A Biomimetic Approach to Treating Aggressive Caries

INTRODUCTION
Ending the margins of full-coverage indirect (lab-fabricated) restorations properly onto sound tooth structure has historically been foundational to successful restorative dentistry, but recent research and experience suggests otherwise. Bonding technology now makes it possible to rebuild teeth to their proper function by replacing dentin, enamel, and the dento-enamel junction (DEJ) with materials of similar properties without resorting to aggressive preparations.

The author describes a protocol for tooth restoration termed biomimetic dentistry, which refers to replicating the biologic and elastic properties of intact teeth. In this manner, the clinician no longer attempts to extend the margin onto tooth structure within the restoration. Instead, one utilizes the ability of composites to create a tight seal with dentin, and to bond to an existing composite biobase (described in the clinical case report below). This departure from the one-piece crown simplifies the restorative procedure and creates a more biologically acceptable restoration, creating a fail-safe and repairable design.

Full-Coverage Versus Partial-Coverage Restorations
Dentists typically create full-coverage crowns when there is tooth breakdown of varying degrees. Often, a partial-coverage restoration would be preferable, since it avoids the gingival sulcus and the weaker gingival third of the tooth. However, the classic G. V. Black preparations present certain procedural challenges, such as the need to balance a retentive taper design against potential undercuts. Accordingly, a large number of full-coverage preparations for crowns are routinely performed.

Biological and Mechanical Compatibility of Crowns
Bacteria are approximately 1.0 μm in size and readily form colonies at a subgingival margin of a full crown. A 100% closed margin is not possible to achieve even within the gingiva-free zone, and since many crown margins are placed into bleeding tissues, the fit is even worse. This bacterial colonization at the open margin can cause localized inflammatory periodontal disease, recurrent caries, cement washout, pulpal death, and, eventually, the potential loss of the restoration or the tooth.

The materials used for full crowns are stiff (very high elastic modulus). However, the dentin substrate, with some flexibility (a lower elastic modulus), transfers occlusal forces to the gingival margin. This push-pull between tooth and crown puts the restoration at odds with the tooth’s normal physiologic bending, and can result in abfraction lesions, cement washout, and recurrent caries.

A Biomimetic Strategy
Biomimetic dentistry uses the intact tooth as the restorative guide by using specific adhesive dental materials to replace the lost enamel, dentin, and the DEJ. The aim of this strategy is not to create the strongest restoration possible, but rather, to create a restoration that is compatible with the mechanical and biologic properties of underlying dental tissues.¹ This is, in part, accomplished by using contemporary dental materials to bond the restoration to the substrate while eliminating full crowns as well as the complicated design of G. V. Black onlay preps.

Successful adhesive dentistry may be expressed as an equation between bonds and stresses, where the bonds are able to withstand the stress.² Failure results when the stresses that are exerted on the restoration, potentially from a caries-infected substrate, an untreated crack, from excessive shrinkage of composite, or from occlusal trauma overwhelming the bond. To achieve restorative success, a protocol has been developed at the Alleman-Deliperi Center for Biomimetic Dentistry (see the website biomimeticdentistryce.com) dividing treatment into “Six Lessons” involving caries and structural compromise removal, development of a secure dentin bond, control of polymerization shrinkage, onlay design, and occlusal harmony.

By following this protocol, the clinician will eliminate troublesome aspects of composite resin dentistry, including post-op sensitivity, gap formation from polymerization shrinkage, and recurrent caries. Furthermore, the clinician will be able to create restorations that are long-lasting, maintainable, and even serviceable.

CASE REPORT
Diagnosis and Treatment Planning
Upon clinical examination of tooth No. 30 (Figure 1), a large composite that had fractured was observed, revealing soft secondary decay. It can be surmised that the bond failure resulted from one or more of the following:

1. Failure to remove caries or cracks completely
2. Failure to create a secure bond with the adhesive
3. Gap formation at the gingival margin
4. Bulk fill of light-cured composite that created stress/strain
5. Incomplete cure of the composite mass.

