Selection of the correct restorative material is fundamental to ensure both normal function and occlusal harmony [9]. A natural phenomenon is represented by the gradual abrasion in the dentition, and this process can be disturbed by the use of restorative materials to replace natural tooth structure [33]. Ultimately, the non-uniform structures and physical aspects between natural teeth and restorative materials result in different degrees of wear [3]. A number of studies have evaluated the long-term clinical behavior of ceramics [24, 34, 35], but studies on the loss of the vertical dimension are limited [36].
For this reason, we carried out the 3D light-optical examination of all-ceramic bridges using the Atos II scanning unit. The abrasive behavior was analyzed on the basis of various parameters using digitized virtual models corresponding to the condition of the dental restorations at the time of insertion and after 3, 5, and at least 10 years. Notably, both a decrease in volume due to wear processes over time and an increase in volume in some places was noted. This phenomenon could be caused by errors that occurred when taking the impression, such as insufficient adhesion of the impression to the impression tray or the localization of relevant areas outside its boundary [37], which were then carried over to the plaster models. Accordingly, to avoid falsification of the measurement results, the value range within the evaluations was adjusted, and the regions with an erroneous increase in volume were excluded. Alternatively, instead of a conventional impression, the use of innovative technology, such as an intraoral scanner, can be considered to take a digital impression. Current studies have reported that digital acquisition of intraoral information is at least comparable to the conventional method, and could even be more precise [38, 39]. However, digital impressions also entail technical limitations and system-specific deviations [39, 40].
Overall, an increasing and significant loss of material was characteristic of the all-ceramic bridge constructions during the entire investigation period, with the frequency of abrasion being higher in the second half of the investigation than in the first 5 years. This finding may be due to the loss of mechanical strength of dental ceramics over time. This could be caused by the different dissolution rates of the components, which could lead to increased surface roughness and accelerate the process of abrasion due to fatigue reactions [41]. Favorable factors here include permanent changes in the pH value in the oral cavity, which can vary between 1 and 10 depending on the nutrition, the drinks consumed, and the bacterial metabolic processes [42], as well as the considerable temperature fluctuations of up to 60° [41, 43]. The resulting roughness in the surface causes exposure of the filler particles and creation of cavities, which both increases plaque accumulation and contributes to enhancing wear [3]. Moreover, the roughness of the ceramic influences the susceptibility of the antagonistic natural tooth enamel to abrasion [44]. In addition, the results indicate that the premolar and molar bridge pontics did not react congruently to wear processes. In the case of the molars, higher individual values were recorded at each follow-up examination.
This was also observed in another in vivo study from 2008, in which crowns made of lithium disilicate were examined regarding their abrasion behavior by means of laser scanning the corresponding plaster models. After 1 year, the mean reduction in the occlusal volume of the crowns was 0.19±0.06 mm3 for premolar restorations and 0.34±0.08 mm3 for molar restorations [45]. Furthermore, this appearance is observed not only in the clinical use of dental ceramics, but also in the context of other material classes. For example, the average wear of metal-free polymer crowns after an observation period of 2 years after insertion on premolars and molars was 44 µm and 84 µm, respectively. Moreover, a significant dependence of the degree of wear on the molar or premolar crown localization was deter-mined [46]. Our results on the average loss of -38 µm for the premolars and -46 µm for the molars were also significantly lower after 3 years of examination. The reason for this could be the occlusal surface, which increases in size with the molar region, resulting in a more pronounced chewing force in the distal part of the jaw [47]. In addition, the occlusion has a significant effect on the process of wear [9].
The fact that abrasion in general is a progressive phenomenon [9] has also been confirmed by other studies that have dealt with the wear behavior of teeth and restorative materials. An example of this is the study by Mundhe et al., in which a comparable study design was used to investigate the wear of the natural, antagonistic tooth enamel in response to definitively cemented crowns in the opposing jaw on a ceramic and metal-ceramic basis in order to investigate the effects of a restoration material used in the oral cavity on the natural tooth enamel. The maximum linear wear was determined by means of plaster models obtained from impressions, which were subsequently digitized using a 3D white-light scanner. The results confirmed that significant wear occurred over time, though the investigation period was only 1 year [48]. Furthermore, a current in vivo study by Esquivel-Upshaw et al. used a 3D laser scanner to illustrate occlusal loss of material not only on monolithic zirconium and metal-ceramic crowns, but also on natural teeth. After 1 year, no significant differences were observed in the wear behavior of the different materials and the natural enamel [49]. In addition, a number of in vitro studies have evaluated the wear potential of various materials. For example, Zurek et al. recording the volume loss of zirconium and lithium disilicate ceramics after a chewing simulation using white-light interferometry as a non-contact, optical method of measurement and a scanning electron microscope. A significantly higher loss of material was recorded for the lithium disilicate samples, with a low abrasiveness of zirconium oxide [36]. D'Arcangelo et al. also carried out an in vitro investigation of the wear resistance of various ceramics under masticatory simulation against a test body made of zirconium oxide. The loss of vertical dimension and the volume decrease were recorded with a 3D scanner [50].
