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Table of Contents
ORIGINAL ARTICLE
Year : 2019  |  Volume : 7  |  Issue : 2  |  Page : 43-45

Comparison of bond strength and fracture toughness of metal–ceramic restorations fabricated by conventional lost-wax technique and selective laser sintering


1 Prosthodontics GDC &H Mumbai, Maharashtra, India
2 Prosthodontics GDC &H Aurangabad, Maharashtra, India

Date of Web Publication29-Aug-2019

Correspondence Address:
Dr. Rajneesh Kumar
House no 92, upper ground, block -I,pocket 3, sector 16, rohini, Delhi 110085.
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/INJO.INJO_26_19

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  Abstract 

Introduction: Porcelain fused to metal is the most common restoration. The success of the prosthesis greatly depends on the mechanical bonding between the metal alloy and the ceramic. Newer techniques such as selective laser sintering (SLS) and computer-aided design are fast replacing the lost-wax technique due to ease of fabrication. This study was carried out to ascertain the best possible technique to ensure success. Materials and Methods: Forty strips of cobalt–chromium (Co–Cr) metal were fabricated and divided into two groups (lost-wax technique vs. SLS technology), which were then bonded with ceramic and compared for bond strength and fracture toughness. Results: SLS group showed higher bond strength than conventional lost-wax technique (CAST) group but lowered fracture toughness. This difference was also found to be statistically significant. Conclusion: The lost-wax technique is a time-tested method that needs not be abandoned until successful evolution of emerging technologies is firmly established.

Keywords: Bond strength, fracture toughness, laser sintering, lost-wax technique, metal–ceramic


How to cite this article:
Kumar R, Pathak R, Singh P, Mahajan S, Tiwari D, Deval P. Comparison of bond strength and fracture toughness of metal–ceramic restorations fabricated by conventional lost-wax technique and selective laser sintering. Int J Oral Care Res 2019;7:43-5

How to cite this URL:
Kumar R, Pathak R, Singh P, Mahajan S, Tiwari D, Deval P. Comparison of bond strength and fracture toughness of metal–ceramic restorations fabricated by conventional lost-wax technique and selective laser sintering. Int J Oral Care Res [serial online] 2019 [cited 2019 Sep 21];7:43-5. Available from: http://www.ijocr.org/text.asp?2019/7/2/43/265816




  Introduction Top


The rampant consumption of soft diet worldwide has multiplied the occurrence of dental caries leading to eventual loss of teeth. Such a scenario warrants rehabilitation; though implant therapy has seen rapid strides, it still remains unavailable to the general populace due to high prices. Partial dentures form the bulk of prosthesis inserted universally. Porcelain fused to metal (PFM) is thus the prosthesis of choice in the so-called “esthetic” zone. PFM restorations are complex and depend on adequate bond between the porcelain and metal for both longevity and esthetics. Traditionally, the metal substrate is manufactured via the “lost-wax technique” introduced by W H Taggart in 1907.[1] The ceramic was then added in an incremental manner.

Though nickel–chromium (Ni–Cr) was the mainstay of PFM in the past, well-founded concerns over toxicity has virtually replaced Ni–Cr alloy by cobalt–chromium (Co–Cr) alloy. Rapid prototyping (RP) is widely used in dental technology today. RP works by directly producing three-dimensionally shape products from computer-aided design (CAD) files or digitally scanned data. RP technologies are broadly divided into stereolithographic, laminated object manufacturing and selective laser sintering (SLS).[1] Any type of metal–ceramic fracture failure can become a costly and time-consuming problem, both in clinics and laboratory. PFM crown has a fracture failure rate from 2.3% to 8%.[2] Dental Co–Cr alloys are widely used by clinicians because of their good mechanical properties and excellent corrosion resistance. Slight mismatch in coefficient of thermal expansion between the alloy and porcelain helps in enhancing the overall strength of restorations. Fracture toughness of the material is also important if the restoration is expected to serve the patient for a long period of time (full-mouth rehabilitation). Fracture toughness value is important for more accurate prediction for long-term clinical performance of restoration.[2] Therefore, in this study, we have attempted to explore the clinical applicability of the SLS method by comparing the bond strength between the metal and ceramic and the fracture strength of the metal itself in metal–ceramic restorations produced using conventional lost-wax technique (CAST) method and SLS method.


