Thermoplastic resins have been used in dentistry for over 50 years. In the meantime, their use has spread due to their superior characteristics, and the interest in polyamide based materials (nylon) have increased. Their ongoing development has yielded new classes of more and more advanced materials and technologies, which make possible new applications for thermoplastic resins in the future. The dentists have to meet growing demands for prosthetic rehabilitation due to population aging and higher requirements on the quality of life. In this article we will talk about physical, mechanical and thermal properties of polyamide materials, surface roughness, flexibility and absence of monomer in comparison with PPMA, and the various applications of polyamide resins in removable dentures.
Polymethylmethacrylate (PMMA) resin has been a commonly used denture base biomaterial since 1937. The properties of favorable working characteristics, ease of manipulation, ability to repair, aesthetic appearance, low cost, acceptability by most of the patients, stability in the oral environment, and accurate fit have contributed to the success of this material.1-4 They are synthetically obtained materials that can be modeled, packed or injected into molds during an initial plastic phase, which solidify through a chemical reaction-polymerisation.5 Its mechanical feature, however, is far from the ideal because it has weak flexural and impact strength and low fatigue resistance. These often lead to denture failure during chewing or when it is dropped.3 Denture fractures is one of the most common clinical problems.6 Other disadvantages of PMMA resin are increased porosity, high water retention, volume variations and irritating effect of the residual monomer. Researches have attempted to improve the mechanical properties of PMMA denture bases by reinforcement with fibers glass or carbon, (Fig.1,2) and also by chemical modification7. Development of alternative materials such as thermoplastic resins has also been reported in the literature.2,8
Thermoplastic resins may be repeatedly softened by heating and hardened by cooling without undergoing a chemical change. They may be considered as being composed of bundles of chainlike molecules (called polymers) of many different lengths and molecular weights. They can be classified as thermoplastic acetal, thermoplastic polycarbonates, thermoplastic acrylic and thermoplastic nylon (polyamides).9However, the desired denture base material has not been developed yet.3
Thermoplastic polyamide (nylon), synthesized by the condensation reaction between a diamine and a dibasic acid,1,10 was first studied as a denture base biomaterial in the 1950s. The early form of polyamides displayed several problems, such as high water absorption, discoloration tendency, surface roughness, bacterial contamination, and difficulty in polishing.1,11It was especially used in exceptional cases like repeated denture fracture and for patients with tissue allergies against acrylic denture base or denture fractures.3 In recent years, polyamide has been attracting attention as a denture base biomaterial due to the advantages of: favorable aesthetic outcome, reflect the color of gingival tissue beneath due to highlight transparency, they have high quality esthetic properties; toxicological safety to patients allergic to conventional metals and resin monomers; higher elasticity than conventional heat-polymerizing resins; high physical strength, flexible and strong structure; heat resistance and chemical resistance, low water absorption and solubility, low porosity.1,9,10,12,13,14 The polyamide resins could be injection-molded, the advantages of using this system lay in the fact that the resin is delivered in a cartridge, thus excluding mixture errors with long-term shape stability, reduces contraction, and gives mechanical resistance to aging.14,15
Some disadvantages of polyamide are also described, mainly the lack of chemical bonding between the base and the acrylic teeth, thus the need for mechanical retention (Fig. 3), and the difficulty in repairing and relining the denture.2,10,14,16
Physical, Mechanical and Thermal Properties of Polyamide Resins in Comparison with PPMA
Nylon is a crystalline polymer whereas polymethylmethacrylate is amorphous.1 This results in lack of solubility in solvents, high heat resistance, and high strength coupled with more ductility.9 Polyamide molecules contain hydrogen bonding, which increases the melting point of the polyamide.1 The outstanding features of the nylons are their toughness, low density, abrasion resistance, higher melting point and resistance to chemical attack. The flexibility coupled with its strength, enables it to resist all normal attempts to fracture.9
The main advantage of nylon lies in exceptional mechanical properties of resistance to shock and repeated stressing, it has higher fatigue resistance compared to PMMA. Polyamide denture bases are strength and lightness. Nylon has higher abrasion resistance, elastic memory, creep resistance and is conductive to cyclic stress.9 The polyamide denture base resins have lower flexural strength at the proportional limit and low elastic modulus12 along with good fracture resistance9.
