A microfilled composite resin is the material of choice for diastema closure because of its excellent polishability and enamel-like luster (Figs. 4-7A and B). If the diastema is very large, the lingual surface of the composite resin could be subjected to high functional stress in patients with heavy centric contacts. In these situations, the dentist may elect to use a hybrid composite resin for the entire restoration or a hybrid on the lingual portion overlaid on the facial surface with a microfilled composite resin. Shine-through is usually not a problem in diastema closure because of the labiolingual thickness of the add-on composite resin in the body area of the clinical crown. In many situations some translucency is desirable because the composite resin thins out at the incisal edge. If shine-through is a problem, follow the procedure described for the Class III and Class IV restorations.
FRC FDPs could be fabricated as surface-retained FDPs, inlay-retained FDPs, full coverage crown-retained FDPs and hybrid FDPs (Vallittu and Sevelius, 2000). Inlay-retained FRC FDPs can be made either directly or indirectly. Direct fabrication of FRC FDPs in the mouth requires high clinical skills for establishing a satisfactory anatomical shape of the pontic. Fracture of the pontics or fractures at the connector area are reported as typical complications with FRC FDPs (Vallittu and Sevelius, 2000). In an attempt to overcome such complications, pontics of different materials and designs (Perea et al., 2014a) or improved FRC-resin interfaces (Wolff et al., 2012) have been suggested.
The inclusion of prefabricated pontics in FRC FDPs in particular may simplify the fabrication technique and provide more predictable outcomes for the final FRC FDP. For this purpose, usually acrylic resin-based or porcelain denture teeth are used as pontic materials for direct and indirect FRC FDPs. The use of prefabricated pontics for direct FRC FDPs allows for proper shaping and finishing of the pontic and decreases plaque accumulation and gingival irritation around the pontic compared to directly made resin composite pontics. Prefabricated pontics such as acrylic resin denture teeth have superior mechanical strength and their occlusal adjustment is easier than those of ceramic ones (Powers, 2012). Modern acrylic resin denture teeth provide higher abrasion resistance, improved adhesive properties and enhanced cosmetic-aesthetic values than their older versions (Kawara et al., 1991; Loyaga-Rendon et al., 2007).
In an attempt to improve strength and crazing resistance of resin denture teeth, new technologies have been implemented, some of which are using blend polymers, interpenetrated polymer networks (IPN), and double cross-linking. Denture teeth could also be made of microfilled and nanofilled resin composite which provide optimal optical and mechanical properties. One of the recent generations of denture teeth are made of nano-hybrid resin composite. Such denture teeth are made of a mixture of urethane dimethacrylate resin matrix and poly(methyl methacrylate) (PMMA) clusters that are encapsulated in the structure (Colebeck et al., 2015). Denture teeth of this kind are fabricated in layers that make them not only look more aesthetic but also provide good adhesive properties. Since the outer layer is made of highly cross-linked PMMA, higher cosmetic-aesthetic values and over time higher wear resistance is expected. The ridge-lap surface of denture teeth is less cross-linked and in some denture teeth brands organic and inorganic filler particles are added to this surface (Stober et al., 2006). The less cross-linked layer in the ridge-lap surface promotes better chemical bonding of the acrylic denture teeth to the FRC framework when such teeth are used as pontics in the fabrication of FRC FDPs.
The composition of acrylic resin denture teeth is mainly based on PMMA beads and pigments that are immersed in a cross-linked polymer matrix. The layer between the PMMA beads and the cross-linked polymer matrix is named as semi-interpenetrating polymer network (IPN). In the process of bonding acrylic resin denture teeth to the veneering resin composite, which is the case when such polymeric teeth are used as pontics in the fabrication of FRC FDPs, a chemical mechanism needs to be involved. This is achieved through the formation of a secondary IPN bonding that results after the dissolution of the ridge-lap surface of the acrylic denture tooth by resin monomers. The polymer dissolved by the molecules of the monomer turns into a gel due to the presence of swelling, facilitating the penetration of the monomers (Lastumäki et al., 2002). Consequently, this mechanism enhances the adhesions of the polymeric pontic to the FRC structure in the FDPs.
