JPOR-250; No. of Pages 7 journal of prosthodontic research xxx (2014) xxx–xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study Manoj Kumar Sundar MDS*, Satish Babu Chikmagalur MDS1, Fayaz Pasha MDS2 Department of Prosthodontics, V.S. Dental College & Hospital, K.R. Road, V.V. Puram, Bangalore 560004, Karnataka, India
article info
abstract
Article history:
Purpose: This study aimed to compare the marginal fit and microleakage of metal laser
Received 19 December 2013
sintered Co–Cr alloy copings and conventional cast Ni–Cr alloy copings using a stereomi-
Received in revised form
croscope.
14 July 2014
Methods: Forty extracted maxillary premolars were randomly divided into two groups. One
Accepted 22 July 2014
group was subjected to coping fabrication using conventional lost wax (LW) technique while
Available online xxx
the other group was subjected to coping fabrication using metal laser sintering (MLS)
Keywords:
addition using images obtained with a steromicroscope and an ImageJ analysis software. All
Crowns
the specimens were cemented using Type 1 glass ionomer cement and were subjected to
technology. The marginal fit of these copings were compared before and after ceramic
Marginal fit
thermocycling. The specimens were evaluated for microleakage using stereomicroscope
Microleakage
and 2% methylene blue die used as a tracer. The data were subjected to statistical analysis
Laser sintering
using paired t-test, Mann–Whitney test and Chi-Square test.
Thermocycling
Results: The mean marginal fit of copings before and after ceramic addition in Group B (MLS) was better than the copings in Group A (LW) and was statistically significant (P < 0.05). The influence of ceramic firing had a significant (P < 0.05) increase in mean marginal gap in Group A (LW) but not in Group B (MLS). And the difference in mean microleakage between the groups was not statistically significant (P 0.05). Conclusion: The copings fabricated using MLS technique had a better marginal fit and an observable decrease in microleakage when compared to the copings fabricated using the conventional lost wax (LW) technique. # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
* Corresponding author. Tel.: +91 9789525412/044 24413908. E-mail addresses: [email protected], [email protected] (M.K. Sundar), [email protected] (S.B. Chikmagalur), [email protected] (F. Pasha). 1 Tel.: +91 9448458424. 2 Tel.: +91 9535208181. http://dx.doi.org/10.1016/j.jpor.2014.07.002 1883-1958/# 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
JPOR-250; No. of Pages 7
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journal of prosthodontic research xxx (2014) xxx–xxx
1.
Introduction
Today a vast majority of restorative procedures in fixed prosthodontics are by metal ceramic restorations [1]. The greatest disadvantage of the metal ceramic restoration is indicated to be the marginal fit associated with metal casting and firing porcelain onto it. Deterioration of the initial fit of the metal coping has been observed after the porcelain firing cycle. Studies on marginal distortion have identified many factors, such as the mismatch of porcelain-metal thermal contraction, alloy type and margin design, as contributing to the distortion. Considerable controversy still exists with regards to the influence of ceramic firing on the marginal fit of metal ceramic crowns [2]. A paradigm shift in the procedure for fabricating the metal coping onto which the porcelain can be fired, has come into vogue with the introduction of metal laser sintering (MLS) technology. This metal laser-sintering (MLS) system is an additive metal fabrication technology, based on information received from three dimensional CAD, in which metal powder is shot selectively using a data file and fused with a laser to laminate approximately a 20–60 mm thick layer with each shooting to complete a metal structure. Advantages of the MLS system include the ease with which it can be fabricated and eliminating a major disadvantage of the lost wax technique, i.e., the casting shrinkage [3]. Other advantages include easy fabrication of complicated shapes, operation of an automatic system, and short working time due to elimination of the procedures of fabricating a wax pattern, investing, burning, and casting works. While an essential condition for a successful dental prosthesis is good marginal fit and least microleakage [4], there is little data on the marginal fit and microleakage of fixed dental prostheses (FDPs) fabricated by the MLS system as compared to the conventional casting procedures [5]. Hence the purpose of this study was to compare the marginal fit and microleakage between the copings fabricated using conventional lost wax technique and metal laser sintering technology.
stops were cut in between the samples and the vacuum formed sheet acted as a spacer for making a two stage putty wash impression. A definitive impression was carried out using a two stage putty wash impression technique [6]. The impression was casted with Type 4 die stone (Kalrock, Kalabhai) and the dies were fabricated. Potable water [7] was used as the storage media for the specimens during the period of study.
2.2.
