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t t r f e o Bio-Emulation: ssence Biomimetically Emulating Nature Utilizing a Histo-Anatomic Approach; Structural Analysis

Panaghiotis Bazos, DDS Emulation, Athens, Greece

Pascal Magne, DMD, MSc, PhD Don and Sybil Harrington Professor of Esthetic Dentistry, Herman Ostrow School of Dentistry, University of Southern California, Oral Health Center, Los Angeles, CA, USA

Correspondence to: Panaghiotis Bazos

33 Vasilissis Sophias Avenue 106 75 Athens, Greece

Tel: +30 210 722 2329; e-mail: [email protected]

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t t r f e o A thorough understanding of the histo- The main goals for this article are to iden-ssence anatomic structures and dynamic light tify and reveal previously unreported interaction of the natural dentition pro- histo-anatomic interrelationships and to vides dental practitioners with the ulti- explain existing ones: the sigmoid curve mate strategic advantage with regard distribution (convex enamel/concave to optical integration of the final resto- dentin), the distinction between dentino- ration. The first part of this article will enamel “junction” (DEJ, visual interface) attempt to provide insight on the three- and dentinoenamel “complex” (DEC, dimensional coronal configuration of functional interphase), and the struc- natural teeth and on the utilization of this tural significance of DEC preservation. knowledge in the clinical and technical restorative approach. (Eur J Esthet Dent 2011;6:8–19)

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t t r f e o Introduction the dental professional endeavorssse ntoce faithfully emulate12-14 the naturally intact In the modern dental practice, the res- tooth, which serves as the model, men- toration and tooth should form a struc- tor and measure. turally adhesive and optically cohesive Despite the aforementioned signifi- medium, which has the ability to with- cant advancements and improvements, stand repetitive multi-axial bio-mechan- re-creating the anatomical form and ical force loads over a prolonged period optical features of the intact tooth re- of time. mains an arduous, challenging and at By means of the evolution, advance- times elusive task, both within the clini- ment, and refinement of adhesive dental cal and technical dental realms. In or- technology1,2 with regard to the bonding der to optimize the optical integration materials available on the market in con- of modern composite resins and silica- junction with the validated clinical pro- based ceramics for restorative dental tocols at hand,3,4 clinicians and techni- emulation, a thorough understanding of cians have the ability to biomimetically the coronal elements (enamel/dentino- reproduce the union between synthetic enamel junction (DEJ)/dentin), their dental materials and natural anatomic three- dimensional configuration and tooth structures.5 respective spatial inter-relationships, is With the perpetual improvement of deemed compulsory. dental restorative materials, their prop- erties now include optical light transmis- sion and color dynamics, adding to the Methodology plethora of choices. With a multitude of shades, translucencies, opacities, ef- To ascertain the morphologic relation- fects and stratification techniques,6-11 ship between the exterior surface of

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/dentinoenamel complex/dentin). Morpho-differentiation is the process whereby individual tooth buds attain recognizable shapes (incisor vs molar).

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t t r f e o the enamel layer and the DEJ, the teeth DEJ as the structural epicenterssence were submerged in 10% hydrochloric acid (HCl, Mallinckrodt Baker Inc, Phil- One must examine the significance of lipsburg, NJ, USA) under ultrasonic vi- the DEJ as an important factor in the un- bration for 20 minutes, which led to se- derstanding of the developmental adap- lective enamel demineralization. tation of the surface pattern of enamel Subsequently, the teeth were soaked expression. in distilled water for 1 hour in order to During odontogenesis, this junctional neutralize the acid and facilitate han- interface serves as the histologic blue- dling. The specimens were photo- print (Fig 1), representing a complex in- graphed (D200, Nikon Inc, Melville, NY, terdigitation zone between two distinct USA) on a custom fabricated tripod jig anisotropic calcified tissues with differ- (XX-Halter, Novoflex, Memmingen, Ger- ent biochemical compositions (Fig 2): many) maintaining standardized illumi- i) enamel serving as the structural pro- nation, exposure settings and perspec- tective shell and ii) dentin serving as tive, prior to and after the acid treatment, the structural dampening core (Fig 3a). in order to ensure proper alignment of The enamel and dentin demarcation is the superimposed images. due to the difference of birefringence

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 orientated apatite crystallites, embedded in an intertubular dentin matrix.

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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. Seen above is a 0.5 mm buccal/lingual histologi- cal section of a maxillary first premolar that was submerged in distilled water and photographed on a black background (a). The same specimen was photographed by transmissive cross-polarized illumination (b). The extended dentinoenamel complex is highlighted (c).

