Nathaniel Lawson

	Reductions in laboratory bills may allow clinicians to pass their savings to their patients or provide services to patients with lower reimbursing insurance plans while still remaining profitable. With the rise of national production laboratories, generic brand ceramics and off-shore milling centers, laboratories now compete to offer ceramic crowns at prices so low that the conscientious dentist must wonder what’s being compromised for this price.

	Reductions in laboratory costs may be beneficially achieved by the efficiency in scale achieved by large production laboratories, reductions in laboratory steps, and time made possible by a digital workflow and lower material costs due to advancements in materials. There are some methods of cost savings, however, that may deteriorate the quality of the final prosthesis such as the use of low-quality materials, design and manufacturing.

	The main dental ceramics used for monolithic crowns are lithium disilicate and zirconia. Lithium disilicate is a material that was initially only manufactured by Ivoclar Vivadent with the brand name IPS e.max. Now, there are several similar types of lithium-containing glass ceramic crowns commercially available. Due to patents from Ivoclar Vivadent, the composition, and therefore the properties, of the newer glass ceramics are noticeably different than IPS e.max. The clinician experienced with the use of IPS e.max shouldn’t assume that similar materials are interchangeable without supporting laboratory and clinical testing.

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Unlike lithium disilicate materials, zirconia is manufactured and distributed with so many different brand names that even those of us who evaluate this material only recognize a small percentage of the commercially available materials. The large pucks from which zirconia crowns are milled start as a fine powder that’s pressed into shape. There are several factors that may influence the quality of a zirconia puck, including the purity of the raw powder, the exclusion of pores and the homogeneity of the packing of the powder into the disc. High-quality manufacturers not only control the manufacturing process but also implement quality control to ensure these standards are met. Some of the issues with low-quality materials may be realized immediately such as altered color/translucency and incorrect fit; however, some errors will only be realized with time such as decreased strength or aging resistance.

	Even though most dental laboratories are digitally designing their crowns through software, the design of a crown isn’t an automatic process. There’s still a designer who digitally adjusts the contours, contacts, margins, emergence profile and occlusion of the crown. Even though the digital designer uses a mouse instead of a waxing instrument, the knowledge of how to design the crown must be possessed by the human being in charge. Additionally, not all contouring of the crown, particularly at the margins, comes out of the mill in its final condition. A technician will often put the finishing touches on the restoration by hand. In order to receive a high-quality restoration, the design and finishing of a restoration should be performed by a skilled and experienced technician.  

	A final technique-sensitive aspect of the crown fabrication procedure is the sintering or crystallization of the ceramic crown. The amount of time and the temperature at which a zirconia crown is sintered will affect its microstructure and ultimately its strength and translucency. Even if the manufacturer’s recommended sintering schedule is followed, a crown sintered in an uncalibrated furnace may actually be exposed to the incorrect sintering temperature.

	In summary, the reduction in laboratory costs for ceramic crowns is beneficial for the business end of a dental practice, but the clinician should be cautious of using bargain basement-priced crowns as likely part of the cost savings affected the final quality of the crown.

Articles

Reductions in lab bills may allow clinicians to pass savings to their patients, but some cost savings may deteriorate the quality of the final prosthesis. 

Saving money on your lab bill at what cost?

	Although posterior Class I and Class II restorations comprise the majority of direct restorations performed in the average dental practice, anterior composite restorations demand a higher level of esthetic requirements (shade match and surface finish) as these restorations are often the “showcase” work in our patients’ mouths.

	This article will address several of the material and

 updates that have improved our ability to more simply and reliably achieve esthetic results with direct composites.

	Understanding classifications of composites: a simplified history of filler particles

 as we know it today was developed in the 1950s by Ray Bowen. At that time, he mixed a bis-GMA resin with silanated 150-micron ground glass filler particles and camphorquinone, a light activator. The most fundamental change between the resin composite of that time and current resin composites is the filler particle. The first major change occurred in the 1970s when submicron (40 nm diameter) round silica particles fabricated from flame pyrolysis were added to the composite to fill in between the large ground glass fillers.

 This formulation was the beginning of the “hybrid” composite as it contained a hybrid of 40 nm fumed silica and 150-micron ground glass. One way to picture these submicron filler particles is to imagine that if the submicron particles were the size of a golf ball, then the 150-micron ground glass fillers would be about the size of a football field.

Read more: How to create restorations that look great seven years later

	The next concept in composites was the development of a composite entirely filled with small 40 nm silica particles to take advantage of the polish that can be obtained with small filler particles. To explain this, think of looking out of the window of an airplane. You would not be able to see a divot in the ground the size of a golf ball, but you would definitely notice a football field-sized crater. The same concept applies to filler size when finishing composites.