Acrylic resin denture teeth are in most cases modified at their ridge-lap surfaces when used as pontics in FRC FDPs in order to create a suitable zone for positioning the FRC material. However, this modification may cut off the least cross-linked area of the acrylic resin denture tooth that is in fact crucial in order to achieve good adhesion to the FRC device (Vallittu, 1995). Therefore, acrylic resin denture teeth that are manufactured already with the space could best be incorporated in an FRC framework in direct/indirect FRC FDPs (Fig. 5.2).
Load-bearing capacities and fracture behavior of pre-shaped acrylic resin denture teeth (Perea et al., 2015b) was reported to increase when resin composite was used to fill the space at the bottom of the pontic once the FRC framework was in place. The highest load-bearing capacity of 1700 N could be achieved with FRC FDPs especially when pre-shaped acrylic resin denture teeth were filled with short FRCs to complete the ridge-lap shape of the pontics, a magnitude of strength which should be sufficient to withstand the masticatory forces in clinical applications.
As an alternative to acrylic resin denture teeth, ceramic teeth could also be used as pontics in indirectly made FRC FDPs. Recently, computer-aided design/computer-aided manufacturing (CAD/CAM) technologies have been implemented in the fabrication of pontics in order to create proper shapes of pontics that could be adhered to the FRC structure. The acceptance of CAD/CAM manufactured prosthetic solutions is increasing as a consequence of unpredictable results with some traditional methods and the time that they require (Li et al., 2014). Additionally, industrially manufactured blocks are more homogeneous compared to those of the handmade ones, presenting advantages in terms of mechanical properties of the final restoration (Hickel and Manhart, 2001). The inclusion of CAD/CAM manufactured pontic teeth may also overcome some of the shortcomings such as delamination of the veneering material (Göhring et al., 2002), discoloration (Monaco et al., 2003), and wear (Behr et al., 2003). High wear resistance and good aesthetic properties of ceramic might also be beneficial for the provision of long-lasting FRC FDPs with good cosmetic-aesthetic values.
One other important aspect that directly affects the fracture resistance of inlay-retained FRC FDPs is the thickness of the pontic material as in the case of inlay-retained FRC FDPs. It is highly recommended to reinforce the inlay-retained FRC FDPs at the gingival side of the pontic due to the localization of high tensile stress in this area (Dyer et al., 2004; Shi and Fok, 2009). Previous research reports revealed that bending the FRC framework close to the gingiva would increase the biomechanics of the final FRC FDP in that deeper positioning of the FRC framework would also allow for obtaining thicker pontic material above the FRC structure (Özcan et al., 2012; Perea et al., 2014a).
An FRC FDP should withstand the masticatory forces that are reported to be in the range of 150 N for anterior and up to 878 N for posterior teeth (Ahlberg et al., 2003; Ferrario et al., 2004). In a recent study, ceramic pontics of FRC FDPs with 4 mm thick showed mean fracture load values of 1667 N (Perea et al., 2014a) being significantly higher than those with resin composite or acrylic resin denture teeth pontics with similar thickness. However, the values obtained with resin composite and acrylic resin denture teeth pontics also exceeded the values reported for masticatory forces on posterior teeth (Perea et al., 2014a).
Considering that failures primarily occur at the pontic area and connector area, it is crucial to pay attention to the characteristics of the connectors when designing an FRC FDP. One solution to overcome the design problem in this area is to place a similar amount of fibers at the bottom of the pontics and the connectors (Vallittu, 1998). The reason for this is that since in those areas the tensile stress is higher, the fibers would follow the direction of the principal stress. Likewise, additional fiber reinforcement close to the prepared teeth was shown to increase the fracture strength of the FRC FDPs and reduce the incidence of fractures in the crowns (Vallittu, 1998).
Composite resins are mostly used for direct techniques. These materials exhibit more than satisfactory compressive strength but lack the tensile and flexural strength required to span long distances. Spaces that result when teeth are lost have necessitated the use of alternative splinting approaches. Conventional composite resin cannot be stretched across a missing tooth space without fracturing owing to the lack of flexural and tensile strength If an internal fiber is incorporated, it adds enough strength that the material can function successfully. The synergistic effect of the compressive strength from the composite resin and the tensile and flexural strength from the internal fiber gives the dentist a stronger, more functional result.
