Restorative Materials and Maintenance Protocols

With 120 million US citizens missing one or more teeth and 36 million missing all of the teeth in one or both of their jaws, proper treatment, lifelong recall, and excellent ongoing care for dental restorations is essential to provide patients with improved quality of life and natural-looking smiles.¹ Dental hygienists and dental technicians can be enlisted to join dentists in their efforts to conserve the oral health and well-being of patients by bridging any gaps in communication and employing proactive measures that contribute to the best treatment outcomes.

Collaborative Oral Care and Maintenance

The maintenance of healthy oral structures and the restorations used to replace them is critical to the long-term success of dental treatments. Hygienists and other dental healthcare providers should educate patients about factors affecting oral health, maintenance protocols to protect restorations, and appliance longevity, among other aspects of home care.² For example, because alcohol can cause a reduction in the bond strength between dental restorations and their underlying tooth structures, patients should be informed that limiting its consumption can be beneficial. Moreover, many mouthrinses contain alcohol; therefore, their ingredients should be carefully investigated. Patients should also be instructed to be cautious when biting on hard or sharp objects, such as ice, nuts, pens, or fingernails, which may apply stress that can result in fractures or breaks. In addition, patients who grind their teeth or participate in sports should be instructed to use a night guard or mouth guard to prevent damage to their dental restorations and natural teeth.³

According to the American College of Prosthodontists, the key steps that patients should follow after receiving restorative treatment include brushing their teeth with fluoridated toothpaste at least twice per day, flossing once per day, using mouthrinse as recommended by the dentist, and, when instructed, wearing a night guard to protect restorations from the harmful effects of parafunctional habits. It is also suggested to advise patients to not smoke or chew tobacco and to avoid a high-sugar diet.¹

Furthermore, to boost patients' motivation and help them achieve better oral health outcomes, multiple behavioral change models exist, including Social Cognitive Theory, the Theory of Planned Behavior, and the Health Action Process Approach.⁴ These can be communicated through a discussion with an oral healthcare team member or via another method, such as through handouts, videos, or a smartphone application.⁵

To help patients with post-treatment home care challenges, synergistic communication should be established between clinicians and their dental technicians, dental hygienists, and in some cases, other team members. This can lead to the development of simple follow-up home care instructions that are provided to patients with each restoration at placement or at dental hygiene visits. In order to provide such instructions, hygienists should possess a working knowledge of the different types of restorative materials, how they are used, and their potential failure modes and requirements for maintenance.

Material Science for the Dental Team

As dentistry has shifted from direct amalgam and indirect metal-based restorations to adhesively bonded direct composite and indirect ceramic restorations, the treatment options, techniques, preparation designs, and maintenance protocols have been adapted to avert failures and ultimately increase the lifetime of patients' dentition.⁶ Today, more materials are being researched, developed, and improved than ever before, bringing alternative treatments for a variety of oral conditions. Because the long-term use of restorative materials necessitates that they be comprised of nontoxic substances or those with low toxicity,⁷ government regulations have increased, adding to the knowledge and responsibilities of dental and laboratory practices. The US Food and Drug Administration regularly issues testing guidelines for the materials used in restorations, prostheses, and appliances.⁸ The longevity of dental restorations depends on many factors, including the practitioner's knowledge and skills, the materials and techniques utilized during treatment, and the patient's physical and cognitive conditions, overall health, medical history, ability to afford care, compliance with maintenance protocols, and more.⁹

Regarding the types of materials used, direct restorations include dental fillings primarily made of amalgam, gold, or composite resin. Indirect restorations, which include fixed and removable prostheses, may be supported by natural teeth or dental implants. Fixed restorations, such as inlays, onlays, crowns, bridges, and full-arch prostheses, can be made of metal alloys (eg, high noble and noble alloys, base metal alloys, titanium alloys), ceramics (eg, powder-liquid, glass-based pressed or machinable, high-strength crystalline or metal-ceramic materials, hybrid ceramics), composite resins, and polyaryletherketone (PAEK) polymers. Most popular dental implants are made of either titanium or zirconia.¹⁰ Removable prostheses, which include partial and full/complete dentures, can be combined with implants as part of overdenture, hybrid, and telescopic approaches. The materials most often used in the fabrication of removable prostheses are polymers, composite resins, and metal alloys.¹¹

