Systematic Research and Analysis of the Tooth Colour
The Basic Research
This book shows the author’s systematic original research of the tooth color ‒ an excerpt from earlier German publication (see e.g. Hoffmann 2000 and 2003) ‒ with highest precision at human teeth and dental shade guides with high-precision measuring systems and high-precision positioning systems in vitro. Due to this scientific basic research, it was possible to quantify and isolate the manifold fundamental factors influencing the colour of teeth and its measurement, for the first time. These include, for example, the light or measuring light and the type of illumination and colour temperature, the optical beam path of the light or the measuring geometry, the observation angle (2°, 10°), the size of the measuring surface and measuring opening, the gloss effect on colour coordinates, the liquid content (with scientific evidence of the relationship between liquid content and tooth colour), effect of drying (dehydration), moisture and rehydration, the correlation between the liquid content and the gloss, the subjectivity of visual subjective shade matching, crown curvature, type of system (spectrophotometer, tristimulus colour measuring device), measuring mode (contact or non-contact), measuring system-object-relation, positioning, repeatability or reproducibility, lens shift, displacement between sample, measuring surface and measuring system and further intra- and interindividual factors. In addition, subjective-visual colour matching and objectified measurements were examined in subjective-objective comparisons using colour coordinates comparisons. The coordinates are taken from shade guide analyses of a preliminary study. All these factors influencing the tooth color are investigated on moist or wet, drier (various specific drying and rehydration states) and dry teeth. The analyses are based on the brightness (L*), colour measurement values, such as a*, b* (CIELAB), C*, h, (CIELCH), ΔE, the metamerism index, of spectral values and curves, patterns (tabs) of dental shade guides (VITAPAN 3D-MASTER, VITAPAN classical) and tooth colour spaces …
As part of this exploration, phenomena (e.g. changes and breaks in behaviour as well as highly individual developments in colour coordinates, paradoxes between the values of subjective determination using shade guide tabs and the values of objective measurements) were identified, and insights into the very complex colour dynamics through dehydration and rehydration were shown (up to more than >8 days). The development of the individual colour coordinates gave information about the liquid flow through the tooth and its tissues (pulp, dentime, enamel), in particular during drying and liquid reabsorption, and liquid dynamics, structure influences and the temporal extent of these processes.
Based on this data, the author had developed several methods for research and practice, suggestions for feasible innovations, such as monitoring of dental treatment to protect against devitalisation by drying, and reconstructing the colour of naturally moist teeth based on teeth that have been already drying for a while, the identification of the living and the dead via the “dental fingerprint” (biometrics) and a novel method of measuring the time of death for forensic medicine. In addition, a temporal drying limit was specified up to which relatively natural, suitable shade values and results are obtained and from which time of drying no shade matching should be carried out in order to obtain reasonable tooth colour results. This book shows the rehydration time (after drying) to get correct values and results again, with relevance for shade matching, dental color measuring and the right tooth color.
Since everything intertwines with everything, it makes little sense to form chapters in the discussion section. Chapters in the discussion have therefore been omitted in order to avoid repetition and for the benefit of the appropriate networking of knowledge. However, in order to be able to find some certain aspects quickly, the section “Some Discussion Keywords and Key Phrases (Selection)” following the table of contents is useable.
The findings also show that teeth are able to store information on, for example, the condition (liquid content, colour and spectral values) and the time within the drying and liquid reabsorption chronology. The author articulates a “dental chronometer” (“tooth clock”), “dental data store” (“tooth data store”) and a “dental memory” (“tooth memory”). Novel reference-dependent and reference-independent method approaches have been described ‒ perhaps the first reference-independent method in natural science.
