The study sample consisted of 10 adult subjects (8 females and 2 males, mean age), whose chief compliance was the need of additive restoration in the anterior maxillary area to enhance the smile aesthetic appearance. Subjects were prospectively recruited from a dental private practice in Catania, from June 2019 to December 2019. This study followed the principles laid down by the World Medical Assembly in the Declaration of Helsinki 2008 Helsinki Declaration on medical protocols and ethics and received positive response by the Approval Board of the School of Dentistry, University of Catania (protocol n. 14/19). Inclusion criteria were: adult subjects requiring aesthetic/functional restorations of the maxillary anterior region (canine to canine), good oral hygiene, periodontal health. Exclusion criteria were: missing teeth in the maxillary anterior region, restoration/cavities, history of orthodontic treatment, misalignments and periodontal defects in the maxillary anterior region, severe bruxism or clenching.
Photographic examination
After the clinical assessment of smile characteristics (occlusal, phonetic, static and dynamic), each patient underwent digital photographic examination, according to previous documented guidelines of virtual smile design project [26]. In this respect, two full-faces photos of the patient, one with slightly disclosed dental arches and one with a maximum smile, were registered. The first photo (F1) of the face was taken with the retractors, with semi-disclosed dental arches, to correctly evaluate the parallelism between the bi-pupillary line and occlusal planes as well as and the congruence between the median and interincisive lines (Fig. 1). The second photograph (F2) of the face was detected by removing the retractors and asking patients to smile to evaluate the orientation of the incisal plane with respect to the curve of the lower lip, as well as the width of the lateral corridors (Fig. 2) [27].
Standardized photographic records were taken using camera D300 (Nikon Corporation, Minato-ku, Tokyo, Japan) equipped with AF-S VR Micro-Nikkor 105 mm f/2.8G IF-ED macro lens (Nikon Corporation, Minato-ku, Tokyo, Japan) and Metz 15 MS - 1 digital flash system, with LumiQuest pocket bouncer, on a Medical Close-up bracket (CLS Wireless Flash System). Subjects were instructed to be seated behind a line drawn on the floor while the camera was placed at a distance of 1.50 m from the patient and at the same height as the patient's face in a vertical position [28]. Subjects were asked to look at the camera in order to get the bipupillary plane as parallel as possible to the horizontal plane. Subjects were asked to wear specific glasses equipped with an optical measurement system that allowed the clinician to consistently placed the photographic markers over the camera digital grid. Also, the photographic markers provided the conversion of pixels into mm, in order to consistently calibrate the images used during the virtual planning flow. This method increases the reliability of multiple images acquisition as well as the trueness in the subsequent virtual smile design process.
CAD-CAM workflow
Step 1- virtual planning
The digital photographs were imported into the 2D DSS system (version 1.11.1-alpha.1, Digital Smile System Srl, Italy) for the realization of the virtual planning of the potential aesthetic rehabilitations of the maxillary anterior region (1.3-2.3), aimed at application of veneers and the digital drawing of the new smile, simulating anterior veneers (1.3-2.3) was performed and shown to the patient. The digital restoration project was then realized (Fig. 3).
Step 2- realization of digitally designed mock-ups
In order to obtain a digital wax-up, the stl. files of the patient’s dental arches were registered and align to both F1 and F2 photographs by using the DSS CAD software (DSS3D. Beta.12977, EGS Srl, Italy). This software allows clinicians to design a three-dimensional digital wax-up using as reference the outlines of the 2D smile design previously performed (Fig. 4). The derived .stl file of the digital wax-up was exported and sent to the digital lab for the realization respectively of mock-up (0.4 mm) in methacrylic photoreactive resin (Formlabs, Photopolymer Resin, Gray (GPWH02), Formlabs Inc. USA) and in polymethyl methacrylate (Synergy Disk Tempo Multi, Opal, Nobil-Metal SPA, Italy) (Fig. 5). For the purpose of the present investigation, the 3D printing machine used was the Formlabs form 2 (Formlabs Inc. USA), featuring SLA 3D printing technology. The milling machine used was the CORITEC IMES-ICORE 250i (imes-icore® GmbH, Eiterfeld, Germany) that featured a 5-axis system; the milling sequence of the workpiece involved three progressive internal and external steps of roughing (2 mm), roughing and finishing (1 mm) and finishing (0.6 mm). In order to assure accuracy of the 3D printing, the following procedure were carried out: 1) the liquid resin and the tank were replaced before each print, 2) the digital mock-up was placed in the midst of the printing plate in order to avoid ovalization of the laser beam, 3) the mock-up was positioned with an inclination between 20° and 40° in order to avoid the deformation of the object under its own weight.