Metal Alloys

According to the American Dental Association, the metal alloys used for dental restorations are classified as high noble, noble, and base metal alloys. High noble and noble alloys, which are the costliest, are indicated for use in inlays, onlays, crowns, fixed bridges, and substructures. However, due to their poor esthetics, they are not indicated for use in anterior teeth. The advantages of high noble and noble alloys include suitable flexural strength, durability, and hardness as well as biocompatibility with the oral structures. Base metal alloys are recommended for crowns, fixed bridges, and removable partial dentures. Although base metal alloys are more affordable, they are more difficult to cast than high noble and noble alloys, lack esthetics, and may result in metal sensitivities.¹⁰

A review of 1,965 articles published between 1942 and 2014 revealed that adverse reactions caused by the release of metal ions or other products of the interaction between the physiologic environment and metals in dental restorative materials may occur. Dental alloys derive their biocompatibility and protection from corrosion by forming a protective surface oxide layer made of the alloy's components. In an environment of dissimilar metals, contact between two different metals or changes in the temperature or pH in the mouth can cause a breakdown in this protection and lead to a galvanic reaction. Corrosion still exists even when the protective layer is intact. Although the metal ions are released more slowly, adverse reactions such as local toxic responses, systemic changes in metabolic processes, or an allergic response to these ions may occur. Allergic responses are often caused by nickel, which is present in most stainless steel, cobalt/chromium, nickel-titanium, and nickel-chromium alloys. If such a reaction occurs, the restoration must be replaced with one that does not contain the allergen. Because no material can be proven to be 100% safe for individual patients, the potential risks and benefits of using metal alloys must be carefully evaluated.⁹

Gold

Gold was first introduced in dentistry more than 2,500 years ago, and its use increased, especially during the last 100 years. Currently, 80 tons of gold are utilized annually for dental applications.¹² Restorations made of cast gold can last for 40 to 50 years, and their excellent marginal fit is a contributing factor. Considering the size of the average bacteria, when compared with the marginal gap at the interface between a tooth and a lithium disilicate or zirconia restoration, the margins of cast gold restorations are excellent.¹³

Titanium

Titanium was introduced in the 1950s as a superior metal for dental implants because of its ability to osseointegrate with natural bone. Being relatively soft, titanium must be combined with other metals (eg, aluminum, vanadium, tantalum, zirconium) to improve its strength, creep, and fracture resistance. As a highly reactive element, it instantly oxidizes in the air, forming a tight but stable 10-nm thick oxide layer, which contributes to desirable properties such as biocompatibility and corrosion resistance. However, because oxidation and the inclusion of oxygen can lead to increased brittleness with loss of strength, titanium must be melted in a vacuum or under inert gas, which makes casting difficult. Therefore, titanium implants are fabricated by milling them from solid bars.¹⁴

The survival rate of titanium implants has been reported to be 96.4% after 10 years.¹⁵ The main concerns associated with titanium implants include the potential for compromised esthetics, the rising incidence of peri-implant mucositis and peri-implantitis, and advanced bone loss resulting from implant failure. Peri-implant mucositis and peri-implantitis develop from the formation of a plaque biofilm on the coronal portion of the dental implant/abutment complex, which initiates a disease process that is very difficult to contain and eliminate. Other factors that contribute to the failure of titanium implants include certain implant/abutment design features or junction surface alterations, a lack of and/or poorly attached keratinized tissue, iatrogenesis, and potential toxicity caused by metals leaching.¹⁶

Composite Resins

Composite resins were introduced in the mid-1950s and evolved around the type and size of the filler content used. The failure rate of composite resin restorations ranges from 1% to 3% in posterior teeth and from 1% to 5% in in anterior teeth.¹⁷ The causes of failures include recurring caries, fractures of the restorations, marginal deficiencies, postoperative sensitivity, and wear, which can be attributed to issues with adhesion, curing, and polymerization shrinkage that results in debonding. Volumetric shrinkage and microleakage are inherent in the use of composite resins, and a lack of structural strength can be an issue in more extensive composite restorations. Factors such as the practitioner's training and technique, curing light ergonomics, patient habits and caries risk, location in the oral cavity, and particle size are among the determinants of long-term success. In addition, each composite material demonstrates different physical and chemical properties, such as wear rate and fracture resistance, which are dependent on proper curing and the degree of conversion achieved. Composite resins are stronger under compression than tension; however, creating rounded contours can reduce areas of tension and the potential for chipping.^17,18

