Needing a tooth replaced, Robert P Crease discovers just how much materials science there is in dentistry
My dentist, Dr Albora, told me I needed a new tooth. He made me bite down on a U-shaped tray filled with polyether, an elastomeric material that’s been used in dentistry for decades. I tried not to gag while the gooey stuff set. Albora then waggled it off and sent the imprint to a company on Long Island that makes false teeth. But as my tooth’s colouring was unusual, Dr Albora said I had to go there myself.
A week later I drove to Marotta Dental Studio, where Igor Binshteyn sat me in a chair, lifted my lip and inspected a tooth to the side of my upper jaw. He was helping to fabricate its replacement and needed to get the exact colour.
As Binshteyn finished, I remarked teasingly on the company’s car park, which was chock full. Do you need that many people to make false teeth? What could possibly be involved to make things smaller than the nail on my little finger?
Binshteyn laughed. “Follow me”.
He opened a door to a huge undivided warehouse-like space about 1200 square metres in size. Some 50 people were working at lines of tables where the steady hum of conversation was punctuated by the noise of ovens, lathes and grinding machines; imagine the sounds of a metalworking shop in a busy train station.
“Do you want a tour?” Binshteyn said.
Of course I did.
Open wide
My tour guide was Marotta’s vice-president, Steven Pigliacelli, who teaches prosthodontics at New York University. A prosthodontist is a dental specialist who has completed dental school and, in the US, has had at least three additional years of advanced training to focus on restoring and replacing missing or damaged teeth. As I learned, prosthodontists must practice materials science.
The lab has two paths for making teeth, Pigliacelli explained, each with different materials and methods. The first is known as “porcelain-fused-to-metal”, or PFM. It begins with technicians applying gypsum – hydrated calcium sulphate – to create stone replicas of the jaw and teeth from the impressions given to them by dentists such as Albora.
Gypsum has different coefficients of expansion for different dental purposes, and textbook formulas give the amounts of liquid for each kind. But Marotta employees found that tiny amounts of water must be added or subtracted to the gypsum to compensate for things like humidity and temperature before the gypsum is used. “It took us months,” Pigliacelli explained, “to realize that how it’s shipped, how it’s stored, what season it is – all these affect the material.”
I watched as a technician sectioned a stone jaw replica to cut out a tooth, turn it into a master model called a die, and test it in an articulator that mimics jaw movements. I saw another wax that die, use a lost wax technique to create a solid gypsum frame, and cast a silver-palladium alloy. That creates the metal inner core for the tooth, called a coping, which is shaped and finished before porcelain is applied. Porcelain is baked onto the coping in another area of the lab.
“The metal and porcelain need closely matched coefficients of thermal expansion or the porcelain can separate from the metal,” Pagliacelli told me. “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.”
Smile for the camera
The second path to making a tooth involves scanning the teeth rather than waxing a die. The electronic files are worked on in the computer, printed out, reworked and 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 used for front teeth.
“So we have two parallel paths, PFM and digital scans”, Pigliacelli said. “It’s usually a personal choice of the dentist”. But external pressures are driving a move away from PFM, he said. Five years ago the price of palladium – a key component of the PFM alloy – soared as it was increasingly used in devices to reduce auto and other emissions. “That’s cut down on the PSM market.”
In another corner of the lab I ran into Binshteyn again, who was using different porcelains and ceramic stains to create colours. “Teeth are not a solid colour,” he said, “but vary in shade from reddish near the gum line to lighter colours toward the biting edge. They also vary in translucency.” (Check this out yourself.) Furthermore, the colour of a tooth must match not its neighbours but the corresponding tooth on the other side of the mouth. To make a natural-looking tooth, Igor picks out a base colour and then layers in shades of porcelain.
As my lab tour went on, Pigliacelli showed me different materials for different applications. One challenge is making teeth for a person with bulimia, whose acidic oral environment can corrode dental materials. Other challenges arise with people who have had failed implants, or temporomandibular joint – or TMJ – a disorder involving jaw joints and muscles.
On the way out I met Lenny Marotta, Pigliacelli’s brother-in-law, who had founded Marotta Studio in 1981. Marotta is fascinated by the history of dental materials, which include bone, ivory, wood, gold and silver. He showed me his collection of early prosthodontic instruments, including milling machines, furnaces, hammers and air compressors.
Later, Marotta also pointed out a strange-looking oven. “That’s for processing vulcanite, a hardened rubber material widely used for dentures before modern plastics.” But he’s been unable to find a museum interested in old prosthodontic stuff. “Nobody knows we exist,” he complained.
The critical point
I’d assumed that materials science was required to make prosthetic teeth biocompatible, corrosion resistant, and able to withstand mechanical motions such as crushing, biting and grinding. I also knew that teeth have to be made economically enough to be widely available, aesthetic enough to be desirable, and versatile enough to withstand a variety of oral environments.
But I had no idea how much materials science was required to bring all these properties together.
Back in his office, Dr Albora installed the tooth and handed me a mirror. I had to pull up my lip to see it way on the side of my mouth. “I’d have to smile before somebody notices it,” I complained. “Well,” he said, “that’s for just in case you do.”
