cad_cam versus directly fabricated restorations

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ARTICLE IN PRESSENTAL-1769; No. of Pages 9

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avai lab le at www.sc iencedi rec t .com

journa l homepage: www. int l .e lsev ierhea l th .com/ journa ls /dema

racture strength of temporary fixed partial dentures:AD/CAM versus directly fabricated restorations

anessa Altb, Matthias Hanniga, Bernd Wöstmannb, Markus Balkenhola,b,∗

Clinic of Operative Dentistry, Periodontology and Preventive Dentistry, Saarland University Hospital, Building 73, D-66421 Homburg,aar, GermanyDepartment of Prosthetic Dentistry, Justus-Liebig-University, D-35392 Giessen, Germany

r t i c l e i n f o

rticle history:

eceived 8 November 2010

ccepted 16 November 2010

vailable online xxx

eywords:

emporary fixed partial denture

nterim restoration

rovisional restoration

racture strength

omposites

-Point bending test

esin

hermocycling

n vitro study

a b s t r a c t

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 man-

ufactured. 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 sub-

jected 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 (˛ = 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 mate-

rials seem to offer clear advantages versus PMMA based materials and should, therefore, be

considered for CAD/CAM fabricated temporary restorations.

© 2010 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

. Introduction

Please cite this article in press as: Alt V, et al. Fracture strength of temrestorations. Dent Mater (2010), doi:10.1016/j.dental.2010.11.012

omputer aided design/manufacturing (CAD/CAM) technolo-ies have gained popularity in recent years for fixed restorativend prosthodontic treatment procedures. Among others, this

∗ Corresponding author at: Clinic of Operative Dentistry, Periodontology3, D-66421 Homburg, Saar, Germany. Tel.: +49 6841 16 24 478; fax: +49

E-mail address: [email protected] (M. Balkenhol).109-5641/$ – see front matter © 2010 Academy of Dental Materials. Puoi:10.1016/j.dental.2010.11.012

process is driven by the growing demand for placing highesthetic all-ceramic restorations [1,2]. At the same time, due

porary fixed partial dentures: CAD/CAM versus directly fabricated

and Preventive Dentistry, Saarland University Hospital, Building6841 16 24 954.

to improvement in physical properties of e.g. zirconia andother ceramics, these materials can be successfully usedalso in stress bearing areas [3]. Apart from the Cerec Sys-tem, most CAD/CAM supported technologies still use labside

blished by Elsevier Ltd. All rights reserved.

dx.doi.org/10.1016/j.dental.2010.11.012


mailto:[email protected]


ARTICLE IN PRESSDENTAL-1769; No. of Pages 9

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Table 1 – Temporary c&b materials under investigation.

Product Manufacturer MRa Shade Batch Composition

LuxatempAM Plus

DMG, Hamburg,Germany

10:1 A2 605703,910935

Urethane diacrylate, aromatic diacrylate, glycolmethacrylate, pigments, additives, stabilizer,silica, glass filler (44 wt.%)

Cercon BasePMMA

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: 0708-475L: 0612-600

P: ethyl methacrylate prepolymers, benzoylperoxide, pigments, TiO2; L: isobutylmethacrylate, di-butyl phthlate,dimethyl-p-toluidine

us mio mo

P, powder; L, liquid. All data reflect information provided by the varioa Mixing ratio dimethacrylates base: catalyst [by volume]; mixing rat

procedures during the manufacturing process (e.g. veneer-ing of zirconia frames/substructures) [4] and in consequencerequire temporary restorations to be fabricated on the pre-pared abutment teeth until the final fixed partial denture (FPD)is placed in situ.

The temporary restorations in turn fulfill a wide range offunctions comprising protection of the prepared tooth struc-ture, pulp and the surrounding periodontal tissues as well as tomaintain oral functions (mastication, phonetics) and esthetics[5,6]. Most of these restorations are fabricated chairside usingan over impression technique in combination with resin basedtemporary crown and FPD materials (t-c&bs) [7,8]. As the time-frame between preparation of a tooth and luting of the finalrestoration might exceed a couple of weeks, the t-c&bs usedto fabricate temporary crowns or FPDs have to meet severalrequirements [5,9].

