The popularity of nanotechnology is on the rise in many fields, and dentistry is no exception. Its effects are especially pertinent in the development of composite resin dental materials used in direct restorations. This is a category whose history is marked by frequent evolution and numerous generations, in what might appear to be a game of materials science whack-a-mole. Early generations offered strength but poor esthetics, only to be replaced with materials that resulted in pretty restorations that lacked durability.
Over the years, composite resin materials have been reformulated and tweaked again and again to address myriad weaknesses such as poor compressive, mechanical and tensile strength, inadequate fracture resistance, subpar wearability, difficult handling characteristics, lackluster polishability, and the tendency to shrink upon polymerization. But while the components that make up these materials include synthetic resin, a coupling agent called silane, colorants and possibly a photoinitiator, the lion’s share of all this tweaking has had to do with filler content.
Filler in composite resins is composed of particles of various inorganic substances ranging from colloidal silica and ceramic to bioglass, titanium, hydroxyapatite, zirconia, and silver. And they may exist as clusters, grains, fibers or films.1 In fact, newer generations of composites have featured an increasing variety of particle shapes and sizes, with positive results. But it’s been found that when such a mix comprises very small particles, even more beneficial composite characteristics can result.
A WORD ABOUT FILLERS
Fillers are a primary component of composite resins used in dental restorations. Ranging from particles of inorganic substances, such as colloidal silica and ceramics, to bioglass, titanium and silver, they affect viscosity as well as various material characteristics.
Generations of composite materials are distinguished from one another by filler density, and particle size, shape and type. The generation of macrofilled composites, which debuted in the 1960s, featured large particles, resulting in poor esthetics as they did not take a polish well. In addition, their large particles were subject to “plucking,” which meant that the particles would pop out, leaving gaps in the surface. Not the look most patients are going for.
Subsequently, microfilled composites, which featured small particles in the 1980s, polished well, but lacked strength due to a low fill ratio. Hybrid composites, also a product of the 1980s, mixed large and small particles and were highly filled. This resulted in a high wear rate and good workability, but they did not retain high polish. Microhybrids were next, offering at least three particle sizes, all of which were smaller than previous iterations. They turned in reasonable results, with less shrinkage, though some clinicians prefer to layer them with a veneer of microfill to boost polishability.
Most recently in the composite lineup are the nanomaterials. Nanofilled composites are said to offer good strength and polishability courtesy of nanoparticles and nanoclusters smaller than 100 nm. Nanohybrids, the more recent of the two, offer a broader mix of particle sizes, all less than 100 nm, and reportedly further elevate esthetic and physical properties, while also decreasing polymerization shrinkage.
LET’S GET SMALL
“Nanotechnolgy, as defined by K. Eric Drexler, PhD, who has been described as the founder of nanotechnology, is the manipulation of matter on the molecular and atomic levels,” says John Comisi, DDS, MAGD, assistant professor at the James B. Edwards College of Dental Medicine, Medical University of South Carolina, in Charleston. “As related to dentistry, nanotechnology is involved in creating materials that can be used for preventing or reversing, via remineralization, dental caries, and/or restoring teeth ravaged by it. By definition, nanomaterials are those materials with components less than 100 nm in at least one dimension.”
Less than 100 nm is reflective of particles found in smoke. That’s pretty darn small. Nonetheless, they make a difference in material performance.
“When inorganic nanoparticles are incorporated in resin-based materials, improved mechanical and physical dental material performance occurs, making the restored areas more resistant to fractures when the patient bites into hard foods, in addition to making the restored area appear smoother,” says Mary Anne Melo, DDS, MSc, PhD, an associate professor and director of the Operative Dentistry Division at the University of Maryland School of Dentistry, Department of General Dentistry in Baltimore. “These improvements, promoted by nanotechnology, greatly improve dental materials and reduce the frequency of material failures due to material fracture.”
But Melo explains that the ultimate success of dental restorative materials is determined by their long-term service inside the mouth, while their clinical success depends on a number of characteristics. “Nanotechnology,” she says, “has contributed to the development of several areas in dental materials sciences, and has been the epicenter of extensive research in recent decades. The initial benefits of nanotechnology are reflected in improved strength, toughness, resistance to wear and enhanced appearance comparable to natural dentition. For example, both strength and optical properties are improved in resin-based dental composites.These are properties that are important for the long-term survival of dental restorations and prosthetic rehabilitation.”
David Hornbrook, DDS, of La Mesa, California, adds increased flexural strength and fracture toughness, lower polymerization shrinkage, and higher polishability to that list of benefits.
