Material Answers: A Restorative Guide

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When it comes to restorative materials, clinicians have no shortage of options. Material selection hinges on a variety of factors, including esthetics, durability, and function—all of which shift in priority based on the patient and clinical situation. Plus, restorative materials include representatives from a broad class of materials, including metals, polymers, ceramics, and composites.¹ With so many elements to consider, choosing the right restorative can present a challenge.

Permanent, esthetic restoratives have long been significant contributors to restorative dentistry and overall oral health.² Consider how each element of the oral cavity (teeth and soft tissue) must function properly to allow for basic actions, such as chewing, swallowing, breathing, and speaking.¹ It’s clear that material selection is critical.

Most restorative materials are characterized by physical, chemical, and mechanical features.¹ Practicing dentistry depends not only on the knowledge of various clinical techniques but also on an understanding of the science behind those applications.¹ Here, we look at the “why” of restorative material selection.

Adhesives and Cementation

The development and regular use of adhesive materials transformed many aspects of restorative dentistry.³ Approaches to cavity preparation have changed in recent years because with adhesive materials, it is no longer necessary to prepare the cavity to provide mechanical retention.³ Adhesive protocols are capable of conserving large quantities of sound tooth structure, which helps eliminate or reduce microleakage, making adhesive critical to the success of contemporary, esthetic restorative materials.³

Adhesives are so important that when it comes to indirect restorations, the bonding protocol is the most crucial factor, according to Olga Zarta, DDS, a prosthodontist and the director of specialization of operative aesthetic materials at Bosque University in Bogota, Colombia, and Rafael Santrich, owner of VM Lab Technologies Inc, in Aventura, Florida. Santrich is also the director of the Top Colombia study group, to which Dr Zarta is a member, and both helped publish “Adhesion Protocols in Dentistry” in September 2021. These protocols are designed like a chart, offering clinicians a helpful resource on what adhesives or cements should be used for different clinical scenarios.

“The issue here is not the cement,” Santrich says. “The issue is how [they] treat the surface of the restoration—let’s say zirconia, glass ceramic, or whatever it is—that’s a big issue.”

Dr Zarta prefers composite cements for both direct and indirect restorative cases. “They have more stability…if we use a flowable composite, [it] goes everywhere,” she says.

Rolando Nuñez, DDS, MSc, manager of clinical research at BISCO, Inc, has a different approach. He prefers to bond in almost every case. “I believe the best way to achieve a good restoration is to bond,” he says. “I particularly prefer to bond everything, whether it’s zirconia, [IPS] e.max, composite—bond everything to both the surface and the tooth.”

However, there is a convenience factor, where clinicians choose to use products that do not offer a high-bond strength because of the preparation’s height and taper, which may provide enough retention without bonding, Dr Nuñez explains. Plus, bonding everything has its limitations.

“Clinically, when you do bonding, you have to isolate,” Dr Nuñez says. “But when you use these materials that are easier to use and moisture tolerant, you don’t need to necessarily fully isolate. You can isolate without having to use a rubber dam to do the procedure.”

Direct Restorations

Although the introduction of CAD/CAM systems has blurred the line between direct and indirect restorative materials, allowing for the in-office production of ceramic restorations,¹ there is still some distinction between direct and indirect materials. Unlike indirect restorations, direct restorations are typically placed onto the tooth structure in 1 visit and do not require laboratory preparation.⁴ Materials for direct restorations are separated into 4 categories, which include the following:⁴

• Amalgam
• Resin-based composites
• Glass ionomer
• Resin-modified glass ionomer

Amalgam

Amalgam was the industry gold standard for over 150 years because of its durability and effectiveness on load-bearing teeth.⁴ However, the days of amalgam are largely gone, explains Dr Nuñez. It’s fallen out of favor because it contains mercury, which patients don’t want in their bodies, and because of its clinical value, he says.

“Amalgam had its place and has its place in history,” Dr Nuñez says. “When done properly, an amalgam can last a long time. That’s something you can’t debate. However, when it’s used in large areas, if the of the preparation is not adequate, amalgam can transfer the load of mastication and induce cracks and fractures.”

