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 relations. Purpose. 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. 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 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 μm (SD 32.50 μm). Plaster master casts produced a mean vertical discrepancy of 42.8 μm (SD 12.17 μm). 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 μm (SD 49.9 μm) and 0 μm (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. Conclusion. 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. (J Prosthet Dent 2001;86:532-8.)