Advances in Dentures: Novel Polymeric Materials and Manufacturing Technologies

Abstract

Acrylic resins dominated dentures technology for several decades. Due to their many disadvantages, new types of polymers, with better properties, suitable for dental prosthodontics applications were constantly attempted. The choice of polymeric materials and manufacturing technologies has experienced significant development in recent years. Different types of thermoplastic injected resins, light-cured resins, or the versatile high-performance polymers are several choices of novel materials for dentures manufacturing. CAD/CAM systems, both substractive and additive, are being considered the most promising choice for the future manufacturing of polymers in dentistry. The chapter is focused on presenting the choices of novel polymeric materials, their manufacturing technologies, and applications in prosthodontics.

1. Introduction

Polymer-based materials play an important role in prosthodontics. They have a wide range of applications, including fixed or removable partial dentures, full dentures, overdentures, provisionals, facial prostheses, splints, and mouthguards. The most commonly used polymers in prosthodontics are polymethyl methacrylate (PMMA), polycarbonate (PC), polyurethane (PU), polyamide (PA), polyoxymethylene (POM), and polyaryletherketone (PAEK) [1]. The PAEK family includes polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), both used in prosthodontics.

Due to the fact synthetic PEEK has been recently investigated as an implant material [2, 3], its area of application has expanded to implantology, as well.

Acrylic resins, which represented an important step forward in dentistry, have been used in prosthodontics since the middle of the twentieth century. The drawbacks of classic acrylic resins, manufactured by means of the thermopolymerization technique, include high polymerization shrinkage, low mechanical resistance, awkward flasking, and difficult processing. The residual monomer may result in increased porosity, allergenic potential, and cytotoxicity. Another disadvantage is due to water sorption, which is a slow process and takes place while wearing the denture for a certain period of time, potentially leading to bacterial colonization. Their main advantage is represented by the affordable cost [4, 5]. New classes of acrylics, with better characteristics are currently available, such as thermoplastic, injected, or CAD/CAM manufactured resins.

Concurrently, not only new polymeric materials, with better properties were developed, but also new manufacturing technologies, such as injection, light-curing, CAD/CAM milling, and 3D printing, have emerged.

Thermoplastic (injected) and CAD/CAM manufactured acrylic prosthodontics are being characterized by better impact resistance, long-term stability, and dense and smooth surface, with low or no porosity. Due to the limited water sorption, their long-term stability is higher, and the absence of residual monomers results in good biocompatibility [6].

2. Novel polymeric materials and manufacturing technologies for prosthodontics applications

2.1 Thermoplastic injected polymers

The most used thermoplastic-injected polymers for prosthodontics manufacturing include PA, POM, PMMA, and PAEK. The material, in granular form, wrapped in special cartridges, is heated and injected by means of special devices, excluding any chemical reaction, thus preventing polymerization shrinkage (Figure 1) [7, 8].

Figure 1.

Thermoplastic polymer for injection, in a granular form; injection unit for thermoplastic resins.

Thermoplastic injected dental polymers are monomer-free, thus nontoxic and biocompatible [9].

They are indicated for manufacturing of removable partial dentures, preformed clasps, removable partial denture frameworks, provisionals, full dentures, orthodontic appliances, anti-snoring devices, mouthguards, and splints [10].

Polyamides are flexible thermoplastics, practically unbreakable, indicated for denture bases in patients with allergies, and retentive dental fields. Based on the flexibility degree, polyamides are classified in superflexible and with medium-low flexibility. The superflexible type is extremely elastic (Figure 2), while the medium-low one is a half-soft comfortable material. The clasps may be manufactured from the same material, ready-made clasps may be used, or even metal clasps may be considered (Figure 2) [11].

Figure 2.

Superflexible polyamide removable partial denture; superflexible polyamide removable partial denture combined with metal clasps.

Polyoxymethylene or acetal resins, characterized by low elasticity and high impact strength, are being indicated for replacing the metal framework and clasps in removable partial dentures. Other indications include provisionals, splints, orthodontic appliances, and Kemeny-type single unilateral partial dentures (Figure 3).

Figure 3.

Removable partial dentures with POM framework and clasps, and PMMA saddles; combination between a thermoplastic PMMA resin saddle, metal, and POM clasps.

