Beyond the Buzzword: Deep Dive into Biomimetic Dentistry with the BC Dental Study Club

The recent BC Dental Study Club course offered a comprehensive exploration into the principles and techniques of Biomimetic Dentistry – an approach focused on restoring teeth to mimic their natural structure, function, and biomechanics for predictable, long-lasting results. This wasn't just about learning rigid protocols; it was about understanding the fundamental science behind adhesive dentistry and how to apply it intelligently in diverse clinical situations.

A core takeaway was the crucial interplay of stress and time in achieving successful adhesive restorations.

Understanding the Dynamics of Stress and Bond Development

Polymerization shrinkage is an inherent challenge with composite materials. The course highlighted that this shrinkage and resulting polymerization stress doesn't simply end when the curing light turns off; it continues to develop over time, potentially leading to residual stress within the confined space of a cavity preparation.

Experiments using glass cartridges, chosen for their modulus of elasticity similar to enamel, effectively demonstrated this phenomenon. These experiments visually showed cracks forming hours, and even days, after the composite was cured within the cartridge, illustrating that curing is merely the initiation of the polymerization reaction.

Simultaneously, the strength of the adhesive bond between the composite and tooth structure develops over time. While the absolute maximum bond strength is theoretically achieved after approximately two weeks, the most significant increase occurs within the first 5 to 10 minutes after curing. The critical insight here is the need to minimize stress on the developing bond during this crucial early phase. If the polymerization stress developing in the composite during this period exceeds the strength of the forming bond, bond failure can occur.

Visualizing these stresses is possible through various methods discussed, including:

• Measuring strain on the tooth.

• Observing cuspal displacement visually or digitally.

• Finite element analysis modeling.

• Analyzing crack formation in the glass cartridge model.

  
  

Challenging Traditional Approaches: The Incremental Layering Question

A surprising finding presented from studies utilizing the glass cartridge model was that traditional incremental layering techniques, specifically using four 1mm horizontal layers, did not effectively reduce residual stress compared to a bulk fill technique in those experimental setups. This directly challenges a long-held belief. While these studies did show that 1mm horizontal layers resulted in higher bond strength compared to larger increments or bulk fill in microtensile bond strength tests, this higher bond strength didn't translate to reduced residual stress or crack formation in the glass model. This distinction between achieving high bond strength and managing residual stress was a key learning point.

However, smaller increments (like 1mm or even 0.5mm) still play a role, particularly in areas where maximizing the bond to compromised tooth structure is critical, such as root caries or carries-affected dentin. A smaller increment in these difficult areas can help encourage bond development, with larger 2-3mm increments used thereafter.

Optimizing Adhesion and Managing Stress: Key Techniques

The course delved into specific techniques designed to control polymerization stress and maximize adhesive success:

1. Indirect Restorations: Techniques like inlays and onlays are inherently stress-reducing because the bulk of the material polymerization occurs outside the tooth. The shrinkage stress within the tooth is limited primarily to the thin layer of luting material used for cementation. In the glass cartridge experiments, the inlay group showed no cracks, demonstrating superior performance over direct techniques in minimizing residual stress. Indirect restorations are often favored for MOD restorations and those with increasing structural loss or cuspal involvement, not necessarily because they are stronger by default, but because they offer better control over material properties and minimize the amount of shrinking material within the tooth.

   
   

2. Immediate Dentin Sealing (IDS): Described as one of the most important aspects of biomimetic dentistry for indirect restorations, IDS involves applying and curing the bonding agent onto the dentin surface immediately after tooth preparation, before taking the impression or scan. The rationale is that freshly cut dentin is the optimal substrate for bonding, and IDS capitalizes on this ideal state.

   
   

   The advantages of IDS are numerous and significant:

   • Improved Bond Strength: Precuring the thin adhesive layer allows the bond to form under minimal stress.

   • Fewer Gap Formation.

   • Decreased Bacterial Leakage: Sealing the dentinal tubules helps prevent contamination.

   • Reduced Dentin Sensitivity: Plugging tubules protects against post-operative sensitivity.

   • Formation of a "Secure Bond": By establishing a strong bond to the dentin early, any potential failure under load is more likely to occur cohesively within the composite buildup or adhesively between the composite buildup and the indirect restoration, rather than at the dentin interface itself. This preserves the dentin seal and simplifies potential repairs.

   • Provides optimal conditions for bond development by allowing it to polymerize with low stress and sufficient time.

     
     

   The typical IDS protocol involves:

   • Surface Preparation: Ideally air abrasion after using a diamond bur, or at least using a medium/fine diamond bur to prepare the dentin surface.

   • Primer Application: Scrubbed onto the dentin for about 20 seconds.

   • Air Drying: Crucially, air drying for 10 seconds to evaporate solvents, ensuring the surface looks moist and shiny, not dry and matted. (Note: Optibond may require a different drying technique aiming for a moist appearance without a specific time).

