Getting flexible: A new twist on denture materials

April 12, 2019

A closer look at flexible dentures and the materials used for them.

Whether they are flexible or conventional, dentures must be strong and durable. They also must fit and function well for patients.

The denture base is a significant factor in all of these areas. More specifically, the denture base materials you use can affect your success in these areas. The Journal of Contemporary Dental Practice published a study in October 2018 suggesting the selection of the proper denture base material is “imperative as it largely affects the overall clinical outcomes and comforts.1

The evolution of denture materials in the last 100 years

The 20th century began the plastic era for full denture bases. In 1937, methyl methacrylate polymers (PMMA) acrylic resins became the denture base material of choice.Replacing the Vulcanite materials that preceded them, PMMA looked great because you could dye it to match the patient’s gums, was easy to fix and affordable.3

Meanwhile, chrome cobalt alloys made the bases for partial dentures. These were durable and functioned well.

However, dentures made with these rigid materials can cause patient discomfort in the beginning. Many times, dentures made of acrylic need adhesive to stay put and partial dentures with metal bases need metal clasps for retention., They can break if they drop, are prone to bacteria growth. and, some people are allergic to the residual monomers in some acrylics.

Related reading: The fascinating history of dentures

Enter flexible dentures

In the 1950s, denture professionals began to use thermoplastic resins for the base.4 Thermoplastics are versatile polymers you can heat and then mold. They are excellent for dental because you can heat up a prostheses and adjust it, if needed. They are also strong and resist shrinking.5

The first flexible denture material was Valplast, a nylon-based material, in 1953.6 However, the prevailing opinion at the time was for patients to chew properly, denture bases needed to be rigid. As a result, the use of flexible materials for dentures was limited.

In the 1960s, Flexite, then called Rapid Injection Systems, developed their line of nylon-based materials. Unlike Valplast, Flexite had a few varieties. However, there still were not many flexible dentures in patients’ mouths.

It wasn’t until the 70s and 80s when flexible dentures became more widely used. Flexible partial dentures were an excellent solution for patients that wanted a “Hollywood Smile,” and had some healthy teeth in their mouth.7

While the most commonly used are nylon, acetal resin, polypropylene, and acrylic resin, the thermoplastics used for flexible dentures include:8

  • Nylon (polyamides)                                                         

  • Polyester (polyethylene terephthalate)

  • Polycarbonates

  • Acrylics (polymethylmethacrylate; PMMA)

  • Polypropylene

  • Acetal resin (polyoxymethylene)

While there are flexible removable complete denture prostheses (RCDPs), usually flexible dentures are removable partial denture prostheses (RPDPs). Flexible RPDPs are also known as non-clasp, metal-free or clasp free dentures.

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Flexible RPDPs present some advantages over standard acrylic RPDPs:

  • They don’t require metal clasps.

  • The natural gum color shows through the material.

  • They are faster to produce than conventional dentures.9

  • The undercuts of the alveolar ridge can improve retention and make it easier to get the denture in the mouth.

  • The thermoplastic materials have improved durability so the base can be thinner than traditional acrylic dentures.

  • The materials present no allergic reaction risk.

  • The transparency of the material and tooth-colored clasps makes them hard to see in the patient’s mouth.10

  • The thermoplastic nylon resin material is thin and flexible which makes them more comfortable for patients to chew and eat.

  • The thermoplastic nylon resists staining and odors and its durability means it is less likely to break when dropped.

  • The thermoplastic nylon resin materials contain no BPAs and are considered the most biocompatible material.11

There are few studies about RPDPs clinical performance. There was some research on patient acceptance of RPDPs and RCDPs. The first study reported few problems, with just two midline fractures disclosed and some reported gradual fading of the color and teeth debonding. However, participants also reported improvements in chewing, talking and comfort with flexible dentures.12

The second study reported that 100percent of the participants preferred flexible dentures.13

Read more: How to fabricate flexible removable partial dentures

A critical disadvantage of flexible dentures is, like the more rigid acrylic bases, they are more vulnerable to bacteria.14 Flexible dentures need careful cleaning and maintenance with non-abrasive cleansers. In the case of partials, the patients should also be diligent about their oral health care routine to prevent any problems in their remaining healthy teeth.

Another disadvantage of the materials, particularly for RPDPs, is they can be more expensive than the conventional RPDPs. Although not significant, the increased cost is attributed to the extra finishing time needed for the material.15

Chances are, your lab has their favorite materials for fabricating RCDPs and RPDPs. The different materials finish, fit and adjust in various levels of difficulty, as well as have different color blending characteristics.16Usually, discussing the case with your lab will help determine what material is best for your situation.

Lee Soroca, president of Flexite, says within their line of seven different types of thermoplastics, they have built in different purposes to accommodate different skill levels of the technician. All of the Flexite thermoplastics are monomer-free resins and do not need two components like some other thermosetting plastics. It is the absence of these monomers that makes it possible for patients with allergy concerns to wear dentures.

Soroca says the different thermoplastics have their own characteristics to fulfill a different need. Some are rigid, some are flexible and the others fall somewhere in between. They also vary in their opacity, from transparent to opaque.  For example, Flexite’s nylon thermoplastics for RPDPs were tested and show to have the flexibility and proportional limits the same as the metals used.

“We've always had the slogan, ‘With Flexite, you have a choice,’” Soroca says, of the company founded in the early 1960s.  “The lab can decide what they like to work with and the patient has a choice what material they'd like.”

Flexite’s newest material is a crystalline nylon called Flexite Supreme-N. Soroca says it can be used for “almost anything” because of its flexibility, fitand finish characteristics. This ubiquity is unusual as many of the thermoplastics used in flexible dentures are usually intended for the type of prosthesis needed, whether a partial or full denture.

