Fabrication of interim crowns and evaluation of intaglio surface trueness

To determine the number of interim crowns (sample size) to be fabricated per finishing line locations, three pilot experiments were performed prior to this study. Based on the results of the pilot experiment, the sample size was determined using power analysis software (G*Power v3.1.9.2; Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany) (N = 16; effect size [f] = 0.63; actual power = 99.11%; power = 99%; α = 0.05).

To obtain a reference virtual model of tooth preparation, a precise surface scanning was performed using a contact scanner (DS10; Renishaw plc, Gloucestershire, UK) (Fig. 1). To obtain a high-resolution virtual model, five standard tessellation language (STL) files were acquired through contact scanning and five STL files were merged after optimization alignment by using a 3D mesh software program (Geomagic Design X; 3D Systems, Rock Hill, USA).

Procedure for intaglio surface trueness of interim crowns fabricated from tooth preparation scanned at four finish lines.

The reference model of tooth preparation was adapted to the conditions of each group and fixed to the reference model without movement. The supragingival finishing line was located approximately 0.5 mm above from level of the gingiva, whereas the subgingival finishing line was located approximately 0.5 mm below from the level of the gingiva. The equigingival finishing line was located at the level of the gingiva. Additionally, at the subgingival finishing line, a gingival displacement cord (# 2 Ultrapak; Ultradent, South Jordan, UT, USA) was packed into the gingival sulcus below the finishing line. The depth of the subgingival finishing line was confirmed using a periodontal probe (CP 15 UNC; HU-Friedy, CHI, USA).

To obtain a test virtual model of tooth preparation, an intraoral scanner (i500; MEDIT, Seoul, Republic of Korea) was used to scan a reference model at the supragingival, equigingival, subgingival, and subgingival finish line locations with gingival displacement cords (N = 16 per locations; Fig. 1). All scanning and analysis procedures were performed by an experienced investigator (K.S.).

Sixteen test virtual models acquired per finishing line locations and a reference virtual model were extracted as STL files for interim crown fabrication. In a dental CAD software program (3Shape, Copenhagen, Denmark), the design of interim crowns was performed under the same conditions of a 60-µm cement space. The STL file of the interim crown designed based on the reference virtual model was designated as a CAD reference model (CRM) for the evaluation of intaglio surface trueness (Fig. 1). Interim crowns designed based on the test virtual model were fabricated using a stereolithography 3D printer (ZENITH; Dentis, Daegu, Republic of Korea) with 0° parallel to the vat bottom. In consideration of the printing and repetition accuracy according to the position of the printed object in the vat, the interim crowns produced in four groups were divided into quarters and adjusted to the same position and number when printing once. For the photopolymerization resin for the interim crown, 3D printing resin (For interim crown; Dentis, Daegu, Republic of Korea) was used. For interim crowns after printing, all residual resin was removed according to the manufacturer’s recommendations, and postphotopolymerization was performed using a light-curing unit (CUREDEN; Kwang Myung DAICOM, Seoul, Republic of Korea). All evaluations were completed within 3 h after printing in consideration of the dimensional change according to the time change after printing. The intaglio surface of interim crowns after all posttreatments were scanned using an intraoral scanner (i500; MEDIT, Seoul, Republic of Korea), and the STL file was designated as the CAD test model (CTM) for the evaluation of the intaglio surface trueness (Fig. 1).

Through the evaluation of the intaglio surface trueness, the accuracy of the intaglio surface of interim crowns manufactured according to the finishing line locations was compared (Fig. 1). CRM and CTM alignment and 3D comparison were performed using a 3D inspection software program (Geomagic Control X; 3D Systems, Rock Hill, SC, USA) (Fig. 1). The area of the intaglio surface was segmented based on the margin of CRM. To evaluate the intaglio surface area in detail, it was divided into the marginal, axial, and occlusal regions. CRM and CTM were aligned based on the segmented intaglio surface, and the root mean square was calculated as follows based on all cloud points of the CRM intaglio surface (Eq. ¹):

where Di represents the gap distance of point i of CRM and CTM and n is the number of all points evaluated.