Comparison of temperature increase in the pulp chamber during the polymerization of materials used for the direct fabrication of provisional restorations

Comparison of temperature increase in the pulp chamber during the polymerization of materials used for the direct fabrication of provisional restorations Konstantinos Michalakis, DDS, PhD,a Argiris Pissiotis, DDS, MS, PhD,b Hiroshi Hirayama, DDS, DMD, MS,c Kiho Kang, DDS, DMD, MS,d and Nikolaos Kafantaris, DDS, PhDe Tufts University, School of Dental Medicine, Boston, Mass; Aristotle University, Thessaloniki, Greece Statement of problem. Polymerization of resin materials used for the fabrication of provisional restorations is associated with an exothermic reaction. This temperature rise may present a serious biological problem, since it can cause iatrogenic thermal trauma to the pulp. Purpose. This in vitro study compared the temperature increase in the pulp chamber of a molar placed in contact with different resins used for the direct fabrication of provisional restorations. Material and methods. Polymethyl methacrylate (Jet), polyethyl methacrylate (Snap), polyvinylethyl methacrylate (Trim), Bis-acrylic composite (Protemp II), and a VLP urethane dimethacrylate (Revotec LC) were compared with respect to their exothermic reaction properties during polymerization. A mandibular molar prepared for a complete coverage restoration was placed in an acrylic resin block. A thermal probe connected to a digital thermometer was placed into the pulp chamber. Specimens were submerged in a water bath to simulate intraoral conditions. The provisional resin materials tested were measured and mixed according to manufacturer’s instructions. The resin mixture was placed into a vacuum-formed acetate template and was then positioned on the prepared molar tooth. The temperature was recorded during polymerization at 30-second intervals until it was evident that the peak temperature had been reached. Temperature increase was measured (8C) for both the initial crown fabrication and the reline procedures. Data were analyzed with descriptive statistics, 1-way analysis of variance, and Tukey Honestly Significant Difference tests (a=.05). Results. One-way ANOVA revealed significant differences (F=57.010, P,.0001) in temperature rise for different provisional resin materials. Mean temperature increase for the provisional crown fabrication ranged from 37.768C for the polyvinylethyl methacrylate to 39.408C for the polymethyl methacrylate. Mean temperature rise for the reline procedures ranged from 36.808C for the polyvinylethyl methacrylate to 37.698C for the polymethyl methacrylate. All of the tested materials produced an exothermic chemical reaction. Conclusions. Polymethyl methacrylate produced the higher exothermic reaction in both initial crown fabrication and reline procedures. Polyethyl methacrylate, polyvinylethyl methacrylate, and Bis-acrylic resins tested were not significantly different from each other. (J Prosthet Dent 2006;96:418-23.)

CLINICAL IMPLICATIONS Clinicians should be aware of increased temperature levels associated with the direct fabrication of provisional restorations and take necessary precautions to minimize iatrogenic trauma to the pulp.

T

he pulp can be subjected to lesions resulting from caries or trauma. The latter can be iatrogenic, resulting from removal of previous restorations,1 tooth

a

Visiting Assistant Professor, Division of Graduate and Postgraduate Prosthodontics, Tufts University, School of Dental Medicine; Clinical Associate, Department of Fixed Prosthodontics, Aristotle University; Private practice, Thessaloniki, Greece. b Associate Professor, Department of Removable Prosthodontics, School of Dentistry, Aristotle University. c Professor, Director of Graduate and Postgraduate Prosthodontics, Tufts University, School of Dental Medicine. d Associate Professor and Associate Director of Graduate and Postgraduate Prosthodontics, Tufts University, School of Dental Medicine. e Professor and Head, Department of Removable Prosthodontics, School of Dentistry, Aristotle University.