  Materials and Methods Top


An in vitro study was carried out to evaluate and compare the bond strength and fracture toughness of metal–ceramic restoration fabricated by conventional lost-wax technique (CAST) and SLS. For fabrication of CAST samples, a strip of dimension 25 × 3 × 0.5 heat cured acrylic resin was formed. A mold of this strip was made by adding silicone. This mold was filled by casting wax to prepare 20 wax sheets, which were casted by conventional lost-wax technique using Co–Cr alloy.

For SLS samples, an standard triangle language or standard tessellation language file is created of dimension 25 × 3 × 0.5 and SLS equipment sinters the Co–Cr powders. The present in vitro study was carried out to compare the bond strength of porcelain to CAST Co–Cr alloy and SLS Co–Cr core materials and fracture toughness of metal substructure. The study comprised 40 specimens, which were divided into two groups. Each group contained 20 specimens. Different groups for bond strength testing were categorized:

Group 1 (G1): Co–Cr alloy specimens made using conventional lost-wax casting method (CAST)

Group 2 (G2): Co–Cr alloy specimens made using SLS.

Bond strength was tested first for both groups. Then fracture toughness of the metal for both groups was tested.


  Results Top


An in vitro study was carried out to evaluate and compare the bond strength and fracture toughness of metal–ceramic specimens fabricated by conventional lost-wax technique and SLS. Different groups for the bond strength testing were categorized:

Group 1 (G1): Co–Cr alloy specimens made using conventional lost-wax casting method (CAST)

Group 2 (G2): Co–Cr alloy specimens made from direct metal SLS.

Comparison of bond strength of CAST and SLS: Mean bond strength of two groups was compared by t test. Results suggest that bond strength of two groups is different. This difference was found to be statistically significant (P < 0.05). SLS samples were found to have greater bond strength than CAST samples [Graph 1].
Graph 1: Comparison of bond strength of CAST and SLS

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Comparison of fracture toughness of CAST and SLS: Mean fracture toughness of both groups was compared by t test is shown in table. Result suggests that fracture toughness of both the groups was different and this difference was found to be statistically significant. CAST group has greater fracture toughness than the SLS group [Graph 2].
Graph 2: Comparison of fracture toughness of CAST and SLS

Click here to view



  Discussion Top


Fixed prosthodontics treatment involves the replacement and restoration of teeth by artificial substitutes that are not readily removable from the mouth. Its focus is to restore function, aesthetics, and comfort. Every patient wants fixed replacement of missing teeth. Fixed prosthodontics can offer exceptional satisfaction for both patient and dentist. For PFM restorations, various methods of fabrication are possible. The longevity of metal–ceramic restorations depends on reliable bonding between metal and ceramic, primarily produced by an oxide layer.[3],[4] In this study, bond strength of metal–ceramic restoration and fracture toughness of metal substructure fabricated by conventional lost-wax technique (CAST) and SLS were evaluated and compared. Similar to our study, increase in bond strength is seen in a study conducted by Bae et al.[1] He compared the fracture strength of the metal and the bond strength in metal–ceramic restorations produced by SLS and by conventional casting (CAST). In his study, the mean bond strength of the SLS group (50.60MPa) was higher than that of the CAST group (46.29MPa), but there was no statistically significant difference. A similar study was carried out by Akova et al.[5],[6],[7],[8] by comparing the shear bond strengths of cast Ni–Cr and Co–Cr alloys and the laser-sintered Co–Cr alloy to a dental porcelain. The mean shear bond strength was highest for the cast Ni–Cr metal–ceramic specimens, but the bond strength was not significantly different from that for the cast Co–Cr metal–ceramic specimens and the laser-sintered Co–Cr metal–ceramic specimens.[9] Another important mechanical property for success of dental restoration is fracture toughness. Fracture toughness of the material is also important if the restoration is expected to serve the patient for a long period of time (full-mouth rehabilitation). Fracture toughness value is important for more accurate prediction for long-term clinical performance of restoration.[8] Bonilla et al.[10],[11],[12] compared the resistance to crack propagation of nine flowable composites as measured by the fracture toughness. The fracture toughness value, Kic (MN/m3/2), for each specimen was measured by the use of a three-point bending mode and a single-edge notched beam at a crosshead speed of 0.125mm/min until it fracture under universal testing machine. A similar study conducted by Knobloch et al.[13],[14],[15] evaluated the relative fracture toughness of three packable composites (Alert, SureFil, and Solitaire), two conventional composites (Herculite and Heliomolar), and one laboratory-processed composite (Belleglass). Testing was performed on a universal testing machine at a displacement rate of 0.5mm/min until it fractured. In this study, fracture toughness was evaluated and it was found that mean fracture toughness of conventional lost-wax samples was greater than SLS samples. This increase in bond strength of SLS can be due to the result of the balling phenomenon occurring in the laser sintering, and the decrease in fracture toughness of SLS group might be due to partial sintering of Co–Cr powder resulting in balling phenomenon, which results in voids in the metal strips. These voids can result in propagation of cracks induced by metal fatigue and thus results in decreased fracture toughness.


