As Mother Nature Intended

Saul Pressner, DMD, past president and president-elect of the Academy of Biomimetic Dentistry, says that the movement is gaining increased traction in the United States approximately 4 decades after Bertolotti brought over principles that he learned in Japan from Takao Fusayama, DDS, PhD.

"Patients really want to prevent their teeth from being filed down unnecessarily. They are realizing that the more that healthy tooth structure is preserved, there is less likelihood for the need of root canal therapy," Pressner says. "The literature shows a greater risk of catastrophic failure associated with teeth that have received endodontic therapy; therefore, biomimetic concepts and protocols attempt to prevent the need for unnecessary endodontic therapy and favorably increase the long-term prognosis of teeth. In addition, the literature supports biomimetic dentistry concepts, and many dentists have observed success with these protocols in their practices."

As with any movement in dentistry, the materials that are at dentists' disposal play a significant role. From direct composites to ceramics and adhesives for indirect restorations, today's materials are making it more feasible than ever to mimic nature.

Generally, biomimetic dentistry can be seen as having two main goals: the preservation of natural tooth structure and the replacement of missing or damaged natural tooth structure using materials and techniques that closely mimic it.

The intent of conserving natural tooth structure is to prolong the lifetime of the tooth. For example, an occlusal onlay on a molar requires the removal of approximately 30.7% less tooth structure than a full-coverage crown. When a full-coverage crown requires replacement due to recurrent caries or fracture, there is often minimal tooth structure remaining onto which clean margins can be placed. In these situations, the tooth may require extraction and need to be replaced with a dental implant. This process is sometimes referred to as the death cycle of a tooth. If an occlusal onlay is placed instead of a crown, more available tooth structure will exist onto which clean margins can be placed. In addition, preserving pulp vitality prevents the structural compromise of teeth and resulting tooth loss that can occur with endodontic access.

The second goal, restoration of tooth structure to mimic its natural form and function, is grounded in the observation that natural teeth can survive for the entirety of an individual's lifetime. Few restored teeth can match this outcome. The natural tooth is therefore a structure that is well designed for oral function. From an engineering perspective, the biomaterial components of the tooth include enamel and dentin. Enamel is the hard, wear-resistant, higher-modulus (ie, stiff), relatively insoluble and strong outer layer, whereas dentin is the lower-modulus and tough inner layer. These two layers are tenaciously bound at the dentinoenamel junction. In the geometry of the natural tooth, the enamel rods are oriented to translate occlusal forces into compressive forces on the dentin.

"I usually utilize nano-filled hybrid materials for direct composite restorations," Pressner says. "If I am performing an indirect restoration, I often use lithium disilicate. For an endodontically treated tooth that has very thin cusps, I will build up the tooth into a biobase using biomimetic protocols and prepare it for an onlay restoration. Lithium disilicate has very similar wear properties when compared with natural tooth structure, and the thin cusps are protected by the onlay restoration. This tooth is then less likely to fracture catastrophically. When the biobase procedure is performed, we use air abrasion, perform immediate dentin sealing, and gradually build up the composite in small, horizontal increments, often utilizing plasma-treated polyethylene fibers to decrease the C-factor, followed by a scan or an impression for the final indirect restoration. Sometimes, a stress-reduced direct composite, per Simone Deliperi, DDS, can be utilized for the final restoration if there is enough healthy tooth structure and cost is a primary factor for the patient."

The biomimetic dentist relies on his or her analysis of how to treat the damaged and remaining tooth structure, a skill set of techniques to remove and replace the damaged tooth structure, and a toolbox of materials that are able to reproduce the physical properties of the natural tooth. In practice, biomimetic dentistry involves two primary principles that are necessary to achieve long-term success. First, the adhesion between the remaining tooth structure and any restorative materials should be maximized. Second, the generation of destructive stress within the tooth during restoration and function should be minimized.

Protocols that can be utilized to increase bond strength include rubber dam isolation, the use of caries removal endpoints to establish a peripheral seal zone, beveling the enamel, airborne particle abrasion, matrix metalloproteinase deactivation, the use of gold standard bonding agents, and immediate dentin sealing.