2.2 Light-cured polymers

Light-cured diacrylic composite resins were initially elaborated as direct esthetic restorative materials, and the indirect type, successfully used in dental laboratories, has been subsequently developed. Their indications include veneering of metal-polymeric crowns and bridges, single-tooth or temporary crowns, inlays, onlays, and repairing damaged porcelain veneers. One of their advantages is the prolonged handling time, as well as good adherence to the metal framework, and low polymerization shrinkage. In certain cases, more than one curing method is used, for example, combination of heat, pressure, and light-curing [12].

The light-curing eclipse resin system (Dentsply-DeguDent) enables the manufacturing of full dentures, removable partial dentures, and splints and allows speedy manufacturing by eliminating time-consuming stages such as pattern modeling, investing, and heat-curing. Composed of three wax-like urethane-based resins: baseplate resin, setup resin, and contour resin, it has no allergic potential.

The first step is mounting the teeth directly on the denture’s light-cured base. The denture itself is used for try-in, and the esthetic and phonetic checking is being carried out. Afterwards, the setup and contour resins are used to finalize the denture, which is light-cured (Figures 4–6) [13, 14].

Figure 4.

Light curing of the denture base and of the finalized denture.

Figure 5.

Teeth mounting on the denture’s light-cured base, by using the setup resin.

Figure 6.

Applying the contour resin, and processing it by using the warm air gun to create a smooth surface.

2.3 CAD/CAM manufactured polymers

CAD/CAM systems, either substractive or additive, have been used in dentistry since 1980s, at first for fixed prosthodontic restorations, later on, expanding to removable prosthodontic restorations, as well as various dental appliances [15].

CAD/CAM systems have three major components. The data acquisition may be carried out by intraoral or extraoral scanning. Extraoral scanning involves data acquisition by means of an impression or a model, which is being converted into virtual models. The second component is the software. Its role is to design the virtual restorations and establish the milling parameters. The third component depends on the type of the system, substractive or additive. A milling machine is used for substractive from a material block (Figure 7). 3D printing device is used for additive manufacturing [16].

Figure 7.

PMMA block; milling artificial teeth from a PMMA block.

The advantages of CAD/CAM systems are reduced number of appointments and the ease to access the previously saved digital data if needed [17].

The high initial cost of the milling machine or 3D printing device may be overcome by referring the data to a milling or 3D printing center, which will handle the manufacturing step.

The most used polymeric materials for CAD/CAM milling are PMMA, composite resins, PU, PC, and PEEK. It is major indications for polymeric materials include provisionals, crowns, copings, mouthguards, short bridges, veneers, inlays, onlays, denture bases, artificial teeth, removable partial dentures framework and clasps, patterns, and models. The bicolored discs allow manufacturing of monolithic total dentures in one uninterrupted milling process.

Probably, the most important advantage of the subtractive method is using homogenous materials. However, its major drawback is due to material loss which leads to higher costs.

The additive manufacturing addresses these drawbacks in the CAM step, and the prosthodontic devices being fabricated by materials layering [18]. The materials present themselves in different forms, depending on the type of 3D printing method used.

The most frequently used 3D printing methods for dental polymers are based on vat photo-polymerization, material jetting, and material extrusion, showing noticeable differences in resolution, accuracy, and repeatability [19].

Currently, vat photo-polymerization technologies are the most used in dentistry, including the stereolithography (SLA) and digital light processing (DLP) methods. The material, in a liquid form, is being selectively light-cured by means of a directed UV-laser beam (SLA) or a UV-light mask (DLP) (Figure 8). Unluckily, both SLA and DLP printed objects need post-processing. The post-processing steps include cleaning with isopropanol, to remove the residual monomer, and post-polymerization (Figure 9) [20, 21, 22].

Figure 8.

SLA 3D printer; 3D printed dental model.

Figure 9.

Post-processing of a SLA 3D printed denture base: Cleaning with isopropanol, and post-polymerization.

Both SLA and DLP allow manufacturing a wide selection of polymeric materials: PMMA, reinforced PMMA, Bis-GMA, and UDMA-based resins, PU [23].

Material jetting (MJT) is a droplet-based and photo-polymerization technique. The material, in a liquid form, is applied, as tiny drops, from the print head directly to the build platform, and needs no post-processing. The layer-by-layer deposition is extremely fast and highly accurate. A special feature of MJT is the possibility of multi-material, multicolor 3D printing [23].

Fusion deposition modeling (FDM) is an extrusion-based printing technique. Thermoplastic materials, in a filament form, are being deposited layer-by-layer [24].

