Long-term effects of storage and thermal cycling on the marginaladaptation of provisional resin crowns: A pilot study

David Ehrenberg, DDS, MS,a Gabriel I. Weiner,b and Saul Weiner, DDSc

New Jersey Dental School, Newark, NJ

Statement of problem. Provisional resin crowns may be used for an extended period while complex treat-ments are completed. The crowns function intraorally; therefore, moisture absorption and thermal cyclingmay affect the physical properties of acrylic resin, causing a change in marginal gap size.

Purpose. The purpose of this pilot study was to examine the effect of long-term water absorption and thermalcycling on marginal gap size of polymethyl methacrylate copolymer and bis-acrylic composite resin crowns.

Material and methods. Specimens (n=10) were fabricated from 2 acrylic resins: a polymethyl methacrylate(Alike) and a bis-acrylic composite resin (Provitec). Specimens were first fabricated on a metal master die. Cus-tom die stems were fabricated for each specimen from a low-fusing alloy (Cerroblend) to eliminate the factor ofpolymerization shrinkage. Specimens were then fitted to assure a standardized, pre-experimental marginal gaprange of #25 mm. Specimens were stored in a humidor at 37�C and 97% relative humidity for 1 year and sub-sequently thermal cycled (5�C to 60�C, 6-second dwell time, for 8000 cycles). Measurements in micrometers ofthe marginal gap were recorded using a microscope equipped with a digital video camera and image analysis soft-ware before and after treatment. A 2-way analysis of variance with a split design was performed for factors ofmaterials and treatment (a=.05).

Results. For the factor of material, there was no significant difference; however, there was a significant differ-ence between treatments, with a significantly greater increase in marginal gap size after thermal cycling(P,.002).

Conclusion. Provisional crowns made from either a bis-acrylic resin composite or a polymethyl methacrylatecopolymer demonstrated loss of marginal adaptation during a simulated long-term period of service.(J ProsthetDent 2006;95:230-6.)

CLINICAL IMPLICATIONS

Provisional crown restorations fabricated from either a polymethyl methacrylate copolymer or bis-acrylic composite resin material would require periodic evaluation and adjustment to maintainmarginal integrity during an extended treatment period.

Autopolymerized acrylic resin materials have beenextensively studied for use in clinical dentistry.1-16

Provisional crowns fabricated from autopolymerizedresin materials have been used to provide provisionalrestorations for prepared teeth prior to completionof definitive fixed prosthodontic treatment.17,18 Thehallmarks of this type of restoration are ease of fabrica-tion and reliability.19,20 The use of polymethyl methac-rylate (PMMA) copolymer, polyethyl methacrylate(PEMA) copolymer, and related copolymers for pro-visional crown materials have been studied.19,21 Inaddition, techniques to optimize use of these materialshave been evaluated.22,23 Fracture strength and margi-nal adaptation are 2 parameters that are importantpredictors for clinical performance. Fracture strengthprovides an estimate of the performance of a restoration

aAssociate Professor, Department of Restorative Dentistry.bVisiting student, Department of Restorative Dentistry.cProfessor, Department of Restorative Dentistry.

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under masticatory forces.24 Marginal adaptation is ameasure of the polymerization shrinkage, stress release,and plasticization of a material when a crown is ce-mented on a tooth for a period of time.21,25,26

Barghi and Simmons27 and Robinson and Hovijitra 28

demonstrated that relining at the time of fabrication im-proved marginal adaptation. Preston29 suggested thatindirect fabrication on a diagnostic cast also improvedmarginal adaptation. Previous work on the effects ofthermal cycling and occlusal loading in vitro, simulat-ing the oral environment, showed that marginal gapsizes increased after treatment, particularly after ther-mal cycling.21,25,26 Relining, however, reduced thechanges observed from thermal cycling.25,26 Moreover,provisional crowns made from different PMMA co-polymer materials show varying effects upon marginaladaptation.21

More recently, bis-acrylic composite resin materialshave been used for provisional crowns.30 It has been re-ported that these materials are stronger and more dura-ble than conventional acrylic resin materials and should

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be substituted for PMMA copolymer resin materials.30

However, there is little research to support these claims.Currently, complex treatment plans that integrate peri-odontal, endodontic, orthodontic, implant, and pros-thodontic treatment are frequently implemented.18,20

These treatment plans require more time to completeand thus delay placement of definitive restorations,requiring longer service of a provisional crown.21,31