Composites mimic the mechanical properties of dentin better than any other material.¹⁹ They also exhibit enamel-like wear properties, which makes them less abrasive to opposing dentition. Unlike with ceramics, adjustments or modifications of composite materials, such as those for marginal breakage or chipping, can be easily accomplished intraorally.¹⁸ Composites are durable, and they demonstrate excellent fractural strength, wear resistance, and gloss retention. Although composites are suitable for bite alterations and testing occlusal changes, they are not suitable for patients with bruxing habits or those with an acidic oral environment or digestive issues such as gastroesophageal reflux disease or bulimia.²⁰ After 5 to 7 years, composite resin restorations may need to be repaired or repolished.²¹ Finishing and polishing of composites results in a highly polished surface, which improves wear resistance, reduces biofilm accumulation, and prolongs the life of restorations.¹⁸

Ceramics

Dental ceramics date back to 1889 when the all-porcelain jacket crown was patented. Porcelain-fused-to-metal (PFM) crowns were developed in the 1950s, porcelain all-ceramic restorations that were strengthened with alumina entered the market in 1965, and pressable ceramics were introduced in the 1980s. In the mid-1990s, the first CAD/CAM ceramic substructure was developed. Lithium disilicate ceramics surfaced in 2005, starting with pressable versions, and the millable versions appeared in 2006. Presently, dental practitioners and material manufacturers are leaning away from the use of metal alloys in favor of all-ceramic restorations.²²

Dental ceramics are inorganic structures made of metallic and semi-metallic elements and oxygen compounds. Ceramics are suitable for many applications; however, they are brittle and exhibit high compressive and low tensile strengths, which makes them prone to fracture and abrasive to opposing teeth. The advantages of ceramics include dimensional stability, high wear resistance, and esthetic properties that closely imitate those of the natural dentition. Thanks to a high glass content and a small number of filler particles controlling the color and other optical effects, including opalescence and opacity, ceramic restorations can blend seamlessly with natural teeth. Over the years, dental ceramics have undergone countless modifications regarding their processing techniques, mechanical properties, bonding procedures, restoration methods, strength, and fracture toughness. They were initially limited for use in the posterior regions, but now, they are recommended for crowns, veneers, fixed partial dentures or bridges, implant-supported restorations, and implant abutments. Although anterior restorations require greater esthetics, posterior restorations must be stronger due to masticatory forces. When choosing a ceramic material, the properties (eg, wear, durability, esthetics), bond strength, and case specifics (eg, underlying tooth color, load, preparation type) should be considered.²³

The side effects of dental ceramics are limited because they are the most inert of all of the materials and are not known to cause biologic reactions.⁹ The 5-year survival rate of an all-ceramic restoration is approximately 95%, with the most common pitfalls being fracture and chipping.²⁴ Fabrication errors and defects such as a poor fit or the presence of porosities often relate to failures in following manufacturers' specifications. Other factors related to the failure of ceramic restorations include occlusal stresses, internal fit, and parafunctional habits. Monolithic all-ceramic rather than layered ceramic prostheses are recommended in high-risk areas. Ultimately, having an understanding of patients' occlusion and oral functions produces the most successful prostheses.²⁴

Zirconia

For more than a decade, zirconia has been used for crowns and other fixed restorations. Its excellent mechanical properties, biologic properties, and esthetics have made it a popular choice among the other modern ceramic materials. Due to the need for a specific bonding protocol and other factors, restoration with zirconia can be challenging. The most commonly reported causes of failure of zirconia crowns involve the luting agents and bonding protocols used as well as chipping of the veneering porcelain in layered zirconia restorations. In one systematic review, 4.5% of the crowns showed chipping after 5 years, and in another systematic review, the 5-year survival rates of zirconia crowns were similar to those of other ceramic crowns and metal-ceramic crowns when used for anterior teeth but on average higher than those for metal-ceramic crowns when used for posterior teeth.²⁵

