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 Table of Contents  
Year : 2019  |  Volume : 11  |  Issue : 2  |  Page : 80-85

The influence of coping designs on labio-marginal towards fracture resistance of metal-porcelain restoration

1 Postgraduate Program in Prosthodontics, Faculty of Dentistry, Universitas Sumatera Utara, Medan, Indonesia
2 Department of Prosthodontics, Faculty of Dentistry, Universitas Sumatera Utara, Medan, Indonesia
3 Department of Mechanical Engineering, Faculty of Engineering, Universitas Sumatera Utara, Medan, Indonesia

Date of Web Publication29-Apr-2019

Correspondence Address:
Jl. Alumni No. 2 Kampus USU, P. O. Box: 20155, Medan
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jioh.jioh_5_19

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Aims: Metal-porcelain restoration on esthetic zones often results in an umbrella effect due to metal collar coping on the labiomarginal area. Therefore, metal collarless design on metal-porcelain restoration was indicated, yet it may influence the fracture resistance of the restoration. This study aimed to determine the effect of metal coping designs on the labiomarginal area against fracture resistance of metal-porcelain restoration. Materials and Methods: A total of 24 samples of metal-porcelain restorations from four designs were fabricated and cemented on 24 metal dies. The study was divided into four groups; Group A with metal collar design, Group B with modified metal collar design, Group C with metal collarless design, and Group D with modified metal collarless design. The measurement of fracture resistance was carried out using universal testing machine (Torsee UTM AMU-10, Tokyo, Japan) at a crosshead speed of 0.05 mm/minute. Loading point was applied at an angle of 45° until the porcelain layer fractured. Results: The mean value and standard deviation of fracture resistance are 2237.56 ± 183.83 N in Group A, 1934.34 ± 152.81 N in Group B, 2049.62 ± 162.58 N in Group C, and 2146.15 ± 210.75 N in Group D. Significant influence of fracture resistance on metal-porcelain restorations was found in all experimental groups (P < 0.05), and there were significant differences between Groups A and B and between Groups B and D (P < 0.05). Conclusion: Metal-porcelain restoration with modified metal collarless design can be an alternative restoration on esthetic zone with high fracture resistance and may be able to prevent umbrella effect on the labiomarginal area.

Keywords: Coping design, fracture resistance, metal collar, metal collarless, metal-porcelain restoration

How to cite this article:
Chihargo, Tamin HZ, Nasution I. The influence of coping designs on labio-marginal towards fracture resistance of metal-porcelain restoration. J Int Oral Health 2019;11:80-5

How to cite this URL:
Chihargo, Tamin HZ, Nasution I. The influence of coping designs on labio-marginal towards fracture resistance of metal-porcelain restoration. J Int Oral Health [serial online] 2019 [cited 2021 May 18];11:80-5. Available from:

  Introduction Top

Metal-porcelain restorations are still widely used for esthetic zone in anterior teeth.[1],[2] Metal structure of metal-porcelain restorations contributes to the level of opacity and less esthetic if it is visually visible on the anterior region. It may also cause the umbrella effect due to the metal coping on the marginal area.[2],[3],[4] Metal coping designs can be vary with some modifications in order to achieve porcelain coverage on the marginal area by replacing it with porcelain margin.[5]

Metal coping designs in metal-porcelain restorations can be classified into metal collar design, modified metal collar design, metal collarless design, and modified metal collarless design.[6] Metal collar design is rarely used in anterior teeth for several esthetic reasons which may cause the umbrella effect which is known as dark color changes of marginal gingiva.[7] Therefore, it is modified by shortening the metal coping on marginal wall, but this modification may cause distortion on porcelain layer due to the porcelain firing process; hence, it affects the fracture resistance of porcelain and marginal fit of the restoration.[2]

The use of metal collarless design in order to meet the esthetic requirement is by replacing the metal collar with porcelain layer on marginal wall, but this design is still less able to solve problem due to cloudy shadows of the opaque layer on marginal areas, especially if the marginal parts of teeth are insufficient.[1] Therefore, metal collarless design is modified by shortening the final edge of the 1–3-mm metal layer on the labial wall.[1],[2],[6] Modified metal collarless design is often referred as porcelain butt joint. However, several studies suggest that this design has disadvantages such as the loss of the ferrule effect of metal collar and unable to withstand pressure during cementation or mastication.[1],[2]

This study evaluates the influence of four coping designs (metal collar design, modified metal collar design, metal collarless design, and modified metal collarless design) toward fracture resistance of metal-porcelain restorations.

