Dentine scattering, absorption, transmittance and light reflectivity in human incisors, canines and molars
Introduction
The development of new substitutes identical to natural teeth requires a comprehensive evaluation of dental structures and a real understanding of its optical behavior. Not only the shape, size, position and surface irregularities of teeth promote a successful esthetic restoration, but also the combination of their optical properties. When light is guided towards a tooth surface, four phenomena can be described at the tooth level: (1) specular reflection and (2) diffuse light reflection at the surface, (3) absorption and scattering of the light flux within the dental structures and (4) specular transmission of the flux through the hard tissues [1]. The correlation between tooth color and the reflectance spectra has been investigated by several authors [2], [3].
Kubelka-Munk described in 1948 [4] a simplified mathematical model of light traveling through translucent materials. Based on the two flux theory of light that comes in and out of a turbid or light scattered material, Kubelka-Munk treated, in a theoretical manner, the reflectance and transmittance of scattering and absorbing materials (homogeneous and nonhomogeneous) placed on different colored backings [5]. They expressed, through hyperbolic functions and practical formulas, the scattering (S) and absorption (K), wavelength dependent parameters of translucent materials, taking into account their thickness and the influence of the background.
For years, various reflectance measurements were performed in order to understand and explain the optical properties of enamel and dentine. The accuracy of Kubelka-Munk reflectance theory was demonstrated several times on tooth structures, in order to confirm whether this theory was capable of an accurate prediction of the reflectance spectra and color for specimens with different thicknesses on varying backings [2], [6]. Kubelka-Munk equations were also used to quantify and calculate the absorption and scattering coefficients of different shades of resin composites [7], [8] and porcelain materials [9], [10], [11]. It has been previously demonstrated that inherent color at infinite thickness, as well as color at any thickness on any backing, and translucency of direct restorative materials at any thickness can be accurately predicted using the Kubelka-Munk reflectance model [12]. The reflectance and transmission of dental enamel was measured by means of an integrating sphere at a wavelength in the range of 200–700 nm [1]. By computing the scattering and absorption coefficients, it was observed that the optical absorption of hard dental tissue is influenced by its organic components, respectively aromatic amino acids. Different studies reported that, by measuring the angular scattering distributions at a precise wavelength, for enamel and dentine samples of different thicknesses, the scattering and absorption coefficients of dentine, contrary to enamel, did not change significantly with wavelength [10], [13]. The enamel scattering is stronger for shorter wavelengths (blue range) than for larger wavelengths (red range). The more the enamel scatters, the lighter the tooth [2]. In addition, the absorption coefficient of enamel is small [1]. Therefore, tooth color and appearance is mainly determined by the properties of dentine. However, it was demonstrated that in dentine the scattering processes are due mainly to the tubules, while other anatomical structures, such as collagen fibrils and the mineral crystals, are playing only a minor role [14], [15], [16]. With the purpose of better understanding the causes of tooth color and in order to explain the intensity of scattered light as a function of the scattering angle, several authors studied the direction of incident and scattered light within dentine. When the scattering coefficient was measured in the plane parallel to the tubules, its value was lower than the one obtained in the plane perpendicular to the tubules [17]. Although several studies on the reflectance spectra of enamel and dentine have been reported, to our knowledge, no comparison of these optical properties between different types of teeth has been performed so far. It is known that the distribution of the hard dental structures varies according to its morphology, as well as the thickness of enamel and dentine layers.
Therefore, the purpose of the present study was to measure and evaluate the spectral behavior of scattering, absorption, transmittance and light reflectivity of incisors, canines and molars human dentine, using Kubelka-Munk reflectance theory. The null hypothesis tested by our study was that scattering, absorption, transmittance and light reflectivity of dentine samples from human incisors, canines, and molars are not significantly different.
Section snippets
Preparation of human dentine samples
This study, approved by the Ethical Board of the local university (decision number 406), was conducted on 81 human teeth (33 maxillary central and lateral incisors, 7 canines and 41 molars). All teeth were extracted for orthodontic or periodontal reasons, cleaned from any debris under water jet with brushes and stored in distilled water at room temperature until their preparation. Teeth of small sizes or with pathological discolorations, cracks or fractures, caries, conservative or prosthetic
Scattering coefficient
The spectral distribution of the Kubelka-Munk scattering coefficient (S) for the mean values of human incisors, canines and molars dentine samples are shown in Fig. 2. The scattering coefficient is wavelength-dependent; therefore, its value varies across the visible spectrum. For short wavelengths, we found the smallest values for S, independently of the type of teeth that was analyzed. For medium wavelengths, slightly higher values (average of 50% increase) of S were found, while for the
Discussion
Highly aesthetical restoration techniques with biomimetic materials are difficult to achieve by the dentist and the dental technician without a complete control over the optical properties of natural teeth. It is commonly agreed that tooth shade perception is the result of the combination of its optical properties [19]. Therefore, when performing shade matching, all tooth properties related to the flux of light encountering the tooth surface or passing through its hard substrates need to be
Conclusions
Within the limitations of this study, the optical properties of human dentine are strongly influenced by the type of tooth. Although in the final appearance and color perception of a tooth there are several other factors involved, the significant differences found between these three types of teeth should be taken into account when performing highly aesthetic dental restorations.
Conflict of interest statement
Conflict of interest none.
Acknowledgment
This paper was published under the frame of the European Social Found, Human Resources Development Operational Program 2007-2013, project no. POSDRU/159/1.5/S/138776.
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