Fortunately, the tooth was aymptomatic and without periapical pathology. The plan was to remove caries, then place the bioliner and biobase according to Biomimetic Principles, followed by a lab-fabricated pressed all-ceramic replacement of the tooth’s enamel.

Figure 1. Pretreatment.	Figure 2. Restoration removed.
Figure 3. Caries staining.	Figure 4. Caries debrided.

Debride Dentin and Remove Cracks
After administration of a local anesthesia (Lignospan Forte 2% [Septodont]), the failed composite was removed, exposing the carious underlying dentin (Figure 2).

Using a tapered, coarse diamond (F82 [Pollard Dental]), the carious dentin was removed along a 2.0 mm periphery of the preparation. Caries Detector Solution (Kuraray Noritake Dental) was applied, then rinsed in order to identify any remaining infected dentin (Figure 3). Next, we removed soft dentin and dentin cracks using the high-speed No. 4 round diamond (801-018C [SS White Burs]). Caries removal continued with additional application and rinsing of the indicator solution. As caries began encroaching on the pulp, we allowed the “pink haze” to remain, reaching the objective caries excavation endpoint (Lesson 1).³ Further excavation of dentin beyond this point was undesirable since the potential for a pulp exposure becomes more likely (Figure 4).

Following caries debridement, the structural viability of the tooth was evaluated. The criteria for removing a cusp is if it too thin (< 2.0 mm); or the isthmus is too wide (> 3 mm); or the box is too deep (> 4 mm)⁴ (which occurred in this case). The cusps were removed using a round-ended tapered diamond (FG 140-014C [Tri Hawk]), followed by a 360° bevel.

Create a Secure Resin-Dentin Bond
One drop from bottles Nos. 1 and 2 of the self-etch adhesive (Clearfil DC Bond [Kuraray Noritake Dental]) were mixed together and allowed to dwell on the preparation for 20 seconds.

Next, the prep was lightly dried with oil-free air, then light-cured for 20 seconds.

Build the Bioliner and the Biobase Minimizing Shrinkage
The bulk of the tooth needs to be restored by creating as low a C-factor as possible to create a low-stress restoration. C-factor is defined as the ratio of bonded surfaces to the unbonded surfaces; keeping in mind that the higher the ratio, the more the contraction force that will get transferred to the margins of the restoration.⁴ Polymerization shrinkage remains a problem with direct composites;⁵ therefore, the composite needs to be polymerized slowly to allow the flow of the material to take up the contraction force of the curing composite. If the contraction force of the polymerizing composite exceeds the bond strength of the adhesive, then gap formation and/or cusp deformation can occur; this can lead to post-op sensitivity and eventual breakdown at the margins. The bioliner, consisting of a thin fiber-reinforced layer of flowable composite (N’Durance Dimer Flow [Septodont]), helps to take up some of the setting contraction force, resulting in stable bond strengths even in cavities with high C-factors.⁶

After placing the matrix, the bioliner was created by adapting and bonding small squares of a fiber net (Ribbond) onto the cut dentin surface. The bioliner was completed by adding a thin layer of a heavily filled, low-shrinkage, flowable composite (N’Durance Dimer Flow). This is allowed to flow around and in between the fiber Ribbond netting, and then it was light cured.

Next, the biobase was built by first creating the proximal walls, beginning with a thin layer of flowable composite (N’Durance Dimer Flow) at the gingival margin, followed by a light-cured sculptable, low-shrinkage composite resin (N’Durance Universal [Septodont]) with a flexural modulus that approximates dentin. The sculptable composite was formed into 1.0-mm thick proximal walls and light cured (Figure 5). This technique is a modification of the method described by Deliperi and Bardwell⁷ that reduces polymerization shrinkage stress while allowing direct light penetration to the gingival margin. Using a low-shrinkage flowable and sculptable composite resins in this manner accomplishes the following:

• Transfers a minimum of force to the margins of the preparation during curing
• Creates a smooth and biologically compatible interface with the gingival tissues
• Allows for complete light penetration throughout the entire composite mass.

