title: Effect of firing cycle and surface finishing on the sag resistance of long-span metal ceramic framework using base metal alloys-an in vitro study.
abstract: BACKGROUND
  The search for the ideal restorative material or combination of materials in dentistry is still the subject of modern dental research. In clinical practice, there are increased numbers of ceramo-metal restorations being fabricated which replace multiple missing teeth in a single framework. The literature is scanty with regard to investigations of the 'sag' resistance of base metal alloys commonly used for ceramo-metal restorations and specifically for long-span restorations. An in vitro study was carried out to investigate the effects of surface finishing and simulated porcelain-firing on the sag resistance of long-span ceramo-metal frameworks using base metal alloys.
  
  
  METHODS
  Four types of alloys were used. A total of 80 samples were selected for the study and they were divided into four groups of 20 samples each. 'As cast' metal specimens in group I, group II and with surface finishing in group III and group IV. Each test sample was mounted on the sample holding accessory of the custom made thermo mechanical analyser and subjected to three firing cycles. Each sample was evaluated for the sag-related deflection recorded by the dial gauge with an accuracy of 1 µm. The data recorded was subjected to a statistical analysis using unpaired t-test to compare the relative difference in the sag-related deflection values.
  
  
  RESULTS
  It was observed that the values for sag-related deflection were significantly less for the 'as cast' samples of all the alloy groups. Nickelchromium (NiCr) and cobaltchromium (CoCr) exhibited value of 19.4 μm and 14.8 μm, respectively.
  
  
  CONCLUSIONS
  Among the four groups of alloys tested, CoCr based alloy exhibited the least amount of sag-related deflection. The results show that the sag-related deflection was less in Co-Cr alloys as compared with the NiCr alloys, still Ni-Cr alloys in fixed dental prostheses frameworks are used due to their desirable physical and chemical properties.
authors.affiliation: Graded Specialist (Prosthodontics), MDC Meerut.
authors.forename: Poonam
authors.initials: P
authors.lastname: Prakash
authors.affiliation: Commanding Officer, 200 MDC, C/o 56 APO.
authors.forename: Dsj
authors.initials: D
authors.lastname: D'Souza
authors.affiliation: Professor, Division of Prosthodontics, Genesis Institute of Dental Sciences & Research, Ferozpur.
authors.forename: Manjit
authors.initials: M
authors.lastname: Kumar
authors.affiliation: Associate Professor, Department of Dental Surgery, AFMC, Pune - 40.
authors.forename: M
authors.initials: M
authors.lastname: Viswambaran
keywords: base metal alloys; sag resistance