Types of Thermoplasic Resins

Properties desirable in thermoplastic denture base resin include high flexural strength, low elastic modulus, and, high impact strength. Various types of thermoplastic resins have been approved for dental use; they include:²⁰

1. Acetyl or acetal resin:²⁴ Acetyl resin also known as polyoxymethylene (POM), is an aesthetic denture base and denture clasp material used since the late ’80s. They are indicated in patients allergic to nickel and chromium alloys.²⁵ They have optimal resilience, modulus of elasticity, impact strength, and resistance to organic solvents, oils, and hot and cold water making them a suitable material for the fabrication of retentive clasps, connectors, and support elements for RDPs (Figure 5).^26,27

   Figure 5 - Acetyl resin NMCD

   

   Figure 5 - Acetyl resin NMCD

2. Polyamide: The first polyamide-based flexible partial denture system (Valplast) was introduced in 1953. The three popular polyamides are detailed below:

   1. Valplast^®: Vaplast is a nylon-based material containing 99.9% polylaurolactam. It is semitransparent pink, odorless, non-allergenic, and has a low elastic modulus.²³ Its flexural strength and flexural modulus are lower compared to other thermoplastic resins, however, only 33% (approximately) higher than PMMA resins, rendering it soft, elastic, flexible, and fracture resistant.²³ Its high elasticity permits its usage in teeth and/or ridges with a large undercut.²³ Its flexibility reduces the likelihood of fracture even when increased occlusal forces or stresses are applied to it. Due to its high fracture resistance, denture bases can be made thinner and lighter, thereby, aiding in improved patient comfort.^18,19 Its disadvantages include discoloration and staining; difficulty in grinding, polishing, clasp adjustment, relining and repairing; and, loss of fit when used to cover a wide area.^18,23

   2. Lucitone FRS: Lucitone FRS is a high-grade microcrystalline polyamide. Lucitone FRS is an esthetic and stable material usually recommended for replacing anterior teeth.²³ This denture base material is generally indicated in areas where the occlusal forces are light and only a few teeth are missing.²³ Its indications can be expanded by incorporating a metal framework in the design of the prosthesis.²³ It is available in a range of flexibility from firm (2mm) to super flexible (1.5mm). Five shades of Lucitone FRS are available including original, light, light reddish pink, dark pink, and clear. It is elastic and fracture-resistant. It is softer than polyester and polycarbonate resins (resulting in improved patient comfort), but harder than Valplast^® (making it more durable.) It provides an optimal fit and is highly resistant to abrasion, stains, and calculus and easy to grind and polish.²³ However, Lucitone FRS^® has a few disadvantages including the risk of fracture if the denture base is made too thin,²⁸ color instability,²⁹ loss of fit with long-term use, and inability to bond to self-curing resins (repair and reline difficultly).

   3. Ultimate: Ultimate is a soft, elastic, and durable thermoplastic material.²³ It permits the fabrication of thin and light denture bases. Incorporation of a metal rest or metal major connector, and wires in the design of the prosthesis improves its hardness, fracture resistance, and permits clasp adjustments.²³ It is not indicated in patients with inadequate restorative space and shallow vestibular depths.²³

3. Polyester resin: EstheShot1 (ES 1) and ES Bright are the new polyester-based thermoplastic resins.²³ They provide the most superior fit,³⁰ and are safe and esthetic. Repair and relines are easily accomplished because they bond to self-polymerizing resins.¹⁹ They have moderate absorbency and are susceptible to discoloration. Their resilience and Rockwell hardness are less than the polycarbonate and polyamide resins making them susceptible to surface roughness and fractures.³¹

4. Polycarbonates: The ability of polycarbonates to bond to self-polymerizing resins, their fit, feel, and the risk of discoloration post immersion in curry is similar to that of PMMA resins.²³ They are indicated in areas where the occlusal forces are light and only a few teeth are missing.²³ Repairs and relines are easy to accomplish chairside because they bond to self-polymerizing acrylic resins.^19,23 A metal framework should be incorporated to provide optimal support and prevent fracture of the resin clasp. The two common polycarbonate-based thermoplastic resins include Reigning^® and Reigning N^®. Reigning N^® is an improvised thermoplastic resin with flexural strength and flexural moduli greater than polyamides and polyesters. Its fit is inferior to the polyester resins, but, superior to the polyamide resins.²³

Table I compares the properties of the three commonly used dental thermoplastic resins.

Note: The polyester and polycarbonate resins are stiffer (due to their high elastic moduli) than the polyamide resins.³² In the presence of a deep undercut, a stiff clasp may exert increased stresses on the abutment tooth or may fracture during the insertion and removal of a prosthesis.³² Hence, in addition to the factors described above, it is also critical to consider the retentive areas and the depth of the undercut of the abutment teeth when selecting the thermoplastic denture base resin for NMCDs.³²