Bio-Emulation: Biomimetically Emulating Nature Utilizing a Histo-Anatomic Approach; Structural Analysis.
Panaghiotis Bazos, DDS 1
Pascal Magne, DMD, MSc, PhD 2
1 Emulation, 33 Vasilissis Sophias Avenue 106 75 Athens, Greece
2 Associate Professor, Herman Ostrow School of Dentistry, University of Southern
California, Oral Hedallth Center, Los Angeles, CA 90089, United States.
Corresponding Author:
Panaghiotis Bazos DDS, 33 Vasilissis Sophias Avenue 106 75 Athens, Greece
Phone: +30 210 722 2329
Email: p_bazos@mac.com
ABSTRACT
A thorough understanding of the histo-anatomic structures and
dynamic light interaction of the natural dentition provides the dental
team with the ultimate strategic advantage with regards to optical
integration of the final restoration. The first part of this article will
attempt to provide insight on the three dimensional coronal
configuration of natural teeth and on the utilization of this
knowledge in the clinical and technical restorative approach.
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INTRODUCTION
In the modern dental practice, the restoration and tooth should unite predictably in such a manner as to form a structurally adhesive and optically cohesive medium, that has the ability to withstand the multi-axial bio-mechanical force loads in repetition over a prolonged period of time. By means of the evolution, advancement and refinement of adhesive dental technology 1,2 with regards to the bonding materials available on the market in conjunction with the validated clinical protocols at hand 3,4 , clinicians and technicians have been provided with the ability to biomimetically reproduce the union between synthetic dental materials and natural anatomic tooth structures. 5 With the perpetual improvement of dental restorative materials with regards to optical light transmission and color dynamic properties given the plethora of choices with a multitude of shades, translucencies, opacities, effects and stratification techniques 6-11 , the dental professional endeavors to faithfully emulate the dental archetype 12-14 ; that of the natural intact tooth, which serves as the model, mentor and measure. Despite the aforementioned significant advancements and improvements, re-creating the anatomical form and optical features of the intact tooth ever remains an arduous and challenging task and at times an elusive one, both within the clinical and technical dental realms. In order to optimize the optical integration of modern composite resins and silica based ceramics per restorative dental emulation, a thorough understanding of the coronal elements, (enamel / dentinoenamel junction / dentin) their three-dimensional configuration and their respective spatial inter-relationships is deemed compulsory.
Histo-Anatomic Development
Fig 1 Bell Stage: Histo-differentiation is the process of transforming a mass of similar looking epithelial cells into morphologically and functionally distinct components (enamel/dej complex/dentin). Morpho- differentiation is the process whereby individual tooth buds attain recognizable shapes (incisor vs molar).
METHODOLOGY
In order to ascertain the morphologic relationship between the exterior surface of the enamel layer and the DEJ, the teeth were submerged in 10% hydrochloric acid (HCl, Mallinckrodt Baker, Inc.,
4 Fig 2 Enamel is the densely mineralized brittle yet hard outer shell of the tooth that envelopes/engulfs the softer dentin core; carbonate-rich hydroxyapatite crystals are arranged in enamel rods. Dentin, conversely, is a collagen rich apatite reinforced bio-composite that is resilient yet tougher than enamel and similar at the nanostructural level to bone. It has a unique structural architecture consisting of dentinal tubules, surrounded by peritubular dentin cylinders of randomly oriented apatite crystallites.
Phillipsburg, NJ, USA) under ultrasonic vibration for 20 minutes, which led to it’s selective enamel demineralization. Subsequently the teeth were quenched in distilled water for 1 hour in order to neutralize the acid and facilitate handling. The specimens were photographed (D200 Nikon Inc., Melville, NY, USA) on a custom fabricated tripod-jig (XX-Halter, Novoflex, Memmingen, Germany) maintaining standardized illumination, exposure settings and perspective, prior to and after the acid treatment, in order to ensure proper alignment of
a
b
c
Fig 3 The DEJ when examined histologically provides a visual interface yet when examined on a bio- mechanical level, it is regarded as a functional interphase. As seen above the frontal longitudinal histological tooth section of a maxillary premolar 0.5mm in thickness was submerged in distilled water and photographed on a black background in depiction (a) . The same specimen was photographed by transmissive cross polarized illumination (b) . Highlighted is the extended DEJ complex. (c)
the superimposed images.