Once the walls were formed, the biobase was completed by bulk filling the large space with a low-shrinkage, dual-cured composite resin (N’Durance Dimer Core [Septodont]). Dual-cure composites exhibit minimal shrinkage and contraction stress, especially when allowed to cure with minimal (one to 2 seconds) or no light activation; properties that make this material ideal for the bulk of the biobase.

Complete the Preparation, Seal, and Make the Impression
The preparation was completed with a 360° bevel. An interocclusal clearance of 2.0 mm was verified as required for the pressed ceramic material chosen (Figures 5 and 6). The case was then sealed by applying the self-etch adhesive (Clearfil DC Bond) onto the preparation surface and polymerizing it. A viscous oxygen barrier solution (DeOx [Ultradent Products]), was applied, the material re-cured, and then the preparation was cleaned and the (vinyl polysiloxane) impression (Elite HD+ Maxi Tray/Light Body [Zhermack]) was made. The patient was then dismissed without a temporary restoration, and with instructions for proper postoperative home care (Figure 7).

Figure 5. Isolation was achieved and the bioliner placed.	Figure 6. Biobase completed.
Figure 7. The completed preparation.	Figure 8. Ready for cementation of the all-ceramic restoration.

Cementation
The fit of the pressed lithium disilicate (IPS e.max [Ivoclar Vivadent]) restoration was verified, and then the intaglio surfaces were cleaned by applying 40% phosphoric acid gel for 20 to 30 seconds. The restoration was then rinsed thoroughly with water and dried with oil-free air. Ceramic Primer (Kuraray Noritake Dental) was then applied to the internal aspect of the casting and then dried with oil-free air.
Next, the pressed all-ceramic restoration was placed onto the model and attached to a cotton tip applicator stick by using wax (Bite Registration Wax [DeLar Corp]) on the end of the stick to enable efficient handling and passing of the restoration immediately prior to cementation.

The tooth was prepared for the cementation by isolating the area with a dental dam. The the tooth was cleaned with plain flour pumice, then rinsed and dried (Figure 8). The restoration was then loaded with cement (SA Cement [Kuraray Noritake Dental]) and seated onto the tooth (Figure 9). Excess cement was removed using a cut cotton roll, and then the restoration was wave-cured for 5 seconds (Translux Power Blue [Heraeus Kulzer]). This short light-curing procedure brought the cement to its gel point, allowing for a slow polymerization and for an easier initial removal of excess cement using curettes and floss.

The final cure was completed with 20-second curing light exposures to the facial, occlusal, and lingual surfaces; followed by further cleanup of any remaining excess cement.

Finally, the occlusion was checked and any needed adjustments made, and then the tooth/restoration interface was polished with rubber points (White Midgets, ultrafine [Dedeco]) and polishing paste (Figures 10 and 11).

Figure 9. The seated lithium disilicate restoration.	Figure 10. Final case (lingual view).

Figure 11. Final case (facial view).

DISCUSSION
Large restorations will succeed if sound principles are followed; these include decay/crack removal, creating a secure bond, lowered C-factor layering, preparation, and occlusion. In this example, a large and failed composite with deep recurrent caries was treated without endodontics, according to principles of biomimetic dentistry.

Rather than attempting to end the all-ceramic restoration on sound tooth structure at the precarious gingival margin, this method creates bulk at the gingival third and seals the dentin using a low-shrinkage composite resin that is securely bonded to the dentin substrate. In addition, this biomimetic preparation makes it possible for the restoration to replicate the intact tooth’s ability to bend/flex, creating a more functionally and structurally harmonious restoration. These restorations allow for a “safe failure” rather than a “catastrophic failure” because carious breakdown typically occurs at the easily accessed restorative-to-composite interface. The biobase is built using light-cured composite resin to establish the proximal walls, followed by the bulk filling of the remainder using dual-cure resin that minimizes the contraction stress during polymerization.

