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19 March 2024: Database Analysis  

Comparative Accuracy of Intraoral and Extraoral Digital Workflows for Short Span Implant Supported Fixed Partial Denture Fabrication: An In Vitro Study

Manawar Ahmad Mansoor ORCID logo1ABCDEF*, Mohammed E. Sayed ORCID logo1ABCDE, Hina Naim Abdul ORCID logo1BCDF, Meshal Saleh Zaidan ORCID logo2BCD, Thamer Mohammad Hakami ORCID logo2BCD, Mohammed Abdullah Dighriri ORCID logo2BCD, Saeed M. Alqahtani ORCID logo3BCDEF, Mohammed A. Alfaifi ORCID logo4CDEF, Majed S. Altoman ORCID logo4BCDE, Hossam F. Jokhadar ORCID logo5BCDE, Saad Saleh AlResayes ORCID logo6DEF, Mohammed H. AlWadei ORCID logo7DEF, Asayil Ibrahim Jundus ORCID logo8DEF, Abeer Mohammed Komosany ORCID logo9DEF, Hind Ziyad AlNajjar ORCID logo10CDEF

DOI: 10.12659/MSM.943706

Med Sci Monit 2024; 30:e943706

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Abstract

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BACKGROUND: The advent of digital impressions using computer-aided design and manufacturing technology (CAD/CAM) has simplified and improved the fabrication of implant prostheses in dentistry. The conventional impression has several drawbacks, including tray selection, material type, impression technique, impression disinfection, and cast model storage. The inaccuracies caused by distortion and contraction of impression material can be minimized with digital impressions. This study aimed to compare digital dental impressions of 10 working casts made using the Pindex laser removable die system to fabricate parallel drill channels vs 10 working casts made using the Di-Lok plastic tray removable die system.

MATERIAL AND METHODS: An implant master die with 2 dental implant analogs was fabricated. Ten working casts using the Pindex laser removable die system with parallel drill channels and 10 working casts using the Di-Lok plastic tray removable die system were fabricated. The working casts were scanned using an extra-oral laboratory scanner and the implant master model was scanned with an intra-oral scanner.

RESULTS: The properties of the casts made using the 2 systems were evaluated and analyzed with ANOVA and post hoc Tukey test. The mean horizontal linear distances between A1B1 (P<0.021), A2B2 (P<0.018), C1D1 (P<0.026), C2D2 (P<0.03), B1C1 (P<0.01), and mean vertical distances between B1A2 (P<0.015), C1D2 (P<0.001), B1B2 (P<0.028), and C1C2 (P<0.001) were significantly different between the Pindex system and Di-Lok tray system as compared to intra-oral scans.

CONCLUSIONS: Complete digital workflow with intra-oral scans were more than the partial digital workflow with extra-oral scans for the Pindex system and Di-Lok tray systems.

Keywords: Dental Implants, Dental Impression Materials, Dental Impression Technique

Introduction

Dental implants have become a successful restorative treatment modality in modern clinical dentistry [1]. The periodontal ligament is absent at the implant-bone interface, so a precise three-dimensional impression is necessary for implant-supported fixed restorations [2,3]. The natural tooth can move by up to 100 μm due to the presence of the periodontal ligament, which compensates for a certain degree of misfit of a fixed partial denture. In contrast, an osseo-integrated implant has minimal movement, in the range of 10 μm or less [4]. All tensile, compressive, and flexural stresses added to an implant-supported restoration due to misfit remain unchanged due to the lack of flexural quality of the implant [5]. The union of any prosthesis, be it fixed or removable, to osseointegrated fixtures is intended to produce a synarthrodial, unified structure in which the supporting implant and the investing bone act as a single unit [6]. However, any misalignment of the prostheses to the osseointegrated fixtures, visible or not, can induce internal stress in the prosthesis. The term “passive fit,” referring to the relationship of a prosthetic superstructure to its underlying implant abutments, appears with increasing frequency in the literature [7]. Restorative dentists have the task of creating the most accurate fit clinically possible to avoid bone strain from uncontrolled loading of implants through the superstructure [8]. The various clinical and laboratory procedures required to fabricate the implant prosthesis to restore the edentulous site are among the many variables that can influence the accuracy of fit achieved. Impression-making technique, impression material used, number of implants used, the degree to which implants and abutments are parallel, production of the master cast, and framework fabrication can accumulatively influence the fit observed by the clinician when the framework is fitted to the implant abutments [9,10]. Impression inaccuracies negatively impact the restoration’s precise fit. Consequently, mechanical problems can arise, such as loosening screws or abutments, fractures of prosthetic parts, or issues with the implant itself. In addition, this can lead to vertical or marginal discrepancies, which can promote plaque formation and have a detrimental effect on the surrounding hard and soft tissue of the implant [11]. Although a true passive fit of multi-implant-supported prostheses to their intra-oral implant abutments does not seem attainable, the degree to which implant prosthesis misfit will lead to complications is unclear. To prevent bone strain resulting from uncontrolled loading of implants through the superstructure, it is highly recommended to perform meticulous and accurate implant prosthodontic procedures to achieve the best possible fit [12].