Among others, the mechanical strength of a t-c&b is of par-ticular importance as this factor might influence the integrityof the temporary restoration during clinical service, when itis exposed to functional loads [10–13]. Hence, determinationof mechanical properties of t-c&bs was the subject of severalstudies [9,10,14–19].

The chairside fabrication of temporary restorations isassociated with a couple of short-comings, affecting themechanical strength as well as its surface texture and pre-cise fit [12,20,21]. e.g. mixing procedures and filling the overimpression might lead to an incorporation of voids, compro-mising the mechanical strength [20]. In addition, studies haveindicated that flexural strength is very low directly after fab-ricating these restorations [12].

CAD/CAM technologies – used to fabricate temporaryrestorations – may solve some of these issues. i.e. usingresin based blanks cured under optimal conditions exhibitincreased mechanical strength and prevent porosities withinthe restorations [2]. In addition, CAD/CAM fabricated tem-poraries reportedly reduce the chairside time and producesuperior results [22].

Therefore, it was the aim of this study to compare themechanical strength of directly fabricated temporary 3-unitFPDs versus identically CAD/CAM fabricated FPDs, milled of


blanks, which were produced under optimal conditions usingthe same materials in a semi-clinical setup.

The null-hypothesis tested was three-fold: the mechanicalstrength of temporary 3-unit FPDs is independent of (1) the

anufacturers.no-methacrylate liquid: powder [volume:mass].

manufacturing process, (2) the t-c&b material used and (3) thestorage condition after fabrication.

2. Materials and methods

The mechanical properties of the different materials andmanufacturing techniques were tested using a semi clinicalsetup on a metal master-model with a 3-unit FPD. SEManalysis of the fractured surfaces was carried out on repre-sentative samples. Table 1 gives an overview of the materialstested including their composition. All materials were usedaccording to the manufacturers’ recommendations. The testswere carried out at ambient laboratory conditions (23 ± 1 ◦C,50 ± 5% rel. humidity).

2.1. Master model

Two resin teeth (no. 25 and 27, frasaco, Tettnang, Germany)were prepared with a shoulder preparation (angle of conver-gence 6◦) for treatment with full crowns. Following this, theroots of the two teeth were completed with wax to simulatea natural root (root length 16 mm). The teeth were dupli-cated and cast using CrCo-alloy (Brealloy C + B 270, Bredent,Germany). A base corpus, representing an alveolar ridge, wasmanufactured (CrCo-alloy), containing two sockets for mount-ing the two teeth in a distance of 12 mm (gap between thesocket and the root: 1 mm). The teeth were fixed inside thesocket with a vinyl-polysiloxane (Monopren Transfer, Ketten-bach, Eschenburg, Germany) (Fig. 1A) [15,23]. This material hadshown to sufficiently simulate the natural tooth movementunder the test conditions, as confirmed by results obtainedfrom a Periotest device (Medizintechnik Gulden, Modautal,Germany). Finally, a jig was fabricated to record the preciseposition of the abutment teeth within the sockets.

A 3-unit master FPD was fabricated (Fig. 1B), cast (Breal-loy C + B 270, Bredent, Germany) and fitted on the abutmentteeth featuring an optimal marginal adaptation (Fig. 1C).The connection area between the abutment teeth and thepontic was 4.0 mm × 3.25 mm (pontic height: 6.3 mm). The


occlusal surface of the pontic was shaped to allow unequivo-cal positioning of a stainless steel spheric in the center of theFPD. The master FPD was digitized using a 3-Shape scanner(Wieland, Pforzheim, Germany) and the STL data set was saved




d e n t a l m a t e r i a l s x x x ( 2 0 1 0 ) xxx–xxx 3

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 overi adet

aH

2

DictmGtMmstor

t

mpression. (D) Temporary resin bridges milled of the self-memporary bridge and spherical prior to fracture testing.

nd imported in the Cercon Brain Expert system (Degudent,anau, Germany) to fabricate identical 3-unit FPDs.

.2. Direct fabrication of temporary FPDs

irect fabrication of FPDs was performed using an overmpression technique [24]. The master FPD was placed inorrect position on the two abutment teeth. Correct posi-ion was confirmed by a jig. A vinyl-polysiloxane impression

aterial (Panasil Putty/Contact Plus, Kettenbach, Eschenburg,ermany) was mixed according to the manufacturers instruc-

ions, dispensed in a metal segment tray (type C 1/2 L3, Carlartin, Solingen, Germany) and placed in position over theaster FPD (one stage putty-wash technique). The impres-

ion was removed after setting and cut into 2 pieces to removehe master FPD without altering/damaging the contour of the


ver impression. The impression tray was used for the correcte-assembly of the two pieces.