Observations have shown that standard PMMA copoly-mer resin crowns degrade with time.21,25,26,31,32 The re-ports that recommend restriction of PMMA copolymercrowns do so based on clinical observation regardingstaining and wear resistance.33 There is little informa-tion regarding changes in marginal gap over longertime periods. Understanding the nature of thesechanges and the mechanisms underlying them may assistin the design of autopolymerizing materials that aremore effective as long-term provisional restorations.Increased porosity, enlargement of marginal gaps, disso-lution of the cement, and fracture can occur.34 How-ever, a progressive change at the marginal areas forbis-acrylic composite resin has not been documented ex-perimentally. The purpose of this pilot study was to com-pare the effects of long-term storage and thermal cyclingon the marginal gap of crowns made from a PMMA co-polymer with that of crowns made from a bis-acryliccomposite resin.

MATERIAL AND METHODS

There were 2 experimental groups (n=10). In the firstgroup, the crowns were made from a PMMA copolymer(Alike; GC America, Alsip, Ill). The second group ofspecimens were made from a bis-acrylic composite resinmaterial (Provitec; GC America). An ivorine mandibularpremolar tooth (Columbia Dentoform, New York, NY)was prepared for a complete cast crown with a chamferfinish line. In the midsection of the buccal root surfaceof the prepared tooth, 3 parallel 1.0-mm-long verticallines (direct buccal, mesial line angle, and distal lineangle) were marked using a scalpel (Bard-Parker No.15 blade; Becton Dickinson and Co, Franklin Lakes,NJ) as reference points for measurement of the marginalgap size. A metal disk (Sears, Chicago, Ill) was attachedwith a mounting screw to the apical end of the preparedivorine tooth for ease in manipulation (Fig. 1, A and B).

Impressions of both the unprepared and preparedtooth were made using a polyvinyl chloride (PVC)pipe fitting (Sears, Chicago, Ill) measuring 2.4 cm indiameter and 3.6 cm long, with a reduced opening at1 end of the PVC pipe measuring 1.6 cm. The PVCpipe was filled, using the larger-diameter opening, withregular-viscosity vinyl polysiloxane impression mate-rial (Examix; GC America) (Fig. 1, C and D). Both ofthe ivorine teeth were inverted and pressed into theirown impression material–filled PVC pipe. When

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polymerization was complete, the impression materialmolds were pushed out of their PVC pipes and slitopen vertically, and the ivorine teeth were retrieved.Each PVC pipe served as a repositioning sleeve to alignand hold the 2 mold halves. The mold of the preparedtooth was termed the master preparation mold (Fig. 1,C), and the mold of the original, unprepared ivorinetooth was termed the master tooth mold (Fig. 1, D).

The master preparation mold was filled with acrylicresin (Pattern Resin; GC America) and repositioned inthe PVC pipe sleeve, and then the acrylic resin was al-lowed to polymerize. Five patterns were made, trimmed,sprued, invested (Vestra Fine; 3M ESPE, St Paul, Minn)in metal rings, placed into a burn-out oven (Acco-Therm II 850; Jelenko, Armonk, NY) until heat-soakedfor 20 minutes at 621�C, and cast with gas and com-pressed air using casting metal (Technic No. 149; Ney,Hartford, Conn). These metal dies were adjusted andpolished as needed to correct for casting imperfections.

Fabrication of the provisional crowns required themaster tooth mold, the cast metal die, and the appropri-ate provisional crown resin. The crown portion of themaster tooth mold (Fig. 1, D) was filled with a standard-ized acrylic resin mixture, a PMMA copolymer (Alike),or a bis-acrylic composite resin (Provitec). The castmetal die of the preparation was lubricated with mineraloil and inserted into the first half of the mold (Fig. 2, A).Then the second half of the mold was closed and com-pletely repositioned into the PVC pipe sleeve untilpolymerization was complete (20 minutes). After 20minutes, the crown was removed, trimmed, and pol-ished. Each metal die was used for the fabrication of4 provisional crowns.

To eliminate the effects of polymerization shrinkageas a variable in this experiment, a low-fusing metalwith a melting point of 70�C (Cerroblend; CerroMetal, Bellafonte, Pa) was used to fabricate customdies that were accurately adapted to the crown marginfor each provisional resin crown (Fig. 2, B). A 2.4-cmthreaded metal rod (1/4-20 thread; Sears, Chicago,Ill) was joined to the apical end of the provisionalcrown–metal die combination with sticky wax (Kerr,Orange, Calif) in alignment with the long axis of thedie. This threaded metal-die assembly was termed thedie analogue.

