Fit of implant-supported fixed prostheses fabricated on master casts made from a dental stone and a dental plaster

Statement of problem. The impression and cast on which an implant-supported fixed prosthesis is fabricated must accurately reproduce the intraoral rel ations. Purpose. The fit of fixed prostheses fabricated on master casts poured in a conventional die stone and in an ultra-low-expansion plaster was investiga ted in vitro. Material and methods. An impression was made of patient replicas with inter -implant abutment distances of 50 and 35 mm. Ten master casts were poured i n a conventional die stone (Velmix, Kerr) and 10 in an ultra-low-expansion plaster (Gnathostone, Zeus). A simulated plaster fixed prosthesis was fabri cated on each master cast and then returned in a random order to the approp riate patient replica. The fixed prostheses were screwed into place on one abutment with a torque of 10 Ncm. Vertical discrepancies were measured at t he other abutment by an operator blinded to the cast on which the fixed pro sthesis was fabricated. A 2-way analysis of variance was performed for dist ance and materials, and significant differences were identified with regres sion analysis. Results. For the 50-mm inter-abutment distance, die stone master casts prod uced a mean vertical discrepancy of 80 mum (SD 32.50 mum). Plaster master c asts produced a mean vertical discrepancy of 42.8 mum (SD 12.17 mum). The m eans were significantly different (P=.01). For the 35-mm inter-abutment dis tance, the mean vertical discrepancy produced from the die stone and plaste r master casts was 84.33 mum. (SD 49.9 mum) and 0 mum (SD 0), respectively. The means were significantly different (P < .001). A significant differenc e was found between the mean vertical seating discrepancies of fixed prosth eses produced from plaster casts with inter-abutment differences of 50 mm a nd plaster casts with inter-abutment distances of 35 mm (P=.003). No signif icant differences were found between mean vertical seating discrepancies fo r fixed prostheses fabricated on die stone casts. Conclusion. In this in vitro study, master casts poured in an ultra-low-exp ansion 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.