The t-c&b materials were mixed according to the manufac-urers’ instructions and dispensed into the over impression

base block. (E) Trimmed temporary bridge. (F) Setup with

from the bottom to the top to prevent incorporation of voids.The filled impression was placed in the correct position ontothe master model. A slot on top of the alveolar ridge ensuredcorrect position of the over impression. The temporary FPDswere carefully removed from the over impression after themanufacturers recommended setting time and excess mate-rial trimmed to precisely fit them on the abutment teeth in thedesired position.

2.3. CAD/CAM fabrication of temporary FPDs

To obtain base blocks of Luxatemp AM Plus and Trim, whichcould be fixed properly in the frames of the Cercon BrainExpert milling device, a siloxane mold (Duosil, Shera, Lem-förde, 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 preventbubble formation. Luxatemp AM Plus was dispensed directlyfrom the mixing tip inside the silicone mold as describedbefore. For both materials, curing took place at 37 ◦C in an



IN PRESSs x x x ( 2 0 1 0 ) xxx–xxx

Table 2 – Results of the three-way ANOVA.

Independent variable Significance

Material p < 0.001Fabrication p < 0.001Storage p < 0.001Material × fabrication p < 0.001Material × storage p < 0.01Fabrication × storage n.s.

ARTICLEDENTAL-1769; No. of Pages 9

4 d e n t a l m a t e r i a l

incubator (Ehret, Emmerdingen, Germany). After curing, baseblocks were stored for at least 1 week prior to milling the FPDs.

The polymerized blocks as well as the Cercon Base PMMAdisks (Degudent, Hanau, Germany) were fixed inside theframes of the Cercon Brain Expert milling device to mill thetemporary FPDs (Fig. 1D). The final resin FPDs were care-fully removed from the blocks and fitted as described before(Fig. 1E).

2.4. Storage conditions

Prior to storage, conformance of the temporary FPDs with themaster FPD was checked regarding the dimensions using a dig-ital caliper (Mitutoyo, Japan). In addition, temporary bridgeswere 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 dimensionswere discarded and new FPDs fabricated.

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

2.5. Fracture test

The CrCo-alloy alveoar ridge master model was fixed on bothsides 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 pon-tic (Fig. 1F) and fracture test started at a crosshead speed of200 mm/min until fracture occurred. Maximum force at frac-ture (Fmax) was recorded (testXpert software, release 11.1,Zwick/Roell, Ulm, Germany).

2.6. SEM analysis

Representative SEM micrographs were taken from the frac-tured surfaces. Fracture surfaces were carefully cut from theFPD segments, stuck to specimen holders (Plano, Wetzlar,Germany) and sputtered with platinum. SEM evaluation was


performed with a scanning electron microscope (type FEI XL30ESEM FEG, FEI Company, Eindhoven, The Netherlands) at anaccelerating voltage of 10–20 kV and a magnification of 125×to 20,000×.

Table 3 – Force at fracture = Fmax in N (mean values and standa

Material Fabrication

1 day 3

Luxatemp AM PlusCAD/CAM# 1115.5 ±Direct* 561.9 ± 7

Cercon Base PMMACAD/CAM# 416.9 ± 8Direct n.a.

TrimCAD/CAM* 379.3 ± 1Direct* 188.1 ± 5

Lower case superscript letters are related to rows and denote results of thSame lower case letters denote groups, which do not differ significantly (#

Material × fabrication × storage n.s.

Dependant variable: force at fracture; n.s., not significant.

2.7. Statistical analysis

Mean values and standard deviations of the maximum forceat fracture were calculated. The influence of the independentvariables (material, fabrication technique, storage) were ana-lyzed for their effects by a three-way ANOVA [25,26].

Post hoc comparisons were carried out using aGames–Howell test (when variances were not homogeneous)and a Tukey’s test, respectively. A t-test for independent sam-ples was used to compare the influence of the manufacturingprocess [25,26]. All statistical analyses were performed usingSPSS for Windows (release 15.01, SPSS Inc., Chicago, IL, USA)on a significance level of 5%.

