson, the national oral health advisor for Delta Dental, one of the nation's largest insurers. In the system, a crown is a crown. Most insurers don't even cover new diagnostic services like the Difoti, meaning that the dentist must somehow absorb the $6,495 cost.

So dentists must analyze their needs before investing in the machines. A dentist who does few crowns or other restorations might be better off sticking with the labs, many of which are starting to use the Cerec technology. In any case, in an already capital-intensive business, the new devices put pressure on the bottom line.

That in turn could be leading to some unnecessary treatments. "The question is, does a dentist make the same clinical decisions when he has a $20,000 machine to pay off," Dr. Anderson cautioned.

Aided by their digital cameras, Dr. Feuerstein said, some dentists are selling cosmetic procedures that they might not have suggested before. "It's like the extreme makeover reality shows," he said. "Dentists will take a patient with 'ugly' teeth and show them what they could look like with facades and laser-shaped gums."

On balance, most dentists view the new dental technologies as a positive force. "Today, cavities are a universal disease," said Dr. Frederick C. Eichmiller, director of the Paffenbarger Research Center of the American Dental Association. "In the future, it won't be." Dr. Eichmiller and his team in Gaithersburg, Md., are developing toothpastes and rinses that will help teeth repair themselves, as skin or bones do. They are also working on "smart fillings" that will detect decay and release a healing mixture of minerals.

What will that mean for the high-tech dentist? "Remember," Dr. Katz said, "the goal of dentistry is to put ourselves out of business."

Correction: Sept. 6, 2003, Saturday

An article in Circuits on Thursday referred imprecisely to a high-tech dental tool, Cerec, which is used to create crowns. Its basic technology was developed in the early 1980's, and a machine on which it is used in many dentists' offices, the Cerec 3, has existed since 2000; Cerec 3D is the latest software for it, introduced this year.