	Unfortunately, adding a filler load of only 40 nm silica particles that is high enough to achieve favorable mechanical properties results in a composite that is too thick to handle in the clinic. Therefore, these fillers were added to resin in a high concentration, completely cured, then crushed into small particles known as prepolymerized filler particles (PPFs). These PPFs were used as the filler particles in microfilled composites. Although microfills achieve a very high polish, these materials fell out of favor due to their inferior mechanical properties, limiting them mostly to anterior restorations.

	The next major development in anterior composites was the nano-filled composite. Unlike the micro-filled composites in which submicron fillers were added as PPFs, nanofilled composites have 5-40 nm silica and zirconia particles clumped together and added as “nano-clusters.” These materials are able to achieve both a high polish and favorable mechanical properties.

	Enhancement of the process of grinding glass fillers has allowed new generations of “hybrid” composites such as micro-hybrid and nano-hybrid composites. These hybrids have smaller and sometimes more homogenously sized ground glass fillers, ranging from a couple microns down to 100 nm.

 In other words, if the original ground glass fillers were the size of a football field, then these glass fillers are closer to the size of a football. They also contain some combination of PPFs, nano-clusters or loose submicron fillers.

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	These categories are all subjective and it is very confusing for the clinician to differentiate materials based on the information given by the manufacturer. In fact, a researcher dissolved out filler particles from commercially available composites and reported that the sizes of the filler particles were unrelated to the category assigned by the manufacturer.

 In general, current generation composites are either hybrids (containing small ground glass fillers and submicron fillers), micro-filled or nano-filled (

). As a rule of thumb, the clinician may ask the manufacturer if its composite has any fillers larger than 500 nm. As the wavelength of visible light ranges from 390-700 nm, particles smaller than this may be visually undetectable after polishing. Additionally, the clinician may consult independent research regarding the gloss, gloss retention and roughness achieved by the composite.

	The chameleon effect: the blending effect of our composite restorations

	The “chameleon effect” can more scientifically be defined as a blending effect, the perception that two colors are closer when juxtaposed than when viewed separately. Practically, this means a composite exhibiting a chameleon effect will blend with the surrounding tooth structure. The esthetic-minded dentist will appreciate this trait because it simplifies shade matching, while the business-savvy dentists can appreciate the potential to reduce the number of shades ordered for the clinic. Several attributes of a composite restoration will increase its blending ability such as increasing its polish, decreasing its thickness and using a more translucent composite.

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	Many of the composites that are advertised as possessing a chameleon effect are in fact more translucent. For example, 

 shows a 1-mm disc of an A2 shade composite possessing the chameleon effect (

), which is more translucent than the A2 body shade of a reference composite (

). A conscientious clinician may fear that this translucency is achieved with a lower filler content and reduction in strength. Composite translucency can be changed by modifying the index of refraction of the resin without affecting filler content and preserving strength.

	Fig. 2                                                                      Fig. 3

	More translucent composites that blend well with surrounding tooth structure greatly simplify the process of performing anterior restorations, but they do have their limitations. If placed over a dark tooth preparation, then the underlying tooth structure is more likely to discolor a translucent composite. Also, when translucent composites are used for Class IV restorations in which the lingual wall is absent, the darkness of the oral cavity will manifest as a greying of the restoration (

). For these “through and through” Class IV preparations, it is beneficial to use a composite to mimic the opacity of dentin. Several composite systems offer different translucencies of composite. For example, the Filtek Supreme Ultra system offers a dentin shade that is about 60-percent more opaque than the body shade and an enamel shade that is about 30 percent more translucent than the body shade (Fig. 2).

  These dentin and enamel shades can be used to very closely match the natural tooth structure, but it requires a significant amount of experience to correctly achieve the desired effect (

	Fig. 4                                                                    Fig. 5

	Polishers: a key to an esthetic restoration

	The polish of a composite restoration can be the key to achieving the esthetic outcome desired by the clinician and patient. But polishing a composite is more than just for the looks. Our patients’ tongues are very sensitive to roughness, with some studies showing that the tongue can detect roughness on the scale of 0.5 microns.

 Composites with roughness values above 0.2 micron are susceptible to plaque accumulation.

 And polishing composites improves their stain resistance.