Step 3- surface-to-surface analysis of milled and prototyped mock-ups
Both milled and prototyped mock-ups were scanned by using optical scanner with structured light technology (SinergiaScan, Nobil-Metal S.p.A, Italy) and the generated .stl files were imported in Exocad software (DentalCad 2.3 Matera, exocad GmbH, Darmstadt, Germania) along with the .stl file of the 3D digital wax-up project. The scan of each prototyped and milled mock-up were registered on this file and the surface-to-surface matching technique was applied to assess the level of trueness of both mock-ups relative to the digital wax-up performed according to the virtual planning.
Step 4- clinical test
In the fourth phase of the protocol, the mock-ups were tested in the participants' oral cavity. At this stage, each patient was subjected to occlusal evaluation to discriminate the prosthetic fitting of both methacrylic photoreactive resin and polymethyl methacrylate mock-ups.
Moreover, specific linear measurements were performed to assess potential dimensional alteration in both milled and prototyped mock-ups throughout each stage of the entire CAD-CAM workflow:
- 1.
Upper right central incisor height (rU1h) = measurement taken from the center of incisal margin to the most cranial point of gingival contour of the upper right central incisor
- 2.
Left right central incisor height (lU1h) = measurement taken from the center of incisal margin to the most cranial point of gingival contour of the upper right central incisor
- 3.
Upper right central incisor width (rU1w) = mesio-distal diameter of upper right central incisor meaured at the equator level
- 4.
Left right central incisor width (lU1w) = mesio-distal diameter of upper left central incisor measured at the equator level
- 5.
Canine-to-canine width (CCw) = mesio-distal diameter of anterior frontal group measured at the equator level from the distal margin of upper right canine to the distal margin of upper left canine.
In particular, the reported measurements were performed on:
- a.
2D digital smile design, by referring to a specific digital caliper in 2D DSS software (Digital Smile System Srl, Italia)
- b.
3D digital smile project, by using linear measurements function in Exocad.
- c.
scanned MRP and PMMA mock-ups, by using linear measurements function in Exocad.
- d.
MRP and PMMA mock-ups, by using digital caliper (Digital Caliper 0–150 mm, Mitutoyo, Japan).
Statistical analysis
All the measurements were recorded on Microsoft Excel® spreadsheet (Microsoft, Redmond, WA, USA) and analyzed using SPSS® version 24 Statistics software (IBM Corporation, 1 New Orchard Road, Armonk, New York, USA) with P values of less than 0.05 considered statistically significant. The Kolmogorov–Smirnov test and Levene’s test were used to assess respectively the normal distribution and the equality of variance of the data recorded. Since data showed normal distribution (p > 0.05) and equality of variance (p >0.05), parametric tests were used to evaluate potentially significant differences between data measurements.
The trueness of both prototyped and milled mock-ups was assessed by using the Paired Student’s t test which compared the percentage of matching of scanned-prototyped and scanned-milled mock-ups with the digital 3D project, according to the surface-to-surface analysis.
The two-way analysis of variance (ANOVA) was used to assess if there were statistical differences among the linear measurements obtained at each stage of the entire CAD-CAM workflow. In particular, each linear measurement (rU1h, lU1h, rU1w, lU1w and CCw) obtained from 1) the original 3D project, prototyped and scanned-prototyped mock-ups, 2) the original 3D project, milled and scanned-milled mock-ups were compared and post-hoc comparison tests were performed to assess crossed differences.