title: Die spacer thickness reproduction for central incisor crown fabrication with combined computer-aided design and 3D printing technology: an in vitro study.
abstract: BACKGROUND
  The inability to control die spacer thickness has been reported. However, little information is available on the congruency between the computer-aided design parameters for die spacer thickness and the actual printout.
  
  
  OBJECTIVE
  The purpose of this study was to evaluate the accuracy and precision of the die spacer thickness achieved by combining computer-aided design and 3-dimensional printing technology.
  
  
  METHODS
  An ivorine maxillary central incisor was prepared for a ceramic crown. The prepared tooth was duplicated by using polyvinyl siloxane duplicating silicone, and 80 die-stone models were produced from Type IV dental stone. The dies were randomly divided into 5 groups with assigned die spacer thicknesses of 25 μm, 45 μm, 65 μm, 85 μm, and 105 μm (n=16). The printed resin copings, obtained from a printer (ProJet DP 3000; 3D Systems), were cemented onto their respective die-stone models with self-adhesive resin cement and stored at room temperature until sectioning into halves in a buccolingual direction. The internal gap was measured at 5 defined locations per side of the sectioned die. Images of the printed resin coping/die-stone model internal gap dimensions were obtained with an inverted bright field metallurgical microscope at ×100 magnification. The acquired digital image was calibrated, and measurements were made using image analysis software. Mixed models (α=.05) were used to evaluate accuracy. A false discovery rate at 5% was used to adjust for multiple testing. Coefficient of variation was used to determine the precision for each group and was evaluated statistically with the Wald test (α=.05).
  
  
  RESULTS
  The accuracy, expressed in terms of the mean differences between the prescribed die spacer thickness and the measured internal gap (standard deviation), was 50 μm (11) for the 25 μm group simulated die spacer thickness, 30 μm (10) for the 45 μm group, 15 μm (14) for the 65 μm group, 3 μm (23) for the 85 μm group, and -10 μm (32) for the 105 μm group. The precision mean of the measurements, expressed as a coefficient of variation, ranged between 14% and 33% for the 5 groups.
  
  
  CONCLUSIONS
  For the accuracy evaluation, statistically significant differences were found for all the groups, except the group of 85 μm. For the precision assessment, the coefficient of variation was above 10% for all groups, showing the printer's inability to reproduce the uniform internal gap within the same group.
authors: Resident, Graduate Prosthodontics, Marquette University School of Dentistry, Milwaukee, Wis.
authors: Lisa N
authors: LN
authors: Hoang
authors: Assistant Professor and Director, Department of General Dental Sciences, Graduate Prosthodontics, Marquette University School of Dentistry, Milwaukee, Wis.
authors: Geoffrey A
authors: GA
authors: Thompson
authors: Assistant Professor and Director, Predoctoral Prosthodontics and Biomaterials, Department of General Dental Sciences, Marquette University School of Dentistry, Milwaukee, Wis. Electronic address: seokhwan.cho@marquette.edu.
authors: Seok-Hwan
authors: SH
authors: Cho
authors: Associate Professor and Director, Department of General Dental Sciences, Graduate Dental Biomaterials, Marquette University School of Dentistry, Milwaukee, Wis.
authors: David W
authors: DW
authors: Berzins
authors: Assistant Professor, Division of Biostatistics, Medical College of Wisconsin, Milwaukee, Wis.
authors: Kwang Woo
authors: KW
authors: Ahn
mesh_terms: D017076:Computer-Aided Design / Q000592:standards; D003442:Crowns / Q000592:standards; D018772:Dental Marginal Adaptation; D017267:Dental Prosthesis Design / Q000295:instrumentation*; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008422:Materials Testing; D003765:Models, Dental; D066330:Printing, Three-Dimensional / Q000592:standards; D011897:Random Allocation; D019279:Resin Cements / Q000737:chemistry; D013499:Surface Properties; D019231:Tooth Preparation, Prosthodontic / Q000295:instrumentation
chemical_list: D019279:Resin Cements