## 4 Discussion  
We evaluated the fracture resistance of three materials commonly used in dentistry to create temporary crowns. Despite the limitations of laboratory fatigue tests which do not replicate the conditions existing in the oral cavity, fracture resistance values provide a good indicator of the clinical performance of materials when assessed under controlled conditions. In order to simulate clinical conditions and after analyzing the literature--in which screw-retained ISPRs proved to be more fragile--we chose to provisionally cement the crowns [12; 13]. PEEK proved to be the strongest material, followed by composite resin, while PMMA had the weakest performance. These results confirm those reported by other studies [1; 2; 3; 7; 12; 14]. As for the average strength values of each material reported in the literature, the 1300.4 N of PMMA was comparable to the values found by [PERSON] et al. [15]. The composite resin, with 1425.9 N, was comparable to the values presented by [PERSON] et al. [7], [PERSON] et al. [1], [PERSON] et al. [12], and [PERSON] et al. [16]. The PEEK value of 2359.5 N was similar to that reported in the review of [PERSON] et al. [17]. The differences found in other studies can be explained by variables in the methodology, such as testing a pontic or a cantilever instead of an abutment crown, or performing fatigue tests before the final fracture test. PEEK is often tested in a hybrid form with a composite veneer, which improves its esthetics but reduces strength. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **M** & **SD** & **Threshold 300.1 N** & **Threshold 966.9 N** \\ \hline Type III & 2359.5 & 50.0 & t\({}_{\{4\}}\) = 1350.4 (\(p<0.001\)) & t\({}_{\{4\}}\) = 907.1 (\(p<0.001\)) \\ Type IV & 1281.7 & 75.5 & t\({}_{\{4\}}\) = 265.4 (\(p<0.001\)) & t\({}_{\{4\}}\) = 85.3 (\(p<0.001\)) \\ Type V & 1434.4 & 49.0 & t\({}_{\{4\}}\) = 411.0 (\(p<0.001\)) & t\({}_{\{4\}}\) = 167.7 (\(p<0.001\)) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of fracture strength by fracture type. Several studies have addressed masticatory strength, with values ranging between 190.42 N and 967 N [1; 2; 3; 18; 19]. According to these studies, all materials tested would show resistance to normal and parafunctional masticatory function. As for the type of fracture, all were classified as catastrophic. Type III fractures--less than half of the affected crown--were observed in PEEK samples, while type IV and V fractures--more than half of the affected crown--were the most prevalent in composite resin (3M(r) ESPE, Minnesota, USA) and PMMA samples. These results are in agreement with those presented by [PERSON] et al. [1] and [PERSON] et al. [2; 3]. Other studies support these results, arguing that PEEK demonstrates better marginal adaptation and fracture resistance when compared to traditional temporary materials. This material has an elastic modulus of 18 GPa when reinforced with carbon, resembling bone tissue. The cross matrix of reinforced carbon fibers provides excellent resistance and flexural resistance, corroborating the results obtained with respect to the maximum fracture values and fracture topography. According to the authors, due to the grayish brown color of PEEK, it is not suitable for monolithic esthetic restorations on anterior teeth. Hence, a more esthetic material like composite resin should be used as a coating to get an esthetic result. Many surface conditioning methods of PEEK to improve bonding with resin composite crowns have been suggested [8; 9; 13; 19; 20]. Any comparisons of results of the present study with other studies should be carried out with caution as the test parameters significantly differed from one study to the other. Furthermore, in vitro wear tests show little correlation with clinical data. In general, material loss under clinical conditions is lower than in laboratory studies. In addition, there is also a high variability of results regardless of the kind of study.