Functional modi ications of denture base resin-A review

Allour Vaishnavi1, Balaji Ganesh S*2, Anjali A K3 1Saveetha Dental College and Hospital, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai 600077, Tamil Nadu, India 2White lab Material Research Centre, Saveetha Dental College and Hospital, Saveetha Institute of Medical and Technical Sciences, Saveetha University, 162, PH Road, Chennai 600077, Tamil Nadu, India 3Department of Pathology, Saveetha Dental College and Hospital, Saveetha Institute of Medical and Technical Sciences, Saveetha University, Chennai 600077, Tamil Nadu, India


INTRODUCTION
Acrylic resins were proposed by Dr. Walter Wright and Vernon Brothers in Philadelphia. In 1936, transparent resin came into existence and also acrylic powder came into use in 1937. 95% of den-tures were made by them. Polymethyl methacrylate, due to its extraordinary aesthetics and ease of processing & repair it was considered as the basic denture base material used in dentistry (Kostoulas et al., 2008). Certain properties such as poor colour stability of self-cured resins, porosity, crazing, warpage, poor adhesion to metal, and porcelain retention were the major disadvantages of this material (Petrescu, 2010). In the area of denture base resins, the new modi ications have been made to overcome the problem. To control the problems of monomer allergy, the hypoallergenic resins were used (Rodriguez, 2013). By using gum toning in the resins can improve denture aesthetics. The present review shows the different growth in the area of denture base resins. In denture base tooth attachment various anthropometric measurements have been used.  (Karacaer, 2003)

Enigma Gum Toning
These Enigma gum toning colour tones were better in custom shade matching of natural gingival tissue. It provides Extra con idence to the patients in the aesthetics of their dentures. These exist in different shades such as ivory, light pink, natural pink, dark pink & light brown shade. These are also very comfortable in aesthetic and health in pregnant women.

Resins With Modi ied Chemical Structure
Hydroxyapatite illers are added to increase fracture toughness. The addition of Al2O3 illers is to increase the lexural strength & thermal diffusivity that could lead to more patient satisfaction. The powder to the liquid ratio by weight is 2.2:1 is the best mix for better treatment for geriatric patients for better oral health. The new additive is triphenyl bismuth (Ph3Bi) which gives radiopacity (Ashok and Suvitha, 2016). Abutment taper, screw access channel engagement, height, axial wall number, abutment platform, and surface roughness are the predominant modi ication variables of implants supported dentures (Basha et al., 2018;Ajay, 2017).

Thermoplastic Resins
Thermoplastic resins are very much advantageous compared to conventional powder-liquid system. They possess magni icent aesthetics which match the tooth structure or tissue coloured material and these materials show a very pleasant feel to the patients. There is no growth of bacteria because they are non porous and although they are non porous to keep them comfortable to patients against their gums they retain a slight amount of moisture.
These resins were very strong and are unbreakable and it also has higher lexibility and also lesser weight. These resins are very little and also it doesn't have monomer material and also it is an alternative resin to conventional resins. They may also be relined & repaired by repressing the restoration (Ashok, 2014).

Thermoplastic Nylon
Polyamide is the basic material used in thermoplastic nylon. The irst lexible thermoplastic, te lon is introduced in thermoplastic resin by the usage of a rapid injection system. It is used for the fabrication of dental appliances. The framework for standard removable partial dentures is built by the pink acrylic denture material. Valplast & lexplate are polyamides (nylon plastics). Tooth-coloured clasp known as clasp-Eze which is made of nylon material was manufactured and patented by the lexite company in 1992 was the irst preformed. It is available for both clear shades and pink colour.

Limitations
Thermoplastic nylon materials were very hard to polish. Conventional tooth-borne rest seats are stronger than thermoplastic nylon.

Thermoplastic Acetal
In 1971 acetal was irst proposed as an unbreakable thermoplastic resin. Rapid injection develops the irst tooth-coloured clasps with a thermoplastic luoropolymer. It has good short-term mechanical properties as a homopolymer, but has better longterm stability as a co-polymer.

Mechanism Of Action
Resin clasps are to be placed more gingivally, to increase its aesthetic appeal beyond the obvious coloured properties.

Thermoplastic Acrylic
Thermoplastic acrylic has very poor impact resistance and excellent lexural and tensile strength. These resins are very popular due to their translucency and vitality. These resins are also best for their aesthetics, very easy to handle, polish and adjust. These acrylics exist in various tooth colours and different gingival colours. These materials are reliable and repairable in its clinical aspects. Flexite M.P is a type of thermoplastic acrylic and it is a special blend of polymers. These were very well known for bruxism appliances as well as dentures because these are unbreakable even it falls and also have the greatest impact rating of any acrylic. These are used in advancements of denture base resins recently by using nanoparticles in denture bases (Venugopalan, 2014;Kannan and Venugopalan, 2018).

Thermoplastic Polycarbonate
Polymer chain of bisphenol-alpha carbonate is known as polycarbonate. Polycarbonate resin is similar to the acetal resin and is also very strong, resists fracturing and is quite lexible. Polycarbonate is not worn as well as acetal during occlusal force and consequently will not maintain for long durations in vertical dimensions. Polycarbonate is ideal for provisional crown & bridges but not suitable for full or partial dentures. The polycarbonate has a natural translucency and also inishes very well, yielding excellent aesthetics. It has the best aesthetic properties.