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preparation procedures,2,3 desiccation,4 or fabrication of provisional restorations.5,6-10 Provisional restorations are used between the initiation of the treatment and placement of the definitive prostheses. Their purpose is the stabilization of occlusion, protection of the teeth and the periodontal tissues, function, esthetic enhancement, and provision of diagnostic information.11-13 Treatment periods may be rather extensive due to orthodontic, periodontic, and endodontic therapies or implant placement.14,15 Evaluation of strategic teeth, occlusal scheme, or alterations of a patient’s occlusal vertical dimension are additional reasons for lengthy therapeutic periods.1 During the provisional restorative phase, the clinician should strive to ensure that all the goals which should VOLUME 96 NUMBER 6

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be fulfilled by an interim prosthesis are achieved. These goals can be described as biologic, diagnostic, esthetic, and mechanical. However, it should be noted that although provisional restorations offer a wide range of therapeutic objectives, their fabrication should be done with care, since the materials used may be detrimental to the pulp if certain precautions are not taken. The fabrication of treatment restorations with a direct technique presents 2 major problems. The first problem consists of the presence of free monomer, which can be harmful to the pulp, especially when placed in direct contact with open dentinal tubules.5 Secondly, most of the materials used for provisional restoration introduce a temperature rise during polymerization.6-10 This temperature rise may present a biological problem, since a previous histologic animal study has demonstrated that healthy pulps failed to recover from an intrapulpal temperature rise of 5.558C in 15% of the situations. When the intrapulpal temperature increased by 11.18C and 16.658C, 60% and 100% of the teeth, respectively, lost vitality.16 Thus, it appears that a material that does not produce an exothermic reaction should be selected or certain precautions should be taken to protect healthy pulpal tissue from losing vitality. The purpose of this in vitro study was to compare the temperature increase in the pulp chamber of a molar placed in contact with different resins used for the direct fabrication of provisional restorations.

MATERIAL AND METHODS A polymethyl methacrylate (PMMA), a polyethyl methacrylate (PEMA), a polyvinylethyl methacrylate (PVEMA), a Bis-acrylic composite, and a visible-lightpolymerizing (VLP) urethane dimethacrylate were compared with respect to their exothermic reaction properties during polymerization (Table I). The materials included in the study were chosen because they are widely used both in the United States and the European Union. A freshly extracted mandibular molar tooth was stored in 1% chloramine solution (Sigma-Aldrich, St. Louis, Mo) for 2 weeks. The tooth was then stabilized with a minimal amount of sticky wax (Kemdent; Purton, Swindon, UK) in a vertical position in a 4.5 3 2.5 3 0.4-mm box made of boxing wax (Hygenic Corp, Akron, Ohio). Autopolymerizing acrylic resin (ProBase; Ivoclar Vivadent, Schaan, Liechtenstein) was then hand mixed and poured under vibration (No. 200 Vibrator; Buffalo Dental Mfg Co, Syosset, NY) in the wax box, covering the root portion of the tooth. The resin block was then placed in a pressure-polymerization apparatus (Ivomat; Ivoclar Vivadent) for 15 minutes at 408C under 4 bar pressure. After complete polymerization of the acrylic resin, an impression of the tooth and the resin block was made using a custom tray made of DECEMBER 2006

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Table I. Resin materials included in study Brand

Jet

Material type

Polymethyl methacrylate Snap Polyethyl methacrylate Trim Polyvinylethyl methacrylate Protemp II Bis-acrylic composite Revotec LC VLP urethane

Manufacturer

Lot/batch no.

Lang Dental, Wheeling, Ill Parkell Biomaterials, Farmingdale, NY Harry Bosworth, Skokie, Ill 3M ESPE, Seefeld, Germany GC Corp, Tokyo, Japan