DEJ AS THE STRUCTURAL EPICENTER OF AN INTERPHASE BETWEEN ENAMEL AND DENTIN
One must examine the significance of the DEJ as an important factor in the understanding of the developmental adaptation of the surface pattern of enamel expression. During odontogenesis this junctional interface serves as the histologic blue print (FIG 1.), representing a complex inter-digitation zone between two distinct anisotropic calcified tissues with different
Fig 4 The coronal dentin surface can be considered a three-dimensional configuration of the dentinoenamel junction 20 . When macroscopically observed, a high degree of conformity exists between the gross form of the dentinoenamel junction and the overlying enamel surface 21-23 , the significant exception being the localized enamel thickness on the buccal and lingual middle thirds of the crown, forming a transitional sigmoidal curve distribution. biochemical compositions (FIG 2): i) enamel serving as the structural protective shell and ii) dentin serving as the structural dampening core. (FIG 3a) The enamel and dentin demarcation is due to the difference of birefringence between the tissues. (FIG 3b) The DEJ is less mineralized than either enamel or bulk dentin, conversely being
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Fig 5 Convex contours of the enamel surface are evident when viewed from the proximal surface, providing a contrast to the sharp, concave relief of the dentin surface. Congruency of micro-expressions between enamel and dentin surface characteristics are depicted by the colored arrows. Note the transparent ridge. Maxillary first pre-molar depicted. richer than either in organic matrix. Microscopically a bi-scalloped surface topography is present, establishing a complex zone capable of plastic deformation while being collagen fibril-reinforced 15 . Considerable interest has been taken with regards to the interconnectivity of the inner aprismatic enamel, the DEJ and the outer layer of dentin, known as the “mantle dentin” approximately 150 microns in thickness, which is synthesized at the onset of
dentinogenesis 16 . This extended dentinoenamel complex (DEC) has been histologically described 17 and observed as a functionally graded interphase between two vastly bio-mechanicaly different tissues, that provides crack tip shielding 18 , partially accomplished by a localized reduction in density and mineralization, in both enamel and dentin as they approximate their junction 19 (FIG 3c.) Fig 6 A pronounced dentin concavity is present on the buccal surface at the junction of the cervical land middle thirds of the posterior dentition, forming a sigmoid curve. This localized enamel over expression may present a selective bio-mechanical reinforcement mechanism to the compressive loads experienced in the posterior dentition. Mandibular second molar depicted.
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Fig 7 The above molar was extracted due to periodontal sequelae. Upon trans-illumination, multiple crack extensions are observed, yet they are contained only within the enamel layer. Since the process is gradual, the cracks will be continuously replenished of protein rich oral fluids, thereby utilizing a self- healing mechanism.
