title: Fit of implant-supported fixed prostheses fabricated on master casts made from a dental stone and a dental plaster.
abstract: BACKGROUND
  The impression and cast on which an implant-supported fixed prosthesis is fabricated must accurately reproduce the intraoral relations.
  
  
  OBJECTIVE
  The fit of fixed prostheses fabricated on master casts poured in a conventional die stone and in an ultra-low-expansion plaster was investigated in vitro.
  
  
  METHODS
  An impression was made of patient replicas with inter-implant abutment distances of 50 and 35 mm. Ten master casts were poured in a conventional die stone (Velmix, Kerr) and 10 in an ultra-low-expansion plaster (Gnathostone, Zeus). A simulated plaster fixed prosthesis was fabricated on each master cast and then returned in a random order to the appropriate patient replica. The fixed prostheses were screwed into place on one abutment with a torque of 10 Ncm. Vertical discrepancies were measured at the other abutment by an operator blinded to the cast on which the fixed prosthesis was fabricated. A 2-way analysis of variance was performed for distance and materials, and significant differences were identified with regression analysis.
  
  
  RESULTS
  For the 50-mm inter-abutment distance, die stone master casts produced a mean vertical discrepancy of 80 microm (SD 32.50 microm). Plaster master casts produced a mean vertical discrepancy of 42.8 microm (SD 12.17 microm). The means were significantly different (P=.01). For the 35-mm inter-abutment distance, the mean vertical discrepancy produced from the die stone and plaster master casts was 84.33 microm (SD 49.9 microm) and 0 microm (SD 0), respectively. The means were significantly different (P<.001). A significant difference was found between the mean vertical seating discrepancies of fixed prostheses produced from plaster casts with inter-abutment differences of 50 mm and plaster casts with inter-abutment distances of 35 mm (P=.003). No significant differences were found between mean vertical seating discrepancies for fixed prostheses fabricated on die stone casts.
  
  
  CONCLUSIONS
  In this in vitro study, master casts poured in an ultra-low-expansion plaster limited to a maximum inter-abutment dimension of 35 mm were more accurate than casts with 50-mm inter-abutment spans or those poured in a conventional die stone.
authors.affiliation: Eastman Dental Institute for Oral Healthcare Sciences, University College London, London, United Kingdom. drmwise@aol.com
authors.forename: M
authors.initials: M
authors.lastname: Wise
mesh_terms: D000704:Analysis of Variance; D002133:Calcium Sulfate / Q000737:chemistry*; D000044:Dental Abutments; D003734:Dental Casting Investment / Q000737:chemistry; D003735:Dental Casting Technique; D015921:Dental Implants; D003761:Dental Impression Technique; D019094:Dental Prosthesis, Implant-Supported; D003779:Denture Design; D003830:Denture, Partial, Fixed; D006801:Humans; D003765:Models, Dental; D012044:Regression Analysis; D016037:Single-Blind Method; D013499:Surface Properties; D019415:Torque
chemical_list: D003734:Dental Casting Investment; D015921:Dental Implants; D002133:Calcium Sulfate