1410-5063 46665 04257 233479 509131

autopolymerizing PMMA (SR Ivolen; Ivoclar Vivadent) and irreversible hydrocolloid material (Blueprint; Dentsply Caulk, Milford, Del). Type III stone (COECAL; GC Europe, Leuven, Belgium) was mixed under vacuum (Vacuum Power Mixer Plus; Whip Mix Corp, Louisville, Ky) and the impression was poured under vibration (No. 200 Vibrator; Buffalo Dental Mfg Co). After setting of the stone, an acetate template (Coping Material; Keystone, Cherry Hill, NJ) was fabricated using a thermal vacuum-forming machine (Tray-Vac; Buffalo Dental). Twenty-four hours later, the tooth was prepared for a complete coverage restoration with a 10-degree axial wall convergence17 and a 360-degree chamfer finish line (Fig. 1). The reduction of the axial walls was 1.5 mm, and the occlusal reduction was 2.0 mm. The reduction was evaluated with the acetate vacuum-formed template fabricated earlier. The apical portion was removed to access the pulp chamber from the apical side of the tooth, and the pulpal tissues were removed. The pulp chamber was then cleaned of all organic remnants using a 5.25% sodium hypochlorite solution (Clorox Co, Oakland, Calif).18 A thermal probe connected to a digital precision thermometer (BAT 8; Bailey Instruments Inc, Saddle Brook, NJ) was placed through the root canal into the pulp chamber, touching the roof of the chamber.9 Amalgam (Dispersalloy; Dentsply Caulk) was then condensed into the pulp chamber, surrounding and stabilizing the thermal probe in position (Fig. 2).19 Afterward, the acrylic resin was replaced over the apical opening. Amalgam condensation around the thermal probe permitted the collection of readings from all directions, since the amalgam is a good thermal conductor.20 All thermal energy released from the exothermic reaction of the acrylic resin polymerization was transmitted from the occlusal and the axial walls to the thermal probe, through the amalgam. A thin layer of petroleum lubricant was applied to the acrylic resin block/prepared tooth/engaged thermal probe assembly, which was then placed in a water bath (Water Bath; Whip Mix Corp, Louisville, Ky) containing distilled water at a temperature of 368C. The assembly was then allowed to 419

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Table II. Descriptive statistics for the temperature increase (8C) (n=10) 95% Confidence interval for mean

PMMA I PMMA R PEMA I PEMA R PVEMA I PVEMA R Bis I Bis R VLP I VLP R Total

Mean

SD

SE

Lower bound

Upper bound

Minimum

Maximum

39.49 37.69 38.31 36.84 37.76 36.80 37.77 36.82 38.83 37.16 37.75

.81 .25 .43 .27 .25 .16 .45 .17 .18 .34 .94

.26 .08 .13 .08 .08 .05 .14 .05 .06 .10 .09

38.90 37.51 38.01 36.65 37.58 36.68 37.44 36.70 38.70 36.91 37.56

40.07 37.87 38.61 37.03 37.94 36.92 38.09 36.94 38.96 37.40 37.93

38.20 37.30 37.80 36.50 37.40 36.60 37.10 36.60 38.50 36.70 36.50

40.50 38.10 39.00 37.30 38.20 37.10 38.60 37.10 39.10 37.60 40.50

I, Initial provisional crown fabrication; R, reline procedure.

Fig. 1. Molar prepared with 360-degree chamfer preparation.

Fig. 2. Radiograph depicting thermal probe and amalgam condensed into pulp chamber.

thermally equilibrate. The water bath was used to simulate intraoral conditions.21,22 The provisional resin materials tested were measured and mixed according to the manufacturer’s instructions. An electronic scale (Galaxy 110; Ohaus, Pine Brook, NJ), with an accuracy of 60.0001 g, was used to weigh the polymer of each resin material. A 1-mL syringe (BS01H2516; Terumo Europe NV, Leuven, Belgium) was used for the measurement of the monomer. The Bisacrylic composite resin was dispensed from the calibrated syringe provided by the manufacturer, while the VLP urethane dimethacrylate was used as provided by the manufacturer. The vacuum-formed acetate template was completely filled with the resin mixture and then positioned on the prepared molar tooth. All excess resin material was removed from the margins of the tooth by the use of an explorer. The temperature was recorded during polymerization at 30-second intervals until no further increase was noted.19 After complete polymerization of the resin material, the template was

removed from the tooth, the provisional crown was retrieved, and the intaglio surface was ground. The assembly was cleaned of any resin residue, and a new thin layer of petroleum lubricant was applied. Afterwards, the assembly was placed in the water bath to thermally equilibrate. A new resin mixture was measured and mixed, as previously described, and placed into the provisional crown. The crown was then positioned onto the prepared molar, the excess was removed from the margins, and the assembly was placed immediately in the water bath. The temperature was again recorded at 30-second intervals until no further evidence of increase was noted.19 The procedure for the VLP urethane dimethacrylate was the same as for the rest of the tested materials, except that a light-emitting diode (LED) polymerizing unit with a wavelength of 430 to 480 nm and a light intensity of approximately 1000 mW/cm2 (Elipar FreeLight 2 LED; 3M ESPE, St. Paul, Minn) was used for the polymerization of this material.23-29 The tip was held

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Fig. 3. Box plots of temperature increase values (8C).