And it’s not just composite resin materials that can benefit from nanotechnology. Hornbrook, an internationally recognized lecturer on a wide range of dental restorative issues, including dental materials, notes “Applications of nanotechnology in dentistry are also seen in bone-grafting materials, and in some of the new ceramic materials at this time.”
Point of Sale | Thinking Small
- Nanoparticles allow for an expanded surface area that can help enhance both physical and esthetic properties of composite resin restorations.
- Thanks to their small size, nanoparticles can penetrate cells to improve delivery of medicaments and enhance infection control.
- Nanomaterials can foster remineralization of damaged teeth.
- Other applications include bone grafting, osseointegration, and use in root canal sealants.
- xpected applications include use with stem cells to regenerate pulp tissue and strengthen dentin, and the development of “smart” materials that can help control caries and biofilm.
STRENGTH IN NUMBERS
But it’s more than the diminutive size that makes nanoparticles so powerful. According to Hornbrook, despite the fact that nano-sized particles have a diameter less than 100 nm, they allow for greater surface area per unit mass when tightly packed together. Says Hornbrook, “This can enhance the durability, wear, and polishability of our restorative materials, and the increased volume of particles in the materials will also enhance other physical properties such as reduced water sorption and increased wear resistance.”
An expanded surface area can also foster catalysis, which can improve things like drug delivery in the medical/dental context.1 Says Comisi, “Dental restorative materials benefit as the particle size of the fillers can be smaller and more densely packed for better wear resistance.”
Particles that exist at such a tiny scale are also said to feature unique physical chemical, electric, magnetic, mechanical and optical properties. Such enhanced properties are known as “quantum effects.”1
“The resulting changes in strengthening mechanisms for dental material properties come from the the resulting distribution of the nanosized particles in the materials’ matrix,‘‘ explains Melo. “The nanoparticles also present high reactivity, which allows a strong chemical reaction with other materials. This approach has been applied in a wide range of dental materials, including composites, bonding systems, imprint materials, ceramics, coatings for dental implants and bioceramics.‘‘
Terms to Learn
- Bis-GMA: Bisphenol A-glycidyl methacrylate; a commonly used resin in dentistry.
- Cariostatic: Stops the progression of caries.
- Compressive strength: Refers to the amount of load pressure or crushin•g an object can take without being destroyed.
- Flexual strength: Refers to how much bending an object can take without deforming.
- Hydrophilic: Water friendly.
- Hydrophobic: Water fearing.
- Nm: Nanometer.
- Osseointegration: Refers to an object becoming one or forming a structural and functional connection, with bone.
- Planktonic pathogens: Pathogens that float rather than being attached to a surface as part of a biofilm community.
- Photoinitiator: A compound that reacts to light and initiates light curing in a composite resin.
- Tensile strength: Refers to the amount of longitudinal stress or stretching an object can take without being destroyed.
Another benefit of the use of nanomaterials in dentistry is the ability to integrate antimicrobial and restorative properties into them. Says Comisi, “We are seeing the research in this arena incorporating antimicrobial agents in nanotubes, and nano-sized calcium and phosphate nanocrystals to help elicit remineralization of dental structures.”
Among the most recent developments involving nanomaterials are restorations that are also designed to stop the progression of caries. “Cariostatic nanocomposites are being researched and developed and are showing great promise,” says Comisi. “These materials will likely have a different resin component than our traditional bis-GMA hydrophobic materials that are commonly used today. A more hydrophilic, structurally stable resin-type base will be used more commonly as we go forward to allow these important nanoparticles, especially the calcium and phosphate nanocrystals, to be readily available for use by the tooth structure in the restorative and prevention process.”
“Nanotechnology has opened the door to new strategies for dental caries management taking advantage of antibacterial properties associated with the use of dental nanomaterials,” says Melo. “These new dental nanomaterials modulate biologic activity, reducing the amount of bacteria present on dental material surfaces.”
For example, metal nanoparticles, such as silver and zinc, with which Melo has worked since 2009, have gained significant interest over the years due to their remarkable antimicrobial properties. “The excellent antibacterial effect of these nanostructured agents is mainly attributed to the high surface area to volume ratio enabling greater presence of atoms on the surface, which provides maximum contact with the environment,” she reports.
Melo says that because these small particles can penetrate cell membranes more easily, “This affects intracellular processes, resulting in higher reactivity and antimicrobial activity. The nanoparticle penetration is particularly valuable because microorganisms in biofilms are more resistant to antibacterial agents than planktonic pathogens, and much more concentrated antibacterial agents may be required for effective treatment.”
