An in vitro comparison of the marginal fit of provisional crowns using the virtual tooth preparation workflow against the traditional technique

Abstract

Aim:

This study investigates the effectiveness of an innovative virtual tooth preparation workflow for the fabrication of dental crowns using cone-beam computed tomography (CBCT) and intraoral scanners (IOSs) with conventional workflow using extraoral/laboratory scanners.

Settings and Design:

This in vitro experimental study was conducted in the laboratory of a university in Chennai, India. The dental laboratory and research facilities at the institution were utilized for the fabrication of the temporary crowns and the data acquisition process.

Materials and Methods:

Institutional approval was obtained from the university. It was basically a comparison between the virtual prep technique using CBCT and IOS and the conventional digital technique using extra oral scanners (EOS) for temporary crown fabrication. The sample size was estimated using an effect size of 1.5004, assuming a normal distribution, a significance level of 0.05, and a power of 0.95 in G power software. Based on this calculation, an extracted second lower molar was used to fabricate 10 samples in each group. The samples were divided into three groups: the CBCT (Group 1), the IOS (Group 2), and laboratory scanner (Group 3 as control) groups. The vertical marginal gap of all the surfaces of the crown was evaluated using a scanning electron microscope.

Statistical Analysis Used:

Data were analyzed using one-way ANOVA using the SPSS software version 26.0, IBM, Armonk, NY, USA.

Results:

Acceptable marginal discrepancy values were obtained in all three groups. There was no significant difference in the marginal discrepancy recorded (P = 0.113).

Conclusion:

Virtual tooth preparation using CBCT and IOSs can be used as an alternative to the conventional workflow for provisional crown and bridge fabrication.

INTRODUCTION

With the advent of digital dentistry, various methods of provisional crown fabrication have been employed lately.[1] Currently, the use of computer-aided design (CAD)/computer-aided manufacturing (CAM) technology is the most preferred method of fabrication following tooth preparation over manual methods.[2] This includes scanning of the prepared tooth surface and then designing the crowns. This is both time-consuming and usually requires an additional clinical appointment.

Virtual tooth preparation workflow revolutionizes the fabrication of temporary crowns by utilizing computer-generated tooth preparation technology.[3,4] This innovative approach eliminates the conventional need for in-mouth tooth preparation followed by prefabricated dental wax-ups, as the prosthesis is readily available even before commencing the tooth preparation process.[5,6] This allows for efficient relining and cementation of the prosthesis in accordance with the patient’s specific preparation requirements. This method has several advantages, such as improved accuracy, reduced time required for crown fabrication, increased efficiency, enhanced understanding of the treatment procedure and outcomes, and greater patient involvement in the treatment process.[7]

Newer methods of data acquisition have been advocated for obtaining three-dimensional (3D) models to fabricate dental prostheses, including conventional extra oral scanners (EOS) and other methods such as intraoral scanners (IOS) and cone-beam computed tomography (CBCT).[8-10] This scan data can be converted into standard tessellation language (STL) file format, which can later be imported into digital dental designing software for fabrication of crowns using virtual tooth preparation workflow.[11-13] Achieving proper adaptation of dental prostheses is crucial for the long-term success of prosthetic treatments.[14,15] Previous studies[16-20] have proposed a clinically acceptable range of 80–200 microns as the permissible difference between the tooth surface and the prosthesis.

This research aims to compare the marginal fit of crowns created using a virtual tooth preparation workflow from CBCT data and intraoral scan with the commonly used digital workflow to fabricate dental crowns and bridges using extraoral scan. The null hypothesis was that there would be no difference in the marginal fit of crowns fabricated using either workflow.

MATERIALS AND METHODS

Study setting and estimation of sample size

In this in vitro study, an extracted lower second molar tooth was used to create 10 crowns for each group from 10 CAM outputs to assess the marginal discrepancy for the four surfaces of each crown (n = 40 for each group). To determine the sample size, the effect size was estimated (dz = 1.5004) assuming a normal distribution, with a = 0.05 and a power of 0.95 (1-b error probability). The sample size calculation was performed using G*Power 3.1.9.3 for Mac OS X®□,[21] based on data from a previously published study.[5] Ethical approval for this study was obtained from the university (approval number: IHEC/SDC/PROSTHO-2002/23/124).