Fabrication of copings
The dies of Group A specimens were coated with two layers of die spacer (DVA Diespacer blue) corresponding to 30 mm [8]. A layer of vaseline was applied over the die models and pattern resin (GC America) was used to fabricate patterns for copings. The thicknesses of the patterns were randomly checked at multiple points using a metal gauge and were kept at a uniform thickness of 0.4 mm The finished resin patterns of the copings of Group A were invested and casted using conventional lost wax (LW) technique with Ni–Cr alloy pellets. The copings for Group B specimens were fabricated using metal laser sintering (MLS) technology (EOS M270). EOS has developed a biocompatible cobalt chrome alloy especially for dental prostheses. It is a cobalt chrome molybdenum-based superalloy designed specifically for porcelain fused to metal (PFM) restorations. The material is available in powder form and has a coefficient of thermal expansion of 14.0–14.5 10 6 m/m 8C and thus provides optimum adhesion of commercial ceramics. The metal laser sintered copings were fabricated using this Co–Cr alloy with the thickness of the coping kept a constant of 0.4 mm with an internal relief of 30 mm, a space for the luting cement. All these necessary data were fed to the software (EOS RP tools) for selective laser sintering. Once the parameters were set a constant, all the die specimens were scanned using the 3D scanner (Scanner 3Shape) and the CAD design of the coping was obtained as an STL data. This data was then used in EOS RP tools software for fabrication of copings using laser sintering technology by vertical stacking of the alloy powder with each increment of layer 20–60 mm thick.
2.
Materials and methods
2.3.
2.1.
Specimen preparation and die fabrication
The obtained copings of the specimens in Groups A and B were transferred onto their respective tooth specimen and analyzed for marginal fit using pictures obtained from a Nikon digital camera mounted to a stereomicroscope [9] (Olympus CHi20) set at 40 magnification and the measurements were calculated (in mm) using ImageJ analysis software. Four images were captured for each specimen which includes four sides of the specimen namely buccal, lingual, mesial and distal. The ImageJ analysis software was first calibrated (Fig. 1) to micrometer scale and the obtained images were analyzed for the marginal fit by marking the distance from the tooth margin to the coping margin. Each sample had four values one from each side of the specimen namely, mid-buccal, mid-lingual, mid-mesial and mid-distal [10]. The values were recorded at the near mid-point of the specimen on each side. This was standardized as the image magnification was same, grids were
Forty extracted, non-carious permanent maxillary premolars were randomly divided into two groups Group A and Group B. All the premolar specimens were prepared to receive a full coverage metal–ceramic restoration with 1.5 mm functional cusp reduction, 1 mm non-functional cusp reduction, 1.2 mm overall axial wall reduction and 1 mm heavy chamfer finish line. All the specimens were mounted in dental plaster in the shape of a horse-shoe, mimicking a dental arch with ten samples in each arch. Four such arches were obtained to facilitate making impressions similar to an intra-oral impression. Each of these arches was pressed with a 1 mm thick soft vacuum formed sheet which provided the space for the light body consistency addition silicone (Aquasil ultra). The pressed vacuum formed sheet was cut to confine only the arch. Four
Assessment of marginal fit
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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Fig. 1 – Calibration of ImageJ analysis software.
Fig. 3 – Stereomicroscopic image of metal ceramic crown on prepared tooth specimen showing marginal gap.
Table 1 – Microleakage scoring criteria [14]. Score
Characteristics
0 1 2 3
No leakages Leakage to one third of axial wall Leakage to two thirds of axial wall Leakage along the full length of axial wall Leakage over occlusal surface
4
Fig. 2 – Stereomicroscopic image of metal copings on prepared tooth specimen showing marginal gap.
used in the software to locate the near mid-point on all four sides. The mean value was recorded as the mean marginal fit in mm (Fig. 2). All the copings of the specimens in Group A and Group B were layered with Duceram plus ceramic material using a conventional layering technique for fabrication of a metal ceramic restoration. All the forty metal ceramic crowns were then assessed for their marginal fit (Fig. 3).
2.4.
Assessment of microleakage
All the metal ceramic crowns were cemented onto their respective specimens using Type 1 glass ionomer cement
(GC gold label luting and lining cement). The cementation procedure was carried out with application of constant pressure of 6 kg (60 N) for 5 min [11]. All the specimens were stored in potable water for 24 h [7] and were later, thermocycled in water with a temperature differential of 50 8C (5–55 8C) for 1500 cycles [12]. Dwell time in each temperature bath was 5 s, with a transfer time of 5 s. The specimens were then immersed in 2% methylene blue dye for a period of 48 h [13]. All the specimens were then cleaned under flowing water for 1 min followed by sectioning the specimens along the buccolingual direction using diamond disk mounted on to a lathe (Rayfoster). The sectioned samples were then viewed under the stereomicroscope (Olympus Chi20 at 40 magnification) and scored for microleakage (Fig. 4) according to the scale adopted by Tjan et al. [14] as explained in Table 1.
2.5.
Statistical analysis
All data were subjected to statistical analysis using paired t-test for comparison of marginal fit and Mann–Whitney test and Chi-Square test for comparison of microleakage between the groups.
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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Table 2 – Mean marginal fit of copings before and after ceramic addition. Group
SD
SE of mean
Mean difference
t
Before ceramic firing 66.244 Group A Group B 56.264
12.791 11.074
2.860 2.476
9.980
2.638
0.012*
After ceramic firing Group A Group B
10.760 11.375
2.406 2.544
17.203
4.913
0.05).
