Fabricating a replacement tooth is a materials-science problem as much as a dental procedure, according to a tour of Marotta Dental Studio in Long Island, New York. The laboratory, which employs about 50 technicians, produces crowns, bridges, and false teeth using two distinct pathways that demand precise control over ceramics, metals, and digital workflows.
The first method, known as porcelain-fused-to-metal, or PFM, begins with gypsum — hydrated calcium sulphate — used to create stone replicas of a patient's jaw and teeth from dental impressions. Gypsum has different coefficients of expansion depending on its intended dental purpose, and textbook formulas specify the amount of liquid for each type. But technicians at Marotta found that tiny adjustments of water must be added or subtracted to compensate for humidity and temperature before the gypsum is used. «It took us months to realize that how it's shipped, how it's stored, what season it is — all these affect the material,» said Steven Pigliacelli, the lab's vice-president, who teaches prosthodontics at New York University.
In the PFM process, a technician sections a stone jaw replica to cut out a tooth, turning it into a master model called a die, and tests it in an articulator that mimics jaw movements. Another technician waxes the die, uses a lost-wax technique to create a solid gypsum frame, and casts a silver-palladium alloy. That forms the metal inner core, or coping, which is shaped and finished before porcelain is applied. The porcelain is then baked onto the coping in a separate area of the lab. «The metal and porcelain need closely matched coefficients of thermal expansion or the porcelain can separate from the metal,» Pigliacelli said. «Every once in a while a company tells us 'We've got this really cool new alloy!', but we need to match the coefficients of expansion in the ceramic material. We're working with sensitive materials for sensitive applications, and we have to experiment with and adapt them. This stuff about materials you don't learn in school but work out in the lab.»
The second pathway relies on digital scanning rather than waxing a die. Electronic files are manipulated on a computer, printed out, reworked, rescanned, and milled by a CAD machine using materials such as zirconia — nicknamed «ceramic steel» — and EMAX, a lithium disilicate glass-ceramic. Zirconia is extremely hard and used for teeth that endure heavy grinding forces, while EMAX is more aesthetically pleasing and often chosen for front teeth. «So we have two parallel paths, PFM and digital scans,» Pigliacelli said. «It's usually a personal choice of the dentist.»
External economic pressures are driving a move away from PFM, however. Five years ago the price of palladium, a key component of the PFM alloy, soared as it was increasingly used in devices to reduce automotive and other emissions. «That's cut down on the PFM market,» Pigliacelli noted.
Color matching adds another layer of complexity. Igor Binshteyn, a technician at the lab, uses different porcelains and ceramic stains to create natural-looking teeth. «Teeth are not a solid color,» he said, «but vary in shade from reddish near the gum line to lighter colors toward the biting edge. They also vary in translucency.» Furthermore, the color of a tooth must match not its neighbors but the corresponding tooth on the other side of the mouth. To achieve this, Binshteyn picks a base color and then layers in shades of porcelain.
The lab also tailors materials for specific medical conditions. One challenge is making teeth for a person with bulimia, whose acidic oral environment can corrode dental materials. Other challenges arise with patients who have had failed implants or temporomandibular joint disorder, a condition involving the jaw joints and muscles. These cases require technicians to select and adapt materials with properties suited to each patient's unique oral environment, demonstrating how dentistry depends on an empirical understanding of how ceramics, metals, and other materials interact.