	Fig. 6                                                                          Fig. 7

	Prior to polishing, the restoration is typically finished with either finishing carbides or diamonds. Finishing carbides are typically available as 8-12 bladed (fine), 16-20 bladed (extra-fine) and 30 bladed (ultra-fine). Finishing diamonds are also available in different diamond grit sizes ranging from about 30 microns (fine) to 15 microns (extra-fine) to 8 microns (ultra-fine). A recent study compared the roughness of two different composites when finished with a 20-grit diamond bur and a 30-bladed tungsten carbide bur. There was about a 10-fold increase in roughness using the diamond finishing bur (Ra = 2.5 microns) as compared to the tungsten carbide finishing bur (Ra = 0.25 microns).

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	Following the shaping and smoothing of restorations that occurs with a bur, they are then finished and polished. Finishing is the recontouring of a restoration whereas polishing is removing surface irregularities. There are various shapes and formulations of composite finishing and polishing tips (

). They all typically contain either aluminum oxide or diamond abrasives. Diamonds are harder and more abrasive than aluminum oxide.

	One of the more popular systems is composed of the 

 (second step) tips (Dentsply Sirona). They are available in a disk, point and cup. The cups can be used for gingival margins and achieving labial contours, whereas the points can be used to achieve labial contours and polish the lingual of anterior teeth. The Enhance Finishing tips contain larger aluminum oxide particles while the PoGo Polishing tips contain smaller diamond particles (

). The abrasive particles are embedded in a urethane dimethacrylate binder. This binder allows these tips to finish/polish aggressively and quickly with high pressure and then smooth with lower pressure. The binder will wear away quickly when high pressure is applied during aggressive finishing/polishing, which limits the lifetime of these tips. Other systems with points and cups use rubber-based binders (

), which will preserve the shape and increase the lifetime of the finisher/polisher, but it does not allow for aggressive finishing/polishing with high pressure.

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	          Fig. 9                                                                    Fig. 10

	The other commonly used tools for finishing and polishing anterior composites are flexible polishing discs (i.e. 

). These contain aluminum oxide particles of different sizes ranging from 5 microns (super fine) to 14 microns (fine) to 29 microns (medium) to 100 microns (coarse). The medium and coarse discs can finish and recontour the restoration. Several studies have compared the polish achieved with flexible polishing discs to the polishing points, with some studies showing an advantage with the points

 Practically, the advantage of polishing discs in anterior restorations is that these discs are flexible and will often help to create smooth contours of the restoration. They can also adapt to embrasure spaces (

	Some of the newest composite polishers are spiral-shaped (Sof-Lex Diamond, 3M ESPE, 

). A recent study determined these polishers were able to achieve a smoother surface than several polishing points.

  In the author’s experience, the spiral-shaped polishers will obtain a very high gloss. Due to their shape, however, they will not achieve any finishing, so any gross irregularities should be removed with a finishing bur prior to using these polishers.

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	Fig. 12                                                               Fig. 13

	The final step in polishing is the application of a polishing paste. The main components include diamond and/or aluminum oxide particles to serve as abrasives with a glycerin-based lubricant.  For the application of pastes, buffing polishers are ideal (

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	2. Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials.  J Am Dent Assoc. 2003 Oct;134(10):1382-90.

	3. Lang BR, Jaarda M, Wang RF. Filler particle size and composite resin classification systems. J Oral Rehabil. 1992 Nov;19(6):569-84.

	4. Paravina R, Westland S, Imai F, Kimura M, Powers J. Evaluation of blending effect of composites related to restoration size. Dent Mater 2006;22(4)299-307.

	5. Ota M, Ando S, Endo H, Ogura Y, Miyazaki M, Hosoya Y. Influence of refractive index on optical parameters of experimental resin composites. Acta Odontol Scand. 2012 Sep;70(5):362-7.

	6. Ryan EA, Tam LE, McComb D. Comparative translucency of esthetic composite resin restorative materials. J Can Dent Assoc. 2010;76:a84.

	7. Jones CS, Billington RW, Pearson GJ. The in vivo perception of roughness of restorations. Br Dent J 2004;196(1):42-5; discussion 31.

	8. Bollen C, Lambrechts P, Quirynen M. Comparison of surface roughness of oral hard materials to the threshold surface roughness for bacterial plaque retention retention: a review of the literature. Dent Mater 1997; 13: 258-269.

	9. Barakah HM, Taher NM. Effect of polishing systems on stain susceptibility and surface roughness of nanocomposite resin material. J Prosthet Dent. 2014 Sep;112(3):625-31.

	10. Daud A, Gray G, Lynch C, Wilson N, Blum I. A randomized controlled study on the use of finishing and polishing systems on different resin composites using 3D contact optical profilometry and scanning electron microscopy. J Dent 2018; 71: 25-30.

