Prosthetic rehabilitation using an implant-supported prosthesis is considered a predictable procedure with favorable success rates [1]. For dental implant procedures, the planning and positioning of the implants should be accurate because of the limitations of the patient’s anatomical condition and restorative requirements [2].

Proper implant positioning is an essential step for treatment planning. Two surgical guide fabrication methods are available, conventional and computer-aided design/computer-aided manufacturing (CAD/CAM).

Conventional surgical guide fabrication is performed directly on gypsum casts [3]. The conventional surgical guide processing method involves duplicating the diagnostic wax model using acrylic resin to fabricate templates as well as metallic pins. The metal pins are used to evaluate the expected implant position in dental computed tomography (CT) scans.

When performing a radiologic evaluation, the conventional surgical guide is positioned on the missing teeth area of partially or fully edentulous patients. The visualization of the rehabilitation plan by scanning the metal pins also improves the presurgical evaluation. However, the transfer of a complicated plan to the surgical field lacks accuracy [2]. The complicated and inaccurate laboratory procedure involved in manufacturing conventional surgical guides can also cause an error in placing the implant in the planned area [4]. Several digital approaches have been developed to improve conventional surgical guides. The use of digitally 3D-printed guides could provide significant benefits in multiple simultaneous implant placements and reduce the time required for incision and flap exposure, as well as post operative complications and the discomfort of the patients [5]. Stereolithographyappearance (SLA), digital light processing (DLP), and liquid crystal display (LCD) are 3D printing technologies, which could be used in dentistry [6].

SLA is a digital method that uses a computer-guided laser beam to polymerize photosensitive liquid acrylic through a series of layers [2]. The method involves laser beam illumination above photosensitive liquid acrylic, which allows the surgical stent to be polymerized in layers according to the computer-designed plan [7].

A DLP printer could fabricate surgical stents in a similar way. A DLP printer uses a projector to project the cross-section image of a surgical stent onto photosensitive liquid resin [8]. LCD is the latest printing technology. The light source and imaging system are the main differences in LCD technology [6]. To fabricate 3D-printed guides using DLP for dental implant surgery, a specialized resin needs to be developed with characteristics such as biocompatibility, mechanical and chemical stability, and tolerance of vertical pressure for implant placement.

In this study, photocurable macromolercular resin (KIS resin; Kuwotech, Gwangju, Korea) was developed as a basic material for DLP. The biological and physical properties of the newly developed resin material were assessed. In addition, the clinical accuracy of digital light-processed stents fabricated with KIS resin was compared to that of conventional stents by measuring the marginal gap and radiological analysis.

Resin synthesis and fabrication

KIS resin for DLP surgical stents

The resin was blended with different oligomers (Bis-EMA, Bis-GMA), monomers (TMPTA, DPHA, and IBOA), additive (TMPDE, TMPTMP) and a single initiator (DPO). The reaction was performed in a 3 neck synthesis device by the regulation of reaction velocity, temperature and other conditions. The KIS resin compound (Kuwotech) was synthesized using Bis-EMA 45 wt%, TMPTA 36.4 wt%, DPHA 10.4 wt%, IBOA 5.2 wt%, DPO 2.5 wt%, TMPDE 0.25 wt%, and TMPTMP 0.25 wt% at 200 rpm for 6 hours (Fig. 1A).

Fig. 1. (A) KIS novel resin. (B) Conventional orthodontic acrylic resin.

Calibration of the physical characteristics of KIS resin

The specimens were manufactured by using DLP-type 3D printer (Gprinter; Gooo3D, Seongnam, Korea). Flexural strength, compressive strength and tensile strength were measured under the guidelines of certified standards. The three-point flexural strength of the specimens was obtained by a universal testing machine (Test One, Siheung, Korea) in accordance with ISO 20795-1 (denture base polymer material).

The compressive strength and tensile strength of the KIS resin were also calibrated using a universal testing machine (Test One). ASTM D695 was used as the guideline for compressive strength, and ASTM D638 was used for tensile strength.

Acrylic resin for conventional surgical stents

A commercial orthodontic acrylic resin (Caulk; Dentsply Sirona, Charlotte, NC, USA) was used to fabricate of conventional surgical stents. Methyl methacrylate in monomer liquid and poly-methylmethacrylate powder were blended to activate chemical polymerization (Fig. 1B).

Cell experiments

Human gingival fibroblasts

The human gingival fibroblast (hGF) cells of healthy gingival connective tissue were obtained by the reconstruction procedures of atrophic gingival ridges. The teeth adhered to the gingival tissue in this study had no bone loss in radiologic evaluation, endodontic involvement, or caries. After local anesthesia was applied to the surgical area, gingival tissue was taken from keratinized gingiva near the surgical area. The gingival connective tissue samples were divided into small pieces and placed in 35-mm² tissue culture dishes containing Dulbecco’s modified Eagle’s medium (DMEM) with 100 units/mL penicillin, 100 μg/mL streptomycin, and 10 % fetal bovine serum (FBS) at 37℃ in a humidified atmosphere of 5% CO₂, and the medium was changed every 3 days [9].