4.
Discussion
Precise marginal and internal fit is one of the most important criteria for clinical success of dental crowns [15–19]. Though theoretically sintering technology seems to overcome the shortcomings of the conventional lost wax technique of fabricating cast copings, not many studies have been reported in the literature about the marginal fit and microleakage of the copings fabricated by the use of metal laser sintering technique. Hence the present study was undertaken to evaluate the marginal fit and microleakage of conventional cast Ni–Cr alloy copings and compare it with the marginal fit and microleakage Co–Cr alloy copings fabricated by metal laser sintering technique. In this in vitro study we defined a perpendicular measurement at the external margin from the coping to the
Fig. 4 – Sectioned specimen for microleakage assessment.
cavo-surface margin of the finish line preparation as a vertical marginal discrepancy [20]. Shiratsuchi et al. [21] suggested that a deep chamfer and a rounded shoulder design facilitate marginal adaptation in comparison to a shoulder design and may be preferred for metal ceramic crowns. The dimension of the full coverage metal ceramic FPDs were constructed according to Shillingburg et al. The prepared specimens were mounted on dental plaster in a horse-shoe shape for convenience of making an impression using a stock tray. Impressions of the prepared specimens were made using the
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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Table 4 – Comparison of mean microleakage between the groups. Group Group A Group B
Mean
SD
SE of mean
Mean difference
1.95 1.25
1.15 0.85
0.26 0.19
0.700
Z 1.976
P-Value 0.050
Table 5 – Association between microleakage and the groups. Microleakage scores
Score Score Score Score Score Total
0 1 2 3 4
Group A
Group B
n
%
n
%
1 8 4 5 2
5 40 20 25 10
3 11 4 2 0
15 55 20 10 0
20
100
20
100
two step putty-wash technique with polyvinyl siloxane impression material [22,23]. In this study a 1 mm thick soft clear ethyl vinyl acetate sheet was pressed under vacuum forming machine, acted as the spacer for the light body consistency addition polyvinyl siloxane impression [6]. Two layers of die spacer was used to create a 30 mm relief while fabricating copings by conventional technique which has reported the least discrepancy in the marginal fit of the copings [8]. Similarly while fabricating the copings by MLS technique a relief of 30 mm was incorporated into the software (EOS RP tools). Considerable controversy still exists with regards to the influence of ceramic firing on the marginal fit of metal ceramic crowns [2]. With regards to Co–Cr alloy copings fabricated by MLS technology, the obtained results in this study showed a marginal improvement in the external marginal fit of the copings though statistically not significant. The reason behind this improvement is unknown. The number of the measurement points per crown used in previous studies has varied considerably. In many studies measurements ranged from 4 to 12 [10]. For the convenience of measurement and ease of repeatability, four potential sites namely mid-facial, midlingual, mid-mesial and mid-distal surfaces were analyzed to measure the marginal gap. The stereomicroscopic images obtained at 40 magnification were calibrated to measure the marginal fit accurately using ImageJ analysis software. ImageJ analysis utilizes pixels as unit of measurement. An image of a ruler was obtained with 40 magnification and the number of pixels between the known distance of 1 mm (1000 mm) was used to determine pixels/mm ratio. Each pixel was measuring 3.87 mm with 1 mm having 0.258 pixels. Accordingly the pixel/mm ratio obtained was set to 0.258. After the calibration of the software to mm scale, the obtained images were analyzed for the marginal fit. During this calibration the two lines of the millimeter scale were relatively thick than the points used for measurement. This might change the ratio by 0.010 pixels which however would not drastically change the measurements made. This was a limitation in this study as there was no ruler with a precise mm scale. Piemjai suggested that a cementation pressure of 300 N would provide for better marginal adaptation. However under
x2
P-Value
4.579
0.313
clinical circumstances a cementation force of 300 N is not practical. The finger pressure which any clinician can apply while cementing a crown varies from 20 to 67 N [11]. During the procedure of cementing the crowns in this study, a 6 kg iron disk was placed over the modified survey arm to apply 60 N [11] of load on to the crown which was placed beneath the other end of the survey arm. Though literature states many variations in the storage period before thermocycling, temperature difference, number of thermocycles, dwell time, transfer time and tracer used for microleakage [7,12,13,24], in this study the specimens were stored in potable water for 24 h before subjecting it to thermocycling. 