	11. Gönülol N, Yilmaz F. The effects of finishing and polishing techniques on surface roughness and color stability of nanocomposites. J Dent. 2012 Dec;40 Suppl 2:e64-70.

	12. Pala K, Tekçe N, Tuncer S, Serim ME, Demirci M. Evaluation of the surface hardness, roughness, gloss and color of composites after different finishing/polishing treatments and thermocycling using a multitechnique approach. Dent Mater J. 2016;35(2):278-89.

Why advances in materials and technique are leading to more reliable outcomes. 

How to achieve esthetic results with direct composites

	It is impossible not to love the vast shade palette, versatile application and minimally invasive preparation design made possible by composite resin restorative materials.  You and many of your colleagues have assuredly admired the beauty of a well-placed resin composite.

	Fast-forward a few years and you may well be one of the thousands of dentists replacing those meticulously placed composite restorations secondary to recurrent decay. Over half of restorations placed by dentists in the U.S. are replacing failing restorations - the vast majority being of a 

Read more: 4 big benefits of resin cements

	Even in patients with meticulous oral hygiene and essentially sugar-free diets, composite resins are inherently susceptible to degradation by not only cariogenic bacteria but also intrinsic enzymes of healthy human saliva.

 In the words of an amazing clinician, well-respected researcher and personal friend, “It should be eye opening that composites do not cure caries and are not perfect!” (Dr. Augusto Robles, 

University of Alabama at Birmingham School of Dentistry

	Posterior resin composite at time of placdement with highly polished surface.

	While a true statement, researchers are making tremendous progress in formulating caries-resistant composite resins. The notion of “bioactive composite resins” has been circulating through the literature since the development of glass ionomer restoratives with a recent resurrection in this field. These composite resin formulations have been deemed capable of releasing fluoride, calcium, phosphorous, or a combination of these elements to aid in remineralizing decalcified areas of the tooth.

	Same composite following six months of clinical service covered in a layer of plaque biofilm.

	In recent years, oral healthcare has been making a transition away from reactionary care to preventative care. For such a transition to be successful, dental materials must also make such a shift. Though bioactive remineralizing 

 are obviously useful, these agents are treating the signs of decay and not the etiology. Hence, anti-microbial composites have made an entrance into the world of dentistry.

	Anti-microbial composites: What are they? 

	All dentists know of the bacterial biofilm that forms over the protective salivary pellicle in the early stages of plaque accumulation. In this cariogenic bacterial metropolis, adhesion proteins as well as acidic metabolites are deposited and the carious process continues.

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	Anti-microbial/anti-biofilm composite resins act to resist the carious process via:

	1.    Non-selectively killing bacteria contacting the restoration.
	2.    Selectively killing pathogenic bacteria contacting the restoration.
	3.    Repelling proteins necessary for bacterial attachment to the restoration.

	Broad spectrum approach: non-selective bacterial destruction

	Let’s begin with the heavy hitters – the agents proven to kill the majority of bacterial species found in the oral cavity. These include chlorhexidine, elemental silver and quaternary ammonium compounds.

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	Popular among the periodontal community, 0.12 percent chlorhexidine gluconate has made a name for itself as Periogard

 oral rinse. Recent studies have aimed to encapsulate chlorhexidine molecules and mesoporous silica nanoparticles in composite resins to create an anti-bacterial powerhouse of a resin restoration. Chlorhexidine’s ability to destroy the outer and inner cell membranes and induce apoptosis is directly proportional to the concentration of the agent.4 More chlorhexidine leads to more bacterial destruction. Sounds great, right? The unresolved problem with this potential miracle-worker is its “burst-release kinetics” from the resin restoration.

 Large amounts of chlorhexidine are released in the first few days, leaving you with a regular old composite restoration after about a week – susceptible to the same destruction as the non-antibacterial resin composites.

	The use of silver as an antibacterial agent predates chlorhexidine (some reports state we are in the sixth millennia of using silver as an antibacterial agent). Elemental silver inactivates bacterial enzymes responsible for synthesizing bacterial DNA replication.

 No DNA replication means no growth nor repair, which results in cell death. Nanoparticles of silver incorporated into composite resins have shown success in preventing microbial biofilm formation and inducing bacterial cell death without reducing bond strength nor discoloring the restoration.

 So, what’s the problem? Similar to chlorhexidine containing resins, these silver nanoparticles portray the dreaded burst-release from their resin matrix. 

	The common denominator of disadvantages for these antimicrobial resins appears to be the sudden massive release of the antimicrobial agent. How do we prevent this from occurring? Maybe we should try covalently bonding the antimicrobial agent to the resin matrix.