Polyacid-modified resin composites were introduced in the 1990s as a new class of materials that aimed to combine the esthetic property of composite with the fluoride-releasing property and adhesion of glass ionomer. These materials have been nicknamed “compomers.” They are similar to composite in that they contain no water and are hydrophobic, set by a polymerization reaction, lack the ability to bond to tooth structure, and require bonding agents of the type used with conventional composite resins.72 Like glass ionomers, they do release fluoride; however, their fluoride release levels are significantly lower than those of glass ionomer cements.72 As a sealant material, polyacid-modified resin composites underperform glass ionomer cements in terms of fluoride release and underperform conventional resin composite materials in terms of retention.73–75
Many composite resins wear much like natural tooth structure and do not cause iatrogenic wear of the opposing dentition. Indirect composite resin laminate veneers are the treatment of choice in many situations:
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Darkly stained teeth. Indirect composite resin can cover dark color without opaquing agents while retaining a vital appearance.
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Conservation of tooth structure. Tooth preparation for composite resin laminate veneers can be more conservative than that for porcelain alternatives because composite resin does not require 0.5 mm thickness, as does porcelain. Composite resin can be much thinner in spots and still function well.
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Fabrication alternatives. Indirect composite resin laminate veneers can be fabricated either in the office or in the dental laboratory. They can be polymerized or processed. They can be made of microfilled, small particle, or hybrid composite resin. The glass in the small particle or hybrid composite resin can be etched with hydrofluoric acid, which provides micromechanical retention rivaling that of etched porcelain.
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Chairside repairs. These restorations can be repaired at the chairside with light-cured composite resins.
The technique described below is for a light-cured hybrid composite resin that is heat tempered, etched with 10% hydrofluoric acid gel, and treated with silane. The silane chemically bonds to the remaining glass particles and then to the luting composite resin, which is used to attach the laminate veneer to the etched enamel surface of the tooth. (Note that techniques may vary among manufacturers.)
Armamentarium.
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Mirror
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Explorer
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Metal “plastic” instrument (e.g., Hu-Friedy, Inc.)
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#12 surgical blade
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Bard parker handle
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Anterior scaler (U-15 Towner, Hu-Friedy, Inc.)
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Medium grit flame or chamfer diamond bur
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Vinyl polysiloxane impression material
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Irreversible hydrocolloid impression material
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Maxillary and mandibular full arch impression trays
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Die stone
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Hybrid composite resin
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Light-cured or dual-cured luting composite resin (see Chapter 12)
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Toaster oven or Coltene oven
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12- and 30-fluted carbide finishing burs (e.g., ET Esthetic Trimming, Brasseler USA)
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Fine finishing diamond burs (e.g., ET Esthetic Trimming, Brasseler USA)
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Rubber composite resin polishing cups (see Chapter 5)
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Composite resin finishing disks (see Chapter 5)
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Composite resin polishing paste (see Chapter 5)
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10% hydrofluoric gel
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37% phosphoric acid gel(see Chapter 5)
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Dentin-enamel bonding resin (see Chapter 3)
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Silane coupling agent
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Intraoral light-curing unit (e.g., Demi Plus LED Dental Curing Light, Kerr Corp.)
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Oil-free pumice
Clinical technique.
1.
Clean the tooth and the neighboring teeth with pumice.
2.
Select the desired shades of composite resin while the teeth are wet with saliva.
3.
Determine the desired alignment of the teeth.
4.
Prepare the eight maxillary anterior teeth by removing small amounts of enamel with a medium grit flame or chamfer diamond bur. If only minimum preparation is necessary to improve alignment and increase facial contour, remove only 0.25 to 0.50 mm of enamel from the facial area and none from the incisal area (Fig. 6-1A). If incisal reduction is necessary, remove 1 to 1.5 mm (Fig. 6-1B).
CLINICAL TIP
Preparation dimensions may vary depending on the manufacturer’s recommendations and the amount of desired color change.
CLINICAL TIP
Preparation dimensions may vary depending on the manufacturer’s recommendations and the amount of desired color change.
5.
Make a full arch impression of the prepared teeth with a vinyl polysiloxane impression material. No retraction cord is needed because the margins are placed at the gingival crest.
6.
Make a full arch irreversible hydrocolloid opposing impression.
7.
Place a provisional restoration if needed (see Chapter 7)
8.
Pour stone casts of both the prepared and the opposing arches. Laminate veneers can be fabricated on the stone cast by using a separating medium or on a flexible cast as described below.
9.