Isolation is critical when performing any type of adhesive dentistry because salivary contamination can decrease the bond strength after each step of bonding. Unlike other isolation methods, rubber dams place a tight seal around the tooth, providing a physical barrier between it and the oral environment. A well-placed rubber dam will protect a tooth preparation from saliva, sulcular fluid, and blood.

David S. Alleman, DDS, and Pascal Magne, DMD, MSc, PhD, thoroughly discuss the concept of respecting caries removal endpoints to establish a peripheral seal zone in "A Systematic Approach to Deep Caries Removal End Points: The Peripheral Seal Concept in Adhesive Dentistry." The article describes a method that involves selectively removing caries from the periphery of the tooth, guided by caries detecting dye that can stain demineralized dentin, while maintaining carious tooth structure in the deepest parts of the preparation to obtain a periphery of sound enamel and dentin that maximizes the bond strength and seals the underlying caries. This concept preserves the pulp vitality of the tooth and is only used when complete caries removal would risk pulpal exposure.

Beveling the enamel in preparations to increase bond strength may be a contentious topic. Although the proponents of beveling enamel margins emphasize that it provides more enamel rods to bond to, critics of the technique caution that the areas of thin composite overlying the enamel bevels may be more susceptible to chipping.

The process of airborne particle abrasion involves the use of an intraoral sandblasting unit to clean the tooth preparation with alumina particles, roughening the surfaces of both the enamel and dentin. Some research has suggested that airborne particle abrasion does not increase or decrease the bond strength to enamel or dentin; however, other research suggests that airborne particle abrasion reduces the thickness of the smear layer on dentin, which facilitates a more favorable bond for self-etch adhesives. Regardless of its effectiveness in improving bond strength, airborne particle abrasion is a useful method of removing any contaminants from the surface of the preparation prior to bonding.

"We know biofilms are there," says Alireza Sadr, DDS, a researcher and clinical associate professor in the Department of Restorative Dentistry at the University of Washington School of Dentistry. "With enamel, if you use the proper pressure and particles for airborne particle abrasion, then you will have a very good result. The bond is strong, and the process is very minimally invasive. With dentin, there is more controversy, but I still believe that overall, treatment of the tooth surface with airborne particle abrasion improves the bond. We should be aware of the possibility of damaging the tooth structure, however, so air particle abrasion is not for untrained hands, but once you are trained and comfortable using it, it is a very essential tool for biomimetic dentistry."

One of the keys to the long-term success of bonded restorations is the preservation of the dentin bonds. Matrix metalloproteinases (MMPs)—enzymes inherent in dentin collagen—present a challenge to the long-term strength of bonds. Once they are activated through phosphoric acid etching, MMPs may enzymatically degrade the dentin bond by degrading the dentin collagen in the hybrid layer. Clinical trials have shown that the application of a 2% chlorhexidine solution prior to the application of a bonding agent results in higher bond strength after 14 to 20 months. The chlorhexidine, it was theorized, deactivated the MMPs. Other methods proposed to deactivate MMPs include the use of benzalkonium chloride and at least one desensitizer product.

"I view degradation of the tooth as a partly autoimmune response of the body," Sadr says. "This response is basically regulated through enzymes such as the MMPs that are bound to the dentin. The way in which a primary tooth exfoliates demonstrates how the human body can break down dentin. The permanent dentin is not that different, and this mechanism of the tooth structure can degrade dentin and dentin bonds. Once that mechanism starts, the only way to stop the loss of tooth structure is by deactivating the MMPs. We cannot look at dentin like a material in carpentry or construction—as an object and not a biological structure. Once you understand that it is a biological structure with a sort of time bomb inside that can degrade it, then you understand how important it is to deactivate that mechanism."

