RESEARCH AND EDUCATION

aAssociate PrTechnology,bAssistant PrcAssociate PrTechnology,dGeneral DeneAssociate Pr

414

Comparison of interim restorations fabricated by CAD/CAMwith those fabricated manually

Mohammad M. Rayyan, BDS, MSc, PhD,a Moustafa Aboushelib, DDS, MSc, PhD,b

Nagwa M. Sayed, BDS, MSc, PhD,c Ahmed Ibrahim, MSc, DDS,d and Ryo Jimbo, DDS, PhDe

ABSTRACTStatement of problem. Interim restorations represent an essential treatment step; however, theoptimal resin material for long-term interim restorations requires investigation.

Purpose. The purpose of this in vitro study was to compare the color stability, water sorption, wearresistance, surface hardness, fracture resistance, and microleakage of computer-aided design/computer-aided manufacturing-(CAD/CAM) fabricated interim restorations with those of manuallyfabricated interim restorations.

Material and methods. Epoxy replicas were made from a prepared maxillary first premolar. Onthe replicas, interim crowns were fabricated and divided into the following groups: CAD/CAMpoly(methyl methacrylate) (PMMA) blocks (CC), autopolymerizing temporary resin (AP), automixtemporary resin (AM), and thermoplastic resin (TP). After cementation, all specimens were sub-jected to thermocycling and dynamic fatigue. The CIE Laboratory color coordinates were thenrecorded before and after immersion in coffee, tea, carbonated cola, and red wine. Watersorption was evaluated by using an immersion technique. Wear resistance was measured in asurface abrasion device. Vickers microhardness was measured on polished specimens. Fractureresistance was evaluated by axial loading with a universal testing machine. Marginal dyepenetration was evaluated by sectioning the interim restorations after immersion in methyleneblue (a=.05).

Results. Colorimetric analysis revealed a large degree of color alteration (DE) in the manuallyfabricated interim restorations: AP = DE of 6.7 ±2); AM = DE of 7.1 ±1.5), and TP = DE of =5.4 ±3.1.The CC group demonstrated color stability (DE=2.1 ±0.2). CAD/CAM interim restorations demon-strated significantly lower water sorption, higher wear resistance, higher surface hardness,and significantly higher fracture resistance (1 289±56N) compared with manually fabricatedinterim restorations (AP=996 ±45, AM=899 ±37, and TP=1179 ±41). The stereomicroscopic exam-ination of sectioned specimens demonstrated the absence of dye penetration in all testedspecimens.

Conclusions. CAD/CAM interim crowns presented stable physical and mechanical properties andmay be used for long-term interim restorations. (J Prosthet Dent 2015;114:414-419)

Interim restorations are usedto protect the pulp againstthermal, mechanical, physical,and bacterial contamination.1,2

As traditional poly(methylmethacrylate)-based resin(PMMA) possesses low me-chanical properties, attemptshave been made to reinforcethis material with fillers suchas glass,3-8 silica,9 carbonfiber,6,8 steel wires,8,10 andpolyaramid.3,11 Other disad-vantages of PMMA interimrestorations include their lackof marginal integrity12,13 andpoor color stability because ofa porous surface that attractsstains.14,15 Additionally, theirhigh polymerization shrink-age, heat generation, watersorption, and degradation ofthe resin matrix are of con-cern.16 Interim restorations arecommonly fabricated fromtemplates of the required mor-phology, which are filled withthe resin material and seated

ofessor, Fixed Prosthodontics, Beirut Arab University, Beirut, Lebanon; and Assistant Professor of Fixed Prosthodontics, Misr University for Science andCairo, Egypt.ofessor, Dental Biomaterials Department, Alexandria University, Alexandria, Egypt.ofessor, Fixed Prosthodontics, Beirut Arab University, Beirut, Lebanon; and Assistant Professor of Fixed Prosthodontics, Misr University for Science andCairo, Egypt.tist, Public Health Service (Folktandvården), Gefle County, Sweden.ofessor, Department of Prosthodontics, Faculty of Odontology, Malmö University, Malmö, Sweden.

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Table 1.Material composition

Trade Name Composition Shade Manufacturer

Cercon basePMMA blocks

CAD/CAM PMMA A2 DeguDentGmbH

Alike MMA/PMMA A2 GC Europe

Acrytemp Bis-acrylic autopolymerizing compound A2 Zhermack

DurAcetal Highly crystalline acetal copolymer resin A2 Myerson LLC

CAD/CAM, computer-aided design/computer-aided manufacture; MMA, methacrylatemonomer; PMMA, poly(methyl methacrylate).