  Materials and Methods Top

Biomechanical preparation of the right central incisor was performed on typodont tooth with a radial shoulder on the marginal surface. Radial shoulder preparation was continued on both mesiodistal of the middle region and merged with chamfer preparation on the linguomarginal surface. Afterward, typodont tooth was mounted into a square metal block (3 cm × 3 cm × 3 cm) [Figure 1]a containing putty polyvinyl siloxane (PVS) impression material and light body PVS (I-SiL, Spident, Korea). The mold was filled self-cured acrylic resin (Hilton, Japan). Twenty-four resin teeth [Figure 1]b were obtained from the mold and then sprued with soft wax inlay (Violet, Tokyo Japan). Thereafter, they were mounted into phosphate-bonded investment material (Deyuan, China), and casting procedures were done by using nickel-chromium/Ni-Cr alloy (Ker-N, Eisenbacher Dental, Germany) with compositions from Ni-61.27%, Cr-26.44%, Mo-10.46%, Mn-0.001%, and C-0.02% in order to get 24 analog teeth from Ni-Cr material. In addition, the metal analog teeth were coated with wax and planted vertically into a square block (3 cm × 3 cm × 3 cm) containing self-cured acrylic resin up to 1 mm apical from the cementoenamel junction. After the resin block had been set, the wax layer on the tooth root was melted by boiling water. The distance between the root of the tooth and the resin block was filled with silicone paste (Clear RTV Silicone, Permatex, USA). The procedure was carried out on 24 metal analog teeth, resulting in 24 metal dies [Figure 1]c, and the impression of 24 metal dies was taken on the modified metal tray [Figure 2] with putty PVS impression materials and light body PVS. Dental stone (Fuji Rock, GC, Japan) was poured into the mold in order to get 24 die stones.
Figure 1: (a) Typodont tooth duplication on the metal block containing polyvinyl siloxane material. (b) Resin dies obtained from typodont tooth duplications. (c) Metal die obtained from casting the resin die and planted on resin block

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Figure 2: The impression result of the metal die using polyvinyl siloxane material on a modified metal tray

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Green inlay wax was applied on the die stone with full coverage to the finishing line of the labiomarginal wall (1.5 mm) for Group A. While on Group B, green inlay wax partially covered (0.8 mm) the finishing line of the labiomarginal wall. On Group C, green inlay wax covered limitedly on the cavosurface angle with the thickness of 0.3 mm. While on Group D, green inlay wax was trimmed 1.5 mm above the cavosurface angle [Figure 3]. The measurements above were carried out using a wax caliper (Caliper Iwanson, Medesy, Italy). Then, the wax sprues were placed on the incisor surface of the wax pattern and invested in a phosphate-bonded investment material which had been mixed in a vacuum mixer (Mixyvac, Manfredi, Italy). The casting procedure was carried out with Ni-Cr alloy. Afterward, the finishing of metal coping was carried out using rotary instruments and sprayed with 50 μm alumina sand on a sandblasting machine (Blasty, Manfredi, Italy). The thickness and expansion of metal coping on the labiomarginal surface was measured with digital vernier calipers (Mitutoyo Co, Kawasaki, Japan).
Figure 3: Wax patterns from Groups A, B, C, and D on stone dies

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Porcelain (VITA VMK Master, VITA Zahnfabrik, Germany) build-up was carried out in Groups A and B which began with two applications of opaque, dentine, and enamel layers. The combustion was carried out using a vacuum furnace (VITA Vacumat 40, VITA Zahnfabrik, Germany) which set at a temperature of 950°C, and the temperature was decreased in accordance with the firing scheme of the manufacturer's instructions. Porcelain margin material was applied to Groups C and D using direct lift technique after the application of the opaque layer was carried out. Porcelain margin was applied at the cervical end and carved with its convexity in order to reduce overcontouring of the restoration. This layer was then condensed and taken to firing procedure. The second porcelain margin layer was applied to repair the marginal area which had not been coated due to the first margin porcelain layer discrepancy after the firing process. Thereafter, dentinal and enamel porcelain layers were applied over opaque and porcelain margin layer, following the porcelain firing procedure. All samples were contoured with abrasive wheels. Measurements were done using a digital vernier caliper in order to ensure the total thickness of metal and porcelain was 1.5 mm. All samples were then continued to the glazing procedure [Figure 4].
Figure 4: Metal-porcelain restorations with four coping designs from Groups A, B, C, and D

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The intaglio surface of the restorations and the surface of metal die were sprayed with 50 μm alumina sand and immersed in ultrasonic cleaner (Fulgor, Med. Pro 3.5 lt, Italy) with distilled water for 10 min and dried in room temperature. Afterward, the restorations were cemented on each metal dies with glass ionomer cement (Fuji 1, GC, Japan) which was mixed in accordance to the manufacturer's instructions and pressed with a static pressure of 15 kg.

