Introduction
When it comes to the establishment of new materials, devices, and methodologies, dental in vitro testing is of particular importance, as it helps to estimate study parameters for subsequent clinical investigations and thereby protects patients from unnecessary detrimental burden. However, constructing dental in vitro models is demanding, because of the limited availability of undamaged human extracted teeth. Moreover, because human teeth cannot usually be obtained from a single individual, in view of standardization, teeth with variable properties (in terms of geometry, size, enamel texture, etc.) hamper the production of a full dentition model which is comparable to the clinical situation. For shear bond strength (SBS) testing, bovine teeth of cattle aged between 2 and 5 years are accepted as substitutes for human teeth according to DIN 13990–1. Studies comparing the influence of different substrates on tensile or shear bond strength showed that the SBS of human and bovine teeth was similar [
1‐
6]. However, due to their size, bovine teeth are not suitable for full dentition models. Generally spoken, metallic teeth provide bond strengths which are above those found for human teeth and stiffer than human teeth. Resin teeth have the disadvantage that they can withstand only rather small oblique forces and are too malleable. A rather new millable fiber-reinforced composite (FRC) seemed to be a good approach for overcoming these shortcomings. Therefore, in the present study, SBS and ultimate load (F
u) tests were performed with the aim of comparing the results of samples made of FRC- or polymethylmethacrylate-based resin (PMMA). The working hypothesis was that both computer-aided design/manufacturing (CAD/CAM) materials show no difference in SBS in comparison to bovine teeth. Moreover, the present study investigated whether teeth made from both CAD/CAM materials have sufficient F
u values above physiological mastication force.
Discussion
The working hypothesis was confirmed, because FRC and PMMA CAD/CAM materials showed no significant difference in SBS compared to bovine teeth with mean SBS values in all test groups between 17 and 18 MPa.
However, PMMA samples were the only ones which showed cohesive fracture within the substrate during SBS testing (all ARI: 4). Hence, PMMA-based CAD/CAM materials might be unsuitable to replace natural teeth in in vitro SBS tests for which the outcome is affected by the adhesive connection between teeth and orthodontic device or dental restoration. In contrast, all FRC samples were associated with an adhesive failure mode (all ARI: 0). Likewise, bovine teeth showed a solely adhesive failure but differed slightly with respect to ARI (mean ARI: 1.6). Therefore, our results, which were in line with previous studies on human teeth (ARI: 1.8; [
19]) and bovine teeth (ARI: 2; [
7]), demonstrate that, among the teeth made of FRC, a similar fracture mode and fracture strength compared to bovine teeth can be achieved. Nevertheless, when precisely examining the bonding area, different ARI values might have to be taken in account. However, this has to be investigated in further studies with different primers and bonding systems.
Furthermore, both FRC and PMMA lower incisor teeth could withstand mean
Fu ranging above the maximum physiological mastication force, which reaches a maximum of 270 N in the axial direction [
20‐
23] and 200 N when tilted by 45° in the sagittal plane [
24]. It is important to note here that
Fu tests were performed tilted by 45° in the sagittal plane in order to simulate a particular critical load case. As long as the load is applied in the tooth axis, phantom teeth made of any restorative material will generally not fracture, since compressive strength values are far above the tensile material strength values. However, since for many treatment-concepts loads tilted with respect to the tooth axis state a more critical case compared with axial loading, phantom teeth are required to withstand in vitro simulations with tilted force application. The initial
Fu values of all incisor teeth except for one (PMMA:
Fu,min = 191 N, see Table
2) were above 200 N. However, nearly half of the PMMA teeth showed fracture values of
Fu < 300 N. Therefore, longer periods of water storage or procedures like chewing simulation or thermocycling prior to
Fu tests might be critical for teeth made of PMMA. If the material’s strength decreases due to aging, the failure rate of phantom teeth below the 200 N threshold will increase and FRC teeth, on the other hand, had a much higher
Fu (
p < 0.001) which exceeded 550 N. With such a safety margin, FRC teeth should not be affected by artificial aging in chewing simulations, which are typically carried out with force magnitudes ranging between 50 and 100 N. In addition, compared to PMMA, the Young’s modulus of FRC is closer to that of either enamel or dentine (Table
1).
Moreover, the present study demonstrated that SBS tests on bovine teeth, the results of which were in line with previous studies [
6,
25,
26], were combined with a particular high standard abbreviation which was about twice as high compared to both CAD/CAM materials. Similar to even higher standard abbreviation values were shown in previous studies, including those with a higher sample size [
3,
7,
27‐
31]. This might be due to the fact that bovine enamel is not as standardized as industrially produced restorative materials, mostly because of the difference in substrate morphology, i.e., perfectly flat disks with FRC and PMMA in contrast to rather planar tooth surfaces with bovine teeth. Moreover, this might explain the different results in studies which tested SBS on bovine teeth before [
16] and may reflect the ongoing discussion on the usage of bovine teeth as an alternative for human teeth in SBS testing [
32]. In contrast, using artificial teeth made from FRC might provide testing under the highest standardization possibilities and therefore allow for easier comparison with other studies. Furthermore, in contrast to human or bovine teeth which are often hard to obtain, CAD/CAM teeth can be easily created in different geometries for any kind of in vitro situation required. In addition, changes within one tooth design can be easily implanted digitally and designs which are made once can be easily manufactured again by use of a 3D printer.
To the best of our knowledge, this is the first investigation to evaluate the suitability of artificial teeth for in vitro testing. We compared artificial teeth with bovine teeth with respect to SBS while withstanding physiological mastication forces. Our study intended to validate artificial teeth for further investigations as for instance in vitro tests on different orthodontic materials and designs such as brackets, fixed retainers, or on prosthodontic adhesive restorations. Comparison between our newly introduced concept and others is limited, since previous studies with artificial teeth solely concentrated on the suitability of different materials for prosthodontic dentures with a focus on maximal stability to the acrylic base [
33‐
38], sufficient optical properties and color stability [
39], or evaluated concepts for educational purposes or validation of endodontic procedures [
40‐
43].
When interpreting the results of the present study, several limitations have to be considered. For our investigation, we chose Transbond XT as the adhesive of choice, because it is widely accepted as the orthodontic gold standard adhesive [
9,
44,
45]. Using FRC teeth with other adhesives might lead to different results. This is also the case for our standardized aging protocol for SBS testing, which stood in agreement with previous studies [
7‐
9], including 24 h of water storage in 37 ± 1 °C water. Differences in the water storage period might affect SBS values. Therefore, both different periods of water storage and using other adhesives have to be investigated by further studies.
To this end, this is the first study which validated artificial FRC CAD/CAM teeth as an alternative to bovine teeth in in vitro testing. Using artificial FRC CAD/CAM teeth might facilitate the easier and more standardized production of dental in vitro models, simplifying the testing of different devices, materials, and methodologies in an in vitro setting in order to protect patients from unnecessary detrimental effects in subsequent clinical studies.
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