Basically, abrasion in the oral cavity usually results from tooth-to-tooth or tooth-to-restoration contact, and this process is generally accelerated by a dental prosthesis. Despite constant technical innovations in the context of current research, no valid in vivo method has been established to objectively assess abrasion behavior [51]. However, the procedure used in this study, the generation of virtual 3D models using the Atos II industrial scanner, proved to be a very practical method for neutrally investigating and displaying the wear behavior of all-ceramic restorations. Thus, our method could be regarded as a unified investigation method to create better comparability within different studies dealing with wear behavior. The results of the present study confirm that zirconia-based all-ceramic restorations are generally suitable for use as prosthetic treatment as described in similar studies [24, 52]. However, not only the wear of the material, but also abrasion of the antagonists caused by the surface interactions determine whether its use in everyday clinical practice can be justified. In this context, the antagonistic wear of enamel against monolithic zirconium dioxide crowns was evaluated in a recent review of the literature. The result was that the wear of the enamel by all-ceramic restorations was similar or greater than the interaction with natural teeth, though it was still less overall than for metal-ceramic restorations [53]. Furthermore, a clinical study was conducted to compare abrasion between monolithic zirconia crowns, natural antagonist teeth, and natural control teeth that were not faced with a restoration, but with natural tooth structure. The measurement was carried out via a laser scan of plaster models obtained from impressions. The mean vertical loss after 2 years was 46 µm for the enamel interacting with the ceramic, whereas the restoration had a mean abrasion of 14 µm. The values for the control teeth were between 19 and 26 µm [54]. The contrary was evaluated in another clinical study in which measurements were made using an intraoral digital impression technique, as significantly higher values were measured with approximate agreement. For enamel or ceramic antagonists, a maximum mean wear of 115 ± 71 µm and 120 ± 27 µm, respectively, was documented after 24 months [55]. These differences may be caused by differences in the surface condition, as this influences the wear behavior. For example, Kaizer et al. demonstrated that zirconia crowns polished in the pre-sintered state with occlusal glass infiltration are not only resistant to the wear phenomenon, but also gentle on the natural antagonist [56].
A comparison of the in vivo abrasive behavior with other classes of materials is difficult because most of the investigations were carried out in the laboratory and clinical data are hardly available, though clinical tests are essential to investigating the complex oral cavity [57, 58]. Furthermore, the correlation of data from in vitro studies evaluating abrasion with those from time-consuming clinical investigations is generally low [59], though different materials can still be evaluated comparatively under standardized conditions [60]. An example of a clinical study is the study by Ohlmann et al. in which the abrasion behavior of metal-free polymer crowns with and without a glass fiber framework in the posterior region was compared to the abrasion behavior of metal-ceramic crowns after 2 years using a 3D laser scanner. A mean occlusal wear of -81.8 µm was measured for the glass-fiber-reinforced polymer-based restorations and -76.8 µm for the polymer crowns without a glass-fiber framework, whereas the metal-ceramic restorations had a mean abrasion of -38.5 µm. This suggests significantly higher abrasion of the polymer-based crowns than the metal-ceramic restorations [61]. A comparison to our results after 3 years of observation also shows that this class of materials exhibits a more pronounced abrasion behavior than all-ceramic restorations. In contrast, the degree of abrasion of the metal-ceramic restorations approaches our mean distance of the complete occlusal surface rate of 48 µm after 3 years.
It would also be possible to carry out measurements based on digital impressions of the restorations in future research, avoiding the conventional working step with the subsequent fabrication of a plaster model. In general, digital impression-taking can be considered an alternative to conventional methods, as they offer comparable accuracy [62]. In addition, impressions taken by conventional methods are generally associated with a high error rate, which is why the quality is unsatisfactory in many cases [37]. The precision of the plaster model is also influenced by a number of different factors, including the impression material, the tray material, the time interval between taking the impression and pouring, and the expansion of the plaster [63, 64].
Notably, the number of subjects undergoing in vivo diagnostics over a longer period of time is currently small [24, 52]. In our study, the abrasion behavior of a total of 15 restorations from 15 different participants was evaluated after 10 to 12 years. Long-term reports are required to obtain meaningful information about the clinical performance of a dental material [24].