Clinical ImplicationsResults of the current study suggest that CAD/CAM-fabricated interim restorations may presentstable, long-term clinical outcomes compared withthose of conventional manually fabricated interimrestorations.

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over the prepared teeth.17,18 This technique requireslaboratory and chairside time because adjustments maybe necessary. However, improved interim restorativematerials with better esthetics and longevity have beenintroduced.19 Bis-glycidyl methacrylate and bis-acryl-based materials are more color stable and have lowerpolymerization shrinkage and better mechanical prop-erties than acrylic resins.20-23

Recently, thermoplastic materials have been intro-duced for the fabrication of interim restorations withthe indirect technique. Because these materials requirespecial equipment and are technique sensitive, the ma-terials have been used with rapid prototyping, includingboth liquid-based stereolithography and powder-based3-dimensional (3D) printing.24-26

Long-term interim restorations are necessary fororal implantation treatment or in situations involvingcomprehensive occlusal reconstruction, where the res-torations could face extended functional loading.27

Thus, the interim material should possess optimummechanical properties, color stability, and marginalintegrity28 and may also serve as a guide for soft tissuehealing.29 Numerous studies have addressed the issue ofstrengthening manually fabricated interim restora-tions.30,31 Computer-assisted design/computer-assistedmachining (CAD/CAM) allows the milling of 3D-designed objects from bulk material, and the technique isreported to provide high precision.32 CAD/CAM interimrestorations are made from preprocessed acrylic resinblocks and are reported to possess better color stabilityand more precise marginal quality than conventionallyprocessed resin.33-35

The purpose of this study was to compare the colorstability, water sorption, wear resistance, surface hard-ness, fracture resistance, and microleakage of CAD/CAM-fabricated interim crowns with those of conven-tionally fabricated interim crowns.

MATERIAL AND METHODS

A maxillary first premolar was prepared to receive aceramic fixed restoration with a 1.5-mm occlusal reduc-tion, 1-mm round finish line, and 6-degree convergenceangle. The prepared tooth was embedded in an epoxyresin (Tri-Epoxy; Keystone Industries) base. A siliconeimpression (Hydrorise; Zhermack) was made of the

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whole assembly and poured to fabricate epoxy resin(Tri-Epoxy) and stone dies (Elite Master; Zhermack SpA).Thereafter, 4 different interim crowns were generated byusing the following procedures.

In the CAD/CAM PMMA (CC) group, dies werescanned (S 50 Zenotec CAD; Wieland Dental) to producea CAD model for a complete maxillary premolar. Pre-molars were milled from a CAD/CAM PMMA block(Cercon base PMMA blocks; DeguDent GmbH). Threesilicone indices were made of the produced CAD/CAMcrown with polyvinyl siloxane (Hydrorise putty; Zher-mack). The indices were used as templates for the othergroups.

In the autopolymerizing temporary resin (AP) group,cocoa butter was applied to the dies to prevent theinterim material from adhering. Acrylic resin (Alike; GCEurope) powder and liquid were mixed according to themanufacturer’s instructions and loaded into the indexand placed on the dies until completely set. The crownswere then removed, finished, and polished using rotaryrubber cups (Komet Dental Gebr Brasseler GmbH).

In the automix temporary resin (AM) group, the2-component automixing cartridge (Acrytemp; Zhermack)resin was loaded into a self-mixing gun and injected intothe silicone index and placed on the corresponding die.After complete setting, the interim crowns were finishedand polished as previously described.

In the thermoplastic resin (TP) group, a silicone indexwas cut into halves, and an access hole was drilled intoeach half. Crystalline acetal copolymer resin (Duracetal;Myerson LLC) was loaded into a special heating gun andinjected into the 2 parts of the mold after being assem-bled on the preheated stone dye. Material properties aresummarized in Table 1.

Zinc oxide-based interim cement (RelyX Temp NE;3M ESPE) was mixed on a waxed paper pad, and a plasticfilling instrument was used to fill each interim restora-tion, which was seated in its corresponding epoxy dieunder a fixed load of approximately 5.0 N. Excess cementwas removed with a cotton pellet after 30 minutes. Thecemented specimens were stored under distilled water atroom temperature for 48 hours before testing.

The 3 coordinates of color (CIELab) were measuredwith a calibrated dental colorimeter (Easyshade Advance;Vita Zahnfabrik) immediately after cementation of the

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Figure 1. Superimposed images before (blue) and after (red) wear testshowing areas of volumetric material loss (yellow).