Regarding adhesive systems, the gold-standard bonding agents recommended by many biomimetic dentists are those that involve separate steps for the primer and the bonding agent. The importance of separating these steps is to separate the hydrophilic components in the primer (eg, monomers such as HEMA) from the hydrophobic ones in the adhesive. Some dentists fear that if both components are mixed together, the resulting hybrid layer and adhesive layer will contain hydrophilic components that will draw in surrounding water, which may hydrolytically degrade the collagen of the hybrid layer. Much evidence supports these gold-standard two-bottle systems, including 13-year clinical trials that demonstrated an 88% clinical success rate for an etch-and-rinse material¹ and a 93% clinical success rate for a self-etch material.²

Immediate dentin sealing is a restorative technique in which a bonding agent is immediately applied to the freshly prepared dentin. The advantages of this technique include that the dentin is sealed while the patient is temporized, which reduces sensitivity and prevents contamination from temporary materials, and that the dentin hybrid layer formed is allowed to mature and strengthen during the temporization phase without experiencing the shrinkage stress that it would if it was placed just prior to overlying layers of composite or a final adhesively bonded indirect restoration.

Methods for decreasing the effects of shrinkage stress include decoupling with time and bonding to dentin before enamel; the use of fibers, chemical cure composites, and materials with an adequate modulus of elasticity; the placement of composite in increments; and the use of indirect or semi-direct materials for enamel replacement.

The concepts of decoupling with time and bonding to dentin prior to enamel are based on the results of many studies. In one study, the data indicated that 3 minutes after light curing, the dentin bond strength was lower than both the enamel bond strength and the 24-hour dentin bond strength.³ Another study showed that the bond between hydroxyapatite (the mineral content of the tooth) and MDP (the adhesive monomer in many bonding agents) formed after 5 minutes.⁴ The results of these and other studies have been applied to develop clinical protocols designed to prevent the application of stress on a bond to dentin before it has completely developed. Therefore, after curing the dentin bonding agent, a delay of at least 5 minutes is recommended prior to applying the first layer of composite in order to allow the bond with the tooth to fully form before the stress of overlying polymerizing composite is applied. Similarly, composite increments are placed without connecting dentin to enamel until the dentin bond has completely developed. The concept is to prevent the curing composite from pulling away from the dentin and damaging the dentin bond because it is more quickly forming a stronger bond to the enamel.

To help further reduce polymerization shrinkage stress, E-glass fibers can be chopped and incorporated into resin composites. A composite shrinks because its individual monomers bond with each other when initiated with a curing light. Prior to polymerization, space exists between these individual molecules; however, in order to polymerize, the molecules must come closer to together. As they come closer together to crosslink with each other, the small loss of intermolecular space multiplied by the massive number of molecules amounts to bulk shrinkage of the total volume of composite. It is theorized that the fibers may help to prevent shrinkage because the individual resin monomers bond with them at the start of polymerization. Because the fibers are rigid and do not shrink, the monomers bound to them will be less likely to travel longer intermolecular distances to crosslink.

Chemical cure composites also present advantages for stress reduction due to their slower polymerization rates, which allow molecules to flow before complete polymerization. And composites used to restore dentin should not have a lower modulus of elasticity than dentin because masticatory compression of a flexible composite will not provide adequate support. "Dentin functions to reinforce the enamel," Bertolotti says. "If you place a material near dentin that approximates a rubber band, then you do not have any reinforcement. So, a stiff hybrid composite, not a microfill, is usually the best choice for dentin replacement."

The placement of composites in smaller increments is largely advantageous regarding shrinkage stress. One could argue that when placing a Class I restoration with a composite that shrinks 1%, it will not make a difference whether it is placed in bulk or in increments because the total amount of net shrinkage that will occur in the preparation is still 1%. However, that assumption would be in error because a composite placed in increments will have a free surface area exposed multiple times, and the free surface area allows flow to compensate for the shrinkage occurring within the material. In addition, if a composite is placed in thin increments, each increment will have a lower C-factor (ie, the ratio of bonded to unbonded surfaces) while polymerizing than a thicker one would if the composite was placed in bulk. In a Class I restoration, a very thin layer will have an equal amount of bound floor and unbound free surface area; however, the bound surface of the preparation walls will be much less when compared with a large increment. Therefore, a thin layer places less stress on the tooth preparation than a thick layer. Conversely, a disadvantage of layering composite is that each time a layer is placed, there is an opportunity for the polymerizing material to deform a surrounding cusp. If multiple increments are placed, the cusps may be pulled inward enough to start a crack.

