# Colour Changes of Acetal Resins (CAD-CAM) In Vivo
###### Abstract
In the surgical-medical field, acetal resins have already been frequently used to make hip prostheses and heart valves. In dentistry, they appeared as an alternative to the poor aesthetics of metal retainers in removable acrylic partial dentures [14; 15] and as an alternative material for patients who are allergic to any of the components of chrome-cobalt alloys [16]. POM has favourable properties [14], such as resistance to deformation [16], wear [17] and water absorption [18]. Few studies have been published on the quality of POM as a temporary restorative material and on assessing its colour stability [9; 11]. Depending on the results obtained on its colour behaviour, it might be possible to recommend its use for a longer period of time. To quantify tooth colour, colourimeters and spectrophotometers [19; 20] are used to provide colour results in coordinates in the CIELAB color space [21] system. The CIELAB color space consists of 3 coordinates: (1) L\({}^{\star}\) (lightness) measures the amount of black and white, (2) a\({}^{\star}\) coordinate (green-red axis) and (3) b\({}^{\star}\) coordinate (blue-yellow axis)*. They are interpreted as follows: the higher the L* coordinate, the greater the amount of white, which gives a brighter tooth; the higher the a\({}^{\star}\) coordinate, the greater the amount of red; the higher the numerical value of the b\({}^{\star}\) coordinate, the greater the amount of yellow [22]. Subsequently, with the aim of each coordinate to represent one of the three dimensions of colour (Lightness, Chroma, and hue), the CIELCh color space appeared. In this nomenclature, the L\({}^{\star}\) coordinate remains identical, representing the lightness*, the C* coordinate represents the chroma (amount of hue) and the angular coordinate h\({}^{\star}\) represents the hue (circular axis; the units were in the form of degrees (or angles), ranging from 0\({}^{\circ}\) (red) through 90\({}^{\circ}\) (yellow), 180\({}^{\circ}\) (green), 270\({}^{\circ}\) (blue) [22]. In the dental field, two formulas have been used to calculate the difference between two colours: (1) Classical Euclidean Formula widely used in dentistry whose equation is \(\Delta\)Eab\({}^{\star}\) = [(\(\Delta\)L*)\({}^{2}\) + (\(\Delta\)a*)\({}^{2}\) + (\(\Delta\)b*)\({}^{\star}\)]\({}^{1/2}\). \(\Delta\)Eab\({}^{\star}\) represented the magnitude of the colour difference [23] and (2) the CIEDE2000 formula (\(\Delta\)E00*) [23; 24] which is closer to the threshold for colour discrimination than the formula \(\Delta\)Eab* and is better suited for interpreting clinical results [25]. \[\Delta\)E00 = \[\left[\left(\frac{\Delta\Gamma^{\prime}}{\text{K}_{\text{L}}\text{S}_{ \text{L}}}\right)^{2}+\left(\frac{\Delta\Gamma^{\prime}}{\text{K}_{\text{C}} \text{S}_{\text{C}}}\right)^{2}+\left(\frac{\Delta\Gamma^{\prime}}{\text{K}_{ \text{H}}\text{S}_{\text{H}}}\right)^{2}+\text{R}_{\text{T}}\left(\frac{ \Delta\Gamma^{\prime}}{\text{K}_{\text{C}}\text{S}_{\text{C}}}\right)\left( \frac{\Delta\Gamma^{\prime}}{\text{K}_{\text{H}}\text{S}_{\text{H}}}\right) \right]^{1/2}\]  
Several research studies have focused on the study of colour in the dental environment through the perceptibility and acceptability threshold (colour differences that can be perceived and accepted by 50% of the observers) [26; 27], in order to categorise colour changes. Several factors are involved in the discolouration of temporary dental restorative materials, including: the composition of saliva (immunoglobulins, enzymes, nitrogenous products and mucins), salivary pH [28] ranging from 5.8 to 7.1 (slightly acidic) [29], and exogenous colouring substances [30; 31]. The aim of the present research in vivo was:  
1. To study whether the degree of discolouration or difference of color, of acetal resin (POM) used as a provisional material in complete upper hybrid prostheses, is significantly related to the weeks of follow-up and to salivary pH in the three thirds of the tooth (gingival third (area 1), middle third (area 2), and incisal third (area 3). 2.