Two Approaches to Creating a Bio-base for Biomimetic Restorations

Principles of Biomimetic Dentistry

Biomimetics is the study of the formation, structure, and function of biologically produced materials. Using this knowledge, the dental industry is able to manufacture products by artificial mechanisms that mimic natural ones. A material formed by a biomimetic technique based on a natural process is called a biomimetic material.³ In modern dentistry, achieving a thorough understanding of the dental hard tissues made it very clear that dentin should be replaced with a dentin-like material, such as composite, and that enamel should be replaced with an enamel-like material, such as ceramic. The foundation for these biomimetic protocols was laid during adhesion's "silent revolution" in dentistry, which occurred during the 1980s and 1990s.⁴

Dentin Bonding and Caries Detection

This foundational knowledge was taken a step further by researchers who distinguished between two distinct layers of carious dentin (ie, infected and affected), each of which exhibited a unique set of dentin adhesion characteristics.^5,6 These researchers were able to bond to affected dentin in a predictable manner by utilizing a then cutting-edge technology known as caries detecting dye.⁷ This innovation enabled a perfect caries removal end point to be visualized in the critically important "peripheral seal zone." A bond to dentin could be formed using recently developed polymerizable monomers that were both hydrophilic and hydrophobic, provided that the dentin surface in question was free of denatured collagen. In this methodical approach, the use of caries detecting dye is informed by anatomic and histologic knowledge in order to reach the appropriate caries removal end point that is required for adhesive restoration.

Today, to better guide clinicians through the process of deep caries diagnosis and removal, laser fluorescence technologies may also be utilized. Using just the tactile and visual methods alone presents a number of drawbacks, which can be avoided by employing this combination of multiple techniques that support one another.^8-10

Development of the Bio-base

After removal of the infected carious dentin, the next step is achieving a strong bond to the remaining affected dentin and placing the composite. This next step is crucial because it forms the strategic base for the indirect restoration that will replace the enamel in the final part of the biomimetic restorative protocol. In biomimetic dentistry, the composite base is referred to as the "bio-base" (Figure 3 through Figure 5).^11,12

The polymerization shrinkage and the shrinkage stress associated with bonding large preparations and placing composite resulted in the development of certain strategies and materials to combat them. One such strategy, "decoupling with time," involves minimizing the polymerization shrinkage stress to the developing dentin bond of the hybrid layer for a certain period of time (ie, 5 to 30 minutes) by keeping the initial increments of composite to a minimum thickness (ie, less than 2 mm).¹³ This minimal thickness prevents the connection, or "coupling," of deep dentin to enamel or superficial dentin before the hybrid layer matures and is close to full strength. This procedure helps to neutralize the negative effects of the "hierarchy of bondability," which states that the shrinkage of composite moves toward the walls of the preparation that are the most mineralized and dry and moves away from the walls of the preparation that are the most moist and organic.

Leveraging Fiber Reinforcement

When creating a bio-base for a bonded partial restoration, the use of fiber-reinforced composite resins can help to improve strength in areas that bear high stress, such as molars. These composites, which feature various short glass fibers incorporated into cross-linked polymer matrices, are formulated to improve mechanical properties, such as compressive strength, flexural strength, fracture strength, fracture toughness, fatigue resistance, and load-bearing capacity.^14,15

Another approach to increase the strength and fracture toughness of the bio-base involves the incorporation of long polyethylene fibers. Due to the design of such fiber-based materials, they also serve as a crack stopping mechanism. The locked-stitch interwoven fibers prevent rapid crack growth and change the direction of occlusal forces, which ultimately dissipates the strain. In a study published in 2006, Belli and colleagues concluded that the use of polyethylene fibers increased the micro-tensile bond strength and lowered the C-factor effect.¹⁶ In addition, Akman and colleagues reported that the placement of polyethylene fibers against the dentin walls decreased the cusp movement under loading of root-filled molars with mesial-occlusal-distal restorations.¹⁷

Ultimately, it is the clinician's choice whether or not to use one of these materials to help ensure that a strong dentin bond is achieved along with a stable composite restoration. The following case reports demonstrate two approaches to creating the bio-base after caries detecting dye has been used to remove all of the infected dentin-one using fiber-reinforced composite and one using a bondable polyethylene fiber reinforcement ribbon.