3. Results

Table 2 shows the results of the three-way ANOVA. Asindicated by the p-values, material, fabrication and storagesignificantly influenced the fracture strengths (p < 0.001). Inaddition, material & fabrication as well as material & storageinfluenced the fracture force in interaction (p < 0.01).

The maximum forces at fracture are given in Table 3. Threemonths water storage and TC lead to a significant decreaseof Fmax for Luxatemp AM Plus, but not for Trim and CerconBase PMMA. Luxatemp AM Plus and Trim showed a signifi-cant higher Fmax, if the FPDs were CAD/CAM fabricated (t-test,p < 0.01).

The directly fabricated FPDs, made of Trim, showed a sig-


nificantly lower Fmax compared to Luxatemp AM Plus (t-test,p < 0.05). In case of CAD/CAM fabrication, the composite basedt-c&b showed significantly higher Fmax values compared tothe two mono-methacrylate based materials (Table 4).

rd deviations).

Storage

7 ◦C 1 week TC 3 months 37 ◦C

198.1a 875.8 ± 145.5b 885.8 ± 256.3b

3.1a 268.4 ± 101.2b 367.4 ± 148.0b

9.8a 325.2 ± 86.4a 423.7 ± 102.8a

n.a. n.a.

72.7a 264.0 ± 38.2a 342.6 ± 103.2a

0.4a 138.5 ± 54.4a 185.0 ± 54.1a

e post hoc tests (influence of storage for the respective test-groups).Tukey test p > 0.05; *Games–Howell test p > 0.05).




d e n t a l m a t e r i a l s x x x ( 2 0 1 0 ) xxx–xxx 5

F nifications of Luxatemp AM Plus fabricated directly (A and B)a differences in surface texture visible.

it(tma

mDPbt

C

ig. 2 – SEM pictures of the fracture surface at different magnd CAD/CAM fabricated (C and D), respectively. No obvious

The morphology of the fracture surfaces analyzed by SEMs shown in Figs. 2–5. Fig. 2 reveals the fracture surfaces ofhe 3-unit FPDs made of Luxatemp AM Plus fabricated directlyFig. 2A and B) and CAD/CAM fabricated (Fig. 2C and D), respec-ively. No differences in morphology were obvious. At higher

agnification (Fig. 3), a tight contact between filler particlesnd polymer are clearly visible.

Fig. 4 shows the fracture surface of the Trim FPDs, whenanufactured directly (Fig. 4A and B) and milled (Fig. 4C and), respectively. No differences in morphology were obvious.orosities, visible in Fig. 4B, are artifacts caused by the electron

Please cite this article in press as: Alt V, et al. Fracture strength of temporary fixed partial dentures: CAD/CAM versus directly fabricatedrestorations. Dent Mater (2010), doi:10.1016/j.dental.2010.11.012

eam in the SEM. Pre-polymers are tightly embedded withinhe matrix. The fracture runs through the pre-polymer beads.

Fig. 5 finally shows the fracture surface of FPDs milled ofercon Base PMMA disks. The morphology is similar to that

Table 4 – Results of the statistical comparison betweenthe materials obtained from the CAD/CAM fabrication.

Storage Cercon BasePMMA

Trim

1 day 37 ◦C# Luxatemp AM Plus p < 0.001 p < 0.001Trim n.s. –

1 week TC*Luxatemp AM Plus p < 0.001 p < 0.001Trim n.s. –

3 months 37 ◦C*Luxatemp AM Plus p < 0.01 p < 0.01Trim n.s. –

Dependant variable: force at fracture = Fmax; n.s., not significant.#Tukey test; *Games–Howell test.

Fig. 3 – SEM pictures of Luxatemp AM Plus at a highermagnification. Note that filler particles are tightlyembedded into the matrix. No disintegration visible.




6 d e n t a l m a t e r i a l s x x x ( 2 0 1 0 ) xxx–xxx

Fig. 4 – SEM pictures of the fracture surface at different magnifications of Trim fabricated directly (A and B) and CAD/CAMsurfaroug

fabricated (C and D), respectively. No obvious differences inthe electron beam in the SEM. Note that fracture line runs th

of the Trim samples. However, pre-polymer beads apparentlyare not bonded tightly within the matrix, as they are partiallyexposed (Fig. 5B).