Table III. One-way ANOVA for intrapulpal temperature increase with different acrylic resins Temperature

Between groups Within groups Total

Sum of squares

df

74.55

Table IV. Tukey HSD test for intrapulpal temperature rise (8C) (n=10)

Mean square

F

P

9

8.28

57.01

,.0001

13.08

90

.15

87.63

99

Subset for alpha=.05

at a 2-mm distance from the provisional crown surface, and a polymerizing time of 45 seconds was used for each of the 5 surfaces. Ten provisional crowns were fabricated for each material and 10 specimens of each material for the reline measurements, providing a total of 100 temperature rise recordings.1 The room temperature was constantly 218C 6 18C. Descriptive statistics, 1way analysis of variance (ANOVA) (a=.05), and Tukey Honestly Significant Difference (HSD) (a=.05) tests were used to determine statistically significant differences in temperature rise for different provisional resin materials.

RESULTS The results of the descriptive statistics for the temperature increase values of the resin materials included in the study are depicted in Table II and Figure 3. The 1-way ANOVA revealed significant differences (F= 57.010, P,.0001) in temperature rise for the provisional resin materials tested (Table III). All of the tested materials produced an exothermic chemical reaction. Mean temperature increase for the provisional crown fabrication ranged from 37.768C for the PVEMA to DECEMBER 2006

Material

PVEMA R BIS R PEMA R VLP R PMMA R PVEMA I BIS I PEMA I VLP I PMMA I Sig.

1

36.80 36.82 36.84 37.16

.524

2

37.16 37.69

.072

3

37.69 37.76 37.77

1.000

4

37.76 37.77 38.31

.053

5

6

38.31 38.83 .084

39.49 1.000

Mean values for groups in homogeneous subsets are displayed. I, Initial provisional crown fabrication; R, reline procedure.

39.408C for the PMMA. Mean temperature rise for the reline procedures ranged from 36.808C for the PVEMA to 37.698C for the PMMA. The Tukey HSD test revealed that PVEMA, Bis-acrylic, PMMA, and VLP urethane dimethacrylate resins were not significantly different when compared in the reline procedure stage. However, the VLP urethane dimethacrylate was significantly different (P,.0001) when compared to PVEMA and Bis-acrylic resins in the initial provisional crown fabrication procedure. The PMMA was significantly different (P,.0001) from the majority of the tested materials, but not from the VLP urethane dimethacrylate. The reline temperature increase of the PMMA resin was not statistically different from those of the PVEMA and Bis-acrylic resins in the initial provisional crown fabrication stage (Table IV). 421

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DISCUSSION This in vitro study demonstrated that PMMA resin produced the highest exothermic reaction of all materials tested. The maximum temperature recorded in this study was 40.508C, and the lowest was 36.508C. A previous study10 reported temperatures ranging from 33.308C to 53.308C. However, the quantity of the provisional resin material used in that study was much larger than that used for the fabrication of a provisional crown, since a polyvinyl mold, the approximate size a maxillary molar size, was filled with the materials tested. Furthermore, that study was not performed in a wet environment of 368C, and the light source was kept away from the thermometer. Nevertheless, the present study confirms the results of previous studies,8,10,19 that PMMA demonstrates the largest temperature increase when compared to other provisional resin materials. Additionally, the results of the current study are similar to those of Moulding and Teplitsky.19 An interesting finding of the present study is that the temperature rise recorded with VLP urethane dimethacrylate is higher than that of PMMA, PVEMA, and Bisacrylic resins. It has been previously reported that the heat release of these materials is less than that of other provisional resin materials.10 Yet, it seems that the polymerization of the VLP urethane dimethacrylate leads to a temperature rise which is the result of both the exothermic reaction process and the energy absorbed during the irradiation.23-27 The temperature increase recorded during the reline procedure of the VLP urethane dimethacrylate was not different from that of the other provisional resin materials tested, with the exception of the PMMA resin. It should be mentioned, however, that the LED polymerizing unit may have contributed to the temperature increase observed during the polymerization of the VLP urethane dimethacrylate material. It is possible that the temperature increase would have been smaller if a conventional halogen light had been used.28 Nevertheless, this requires further research since there is a study supporting the opposite finding.29 Zach and Cohen16 demonstrated that a temperature increase of 2.38C produced minimal intrapulpal changes. These were confined to the odontoblasts that were next to the area of the thermal injury. When the intrapulpal temperature increased by 5.68C, a remarkable pulpal response was noted, including destruction of most of the odontoblasts, displacement of nuclei into the dentinal tubules, reduction of the thickness of uncalcified predentin, denaturation of the matrix, and destruction of TomesÕ fibrils in the area of dentin immediately adjacent to the enamel that was touched by the instrument causing the thermal injury. However, after 56 days, 85% of the teeth subjected to this thermal trauma successfully overcame the inflammatory