Throughout the past few decades, use of digital devices has become increasingly common in dental practices. Implant-supported restorations can now be fabricated using a computerized workflow technology. The digital workflow commences with digital impressions, which convert the intra-oral images into a virtual model [13]. Digital impressions of a patient’s mouth made using an intra-oral scanner or scanning a physical impression using a laboratory scanner are now possible with the help of computer-aided design and computer-aided manufacturing (CAD/CAM) [14]. Digital impressions are taken by intra-oral scanners (IOS), which collect information on reproducible intra-oral tissues and projects on the hardware display as appearing natural. Some benefits of digital impressions are reduced distortion risk during impression taking and model construction, as well as enhanced patient comfort [15]. Conventional impressions are used over a long period of time in fixed prosthetics supported by dental implants. However, the conventional impression has several limitations, such as the selection of impression tray, type of impression material, the impression technique, the time required, disinfection of the impression, preparation of the working cast model, and the difficulty of storing the study model [16]. Because of these drawbacks, scientists have been searching for substitute impression methods (ie, the digital impressions) [17]. One of the factors that can impact the working cast’s quality is the setting expansion of the die stone. Traditionally, the first pour is made using type IV and type V dental stones, whereas the base pour is made using type III dental stone in the impression, commonly used in the Pindex system. A laser beam light source is used by the Pindex machine to drill tiny holes in the underside of the working molds, facilitating precise fabrication of detachable dies in dentistry. In addition to providing a functional base, these parallel-drilled holes offer stability and support [18]. It is impossible to precisely replicate the original tooth location in the working cast due to setting expansion [19–21]. Die systems have advanced to the point where the elimination of the second pour in the “double-pour” system has reduced the effect of setting expansion of the base on the inter-die position. The process of die fabrication has become more efficient after the advent to these advanced methods, which require a prefabricated plastic base commonly referred to as a “die tray” [22]. Precise reassembly of the sectioned dies requires use of a plastic partially articulated tray. After the impression is poured, the cast is cut to a unique horseshoe shape. The cast is seated as the tray is refilled with a second mixture. After the stone solidifies, the tray is taken apart, the preparation is sawn on both sides, and the finished die is trimmed. After the tray is put on an articulator, the cast and die can be put back together [23]. The present study used the first approved dental implant system (Triswiss, Matrix, Switzerland) of its kind, specifically designed for digital manufacturing technologies – computer-aided designing and computer-aided manufacturing (CAD/CAM) milling and 3D printing – and incorporating a new method in which prostheses can be attached directly to the implant without the need to use an abutment. This in vitro investigation aimed to assess the precision of digital impressions (DI) made using an intra-oral scanner (IOS) (ie, full digital flow) compared to that of an extra-oral scanner used for conventional impression (CI) (ie, partial digital flow) with the 2 most commonly used die systems, the Pindex system and the Di-Lok tray system. The null hypothesis tested was that digital impressions (DI) taken with an intra-oral scanner (IOS) and the extra-oral scanner for conventional impressions (CI) after preparing the working casts with the Pindex system and the Di-Lok tray system would generate similar accuracy with impressions taken. Therefore, this study aimed to compare digital dental impressions of 10 working casts using the Pindex laser removable die system to fabricate parallel drill channels vs 10 working casts using the Di-Lok plastic tray removable die system.