“The crystalline nylons and the polyolefins are state-of-the-art today,” Soroca says.

Soroca explains that as a dental material manufacturer, he is always testing new materials for dental-very few “make the grade.”

“They just don't have the fit,” he explains. “They also don't have the flexibility, or they don't color right or they're not suitable for dentistry.”

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When it comes to materials, Soroca, who has nearly 30-years’ experience in material for flexible dentures, says the different materials can improve the prosthetic outcomes. For example, Soroca recommends the super flexible nylons and the polyolefins for new technicians still developing their skills in blocking out and design. A more highly-skilled technician can use crystalline nylon, which is less flexible but has a precise fit. He also recommends the crystalline nylon for full dentures where there are allergy concerns.

Mary Ann Salcetti, DDS, PC, and a visiting instructor for Spear Education, writes in an article for Spear Education that one of the significant concerns for dentists regarding flexible RPDPs is changing their polishing habits.17 With acrylics, you polish with fast, pressure-applied motion. However, Dr. Salcetti suggests that the materials for flexible RPDPs require a different approach for polishing, which takes longer than traditional acrylics.  

“Whether or not you are using Valplast, Flexite, TCS or Iflex, each company has a system for polishing,” Dr. Salcetti writes. “If you are going to provide these partial dentures, be sure you have the armamentarium to polish them well.” 

Related reading: The biggest evolution in dentures is almost here

For polishing, Dr. Salcetti advises clinicians to round the edges of the rubber wheels to avoid cutting the nylon with a sharp edge. If she has any areas where the rubber wheel can’t polish, she uses a Robinson wheel in a horizontal motion with “a light dusting technique.”  

RCPDs and RPDPs have been around for almost 70 years, and so have the materials that are used to make them. Finding the right materials is crucial to your prosthetic outcome for fit, form and function. With the latest developments in thermoplastic materials for denture and partial denture bases, flexible dentures could provide a new twist for a comfortable and esthetic solution for your patients.

References

1.Singh, Reshu et al. "Comparison Of Flexural Strength And Surface Roughness Of Two Different Flexible And Heat Cure Denture Base Material: An In Vitro Study". The Journal Of Contemporary Dental Practice, vol 19, no. 10, 2018, pp. 1214-1220. Jaypee Brothers Medical Publishing, doi:10.5005/jp-journals-10024-2407.

2. Khindria S K, Mittal S, Sukhija U. Evolution of denture base materials. J Indian Prosthodont Soc [serial online] 2009 [cited 2019 Apr 5];9:64-9. Available from: http://www.j-ips.org/text.asp?2009/9/2/64/55246

3. Ibid.

4. Polyzois, Gregory et al. “Flexible Removable Partial Denture Prosthesis: A Survey of Dentists' Attitudes and Knowledge in Greece and Croatia.” Acta stomatologica Croatica vol. 49,4 (2015): 316-24. doi:10.15644/asc49/4/7

5. Mayer, Melissa. “What Is a Thermoplastic Polymer?” sciencing. Com. 30 April 2018. Web. 5 April 2019. <https://sciencing.com/thermoplastic-polymer-5552849.html>.

6. Nobilium. “Nobilium: History of flexible partials.” us.dental-tribune.com. 28 June 2010. Web. 6 April 2019. < https://us.dental-tribune.com/news/nobilium-history-of-flexible-partials/.>

7. Ibid.

8. Polyzois, Gregory et al. “Flexible Removable Partial Denture Prosthesis: A Survey of Dentists' Attitudes and Knowledge in Greece and Croatia.” Acta stomatologica Croatica vol. 49,4 (2015): 316-24. doi:10.15644/asc49/4/7

9. Green, Jenny. “What are Flexible Dentures?” colgate.com. Web. 5 April 2019. < https://www.colgate.com/en-us/oral-health/cosmetic-dentistry/dentures/what-are-flexible-dentures-1216>.

10. Polyzois, Gregory et al. “Flexible Removable Partial Denture Prosthesis: A Survey of Dentists' Attitudes and Knowledge in Greece and Croatia.” Acta stomatologica Croatica vol. 49,4 (2015): 316-24. doi:10.15644/asc49/4/7

11. Salcetti, Mary Anne. “The Pros and Cons of Thermoplastic Partial Dentures.” www.speareducation.com. Web. 6 April 2019. < https://www.speareducation.com/spear-review/2015/09/the-pros-and-cons-of-thermoplastic-partial-dentures>.

12. Ibid.

13.Singh, J P et al. “Flexible denture base material: A viable alternative to conventional acrylic denture base material.” Contemporary clinical dentistry vol. 2,4 (2011): 313-7. doi:10.4103/0976-237X.91795

14.Green, Jenny. “What are Flexible Dentures?” colgate.com. Web. 5 April 2019. < https://www.colgate.com/en-us/oral-health/cosmetic-dentistry/dentures/what-are-flexible-dentures-1216>.

15. Salcetti, Mary Anne. “The Pros and Cons of Thermoplastic Partial Dentures.” www.speareducation.com. Web. 6 April 2019. < https://www.speareducation.com/spear-review/2015/09/the-pros-and-cons-of-thermoplastic-partial-dentures>.

16. Ibid.

17. Salcetti, DDS, PC, Mary Anne. “The Pros and Cons of Thermoplastic Partial Dentures.“ www.speareducation.com. Web. 6 April 2019. < https://www.speareducation.com/spear-review/2015/09/the-pros-and-cons-of-thermoplastic-partial-dentures>.