Because of its metal composition, amalgam transfers energy from the load during mastication, which can result in cracks in the material, Dr Nuñez explains. Amalgam is a mixture of metals—approximately half its weight consists of a powder alloy that is made up of silver, tin, and copper, whereas the other 50% of its weight is made up of elemental (liquid) mercury.⁴ Mercury is used to bind the alloy particles into a pliable, durable filling.^5,6

The issue with mercury isn’t merely its presence in the oral cavity. Instead, the issue arises when the abrasion from brushing or chewing creates heat, causing the mercury to emit a gas that is then inhaled into the lungs.⁵ High exposure to mercury vapor has been associated with adverse effects on the brain and kidneys.⁶ However, the US Food and Drug Administration does not recommend removing amalgam fillings unless recurrent caries develops.⁶

Resin Composites

Developed as a more esthetic alternative to amalgam, resin composites generally consist of an organic polymerizable resin matrix, inorganic filler material, and a coupling agent.⁴ However, despite the efforts to minimize mercury-based amalgam, composite’s durability, strength, and cost have prevented it from entirely replacing the need for amalgam.⁴ “Composites are way more forgiving than amalgam based on their interaction with the tooth,” Dr Nuñez says.

Many resin composite materials need to be light cured, and adhesion to tooth structure requires intermediary agents containing reactive chemicals.⁴ Contemporary developments in resin composite materials include modifications in curing type, filler particles, and resin composition.³ An ncreased depth of cure and reduced shrinkage and curing time have driven recent technology.⁴

Resin composites are usually classified by resin type, particle filler size, or curing type, although the 3 classifications below are not exclusive and often overlap:⁴

• Resin matrix
• Filler particle classification
• Curing type

Other than these classifications, composites are also characterized by type, which include the following:

• Universal. As the name suggests, universal composites are formulated to be used anywhere in the mouth and can produce long-lasting, natural-looking restorations.
• Flowable. These composites have more fluidity to their composition and are typically used for small restorations or as bases and liners.
• Bulk fill. Unlike universals and flowables, bulk-fill composites can be layered thicker in the preparation with less curing. Bulk fills are often used in the posterior, but, like all materials, their use is constantly evolving.

Generally, Dr Nuñez finds universal composites to be the most adaptable composite material. “When you think about a universal composite, it’s an interesting material because it gives versatility to the clinician, so [they] can use it in a wide array of clinical situations when it comes down to direct restorations,” he says. “They have the physical properties, the strength, [and the] esthetics. They’re able to hold the gloss over time, [and they have] great wear resistance, so they’re very versatile.”

Because of their versatility, Dr Nuñez feels every clinician should have universal composites as part of their armamentarium. However, universal composite isn’t without its limits, he adds.

“The frustration for me, regarding the use of that product, is that sometimes certain clinicians have higher esthetic demands,” Dr Nuñez says. “There are some composites out there that are very specific for high-end esthetic cases. So, even though these universal composites will do the job, sometimes it is required for a more specific product.”

When it comes to flowables, Dr Nuñez believes it’s important to have a good flowable composite on hand. Even though flowables should not be a clinician’s first choice in many cases, the material can get you by, he says. “Their polishability [and wear resistance] are not as good, so the frustration could come from using it and expecting better results than you’re actually going to achieve,” Dr Nuñez says.

However, that does not mean flowables do not have a place in the operatory; it’s a matter of using a flowable composite for the right case. “There are times [when] flowable is indicated, but sometimes there is a little misuse or overuse of flowables, [which] can lead to issues when it comes down to performance,” Dr Nuñez says.

Glass Ionomers

Although the phrase glass ionomer is technically incorrect, it’s commonly used to refer to glass polyalkenoate cements, which combine glass powder with polyacrylic acid as a biocompatible adhesive.⁴ Glass ionomer was initially intended as a cement and cavity liner (mainly for nonload-bearing fillings), but the material has been developed to improve its mechanical properties and water solubility.⁴ This material is particularly efficient at fluoride recharging, although how much this actually prevents caries is still debated.⁴

Newer generations of glass ionomers have improved their wear resistance, Dr Nuñez explains. However, they should be used only in the posterior region, he says. “In the anterior region, there is no way a glass ionomer can compete with the esthetic result that a universal composite can achieve. I would probably choose that glass ionomer to be used as a base in the posterior region, and then covered with composite. I wouldn’t use it as my final restorative.”

This is why Dr Nuñez believes universal composite is the best bet for direct restorations. “Universal composites will perform well in most clinical scenarios,” he says. “If esthetics is not out of high demand…the universal composite should be the material of choice.”