Material and Methods

FABRICATION OF THE IMPLANT MASTER DIE:

An implant master die was fabricated with missing #44, #45, and #46 teeth to simulate a mandibular partially edentulous arch. Two equal-sized vertical holes of diameter 5 mm and length 10 mm corresponding to the size of an implant analog (D5/L10) were drilled in the #44 and 46# teeth areas of the model. Stainless-steel dental implant analogs (Triswiss, Matrix, Switzerland) were cemented into their respective drilled holes with self-cure pattern resin (GC, USA). A 1-mm diameter hole was made with a round diamond bur at the distobuccal region on the occlusal surface of tooth #47 (reference point A1) and the mid-incisal surface of tooth #43 (reference point D1) for horizontal linear dimension measurements. Similarly, 4 small holes were made by a round diamond bur in the buccal vestibular region 5 mm below teeth #47, #46, #44, and #43 as reference for vertical measurement and labeled A2, B2, C2, and D2 (Figure 1). Impression copings/scan bodies (Triswiss, Matrix, Switzerland) were tightened with a hex driver (TRI® Prosthetic Driver-Short, Switzerland) to the implant analogs.

FABRICATION OF SPECIAL CUSTOM TRAYS:

A 3-mm-thick pink wax sheet (2 layers, Cavex Set Up regular modeling wax, the Netherlands) spacer design was uniformly applied to the implant master die [24]. Three tissue stops, 1 in the anterior and 2 in the posterior region, were made in wax spacers and adapted on the implant master die to ensure correct tray position [25]. To ensure uniform thickness of the spacer for all 20 samples of special trays, the implant master die and the wax spacer were duplicated using putty impression material. The duplicated cast was used to make the special trays [26,27]. The tray adhesive (3M Express VPS Silicone Tray Adhesive, Germany) was applied to the custom tray and left to dry for 15 min. To keep the impression material from entering the impression coping, modeling wax was used to seal off the opening [28].

FABRICATION OF IMPLANT IMPRESSIONS FOR THE PINDEX SYSTEM AND DI-LOK TRAY SYSTEM:

A total of 20 impressions of this implant master die – 10 for each die system – were made using polyvinylsiloxane impression material (I-SIL, Korea) using the open-tray impression technique [29]. Sample size was estimated assuming that the post hoc Tukey test would be used to compare the accuracy in the 3 groups (the intraoral scan group, the Pindex system group, and the Di-Lok tray system group). A study powered at 1-beta=0.95 to detect a difference with alpha=0.05 would require a total of 30 scan images (10 for the intraoral scan group, 10 for the Pindex system group, and 10 for the Di-Lok tray system group; and 20 working casts (10 for the Pindex system group and 10 for the Di-Lok tray system group). The mixture of base and catalyst impression material was injected, loaded into the custom-made tray, and left for 6 min until the material had completely set [30].

FABRICATION OF WORKING CASTS:

Before pouring the cast, the impression was disinfected, allowed to completely set for 30 min, and checked for any defects or flaws in the abutment area [7]. We ensured that the impression was free of defects by briefly rinsing it with water and letting it air dry before pouring [31]. To avoid rotation of the impression copings or deformation of the impression material itself, implant analogs were secured into the impression copings using a minimal tightening force while carefully holding the tray [32]. Using an electronic weighing equipment (Scienish Digital Electronic Scale, Guangdong Province, China), 100 g of type IV die stone (Snow Rock Lab stone type IV, USA) were weighed to the nearest 0.01 g. Next, using a measuring cylinder, 23 mL of water were measured to the nearest 0.1 mL in a sanitized rubber mixing bowl, to which type IV die stone was gradually added with a hand-mixing stainless-steel spatula for 10 s [33]. Subsequently, the mixture was stirred in a mechanical vacuum mixer for 30 s, yielding a smooth, uniform, and bubble-free mix. Die stone was applied in increments until the impression was fully filled over a period of 30 s. The poured impressions were left to harden for 30 min. Once the casts were removed from the impressions, the casts were placed on a table to dry [34,35]. In the partial digital workflow, the model-carrying table of the bench-top scanner has limitations in axial rotations for complete imaging, and the neighboring proximal surfaces block the scan beams and result in incomplete scanning [36]. To prevent this, sectioning and referencing were done utilizing the Pindex and Di-Lok tray systems, as 2 techniques commonly used in partial digital prosthesis fabrication, for selective removal of proximal parts upon sequential scanning. The impression was held tilted on the vibrator. The type IV die stone was gently applied first from the periphery of the impression to reach the deepest area [37].

FABRICATION OF IMPLANT DIE WITH DI-LOK TRAY SYSTEM:

Ten casts (numbered 1 through 10) were trimmed to a “horseshoe” configuration by the center-grinder machine for the Di-Lok tray system (Accu-trac, Carson Dental, Freud Dental Company). The casts were able to fit inside the Accu-trac Tray due to its horseshoe shape [38]. B.P. Blade was used to create retentive grooves on the inferior surface of the casts. The cast was soaked in water for 10 min. Vacuum-mixed type 4 die stone was filled into the retentive grooves on the cast and then vibrated into the die tray [39]. The casts were placed into the trays and excess stone was removed. After allowing the casts to set for 30 min, the hinged arm of the tray was unlocked, and the casts were removed from the die trays [40]. After 24 h, vertical saw cuts were made through the indentations on the base using a jeweler’s saw [41]. The dies were trimmed off all rough surfaces. Each die was inserted and removed 30 times from the die tray to simulate average laboratory use [42], then the parts of the casts were reassembled in the tray. The hinged arm of the tray was returned and locked into position (Figure 2).

FABRICATION OF IMPLANT DIE WITH THE PINDEX SYSTEM:

Ten more casts, numbered 11 through 20, were chosen to be used with the Pindex system (Coltene/Whaldent AG, Switzerland). To prevent undercuts, these casts were trimmed and shaped by a center-grinder machine. The light source of the Pindex machine was used to drill holes in the bottom of the casts that mirrored the center of the transfer copings [43]. Double straight dowel pins with common head (Twin Pin, R and D Dental, Korea) were cemented into their respective holes with cyanoacrylate adhesive. Metal sleeves were fitted into each cemented pin along with rubber caps. Next to the die pins, on the detachable die parts, separating medium was applied using a paint brush and allowed to dry for 10 min. On the inferior surface of the casts, undercut grooves were prepared with B.P. Blade for retention. Prepared casts with the pins were positioned onto the base formers. After pouring type III dental stone mix into the base former and onto the cast, it was carefully vibrated into place and left for 60 min to set [44]. Twenty-four hours after removal of the casts from the base former, casts were subjected to die cutting. The position of each saw cut was marked with a pencil, which was kept slightly converging towards the pins for easy removal of the dies and to prevent undercuts. With a jeweler’s saw frame and blade, dies were sectioned. The saw cuts were made such that they taper slightly, with the base surface being closer and the occlusal surface being the furthest apart. The die stone that was poured first was cut through. Dies were removed gently by tapping. Each cast was sectioned, leaving the base intact. The dies were cleared of any loose debris. To avoid binding, dies were carefully cut with a B.P. Blade. Sectioned dies obtained for each master cast were removed and seated 30 times into their respective positions to simulate the average number of handlings during the laboratory procedure, then the dies were carefully seated back in position (Figure 3).

PARTIAL DIGITAL SCAN OF IMPLANT WORKING CAST DIES:

The scan bodies (Triswiss, Matrix, Switzerland) were tightened on implant analogs #44 and #46 on the working casts and labeled with reference points B1 and C1, respectively. Each working cast was scanned by an extra-oral laboratory scanner (3Shape, Copenhagen, Denmark), and images were saved as STL (Standard Tessellation Language) files for subsequent analysis (Figure 1).

