Fracture strength of temporary fixed partial dentures: CAD/CAM versus directly fabricated restorations
Introduction
Computer aided design/manufacturing (CAD/CAM) technologies have gained popularity in recent years for fixed restorative and prosthodontic treatment procedures. Among others, this process is driven by the growing demand for placing high esthetic all-ceramic restorations [1], [2]. At the same time, due to improvement in physical properties of e.g. zirconia and other ceramics, these materials can be successfully used also in stress bearing areas [3]. Apart from the Cerec System, most CAD/CAM supported technologies still use labside procedures during the manufacturing process (e.g. veneering of zirconia frames/substructures) [4] and in consequence require temporary restorations to be fabricated on the prepared abutment teeth until the final fixed partial denture (FPD) is placed in situ.
The temporary restorations in turn fulfill a wide range of functions comprising protection of the prepared tooth structure, pulp and the surrounding periodontal tissues as well as to maintain oral functions (mastication, phonetics) and esthetics [5], [6]. Most of these restorations are fabricated chairside using an over impression technique in combination with resin based temporary crown and FPD materials (t-c&bs) [7], [8]. As the timeframe between preparation of a tooth and luting of the final restoration might exceed a couple of weeks, the t-c&bs used to fabricate temporary crowns or FPDs have to meet several requirements [5], [9].
Among others, the mechanical strength of a t-c&b is of particular importance as this factor might influence the integrity of the temporary restoration during clinical service, when it is exposed to functional loads [10], [11], [12], [13]. Hence, determination of mechanical properties of t-c&bs was the subject of several studies [9], [10], [14], [15], [16], [17], [18], [19].
The chairside fabrication of temporary restorations is associated with a couple of short-comings, affecting the mechanical strength as well as its surface texture and precise fit [12], [20], [21]. e.g. mixing procedures and filling the over impression might lead to an incorporation of voids, compromising the mechanical strength [20]. In addition, studies have indicated that flexural strength is very low directly after fabricating these restorations [12].
CAD/CAM technologies – used to fabricate temporary restorations – may solve some of these issues. i.e. using resin based blanks cured under optimal conditions exhibit increased mechanical strength and prevent porosities within the restorations [2]. In addition, CAD/CAM fabricated temporaries reportedly reduce the chairside time and produce superior results [22].
Therefore, it was the aim of this study to compare the mechanical strength of directly fabricated temporary 3-unit FPDs versus identically CAD/CAM fabricated FPDs, milled of blanks, which were produced under optimal conditions using the same materials in a semi-clinical setup.
The null-hypothesis tested was three-fold: the mechanical strength of temporary 3-unit FPDs is independent of (1) the manufacturing process, (2) the t-c&b material used and (3) the storage condition after fabrication.
Section snippets
Materials and methods
The mechanical properties of the different materials and manufacturing techniques were tested using a semi clinical setup on a metal master-model with a 3-unit FPD. SEM analysis of the fractured surfaces was carried out on representative samples. Table 1 gives an overview of the materials tested including their composition. All materials were used according to the manufacturers’ recommendations. The tests were carried out at ambient laboratory conditions (23 ± 1 °C, 50 ± 5% rel. humidity).
Results
Table 2 shows the results of the three-way ANOVA. As indicated by the p-values, material, fabrication and storage significantly influenced the fracture strengths (p < 0.001). In addition, material & fabrication as well as material & storage influenced the fracture force in interaction (p < 0.01).
The maximum forces at fracture are given in Table 3. Three months water storage and TC lead to a significant decrease of Fmax for Luxatemp AM Plus, but not for Trim and Cercon Base PMMA. Luxatemp AM Plus
Discussion
This study aimed at investigating the influence of fabrication technique, material and storage condition on the fracture strength of temporary 3-unit FPDs. Summarizing the results requires rejection of all parts of the null-hypothesis, as the fracture strength was dependent on the material used, the fabrication technique as well as the storage condition.
When fabricating temporary crowns and FPDs, the quality of the final restoration is strongly dependent on the technique used as well as the
Conclusions
Under the limitations of this in vitro study, the following conclusions can be drawn:
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CAD/CAM fabricated FPDs exhibit a higher mechanical strength compared to directly fabricated FPDs, when manufactured of the same material.
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Composite based materials seem to offer clear advantages regarding mechanical strength versus PMMA based materials and should, therefore, be considered materials for future CAD/CAM manufactured temporary crowns and FPDs.
Acknowledgements
We are indebted to the various manufacturers for donating the materials used in the present study. We would like to thank Dr. U. Schusser and Dr. L. Völkl from Degudent for supporting the CAD/CAM fabrication of the FPDs. The authors would further like to acknowledge the support of Mr. U. Heun (Dept. of Prosthodontics, Jutus-Liebig-University, Giessen, Germany) as well as Mr. N. Pütz (Institute of Anatomy, Saarland University, Homburg, Germany) for their support during mechanical testing and
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