Influence of distal prosthetic extension on structural deviation of metal-acrylic all-on-four prostheses: an in vitro study

The All-on-Four concept rehabilitates the edentulous arch with two anterior axial and two posterior tilted implants, but the segment extending distally beyond the implants may affect its structural accuracy. Limited information exists on how the distal prosthetic extension distance affects the structural deviation of completed metal-acrylic hybrid prostheses after acrylic processing. This in vitro study evaluated the effect of three distal prosthetic extension distances on structural deviation in selective laser melting (SLM)-fabricated cobalt-chromium (Co-Cr)/polymethyl methacrylate (PMMA) All-on-Four hybrid prostheses. An edentulous maxillary model restored according to the All-on-Four concept was used. Co-Cr frameworks designed with the same intended CAD cross-section and three distal prosthetic extension distances, defined as the straight-line anteroposterior distance from the incisive papilla to the distal prosthetic endpoint, were fabricated by SLM: 26.5, 32.0, and 39.0 mm ( n = 10 per group; N = 30). Frameworks were veneered with autopolymerizing PMMA acrylic resin. The completed prostheses were digitized with an industrial optical scanner and compared with the computer-aided design (CAD) framework reference in metrology software. Linear deviations in the distances between pairs of control rod endpoints and between pairs of abutment centers, as well as XYZ coordinate deviations of the control rod center points, were analyzed. P1-P2 linear deviation differed significantly among all groups ( P < .001), increasing from 0.183 ± 0.110 mm at 26.5 mm to 0.352 ± 0.123 mm at 32.0 mm and 0.482 ± 0.116 mm at 39.0 mm. For P3-P4, A1-A2, and A3-A4, the 39.0-mm group showed significantly greater deviations than the shorter-extension groups. The greatest changes occurred along the posterior vertical z-axis, with median P3Z and P4Z deviations reaching 0.600 and 0.595 mm, respectively, at 39.0 mm. Increasing distal prosthetic extension distance affected structural deviation in SLM-fabricated Co-Cr/PMMA All-on-Four hybrid prostheses. The 39.0-mm extension produced the greatest post-processing deviation, particularly along the posterior vertical axis.