SCIENCE / MEDICINE : Crowning Achievement : Computers: The technology that designs better cars now may do the same for replacement teeth. It may mean lower costs and more convenience for patients.
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, for computer-aided design/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 delicately chisel its contours. A system in the final stages of testing at USC’s School of Dentistry can accomplish all this 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 has already 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’ 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.
Dr. Jack D. Preston, professor of esthetic dentistry at USC and Duret’s co-director of the imaging division, said unequivocally: “This is the beginning of an entirely new technology while what we’ve done with porcelain and metal has reached the end of the line.”
The American Dental Assn., however, takes a more conservative view.
“There is a tendency to get all excited about things too prematurely,” said Dr. Charles Schoenfeld, the ADA’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’. It is designed to do crowns and bridges as well as partial restorations and is under review by the FDA for these 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 take the project from his garage workbench in a small town in the French Alps to USC. Along the way, he was jeered by French colleagues who thought his concept 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 in 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,” Duret 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’s 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.
Duret’s consists of three desk-top computers, one on a counter near the patient’s chair, a second in a side room about the size of a walk-in closet and the third in a small room beyond that.
The first computer receives images from a camera that photographs the patient’s tooth--filed and otherwise conventionally prepared for a crown--from about 10 angles. The camera, shaped like a wand to make it easily maneuverable inside the patient’s mouth, uses low-power laser beams to gather and feed to the computer information about contour and volume.
The computer correlates all the angles to produce a three-dimensional picture of the patient’s tooth on the screen. Then the second computer goes to work.
This is the designing computer. Programmed into its memory are standard tooth designs--the same designs dental students must commit to memory in order to sculpt anatomically accurate dental protheses.
The image of the standard tooth--say, an incisor--is superimposed by the computer on the 3-D picture of the patient’s tooth. The dentist then modifies the outline, grooves and contours of the standard tooth to customize the crown’s fit. Because the dentist also has taken pictures of the opposing and adjacent teeth, the newly designed tooth can be visualized on the computer screen exactly as it will sit in the mouth, allowing adjustments in shape and fit without discomfort to the patient.
Satisfied with the design, the dentist has the second computer send it to the third one, which guides a milling machine. Because dental CAD/CAM uses milling technology instead of molding, it opens the door to new prosthetic materials, Preston said. Porcelain and metals have been the standard for crowns and bridges because molding requires materials that can be worked in liquid form.
The final step is coloring the crown so it matches the neighboring teeth. This is still done by hand, although Preston said it too may soon be taken over by computer.
For those working with dental CAD/CAM, the introduction of computer technology to this largely mechanical aspect of dentistry is logical, perhaps even overdue, considering the computer’s established role in the auto industry and other fields marrying design and manufacture.
But a dental crown is not a car, rolled off the assembly line with standardized design and function, the ADA’s Schoenfeld cautioned.
“It has to fit the patient’s tooth precisely, it has to restore contact with the adjacent teeth and it has to function properly in speech and deflection of food during chewing,” he said. “That’s a tall order for a computer program to fill.”
Duret and other CAD/CAM researchers believe that their computers are doing that now. They also think dentists and dental laboratories that make prostheses will want the systems because of advantages in cost and convenience.
“The dental laboratory will see an increase in productivity with machines that do not get sick. The dentist will increase productivity because less time will be required with the patient, and it will also be more comfortable for the patient,” Haas said.
Haas’ 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.
Bytes for a Better Bite
Dr. Francois Duret’s method of tooth reconstruction uses three desk-top computers, one on a counter near the patient’s chair, a second in a side room about the size of a walk-in closet and a third in a small room beyond that.
1. Laser Photography
Dentist inserts a wand-shaped camera into the mouth to photograph the patient’s tooth from about 10 angles. The camera uses low-power laser beams to gather and feed information to the computer about the contour and volume.
2. Computer-aided Design
The computer correlates all the angles to produce a three-dimensional picure of thre patient’s tooth on the screen. The picture is then transmitted to a designing computer where a replacement tooth is outlined. The dentist modifies the outline to customize the crown’s fit. Because the dentist also has taken pictures of the opposing and adjacent teeth, the newly designed tooth can be visualized on the computer screen exactly as it will sit in the mouth.
3. Milling the Crown
Satisfied with the design, the dentist has the second computer sent it to the third one, which guides a milling machine. Because dental CAD/CAM uses milling technology instead of molding, it opens the door to new prosthetic materials, Porcelain and metals have been the standard for crowns and bridges because molding requires materials that can be worked in liquid form.