Scanning and manufacturing protocol

This study was a comparison between the virtual prep technique using CBCT and IOS and the conventional digital technique using EOS for temporary crown fabrication.

The samples were divided into three groups: test Group 1 (CBCT group), test Group 2 (IOS group), and control Group 3 (EOS) [Figure 1]. The samples in the test groups were fabricated similarly in each step except for their method of data acquisition. The scans obtained from each of these methods were exported in STL file format into CAD software (3Shape, Niels Juels Gade 13, 1059 Copenhagen K Denmark), where a virtual tooth preparation workflow was followed. This involved digitally removing the necessary tooth structure to accommodate the temporary crown restoration while ensuring proper retention and esthetics by fine-tuning the preparation parameters, such as the path of insertion, depth, taper, and margin design, to achieve the desired dimensions and characteristics. This step allowed for customization based on the specific requirements of the design and was standardized for all three groups. The margin line was kept at the cementoenamel junction to follow the crown contour. The crown fabrication process incorporated specific set cement space dimensions, with margins allowing 20 μm, whereas the rest of the internal spaces allowed for 60 μm space. Once satisfied with the virtual tooth preparation, the pertinent information was transferred to CAM software (CORiTEC iCAM V5; imes-icore GmbH) and milling (CNC; CORiTEC 550i; imes-icore GmbH). Utilizing a cutting-edge CNC machine (CORiTEC, imes-icore GmbH), the crowns were milled meticulously from polymethyl methacrylate (PMMA) blocks. It is noteworthy that the PMMA crowns underwent no further treatment or adjustments subsequent to the milling process.

Figure 1.

Flowchart describing the methodology for fabricating the temporary PMMA crowns using three different scanning protocols. CBCT: Cone-beam computed tomography, DICOM: Digital Imaging and Communications in Medicine, STL: Standard tessellation language, D2P: DICOM to print, CAM: Computer-aided manufacturing, IMES: Integrated mechanics electronics software, SEM: Scanning electron microscope

Cone-beam computed tomography group (experimental Group 1)

The tooth was scanned using CBCT (Carestream CS9600, Carestream Dental India, Mumbai, Maharashtra, India) at the recommended setting parameters (voxel resolution: 0.125 mm at the resolution settings of 0.200 mm, 80 kV, 2 mA, 11.4 s, 582 mGy·cm², and field of view 5 × 5) in Digital Imaging and Communications in Medicine (DICOM) format. This was later converted using DICOM to Print (D2P) software (3D systems, Rock Hill, South Carolina) into a STL format. This 3D model of the tooth was later imported into 3Shape dental designing software to fabricate crowns using a virtual tooth preparation workflow [Figure 2].

Figure 2.

Cone-beam computed tomography (CBCT) scanning protocol of crown fabrication (a: CBCT recorded in DICOM format, b: file converted to standard tessellation language [STL] file format, c: Crown designed on 3Shape dental manager using the virtual tooth preparation workflow, d: Final crown prosthesis STL file)

Intraoral scanners group (experimental Group 2)

An intraoral scan of the tooth specimen was recorded using the 3Shape Trios IOS (3Shape, Niels Juels Gade 13, 1059 Copenhagen K Denmark) using a standardized methodology. The scanning sequence followed a systematic approach, beginning with the occlusal surface and progressing to other surfaces of the tooth in a sweeping motion to capture the entire tooth area. Proper illumination was maintained throughout the procedure to ensure clear visualization, and a dry field using cotton rolls was used to optimize the scanning accuracy and reduce potential interferences from saliva or moisture. The scan was then processed and imported as a STL file format to obtain a 3D model of the scan, which was later imported into the 3Shape dental designing studio for designing the temporary crowns using the virtual tooth preparation workflow (3Shape, Niels Juels Gade 13, 1059 Copenhagen K Denmark) [Figure 3].

Figure 3.

Intraoral scanning protocol of crown fabrication (a: Intraoral scanning using 3Shape Trios intraoral scanner, b: Standard tessellation language [STL] file format obtained, c: Crown designed on 3Shape dental manager using the virtual tooth preparation workflow, d: Final crown prosthesis STL file)

Specimen preparation

Tooth preparation was done on the extracted tooth sample by an experienced prosthodontist (V.K) to receive an anatomically contoured PMMA temporary crown. A meticulous tooth preparation technique involved using a diamond rotary instrument (TF 12; MANI, INC., Tochigi 321-3231, Japan) and a high-speed handpiece (Nsk Pana Air FX SB02; NSK, Kanuma, Tochigi 322-8666, Japan) with coolant to a create precise 1.0 mm shoulder margin encircling the tooth.