Zirconia is also used in the fabrication of implants. Its advantages include biocompatibility with the surrounding tissues, osteoconductivity, and reduced plaque on the implant surfaces, which promotes better healing and successful treatment. Recent studies have concluded that zirconia osseointegrates as well as or better than titanium due to the high level of contact between the implant material and the jawbone. Zirconia's physical properties include acceptable flexural strength and resistance to fracture, a low modulus of elasticity, and low thermal conductivity. This material is chemically inactive and suitable for patients with metal allergies or sensitivities or for those who desire metal-free dentistry. Unlike titanium, zirconia implants do not result in dark shadows around the gingival margin, which is especially problematic in areas with thin gingival tissue. Studies published between 2015 and 2020 have shown the survival rate of zirconia implants to be 87% to 100% at 7.8 years.²⁶ The downside of using zirconia for implants is seen in the low temperature degradation that occurs in the presence of moisture, hydrogen, and water, which can result in a loss of strength and other physical properties. More research is needed to understand the effect of low temperature degradation on zirconia's strength, density, toughness, and longevity.²⁷

Hybrid Ceramics

Hybrid ceramic materials only entered the dental market during the most recent decade. Consisting of matrices that are highly filled, hybrids can be either polymer-infiltrated ceramics or ceramic-infiltrated polymers.^28,29 Hybrid ceramic materials are not brittle, and they are fracture-resistant, wear-resistant, and kind to the opposing dentition. When compared with other ceramic materials, hybrid ceramics allow more stresses to be absorbed without resulting in permanent deformation or failure, but if failure occurs, repairs can be easily performed intraorally. However, they are only recommended for single tooth restorations such as veneers, inlays and onlays, crowns, and implant-supported crowns. Hybrid ceramics require less time for milling and processing than other ceramic materials, which preserves the lifetime of milling burs, and once milled, they do not require sintering or crystallization firing. The final smoothness and gloss of hybrid ceramic restorations are achieved with surface polishing.²⁸ When compared with other ceramic materials, milled hybrid ceramic restorations can achieve finer margins, and the 0.5-mm minimal thicknesses can offer an advantage in occlusal wear cases.¹⁸ Table 1 includes an in-depth comparison of hybrid ceramics with various other ceramics.^16,23,30-33

In summary, glass-ceramics possess superior optical properties, zirconia offers the highest strength and fracture toughness, and hybrid ceramics are advantageous regarding fracture resistance, resilience, shock absorption, milling efficiency, polishability, and accuracy.²⁸

PAEK Polymers

During the 20th century, as patients developed preferences for metal-free, tooth-colored, and lightweight prostheses, a new family of polymer materials, PAEK polymers, entered the dental market. Polymethyl methacrylate (PMMA), a plastic-like material used as acrylic for dentures, offered a less expensive, lighter, and more esthetic alternative to metal denture bases, which had numerous drawbacks beyond their esthetics, including the potential for hypersensitivity, oral galvanic reactions, unfavorable tissue reactions, abutment teeth osteolysis, and the production of biofilm. The disadvantages of early PAEK polymers such as PMMA included that they had a high coefficient of thermal expansion and a low elasticity modulus, degraded more quickly than metal, and could become cytotoxic due to chemical leaching. Therefore, in more recent years, new ultra-high performance PAEK polymers have been developed, including polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), which have risen to the top and are used in the fabrication of dental implants, temporary abutments, obturators, and clasps for dentures as well as in other applications. PEEK and PEKK have excellent biologic, mechanical, cosmetic, and handling characteristics; are similar to bone and natural tooth structures; are nonallergenic; and provide acceptable esthetics, which makes them great substitutes for metals and other materials.³⁴

PMMA, which is used extensively for digital dentures, has been modernized to become more esthetic, denser, and offer lower surface porosity. It can now be used for full-arch temporary restorations without reinforcement. Modern polymer frames last approximately 7 years and provide more comfort than metal frames. In addition, they can be easily combined with other materials, which permits lithium disilicate denture teeth to be bonded to them and composite resin to be added to mimic the oral tissues.³⁵

Conclusion

By staying on top of the latest developments in restorative materials and understanding the obstacles facing modern society, hygienists and other dental healthcare providers can help to open new communication channels to advance our nation's oral health. Interprofessional communication outside of common patient-doctor channels can significantly benefit treatment outcomes. Motivating patients to comply with oral hygiene protocols via direct or indirect communication helps to prolong the life of their restorations, prevent financial losses, and reduce the need for restorative interventions. While dentists, hygienists, and laboratory technicians become inundated with new evidence and their responsibilities expand, they can provide each other and their patients with valuable mutual assistance. Dental hygienists and dental technicians have much in common, and they can share their expertise regarding up-to-date hygiene protocols, home care instructions, information about restorative materials, and maintenance trends.

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