A wide range of thermoplastic polymeric materials, including polyesters, PA, PU, PC, acrylonitrile butadiene styrene, and PEEK are being used.

MJT and FDM also allow a multi-material, multicolor 3D printing mode.

The applications of 3D printing include models, custom trays, dental bites, provisionals, crowns, full dentures, removable partial dentures frameworks and clasps, artificial teeth, try-ins, surgical/implant guides, templates, implants, orthodontic appliances, and mouthguards. Flexible polymers may be used to manufacture 3Dprinted dentures bases and mouthguards [18].

2.4 Shape memory 4D printed polymers

4D printing, incorporating time as the fourth dimension, enables printing constructs, which are capable to transform over time, under different stimuli, allowing the creation of complicated structures with on-demand dynamically controllable shapes and functions (Figure 10) [25]. Shape memory polymers are capable of remembering permanent shapes, present the capability to be deformed temporally, and return to their original shape under an external stimulus [26].

Figure 10.

Comparative schematics of 3D and 4D printing technologies.

4D printing has proven its efficiency in dentistry due to the dynamic oral environment, which undergoes continuous changes of temperature and humidity [27].

4D printed denture bases adapt to the occlusion forces, including eating and drinking patterns, and are being characterized by similar elasticity and thermal properties as the oral tissues. In the case of residual ridge resorption, using smart materials to compensate for bone loss has been attempted. Other applications of 4D printing in prosthodontics include crown copings, removable partial dentures frameworks, orthodontic appliances, surgical guides, and implants [28].

2.5 High-performance polymers

High-performance polymers are characterized by the capability to preserve their mechanical, thermal, and chemical properties when submitted to various extreme environmental conditions [29].

The most used high-performance polymers for dental application are PEEK and PEKK-based, both belonging to the PAEK family. PEEK and PEKK are ketone-based, thermoplastic polymers, with excellent mechanical and chemical resistance, and low water sorption, being highly biocompatible, and displaying an elasticity comparable to bone [30]. Compared to acrylic resins, PAEK has no residual monomer content and has no allergenic potential. Due to its corrosion resistance, PAEK is considered as an alternative to metallic restorations. It has the advantage to be lightweight (Figure 11), but, because of its opacity and whitish-gray color, needs to be veneered, when used for fixed restorations. Because of its bone-like elasticity module, it is considered a good option for Ti implants.

Figure 11.

A milled removable partial denture framework, made of PEEK (including clasps), weight only 1.36 grams. The removable partial denture, including the acrylic saddles, weight only 3.36 grams.

Its indications include copings, bridges infrastructure, removable partial dentures framework and clasps, implants, and implant abutments (Figures 12 and 13). It may be optimized by adding ceramic or hydroxyapatite nanoparticles and carbon fibers [31, 32].

Figure 12.

PEEK and PEKK bridge infrastructures, respectively.

Figure 13.

PEEK removable partial denture framework and clasps.

In the case of PEEK implants, because the material is bioinert, it lacks osseointegration, so coating is needed [2, 33]. PEEK is also considered a choice for scaffold manufacture, despite its non-degradability, when blended with biodegradable polymers, such as poly(glycolic acid) and polyvinyl alcohol [34, 35].

PEEK in granular form and PEKK in ingots form may be manufactured by injection (Figures 14 and 15). PEEK and PAEK blocks are being used for milling by CAD/CAM substractive systems, (Figure 16), and, more recently, PEEK in a filament form has been used for 3D printing, namely by FDM, or SLS, which uses the material in a powder form [36, 37].

Figure 14.

PEEK in granular form and the injection unit.

Figure 15.

PEKK ingots, the preheating, and the injection units.

Figure 16.

PEEK block for CAD/CAM milling; milling of a PEEK block.

3. Conclusion

Multiple options of novel polymeric materials for prosthodontic applications are currently available. Various technologies have been attempted to replace the classic thermopolymerization method used for PMMA dentures. Each material and technology has its advantages and drawbacks, and the best choice should be made considering the status of the patient, the available technological infrastructure, and the involved costs. The future in prosthetics comes with the emerging new smart polymeric materials, which are able to self-adapt in the continuously changing conditions of the oral environment.

Acknowledgments

The authors would like to thank Professor Cristina-Maria Bortun for her valuable contribution and support.

Conflict of interest

The authors declare no conflict of interest.