2. Literature Review
Colours determine every personʼs life. They make life as we know it possible in the first place. If mankind would not know any colours and thus also contrast the world would not be tangible, objects would be indistinguishable from their surroundings, there would be no optically descriptive language and no optically orienting movement – the human would not be human. And colours arouse emotions, influence feelings and have their own aesthetics. In dentistry, dental or tooth colour aesthetics ‒ the authorʼs word creation of dental or tooth colour and aesthetics ‒ is one of two pillars (colour and shape) of dental aesthetics and at the same time the more decisive one for a successful restoration. Deviations in the dental colour are perceived even before deviations in the tooth shape and may be perceived as disturbing, as the author of the current work knows to report on the basis of his experience. Dentistry as a whole, on the other hand, can be divided into function (masticatory function, health) and aesthetics. Nothing was more obvious than to understand the emergence of the tooth colour as a whole as a system of manifold influencing factors. It was not expedient to deal with a single aspect or to investigate a single question. It was necessary to systematically analyse the tooth colour and to investigate it with high-precision measuring systems in order to extract from it the secret of the orchestral interplay of the factors that make up it.
Before and in 2000, the author of this present work had isolated and clearly quantified the factors that form the dental colour of human teeth and which influence the origin or development and perception of color, measurement and visual colour determination or dental shade matching (factors of influence on the dental color). At that time, there were 44 [1‒44] metrological studies, essentially. Of these, 19 were based on spectrophotometer measurements (see bibliography ‒ bold and italic, [Bolt et al. 1994, el-Sayed et al. 1994, Fay et al. 1999, Groh et al. 1992, Horn et al. 1998, Ichesco et al. 1991, Ishikawa-Nagai et al. 1992, 1994, Kleber et al. 1998, Koertge et al. 1998, Leard and Addy 1997, OʼBrien and Groh 1990, 1989, 1997, Takeda et al. 1996, van der Burgt et al. 1990 , White and OʼBrien 1989, Zhu et al. 1998]) and 25 (see bibliography ‒ bold [Belli et al. 1997, Douglas 1997, Goldstein and Schmitt 1993, Goodkind and Schwabacher 1987, Johnston and Kao 1989, Kowitz et al. 1994, Lenhard 1996, Lu and Zhao Y. 1993, Ma et al. 1999, Manly 1947, Matis et al. 1998, Matis 1998, Millstein et al. 1988, Nakamura et al. 1993, Nathoo et al. 1994 , Nissan et al. 1992, OʼBrien et al. 1989, Okubo et al. 1998, Ouellet et al. 1992, Rosenstiel et al. 1991, Rosenstiel et al. 1996, Rosenstiel et al. 1989, Rustogi and Curtis 1994, Seghi et al. 1990, Seghi et al. 1989]) were performed with a simpler colorimeter. They mostly dealt with the color of artificial material (no natural or human teeth) or a very specific question, for example, such as the effects of toothpaste and bleaching or the influence of certain substances, such as chlorhexidine, fluoride gels, coffee or tea (see bibliography).
It is a widely observed phenomenon that teeth that are dried with rubber dam or cotton rolls or that have been extracted will appear brighter. A number of textbooks have indirectly mentioned this phenomenon in the context of chairside shade matching for dental restorations. But there was no scientific proof of this. This would only be possible if both the brightness or colour values as well as the weight or weight loss could be measured as an expression of the drying and the resulting curves could be covered and measured values curves have peculiarities that could only be explained by drying.
While there have been numerous publications (see discussion section) in the course of the 20th century on water and water content in teeth and in their hard tissues, it is surprising that no study has so far dealt with this fundamental issue of brightness or tooth colour and tooth colour values (e.g. L*a*b*C*h etc.) depending on the liquid content, drying and liquid absorption processes. One aim of the investigations was therefore to establish the relationship between liquid content, liquid loss (drying, dehydration), liquid absorption (rehydration) and tooth color. This basic research has also been developed to provide information on various color influencing factors, the liquid flow within the tooth, the role of the dental liquid and the dynamics of the tooth liquid in and on the tooth tissue during drying and redydration, information on which more far-reaching conclusions will be based.
Bolt et al. (1994) could indeed mention an interesting effect in terms of the of window size (spectrophotometer equipped with external windows of 3, 4, and 5 mm). However, this influence may have been significantly influenced, for example, by very rapid drying processes after storage in particularly volatile liquid (formalin) and during the time of mounting and changing of the “windows” and thus this factor of influence has not been clearly isolated.
In fact, there is also a double effect in this study. The measuring light directed onto the sample and the light reflected by the sample must pass through the measuring opening and are influenced by the window size.