	Quaternary ammonium methacrylate compounds

	Similar to the chlorhexidine products, quaternary ammonium compounds (QAMs) have a positive track record among the dental community. Cetylpyridinium chloride is a QAM found in many “anti-plaque” and “intense germ-killing” oral rinses. QAMs can covalently bind to the composite resin matrix and thus be slowly released as the restoration is subjected to enzymes of the oral cavity.

 The resulting decrease in biofilm formation and lactic acid concentration around QAM containing composite resins has been reported to remain consistent for greater than 12 months. 

Read more: Using silver diamine fluoride as a new caries management solution

	How do they work? QAMs have a net negative electromagnetic charge. The bacterial cell surface has a net positive electromagnetic charge. The negatively charged QAMs attach to the positively charged proteins on the bacterial cell surface and block the transport of water, ions and metabolic products across the bacterial cell membrane.

 If a sufficient number of transport proteins on the cell membrane are blocked, the bacterial cell dies via either accumulation of metabolic products inside the cell or increased intracellular osmotic pressure (the cell explodes). QAMs of various chain lengths have been researched. The most effective QAMs appear to be those of larger chain lengths (chain length of roughly 16).8 In other words, larger QAMs lead to better blockage. 

	Targeting the bad bugs: selective bacterial destruction

	The rich and diverse oral microbiome consists of anywhere between 500 and 700 different bacterial species,

. Relatively few of these hundreds of species are responsible for diseases of the oral cavity. To promote oral health, the ideal antimicrobial agent will kill and/or prevent proliferation of disease-causing bacteria and allow protective bacteria to take over the oral niche. 

	This is where we introduce the quaternary ammonium compound dimethylaminododcyl methacrylate (DMADDM). Recent studies incorporating DMADDM into composite resins have reported decreased numbers of streptococcus mutans (the No. 1 bacteria associated with caries) and streptococcus sanguinis alongside unchanged numbers of streptococcus gordonii (bacteria associated with sound enamel structure).

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	Though the reasoning behind this finding of increased susceptibility of certain streptococcal species to DMADDM has not yet been elucidated, such an agent shows potential for effective utilization in anti-biofilm composite resins.

	Though QAM-containing composite resins show enormous promise, their ability to induce bacterial cell destruction is directly related to contact occurring between the QAM-containing resin and the bacterial cells. In the oral environment, a uniform protein coating develops over the surface of the tooth and serves as a barrier between bacteria and the QAM containing resin composite. If the QAMs are unable to directly bind to the bacterial cell surface, bacterial cell destruction cannot occur.

	Bacteria cannot destroy that which they cannot touch

	The most prevalent protein-repelling biomimetic material among the medical and dental community is a mouth-full: 2-methacryloyloxyethyl phosphorylcholine (“MPC” for short). Joint replacement prosthesis, coronary stents and other medical devices with MPC coatings have gained FDA approval and are regularly utilized in clinical settings.  The polymerization of this zwitterionic phosphorylcholine moiety-containing custom methacrylate

 (say that five times fast) with the conventional resin composite matrix has allowed for the creation of timed-release, protein-repellant composite resin materials. 

	How does it work? MPC is a hydrophilic molecule, meaning it attracts water. A completely hydrated MPC polymer contains a significant amount of free circulating water particles capable of binding to any nearby protein molecules. Instead of the protein molecules binding to the resin material, they bind to these circulating free water molecules and are readily cleared from the resin surface. 

	Though MPC has not been shown to directly kill bacterial cells, the sustained release and ability to prevent bacterial binding to the restoration surface makes its incorporation into dental composites one of the most promising antimicrobial agents.

	Another approach to prevent biofilm attachment is targeting the enzymes that streptococcus mutans uses to produce “sticky” glucan polymers. These glucan polymers help attach the plaque biofilm to the tooth. Small molecules have been identified that inhibit these enzymes.

 Rat models have shown that application of a solution of this small molecule to teeth can reduce the incidence of caries. A next step would be to incorporate these small molecules into a composite resin. An advantage of this approach is that it would prevent tooth demineralization without significantly altering the intraoral microbial flora. 

Read more: Study finds virulent bacteria linked to caries in children

	We have the technology, now we need to get it into the clinic

 are all promising solutions to create a resin composite that will resist its biggest threat: acid-producing bacterial plaque. To the best of the knowledge of these authors, there is not yet a commercially available restorative material utilizing any of these approaches. A scan through the abstracts of the most recent 

 meeting shows that these technologies are starting to translate from concept to reality. With time for product development and clinical evaluation, hopefully these materials will arrive in our clinics soon.   

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Researchers are making tremendous progress in formulating caries-resistant composite resins. 