# Colour Changes of Acetal Resins (CAD-CAM) In Vivo
###### Abstract  
To quantify the discolouration of the temporary acetal resins in vivo, based on the weeks of follow-up and the salivary pH in the three thirds of the tooth. To find out if the final CIELAB coordinates can be predicted from the initial colour coordinates, the salivary pH, the situation (in thirds) and the weeks of follow-up. Colour coordinates (L, C, and h) were recorded by spectrophotometry in 13 participants fitted with hybrid provisional complete dentures made of acetal resin. Colour recordings were made on the day of placement and after several weeks of follow-up (6 to 31 weeks). Salivary pH was also measured as a predictor variable for colour change. The ANOVA statistical test and regression models have been used. The highest colour difference according to \(\Delta\)Eab* was 27.46 units after 15 weeks of follow-up and the lowest was 7.34 units after 17 weeks of follow-up. Neither in the cervical nor in the middle third any regressor variable (initial L*, initial C*, initial h*, salivary pH and weeks of follow-up) was able to significantly predict any of the final colour coordinates (\(p>0.05\)). The colour change of the temporary acetal resins used exceeds the threshold of clinical acceptability, and it is not acceptable to maintain satisfactory aesthetics. The weeks of follow-up and the salivary pH are not capable of satisfactorily predicting the final color coordinates of the acetal resins. acetal resins; spectrophotometer; CIELAB colour space; difference of colour formulae; interim dental restoration; dentistry; temporal resin +
Footnote †: journal: Medical  
or maxillofacial prosthesis; often such prostheses are used to assist in determination of the therapeutic effectiveness of a specific treatment plan or the form and function of the planned definitive prosthesis\" [7]. The proper fabrication of a temporary fixed prosthesis plays a major role in the success or failure of definitive restorative treatment. Implant-supported fixed provisional dental restorations, especially in fully dedentulous patients, serve as a mock-up to assess whether the more costly definitive fixed dental restorations fit the patient's aesthetic needs and preferences, providing a predictability factor. On the one hand, the patient \"tries in\" the provisional fixed prosthesis and can express their satisfaction or dissatisfaction. On the other hand, the clinician assesses the adaptability of the dental and/or gingival structures [8] and thus generates a more approximate judgement to modify the treatment plan if necessary, minimising costs. Nowadays, the high aesthetic demands and the reduction of waiting times to get patients' teeth in place have led to a remarkable improvement in the field of provisional restorations. Temporary restorations can be fabricated using conventional techniques (often self-curing polymeric materials) and by CAD/CAM (Computer-aided design/Computer-aided manufacturing) techniques [9; 10]. The materials used for the fabrication of provisional prostheses need to fulfil certain characteristics: they must preserve the health of adjacent periodontal tissues, be biologically inert, resist masticatory functional loads, offer adjusted handling and working time, provide good occlusion, adequate phonetics, satisfactory aesthetics, colour stability, low cost, speed of fabrication, and ideally, offer the possibility of being repaired/modified [11; 12]. Within the group of temporary materials derived from acrylic resins, one of the most widely used and studied is polymethylmethacrylate (PMMA) [9; 13]. Acetal resins have recently been introduced for the purpose of making temporary dental prostheses by CAD-CAM, although their use is not yet widespread. Acetal resins are thermoplastic polymers derived from formaldehyde. They are usually coloured to more closely mimic tooth colour, as their natural colour is white. The homopolymer of acetal resin is called polyoxymethylene (POM) and its composition is a chain of methyl groups alternating with oxygen molecules [14].