Observation of cell attachment and spreading using scanning electron microscopy

Two types of disks were fabricated using KIS resin material and conventional resin. Scanning electron microscopy (SEM, S-4700; Hitachi, Tokyo, Japan) images of the disks seeded with hGF cells were taken to evaluate hGF cell morphology, attachment, and early growth. hGF cells were seeded at a density of 1×10⁴ -cells/mL with a-MEM media containing 10% FBS. After incubation for 2 days, the disks were washed with phosphate-buffered saline (PBS) and then fixed with 2.5% glutaraldehyde in 100 mM cacodylate buffer (Sigma-Aldrich, St. Louis, MO, USA). The samples were dehydrated in increasing concentrations of ethanol (30%, 60%, 95%, and 100%), immersed in hexamethyldisilazane (Sigma) for 15 minutes and mounted on aluminum stubs immediately followed by coating with platinum [10]. SEM images were taken twice in each group at ×100, ×250, and ×1,000 magnifications.

Animal study

Ethics

Two adult male beagle dogs (18-months-old, approximate weight of 15 kg; Genia, Seongnam, Korea) were treated in the animal study. The animals had ad libitum access to water and a standard laboratory dog food diet throughout the study. Individual housing was provided for the animals. The study protocol of this study was approved by the Ethics Committee of Chonnam National University for animal experiments (CNU IACUC-YB-2017-26).

Fabrication of the surgical guides

The surgical guides used in this study were fabricated by 2 different methods, the DLP method and the conventional method. Premolars (P1–P4) were extracted from the beagle dogs. One month after extraction, impressions were taken to fabricate the implant surgical stents. Implant surgical stents were fabricated by the DLP method for the left side of the jaw and the conventional method for the right side of the jaw.

(1) Digital light processing surgical stent

Computer software (3 shape Dental system; 3 Shape, Copenhagen, Denmark) was employed for converting the DICOM files of the cone-beam computed tomography (CBCT) images to STL format. Using the computer software (3 Shape), the implant placement position and axis of the fixture were virtually selected. The implant surgical stents were designed according to a virtual plan [11]. The surgical stent design was exported in STL format and the completely designed files were transferred to the 3D printer software [12].

The surgical stents were fabricated by a DLP 3D-printer (Gprinter; Gooo3D) with the following settings: an additive layer thickness of was 50 μm and a photo-polymerization time of 2 seconds. The KIS resin was used as the basic material for the surgical guide.

The fabricated surgical stent was separated from the vat following ultrasonic cleaning. After the first ultrasonic cleansing was done, the supporting part was removal, a second ultrasonic cleaning were performed and the DLP surgical stent was ready to use [12].

(2) Conventional surgical stent

Conventional surgical guides were fabricated with auto-polymerizing acrylic resin on a gypsum model. A diagnostic wax model was duplicated with Omnivec and acrylic resin. Metal pins were positioned in the center of the pontic area. Powder polymer and monomer in liquid were mixed in a rubber cup and poured into the base plate of the wax-boxed model.

After complete polymerization of the resin material, the metal pins were removed. Then, metal sleeves were placed at the hole and the surgical stent was ready to use.

Marginal gap analysis

The surgical guides fabricated by the two methods were positioned on duplicate models. The samples were cross-sectioned horizontally using thin disks on the cast, as shown in Fig. 2. The marginal gap was analyzed by measuring the internal fit of the surgical guide on the cast as shown in Fig. 3 [3]. The average value of marginal gap was measured on ×30 SEM images on horizontal plane.

Fig. 2. Marginal fit analysis on gypsum duplicated model (A–D, ×30); (A, B) DLP printed stent, (C, D) conventional type stent. DLP, digital light processing.

Fig. 3. (A) Marginal gap analysis of conventional stent on gypsum model. (B) DLP printed stent on gypsum model. DLP, digital light processing.

Surgical procedures

All surgeries were performed by a skilled periodontist. The two animals underwent local anesthesia with a 2% lidocaine hydrochloride. The right side of the mandible received the conventional stent and the DLP stent was used on the left side, as shown in Fig. 4. The surgery started with a mid-crestal incision and a vertical incision made in the mesial margins of the crestal incision. The surgical guides were carefully and properly seated on the teeth adjacent to the missing area. To expose the bone surface area for implant placement, full-thickness flap elevation was done buccally and lingually. Consecutive drilling for osteotomy was performed with a KIS implant surgical kit according to the manufacturer’s insrtuctions. Three dental implants (KISplant, diameter 3.5, length 7 mm) were placed in each side of the mandible [13].