1500 cycles with a temperature difference of 50 8C (5–55 8C) with a dwell time of 5 s in each temperature bath and a transfer time of 5 s. The specimens were then soaked in 2% methylene blue dye solution for 24 h to allow the penetration of the dye through the margins and to assess the marginal microleakage. Microleakage was recorded by a single operator according to the scale adopted by Tjan et al. [14] as it allows a better evaluation of microleakage in complete crown restorations. The measurements made in this study were all well within the clinically acceptable range as stated in other studies [25–28]. From the results of this study, it was observed that the mean marginal fit of copings in Group B (56.264 mm) was better than the mean marginal fit of the copings in Group A (66.244 mm). Higher mean marginal fit of copings after ceramic addition was recorded in Group A (70.833 mm) compared to Group B (53.630 mm). And the influence of the ceramic firing has increased the mean marginal gap by 4.589 mm in Group A and decreased by 2.634 mm in Group B. The mean microleakage scores were 1.95 for Group A and 1.25 for Group B which does not show a significant difference in microleakage between the two groups. However when the percentage distribution of the microleakage score was analyzed, though they are not statistically significant, it is seen that 70% of the specimens of Group B had the least microleakage scores confining to the cervical third of the tooth (score 0 and 1). While only 45% of Group A samples had the least microleakage scores confining to the cervical third (score 0 and 1). It is also seen that none of the samples had a score of 4 in Group B while
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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10% of the samples had a score of 4 in Group A. Further increase in sample size may help us in finding the difference between the microleakage scores. Marginal fit and microleakage are factors which play a role in the success of a fixed dental prosthesis. An increase in marginal discrepancy will increase the microleakage proportionally thereby increasing the failure rate of a fixed dental prosthesis. Other factors, such as the mechanical properties of the luting cement and the adhesion between cement and tooth structure can also promote microleakage. Though the copings which were fabricated using pattern resin for the conventional lost wax technique was evaluated by using a metal caliper at multiple points, the uniformity of the thickness is subject to variations unlike the copings fabricated by metal laser sintered technique. Probably this is one known limitation of this study.
5.
Conclusion
Within the limitations of this study the following conclusions were drawn: (a) The marginal fit of copings before and after ceramic addition, fabricated using MLS technology was statistically found to be better than those fabricated using the conventional lost wax technique. (b) The copings fabricated using the conventional lost wax technique showed an increase in marginal discrepancy after ceramic addition which was statistically significant. (c) There was an observable decrease in the marginal discrepancy of the copings fabricated using MLS technology after ceramic addition as compared to the marginal discrepancy of the copings before ceramic addition. (d) There was no significant difference between the microleakage score between the two groups.
Conflict of interest We do not have any conflict of interest.
Acknowledgements The authors gratefully acknowledge the excellent technical support of the following dental laboratories: Dental Arts, Bangalore and Dental Ceramists Pvt. Ltd., Mumbai.
references
[1] Stephen F, Rosensteil. Contemporary fixed prosthodontics. 4th ed. Missouri: Mosby Inc., an affiliate of Elsevier Publishers; 2008. [2] Gemalmaz D, Alkumru HN. Marginal fit changes during porcelain firing cycles. J Prosthet Dent 1995;73: 49–54. [3] Anusavice KJ. Phillips’ science of dental materials. 11th ed. Philadelphia: W.B. Saunders; 2003. p. 565, 584, 585.
[4] White SN, Sorensen JA, Kang SK, Caputo AA. Microleakage of a new crown and fixed partial denture luting agent. J Prosthet Dent 1992;67:156–61. [5] Kim KB, Kim JH, Kim WC, Kim HY, Kim JH. An evaluation of marginal fit of three-unit fixed dental prostheses fabricated by direct metal laser sintering system. Dent Mater 2013;July (29):e91–6. [6] Wu AY-J, Donovan TE. A modified one-step putty-wash impression technique. J Prosthet Dent 2007;98:245–6. [7] Schmid-Schwap M, Graf A, Preinerstorfer A, Watts DC, Piehslinger E, Schedle A. Microleakage after thermocycling of cemented crowns – a meta-analysis. Dent Mater 2011;27(September):855–69. [8] Soriani NC, Leal MB, Paulino SM, Pagnano VO, Bezzon OL. Effect of the use of die spacer on the marginal fit of copings cast in NiCr, NiCrBe and commercially pure titanium. Braz Dent J 2007;18:225–30. [9] Jahangiri L, Wahlers C, Hittelman E, Matheson P. Assessment of sensitivity and specificity of clinical evaluation of cast restoration marginal accuracy compared to stereomicroscopy. J Prosthet Dent 2005;93:138–42. [10] John A, Sorensen D. A standardized method of determination of crown margin fidelity. J Prosthet Dent 1990;64:18–24. [11] Piemjai M. Effect of seating force, margin design and cement on marginal seal and retention of complete metal crowns. Int J Prosthodont 2001;14:412–6. [12] Gale MS, Darvell BW. Thermal cycling procedures for laboratory testing of dental