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reaction. The present study demonstrated that the temperature increase resulting from the polymerization process of the provisional resin materials ranged between 1.18C and 4.58C for the initial crown fabrication, and between 0.58C and 2.18C for the reline procedures. According to Zach and Cohen,16 the intrapulpal changes resulting from the temperature increase during polymerization of the materials will range from minimal to significant. The majority of the pulp tissues in their study recovered. However, a number of properties should be considered when making a material selection, one being the presence of a low exothermic reaction. Precautions that may minimize thermal trauma to teeth include the use of molds other than vacuum-formed templates, such as vinyl polysiloxane or irreversible hydrocolloid impressions, as demonstrated by Moulding and Teplitsky.19 According to that study, when autopolymerizing resins are used in combination with molds made from siloxane and irreversible hydrocolloid materials, they present lower temperature rises because the bulk of these materials can act as a heat sink and, thus, dissipate heat from the tooth. Additionally, the water present in the irreversible hydrocolloid provides a cooling medium. Other safety measures include the repeated removal and replacement of the template on the prepared teeth and the use of air-water spray to minimize the heat increase. The present study has also demonstrated that the intrapulpal temperature rise is always smaller during reline procedures. This is probably due to the fact that a smaller volume of resin material is used for relining. Since the temperature increase during relining is always smaller, it would be beneficial to fabricate the provisional restorations on a stone cast and then perform the reline procedures on the prepared teeth. These precautions may help avoid iatrogenic trauma to the pulp. It should be noted that this is an in vitro study and, therefore, has some limitations. These include the use of amalgam in the pulp chamber and the use of acrylic resin to surround the prepared tooth. The use of a water bath was deemed necessary to simulate intraoral conditions. Further in vivo animal studies are necessary to verify the results of this in vitro study.

CONCLUSIONS Within the limitations of this in vitro study, the following conclusions were drawn: 1. The PMMA resin produced a significantly higher (P,.0001) exothermic reaction when compared to the rest of the materials included in this study. This finding applied to both the initial crown fabrication and the reline procedures. 2. The PEMA, PVEMA, and Bis-acrylic resins were not significantly different in terms of temperature rise

