Stress Distribution within Intact Teeth
"Physiologic performance of intact teeth is the result of an intimate and balanced relationship between biologic, mechanical, functional, and esthetic parameters. " -Pascal Magne
Histo-Anatomic Approach and Dynamic Light Interaction
Three things that need to be considered when attempting to restore the optical features of an intact tooth, they are: the inherent translucent nature of enamel, the dentinoenamel complex (DEC), and dentin.
The difficulty with applying all of these concepts is finding the right material. Translucent materials can easily alter colors due to their interaction with light. Comparing the three, enamel is translucent, the DEC is transparent, while the dentin is opaque. When considering these terms, finding the right restorative material is of the utmost importance to match the existing color palate of the intact tooth. To make things more complicated, surface texture, gloss, and luster need to also be remembered.
The way light interacts with the tooth and the exact layers of it has dramatic effect on how we look at an intact tooth. Light reacts to teeth in four ways:
-
Color: the light is scattered with the layers of the tooth and later on, reflected out
-
Transparency, translucency, opacity: light is transmitted and diffused though the tooth
-
Energy: light is absorbed and transferred into heat, energy, photoluminescence
-
Fluorescence: light gets re-radiated with a lower energy source
Drs Panaghiotis Bazos and Pascal Magne go more into detail in their fantastic article. To fully understand biomimetics, one must look at the layers and understand what the properties are of each layer and how they interact with the rest of the mouth, in this case, esthetics. Please take a look at the article, which can be found here, to understand how dynamic light impacts the natural dentition.
Histo-Anatomic Approach, Structural Analysis
The three dimensional coronal configuration can become complicated when considering the dentinal structure along with the enamel morphology. For example, dentin has a concave pattern while enamel has a convex pattern. The goal of restoring a tooth, is that we can match the existing properties and have the ability to withstand multi-axial bio-mechanical forces over any given time. Fortunately, we have the technology and capacity to be able to mimic these structures so that we are able to give a lasting prognosis for the tooth and essentially be the last restoration that tooth will need if done properly. With the improvements of composites, we are able to replicate the color dynamics, translucencies, opacities, and shades which allow us to replicate the natural tooth with more exactness. Natural teeth serve to be a guide for us to follow. We must try to replicate with precision the existing structures of the dentine-enamel junction (DEJ), their three-dimensional configuration, and their correct spacial relationships in regards to the existing coronal elements. Often times this is no easy task, both with direct and indirect restorations. Once these skills are mastered, the restorations can easily be given predictable qualities.
Please review Drs Panalhiotis Bazos and Pascal Magne’s article which beautify explains in more depth the importance, interactions, and the actual histo-anatomical approach to the dentition. It is vital to understanding the concepts of biomimetics.
CAD/CAM vs. Composite: Which is better?
There has been quite the discussion on which of CAD/CAM or composites are better for a large restoration. If needing to fill a large MOD defect (5 mm depth by 5mm bucco-palatal width), the results and opinions have varied. However, it wasn't until a recent article by S Batalha-Silva et al, that really clarified the results for us. The in vitro study attempts to reduce variables while placing them in a controlled environment that can be easily reproduced. The research that was preformed was a fatigue resistance and crack propensity of large resin restorations. Paradigm MZ100 and Miris composites were used. After a series of fatigue testing, the survival rate of Paradigm MZ100 CAD/CAM group was 100% with minimal new crack propensity of 7%. The survival rate of Miris 2 composite direct restorations was 13% while their crack propensity was 47% with more severe cracks.
The longevity of a restoration is influenced greatly on things like, materials used, patient conduct, and dentist’s skills. One of the major problems, however, is the polymerization shrinkage stress of the the composite resins, especially with high C-factor defects. Strong adhesives can cause the composite material to exert cuspal stress which leads to movements and cracking. The best way to avoid polymerization shrinkage is to use post-polymerized restorations, composite resin inlay/onlays fabricated by CAD/CAM. It has been noted that some have found no clinical advantage of CAD/CAM milling over incremental direct restorations after a 5 year recall. Others have found a 85% survival rate of 3+ surface of direct composite restorations after 12 years. In spite of all of these recent reports, Indirect restorations are still considered the gold standard of restoring large defects. There are still the same polymerization shrinkage stresses but the forces are minimized in luting restorations because it is restricted to a thin cement layer, thus leading to superior marginal quality. It also needs to be noted that the luting procedure is the gold standard by reducing cracks in the cusp base. With cracks come possible microleakage and postoperative sensitivity. Other advantages that indirect restorations have is that they can provide anatomic contours, marginal adaptation, appropriate proximal contact, contour and occlusion because they are designed with the CAD/CAM program. The composite CAD/CAM blocks present acceptable wear properties. They are less brittle than porcelain and can be used in thinner layers, allowing more conservative preparation designs. The use of CAD/CAM, Paradigm MZ100 blocks, and immediate dentin sealing has proven to be a superior, conservative, and reliable biomimetic restoration.
With all of this information, it cannot be concluded that a large direct MOD defect is contraindicated for restoration with a direct composite, even with the higher crack propensity and failure at a higher load. These direct composite restorations can be beneficial to patient and practitioners who have limited access to newer CAD/CAM materials.
The article presents a myriad of information that can be further dissected down into other posts if time allowed. The research preformed is very thorough with lots of details. Please read the article which can be found here.