restorations. J Dent 1999;27(February):89–99. [13] Crim GA, Swartz ML, Phillips RW. Comparison of four thermocycling techniques. J Prosthet Dent 1985;53(January):50–3. [14] Tjan AH, Dunn JR, Grant BE. Marginal leakage of cast gold crowns luted with an adhesive resin cement. J Prosthet Dent 1992;67(January):11–5. [15] Bindl A, Mormann WH. Marginal and internal fit of allceramic CAD/CAM crown copings on chamfer preparations. J Oral Rehabil 2005;32:441–7. [16] Valderhaug J, Birkeland JM. Periodontal conditions in patients 5 years following insertion of fixed prostheses. Pocket depth and loss of attachment. J Oral Rehabil 1976;3:237–43. [17] Valderhaug J, Heloe LA. Oral hygiene in a group of supervised patients with fixed prostheses. J Periodontol 1977;48:221–4. [18] Lang NP, Kiel RA, Anderhalden K. Clinical and microbiological effects of subgingival restorations with overhanging or clinically perfect margins. J Clin Periodontol 1983;10:563–78. [19] Foster LV. Failed conventional bridge work from general dental practice: clinical aspects and treatment needs of 142 cases. Br Dent J 1990;168(March):199–201. [20] Hung SH, Hung KS, Eick JD, Chappell RP. Marginal fit of porcelain fused to metal and 2 types of ceramic crowns. J Prosthet dent 1990;63:26–31. [21] Shiratsuchi H, Komine F, Kakehashi Y, Matsumura H. Influence of finish line design on marginal adaptation of electroformed metal–ceramic crowns. J Prosthet Dent 2006;95:237–42. [22] Wu AY, Donovan TE. The use of vacuum-formed resin sheets as spacers for putty-wash impressions. J Prosthet Dent 2007;97:54–5. [23] Monzavi A, Siadat H. Use of wax spacers for putty-wash impression of implant snapon impression copings. J Prosthet Dent 2005;93:494. [24] Crim GA, Garcia-Godoy F. Microleakage: the effect of storage and cycling duration. J Prosthet Dent 1987;57(May):574–6.
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
JPOR-250; No. of Pages 7 journal of prosthodontic research xxx (2014) xxx–xxx
[25] Christensen GJ. Marginal fit of gold inlay castings. J Prosthet Dent 1966;16:297–305. [26] Gavelis JR, Morency JD, Riley Sozio RB. The effects of various finish line preparations of the marginal seal and occlusal seat of full crown preparations. J Prosthet Dent 1981;45:138–45.
[27] Fransson B, Øilo G, Gjeitanger R. The fit of metal–ceramic crowns. A clinical study. Dent Mater 1985;1:169–74. [28] Valderrama S, Van Roekel N, Andersson M, Goodacre CJ, Munoz CA. A comparison of the marginal and internal adaptation of titanium and gold–platinum–palladium metal ceramic crowns. Int J Prosthodont 1995;8:29–37.
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/jpor
Original article
Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study Manoj Kumar Sundar MDS*, Satish Babu Chikmagalur MDS1, Fayaz Pasha MDS2 Department of Prosthodontics, V.S. Dental College & Hospital, K.R. Road, V.V. Puram, Bangalore 560004, Karnataka, India
article info
abstract
Article history:
Purpose: This study aimed to compare the marginal fit and microleakage of metal laser
Received 19 December 2013
sintered Co–Cr alloy copings and conventional cast Ni–Cr alloy copings using a stereomi-
Received in revised form
croscope.
14 July 2014
Methods: Forty extracted maxillary premolars were randomly divided into two groups. One
Accepted 22 July 2014
group was subjected to coping fabrication using conventional lost wax (LW) technique while
Available online xxx
the other group was subjected to coping fabrication using metal laser sintering (MLS)
Keywords:
addition using images obtained with a steromicroscope and an ImageJ analysis software. All
Crowns
the specimens were cemented using Type 1 glass ionomer cement and were subjected to
technology. The marginal fit of these copings were compared before and after ceramic
Marginal fit
thermocycling. The specimens were evaluated for microleakage using stereomicroscope
Microleakage
and 2% methylene blue die used as a tracer. The data were subjected to statistical analysis
Laser sintering
using paired t-test, Mann–Whitney test and Chi-Square test.
Thermocycling
Results: The mean marginal fit of copings before and after ceramic addition in Group B (MLS) was better than the copings in Group A (LW) and was statistically significant (P < 0.05). The influence of ceramic firing had a significant (P < 0.05) increase in mean marginal gap in Group A (LW) but not in Group B (MLS). And the difference in mean microleakage between the groups was not statistically significant (P 0.05). Conclusion: The copings fabricated using MLS technique had a better marginal fit and an observable decrease in microleakage when compared to the copings fabricated using the conventional lost wax (LW) technique. # 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
* Corresponding author. Tel.: +91 9789525412/044 24413908. E-mail addresses: [email protected], [email protected] (M.K. Sundar), [email protected] (S.B. Chikmagalur), [email protected] (F. Pasha). 1 Tel.: +91 9448458424. 2 Tel.: +91 9535208181. http://dx.doi.org/10.1016/j.jpor.2014.07.002 1883-1958/# 2014 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved.
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
JPOR-250; No. of Pages 7
2
journal of prosthodontic research xxx (2014) xxx–xxx
1.
