title: A clinical pilot study of the dimensional accuracy of double-arch and complete-arch impressions.
abstract: BACKGROUND
  It has been suggested that articulated casts fabricated with the double-arch impression technique may have superior occlusal accuracy than those fabricated with a single complete-arch impression. However, lack of tray rigidity may lead to flexure of the impression/tray complex, resulting in inaccurate dies in the bucco-lingual dimension.
  
  
  OBJECTIVE
  This clinical pilot study compared the dimensions of dies fabricated with 3 types of double-arch impressions to dies fabricated with the conventional complete-arch, custom tray method.
  
  
  METHODS
  Thirty-five addition silicone impressions were made of cast metal copings cemented onto natural teeth prepared as complete-crown abutments. Four combinations of tray types and impression material viscosity were used: (1) complete-arch, custom acrylic trays loaded with heavy-bodied material; (2) double-arch, disposable plastic trays loaded with heavy-bodied material; (3) double-arch, disposable plastic trays loaded with putty material; and (4) double-arch, reusable brass metal trays loaded with heavy-bodied material. Immediately prior to tray insertion, light-bodied impression material was syringed over all copings as a wash. The 4 copings were fabricated from cast gold and simulated metal-ceramic complete-crown thimbles with polished collars and had "projections" on the occlusal surfaces. The impressions were poured in type IV die-stone. Bucco-lingual and inter-abutment dimensions were measured. The differences between the stone dimensions and cast metal control dimensions were calculated and converted to percent dimensional change. The data were analyzed with 1-way analysis of variance, Student t tests, and Mann-Whitney tests (P<.05).
  
  
  RESULTS
  The plastic double-arch tray loaded with heavy-viscosity addition silicone and a low-viscosity wash produced the least accurate combination inter- and intra-abutment dimensions. For this protocol, 1.17% mean dimensional change was recorded. This result was significantly different than that obtained for the other 3 impression methods tested. No significant differences were found between the complete-arch method and protocols in which putty was loaded in a plastic or metal tray.
  
  
  CONCLUSIONS
  Within the limitations of this pilot study, the more rigid tray/impression material combinations more accurately replicated stone dies.
authors.affiliation: Dental School, University of Queensland, Brisbane, Queensland, Australia. jeffcox74@hotmail.com
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Cox
authors.forename: Robert L
authors.initials: RL
authors.lastname: Brandt
authors.forename: Harold J
authors.initials: HJ
authors.lastname: Hughes
mesh_terms: D000704:Analysis of Variance; D003760:Dental Impression Materials; D003761:Dental Impression Technique / Q000295:instrumentation; D006801:Humans; D003765:Models, Dental; D010865:Pilot Projects; D015203:Reproducibility of Results; D012833:Siloxanes; D018709:Statistics, Nonparametric
chemical_list: D003760:Dental Impression Materials; D012833:Siloxanes