Fracture strength of temporary fixed partial dentures: CAD/CAM versusdirectly fabricated restorations

Abstract

Objectives

This study aimed at investigating the influence of fabrication method, storage condition and material on the fracture strength of temporary 3-unit fixed partial dentures (FPDs).

Methods

A CrCo-alloy master model with a 3-unit FPD (abutment teeth 25 and 27) was manufactured. The master model was scanned and the data set transferred to a CAD/CAM unit (Cercon Brain Expert, Degudent, Hanau, Germany). Temporary 3-unit bridges were produced either by milling from pre-fabricated blanks (Trim, Luxatemp AM Plus, Cercon Base PMMA) or by direct fabrication (Trim, Luxatemp AM Plus). 10 FPDs per experimental group were subjected either to water storage at 37 °C for 24 h and 3 months, respectively, or thermocycled (TC, 5000×, 5–55 °C, 1 week). Maximum force at fracture (Fmax) was determined in a 3-point bending test at 200 mm/min. Data was analyzed using parametric statistics ( α = 5%).

Results

Fmax values ranged from 138.5 to 1115.5 N. FPDs, which were CAD/CAM fabricated, showed a significant higher Fmax compared to the directly fabricated bridges ( p < 0.05). TC significantly affected Fmax for Luxatemp ( p < 0.05) but not for the PMMA based materials ( p > 0.05). CAD/CAM milled FPDs made of Luxatemp showed significantly higher Fmax values compared to Trim and Cercon Base PMMA ( p < 0.05).

Significance

CAD/CAM fabricated FPDs exhibit a higher mechanical strength compared to directly fabricated FPDs, when manufactured of the same material. Composite based materials seem to offer clear advantages versus PMMA based materials and should, therefore, be considered for CAD/CAM fabricated temporary 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 . 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 . 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) 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 . 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) . 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 .

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 . Hence, determination of mechanical properties of t-c&bs was the subject of several studies .

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 . e.g. mixing procedures and filling the over impression might lead to an incorporation of voids, compromising the mechanical strength . In addition, studies have indicated that flexural strength is very low directly after fabricating these restorations .

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 . In addition, CAD/CAM fabricated temporaries reportedly reduce the chairside time and produce superior results .

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.

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).

Table 1
Temporary c&b materials under investigation.
Product Manufacturer MR a Shade Batch Composition
Luxatemp AM Plus DMG, Hamburg, Germany 10:1 A2 605703, 910935 Urethane diacrylate, aromatic diacrylate, glycol methacrylate, pigments, additives, stabilizer, silica, glass filler (44 wt.%)
Cercon Base PMMA Degudent, Hanau, Germany n.a. B2 005366122220 Highly cross-linked methyl methacrylate, pigments, benzoyl peroxide (<1 wt.%)
Trim Bosworth, Skokie, Ilinois, USA 1:2.3 Light P: 0708-475
L: 0612-600
P: ethyl methacrylate prepolymers, benzoyl peroxide, pigments, TiO 2 ; L: isobutyl methacrylate, di-butyl phthlate, dimethyl-p-toluidine
P, powder; L, liquid. All data reflect information provided by the various manufacturers.

a Mixing ratio dimethacrylates base: catalyst [by volume]; mixing ratio mono-methacrylate liquid: powder [volume:mass].

Master model

Two resin teeth (no. 25 and 27, frasaco, Tettnang, Germany) were prepared with a shoulder preparation (angle of convergence 6°) for treatment with full crowns. Following this, the roots of the two teeth were completed with wax to simulate a natural root (root length 16 mm). The teeth were duplicated and cast using CrCo-alloy (Brealloy C + B 270, Bredent, Germany). A base corpus, representing an alveolar ridge, was manufactured (CrCo-alloy), containing two sockets for mounting the two teeth in a distance of 12 mm (gap between the socket and the root: 1 mm). The teeth were fixed inside the socket with a vinyl-polysiloxane (Monopren Transfer, Kettenbach, Eschenburg, Germany) ( Fig. 1 A) . This material had shown to sufficiently simulate the natural tooth movement under the test conditions, as confirmed by results obtained from a Periotest device (Medizintechnik Gulden, Modautal, Germany). Finally, a jig was fabricated to record the precise position of the abutment teeth within the sockets.