After the stone is fully set, soak the cast of the prepared arch in water for 10 minutes and make an irreversible hydrocolloid impression of the cast.
CLINICAL TIP
Soaking the stone in water before making the irreversible hydrocolloid impression prevents the irreversible hydrocolloid from adhering to the stone.
10.
Inject a vinyl polysiloxane impression material (medium to heavy viscosity) into the irreversible hydrocolloid impression and form a flexible cast (Fig. 6-1C). This technique was first developed by Dr. K. Michael Rhyne for use in indirect composite resin inlay fabrication.
CLINICAL TIP
A flexible working cast does not require a separating medium, nor is it susceptible to breakage. The chance of chipping the restoration upon removal from the working cast is slight.
11.
On the flexible cast, fabricate composite resin veneers using a technique similar to that described for direct intraoral application (Fig. 6-1D).
CLINICAL TIP
To achieve a vital, natural appearance, apply layers of dentin, enamel, and incisal shades and polymerize each layer for 40 seconds (Fig. 6-1E).
12.
Remove the laminate veneers from the flexible cast.
13.
Contour and polish the laminate veneers using 12- and 30-fluted finishing carbide burs in a high-speed handpiece or porcelain contouring and polishing wheels on a lathe.
CLINICAL TIP
Fabricating every other laminate veneer to completion before fabricating the adjacent laminate veneer allows for good interproximal contours and contacts.
14.
Place the laminate veneers on the original stone cast to check the fit and margins; adjust further if necessary (Fig. 6-1F).
15.
Heat treat the laminate veneers in boiling water or a heat device, such as the Coltene unit, for 10 minutes to achieve the heat-curing benefits.
16.
Acid etch the lingual side of the laminate veneers with 10% hydrofluoric acid gel for 30 seconds (Fig. 6-1G) or lightly sandblast with a microetcher or air abrasion unit and rinse thoroughly.
CLINICAL TIP
Handle hydrofluoric acid carefully because it is caustic.
17.
Evaluate the internal surfaces of the laminate veneers to ensure that an etched surface has been achieved (Fig. 6-1H).
18.
Clean the teeth with No. 4 fine pumice in a prophylaxis cup, rinse, and dry with water-free and oil-free air.
CLINICAL TIP
At the delivery appointment, use cheek and lip retractors to isolate the teeth. With this technique no cotton rolls or rubber dam is needed.
19.
Clean the teeth with No. 4 fine pumice in a prophylaxis cup, rinse, and dry with water-free and oil-free air.
20.
Use 37% phosphoric acid for 15 seconds to etch the enamel and remove the smear layer from any exposed dentin surface of the first central incisor (Fig. 6-1I).
21.
Rinse thoroughly.
22.
Leave the tooth surface slightly moist for wet bonding.
23.
Using a brush, apply silane coupling agent to the internal surface of the laminate veneers and air dry.
CLINICAL TIP
Silane is generally indicated for hybrid, microhybrid, and nanohybrid composite resins and generally contraindicated for microfilled composite resins. Check the manufacturer’s recommendation.
24.
Liberally coat the etched surfaces with a hydrophilic primer from a fourth generation dentin and enamel bonding agent (Fig. 6-1J) and dry the primer with oil-free and water-free air until the surface appears glossy without being wet. This indicates that the “hybrid” layer has been established in the dentin and the enamel is thoroughly coated with the resin in the primer.
25.
Paint a thin layer of bonding resin onto the internal surface of the laminate veneers.
26.
Apply a luting composite resin to the internal surface of one of the laminate veneers. Place the laminate veneer on the prepared tooth and remove excess luting composite resin with a brush dipped in bonding agent (Fig. 6-1K).
27.
Polymerize for 40 seconds on the facial and lingual surfaces of the tooth (Fig. 6-1L).
28.
Remove excess polymerized luting composite resin with a #12 surgical blade or a scaler (Fig. 6-1M).
29.
Place the other laminate veneers in the same fashion.
30.
Finish the margins with 12- and 30-fluted carbide finishing burs, fine diamonds, rubber polishing cups, finishing disks, or other composite resin finishing techniques (Fig. 6-1N-P).