However, in the studies of the present work, the effect was analyzed in such a way that the microscopic chromameter used allows only the reflected light to pass through the measuring opening and the switching from 1.8 mm to 0.3 mm measuring opening size and vice versa in a fraction of a second via the intended switch. Measurements on these two different measuring surface sizes were therefore carried out without any significant delay and without any significant drying influence. The emitted light was only sent through an aperture during the contact procedures. In addition, influencing factors, such as drying, could be quantified and thus taken into account in the analyses of the present work.
Positioning, i.e. the object-system relation, has an influence on the values and their reproducibility. The reproducibility in vitro with jig (see Douglas 1997), an intraoral positioning aid, was better and already good, but not sufficient enough for basic research with the highest precision, so that the decision was made in favour of the present in vitro studies with high-performance precision systems (high-end systems for this particular purpose ‒ tooth colour measurement with highest-precision positioning) and with novel high-precision positioning systems – self-developments. The precision and repeatability ‒ see Douglas (1997), Goldstein and Schmitt (1993) ‒ can thus be very significantly increased by the author of the present work. This was one of the prerequisites for the isolation of individual influencing factors and the recording of the temporal extent of drying (˃ 1 week) and the answer to the question: how long does a tooth dry?
Another key was the choice of systems. Two measuring systems were identical in construction, from the same manufacturer and differed only in the measuring geometry. One of the measuring geometries worked with gloss inclusion (specular included) and the other with gloss exclusion (specular excluded). Thus it was possible to determine the differences between the values and curves of these systems ‒ specular included and specular excluded ‒ and in consequence the gloss influence on the colour coordinates of moist, drying, dryer and dry teeth. At the same time, it was possible to determine the proportion of liquid in the dental gloss and to answer the questions: what influence does the tooth have and what influence does dental liquid have on the dental gloss? And what influence does the gloss have on the tooth colour and the color coordinates?
An analysis and also a proof are characterized by not only suspecting, but also isolating, recognizing and quantifying in extent. It must therefore somehow be possible to eliminate, to reduce sufficiently or to calculate further influencing factors to an insignificant level. For a clear analysis, the single influencing factor must remain – overlay-free. An analysis cannot be carried out without isolation. Without real or computational freedom of overlay, an explanation would remain only a hypothesis. A study using apparatuses of different measuring geometries, different manufacturers, (and thus also) different device concepts, device compositions and device tunings and possibly other overlapping influencing factors (e.g. positioning) will not be able to prove that, for example, the measuring geometry is a possible source of influence and it will certainly not be able to determine the extent of a conceivable influence. This is all the more true if the values – in particular obtained from flat surfaces with different systems – are within the framework of the device conformity (i.e. the deviation of the same systems of the same manufacturer) or the additional system influence (different systems of the same manufacturer) or “manufacturer influence” (different systems of different manufacturers) and are therefore without validity. In addition, this does not say anything about the application on curved surfaces, such as a tooth. On the other hand, within the current studies, the extent of the influence of the measuring geometry on human teeth and shade guide tabs (curved surface) was examined and isolated for the first time, and thus evidence was also provided that the measuring geometry actually has an influence on colour detection and all colour coordinates by using identical systems from the same manufacturer, which differed only in the measuring geometry.
Systematizing and geometrically arranging or presenting colours, as in a room, goes back to the Greek philosophers Plato and Aristotle. The measuring general colour science knows the so-called colour space. And in dentistry, Goodkind and Schwabacher may have measured a room in 1987 that could be called tooth colour space. In addition, VITA points out in connection with its VITAPAN 3D-MASTER that it is based on a tooth colour space. For the first time, tooth colour spaces of moist or wet, drying, drier and dry teeth were shown in the course of the work of the present author. Measurements were made for the first time with special high-performance spectrophotometers. They are representations that provide insight into the drying-based population dynamics and insight into something that is likely to happen after the death of teeth and humans – the teeth become brighter and different in colour, translucency and opacity.
Forensic dentistry, for example, deals with the identification of people and dead. Identification or authentication – in the broadest sense the verification of an alleged identity – is becoming increasingly important for the security of people and data, even in everyday life and in a modern society, as is common in specific areas such as a high-security area (e.g. face recognition). According to the author of the current work who hereby lays the scientific foundation stone, both in forensic medicine for the identification of the dead and in this safety-relevant biometrics of living persons, it would be interesting of to use tooth colour and dental reflection spectra directly or via image technology for this purpose.