Aside from their cariostatic and antibacterial abilities, nanomaterials can also be used to foster remineralization of teeth, which is a hot issue right now in light of the current emphasis on minimally invasive dentistry. Says Melo, “Based on recent research findings in the preventive dentistry field, nanotechnology is promising to yield novel strategies for remineralization of decayed areas. In this approach, dental restorative nanomaterials containing nanoparticles of calcium phosphate can release calcium and phosphate ions, which are inorganic components of the tooth, affecting the de-/re-mineralization balance. This way, the mineral loss process, which is very common during the caries progression, can be avoided. The small size and high reactivity of the nano-calcium phosphate compounds make them good candidates to be anti-demineralizing and remineralizing agents providing bioavailable calcium and phosphate ions. Research has demonstrated that calcium phosphate nanoparticles have the advantage of better ion-release profiles due to their small size.”
But caries management is just the beginning. Nanomaterials also are reportedly able to mimic some mechanical and structural properties of native tissue, and are said to have the ability to promote biointegration.1 For example, Hornbrook notes that nanomaterials are making an impact in bone grafting, encouraging osseointegration of implants through modification of implant surfaces. This is accomplished via application of nano coatings, which offer the kinds of surface textures and chemistry most likely to form a biomechanical anchor to bone rather than to fibrous tissue. These coatings include nanostructured diamond, nanostructurally processed hydroxyapatite and nanostructured metalloceramics.2 Nanomaterials can also be used in conjunction with stem cells to aid in regeneration of pulp tissue and to strengthen dentin. Other applications include treatment for hypersensitivity, and enhancement of root canal sealers.1
Although Hornbrook observes that the category of cariostatic nanocomposites has huge potential, he notes that it is still in its infancy. “In the future,” he says, “we may be able to use an antimicrobial on a carious lesion that targets just the bacteria, followed by nanotechnology that remineralizes the defective tooth structure, and targets cells that will actually regenerate new dentin and enamel.”
Similarly, Melo notes, “Nanotechnology provides the potential for “smart” behavior — materials that can react to changes in the environment to bring about beneficial changes in properties. For example, dental fillings made of a smart material could present a particular property in response to an environmental stimulus, such as a low pH stress, and demonstrate a pH-neutralizing behavior. This possibility is significant to the role of dental materials in controlling oral biofilms and combating dental caries.”
Melo observes that this could come in particularly handy in cases involving implants and marginal restorative failures. “One of the most important causes of implant-related infections and recurrent decay around dental fillings are microbial biofilms, which readily form on all currently employed dental restorative materials,” she says. “Recurrent decay at the margin of a restoration and has been widely considered the most important and common reason for restoration replacement, regardless of the restorative material type.”
STEADY AS SHE GOES
There is no doubt that the general dentist is already benefitting from nanotechnology, with more benefits to come. Says Comisi, “As the research is showing, there is a need to continue to improve our materials so that they can be more fracture resistant and more, what I like to call, ‘reservoir restorative materials’ that can provide the backbone for prevention of decay and remineralization of decay-damaged dental structure.”
“Besides improving the physical properties of our restorative materials, the use of nanotechnology is really limitless in dentistry,” concludes Hornbrook. “The future applications that I would like to see would be delivery of therapeutic medicaments and substances. This could include early cancer detection, delivery of antibacterials in periodontal pockets, delivery of local anesthetics, and adaptation of restorative materials to healthy dentin and enamel.”
Melo believes there are two primary aspects that will guide the success of the dental materials ﬁeld over the near term: the furtherance of in vivo clinical studies in humans to identify and validate the benefits from nanotechnology in dental materials through enhanced understanding of nanoparticle-bacteria interactions, particle transport, and long-term dental material efficacy; and additional studies on the improvement of bonding of the materials to the tooth for long-term, quality restorations.
“I believe that the adoption of nanotechnology has noticeably improved many of the properties of composite resins, which may guide preventive and restorative dentistry, resulting in stronger, more efficient, and less invasive approaches in the future,” says Melo.
- Shashrirekha G, Jena A, Mohapatra S. Nanotechnology in dentistry: clinical applications, benefits, and hazards. Compend Contin Educ Dent. 2017;38:e1–e4.
- Sree L, Balasubramanian B, Deepa D. Nanotechnology in dentistry — a review. International Journal of Dental Sciences and Research. 2013;1:40–44.
Featured Image by LEONIDSAD/ISTOCK/GETTY IMAGES PLUS
From MENTOR. March 2018;9(3): 26-30.