Dental composite resin is a tooth-colored restorative material used to replace a decayed portion of tooth structure. Its esthetic appearance is the main advantage over the conventional dental amalgam. Typical composite resin is composed of a resin-based matrix, such as bisphenol A-glycidyl methacrylate and inorganic filler like silica. The filler gives the composite improved mechanical property, wear resistance, and translucency. Functionalized SWNT has been applied to the dental composite to increase its tensile strength and Young’s modulus to help improve the longevity of composite restoration in oral cavity. Addition of functionalized SWNT increased its flexural strength significantly by absorbing more stress [234]. However, further effort in development of CNT-reinforced composite resin has been hampered because of its dark color primarily from CNT, which is a major drawback for esthetic composite resin.
CNT has been applied to the interface of dentin and composite resin to compensate for micro-leakage development in long-term use, which is a major cause of restoration failure. Once micro-leakage develops between tooth and composite resin interface, it works as a nidus for bacterial colonization; thus, secondary decay can develop. CNT has shown the potential to provide protection against bacteria and initiates the nucleation of HA on its surface [235]. Studies have reported that hydrophobic interaction between CNTs and exposed collagen fibers from dentin as a mechanism for CNT’s attachment to the dentin surface [236] and that the bond strength between CNT-coated dentin and composite resin restoration material was not affected by the presence of the CNT [235]. The presence of CNT at the interface of dentin and composite resin can reduce the chance of secondary decay development in the long term by providing protection against decay inducing bacteria and initiating HA nucleation on its surface. However, the gray discoloration (Figure 3.3) at the dentin–composite resin interface due to CNT needs to be overcome to make this application a reality.
Figure 3.3. Photographs of tooth slices coated with CNTs. (A) Nontreated tooth slice (control), (B) transverse view of CNT-coated tooth slice, and (C) sagittal view of CNT-coated tooth slice.
From Ref. [235].
One of the most common complications of denture prostheses is the cracking of denture base from either accidental dropping or long-term fatigue failure. Denture base is usually made of PMMA because of its excellent esthetics, low density, low cost, and ability to be repaired. However, it has relatively low fracture strength which makes a denture base vulnerable to crack by either impact or flexural fatigue under chewing [237]. Recently, MWNT (0.1–1.0 wt%) has been incorporated into PMMA to increase flexural strength and fracture toughness of denture base materials [238]. A similar application of MWNT (0–10 wt%) to PMMA-based bone cement used in the orthopedic area has shown to improve the fatigue performance of bone cement [239]. Authors of both studies found that loading of MWNT in PMMA improved flexural strength and fatigue performances of polymers in a dose-dependent manner. It was speculated that well-dispersed MWNT was able to reinforce PMMA matrix prior to crack initiation and to arrest/retard early phase of crack propagation. Even with the significant improvement in mechanical properties, resultant black color of the denture base remains as a disadvantage of CNT application.
Composite resin is a direct-fill, tooth-colored restorative material (Figure 10-11). Composites were first used to restore anterior teeth but are now routinely used in conservative occlusal and proximal preparations on posterior teeth as well. Composite resin restorations exhibit excellent color-matching characteristics and the material is versatile and relatively easy to manipulate. Light-cure composite material has almost unlimited working time. Disadvantages include the possibility of microleakage, staining, and wear, especially when used in large posterior preparations. Composite restorations can fail because of secondary caries, fracture of the restoration, or fracture of adjacent tooth structure. It is frequently possible to repair previously placed composite restorations with new composite material and, thus, extend the life of the initial restoration. Composite resin restorations are more technique-sensitive than amalgam restorations. Isolation of the operative field from contamination (e.g., saliva, gingival sulcular fluid, hemorrhage, handpiece lubricants, excess water) is necessary for good bonding and long-term success of the restoration.7 From a treatment planning perspective, posterior composites are usually advocated: (1) for relatively small preparations, (2) when all margins of the preparation can be isolated and dried, (3) when esthetics is an overriding concern for the patient, and (4) when the patient has a documented allergy/sensitivity to metallic alternatives. Outside the United States, composite is often the preferred material for all direct-fill posterior restorations.
Composite resins generally consist of a resin polymer matrix, inorganic filler, coupling reagent, coloring agent, and initiator [59]. Three key properties of composite resins used in dental applications are mechanical, physical, and esthetic qualities all of which can be enhanced by silica. Although silica has long been used as the reinforcing filler, the potential novel properties introduced by the nanoscale and various synthesis and surface modifications have only begun to be explored in dentistry. Recent studies have begun testing the effects of altering size and surface properties on the functional properties of silica-based nanomaterials in composite resins.