All samples were tested using universal testing machine (Torsee UTM AMU-10, Tokyo, Japan). The sample was placed at an angle of 45° from the long axis of the tooth, and the pressure was applied at 2 mm below the incisal end of the palatal surface to simulate contact from the mandibular incisor [Figure 5]. This was based on the normal occlusion of Class I angle of the average adult.[8] A metal rod with a diameter of 3 mm with crosshead speed of 0.05 mm/min was used during the test until the porcelain was fracture.
Figure 5: The position of sample testing on universal testing machine

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Statistical analysis (IBM SPSS, version 21 × 86, Indonesia) was carried out using one-way ANOVA and post hoc/Turkey's honesty significant difference (HSD) tests.

  Results Top

The values of fracture resistance recorded in computerized by the form of Kilogram-Force were converted into Newton (N), and the value of load required fracturing from the samples of different groups reveal that 2440.43 N of Group D was maximum and 1769.14 of Group B was least [Table 1]. [Table 2] reveals the results of statistical analysis (ANOVA) of maximum load required to fracture metal-porcelain restorations in different coping designs. It can be observed that there was a statistically significant difference in the mean fracture resistance within the groups with P = 0.04.
Table 1: Statistic analysis with descriptive data of fracture resistance

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Table 2: One-way ANOVA test

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[Table 3] reveals the results of the post hoc/Turkey's HSD test, there was a significant difference between metal collar design and modified metal collar design in fracture resistance of metal-porcelain restorations with P = 0.01, there was a significant difference between modified metal collar design and modified metal collarless design in fracture resistance of metal-porcelain restorations with P = 0.04.
Table 3: Result of the post hoc/Turkey's honestly significant difference test

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  Discussion Top

The presence of metal coping is necessary considered to provide mechanical resistance to fracture, but this provides a disadvantage in the esthetic term.[9] Several studies showed that the metal collar can reduce light transmission to the hard tissues of the teeth, causing dark appearance on the root surface of the teeth and gingival. Therefore, it is necessary to design a restoration that has the advantage of the metal-porcelain structural properties and the esthetic quality of full porcelain, especially in the marginal third of the restoration. These requirements have led to the development of metal-porcelain restoration design in marginal areas, which is also known as metal collarless design.[10]

In this study, metal collar design was used as a control group due to various studies, stated that this design had the highest fracture resistance.[1],[10],[11],[12] In addition, the thickness of all samples of metal-porcelain restorations tested varied from 1.5 mm to 1.7 mm and also had differences in marginal area because the metal coping in the cervical area was reduced horizontally and vertically and replaced with porcelain layers, respectively. Porcelain must also have a minimum thickness that is suitable for the esthetics. The minimum thickness of porcelain is 0.7 mm, and the expected thickness is 1.0–1.5 mm. The extensions of porcelain thickness for >2.0 mm will be susceptible to fracture.[13],[14],[15] It is supported by the research conducted by Corciolani and Vichi which stated that the thickness of metal-porcelain restorations should be below 1.5 mm.[16]

The values of fracture resistance for each sample in one group were quite varied, even though the values were still within the scope of the homogeneous data. Gardner et al. revealed that fracture resistance values could be vary due to discrepancies in porcelain thickness, condensation, and modification of marginal porcelain in each sample.[17] Some studies used loading point that simulates centric occlusion of incisors.[10],[11],[17] In this study, the load was placed on the palatal surface 2–4 mm below the incisal edge. The tip of the round-ended plunger rod could slide along the palatal surface of the sample when the fracture test was carried out; this may also cause the fracture resistance values to be inconsistent between samples which could be seen from the standard deviation values that vary from each sample group. This also happened in a study from Yoon et al. which stated that this condition simulated the clenching process in centric occlusion or contact occlusion without posterior teeth.[1]

