The Hybrid Layer - The Essential Bond in Restorative Dentistry
While immediate dentin sealing is an increasingly recognized technique in restorative dentistry, discussion of the resulting hybrid layer is often neglected. Yet the hybrid layer is the most essential parts of an adhesive restoration. Without a hybrid layer, exposed dentin leaves restorations susceptible to redecay, crack initiation and post-operative sensitivity, but with predictable bonding and hybrid layer formation, restorations have been shown to stay bonded and symptom-free for decades.
Decoupling with Time is an essential step in hybrid layer development, as shown in this case photo by Dr. Davey Alleman, DMD.
What is the hybrid layer?
The hybrid layer is the connection of restorative material to the bonding surface of the tooth — dentin or enamel. Unlike retention form restorations where restorative material is physically held by the tooth structure, adhesive dentistry allows the restorative material to connect at the molecular level to the tooth. When successfully formed, a hybrid layer is fully connected to the tooth at a strength similar to that at which a tooth is connected to itself (30-50 MPa).
Resin coating slide from the Biomimetic Mastership.
Early hybrid layer research
The hybrid layer was named by Nobuo Nakabayashi in 1982 and further investigated in Bart Van Meerbeek’s 1993 dissertation and research by Junji Tagami in 2009. There are four identified interdiffusionary zones (layers) of a hybrid layer that all connect with polymerizing molecules in different ways:
Top interdiffusionary zone (closest to composite)
Middle interdiffusionary zone
Bottom interdiffusionary zone (closest to dentin)
Acid-base resistance zone, also called super dentin (Identified in 2009 by Junji Tagami at IADR in Miami Beach)
Research of hybrid layer development continues to this day, most notably by Bart Van Meerbeek, who researches dental adhesives and their reactions in the hybrid layer.
Understanding hybrid layer development makes restoring deep defects like this predictable for long-lasting outcomes. Case by Dr. Davey Alleman, DMD.
How the hybrid layer affects restorative dental outcomes
As a result of immediate dentin sealing, the hybrid layer connects and seals remaining tooth structure before doctors continue to enamel replacement. A hybrid layer that is fully bonded to the dentin with no gaps can protect the pulp from reinfection and support the tooth structure to prevent crack initiation — two leading causes of post-operative sensitivity. When connected at a strength that mimics a natural tooth (30-50 MPa) the hybrid layer can also withstand long-term occlusal stresses, staying bonded for decades.
After the hybrid layer is fully developed, the biobase is complete and ready for enamel replacement following Dr. David Alleman’s Six Lessons Approach protocols. Case by Dr. Davey Alleman, DMD.
Challenges for practitioners when creating a hybrid layer
Bonding to dentin, which has a larger percentage of water in its makeup than enamel, is complex even in the simplest cases. Advancements in dental bonding systems in recent decades allow bonds to dentin that mimic the connective strength of a natural tooth (30-50 MPa), but these tests do not account for variables like the hierarchy of bondability and c-factor. Add to this conservative principles of leaving caries affected dentin and caries infected dentin under the hybrid layer to allow the pulp to heal and doctors are faced with a number of challenges to overcome when establishing a fully bonded hybrid layer. Relying on a gold-standard dental bonding system alone will not create a long-lasting hybrid layer; techniques make all the difference.
Watch this Six Lessons Approach Podcast episode to learn more about overcoming practical challenges when establishing a hybrid layer.
How biomimetic restorative dentistry develops predicable hybrid layers
Biomimetic dentistry is a bottom to top approach. This means the foundation of the tooth is more important to the tooth’s longevity than the occlusal top. When Dr. David Alleman began his own research into eliminating debonding restorations and post-operative sensitivity in 1998, he organized these techniques into his Six Lessons Approach to Biomimetic Restorative Dentistry.
The Six Lessons Approach is a hierarchy, with Lesson 1: Diagnosis and Treatment of Caries being the most important, followed by Lesson 2: Structural Analysis and Treatment of Cracks Into Dentin and then Lesson 3: Immediate Dentin Sealing and Resin Coating. Without consistent treatment of caries and cracks, even a well-bonded hybrid layer will yield unpredictable results.
This case by Dr. Davey Alleman, DMD shows start to finish how the Six Lessons Approach is used to develop long-lasting hybrid layers that get bonded, but more importantly, stay bonded.
When followed in its entirety, the Six Lessons Approach restores a tooth so it is bonded side to side, front to back and top to bottom, just like a natural tooth is connected to itself. The hybrid layer is essential in that connection. Doctors can learn more about predictable bonding protocols that prevent endodontic treatment and conserve tooth structure at Alleman Center biomimetic dentistry training programs.