Fig. 1
(A) Prepared teeth 25 and 27 made of CrCo-alloy fixed in position in the sockets of the artificial alveolar ridge. (B) Master bridge made of CrCo-alloy. (C) Master bridge placed in correct position on the base model prior to making the over impression. (D) Temporary resin bridges milled of the self-made base block. (E) Trimmed temporary bridge. (F) Setup with temporary bridge and spherical prior to fracture testing.

A 3-unit master FPD was fabricated ( Fig. 1 B), cast (Brealloy C + B 270, Bredent, Germany) and fitted on the abutment teeth featuring an optimal marginal adaptation ( Fig. 1 C). The connection area between the abutment teeth and the pontic was 4.0 mm × 3.25 mm (pontic height: 6.3 mm). The occlusal surface of the pontic was shaped to allow unequivocal positioning of a stainless steel spheric in the center of the FPD. The master FPD was digitized using a 3-Shape scanner (Wieland, Pforzheim, Germany) and the STL data set was saved and imported in the Cercon Brain Expert system (Degudent, Hanau, Germany) to fabricate identical 3-unit FPDs.

Direct fabrication of temporary FPDs

Direct fabrication of FPDs was performed using an over impression technique . The master FPD was placed in correct position on the two abutment teeth. Correct position was confirmed by a jig. A vinyl-polysiloxane impression material (Panasil Putty/Contact Plus, Kettenbach, Eschenburg, Germany) was mixed according to the manufacturers instructions, dispensed in a metal segment tray (type C 1/2 L3, Carl Martin, Solingen, Germany) and placed in position over the master FPD (one stage putty-wash technique). The impression was removed after setting and cut into 2 pieces to remove the master FPD without altering/damaging the contour of the over impression. The impression tray was used for the correct re-assembly of the two pieces.

The t-c&b materials were mixed according to the manufacturers’ instructions and dispensed into the over impression from the bottom to the top to prevent incorporation of voids. The filled impression was placed in the correct position onto the master model. A slot on top of the alveolar ridge ensured correct position of the over impression. The temporary FPDs were carefully removed from the over impression after the manufacturers recommended setting time and excess material trimmed to precisely fit them on the abutment teeth in the desired position.

CAD/CAM fabrication of temporary FPDs

To obtain base blocks of Luxatemp AM Plus and Trim, which could be fixed properly in the frames of the Cercon Brain Expert milling device, a siloxane mold (Duosil, Shera, Lemförde, Germany) was fabricated (50 mm × 50 mm × 20 mm). Trim was mixed according to the manufacturer’s instructions and filled into the mold from the bottom to the top to prevent bubble formation. Luxatemp AM Plus was dispensed directly from the mixing tip inside the silicone mold as described before. For both materials, curing took place at 37 °C in an incubator (Ehret, Emmerdingen, Germany). After curing, base blocks were stored for at least 1 week prior to milling the FPDs.

The polymerized blocks as well as the Cercon Base PMMA disks (Degudent, Hanau, Germany) were fixed inside the frames of the Cercon Brain Expert milling device to mill the temporary FPDs ( Fig. 1 D). The final resin FPDs were carefully removed from the blocks and fitted as described before ( Fig. 1 E).

Storage conditions

Prior to storage, conformance of the temporary FPDs with the master FPD was checked regarding the dimensions using a digital caliper (Mitutoyo, Japan). In addition, temporary bridges were checked for bubbles, voids and other pre-damages at 40× magnification under a light microscope (M420, Leica, Wetzlar, Germany). FPDs with pre-damages and incorrect dimensions were discarded and new FPDs fabricated.