Introduction
Today a vast majority of restorative procedures in fixed prosthodontics are by metal ceramic restorations [1]. The greatest disadvantage of the metal ceramic restoration is indicated to be the marginal fit associated with metal casting and firing porcelain onto it. Deterioration of the initial fit of the metal coping has been observed after the porcelain firing cycle. Studies on marginal distortion have identified many factors, such as the mismatch of porcelain-metal thermal contraction, alloy type and margin design, as contributing to the distortion. Considerable controversy still exists with regards to the influence of ceramic firing on the marginal fit of metal ceramic crowns [2]. A paradigm shift in the procedure for fabricating the metal coping onto which the porcelain can be fired, has come into vogue with the introduction of metal laser sintering (MLS) technology. This metal laser-sintering (MLS) system is an additive metal fabrication technology, based on information received from three dimensional CAD, in which metal powder is shot selectively using a data file and fused with a laser to laminate approximately a 20–60 mm thick layer with each shooting to complete a metal structure. Advantages of the MLS system include the ease with which it can be fabricated and eliminating a major disadvantage of the lost wax technique, i.e., the casting shrinkage [3]. Other advantages include easy fabrication of complicated shapes, operation of an automatic system, and short working time due to elimination of the procedures of fabricating a wax pattern, investing, burning, and casting works. While an essential condition for a successful dental prosthesis is good marginal fit and least microleakage [4], there is little data on the marginal fit and microleakage of fixed dental prostheses (FDPs) fabricated by the MLS system as compared to the conventional casting procedures [5]. Hence the purpose of this study was to compare the marginal fit and microleakage between the copings fabricated using conventional lost wax technique and metal laser sintering technology.
stops were cut in between the samples and the vacuum formed sheet acted as a spacer for making a two stage putty wash impression. A definitive impression was carried out using a two stage putty wash impression technique [6]. The impression was casted with Type 4 die stone (Kalrock, Kalabhai) and the dies were fabricated. Potable water [7] was used as the storage media for the specimens during the period of study.
2.2.
Fabrication of copings
The dies of Group A specimens were coated with two layers of die spacer (DVA Diespacer blue) corresponding to 30 mm [8]. A layer of vaseline was applied over the die models and pattern resin (GC America) was used to fabricate patterns for copings. The thicknesses of the patterns were randomly checked at multiple points using a metal gauge and were kept at a uniform thickness of 0.4 mm The finished resin patterns of the copings of Group A were invested and casted using conventional lost wax (LW) technique with Ni–Cr alloy pellets. The copings for Group B specimens were fabricated using metal laser sintering (MLS) technology (EOS M270). EOS has developed a biocompatible cobalt chrome alloy especially for dental prostheses. It is a cobalt chrome molybdenum-based superalloy designed specifically for porcelain fused to metal (PFM) restorations. The material is available in powder form and has a coefficient of thermal expansion of 14.0–14.5 10 6 m/m 8C and thus provides optimum adhesion of commercial ceramics. The metal laser sintered copings were fabricated using this Co–Cr alloy with the thickness of the coping kept a constant of 0.4 mm with an internal relief of 30 mm, a space for the luting cement. All these necessary data were fed to the software (EOS RP tools) for selective laser sintering. Once the parameters were set a constant, all the die specimens were scanned using the 3D scanner (Scanner 3Shape) and the CAD design of the coping was obtained as an STL data. This data was then used in EOS RP tools software for fabrication of copings using laser sintering technology by vertical stacking of the alloy powder with each increment of layer 20–60 mm thick.
2.
Materials and methods
2.3.
2.1.
Specimen preparation and die fabrication
The obtained copings of the specimens in Groups A and B were transferred onto their respective tooth specimen and analyzed for marginal fit using pictures obtained from a Nikon digital camera mounted to a stereomicroscope [9] (Olympus CHi20) set at 40 magnification and the measurements were calculated (in mm) using ImageJ analysis software. Four images were captured for each specimen which includes four sides of the specimen namely buccal, lingual, mesial and distal. The ImageJ analysis software was first calibrated (Fig. 1) to micrometer scale and the obtained images were analyzed for the marginal fit by marking the distance from the tooth margin to the coping margin. Each sample had four values one from each side of the specimen namely, mid-buccal, mid-lingual, mid-mesial and mid-distal [10]. The values were recorded at the near mid-point of the specimen on each side. This was standardized as the image magnification was same, grids were
Forty extracted, non-carious permanent maxillary premolars were randomly divided into two groups Group A and Group B. All the premolar specimens were prepared to receive a full coverage metal–ceramic restoration with 1.5 mm functional cusp reduction, 1 mm non-functional cusp reduction, 1.2 mm overall axial wall reduction and 1 mm heavy chamfer finish line. All the specimens were mounted in dental plaster in the shape of a horse-shoe, mimicking a dental arch with ten samples in each arch. Four such arches were obtained to facilitate making impressions similar to an intra-oral impression. Each of these arches was pressed with a 1 mm thick soft vacuum formed sheet which provided the space for the light body consistency addition silicone (Aquasil ultra). The pressed vacuum formed sheet was cut to confine only the arch. Four
Assessment of marginal fit
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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Fig. 1 – Calibration of ImageJ analysis software.