Fracture resistance of metal-porcelain restorations is also influenced by the modulus of elasticity of the die material. The use of natural tooth can cause difficulties in standardization because natural tooth shows large variations depending on the anatomy, age of the patient, and the time and method of storing teeth after extraction. Several studies used metal, brass, epoxy resin, and acrylic resin as supporting materials for placement of metal-porcelain restorations. In this way, tooth preparation can be standardized and has the same physical properties of the material.[18] Therefore, metal die from Ni-Cr material with the form of a central incisor which had been prepared was used in this study. This was also supported by a study conducted by Sagsoz et al. who stated the use of Ni-Cr as a support for final restoration had a higher fracture resistance than the use of epoxy resin and natural teeth.[18] In this study, the root surface of metal die was coated with silicone paste below 1 mm from marginal boundaries to simulate periodontal ligaments and the surrounding anatomical structures. This is supported by the research conducted by Lertchirakarn et al. and Sirimai et al. who stated that the use of rigid material to attach the extracted tooth can also cause the load value to be distorted and affect the fracture resistance of metal-porcelain restorations.[19]

The analysis results by post hoc/Turkey's HSD test revealed that there was a significant difference between metal collar design and modified metal collar on fracture resistance of metal-porcelain restorations (P < 0.05). This can be due to the porcelain layer on the marginal part is supported by metal on metal collar design, while the metal support on modified metal collar design was reduced by 0.8 mm and replaced by 0.7 mm porcelain layers. This reduction was based on the consideration of the thickness of porcelain layer should be at least 0.7 mm, and the total thickness of the porcelain-metal restoration should be 1.5 mm and not exceed over 2.0 mm because the porcelain material will be vulnerable to fracture.[13],[14],[15] In addition, direct lift techniques were used for porcelain build-up in marginal areas was a sensitive technique. If the condensation procedure of porcelain in the marginal area was not reached, the cracks or voids could be trapped in the porcelain thickness. Even though the direct lift technique was done properly, porcelain on the marginal area could change its shape in the internal surface when the porcelain firing procedure was carried out. The shape of porcelain could change due to porcelain shrinkage or by the gravity. Afterward, the laboratory technician must grind and repair the internal surface of the porcelain layer on the marginal area in order to adapt the restoration on the die stone, resulting the internal surface of porcelain on the marginal area became rounded which might cause the inner gap became larger and required the porcelain layering and firing procedure to be carried out again. The repetition of porcelain firing procedure might also give negative effect due to the high temperatures such as increasing the inter-surface pressure and uncontrolled formation of the oxide layer. Repeated porcelain firing procedure might also change the coefficient of thermal expansion of porcelain and metal coping, resulting unexpected residual tensile stress on porcelain layer during the cooling process to room temperature, and residual tensile stress could trigger cracks on porcelain. These phenomena might cause the metal collarless design fracture of metal-porcelain restoration became lower.[1],[15]

[Table 2] reveals that there was a significant difference between modified metal collar design and modified metal collarless design. This may be due to the thickness of porcelain on marginal areas was achieved by 1.5 mm on modified metal collarless design. This study was also in accordance with the research conducted by Gardner et al., in which the load required for porcelain fracture on metal-porcelain restorations with the metal collarless design was significantly higher than metal-porcelain restorations with the metal collar design.[11] In addition, porcelain margin material has a higher alumina crystalline content. Alumina has a high modulus of elasticity and is the strongest oxide. Cracks cannot pass through alumina particles easily as they pass through a glass matrix. Glazed porcelain margin is also effective in reducing crack propagation on the outer surface and maintaining the surface under compression.[11]

Based on the analysis of visual assessment and coding, the fracture location of metal-porcelain restorations is divided into two surfaces, namely, the labial and palatal surfaces which divided into four quadrants, the fracture location of the metal collar design on the labial surface was commonly in mesioincisal and mesiomarginal, whereas fracture was found on mesiomarginal and distomarginal of the palatal surface. While in modified metal collar modification, the fracture location on the labial surface was more present on the mesioincisal and distomarginal, whereas on the palatal surface, there was more on the distomarginal. In metal collarless design, the location of the fracture on the labial surface was commonly present on the mesioincisal, mesiomarginal, distoincisal, and distomarginal, whereas on the palatal surface, there was fracture more on the distomarginal. In the modified metal collarless design, the location of the fracture on the labial surface was found more on distoincisal and distomarginal, whereas on the palatal surface was on mesiomarginal. Therefore, it could be concluded that the highest average fracture location was on marginal area. This was also supported by the results of the one-way ANOVA test from this study which stated that metal coping design may provide an effect on fracture resistance in a metal-porcelain restoration.[8],[11],[17]