Fig. 4. Fabrication of surgical guides and implant placement on beagle dog jaw; (A, B) Conventionl stent, (C, D) and DLP printed stent. DLP, digital light processing.

Radiological analysis

To evaluate the accuracy of the two surgical stents, intraoral radiographs (#4+ image plate, Vista Scan; Durr dental, Bietigheim-Bissingen, Germany) were taken before surgery and after the implant installation. The preoperative images and postoperative images were aligned pair-wise to overlap, and software (Photoshop 7.0; Adobe, San Jose, CA, USA) was used to measure the distance between the planned position and the placed position of each group at three longitudinal levels, coronal, middle, and apex [4,11].

The angular deviations of the planned position and placed position in each group were measured on the central axis of the placed fixtures, as shown in Fig. 5. The angular deviations were calibrated using computer software (AutoCAD; Autodesk, San Rafael, CA, USA).

Fig. 5. Angular and linear deviation between planned and placed implant axis; (A) DLP printed stent, (B) conventional type stent. DLP, digital light processing.

Statistical analysis

Statistical analyses were performed using a statistical software package (SPSS version 25.0; IBM, Tokyo, Japan). The Mann–Whitney U-test was used to compare linear deviations and an independent t-test was performed to compare of angular deviations. A p-value of less than 0.05 was considered statistically significant.

Physical characteristics of the resin material

The results of physical tests on the KIS resin showed 107.9 MPa of flexural strength and 78.6 MPa of compressive strength. The tensile strength of the KIS resin was 60.6 MPa. The physical characteristics of the KIS resin met the ISO 20795-1 standard for flexural strength, ASTM D695 for compressive strength, and ASTM D638 for tensile strength.

SEM analysis of fibroblastic cell proliferation

The SEM images of hGF cell attachment to the surface of all resin disks after 3 days of culture are shown in Fig. 6. For each image, the cell morphology and cell attachment were evaluated on SEM images. The cells on the KIS resin surfaces were polygonal in shape and attached well to the resin surfaces. The cells had many cytoplasmic extensions connecting them in a spindle shape.

Fig. 6. The SEM images of cell attachment on resin disk of each group (A: ×100, B: ×250, C: ×1,000, D: ×100, E: ×250, F: ×1,000); (A–C) 3 day observation on KIS resin, (D–F) conventional resin. SEM, scanning electron microscopy.

Similar patterns were seen in the cells on the conventional resin surfaces. However, some cells were scattered and decreased cell attachment was observed. Oval-shaped and unconnected cells were observed on the conventional resin surfaces.

Marginal gap analysis

The internal fit was estimated by the distance from the stent inner wall to the tooth surface on a duplicate gypsum model. A better fit of the DLP-printed surgical guide on the dentition model was observed. In the ×30 SEM images, DLP-printed surgical guide was tightly positioned at dentition. The mean attachment gap value on the DLP-printed surgical guide was 56 μm, whereas the stent fabricated by the conventional method had an average attachment gap of 95 μm.

Radiographic analysis

The linear deviations measured at 3 longitudinal levels (coronal, middle, and apex) of the planned and positioned implant axis showed significantly minimized differences in the coronal and middle parts of both groups, as shown in Table 1. The linear deviations at the coronal level were 1.24±0.8 mm in the conventional stent group and 0.40±0.4 mm in the DLP stent group.

Similar patterns were observed at the middle level of the implant position, with a linear deviation of 1.15±1.3 mm in the conventional stent group and 0.23±0.2 mm in the DLP stent group.

Focused on angular deviation, the DLP stent group showed significantly favorable clinical data. The mean angular deviations in the DLP group was 2.0°±2.0°, which was significantly lower than the angular deviation in the conventional stent group at 7.7°±4.3° (p<0.05). Implant placement using DLP stents showed improved implant positioning and axis control.

Computer-guided implant placement has brought out technical improvements to the field of implant dentistry. The ability to determine implant placement position for desired prosthetic outcomes prior to implant surgery is a major benefit of this technology [14].

SLA was the earliest method for rapid prototyping [6]. The method has been used in various industrial areas and adopted for the fabrication of dental implant surgical stents [15]. However, the SLA printing rate is relatively lower than DLP printing due to its curing rate, which depends on the movement of the laser beam [6].

DLP printers improved the printing rate of SLA printer by using a projector rather than a laser beam. DLP printers showed significantly faster print times.

LCD printers are the latest technology. However, the light intensity of an LCD printer is weak, and partial light leakage from an LCD printer could cause inferior precision [6]. Thus, a DLP printer was used to fabricate the surgical stents in this study.