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for either the initial provisional crown fabrication (P=.053) or the reline procedures (P=.0524). 3. The VLP urethane dimethacrylate produced a significantly higher (P,.0001) intrapulpal temperature rise than that of PVEMA and Bis-acrylic resins in the initial crown fabrication. However, no significant difference in temperature rise was detected (P=.0524) when the previously mentioned materials were compared in the reline procedure. REFERENCES 1. Wittrock JW, Morrant GA, Davies EH. A study of temperature changes during removal of amalgam restorations. J Prosthet Dent 1975;34:179-86. 2. Bhaskar SN, Lilly GE. Intrapulpal temperature during cavity preparation. J Dent Res 1965;44:644-7. 3. Forsell-Ahlberg K, Edwall L. Influence of local insults on sympathetic vasoconstrictor control in feline dental pulp. Acta Odont Scand 1977;35:103-10. 4. Morrant GA. Dental instrumentation and pulpal injury. II—clinical considerations. J Brit Endod Soc 1977;10:55-63. 5. O’Brien WJ. Dental materials and their selection. 3rd ed. Chicago: Quintessence; 2002. p. 121-2. 6. Wolcott RB, Paffenbarger GC, Schoonover IC. Direct resinous filling materials: temperature rise during polymerization. J Am Dent Assoc 1951;42: 253-63. 7. Grossman LI. Pulp reaction to the insertion of self-curing acrylic resin filling materials. J Am Dent Assoc 1953;46:265-9. 8. Plant CG, Jones DW, Darvell BW. The heat evolved and temperatures attained during setting of restorative materials. Brit Dent J 1974;137:233-8. 9. Grajower R, Shaharbani S, Kaufman E. Temperature rise in pulp chamber during fabrication of temporary self-curing resin crowns. J Prosthet Dent 1979;41:535-40. 10. Driscoll CF, Woolsey G, Ferguson WM. Comparison of exothermic release during polymerization of four materials used to fabricate interim restorations. J Prosthet Dent 1991;65:504-6. 11. Rosenstiel SF, Land MF, Fujimoto J. Contemporary fixed prosthodontics. 4th ed. St. Louis: Mosby; 2006. p. 466-504. 12. Malone WF, Koth DL. Tylman’s theory and practice of fixed prosthodontics. 8th ed. St. Louis: Ishiyaku EuroAmerica; 1989. p. 255-71. 13. Shillingburg HT, Hobo S, Whitsett LD, Jacobi R, Brackett SE. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence; 1997. p. 225-7. 14. Lui JL, Setcos JC, Phillips RW. Temporary restorations: a review. Oper Dent 1986;11:103-10. 15. Fisher DW, Shillingburg HT Jr, Dewhirst RB. Indirect temporary restorations. J Am Dent Assoc 1971;82:160-3. 16. Zach L, Cohen G. Pulp response to externally applied heat. Oral Surg Oral Med Oral Pathol 1965;19:515-30.

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17. Goodacre CJ, Campagni WV, Aquilino SA. Tooth preparations for complete crowns: an art form based on scientific principles. J Prosthet Dent 2001;85:363-76. 18. Beltz RE, Torabinejad M, Pouresmail M. Quantitative analysis of the solubilizing action of MTAD, sodium hypochlorite, and EDTA on bovine pulp and dentin. J Endod 2003;29:334-7. 19. Moulding MB, Teplitsky PE. Intrapulpal temperature during direct fabrication of provisional restorations. Int J Prosthodont 1990;3:299-304. 20. Anusavice KJ. PhillipsÕ science of dental materials. 11th ed. Philadelphia: W. B. Saunders; 2003. p. 495-543. 21. Besnault C, Attal JP. Influence of a simulated oral environment on dentin bond strength of two adhesive systems. Am J Dent 2001;14:367-72. 22. Walker MP, Spencer P, Eick JD. Effect of simulated resin-bonded fixed partial denture clinical conditions on resin cement mechanical properties. J Oral Rehabil 2003;30:837-46. 23. Mc Cabe JF. Cure performance of light-activated composites by differential thermal analysis (DTA). Dent Mater 1985;1:231-4. 24. Lloyd CH, Joshi A, McGlynn E. Temperature rises produced by light sources and composites during curing. Dent Mater 1986;2:170-4. 25. Masutani S, Setcos JC, Schnell RJ, Phillips RW. Temperature rise during polymerization of visible light-activated resins. Dent Mater 1988; 4:174-8. 26. Smail SR, Patterson CJ, McLundie AC, Strang R. In vitro temperature rises during visible-light curing of a lining material and a posterior composite. J Oral Rehabil 1988;15:361-6. 27. Hansen EK, Asmussen E. Correlation between depth of cure and temperature rise of a light-activated resin. Scand J Dent Res 1993;101:176-9. 28. Hannig M, Bott B. In-vitro pulp chamber temperature rise during composite resin polymerization with various light-curing sources. Dent Mater 1999;15:275-81. 29. Tarle Z, Meniga A, Knezevic A, Sutalo J, Ristic M, Pichler G. Composite conversion and temperature rise using a conventional, plasma arc, and an experimental blue LED curing unit. J Oral Rehabil 2002;29:662-7. Reprint requests to: DR KONSTANTINOS MICHALAKIS 3, GREG. PALAMA STR. THESSALONIKI 546 22 GREECE FAX: 30 2310 272-228 E-MAIL: [email protected] 0022-3913/$32.00 Copyright Ó 2006 by The Editorial Council of The Journal of Prosthetic Dentistry.

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