   • Adhesive Application: Applied, wicking up excess with a clean micro brush. Two coats are often recommended, cured after each, to achieve a thickness of around 100 microns, sufficient to withstand later sandblasting.

   • Resin Coating (Optional but Recommended): Applying a thin layer (around 0.5mm or less) of flowable composite (like Majesty Flow) on top of the cured adhesive. This elastic layer further protects the developing dentin bond from stress during subsequent buildup or cementation. It helps fight moisture ingress, especially important in difficult isolation situations. It is spread with an instrument like the green instrument or a probe, avoiding contact with enamel margins if possible.

   • Curing: Curing the adhesive (and resin coating if used) for 30 seconds (manufacturer may say 10s, but 30s is recommended for decreased permeability).

     
     

3. Restorative Foundation (Biobase): Following IDS in indirect cases, a restorative foundation (also called a biobase) is often placed. Its purpose is not for retention like a traditional buildup, but to create a more favorable shape and geometry for the indirect restoration.

   
   

   Key objectives of the restorative foundation include:

   • Blocking out undercuts.

   • Filling voids or concavities.

   • Eliminating or reducing box forms, creating gradual, smooth transitions.

   • Making the prep shape more uniform for consistent restoration thickness.

   • Elevating deep areas.

     
     

   Materials used include EverX Flow, EverX Posterior (less aesthetic due to grayness), APX, or even Majesty Flow for small areas or smoothing. For light curing effectiveness and biomechanical performance, the buildup is typically kept to two to three millimeters. Fiber-reinforced composites like EverX Flow are particularly valuable for larger buildups, especially in endo-treated teeth or high C-factor areas, as they help manage polymerization stress. Building up large areas might involve layering, ensuring each layer is within curing depth limits and managing stress by decoupling from enamel if possible or using fiber-reinforced material.

   
   

4. Deep Margin Elevation (DME): This technique is specifically used to manage localized deep subgingival margins, typically those deeper than 2mm, by elevating them coronally with a bonded composite buildup.

   
   

   The primary goals of DME are to improve:

   • Access and Isolation: Turning a subgingival margin into a supragingival or equigingival one dramatically improves the ability to isolate the area.

   • Impression/Scanning Accuracy: A clean, visible margin is essential for accurate digital scans or traditional impressions.

   • Delivery of the Final Restoration: Seating the restoration becomes significantly easier with a coronally elevated margin.

     
     

   The DME protocol involves careful isolation (rubber dam) and often the use of specialized matrix bands. These bands are customized, sometimes by cutting them shorter while leaving the ends full height for the retainer, allowing them to slide down the contour subgingivally. Early placement of the matrix band can help protect gingiva during prep. Fibrotomy (severing tissue attachment with an instrument like a chord packer or blue instrument) is often mandatory to create space for the band to go down to the necessary depth, sometimes close to or at the bone level, requiring careful identification of the bone margin. For very deep margins, a "matrix within a matrix" technique can be employed, using a second band placed inside the first to ensure tightness and adaptation.

   
   

   Research discussed supports the use of DME, finding that from a biomechanical standpoint, it seems advantageous compared to deep onlay restorations. Placing composite in the deep margin area helps replicate the lost cervical dentin and root structure, improving stress distribution. Studies have also shown better marginal integrity with DME, particularly when using layered techniques for the composite buildup (e.g., three 1mm layers showed better margin quality after cycling compared to one layer). While historical concerns about biologic width invasion exist, clinical experience and studies suggest that with a proper seal, tissue health (probed pocket depths of 3-4mm, no bleeding on probing, similar to implants) can be maintained, making it a viable alternative to crown lengthening or extraction in many cases.

   
   

5. Stress-Reduced Direct Composite Techniques: While indirects are preferred for significant structural loss (like MODs or cuspal coverage), direct techniques are viable for less compromised teeth (Class I, II, simple Class III). The course described a stress-reduced approach for larger direct composites.

   Key elements include:

   
   

   • Resin Coating: Applying a 0.5mm layer of flowable composite (Majesty Flow) as an elastic layer over the cured adhesive on the dentin.

   • Dentin Replacement/Buildup: Using a fiber-reinforced composite like EverX Flow (max 3mm thickness) or a regular composite like APX (max 2mm thickness or using smaller increments if needed) to replace the dentin layer. This layer stays slightly shy of the enamel margin, avoiding bonding to enamel at this stage to prevent polymerization stress from pulling towards the enamel.

   • Decoupling: Restoring cusps one at a time or in sections (e.g., mesial buccal, distal lingual) rather than bonding everything across the tooth simultaneously. This minimizes cuspal deflection caused by polymerization shrinkage. The technique involves placing composite for one cusp, adapting it with instruments/brushes, and pushing it away from the opposing cusp to create separation before curing.