4. Discussion

This study aimed at investigating the influence of fabricationtechnique, material and storage condition on the fracturestrength of temporary 3-unit FPDs. Summarizing the resultsrequires rejection of all parts of the null-hypothesis, as thefracture strength was dependent on the material used, thefabrication technique as well as the storage condition.

When fabricating temporary crowns and FPDs, the qual-ity of the final restoration is strongly dependent on thetechnique used as well as the accuracy applied duringmanufacturing. Therefore a semi-clinical setup with a mastermodel was selected to simulate the clinical situation atbest [15,27]. The connector between the abutment crownand pontic (4.0 mm × 3.25 mm) was chosen higher thanthe manufacturer’s recommended minimum for Cercon BasePMMA (9 mm2). The Fmax values obtained have to be regardedprimarily as relative values, as the individual dimensions ofa FPD is an important factor influencing the fracture strength[24]. Another important test parameter is the cross-head


speed, which was defined as 200 mm/min to simulate a quickclosure of the mouth.

A temporary luting agent was omitted on purpose toexclude it as additional influencing variable. It might be spec-

ce texture visible. Porosities in (B) are artifacts, caused byh the pre-polymerized particles.

ulated that the luting agent would have increased the fracturestrength; however, this issue should be addressed in furtherstudies. As storage time and condition reportedly affect themechanical properties of t-c&bs, 3 months storage in water aswell as TC were applied to simulate artificial aging as describedelsewhere [15,24,28]. To be able to directly compare the fabri-cation techniques applied, blanks for the CAD/CAM millingprocess were fabricated using the same materials in additionto testing a commercially available resin base blank (CerconBase PMMA).

The maximum force at fracture was significantly affectedby the storage condition for the composite based t-c&b(p < 0.05), whereas no significant influence of storage wasnoticeable for the two mono-methacrylate based materials(p > 0.05). Nevertheless, there was a tendency for lower Fmaxvalues, when the latter two materials had undergone TC.

These results are in accordance with literature, indicatingan aging effect caused by water storage and TC, respectively[28–32]. This effect might be explained by the water uptakeof the t-c&bs, acting as a plasticizer [15]. Furthermore, cer-tain degradation phenomena on a molecular level betweenfiller and matrix might have played a role in case of LuxatempAM Plus [29,30,33,34], although not obvious in the SEM micro-graphs.


In addition, the effect seems to be more pronounced, ifthe FPDs are fabricated directly. It is hypothesized that apolymerization inhibiting effect of oxygen dissolved in water[35] is the reason for this phenomenon, as the material comes



ARTICLE INDENTAL-1769; No. of Pages 9

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Fig. 5 – SEM pictures of the fracture surface at differentmagnifications (A and B) of Cercon Base PMMA. Note thatsome of the pre-polymerized particles are exposed, i.e. thef

io

fibatcrwta

msCfspitH

racture line runs around the pre-polymerized particles.

n contact with water/humidity at a stage, when the processf radical polymerization is still in progress [12].

In contrast, CAD/CAM manufactured FPDs were fabricatedrom base blocks, cured under optimal conditions, without thenterference of water. During the storage period of these baselocks at ambient conditions, post polymerization processess well as relaxation phenomena take place [33,36]. This leadso an improvement of physical properties, as observed in theurrent experiment. This is in accordance with published data,eporting higher flexural strength of composite based resins,hen specimens are cured and stored at air [28,37]. In addi-

ion, radicals may be active over a period of 7 days, leading tosignificant post polymerization [38].

When inserting milled FPDs, the t-c&bs have reached opti-al physical properties, and thus featuring a high fracture

trength. Clinically, this has two positive effects: First of allAD/CAM fabricated FPDs have a higher resistance against

racture from the moment of placing the restoration, andecond, the absolute resistance against fracture is higher com-


ared to FPDs, fabricated directly. This may be of particularmportance for long span bridges and in cases, where theemporary restoration is intended as long term provisional.ence, when used according to the manufacturers’ recom-

PRESS( 2 0 1 0 ) xxx–xxx 7

mendations, CAD/CAM fabricated Cercon Base PMMA bridgesshow clear advantages.