The fracture pattern was also evaluated, and irregular pattern was found in every sample of each group, such as adhesive failure, cohesive failure, and its combination on different surfaces. However, to see the fracture pattern in detail, further research may be needed with the use of Structural equation modeling analysis. In addition, before fracture resistance test was conducted, color measurement of metal-porcelain restorations was also carried out using the Colorimeter CS-10 (Hangzhou CHNSpec Tech Co. Ltd, China) in every samples on each groups, and compared to the Vita 3D Master shade guide (3M2). The color test results showed that the L*a*b* value of metal-porcelain restoration with collarless design nearly approached the value of L*a*b* of the Vita 3D Master shade guide (3M2). Thus, it could be concluded that metal-porcelain restorations with metal collarless design and modified metal collarless design have higher esthetic value compared to other designs of metal-porcelain restoration. In addition, the metal-porcelain restorations with collarless design may also allow the light transmission to the adjacent root structure near the gingival margin, which able to prevent the umbrella effect.[2],[3],[4]

As for the researchers' limitations in controlling the metal coping thickness equally on each coping surface and the repetition of firing and glazing cycles in some samples were not the same, further studies are required to control the thickness of metal coping using CAD/CAM tools and to evaluate the influence of firing and glazing cycles on collarless designs of metal-porcelain restorations.

  Conclusion Top

Within the limitations of this study, it can be concluded that:

  1. Metal porcelain restoration with modified metal collarless design has almost the same resistance in fracture as metal collar design and may also be able to prevent umbrella effect
  2. The thickness of the porcelain layer on marginal areas which may provide highest esthetic value in modified metal collarless design of metal-porcelain restorations can be achieved if the tooth preparation performed by the dentist in the clinic has a minimum thickness of 1.5 mm
  3. Modified metal collarless design with 1.5 mm reduction of metal coping on the labiomarginal area can be recommended for patients indicated to use metal-porcelain restorations in the anterior tooth as the final restoration of a prosthetic treatment.

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  References Top

Yoon JW, Kim SH, Lee JB, Han JS, Yang JH. A study on the fracture strength of collarless metal-ceramic fixed partial dentures. J Adv Prosthodont 2010;2:134-41.  Back to cited text no. 1
Fahmy AM. Comparison of marginal fit between collarless metal ceramic and two all ceramic restorations. J Am Sci 2012;8:528-34.  Back to cited text no. 2
Chatterjee U. Margin designs for esthetic restoration: An overview. J Adv Oral Res 2012;3:7-11.  Back to cited text no. 3
Oh JW, Song KY, Ahn SG, Park JM, Lee MH, Seo JM, et al. Effects of core characters and veneering technique on biaxial flexural strength in porcelain fused to metal and porcelain veneered zirconia. J Adv Prosthodont 2015;7:349-57.  Back to cited text no. 4
Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-to-metal marginal designs in vivo: A scanning electron microscope study. J Prosthet Dent 1985;53:24-9.  Back to cited text no. 5
Chihargo, Tamin HZ. Role of coping designs against fracture resistance of porcelain in metal-porcelain fixed partial denture: A review. Int J Multidiscip Res 2017;3:44-9.  Back to cited text no. 6
Rosenstiel SF, Land MF, Fujimoto J. Contemporary Fixed Prosthodontics. 4th ed. St. Louis: Mosby Elsevier; 2006. p. 272, 615.  Back to cited text no. 7
Danappanavar PM, Nanda Z, Bhaskar M, Gowd V, Molugu M, Reddy KA, et al. Comparative evaluation of resistance failure in nonprecious metal-ceramic restoration at the incisal edge with varying thickness under different application of load: An in vitro study. J Contemp Dent Pract 2011;12:434-40.  Back to cited text no. 8
Shirakura A, Lee H, Geminiani A, Ercoli C, Feng C. The influence of veneering porcelain thickness of all-ceramic and metal ceramic crowns on failure resistance after cyclic loading. J Prosthet Dent 2009;101:119-27.  Back to cited text no. 9
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Powers JM, Sakaguchi RL. Craig's Restorative Dental Materials. 12th ed. St. Louis, Missouri: Mosby Elsevier; 2006. p. 143.  Back to cited text no. 13
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Corciolani G, Vichi A. Evaluation of measurement repeatability of clinical and spectrophotometers for color matching technique. J Int Dent S Afr 2006:8:62-70.  Back to cited text no. 16
Vernekar NV, Jagadish PK, Diwakar S, Nadgir R, Krishnarao MR. Alternate metal framework designs for the metal ceramic prosthesis to enhance the esthetics. J Adv Prosthodont 2011;3:113-8.  Back to cited text no. 17
Sagsoz NP, Yanikoǧlu N, Sagsoz O. Effect of die materials on the fracture resistance of CAD/CAM monolithic crown restorations. Oral Health Dent Manag 2016;15:165-68.  Back to cited text no. 18
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2], [Table 3]


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