EOS group (Group 3 control)

The prepared tooth was used to record a two-stage putty impression, and a dental stone model was poured to obtain a positive replica of the tooth preparation. This model was then scanned using an extraoral laboratory scanner (3Shape E4 laboratory scanner, 3Shape, Niels Juels Gade 13, 1059 Copenhagen K, Denmark) using a standardized methodology. The scan obtained was later used for designing the PMMA temporary crown using the conventional crown fabrication workflow [Figure 4]. The CAM output obtained was then used to mill 10 samples of this design.

Figure 4.

Conventional tooth preparation workflow (a: Tooth preparation done on the sample to accept polymethyl methacrylate (PMMA) crowns followed by. b: recording two stage putty impression. c: Tooth preparation scan to accept. d: PMMA crown fabrication)

Marginal fit evaluation

The vertical marginal gap of all samples was evaluated using a scanning electron microscope (SEM) (JSM-IT800, JEOL Tokyo, Japan). Before analysis, the specimens were coated with 24-carat gold using a metallizer (Q15RS, Quorum Technologies, Sussex, UK) to prevent any electron distortion. The SEM captured an image of the specimen at a magnification of ×250 using an energy dispersal detector (Link Pentafet, Oxford Instruments, Abingdon, UK). The image was then transferred to a computer with INCA Suite 4.04 software (Oxford Instruments; Abingdon, UK) to digitize and capture the image. To ensure consistency, a marker pen (Sakura marker, Sakura Color Product Corporation, Japan) was used to mark three points on the middle of each surface of each crown. This ensured that the measuring area was standardized and the positions were repeatable for all crowns. The marginal fit was determined by measuring the average of 360 measurements (3 markings for each of the 4 surfaces per tooth for 30 crowns) to calculate the marginal gap of each crown [Figure 5].

Figure 5.

Scanning electron microscope analysis (×250) of the marginal discrepancy of the (a) control and (b) cone-beam computed tomography and (c) intraoral scanner groups

Statistical analysis

The information was gathered and organized using Google Sheets, then transferred to statistical software, SPSS version 26.0, IBM, Armonk, NY, USA, to get statistical significance at α =0.05. The data showed a normal distribution based on the Shapiro–Wilk test. The overall marginal discrepancy of each surface was determined using a one-way ANOVA.

RESULTS

The average marginal gap for all three groups was found to be 113.37 ± 45.758 μm (Group 1 = CBCT), 127.82 ± 43.655 μm (Group 2 = IOS), and 107.30 ± 44.893 μm (Group 3 = EOS) [Table 1]. The mean difference among groups was not significant (P > 0.05), indicating the acceptance of the virtual preparation technique using CBCT and IOS. Although the difference was recorded, they were all in the range of clinically acceptable limits for temporary crowns (<200 μm). When comparing discrepancy on each surface, more discrepancies were observed on the buccal surface, followed by the lingual, mesial, and distal surfaces, respectively [Figure 6]. However, no statistically significant differences were seen among surfaces while evaluating for marginal discrepancy (P > 0.05) [Table 2].

Table 1.

Mean marginal discrepancy recorded for all three groups derived using a one-way ANOVA test accepting the null hypothesis (P>0.05)

Group	n	Mean±SD (µm)	F	P	Null hypothesis
CBCT	40	113.37±45.758	2.218	0.113#	Accepted
IOS	40	127.82±43.655
EOS (control)	40	107.30±44.893

^#No statistical significant at P>0.050. SD: Standard deviation, CBCT: Cone-beam computed tomography, IOS: Intraoral scanners, EOS: Extra oral scanners

Figure 6.

Bar graph depicting the mean marginal discrepancy of all three groups recorded for each surface of the tooth was derived using a one-way ANOVA test (F = 2.218). CBCT: Cone-beam computed tomography, IOS: Intraoral scanners

Table 2.

