RESUMEN
PURPOSE: To compare the three-dimensional (3D) accuracy of conventional impressions (CIs) with digital scans (DSs) using an intraoral scanner (IOS) with intraoral scan bodies (SBs) and varying buccolingual interimplant angulations. A secondary aim was to measure the SB machining tolerance and height with and without application of torque. MATERIALS AND METHODS: Three master models (MMs) with two implants simulating an implant-supported three-unit fixed partial denture for bone-level implants were used. The implants had buccolingual interimplant angulations of 0, 10, and 20 degrees. Test models for the CI test groups were made with impression copings and polyether impressions. SBs were attached to the MMs, tightened to 15-Ncm torque, and scanned by an IOS for the DS test groups (six test groups, n = 5). A coordinate measuring machine measured linear distortions (dx, dy, dz), 3D distortions (dR), angular distortions (dθx, dθy), and absolute angular distortions (Absdθx, Absdθy) of the physical CI test models and STL files of the DS virtual models relative to the MMs. Metrology software allowed both physical and virtual measurement of geometric targets that were comparable and allowed computation of relative displacements of implant centroids and axes. RESULTS: Mean dR ranged from 31 ± 14.2 to 45 ± 3.4 µm for DS and 18 ± 8.4 to 36 ± 6.5 µm for the CI test groups. Mean Absdθx ranged from 0.041 ± 0.0318 to 0.794 ± 0.2739 degrees for DS and 0.073 ± 0.0618 to 0.545 ± 0.0615 degrees for the CI test groups. Mean Absdθy ranged from 0.075 ± 0.0615 to 0.111 ± 0.0639 degrees for DS and 0.106 ± 0.0773 to 0.195 ± 0.1317 degrees for the CI test groups. Two-way analysis of variance showed that the impression technique (P = .012) and implant angulations (P = .007) had a significant effect on dR. Distortions were mostly in the negative direction for DS test groups. Perfect coaxiality of the SB with the implant was never achieved. For SB to implant machining tolerances, the mean absolute horizontal displacement ranged from 4 ± 1.2 to 7 ± 2.3 µm. The SB dz was -5 ± 3.2 µm, which increased in the negative direction to -11 ± 4.9 µm with torque application (P = .002). CONCLUSION: Distortions were found for both DS and CI test groups. The best accuracy was obtained with CIs for parallel implants. With angulated implants, conventional and DSs were not significantly different. Excessive torque application that causes negative dz for SB fit would position the virtual implant at a deeper location compared with reality, resulting in possible framework misfit.
RESUMEN
PURPOSE: The purpose of this study was to compare the three-dimensional accuracy of indirect plastic impression copings and direct implant-level impression copings from three implant systems (Nobel Biocare [NB], Biomet 3i [3i], and Straumann [STR]) at three interimplant buccolingual angulations (0, 8, and 15 degrees). MATERIALS AND METHODS: Two-implant master models were used to simulate a three-unit implant fixed partial denture. Test models were made from Impregum impressions using direct implant-level impression copings (DR). Abutments were then connected to the master models for impressions using the plastic impression copings (INDR) at three different angulations for a total of 18 test groups (n = 5 in each group). A coordinate measuring machine was used to measure linear distortions, three-dimensional (3D) distortions, angular distortions, and absolute angular distortions between the master and test models. RESULTS: Three-way analysis of variance showed that the implant system had a significant effect on 3D distortions and absolute angular distortions in the x- and y-axes. Interimplant angulation had a significant effect on 3D distortions and absolute angular distortions in the y-axis. Impression technique had a significant effect on absolute angular distortions in the y-axis. With DR, the NB and 3i systems were not significantly different. With INDR, 3i appeared to have less distortion than the other systems. Interimplant angulations did not significantly affect the accuracy of NBDR, 3iINDR, and STRINDR. The accuracy of INDR and DR was comparable at all interimplant angulations for 3i and STR. For NB, INDR was comparable to DR at 0 and 8 degrees but was less accurate at 15 degrees. CONCLUSIONS: Three-dimensional accuracy of implant impressions varied with implant system, interimplant angulation, and impression technique.