The bioliner and biobase are designed to reduce internal strain on the restoration. First, the contraction force of the curing composite was reduced by adapting squares of a polyethylene fiber net to the floor of the preparation. Secondly, the large bulk of the biobase was created using a low-shrinkage, dual-cured core material (N’Durance Dimer Core) that cures slowly and with minimal contraction force. With these 2 techniques, the clinician can place larger increments into the preparation without creating internal stress within the restoration. The polyethylene fiber bioliner also has been shown to redirect potentially destructive occlusal forces⁸ that would otherwise develop cracks that could cause injury to the pulp and/or restoration failure. The bioliner creates another place for a “safe failure” between itself and the biobase, rather than risking gap formation due to polymerization shrinkage at the resin-dentin interface.

The biobase itself is a departure from the traditional core buildup that is often used to create the regular contours of the full-coverage preparation—since it will serve to support the “top” that will be later bonded—but is also the gingival part of the restoration. The top two thirds of the restoration is then prepared for a bonded restoration by designing a preparation with a shallow concavity (reducing contraction stress) and with a 360° bevel, greatly simplifying the preparation, impression, and cementation procedures.

CLOSING COMMENTS
When faced with substantial carious breakdown, clinicians have a biomimetic protocol to create multicomponent restorations that preserve and protect the remaining tooth structure in place of full crowns. Biomimetic restorations permit flexion of tooth/restoration, reducing the push-pull cement washout.

Biomimetic techniques often make for easier and less stressful preparation, impression, and cementation procedures. In addition, these procedures allow the replication of the biomechanics of the intact tooth. Teeth that are restored in this way are less likely to experience pulpal or periodontal breakdown, and they have a built-in fail-safe system that allows the restoration to be easily repaired if needed at a later time.

References

1. Magne P, Belser UC. Rationalization of shape and related stress distribution in posterior teeth: a finite element study using nonlinear contact analysis. Int J Periodontics Restorative Dent. 2002;22:425-433.
2. Unterbrink GL, Liebenberg WH. Flowable resin composites as “filled adhesives”: literature review and clinical recommendations. Quintessence Int. 1999;30:249-257.
3. Iwami Y, Shimizu A, Narimatsu M, et al. The relationship between the color of carious dentin stained with caries detector dye and bacterial infection. Oper Dent. 2005;30:83-89.
4. Feilzer AJ, de Gee AJ, Davidson CL. Setting stress in composite resin in relation to configuration of the restoration. J Dent Res. 1987;66:1636-1639.
5. de Gee AJ, Feilzer AJ, Davidson CL. True linear polymerization shrinkage of unfilled resins and composites determined with a linometer. Dent Mater. 1993;9:11-14.
6. Belli S, Dönmez N, Eskitaşcioğlu G. The effect of c-factor and flowable resin or fiber use at the interface on microtensile bond strength to dentin. J Adhes Dent. 2006;8:247-253.
7. Deliperi S, Bardwell DN. An alternative method to reduce polymerization shrinkage in direct posterior composite restorations. J Am Dent Assoc. 2002;133:1387-1398.
8. Belli S, Eskitaşcioğlu G. Biomechanical properties and clinical use of a polyethylene fibre post-core material. International Dentistry South Africa. 2006;8:20-26.

Dr. Cohen is an instructor at the Alleman-Deliperi Center for Biomimetic Dentistry and has been in the private practice of restorative dentistry since his graduation from Temple University School of Dentistry in 1982. He has lectured nationally and published webinars and papers on biomimetic dentistry. He can be reached at (215) 579-9985 or via email at rgc7157@gmail.com.

Disclosure: Dr. Cohen is a consultant to Septodont.