The values obtained for the PMMA based materials arein a range described in literature, taking into considera-tion the differences in dimensions of the FPDs [10,19,23,24].The results obtained for Luxatemp AM Plus after 24 h waterstorage concord with results published by Lang et al. [15].However, in contrast to our results, Lang et al. observed anincrease in fracture strength after thermo-mechanical load-ing.

The direct fabrication of the 3-unit FPDs significantlyaffected the Fmax values for both materials (Luxatemp AMPlus, Trim). As the SEM analysis of the fracture surfaces didnot show any obvious signs of porosities or voids in thegroup of directly fabricated FPDs, it is hypothesized that thehigher Fmax values are primarily related to a higher load bear-ing capacity of the polymer network caused by the effectsdescribed before [28,36].

Although not significant, there was a tendency for higherFmax values of Cercon Base PMMA FPDs in comparison toFPDs, made of Trim. This difference is most likely related tothe different chemical composition of the materials used. Themanufacturer of Cercon Base PMMA claims that the materialis composed of highly cross-linked polymethyl methacrylate.Regular PMMA has a glass transition temperature of 125 ◦C[39], if not cross-linked. Cross-linking even increases the glass-transition temperature and in turn the mechanical strengthof the product. As denoted by the SEM micrographs, thepre-polymers seem to be not tightly embedded within thematrix. Hence it may be hypothesized that optimizing the pro-duction process of Cercon Base PMMA with tight embeddedpre-polymers might even increase the mechanical strength ofthe product.

In contrast, Trim consists of isobutyl methacrylate withembedded pre-polymers, which has a lower glass transitiontemperature (70 ◦C) compared to PMMA [39]. The softeneradded (di-butyl phthlate) additionally decreases mechanicalstrength and in turn explains the lower Fmax values observedfor the Trim FPDs. In contrast to Cercon Base PMMA, the pre-polymers were tightly embedded within the matrix as shownby the SEM analysis.

Depending on the cross-head speed, a deformation of spec-imens made of PMMA materials maybe observed instead of afracture [15,40]. This can readily be explained by their chem-ical base structure [40]. In the current experiment, however,all FPDs broke due to the high cross-head speed applied(200 mm/min).

Finally, the composite based Luxatemp AM Plus showedthe highest mechanical strength when used indirectly, abouttwice as high as the FPDs made of Cercon Base PMMA. Thisresult is readily explained by the difference in composition,as Luxatemp is a composite based material using well estab-lished dimethacrylate monomers as well as reinforcing fillerparticles. The higher mechanical strength of composite basedt-c&bs in comparison to traditional mono-methacrylates is inaccordance with literature [15,40].


As denoted by the SEM micrographs of the Luxatemp sam-ples, silanization process of the filler particles seem to be veryeffective, as they are tightly embedded within the matrix with-out showing any signs of disintegration.



INs x x

r

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According to the instruction for use, Cercon Base PMMA isrecommended for FPDs with a maximum span of one pontic.If the clinical situation requires temporary FPDs with morethan one pontic, t-c&bs of higher mechanical strength mightbe required. Therefore, manufacturers are encouraged to offercomposite based blanks for CAD/CAM fabrication of FPDs.

In addition, higher strength materials might feature ahigher longevity without failure in clinical situations, whichrequire long term provisional treatment. Such clinical sit-uations may e.g. be a comprehensive re-adjustment of theentire occlusion or periodontal pre-treatment with unknownprognosis of the abutment teeth. However, the question oflongevity should be addressed in further clinical studies.

5. Conclusions

Under the limitations of this in vitro study, the following con-clusions can be drawn:

• CAD/CAM fabricated FPDs exhibit a higher mechanicalstrength compared to directly fabricated FPDs, when man-ufactured of the same material.

• Composite based materials seem to offer clear advantagesregarding mechanical strength versus PMMA based materi-als and should, therefore, be considered materials for futureCAD/CAM manufactured temporary crowns and FPDs.

Acknowledgements

We are indebted to the various manufacturers for donating thematerials used in the present study. We would like to thankDr. U. Schusser and Dr. L. Völkl from Degudent for support-ing the CAD/CAM fabrication of the FPDs. The authors wouldfurther 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 Univer-sity, Homburg, Germany) for their support during mechanicaltesting and performing the SEM micrographs, respectively.

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cad_cam versus directly fabricated restorations

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