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interim restorations. A dark box was used to preventexternal light reflection. Each cemented specimen wasstored for 1 week in 1 of the following solutions: coffee(Nescafe Gold; Nestle), tea (Lipton Yellow Label; Uni-lever), carbonated cola (Pepsi; PepsiCo), and red wine.The solutions were replaced every 6 hours, and the sur-face of each restoration was manually brushed withtoothpaste. Color parameters were measured again after6 weeks, and the change in color was evaluated from thefollowing equation:

DE=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðL2−L1Þ2+ða2−a1Þ2ðb2−b1Þ2

q;

where Lab1 is the color coordinate after cementationand Lab2 values are the color coordinates after immer-sion in the staining solutions. DE of >4.2 was consideredunacceptable color change as observed by the nakedeye.36

Uncemented specimens were immersed in distilledwater for 3 months at a temperature of 37�C. Specimenswere weighed on a digital balance (PXC 200 series;Intelligent Weighing Technology, Inc). The increase inweight from new to immersed specimens was used as thewater sorption ratio.37

Cemented specimens were highly polished with abristle brush and 0.05-mm diamond paste. The polishedspecimens were placed in a mastication simulator (CS4.8; SD Mechatronics) device, and a paste of rice shellswas added as the abrasive material. The specimens weresubjected to 2 million cycles at a load of 40 N and werethen ultrasonically cleaned, dried, and gold sputtercoated before scanning electron microscopy imaging (XL30; Philips). Each specimen was digitally photographed(D7000; Nikon Corp) before and after masticationsimulation, and the images were superimposed to detectareas of material loss. The percentage of weight loss wasused to indicate material loss from friction and wear.

Highly polished disks (10×3 mm) were fabricatedfrom a mold for all manually fabricated groups and milledfor the CAD/CAM group. All disks were subjected to theVickers microindentation test under a load of 4.9 N for 30seconds contact time. Indentation diagonals were used tocalculate surface hardness.38

A plastic mold (18×3×2 mm) was used to prepare bar-shaped specimens of the manual interim materials,whereas the CAD/CAM milling block was sectioned witha precision cutting machine (IsoMet 5000 Linear preci-sion saw; IsoMet Corp) to obtain bars of the previousdimensions. Specimens were loaded in a 4-point bendingtest (18- and 10-mm roller supports) until fracture with auniversal testing machine (Accuforce Elite test stand:Ametek) at a crosshead speed of 0.5 mm/second.39

Fracture resistance was measured by applyingaxial load to the cemented interim restorations with auniversal testing machine (Accuforce Elite test stand;

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Ametek) loaded at a crosshead speed of 1 mm/minuteuntil failure occurred. The load deformation curve wasused to accurately mark the fracture point.40

Cemented interim restorations were thermocycled(50 000 cycles, 5 to 60�C, and dwell time of 3 minutes) andimmersed in methylene blue dye for 24 hours. Specimenswere sectioned with a precision cutting machine (IsoMet5000 linear precision saw; IsoMet Corp), and cut sectionswere examined under an optical microscope (SZ 11;Olympus Corp) at ×30 magnification to evaluate theextent of dye penetration. Dye penetration was tracedalong the cement film thickness in micrometers.

Data were analyzed with 1-way analysis of variance(ANOVA) and the Bonferroni post hoc test (a=.05) withstatistical software (SPSS v10; SPSS Inc). According tosample size (n=20 per test), medium effect size, the testof choice had adequate power to detect clinically relevantdifferences. To determine the homogeneity of variance,F-tests were used. Where an assumption of homogeneityof variance for statistical analysis was violated, multiple F-tests were used with adjusted table values where indi-cated to confirm the ANOVA results.41 The alpha levelfor statistical significance was fixed at .05.

RESULTS

Colorimetric analysis of data revealed unacceptable colorchange of (DE > 7.7) in manually fabricated specimens,whereas CAD/CAM specimens maintained their originalcolor (DE < 2.4). All manually fabricated specimens wereassociated with a significant decrease in the L value(restorations having darker color) and a significant in-crease in the yellow scale (b*). Moreover, these restora-tions were occasionally associated with observablesurface scratches and air bubbles, which attracted furtherstaining.

Manually fabricated interim restorations were asso-ciated with significantly higher (F=32, P<.02) water

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Figure 2. Scanning electron micrographs (×5000 magnification) showing wear of resin matrix. A, CAD/CAM interim restoration. B, manually fabricatedinterim restoration. CAD/CAM, computer-aided design/computer aided manufacture.