  Conclusion Top


Within the limits of this study, it was concluded that samples fabricated using SLS showed greater bond strength of ceramic to metal than samples fabricated by conventional lost-wax technique. Metal samples fabricated using SLS showed lower fracture toughness than samples fabricated by conventional lost-wax technique. Results showed statistically significant difference between values of the two groups. Results showed that the bond strength of two methods was greater than the optimum strength of metal–ceramic restoration.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Bae EJ, Kim JH, Kim WC, Kim HY Bond and fracture strength of metal-ceramic restorations formed by selective laser sintering. J Adv Prosthodont 2014;6:266-71.  Back to cited text no. 1
    
2.
Balkenhol M, Köhler H, Orbach K, Wöstmann B Fracture toughness of cross-linked and non-cross-linked temporary crown and fixed partial denture materials. Dent Mater 2009;25: 917-28.  Back to cited text no. 2
    
3.
Anusavice KJ Phillips Science of Dental Material. 11th ed. Philadelphia, PA: Saunders; 2003. p. 562-63.  Back to cited text no. 3
    
4.
Yilmaz H, Dinçer C Comparison of the bond compatibility of titanium and an NiCr alloy to dental porcelain. J Dent 1999;27:215-22.  Back to cited text no. 4
    
5.
Akova T, Ucar Y, Tukay A, Balkaya MC, Brantley WA Comparison of the bond strength of laser-sintered and cast base metal dental alloys to porcelain. Dent Mater 2008;24:1400-4.  Back to cited text no. 5
    
6.
Son MK, Choe HC Evaluation of interfacial bonding strength between laser textured metal coping and porcelain. Proc Eng 2011;10:2286-91.  Back to cited text no. 6
    
7.
Serra-Prat J, Cano-Batalla J, Cabratosa-Termes J, Figueras-Àlvarez O Adhesion of dental porcelain to cast, milled, and laser-sintered cobalt-chromium alloys: shear bond strength and sensitivity to thermocycling. J Prosthet Dent 2014;112:600-5.  Back to cited text no. 7
    
8.
Prasad NM, Jyothi BN, Sabitha MR A comparative evaluation of shear bond strength of porcelain fused to metal substructure fabricated using conventional and contemporary techniques: an in vitro study. Int J Med Res Health Sci 2015;4:186-92.  Back to cited text no. 8
    
9.
Pretti M, Hilgert E, Bottino MA, Avelar RP Evaluation of the shear bond strength of the union between two CoCr-alloys and a dental ceramic. J Appl Oral Sci 2004;12:280-4.  Back to cited text no. 9
    
10.
Bonilla ED, Yashar M, Caputo AA Fracture toughness of nine flowable resin composites. J Prosthet Dent 2003;89:261-7.  Back to cited text no. 10
    
11.
Knobloch LA, Kerby RE, Pulido T, Johnston WM Relative fracture toughness of bis-acryl interim resin materials. J Prosthet Dent 2011;106:118-25.  Back to cited text no. 11
    
12.
Wang RR, Lu CL, Wang G, Zhang DS Influence of cyclic loading on the fracture toughness and load bearing capacities of all-ceramic crowns. Int J Oral Sci 2014;6:99-104.  Back to cited text no. 12
    
13.
Knobloch LA, Kerby RE, Seghi R, Berlin JS, Clelland N Fracture toughness of packable and conventional composite materials. J Prosthet Dent 2002;88:307-13.  Back to cited text no. 13
    
14.
Rao S, Chowdhary R Comparison of fracture toughness of all-ceramic and metal-ceramic cement retained implant crowns: an in vitro study. J Indian Prosthodont Soc 2014;14:408-14.  Back to cited text no. 14
    
15.
Jassim HH Evaluation of the shear bond strengths between two alternative metal alloys and porcelain. MDJ 2013;10:161-6.  Back to cited text no. 15
    


    Figures

  [Graph 1], [Graph 2]



 

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   Materials and Me...
  Results
  Discussion
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