Effect of subgingival depth of implant placement on the dimensional accuracy of the implant impression: An in vitro study

Statement of problem. In some instances, an implant needs to be placed deep subgingivally, which may result in a less accurate impression of the implant. Purpose. The purpose of this study was to evaluate the effect of subgingival depth of implant placement on the accuracy of implant impressions. Material and methods. A stone master model was fabricated with 5 implant analogs (RN synOcta analog), embedded parallel to each other, at the center (E) and the 4 corners (A, B, C, and D). The vertical position of the shoulders of the implants was intentionally different among the implants: A and E were flush with the top surface of the model; B was 2 mm below, and C and D were 4 mm below the surface. The horizontal distances of implants A, B, C, and D from E were measured with a measuring microscope. A cross-shaped metal measuring bar was then fabricated and connected to E, with the arms of the casting designed to be 2 mm above the top surface of the model and incorporating a reference mark. With the measuring bar connected to E, the vertical distances from the apical surface of A, B, C, and D to the measuring reference marks were measured with a digital micrometer. The body of the impression coping for implant D was modified by adding 4 mm of additional impression coping, while standard impression copings were used for all other implants. Open tray impressions were made using medium-body polyether material (Impregum Penta) or a combination of putty and light-body vinyl polysiloxane (VPS) material (Elite HD+) (n=15). Then casts were poured with type IV dental stone. The vertical and horizontal distances of the casts were measured with the methods outlined above for the master model. The distortion values that were determined as differences between the measurements of the master model and those of the casts were collected for statistical analysis. Two-way and 1-way repeated measures ANOVA followed by Tukey's HSD test were performed to compare the distortion values (α=.05). Results. For vertical measurements, 2-way repeated measures ANOVA showed no significant depth ( P =.36), material ( P =.24), or interaction effects ( P =.06). However, it showed significant depth effect for horizontal measurements ( P =.01). Within the polyether group, 1-way repeated measures ANOVA showed significant differences in horizontal measurements among the implants with different depths ( P =.03). The post hoc Tukey's test showed that the impression of 4-mm-deep implants with normal impression copings (C) was significantly less accurate than impressions of 0-mm-deep implants (A) ( P =.02). Within the VPS group, there was no significant difference among the implants with different depths ( P =.09). Conclusions. There was no effect of implant depth on the accuracy of the VPS group. However, for the polyether group, the impression of an implant placed 4 mm subgingivally showed a greater horizontal distortion compared to an implant placed more coronally. Adding a 4-mm extension to the retentive part of the impression coping eliminated this difference. (J Prosthet Dent 2008;99:107-113)