A vinyl polysiloxane master die mold was fabricatedfor the crown-die analogue assembly using a techniquesimilar to that described previously for the master toothmold. The mold was removed from the PVC pipe fitting,vertically cut open, and the die analogue and provisionalcrown were removed. The provisional resin crown wasplaced into the left part of the master die mold; themaster die mold was assembled and reinserted into thePVC pipe sleeve. The metal (Cerrometal) was slowlyheated (melting point 70�C) in a metal ladle held overan open flame, then poured into the mold (Fig. 2, B)

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Fig. 1. A, Unprepared ivorine mandibular premolar tooth with metal disk attached to apical end. B, Ivorine mandibular premo-lar tooth prepared for complete cast crown with chamfer finish line and metal disk attached to apical end. Vertical black linesindicate 2 of 3 measurement points: direct facial and mesial line angle. C, Master preparation mold, consisting of PVC pipefitting and split impression of prepared tooth, used to form accurate acrylic resin pattern of prepared ivorine tooth. Half ofimpression of prepared tooth and cut-away section of PVC pipe fitting shown. D, Master tooth mold of original, unpreparedivorine tooth.

Fig. 2. A, Master tooth mold cut-away showing metal die with fabricated resin crown. B, Master die mold cut-away containingcustom metal die and provisional resin crown assembly.

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through the opening at the apical end until filled andallowed to cool. The master die mold was opened, andthe acrylic resin crown-custom die combination wasremoved. The stem of the low-fusing metal die wastrimmed with acrylic burs (Brasseler USA, Savannah,Ga) and rethreaded with a threading die (No. 9F 1/4-20; Sears).

The interface of the provisional crown to the low-fus-ing metal die (Fig. 3, A and B) was inspected to ensure aclose adaptation using a microscope (SZX12 Olympus;Olympus America, Melville, NY) with a 0.5 lens at316 magnification and equipped with a digital imag-ing system (DEI-750; Optronics, Goleta, Calif). Thedigital photo was loaded into a microcomputer, opened,and measured using the image analysis software(Bioquant Nova; Bioquant Image Analysis Corp,Nashville, Tenn). If the marginal gap of the provisionalresin crown was .25 mm, the internal adaptationwas evaluated with a silicone disclosing material (FitChecker; GC America, Alsip, Ill). The binding pointswithin the provisional resin crown were relieved usinga number 6 round bur (Brasseler USA) in a straighthandpiece (KaVo, Lake Zurich, Ill) and the marginalgap of the provisional resin crown remeasured. Addi-tionally, the metal die was burnished against the crownmargin using a number 8 round bur (Brasseler USA)to further ensure a marginal gap of #25 mm. Specimenswith a marginal gap size still .25 mm were remade. Thedie stem of each crown was numbered and the provi-sional crown cemented (Tempbond; Kerr, Orange, Ca-lif) under finger pressure. Excess cement was removedunder 32.5 magnification. All 20 specimens were thenstored in a humidor at 37�C and at 97% relative humid-ity for 12 months. Treatment consisted of thermalcycling using water baths (RTE-220; Thermo Neslab,Inc, Newington, NH) at 5�C and 60�C for 8000 cycles,with a dwell time in each bath of 6 seconds.20 A robotarm (Tech Mover; Questech Inc, Farmington Hills,Mich) moved the specimens between the 2 water baths.Water bath temperatures were verified for accuracy witha sensor (RS-2 Remote Sensor; Thermo Neslab, Inc).During thermal cycling, specimens were placed into aholder made from a plastic container with drainage holesdrilled through its sides and suspended from the robot’sjaw. The robot was activated to move the specimens intoeach water bath for a dwell time of 6 seconds and re-peated the program for 8000 cycles, which were com-pleted in 93.3 hours.

After the thermal cycling was concluded, the marginalgaps of each of the specimens were again imaged and mea-sured using a microscope (SZX12 Olympus; OlympusAmerica) digital imaging system (DEI-750; Optronics)and a microcomputer with software (Bioquant Nova;Bioquant Image Analysis Corp). Measurements wererecorded in a spreadsheet (Excel 2003; Microsoft,Redmond, Wash) after the storage period and the thermal

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cycling procedure. In accordance with a previously pub-lished technique,21,25,26,31,32 each specimen was measuredat 3 sites along the crown-die assembly circumference: the

Fig. 3. Provisional resin crown die interface. A, Precementa-tion; direct facial marginal gap between arrows 4.5 mm. Orig-inal magnification 316. B, Precementation; direct lingualmarginal gap between arrows 4.5 mm. Original magnification316. C, Postcementation and posthumidor storage; directfacial marginal gap between arrows 325.5 mm. Originalmagnification 325.

