The science behind removables

The surge in the demand for removable prosthodontics may be taking some dental laboratory owners somewhat by surprise, but others had the foresight to prepare for a rise in denture caseloads through formal education and training as well as their own day-to-day working experience. Tom Zaleske, AS, owner of Matrix Dental Laboratory & Consulting in Crown Point, Ind., has spent the past 25 years perfecting his denture craftwork as well as his business management philosophy so that each meets his personal and professional specifics.

"The reason I'm in removable prosthetics has nothing to do with anything other than me looking at demographics of what the 20-year window was going to look like for me when I hit 50 and where I was going to be," said Zaleske. "I chose in school to specialize in removable prosthetics because I saw that there was going to be a dire need for it. And it turned out that I was right."

Working exclusively in removable prosthodontics, Zaleske shares his experience and knowledge through lectures around the country as well as in his role as Technical Coordinator for Keystone Industries, providing telephone support for technicians. He emphasizes that technicians need to position themselves as the "go-to technical resource" for their dentist-clients not only to strengthen the technician-clinician partner relationship but also to ward off competition.

"Every large megabox lab has got a denture department. If they don't, they're getting one. If you're going to compete by buying equipment, you're done. You'll never win that battle because all a guy's got to do is buy the same thing you have and now you're at the same level," he warned. "You have to develop techniques and offer services that take time and a learning curve, which eliminates 95% of your competition because now most labs don't have the time or the inclination to want to do that. So now you're dealing with 5% of the guys who understand the equation, and those are the guys you compete against."

In his courses, Zaleske gets right down to the very elemental makeup of denture materials, laying the groundwork for working with the materials. “You've got to understand the chemistry of the product,” he said. “Understand that denture base acrylics form molecular chains, polymer chains. The longer the chains, the fewer the breaks in the chains, obviously the stronger the product. All of that is involved in time. What's critical is that technicians understand that there are basic time-involved procedures that when they are shortcut, they eliminate that baseline strength and start to compromise in areas where you can't track down where the problem might be coming from.”

Hot vs. cold

He advocates using a heat-cure acrylic that affords more control over the polymerization process by the technician. “The big thing to understand is the differences between a cold-cure and a heat-cured resin is that polymer chains develop because there's heat induced the product. The auto-curing material uses chemistry to produce heat that forms the chains, but once the chemistry burns out, that's it,” he said. “The heat-cured acrylic does have some initiation chemicals that get the reaction going, but the actual polymerization chains develop through outside heat source, which then you can control the duration and the intensity. By being able to go low and slow, you're able to extend the duration of the polymerization and also the amount that it produces. You are able to form longer chains, and you're able to drive off any of the chemistry that might remain after polymerization has occurred.”

He added that auto-cure resins can be very technique sensitive and require care in mixing the correct ratios. In addition, it is likely to have some leftover residual monomers that can cause problems when leftover monomers aren't used up by either the chemistry or heat, which can lead to various problems.

Low and slow

Zalaske’s “low and slow” approach to heat-curing starts with hand-mixing just enough acrylic that he can pack within the recommended working time. “You have roughly about 9 minutes before the material reacts to where it’s ready to pack, and you only have a window of 4 to 5 minutes between the time that it’s ready and the time that you start to enter where the product starts to get stiff and dry and doesn’t bond well to teeth.”

His advice to technicians facing 10 or more cases that need packing is to take it in stages. Mix enough acrylic for 4 or five cases, let it react and get ready, then mix up another batch; while the second batch reacts, the technician has enough time to pack the molds properly using the first batch, all before the second batch is ready for packing.

Once the acrylic is ready for packing, Zaleske hand packs the mold a bit at a time, using pneumatic flask pressure between increments, then lets it bench set for 20 minutes to allow the heat caused by the initial chemical curing to dissipate before placing the clamped flask into a water bath for heat curing.

He strongly warns against placing a prepared flask directly into a heated water bath rather than heating the water after placing the flasks inside. “A lot of guys have water that's warm because they don't want to wait for it to reach 165°F. They pack the flask by overfilling it, then squeeze it a bunch of times until it doesn't squeeze out any more. Then they clamp it and stick in water that's 165°F. One of the biggest reasons you have porosities is because the acrylic on the inside of the mold that you just packed is going through its initial stage, which generates its own heat initially and kicks the reaction off. When they drop it in 165°F water, it's now getting heat from the outside, and it's generating heat from the inside. All of this has to do with climb rate accelerating; it reaches 163° or above inside the mold a lot quicker because it's already generating heat itself on the inside.”

Next, it’s time for it to sit in the hot water. “Nothing comes out of my lab that isn’t cured 8 hours at 163°F,” said Zaleske. “It goes long, and it never goes into ice-cold water to cool it at the end of the cure. It’s always brought down at a slow temperature.”

This extended, controlled process is designed to allow material to transform from monomers to longer polymer chains than possible in faster auto-cure resins. The shorter cure time causes more breaks in the polymer chains, which results in porosity that can in turn cause malodor and discoloration.

Quality dictates time

“I would say that 80% of the problems out there with acrylics-whether it’s the bonding of teeth, a patient has a bad taste in their mouth, porosity-all have something to do with the technician trying to cut time,” said Zaleske. “They don’t know how to be proactive. The doctor says he needs it tomorrow, and they are trying to find a way to get it done as quick as they can. A lot of times, that means using 20-minute monomer, which does not allow enough time for good polymerization.”

Which is where knowledge and skill come into play. By being the resource of information that clinicians not only want but often need, the lab can be proactive with clients. “Labs want to be amenable to the doctor. There’s so many labs that are hungry out there that they’ll do anything to get an account” said Zaleske. “They feel if they can’t do it in a day, then the doctor will find somebody who can. I cannot do what they expect me to do if I’m always pushed into a corner to perform in the shortest period of time.”

By being strong with clients, he has set the rules for them, not the other way around. “I don’t let time dictate quality. Quality dictates time.”