Indirect Restorations

The question of indirect restoration materials is somewhat more challenging. To start with, indirect restoratives vary in terms of physical and mechanical properties, clinical indications, and composition—all of which clinicians must consider when selecting material for an indirect restoration.⁷

Indirect restorations are based on impressions and are usually fabricated at a dental laboratory. However, the recent advancement of CAD/CAM has blurred the lines between direct and indirect materials, allowing clinicians to fabricate restorations in-house for same-day dentistry.

Indirect restorations can be cemented or bonded to the tooth. Indirect restorations generally consist of 5 categories of materials, including the following:⁷

• Noble metal alloys
• Base metal alloys
• Ceramics
• Resin-based composites
• Metal ceramics

Although metals have been used throughout history because of their strength and durability, ceramics have overtaken metals in contemporary dentistry, borne out of a desire for tooth-colored materials.⁷ However, ceramics, such as zirconia, silicate glasses, porcelains, and glass ceramics, are susceptible to fracturing and chipping.⁷ Plus, their hardness may cause wear damage on opposing teeth, leading to concerns about longevity.⁷

“Zirconia has come a long way when it comes to esthetics,” Dr Nuñez says. “The main reason to use zirconia is [its] resistance. So, in certain areas where you want that resistance, mainly in the posterior region, zirconia became popular.”

When zirconia was first introduced on the market, it was not esthetic. “It was dull looking [and], opaque,” Dr Nuñez says. “In the anterior region, you want something a little more translucent [and] more esthetically pleasant.”

Thanks to more modern developments of zirconia, the material is more translucent and esthetic than it was previously. This has led to more clinicians using zirconia for a broader range of cases, Dr Nuñez says. However, Ivoclar’s IPS e.max is a glass ceramic that has become incredibly popular thanks to its wide range of indications. And there are other glass ceramics that work well for CAD/CAM, he says.

Porcelain fused to metal crowns can be more cost effective in certain parts of the world. However, despite their long track record of clinical performance, they’re outdated and have been outpaced by materials with better metaphysical properties, Dr Nuñez explains.

As for resins, Dr Nuñez says it’s a matter of philosophy. “Composites are great for onlays, for example—some people like to do composite veneers,” he says. “One of the main reasons people like composites is they’re easily repaired in case they chip or crack. Then you can bind to them fairly easy intraorally.”

For anterior restorations, Dr Nuñez would choose a glass ceramic, given its esthetics. In the posterior region, he would lean toward a zirconia crown. With the material choices dentists have, there is no right or wrong answer here, he adds.

“Nowadays, there is no strict rule. I don’t have one, because I feel all these materials are suitable for any indication between these 2. When it comes to composites, I will probably stick with onlays, which are in the posterior region. I wouldn’t go with a composite in the anterior region, when it comes to indirects,” Dr Nuñez says.

References

1. Sakaguchi R, Ferracane J, Powers J. Craig’s Restorative Dental Materials. 14th ed. Elsevier; 2019.
2. Denson N, Wells M, Tipton DA, Godoy FG, Babu JP. Bacterial adhesion and biofilm formation on direct, tooth-colored restorative materials: an in vitro study. Adv Dent & Oral Health. 2018;8(3):555736. doi:10.19080/ADOH.2018.08.555736
3. Sofan E, Sofan A, Palaia G, Tenore G, Romeo U, Migliau G. Classification review of dental adhesive systems: from the IV generation to the universal type. Ann Stomatol (Roma). 2017;8(1):1-17. doi:10.11138/ads/2017.8.1.001
4. Materials for direct restorations. American Dental Association. Updated March 9, 2021. Accessed June 9, 2022. https://www.ada.org/resources/research/science-and-research-institute/oral-health-topics/materials-for-direct-restorations
5. Patel N. The case against amalgam fillings. Forbes. October 21, 2019. Accessed June 9, 2022. https://www.forbes.com/sites/forbesbooksauthors/2019/10/21/the-case-against-amalgam-fillings/?sh=5941d8954d1e
6. Dental amalgam fillings. FDA. Updated February 18, 2021. Accessed June 8, 2022. https://www.fda.gov/medical-devices/dental-devices/dental-amalgam-fillings
7. Materials for indirect restorations. American Dental Association. Updated February 9, 2021. Accessed June 10, 2021. https://www.ada.org/resources/research/science-and-research-institute/oral-health-topics/materials-for-indirect-restorations#:~:text=Indirect%20restorations%20generally%20consist%20of,composites%2C%20and%20metal%2Dceramics