Table 2. Physical and mechanical properties of different test groups

Material DE

WaterSorption(mm/mm3)

Wear(wt%)

Micro-Hardness(VHN)

FlexureStrength(MPa)

FractureResistance

(N)

CAD/CAMCercon

2.1 ±0.2 8.7 ±0.7 0.0012 21.2 ±1 142 ±12 1 289 ±56

Alike 6.7 ±2 28.5 ±2 0.0021 16.3 ±2.1 111 ±9 996 ±45

Acrytemp 7.1 ±1.5 19.6 ±3 0.0015 18.5 ±1 118 ±8 899 ±37

DurAcetal 5.4 ±3.1 7.4 ±1.3 0.0011 19.2 ±2 126 ±2.5 1 179 ±41

CAD/CAM, computer-aided design/computer-aided manufacture; DE, color difference; VHN,Vickers hardness number.

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sorption than acetal-based and CAD/CAM interim res-torations. Image superimposition of specimens indicatedvolumetric material loss at the cusp tips and slopes of allused materials (Fig. 1). Scanning electron microscopy(SEM) images of specimens placed in an accelerated weardevice revealed deterioration of the resin matrix in allspecimens, which was observed to a lesser extent inCAD/CAM-fabricated specimens (Fig. 2). CAD/CAMinterim restorations were also associated with signifi-cantly higher fracture resistance (F=14, P<.02) and sur-face hardness than manually fabricated interimrestorations (Table 2). Stereomicroscopy examination ofthe sectioned specimens demonstrated no dye penetra-tion in any of the tested specimens and no discolorationof the interim cement (Fig. 3).

DISCUSSION

This study investigated and compared material proper-ties of CAD/CAM-fabricated interim crowns with thoseof conventional, manually fabricated interim crowns.Results of the study indicated that CAD/CAM-fabricated crowns presented better color stability,higher mechanical properties, and better fit than theother groups tested.

The composition of the resin matrix and its poly-merization methods may have great effect on its colorstability as discoloration of light-polymerized materialsbegins soon after its exposure to the oral cavity.15

Although there may be differences in the progressionrate depending on their composition, polymerized resinmaterials gradually deteriorate. Moreover, as expected,higher water sorption values were recorded for themanually fabricated interim specimens, which is mostlikely related to the polymerization method, higher re-sidual monomer ratio, and higher porosity.

Published reports show that inferior surface hardness,flexure strength, and fracture resistance are reported

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for acetal-based interim restorations compared withCAD/CAM-fabricated interim restorations.21-23 Howev-er, evidence is lacking with regard to the stability ofacetal-based material under the influence of water, andthis would be a consideration for future investigations.However, all specimens from the acetal-based groupexceeded 600 N, which is the threshold for withstandingocclusal load in the posterior region.42

CAD/CAM PMMA blocks are industrially polymer-ized under optimum manufacturing conditions. Suchconditions offer those interim restoration better me-chanical properties than those that are manually fabri-cated.27-29 Their good mechanical properties represent asolution for long-term interim restorations wherestrength and color stability are required. Moreover, theimproved fit of the milled CAD/CAM products shouldlower the risk of bacterial contamination of the tooth andprevent damage to the pulp from excessive temperaturechanges.43

An interesting finding was the absence of dye pene-tration of all tested specimens after 50 000 cycles ofthermocycling. The interim cement used and the accu-racy of marginal fit (Fig. 3) may well have resulted in anexcellent marginal seal and resisted dye penetration. Thisis in contrast to other reports in which microleakage frominterim restorations was commonly observed after theuse of interim cements.12,13 This is an indication that the

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Figure 3. A, Section of representative CAD/CAM interim restoration. No microleakage was observed (×6 magnification). B, Higher magnification ofcervical region (×20 magnification) CAD/CAM, computer-aided design/computer aided manufacture.

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quality of the conventional interim restoration is tech-nique and experience sensitive and could depend onthese factors. From this perspective, the CAD/CAMfabricated interim crown may be beneficial because of itsreproducibility and accuracy.

CONCLUSIONS

Within the limitations of the present in vitro study,CAD/CAM interim crowns showed improved color sta-bility and physical and mechanical properties comparedto conventionally fabricated crowns. CAD/CAM fab-rication is applicable for long-term clinical interimrestorations.

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Corresponding author:Dr Ryo JimboDepartment of ProsthodonticsMalmö University, 205 06 MalmöSWEDENEmail: ryo.jimbo@mah.se

AcknowledgmentsThe authors acknowledge all students and collaborators involved in the project.

Copyright © 2015 by the Editorial Council for The Journal of Prosthetic Dentistry.

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