In addition, this research was based on the exploration and quantification of influencing factors. This includes, for example, the light or measuring light and the light types of different colour temperatures, the beam of the light or the measuring geometries (places of light sources and sensors in relation to the sample), the observation angle (2°, 10°), the size of the measuring surface and the measuring opening, the gloss, the liquid content (with scientific proof of the relationship between liquid content and tooth colour), effect of drying and liquid reuptake (dehydration, rehydration), reversibility of these processes, the proportion of the liquid content in the gloss effect, the subjectivity of visual determination, crown curvature, system type (spectrophotometer, tristimulus colorimeter), measurement mode (contact or non-contact), measurement system-object relation, positioning, repeatability or reproducibility. In addition, subjective-visual determinations and objective measurements were investigated in subjective-objective comparisons via value comparisons. All these influencing factors are analyzed not only on moist, but also on drier (various specific drying or rehydration states) and dry teeth based on, among other things, brightness (L*), colour coordinates such as a*, b*, C*, h, metamerism index, spectral values, tooth colour samples of shade guides and tooth colour spaces. Further and more specifying publications, in particular on this, will follow shortly.
Dinslaken, May 2005 André Hoffmann
André Hoffmann systematically researched the tooth colour and its origin with highest precision - for the first time in 2000. This book shows an excerpt of this systematic original research at human teeth and dental shade guides with high-precision measuring systems and high-precision positioning system in vitro ‒ with the highest precision. Due to his basic scientific research, he was able to quantify and isolate manifold factors influencing the colour of teeth. These include, for example, the light or measuring light and the type of light (illuminant) and illumination and colour temperature, the optical beam path of the light or the measuring geometry, the observation angle (2°, 10°), the size of the measuring surface, and measuring opening, the gloss, the liquid content (with scientific evidence of the relationship between liquid content and tooth colour), effect of drying, moisture and rehydration, the correlation between the liquid content and the gloss effect, the subjectivity of visual subjective shade matching, crown curvature, type of system (spectrophotometer, tristimulus colorimeter), measuring mode (contact or non-contact), system-object-relation, positioning, repeatability or reproducibility, lens shift, displacement between sample and measuring surface and further intra- and interindividual factors. In addition, subjective-visual determinations and objectified measurements were examined in subjective-objective comparisons using colour coordinates comparisons. All these influencing factors are investigated on moist, drying, drier (various specific dehydration and rehydration states) and dry teeth based on the brightness (L*), on colour measurement values, such as a*, b* (CIELAB), C*, h, (CIELCH), ΔE, on the metamerism index, on spectral values and curves, tabs of dental shade guides and on tooth colour spaces …
As part of this exploration, phenomena (e.g. changes and breaks in behaviour as well as highly individual developments in colour values, paradoxes between the values of subjective determination using tooth shade patterns and the values of objective measurements) were identified and insights into the very complex colour dynamics through dehydration and rehydration were shown (up to more than >8 days). The development of the individual color measurement values was based on the liquid flow through the tooth and its tissues, in particular during drying and rehydration, and gave information about dynamics and the temporal extent of these processes.
On the basis of this data, Hoffmann had developed several methods for research and practice, suggestions for feasible innovations, such as monitoring of dental treatment to protect against pulp damage based on drying, and reconstructing the color of naturally moist teeth on those that have already dried, the identification of the living and the dead, human and animals via the “dental fingerprint” and a novel method for measuring the time of death for forensic medicine.
He also described a time limit of drying up to which relatively natural, suitable color values and shade matching results can be obtained and after which no color determination should be carried out; and he established the rehydration time after the end of the drying or dental treatment, which must be waited in order to regain a natural tooth color and to get correct values and results again.
His findings also show that teeth are able to store information on, for example, the condition (liquid content, colour values) and the time within the drying and liquid reabsorption chronology. The author articulates a “dental chronometer” (“tooth clock”), “dental data store” (“tooth data store”) and a “dental memory” (“tooth memory”) and believes that significant progress in this area may include and could be achieved via a neural network for colour measurement apparatus.