Though nylon has superior mechanical properties than any other non-metallic base yet there are some serious limitations such as processing difficulties and dimensional changes. Nylon is hygroscopic; its moisture content varies slowly with the surrounding conditions. On immersion in water the material swells, i.e. there is linear expansion. Processing the denture base materials produced unequal deformation in different dimensions (anterior-posterior and cross-arch). The magnitude of this dimensional change depends on the conditions of molding, shape of the mould, and direction in which it is measured.9
Nylon has low coefficient of linear expansion and galvanic conductance.9 Less sorption and solubility of thermoplastic polyamide nylon resin would extremely decrease porosity of the denture base and thus promote hygiene maintenance.13
Surface Roughness of Polyamide Resins in Comparison with PPMA
Surface roughness is an important factor, which affects the clinical life of materials and resistance to plaque formation. Surface roughness is related to the abrasion of materials. Rough denture surface makes accumulation of microorganisms easier and a higher level of biofilm formation occurs compared to smooth surfaces. Rough surfaces also affect staining resistance, health of oral tissue, comfort of the patient, aesthetics and retention of the dentures directly or indirectly.14
Polyamide denture base material when polished with conventional laboratory technique became more than 7 times smoother whereas processed PMMA when polished became more than 20 times smoother using the same polishing technique. However the surface roughness of polyamide is well within the accepted norm of 0.2 μm Ra. Polyamide produces a clinically acceptable smoothness after conventional polishing.2
Unilateral or bilateral undercuts are frequently encountered and may complicate successful fabrication of denture prosthesis. Management of these situations conventionally includes alteration of the denture prosthesis bearing area, adaptation of the denture base,careful planning of the path of insertion and the use of resilient lining material.An alternative denture prosthesis design in which optimal flange height and thickness can be achieved is by using flexible denture basematerial.Polyamide denture base materials are more flexible than the commonly used PMMA.2,13The flexibility of nylon varies greatly depending on the type of molding powder used, temperature of injection, pressure of injection.9
With thermoplastic materials, the clasps are made of the same material as the denture base (Fig. 4), when using superflexible polyamide or we used ready-made clasps, in the case of using medium-low flexibility polyamide. When manufacturing polyamidic dentures, the support elements blend in with the rest of the denture, as they are made of the same material.17
Material Free of Monomer
Complete biocompatibilityis a major advantage of polyamide resins, because the material is free of monomer and metal, these being the principle causes of allergic reactions in conventional denture materials.
Applications of Polyamide Resins in Removable Dentures
Flexible denture bases may be indicated in patient’s requiring replacement of teeth in esthetic zone, patient’s with restricted mouth opening13, severe soft and hard tissue undercuts2,13,14, sensitivity to PPMA monomer or metal2,13,14,18, bruxism cases, thin mucosa and excessive bone resorption, cases where the patient cannot tolerate the force applied by the denture, very old patients with low motor capacity14, temporary prosthesis after implants13,14, precision attachment, combination with metal framework, single cast partial dentures, preformed partial denture clasp, immediate dentures9, space maintainers13, occlusal appliances18, flexible tooth born partial denture framework9,18,denture bases of RPDs without metal clasps, the flexibility of polyamide allows retentive elements that match the color of the gums or teeth11,12,18.
Thermoplastic resins have been used in dentistry for many years. During that time the applications have continued to grow and the interest in these materials by both the profession and the public have increased. The materials have superior properties and characteristics and provide excellent esthetic and biocompatible treatment options.
Polyamide denture base materials proved to be a useful alternative to conventional denture base resins in clinical situations, including patients who demonstrate a certain degree of tissue undercuts or repeated fracture of dentures and also those that have sensitivity or allergy to methyl methacrylate monomer.
In spite of the various advantages and indications of thermoplastic polyamide resin, further long-term studies are recommended to assess the overall usefulness of the material.
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By Dr. Danielle El Hakim, Dr. Mireille Rahi, Dr. Najib Abou Hamra, Dr. Elias Smaira