International Journal of Oral Care and Research

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


Rajneesh Kumar1, Rupali Pathak1, Priya Singh2, Sonali Mahajan2, Devesh Tiwari1, Prachi Deval1,  
1 Prosthodontics GDC &H Mumbai, Maharashtra, India
2 Prosthodontics GDC &H Aurangabad, Maharashtra, India

Correspondence Address:
Dr. Rajneesh Kumar
House no 92, upper ground, block -I,pocket 3, sector 16, rohini, Delhi 110085.
India

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.



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;7:43-45


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 2021 Oct 24 ];7:43-45
Available from: https://www.ijocr.org/text.asp?2019/7/2/43/265816


Full Text



 Introduction



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



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



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 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}

 Discussion



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



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.

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