Fig. 3 – Stereomicroscopic image of metal ceramic crown on prepared tooth specimen showing marginal gap.
Table 1 – Microleakage scoring criteria [14]. Score
Characteristics
0 1 2 3
No leakages Leakage to one third of axial wall Leakage to two thirds of axial wall Leakage along the full length of axial wall Leakage over occlusal surface
4
Fig. 2 – Stereomicroscopic image of metal copings on prepared tooth specimen showing marginal gap.
used in the software to locate the near mid-point on all four sides. The mean value was recorded as the mean marginal fit in mm (Fig. 2). All the copings of the specimens in Group A and Group B were layered with Duceram plus ceramic material using a conventional layering technique for fabrication of a metal ceramic restoration. All the forty metal ceramic crowns were then assessed for their marginal fit (Fig. 3).
2.4.
Assessment of microleakage
All the metal ceramic crowns were cemented onto their respective specimens using Type 1 glass ionomer cement
(GC gold label luting and lining cement). The cementation procedure was carried out with application of constant pressure of 6 kg (60 N) for 5 min [11]. All the specimens were stored in potable water for 24 h [7] and were later, thermocycled in water with a temperature differential of 50 8C (5–55 8C) for 1500 cycles [12]. Dwell time in each temperature bath was 5 s, with a transfer time of 5 s. The specimens were then immersed in 2% methylene blue dye for a period of 48 h [13]. All the specimens were then cleaned under flowing water for 1 min followed by sectioning the specimens along the buccolingual direction using diamond disk mounted on to a lathe (Rayfoster). The sectioned samples were then viewed under the stereomicroscope (Olympus Chi20 at 40 magnification) and scored for microleakage (Fig. 4) according to the scale adopted by Tjan et al. [14] as explained in Table 1.
2.5.
Statistical analysis
All data were subjected to statistical analysis using paired t-test for comparison of marginal fit and Mann–Whitney test and Chi-Square test for comparison of microleakage between the groups.
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
JPOR-250; No. of Pages 7
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Table 2 – Mean marginal fit of copings before and after ceramic addition. Group
SD
SE of mean
Mean difference
t
Before ceramic firing 66.244 Group A Group B 56.264
12.791 11.074
2.860 2.476
9.980
2.638
0.012*
After ceramic firing Group A Group B
10.760 11.375
2.406 2.544
17.203
4.913
0.05).
4.
Discussion
Precise marginal and internal fit is one of the most important criteria for clinical success of dental crowns [15–19]. Though theoretically sintering technology seems to overcome the shortcomings of the conventional lost wax technique of fabricating cast copings, not many studies have been reported in the literature about the marginal fit and microleakage of the copings fabricated by the use of metal laser sintering technique. Hence the present study was undertaken to evaluate the marginal fit and microleakage of conventional cast Ni–Cr alloy copings and compare it with the marginal fit and microleakage Co–Cr alloy copings fabricated by metal laser sintering technique. In this in vitro study we defined a perpendicular measurement at the external margin from the coping to the
Fig. 4 – Sectioned specimen for microleakage assessment.
cavo-surface margin of the finish line preparation as a vertical marginal discrepancy [20]. Shiratsuchi et al. [21] suggested that a deep chamfer and a rounded shoulder design facilitate marginal adaptation in comparison to a shoulder design and may be preferred for metal ceramic crowns. The dimension of the full coverage metal ceramic FPDs were constructed according to Shillingburg et al. The prepared specimens were mounted on dental plaster in a horse-shoe shape for convenience of making an impression using a stock tray. Impressions of the prepared specimens were made using the
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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Table 4 – Comparison of mean microleakage between the groups. Group Group A Group B
Mean
SD
SE of mean
Mean difference
1.95 1.25
1.15 0.85
0.26 0.19
0.700
Z 1.976
P-Value 0.050
Table 5 – Association between microleakage and the groups. Microleakage scores
Score Score Score Score Score Total
0 1 2 3 4
Group A
Group B
n
%
n
%
1 8 4 5 2
5 40 20 25 10
3 11 4 2 0
15 55 20 10 0
20
100
20
100
two step putty-wash technique with polyvinyl siloxane impression material [22,23]. In this study a 1 mm thick soft clear ethyl vinyl acetate sheet was pressed under vacuum forming machine, acted as the spacer for the light body consistency addition polyvinyl siloxane impression [6]. Two layers of die spacer was used to create a 30 mm relief while fabricating copings by conventional technique which has reported the least discrepancy in the marginal fit of the copings [8]. Similarly while fabricating the copings by MLS technique a relief of 30 mm was incorporated into the software (EOS RP tools). Considerable controversy still exists with regards to the influence of ceramic firing on the marginal fit of metal ceramic crowns [2]. With regards to Co–Cr alloy copings fabricated by MLS technology, the obtained results in this study showed a marginal improvement in the external marginal fit of the copings though statistically not significant. The reason behind this improvement is unknown. The number of the measurement points per crown used in previous studies has varied considerably. In many studies measurements ranged from 4 to 12 [10]. For the convenience of measurement and ease of repeatability, four potential sites namely mid-facial, midlingual, mid-mesial and