   • Enamel Layering: Adding the final enamel layer(s) cusp by cusp (typically 1.5-2mm thick), building anatomy.

   • Instruments: Utilizing specific instruments like the green instrument, brushes, and spatulas for precise placement, adaptation, and shaping.

   • Air Block: Using glycerin after the final cure of each increment or the entire restoration to ensure complete polymerization of the surface layer by blocking oxygen inhibition.

     
     

Indirect Restoration Cementation: The Heated Composite Protocol

Cementing indirect restorations adhesively is critical. The course advocated for using heated composite as the luting agent. Heating the composite lowers its viscosity, allowing for better adaptation, thinner bond lines, and potentially improved mechanical properties. Specific materials like APX were mentioned as suitable due to their low filler size even when heated.

Advantages of heated composite cementation include:

• Increased fracture resistance of the restoration.

• Superior resistance to wear at the margin.

• Extended working time (as it's light-cure only, ambient light must be controlled).

• Excellent color stability.

• Highly polishable margins that blend seamlessly.

  
  

The detailed cementation protocol involves careful conditioning of both the tooth and the restoration:

1. Try-in: The restoration is tried in to check fit. This is ideally done after air abrasion of the prep and under rubber dam isolation.

   
   

2. Restoration Conditioning:

   • Hydrofluoric (HF) Acid Etching: The intaglio surface of the restoration is etched with HF acid (concentration and time depend on the ceramic material).

   • Rinsing: Thorough rinsing to remove etching debris.

   • Phosphoric Acid "Shampooing": Scrubbing phosphoric acid onto the etched surface to clean off any residual debris, followed by another thorough rinse.

   • Ultrasonic Cleaning: Cleaning the restoration in distilled water in an ultrasonic cleaner to remove any remaining microscopic particles.

   • Silane Application: A silane coupling agent is applied in a thin layer, air-dried, and heat-dried to facilitate the condensation reaction and water evaporation.

   • Adhesive Application: Immediately before cementation, a thin coat of adhesive is applied to the conditioned intaglio surface of the restoration but is not cured.

     
     

3. Tooth Conditioning: Ensure the IDS and restorative foundation are clean and ready for bonding. Air abrasion of the prep is a key step here to clean the surface and remove any remaining provisional cement or debris.

   
   

4. Cementation: The heated composite is applied to the restoration. The restoration is carefully seated, ensuring full seating. Excess cement is removed using instruments like the green instrument, explorers, or even super floss.

   
   

5. Curing: The restoration is light-cured for a total recommended time of two minutes, curing from multiple surfaces (buccal, lingual, occlusal). Glycerin (air block) is applied to the margins for the final cure to ensure complete polymerization of the surface layer.

Preparation Principles and Technology

Biomimetic preparation diverges from rigid, geometric designs. The focus is on preserving tooth structure, removing only compromised tissue, and creating rounded internal line angles to distribute stress effectively. Caries removal often utilizes diamond burs at lower speeds with electric handpieces for control. Caries-detecting dye is used to confirm complete caries removal, though slightly demineralized but not infected dentin that stains pink is acceptable to leave, accepting a small reduction in bond strength but preserving tooth structure. Air abrasion is valuable for cleaning prepared surfaces before bonding or taking scans/impressions.

The course also touched upon the exciting potential of 3D printing for indirect restorations, with advancements in materials (higher filler content composites) promising improved polishability and strength in the near future. While milling remains prominent, 3D printing offers a potentially efficient alternative for certain applications.

Finishing and Troubleshooting

• Staining and Glazing: Ceramic restorations can be characterized using luster pastes or stains, often diluted with glaze. These are applied and fired to achieve desired aesthetics. A die can be helpful for this process. Cleaning excess stain can be done with instruments or, for surface stain, using air abrasion.

• Air Abrasion (Sandblasting): Used extensively for cleaning prep surfaces, conditioning indirect restorations, and even as a finishing/cleanup step for composite restorations, using low pressure and fine powder (e.g., 27 micron aluminum oxide). It can remove excess stain or polish surfaces.

• Post-Operative Sensitivity: If sensitivity occurs related to gaps, Teethmate Desensitizer (calcium phosphate powder mixed with sterile water) can be used to plug dentinal tubules. It is mixed into a thick paste and scrubbed onto the affected surface.

• Adding Composite After Curing: If needed, composite or flowable can be added to a cured restoration, but it requires sandblasting/air abrading the surface first to ensure proper adhesion.

  
  

In conclusion, the BC Dental Study Club's course provided a robust, science-backed approach to biomimetic dentistry, emphasizing the control of stress and time through techniques like Immediate Dentin Sealing, Restorative Foundations, Deep Margin Elevation, and optimized direct composite layering and indirect cementation protocols. By understanding and applying these principles, dentists can achieve restorations that not only look natural but also mimic the natural tooth's behavior, contributing to long-term clinical success.

Dr. Noor N. AyToghlo