between the tissues (Fig 3b). The DEJ Microstructure: enamel vs dentin is less mineralized than either enamel or bulk dentin, conversely being richer The microstructure of enamel is domi- than either in organic matrix. Microscopi- nated by hydroxyapatite crystal-rich cally, a bi-scalloped surface topography enamel rods, cemented together by an is present, establishing a complex zone organic matrix protein polymer. Brittle capable of plastic deformation while be- yet stiff, enamel undergoes only mini- ing collagen fibril-reinforced.15 mal deformation while transferring loads Considerable interest has been to the underlying dentin. The key to the shown in the interconnectivity of the unusual properties of enamel lies in its inner aprismatic enamel, the DEJ and unique three-dimensional structural the outer layer of dentin, known as the arrangement, which consists of very long “mantle dentin” approximately 150 mi- rods of carbonated apatite arranged in crons in thickness, which is synthe- directional bundles. These bundles are sized at the onset of dentinogenesis.16 progressively interwoven at higher hier- This extended dentinoenamel com- archical levels. The rise in crack growth plex (DEC) has been histologically de- resistance is largely attributed to a com- scribed17 and observed as a function- bination of mechanisms, which include ally graded interphase between two crack bridging, crack bifurcation and vastly bio-mechanically different tissues crack curving, induced by prism decus- that provides crack tip shielding.18 This sation of the inner enamel.24 is partially accomplished by a localized The microstructure of coronal dentin reduction in density and mineralization, appears to be that of a mineralized colla- in both enamel and dentin as they ap- gen fiber bio-composite, the intertubular proximate their junction19 (Fig 3c). dentin being the matrix and the dentin

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Fig 4 The coronal dentin surface can be considered a three-dimensional configuration of the DEJ.20 When macroscopically observed, a high degree of conformity exists between the gross form of the DEJ 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 sigmoid curve distribution.

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Fig 5 Convex contours of the enamel surface are evident when viewed from the proximal surface, provid- ing 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. Maxillary first premolar depicted.

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Fig 6 A pronounced dentin concavity is present on the buccal surface at the junction of the cervical and middle third of the posterior dentition, forming a sigmoid curve. This localized enamel overexpression 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 reasons. 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 with protein rich oral fluids, thereby utilizing a self-healing mechanism.

tubule lumens with their associated cuffs the selective adaptation of teeth, with of peritubular dentin forming the cylin- the DEC proving to be the most intricate drical fiber reinforcement. Dentin pos- of mechanisms, imparting the structural sesses both elastic and plastic material efficiency of an interconnecting network, properties, which vary significantly from where the various structural elements region to region. Uncracked- ligament function in unison rather than remain- bridging presents as the prevalent crack ing independent from each other. The shielding mechanism observed in the DEC therefore is considered a func- hydrated dentin core.25 tional shielding mechanism, that should be preserved whenever possible during Macrostructure: convex enamel clinical restorative procedures. vs concave dentin Bio-mechanical forces are thus al- lowed to transmit freely through the sur- Most topographic structures are related faces, dissipating throughout this struc- to the different functional roles of the turally fluid medium. As a consequence, enamel and dentin surfaces. The robust, controlled crack extensions are fre- rounded convex contours of the enamel quently expected to form and progress surface provide strength to a tissue sub- steadily during a lifetime. This occur- jected to direct masticatory stresses and rence is validated in the intact teeth of occlusal loads. In contrast, the sharp, older adults, particularly in trans-illumi- concave relief of the dentin surface pro- nated views (Fig 7). vides a stable support for the enamel Lacking awareness of this structur- shell (Figs 5 and 6). ally advantageous non-uniform distribu- From a bio-mechanical viewpoint, tion of enamel/dentin gives rise to opti- harmony between the ectodermal and cal integration nuances. This frequently mesodermal tissues was necessary for puzzles the restorative team due to the

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Table 1 Visual observations of the posteriort t r f fact that traditional dental morphology e o enamel/dentin surface correlations. ssence curriculums focus primarily on external enamel surface characteristics, over- Morphologic Enamel Dentin features surface surface simplifying the subsurface union with dentin, thus assuming uniform distribu- Marginal ridges Rounded Sharp tion. Hence the visual correlations be- tween the enamel/dentin elements are Buccal cusp/s Rounded Sharp deemed of significant value (Table 1). Lingual cusp/s Rounded Sharp Stone replicas (Pearl White, GC Fuji Rock EP, Alsip, IL, USA) were made in Buccal surface Convex Concave26,27 order to better assess surface topogra- Lingual surface Convex Concave phy and characteristics (Fig 8).

Occlusal fissures Present Absent

Discussion

The purpose of this quest is to assist With the advent of adhesive den- dental clinicians, technicians and stu- tal technology, the restorative team is dents in these disciplines, using proper enabled to provide minimally invasive visualization and sound understanding treatment, without being obligated to of spatial ordering among the enamel sacrifice additional tooth structure in and dentin structural elements. Once an effort to establish traditional funda- this is mastered, reconstruction of the mental requirements of resistance and dentition takes on proficient and pre- retention. dictable qualities. This knowledge may be universally With the fact that the design of the applied as a foundation for developing intact tooth is unrivaled on a micro- novel stratification techniques when fab- structural level, one must be inspired ricating restorations in either composite to endeavor towards macro-structural resins or etchable ceramics, with either emulation with the current bio-materials conventional (refractory die or thermo- available. pressed) or contemporary (CAD/CAM Armed with this knowledge of the or 3D printing) methodologies. Bio-Emulation model, commencing di- rect and indirect adhesive dental resto- rations takes on a refined and intuitive Conclusion manner, rather than an over-simplified automated one. The refinement is not This article presented essential histo- that of simplification, yet one that thrives anatomic elements, such as the sigmoid on a thorough understanding of the in- curve distribution (convex enamel/con- nate structural complexity of the intact cave dentin), the distinction between tooth; it relies on powerful yet efficient dentinoenamel “junction” and dentino- spatial ordering principles of the analo- enamel “complex” and the structural gous dental structures. significance of DEC preservation.

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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 first molar depicted.

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