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mesiofacial line angle, the direct facial, and the disto-facial line angle. Each site was measured 3 times, andthe results were averaged. The operator’s measure-ment technique error, 612 mm, was determined byrepeating a series of 10 marginal gap measurementson the same specimen. A 2-way repeated measuresANOVA with a split design was done for factors ofmaterials (Alike vs. Provitec) and treatment (storagevs. thermal cycling) using statistical software (SPSS11.5; SPSS Inc, Chicago, Ill) with a=.05.

RESULTS

The mean gap sizes at each of the 3 measurement sitesfor both the PMMA copolymer resin (Alike) and bis-acrylic resin composite (Provitec) specimen groups wereaveraged and are listed in Table I. All provisional crownsremained intact and cemented during the storage andtreatment stages. The mean marginal gap size after stor-age for the PMMA copolymer specimens was 322.8 6

81.2 mm, and for the bis-acrylic composite resin speci-mens was 323.4 6 98.4 mm (Fig. 3, C). For the storageportion of the protocol, the marginal gaps of bothmaterials increased to a level that greatly exceeded theacceptable marginal gap size of 50 to 100 mm for ametal-ceramic restoration.3,11,12

Following the initial storage portion of the protocol,the specimens were subjected to 8000 cycles of thermalcycling between 5�C and 60�C, and demonstrated a fur-ther significant enlargement of the marginal gap sizes.This was true for both types of specimens. The meangap sizes at each of the 3 facial sites for both the Alikeand Provitec specimens, after thermal cycling, were aver-aged and are listed in Table II. The mean marginal gapsize for the PMMA copolymer group was 421.7 6

102.3 mm, and for the bis-acrylic composite resin groupwas 499.1 6 192.3 mm. There was no significant differ-ence between materials; however, there was a significantdifference between treatments, with a significantlygreater increase in marginal gap size after thermal cy-cling (P,.002) (Table II).

DISCUSSION

This pilot study evaluated marginal gap, a major pa-rameter related to the clinical performance for provi-sional acrylic resin restorations. The findings suggestthat provisional resin crowns made from bis-acrylic

Table I. Mean marginal gap values (mm) (n=10)

Material

Prethermal cycling

treatment

Postthermal cycling

treatment

Bis-acrylic composite resin 323.4 6 98.4 499.1 6 192.3

PMMA copolymer 322.8 6 81.2 421.7 6 102.3

Average 323.2 6 87.8 460.4 6 155.1

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composite resin are affected in a manner similar tocrowns made from PMMA copolymer. The marginalgap changes were similar for both materials. This findingis significant because the clinical performance of com-posite resin materials used for direct restorations doesnot demonstrate these types of changes in the oral cavityover time,1,9,14 and bis-acrylic provisional crowns, afterwater storage for a week, have shown only a 10- to20-mm increase in marginal gap size.24

There are several possible explanations for these ob-servations. First, the bis-acrylic composite resin materialused for provisional restorations may vary in composi-tion from the direct composite resin restorations by fillerpercentage and filler type. The handling characteristicsdeveloped for the provisional crown material may havebeen tailored for specific needs, such as ease of flow, min-imal polymerization shrinkage, and quick chemicalpolymerization, which may have left the restorative ma-terial more susceptible to dimensional distortion by wa-ter sorption and thermal cycling. Secondly, the bulktechnique used for making a provisional crown differsfrom the incremental technique used with direct com-posite resins, so the overall polymerization shrinkagemay be of a greater percentage for the bis-acrylic provi-sional crown material. Nevertheless, both Alike andProvitec showed marked increases in marginal gaps afterboth storage and thermal cycling. In a previous study,crowns made from Alike stored in the humidor for aweek and treated with thermal cycling demonstrated asmaller increase in marginal gap size than was noted inthis extended storage condition.21 In this study, the syn-ergistic effects of storage and thermal cycling may haveresulted in marginal gap increases of approximately 0.5mm. It was recognized that this storage period was lon-ger than usually observed clinically. However, from a sci-entific point of view, the storage phase of the protocolprovided evidence that the marginal deteriorationpreviously observed for shorter lengths of time will con-tinue to progress and can increase the risk for recurrentcaries and gingival inflammation.