Applications Of Thermoplastic Resins
There are various applications of thermoplastic resins those are to performs partial denture clasp, lexible tooth-borne partial denture framework, single cast partial dentures, temporary crowns and bridges, provisional crowns and bridges, occlusal appliances, implant abutments, orthodontic and sleep apnea appliances (Pfeiffer and Rosenbauer, 2004).

Hypoallergenic Resins
These resin materials act as a substitute for polymethyl methacrylate in allergic patients. PMMA has higher water solubility than terephthalate based resin such as promysan, thermoplastic. PMMA is substituted by the light activated indirect composite containing methane dimethacrylate (UDMA) for the patients prone to hypersensitivity .

High impacted resins
Rubber particles are transferred into MMA to strengthen bonding ability with PMMA. Due to the greater impact strength, shows the patients having the frequent lose in dentures for example in parkinsonism, senility. They are available as a powder-liquid system & processing is the same as heat cure resins (Ángel Serrano-Aroca and Llorens-Gámez, 2017).

Fiber reinforced resins
Due to low impact strength and fatigue resistance, the major problem is associated with PMMA Due to the combined effect of fatigue and impact strength causing maxillary fractures, whereas the mandibular fractures caused by impact strength.
Especially in the maxilla, the midline fractures are most common if there a proper bonding, iber reinforcement shows 1000% increase in strength than non-reinforced (Ellakwa et al., 2008).

Metal Fiber Reinforced
Metal ibres had a property of unaesthetic and also adhesion was very poor between the wire and acrylic resin and also expensive. The metal is also much prone to the corrosion, because of these disadvantages these materials have not been used in procedures of dentistry (Mohamed, 2004).

Carbon / Graphite Fiber Reinforced
Carbon ibers (65-70 mm length, 5 % by weight) during packing the silane coupling agents placed were tested (Pesci-Bardon et al., 2006), these have a property of anisotropic that gives greatest reinforcement in denture base resins. The availability of carbon graphite ibers are chopped, woven, braided & tubular provides a more even distribution of reinforcement, high iller loading & easy handling because of iber bundles. It increases lexural strength, impact strength will strengthen the resin (Fan, 2011).

Disadvantages
This material shows poor aesthetic property by the formation of the black colour but that can be compensated by an opaquer. It may lead to a weakening of the inished prosthesis and the polishing of the material is very dif icult (Gjorgievska et al., 2013). In adding to this lateral spreading of ibres during pressing is a major disadvantage.

Aramid Fiber Reinforced
Aramid Fiber reinforced material has a property of increasing the strength of the prosthesis but it also possesses unaesthetic and also it is very strenuous to polish, so that it is used mostly for posteriors in which the areas where aesthetics are not very much salient.

Polyethylene Fiber Reinforced
Multi iber polyethylene strands are reduced to 65 mm length and for adhesion improvement surface is examined with epoxy-resin (to improve adhesion) and are placed in the resin during packing (Phoenix, 2004). Due to the increase in strength and stiffness in unidirectional, it develops anisotropic property (Yadav and Elkawash, 2011).

Advantages
This material shows the greatest aesthetic property that means almost invisible in nature and also greatest impact strength and modulus of elasticity. It also shows the insigni icant increase in case of lexural property (Castro and De, 2016).
It also shows a decrease in transverse strength but it may increase with an increase in polyethylene iber reinforcement is done Triad VLC present for exam-ple in implant-supported prosthesis resin (Keenan et al., 2003).

Disadvantages
Placement and inishing are dif icult as ibers tend to protrude outside the mold. Moreover, polyethylene (even with plasma) does not bond (Ariga, 2018).

Glass Fibres
The denture base stability can be increased and stress transfer can be reduced by adding glass ibers (Jyothi, 2017;Duraisamy, 2019). 6 mm chopped glass ibers with 5% iber in combination with injection molding technique will result in an increase in transverse strength, elastic modulus & impact strength (Selvan and Ganapathy, 2016). The plasma polymerization technique using HEMA, EDA is used to modify Glass ibers. The discrepancy of denture base resins are marginal yet severely affects the long term success of all ceramic complete veneer crowns (Ganapathy, 2016).

Advantages
These materials have the best aesthetic property and it is the excellent option for making denture base. These have excellent lexural properties, fatigue resistance and also the property of polishing. These materials can have an ability of resisting extremely high temperature, moisture and oil. In future to this natural substitutes can be tested (Subasree et al., 2016).

E-glass Fibers
Computer impregnated with a PMMA (porous polymer) and silane coupler that allows dissolution bonding to acrylic is known as E-glass. (e.g. Preat Perma Fiber) (Jain et al., 2017).

Advantages
There are two forms (mesh & iber) and are translucent providing aesthetics. Due to glass iber bonding, they also have more strength. (Vijayalakshmi and Ganapathy, 2016).

CONCLUSION
The various functional modi ications and advancements in denture base resins have provided supporting results in dentistry. In its advancements, it has recti ied the various problems and disadvantages caused by polymethyl methacrylate to the patients. In Future, many more advancements and modi ications are required to provide better treatment and care to the patients oral health.