"We try to minimize shrinkage stresses utilizing biomimetic protocols because composites do shrink," Pressner says. "By using materials and techniques that minimize shrinkage, less stresses are being placed on the surrounding tooth walls, which decreases the chances of catastrophic tooth fracture. By performing techniques that involve incremental placement of composite in horizontal layers and utilizing fiber where appropriate, we are reducing polymerization stresses on the dentin walls and also allowing for a stronger bond to dentin and decreasing stresses within the polymerizing composite. Many dentists trained in these techniques use self-etching primers to obtain a higher bond strength to dentin."

Biomimetic dentistry relies on an analysis of missing tooth structure, cracks present in the tooth, and the patient's occlusion to determine if a tooth should be restored with direct composite or if it would be better treated with an indirect ceramic or composite restoration. As Pressner alluded to, however, the use of certain materials can be more conducive to the goals of biomimetic dentistry.

Composite resins are a favorite of biomimetic dentists, whether as an alternative to metal posts, zirconia crowns, or other materials. "A metal post right down the center of the tooth is a stress raiser, and it can cause the tooth root to fracture," Bertolotti says. "A biomimetic dentist would instead use a composite resin that is matched to the mechanical properties of the dentin in the region to get a tooth that flexes, functions, etc, just like a natural tooth. Similarly, zirconia is a great material for adhesive bridges as an alternative to implants, but for most indirect restorations, lithium disilicate comes closer to mimicking enamel despite being slightly stiffer than it. They are not far apart."

Among composites, some are able to mimic the mechanical properties of dentin better than others. Bertolotti prefers a dual-cure, bulk-fill flowable composite that creates no gaps between the material and the tooth due to directed shrinkage polymerization.

"Most composites are in the same elastic modulus range—except for microfills, which are more flexible—but eliminating the gap between the composite and the tooth by directed shrinkage is more important and one of the biggest developments in dentistry in the past 20 years," Bertolotti says.

Obviously, any dentist's favorite biomaterials should be enamel and dentin. Sadr hopes that more of the restorative materials developed in the coming years are better able to mimic those natural hard tissues. "Resin-based composites are moving in the right direction because of the functional monomers," he says. "That is what we have in modern bonding. Both the composites and bonding agents are forms of polymers—monomers that turn into polymers. Of course, fiber-reinforced composite takes it one step further. These are the materials that are changing the way we practice."

Research is even being performed to develop antibacterial composite materials, and at least one such material has already received US Food and Drug Administration approval. "I think we will see more materials like these as well as composites with even less polymerization shrinkage in the future," Pressner says. "I do feel that we will continue to see improvements in materials and techniques in biomimetic dentistry because patients will increasingly demand that we save more of their healthy tooth structure and request these services more frequently."

A biomimetic dental practice may differ from a more traditional one in that there may be less use of full-coverage crowns, root canal treatments, and endodontic posts as well as increased use of partial-coverage restorations and direct bonded restorations; however, biomimetic dentistry can mean different things to different practitioners.

"Biomimetic dentistry is a way to improve patients' quality of life, and that should be the goal of any healthcare professional," Sadr says. "We have been consumed with restoration longevity for a very long time, but biomimetic dentistry is the way to increase tooth longevity for the purpose of improving patients' quality of life. It is a series of approaches in preventive and restorative dentistry, patient management, disease management, disease control, and maintenance that are geared toward the improvement of oral health and the increased longevity of the tooth versus the longevity of the restorations."

As in every aspect of dentistry, what is best for one patient might not be appropriate for another. For example, esthetic considerations must be balanced with functional ones. "I might compromise slightly if something looks a little better," Bertolotti says. "You need to balance both in a sensible way. The ultimate objective still must be to keep the tooth intact."

The following resources are available for more information about biomimetic dentistry:

Academy of Biomimetic Dentistry
aobmd.org

Alleman Center of Biomimetic Dentistry
drdavidalleman.com

Nejad Institute
nejadinstitute.com

Dental Digest Podcast
dentaldigestinstitute.com/dental-podcast