mid-distal surfaces were analyzed to measure the marginal gap. The stereomicroscopic images obtained at 40 magnification were calibrated to measure the marginal fit accurately using ImageJ analysis software. ImageJ analysis utilizes pixels as unit of measurement. An image of a ruler was obtained with 40 magnification and the number of pixels between the known distance of 1 mm (1000 mm) was used to determine pixels/mm ratio. Each pixel was measuring 3.87 mm with 1 mm having 0.258 pixels. Accordingly the pixel/mm ratio obtained was set to 0.258. After the calibration of the software to mm scale, the obtained images were analyzed for the marginal fit. During this calibration the two lines of the millimeter scale were relatively thick than the points used for measurement. This might change the ratio by 0.010 pixels which however would not drastically change the measurements made. This was a limitation in this study as there was no ruler with a precise mm scale. Piemjai suggested that a cementation pressure of 300 N would provide for better marginal adaptation. However under
x2
P-Value
4.579
0.313
clinical circumstances a cementation force of 300 N is not practical. The finger pressure which any clinician can apply while cementing a crown varies from 20 to 67 N [11]. During the procedure of cementing the crowns in this study, a 6 kg iron disk was placed over the modified survey arm to apply 60 N [11] of load on to the crown which was placed beneath the other end of the survey arm. Though literature states many variations in the storage period before thermocycling, temperature difference, number of thermocycles, dwell time, transfer time and tracer used for microleakage [7,12,13,24], in this study the specimens were stored in potable water for 24 h before subjecting it to thermocycling. 1500 cycles with a temperature difference of 50 8C (5–55 8C) with a dwell time of 5 s in each temperature bath and a transfer time of 5 s. The specimens were then soaked in 2% methylene blue dye solution for 24 h to allow the penetration of the dye through the margins and to assess the marginal microleakage. Microleakage was recorded by a single operator according to the scale adopted by Tjan et al. [14] as it allows a better evaluation of microleakage in complete crown restorations. The measurements made in this study were all well within the clinically acceptable range as stated in other studies [25–28]. From the results of this study, it was observed that the mean marginal fit of copings in Group B (56.264 mm) was better than the mean marginal fit of the copings in Group A (66.244 mm). Higher mean marginal fit of copings after ceramic addition was recorded in Group A (70.833 mm) compared to Group B (53.630 mm). And the influence of the ceramic firing has increased the mean marginal gap by 4.589 mm in Group A and decreased by 2.634 mm in Group B. The mean microleakage scores were 1.95 for Group A and 1.25 for Group B which does not show a significant difference in microleakage between the two groups. However when the percentage distribution of the microleakage score was analyzed, though they are not statistically significant, it is seen that 70% of the specimens of Group B had the least microleakage scores confining to the cervical third of the tooth (score 0 and 1). While only 45% of Group A samples had the least microleakage scores confining to the cervical third (score 0 and 1). It is also seen that none of the samples had a score of 4 in Group B while
Please cite this article in press as: Sundar MK, et al. Marginal fit and microleakage of cast and metal laser sintered copings—An in vitro study. J Prosthodont Res (2014), http://dx.doi.org/10.1016/j.jpor.2014.07.002
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10% of the samples had a score of 4 in Group A. Further increase in sample size may help us in finding the difference between the microleakage scores. Marginal fit and microleakage are factors which play a role in the success of a fixed dental prosthesis. An increase in marginal discrepancy will increase the microleakage proportionally thereby increasing the failure rate of a fixed dental prosthesis. Other factors, such as the mechanical properties of the luting cement and the adhesion between cement and tooth structure can also promote microleakage. Though the copings which were fabricated using pattern resin for the conventional lost wax technique was evaluated by using a metal caliper at multiple points, the uniformity of the thickness is subject to variations unlike the copings fabricated by metal laser sintered technique. Probably this is one known limitation of this study.
5.
Conclusion
Within the limitations of this study the following conclusions were drawn: (a) The marginal fit of copings before and after ceramic addition, fabricated using MLS technology was statistically found to be better than those fabricated using the conventional lost wax technique. (b) The copings fabricated using the conventional lost wax technique showed an increase in marginal discrepancy after ceramic addition which was statistically significant. (c) There was an observable decrease in the marginal discrepancy of the copings fabricated using MLS technology after ceramic addition as compared to the marginal discrepancy of the copings before ceramic addition. (d) There was no significant difference between the microleakage score between the two groups.
Conflict of interest We do not have any conflict of interest.
Acknowledgements The authors gratefully acknowledge the excellent technical support of the following dental laboratories: Dental Arts, Bangalore and Dental Ceramists Pvt. Ltd., Mumbai.
references
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