Case Report 1

A 40-year-old male patient presented with a fractured restoration on his mandibular left first molar (tooth No. 19) that demonstrated occasional sensitivity (Figure 6). His dental history revealed that the amalgam restoration, which had been placed almost 15 years previously, had subsequently been repaired twice during the past 2 years. The clinical exam revealed that, in addition to the fractured restoration, tooth No.19 had also sustained a distolingual cusp fracture. Although the tooth structure was heavily compromised and required an extensive restoration, the pulpal status was determined to be sound, both clinically and radiographically. Following a comprehensive evaluation, the decision was made to pursue a bonded partial restoration.

The first step would be the creation of a bio-base. After profound anesthesia was achieved, the tooth was isolated, and the failing restoration was removed. Caries detection dye was used to achieve the end point for caries removal (Figure 7). Once the cavity was cleaned up, a two-step self-etch adhesive with fluoride-releasing capability (CLEARFIL^™ SE Protect, Kuraray Noritake) was applied per the manufacturer's instructions. Next, a very thin layer of a universal flowable composite (G-ænial^™ Universal Flo, GC America) was placed and polymerized. This was followed by a thin layer of a fiber-reinforced flowable composite (everX Flow^™, GC America), which was  polymerized. The cavity was then restored with the packable version of the fiber-reinforced composite (everX Posterior^™, GC India) and polymerized (Figure 8). To complete the bio-base, a layer of light-cured universal flowable composite (G-ænial^™ Universal Flo, GC America) was used to cover the fiber-reinforced composite (Figure 9). This was placed to ensure that the fibers in the fiber-reinforced composite were well covered. Once the bio-base was completed, the tooth was prepared for a tabletop restoration.

Case Report 2

A 28-year-old female patient presented with a fractured amalgam restoration on her mandibular left first molar (tooth No. 19). Her dental history revealed that she had the tooth restored during high school and that the restoration had fractured one week prior to her presentation. A thorough clinical and radiographic examination revealed that the fractured amalgam restoration had associated secondary decay at the distal aspect (Figure 10). In addition, a new carious lesion had developed at the tooth's mesial aspect. The pulpal status was determined to be sound, so the tooth was cleared to undergo a complex restorative procedure.

After the tooth was anaesthetized, a rubber dam was applied. The remaining part of the fractured restoration was carefully removed, and caries detection dye was applied to achieve the caries removal end point. Following proper preparation of the mesial and the distal aspects of the cavity (Figure 11), a two-step self-etch adhesive with fluoride-releasing capability (CLEARFIL^™ SE Protect, Kuraray Noritake) was applied per the manufacturer's instructions. Next, a thin layer of superfilled flowable composite resin (CLEARFIL^™ AP-X Flow, Kuraray Noritake) was applied to the floor of the cavity and uniformly spread with a sharp probe. A 3-mm piece of bondable polyethylene fiber reinforcement ribbon (Ribbond^®, Ribbond) was then cut, soaked in modelling resin (Modeling Resin, Bisco) (Figure 12), and carefully adapted into the deepest portion of the cavity (Figure 13). To cover up the fiber ribbon, a thin layer of the flowable composite resin was placed, and then it was polymerized. Later, the cavity was restored with the packable version of the composite resin (CLEARFIL^™ AP-X, Kuraray Noritake) (Figure 14).

Conclusion

In both of these case reports, the time taken for the entire procedure-from the start of bonding to the polymerization of the final layer of composite-was monitored with a stopwatch. This helped to ensure that time was allowed for decoupling. The minimum recommended decoupling time is 5 minutes.

Both case reports present valid approaches to achieving the bio-base, and each have their protocols to be followed. Studies have shown that structurally compromised teeth benefit from the addition of fiber reinforcement to improve their fracture resistance when a bonded partial restoration is to be placed. It is incumbent upon the clinician to carefully analyze each clinical situation and decide what type of reinforcement is appropriate.

About the Author

Mohan Bhuvaneswaran, MDS
Accredited Member
American Academy of Cosmetic Dentistry
Adjunct Professor
Airlangga University
Surabaya, Indonesia
Director
Vignesh Dental Hospital
Chennai, India