A Comparison of the Passive Fit of a 3-Unit Implant-Supported Fixed Partial Denture Fabricated by Lost-Wax Casting, Milling Soft Metal Blocks, or Direct Metal Laser Sintering: an in Vitro Study.

Statement of problem. Stresses induced by implant-supported prostheses are related to mechanical complications. However, how the manufacturing method affects the generated stresses is unclear. Purpose. The purpose of this in vitro study was to compare the passive fit of 3-unit cobalt-chromium partial prostheses fabricated by using 3 different methods: conventional casting (milled resin with lost-wax method), machined (milling soft metal blocks), or sintered (direct metal laser sintering). Material and methods. Thirty 3-unit implant-supported cobalt-chromium fixed prostheses were fabricated, 10 with each method. Four strain gauges were placed over an in vitro resin model with 2 implants in it; 2 gauges were placed above and 2 on the sides to measure the microdeformations caused by placing the structures above the implants in longitudinal flexion, vertical flexion, or torsion, before and after tightening the screws. An inferential analysis was carried out by using a statistical software program, and the Wilcoxon and Kruskal-Wallis tests were applied and then complemented with the Mann-Whitney test for multiple comparisons with the Bonferroni correction (alpha=.01). Results. The mean longitudinal deformation, vertical flexion, and torsional deformation values increased after tightening the screws with each of the 3 fabrication methods. Significant differences (P<.01) were found for all 3 methods before and after screw tightening. After screw tightening, the machined structures, followed by sintered structures, and the cast structures (P<.05) had smaller deformations in the X, Y, and Z dimensions. The longitudinal deformation was less in machined structures, and vertical flexion deformation was less in machined and sintered structures than in cast structures (P<.05). Conclusions. The deformations registered in the in vitro model increase considerably after screw tightening, which indicates the lack of passive fit. Machined structures provide a better passive fit, followed by sintered structures, and the cast structures had the poorest fit.