MICROSTRUCTURE: ENAMEL VS. DENTIN
The microstructure of enamel is dominated by hydroxyapatite crystal-rich enamel rods, cemented together by an organic matrix protein polymer. Enamel being brittle yet stiff, undergoes only minimal deformation while transferring loads to the underlying dentin. The key to the unusual properties of enamel lies in its unique three dimensional structural arrangement, which consists of very long rods of carbonated apatite arranged in directional bundles. These bundles are progressively interwoven at higher hierarchical levels. The rise in crack growth resistance is largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which are induced by prism decussation of the inner enamel. 24 The microstructure of coronal dentin appears to be that of a mineralized collagen fiber bio-composite, the intertubular dentin being the matrix and the dentin tubule lumens with their associated cuffs of peritubular dentin forming the cylindrical fiber reinforcement. Dentin posses both elastic and plastic material properties that vary significantly from region to region and along different orientations within very small distances. Uncracked-ligament bridging presents as
the prevalent crack shielding mechanism observed in the hydrated dentin core. 25
MACROSTRUCTURE: CONVEX ENAMEL VS. CONCAVE DENTIN
Most topographic structures are related to the different functional roles of the enamel and dentin surfaces. The robust, rounded convex contours of the enamel surface provide strength to a tissue subjected to direct masticatory stresses and occlusal loads. In contrast, the sharp, concave relief of the dentin surface provides a stable support for the enamel shell. (FIG 5+6.) From a bio-mechanical viewpoint, harmony between the ectodermal and mesodermal tissues was necessary for the selective adaptation of teeth, with the DEC proving to be the most intricate of mechanisms, imparting the structural efficiency of an interconnecting network, where the various structural elements function in unison rather than remaining independent from each other. The DEC therefore is considered as functional shielding mechanism, that should be preserved, whenever possible during clinical restorative procedures. Bio-mechanical force loads are thus allowed to transmit freely through the surfaces, dissipating throughout this structurally fluid
Morphologic Features
Enamel Surface
Dentin Surface
Marginal Ridges
Rounded
Sharp
Buccal Cusp/s
Rounded
Sharp
Lingual Cusp/s
Rounded
Sharp
Concave 26,27
Buccal Surface
Convex
Lingual Surface
Convex
Concave
Occlusal Fissures
Present
Absent
Table 1. Visual Observations of the Posterior Enamel/Dentin Surface Correlations
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Fig 8 Stone replicas facilitate visual assessment and rumination of the variability between enamel and dentin surface topography. Thorn-like dentin tips are connected by sharp ridges defining a constricted occlusal table when compared to that of the enamel counterpart. This cognitive paradigm shift may enable pathways towards improving current restorative stratification techniques and inspiring new bio- material innovations by structural and optical design. Maxillary 1st Molar in depiction.
medium. As a consequence controlled crack extensions are frequently expected to form and progress steadily during a lifetime
of functional loading. This occurrence is validated in the intact teeth of older adults, particularly upon trans-illuminated views. (FIG 7.) Lacking awareness of this structurally advantageous non-uniform distribution of enamel/dentin, gives rise to optical integration nuances, frequently puzzling the restorative team due to the fact that traditional dental morphology curriculums focus primarily on external enamel surface characteristics, oversimplifying the subsurface union with dentin, thus assuming uniform distribution. Hence visual correlations between the enamel/dentin elements is deemed of significant value. (Table 1.) Stone replicas (Pearl White, GC Fuji Rock EP, Alsip, IL) were made in order to establish inter- relationships and better assess surface topography and characteristics. (FIG 8.)
DISCUSSION:
The purpose of this endeavor is to facilitate dental clinicians and technicians and students in these disciplines, in the proper visualization and sound understanding of spatial ordering amongst the enamel and dentin structural elements. Once this is mastered, reconstruction of the dentition takes on a proficient and predictable manner. Provided the fact that the divine design of the intact tooth is unrivaled on a micro-structural level, one must be inspired to endeavor towards the macro-structural emulation with the current bio-materials at hand. Armed with this knowledge of the Bio-Emulation model, commencing direct and indirect adhesive dental restorations take on a refined and intuitive manner, rather than an over-simplified automated one. The refinement we mean is not that of simplification, yet one that thrives on thorough understanding of the innate structural complexity of the intact tooth; it relies on powerful yet efficient spatial ordering principles of the analogous dental structures.
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With the advent of adhesive dental technology, the restorative team is enabled to embrace minimally invasive treatment modalities, without being obligated to sacrifice additional tooth structure in an effort to establish traditional fundamental requirements of resistance and retention. This knowledge may be universally applied as a foundation for developing novel stratification techniques when fabricating restorations in either composite resins or etchable ceramics.
ACKNOWLEDGEMENT:
“I praise you, for I am fearfully and wonderfully made. Wonderful are your works; my soul knows it very well.” Psalm 139:14
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