Storage took place in a water bath at 37 ̊C for 24 h or 3 months, respectively ( n = 10 per material and storage condition). A third group of 10 specimens was subjected to thermocycling for 1 week (TC, 5–55 ̊C, 5000 cycles, dwell time 50 s).

Fracture test

The CrCo-alloy alveoar ridge master model was fixed on both sides in a universal testing machine (type 1454, Zwick/Roell, Ulm, Germany) for fracture testing. A stainless steel spheric (Ø 12.5 mm) was centered on the occlusal surface of the pontic ( Fig. 1 F) and fracture test started at a crosshead speed of 200 mm/min until fracture occurred. Maximum force at fracture (Fmax) was recorded (testXpert software, release 11.1, Zwick/Roell, Ulm, Germany).

SEM analysis

Representative SEM micrographs were taken from the fractured surfaces. Fracture surfaces were carefully cut from the FPD segments, stuck to specimen holders (Plano, Wetzlar, Germany) and sputtered with platinum. SEM evaluation was performed with a scanning electron microscope (type FEI XL30 ESEM FEG, FEI Company, Eindhoven, The Netherlands) at an accelerating voltage of 10–20 kV and a magnification of 125× to 20,000×.

Statistical analysis

Mean values and standard deviations of the maximum force at fracture were calculated. The influence of the independent variables (material, fabrication technique, storage) were analyzed for their effects by a three-way ANOVA .

Post hoc comparisons were carried out using a Games–Howell test (when variances were not homogeneous) and a Tukey’s test, respectively. A t -test for independent samples was used to compare the influence of the manufacturing process . All statistical analyses were performed using SPSS for Windows (release 15.01, SPSS Inc., Chicago, IL, USA) on a significance level of 5%.

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).

Table 1
Temporary c&b materials under investigation.
Product Manufacturer MR a Shade Batch Composition
Luxatemp AM Plus DMG, Hamburg, Germany 10:1 A2 605703, 910935 Urethane diacrylate, aromatic diacrylate, glycol methacrylate, pigments, additives, stabilizer, silica, glass filler (44 wt.%)
Cercon Base PMMA Degudent, Hanau, Germany n.a. B2 005366122220 Highly cross-linked methyl methacrylate, pigments, benzoyl peroxide (<1 wt.%)
Trim Bosworth, Skokie, Ilinois, USA 1:2.3 Light P: 0708-475
L: 0612-600
P: ethyl methacrylate prepolymers, benzoyl peroxide, pigments, TiO 2 ; L: isobutyl methacrylate, di-butyl phthlate, dimethyl-p-toluidine
P, powder; L, liquid. All data reflect information provided by the various manufacturers.

a Mixing ratio dimethacrylates base: catalyst [by volume]; mixing ratio mono-methacrylate liquid: powder [volume:mass].

Master model

Two resin teeth (no. 25 and 27, frasaco, Tettnang, Germany) were prepared with a shoulder preparation (angle of convergence 6°) for treatment with full crowns. Following this, the roots of the two teeth were completed with wax to simulate a natural root (root length 16 mm). The teeth were duplicated and cast using CrCo-alloy (Brealloy C + B 270, Bredent, Germany). A base corpus, representing an alveolar ridge, was manufactured (CrCo-alloy), containing two sockets for mounting the two teeth in a distance of 12 mm (gap between the socket and the root: 1 mm). The teeth were fixed inside the socket with a vinyl-polysiloxane (Monopren Transfer, Kettenbach, Eschenburg, Germany) ( Fig. 1 A) . This material had shown to sufficiently simulate the natural tooth movement under the test conditions, as confirmed by results obtained from a Periotest device (Medizintechnik Gulden, Modautal, Germany). Finally, a jig was fabricated to record the precise position of the abutment teeth within the sockets.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Fracture strength of temporary fixed partial dentures: CAD/CAM versusdirectly fabricated restorations
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