A clinical goal of a provisional restoration is to have aminimal marginal gap and provide the prepared toothstructure with adequate protection from thermal,

Table II. ANOVA statistics for prethermal and postthermalcycling marginal gap data

Source

Type III sum of

squares df

Mean

square F Significance

Treatment pre/

post cycling

188474.3 1 188474.3 12.831 0.002

Between groups 15163.8 1 15163.8 0.884 0.36

Treatment and

group

14688.9 1 14688.9 1.005 0.329

Interaction

Error 264399.5 18 264399.5

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chemical, and bacterial factors.18,20 The literature sug-gests an acceptable marginal gap range of 50 to 100mm for a metal ceramic restoration.3,11,12 The size ofthe marginal gap for a provisional crown should beheld to similar standards, since maintenance of healthygingival and pulpal tissue is an important goal for theprovisional crown as well. The average marginal gapsfound in this study were 323.2 6 87.8 mm after storageand 460.4 6 155.1 mm after thermal cycling. These gapswould not be clinically acceptable since they could resultin a loss of the cement seal, with resultant dentinalsensitivity, potential for recurrent decay, and gingivalinflammation.10

The mechanisms underlying the marginal gap en-largement have been discussed in the literature withregard to PMMA copolymer. Perhaps the interactionbetween the heat of fusion of the low-fusing die materialand the resin may have resulted in a change to the mate-rial’s physical properties that increased its susceptibilityto water sorption and distortion of the marginal adapta-tion. The glass transition temperature for the PMMAcopolymer is not readily available, but PMMA has aglass-transitional temperature of 105�F.13 Acrylic resincrowns are observed to contain voids, encounter poly-merization stresses, retain residual unreacted monomer,and demonstrate crack propagation from thermal andocclusal stresses transmitted to the marginal area.1,6,10

These resins also absorb water,21 and the marginal areasare particularly vulnerable because they are thin and con-tain voids and areas of residual monomer that can takeup much of their cross-sectional area.15 The moist envi-ronment of the oral cavity can leach out the residualmonomer, increasing the void concentration in the mar-ginal area of a provisional crown and resulting in areas ofincreased fracture potential.8 Water sorption and sali-vary esterases may reduce the polymer chain lengthsand allow fatigue of the residual resin at the marginalareas.2,33 Fatigue results in a release of residual stressesand loss of resin integrity at the marginal areas.4,5

Temperature fluctuations cause expansion and contrac-tion of the resin at the marginal areas, promoting crackpropagation through areas of porous resin, which, incombination with stress from occlusal forces, couldresult in increases in marginal gap sizes.21

In view of these results, it would appear thatautopolymerizing PMMA copolymers and bis-acryliccomposite resin may react in a similar manner and arenot well suited for use in long-term provisional crowns.Use of materials processed with heat and pressure wouldresult in an improved density with a greater degree ofpolymerization, thereby reducing free monomer andvoids in the resin.17 Such materials may be more suitablefor long-term provisional restorations. However, moreresearch is required to confirm this approach and to de-velop autopolymerizing materials that have improvedphysical properties and are easy to manipulate.

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Application of this study clinically is limited becauseof several considerations. First, the provisional crownsreceived no occlusal loading or thermal cycling duringthe humidor storage period; thermal cycling was doneat the end of the long storage period. The use of fingerpressure to seat the provisional crown during cemen-tation, although used clinically, was not standardizedand may have been an additional uncontrolled variable.Even though numerous studies21,25,26,31,32 have evalu-ated marginal gap-size changes of PMMA copolymermaterials without utilization of a mechanical jig to en-sure uniform cementation pressure for each specimen,future studies may consider use of a mechanical jig tofurther standardize experimental protocols. In addition,marginal gap measurements should be made after fabri-cation, cementation, humidor storage, and thermal cy-cling to more completely document the marginal gapchanges associated with use of provisional resin crowns.Also, more detailed research to follow this pilot studyshould evaluate all surfaces of the crown-die interface.The humidor storage treatment, while interesting, isnot as clinically relevant as combined occlusal loadingand thermal cycling.

CONCLUSION

Within the limitations of this pilot study, the follow-ing conclusions were made:

1. The long-term exposure of provisional resin crownsto moisture is associated with an increased marginalgap size.

2. Thermal cycling of resin crowns, PMMA copolymerresin or bis-acrylic resin composite, is associatedwith significant (P,.002) increases in marginal gapsize.

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NEW JERSEY DENTAL SCHOOL

110 BERGEN STREET

NEWARK, NJ 07103

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Copyright � 2006 by The Editorial Council of The Journal of Prosthetic

Dentistry.

doi:10.1016/j.prosdent.2005.12.012

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