The physical properties of a newly developed resin for DLP were assessed by various methods. The results of the physical tests of the KIS resin showed 107.9 MPa of flexural strength and 78.6 MPa of compressive strength. The tensile strength of the KIS resin was 60.6 MPa. Park et al. [16] studied the flexural strength of the conventional orthodontic acrylic resin used in this study. The flexural strength of Caulk orthodontic resin was 11,249 pounds-per-square-inch (PSI) and was calculated as 77.6 MPa [16]. The KIS resin showed more improved flexural strength than the conventional resin.

Aguirre et al. [17] compared the flexural strength of 3 different types of denture base resins that were fabricated by the injection method, compression method, and CAD-CAM milling. They reported a significantly higher flexural strength in the CAD-CAM-milled group. A higher degree of conversion could have affected the higher flexural strength of the CAD-CAM specimens [18].

In SEM analysis conducted to observe of cell attachment and spreading, similar morphology was observed on the conventional acrylic resin and the KIS resin disks. Those two types of resin showed favorable hGF cell responses.

Other studies reported that acrylic resin- based materials showed cytotoxicity due to their methyl methacrylate compositions [19]. Residual monomers from incomplete conversion into polymers cause toxic components to leach into the oral environment [20]. However, the CAD-CAM-type pressed acrylic resins were polymerized under conditions that prevented the release of toxic components such as residual monomers [21].

Although most studies regarding the toxic effects of CAD-CAM materials have focused on conventional acrylic resin, bis-acrylic resin and CAD-CAM-type pressed acrylic resin, this study reported the cell compatibility of 3D-printed resin [22].

In the SEM images in this study, favorable cell adhesion and connections of dendritic cell processes were observed on the KIS resin surface. There were more polygonal-shaped cells on the KIS resin specimens. The results indicated the improved biocompatibility of the newly developed KIS resin.

A marginal gap analysis was executed to analyze the difference in the accuracy of the internal fit between the conventional guides and those fabricated by DLP.

Reyes et al. [3] investigated the fit of surgical stents by comparing conventional-type and CAD-CAM fabrication methods. The study reported that the conventional guides showed a better fit on a Kennedy class 3 cast with Type V dental stone. However, the 3D printing type of CAD-CAM guide had a better fit on Kennedy class 2 casts [3].

In this study, the premolars (P1–P4) of beagle dogs were extracted. 3D-printed resin stents were fabricated and adapted on adjacent canines and first molars. The results showed that the DLP surgical guides had a better internal fit than the conventionally fabricated guides. The average gap attachment in the DLP surgical guides was 56 μm and less than the average attachment gap of the conventional guides at 95 μm.

The DLP-printed surgical guide was tightly positioned at dentition. However, the stent fabricated by the conventional method had a gap of over 100 μm.

A comparison of the results of this study with previous studies indicated that further studies of repeated measures with multiple casts of different edentulous types should be conducted. .

This study found that the differences in the deviations and angulations between the planned and placed horizontal implant positions using the DLP method were smaller than the implants placed using conventionally fabricated guides.

The linear deviation of the implant position was significantly minimized at the coronal and middle levels. Even though it was not statistically significant, the horizontal deviation at the apical level showed a trend similar to that at the superior level. This study had a relatively limited study sample, so the statistical results of the lateral deviations at the apical level should be accepted with caution.

Several factors could have influenced the degree of difference between the planned and actual implant axial direction, such as structural stability of the guide, surgical accuracy when fitting these stents, study model construction, the accuracy of the 3D printers, and measurement accuracy. The movement of surgical stents during surgery and the reproducibility of the guide position between the radiologic evaluation and surgery could also generate errors [23].

The 3D-printed stent had the structural components of metal sleeve, which could be related to surgical accuracy. The conventional stent was sleeveless. Therefore, further studies comparing 3D-printed stents with and without metal sleeves are required.

The continued development of digital technology such as intraoral scanners, 3D printers, and CAD-CAM machines could introduce a fully digital workflow for planning guided implant surgery. Finally, an image fusion technique was used to match superimposed CBCT scan images and intraoral scans [24]. The surgeon could produce a virtual implant surgery plan for proper positioning considering the anatomic condition and prosthetic outcomes. Further studies should be conducted to evaluate of improvements in surgical stents using novel image fusion techniques.

In this study, the KIS resin developed for intended use in fabricating implant surgical stents by the DLP method was investigated. The physical properties of KIS resin showed the potential for use as a basic material in the DLP fabrication of implant surgical stents and favorable cell viability.

Implant surgical stents fabricated by the DLP method were evaluated as a favorable tool for implant placement in the proper position. The results of this study showed that DLP-printed surgical stents had significantly decreased deviations of the implant insertion angle and horizontal position between the planned and actual placement state compared to conventional stents.

None.

The authors declare that they have no competing interests.