Using the sol–gel process, Kim et al. [54] synthesized spherical silica nanoparticles having different sizes (from 5 to 450 nm) that were tested for dispersion in, and adhesion to, a resin matrix of 70 wt% bisphenol-α-glycidyl methacrylate (bis-GMA) and 30 wt% triethyleneglycol dimethacrylate (TEGDMA). This study determined that particles with γ-MPS-modified surface were more adhesive and had better dispersion than nontreated particles regardless of size. A similar study used silica nanoparticles with a size range of 20–50 nm and filler mass fractions of 20%, 30%, 40%, and 50% [53]. These composites were compared to a conventional composite containing 10–40 µm silica particles. The use of nanosized silica resulted in increased mechanical properties with mass fractions up to 40% producing an increase in fracture toughness, flexural strength, and hardness in comparison to control. A third study recently tested similar spherical nanosilica fillers with a size range of 10–20 nm for dispersion, surface roughness, and flexural strength [60]. Two filler ratios were tested, 30 and 35 wt%. The surface modification of γ-MPS was determined important for use in the resin matrix and the higher filler ratio decreased surface roughness but decreased flexural strength relative to the lower filler ratio.
Spherical particle may not be the only shape that can be used to enhance composite resins, as other silica-based nanomaterials are now being tested. Tian et al. [56] used fibrillar silicate (diameter in tens of nm and length in µm) in small mass fractions (1% and 2.5%) and determined that uniform impregnation of fibrillar silicate into dental resins significantly improved mechanical properties such as flexural strength, elastic modulus, and work to fracture. MSNs have also recently been explored for enhanced properties in dental composites. The particles were synthesized using the nonsurfactant templating method in the 500 nm range and the composites prepared using combinations of MSNs and nonporous fillers [61]. The authors concluded that including porous fillers increased mechanical properties potentially due to the interconnecting pores. These studies identify the potential benefit of using nanosilica in composite resins and highlight the potential of manipulating size, shape, and surface modifications for increased performance.
Composite resin or (eventually) porcelain veneers to improve shape.
•
The profile of the tooth is narrower at the gingival margin than a normal-sized tooth (emergence profile), and there is therefore a limit to how large the tooth can be enlarged with a restoration, without producing an overhang in the gingival region or an unsightly interdental shadow.
•
Orthodontic alignment and extraction of the tooth may be required and other techniques such as autotransplantation and implants should also be considered.
Clinical Hint – Acid-etch composite resin build-ups
In patients with missing teeth, the central incisors are often conical in form. When closing an anterior diastema, it is often preferable to add composite to the distal aspect of the crown rather than the mesial. The diastema can be closed orthodontically to avoid a ‘flared’ appearance of the tooth crown that tends to look artificial. A more vertical mesial proximal surface and the addition of composite to the distal surface give a better appearance with a more normal distal angle and arch form.
Synthetic resins are organic (carbon-based) materials with molecules that can be made to link up with one another into larger units. These basic building blocks are called monomers and the larger groups they form are called polymers. There are many kinds of linking arrangements and many different ways of stimulating the link-up (or polymerization).
Emulsions are usually water-based and consist of resins most frequently based upon styrene, butadiene, or acrylic polymers (thermoplastic materials) which harden when the water or other solvent evaporates, allowing the polymer groups to coalesce.
Alternatives to thermoplastic emulsions are thermosetting polymers (sometimes dissolved in organic solvent) that undergo a permanent chemical change when they cure. The reaction that transforms the resin liquid or paste into a solid tends to continue long after the initial cure is complete. The result is a harder and more chemically impervious material than is the case with the emulsion types referred to above.
In general, and taking the above into consideration, synthetic resins are suitable where seamless surfaces are required to minimize the occurrence of trapped dirt or contamination, and to provide easily cleaned hygienic surfaces (i.e., good British Standard (BS) 4247 rating). Coupled with this facility are the factors of excellent serviceability, safety and, if applied correctly, esthetics. Furthermore, they can act as binders in screeds and provide, as in the case of polyurethanes (PUs), insulating and also roof stabilization foams, to mention just a few applications.
No one resin type will provide satisfaction in all situations encountered. What follows is not a complete statement of the advantages and disadvantages of the various types, but it will give a general idea as to how careful specifiers have to be.