Marginal discrepancy of all three groups recorded for each surface of the tooth was derived using a one-way ANOVA test

Group	Surface	n	Mean±SD	F	P
CBCT	Buccal	10	131.5±47.394	2.287	0.095#
	Lingual	10	130.3±49.481
	Mesial	10	89.6±26.738
	Distal	10	102.1±47.021
IOS	Buccal	10	122.2±41.912	0.336	0.800#
	Lingual	10	121.6±49.567
	Mesial	10	139.3±55.886
	Distal	10	128.2±26.402
EOS (control)	Buccal	10	111.2±52.855	0.391	0.760#
	Lingual	10	103.8±49.714
	Mesial	10	117.6±49.055
	Distal	10	96.6±28.040

^#No statistical significant at P>0.050. SD: Standard deviation, CBCT: Cone-beam computed tomography, IOS: Intraoral scanners, EOS: Extra oral scanners

DISCUSSION

The findings of this investigation indicate that the null hypothesis was accepted, as there were no statistically significant disparities observed in the marginal discrepancy of all three groups, indicating similar outcomes between the experimental groups (virtual preparation using CBCT and IOS) and the control group (conventional preparation and scanning using EOS). Analyzing the results of the present study, it was found that crowns fabricated using the conventional workflow exhibited the lowest marginal gap measurement (107.30 ± 44.893 μm), and although the crowns fabricated using the virtual tooth preparation workflow displayed higher values of vertical marginal discrepancies (CBCT: 113.37 ± 45.758 μm, IOS: 127.82 ± 43.655 μm), the mean gaps for all three groups fell within the clinically acceptable range.

The significance of achieving precise marginal fit has been extensively discussed in various publications.[22-25] While horizontal discrepancies may allow for clinical adjustments, vertical marginal discrepancies pose a greater challenge and are less amenable to compensation or correction, potentially leading to complications and failures, as reported by Holmes et al.[26] Therefore, this study specifically focused on evaluating the vertical marginal gap. Recent investigations have established that marginal gaps ranging from 80 to 200 microns are clinically acceptable for ensuring the longevity of cemented crowns.[27] It is worth noting that no adjustments were made to the crowns before measuring the marginal fit. However, had the prosthodontist done adjustments on the intaglio surface of the crowns as one would normally do in a clinical or laboratory setting, the marginal gap would have been even lesser.

Traditional methods of fabricating temporary crowns include tooth preparation, which is followed by recording impressions and then using a putty index on a wax mockup or a time-consuming digital design of crowns on the prepared tooth surface. The final temporaries obtained after polishing may also injure the soft tissues around the prepared teeth. This process can be time-consuming and dependent on the expertise and precision of the technician or operator.[28,29] By digitally fabricating crowns before tooth preparation in a clinical setting, several advantages can be achieved. First, it allows patients to visualize the temporary prosthesis beforehand, enhancing their understanding of the treatment plan and instilling confidence in the dentist’s capabilities. In addition, digital fabrication using CBCT data enables accurate registration of teeth, arch, and bone anatomy, utilizing the patient’s preexisting diagnostic radiographic records (CBCTs) where implants could not be placed due to insufficient bone quality and quantity. This, along with intraoral scans eliminate the need for diagnostic cast pouring and mounting, streamlining the process.[30] This workflow also acts as a template to achieve a desirable amount of tooth preparation, thereby allowing better insertion direction. It is also important to acknowledge certain limitations of this study, including the potential requirement for relining the temporary crowns in a clinical setting due to the virtual tooth preparation workflow. Furthermore, artifacts in CBCT images may be a consideration, particularly in cases with metal restorations. Nonetheless, within the scope of this study, the use of preexisting diagnostic CBCT data and intraoral scans holds promise for fabricating crowns in proper occlusion before tooth preparation, with minimal adjustments needed. Moving forward, future research in this field should evaluate the clinical outcomes of the prosthesis fabricated using the same workflow. Such investigations will shed light on the practical implications and efficacy of the digital approach.

CONCLUSION

As per the findings reported in this study, the following conclusions can be drawn:

1. The virtual tooth preparation workflow yields a similar marginal fit when compared to the commonly used digital workflow

2. Diagnostic CBCTs and IOS can be used as an alternative to conventional methods of provisional crown fabrication (EOS) as they provide better accuracy, reduced time required for crown fabrication, increased efficiency, enhanced understanding of the treatment procedure and outcomes, and greater patient involvement in the treatment process.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.