FULL DIGITAL SCAN OF IMPLANT MASTER DIE WITH INTRA-ORAL SCANNER:

The intra-oral scanning for the master model was performed 10 times by the intra-oral scanner Medit® i700 (Model MD-IS0200, Korea) from occlusal, lingual, and buccal surfaces. The images obtained were named and saved in an STL file for subsequent analysis. The linear horizontal dimensions A1B1, A2B2, B1C1, C1D1, and C2D2, and vertical dimensions A2B1, B1B2, C1D2, and C1C2 were analyzed using Medit Scan for Clinics software.

STATISTICAL ANALYSIS:

The detailed evaluations were provided as STL files, analyzed with Medit Scan for Clinics software, which showed substantial differences between the complete digital workflow with intra-oral scan and partial digital workflow with conventional impression with bench-top scan for the Pindex system and Di-Lok tray system. SPSS Statistics for Windows, Version 19.0 (USA) was used to analyze the quantitative data. The mean and standard deviation were calculated for all variables. Data were collected from each group to compare the linear horizontal dimensions of A1B1, B1C1, C1D1, A2B2, and C2D2, as well as the vertical and diagonal dimensions of A2B1, B1B2, C1D2, and C1C2. The comparison was made using a complete digital workflow with intra-oral scan and a partial digital workflow with conventional impression using the bench-top scan for the Pindex system and Di-Lok tray system. Comparisons were made using one-way analysis of variance (ANOVA). The post hoc Tukey test was conducted for pairwise comparisons, with P<0.05 considered statistically significant.

Results

MEASUREMENT OF HORIZONTAL DISTANCES:

The measured horizontal distances were A1B1, A2B2, B1C1, C1D1, and C2D2 for intra-oral scan, Pindex system, and Di-Lok tray system. The mean difference and standard deviation were calculated for each distance. For the horizontal distances, the mean difference and standard deviation for intra-oral scan was 25±67 mm for A1B1, 32±49 mm for A2B2, 33±74 mm for C1D1, 35±83 mm for C2D2, and 44±69 mm for B1C1. The mean difference and standard deviation for the extra-oral bench-top scan for the master casts fabricated using Di-Lok tray for the horizontal distance A1B1 was 81±162 mm, A2B2 was 85±142 mm, C1D1 was 79±93 mm, C2D2 was 81±114 mm, B1C1 was 83± 93 mm.

MEASUREMENT OF VERTICAL DISTANCES:

The vertical distances measured were B1B2 and C1C2 for intra-oral scan, Pindex system, and Di-Lok tray system. The mean difference and standard deviation were calculated for each distance. For intra-oral scan, the vertical distance was 33±116 mm for B1B2 and 35±30 mm for C1C2, which was well within the threshold value of 100 mm. However, for the extra-oral bench-top scan for the master casts fabricated using Di-Lok tray, the vertical distance for B1B2 was 82±95 mm, and for C1C2 it was 81±110 mm (Table 1). The mean difference and standard deviation for the bench-top scans for the master casts fabricated using Pindex system for the horizontal distance were 147±148 mm for A1B1, 142±171 mm for A2B2, 136±155 mm for C1D1, 136±90 mm for C2D2 and 129±75 mm for B1C1. The mean difference and standard deviation for the bench-top scans for the master casts fabricated using the Pindex system for the vertical distances were 132±86 mm for B1B2 and 128±125 mm for C1C2 (Table 1).

MEASUREMENT OF DIAGONAL DISTANCES:

B1A2 and C1D2 were the diagonal measurements for intra-oral scan, Pindex system, and Di-Lok tray system. The mean difference and standard deviation were calculated for each distance. The diagonal distances for intra-oral scan were 28±138 mm for B1A2 and 25±52 mm for C1D2 (Table 1). However, for the extra-oral bench-top scan for the master casts fabricated using Di-Lok tray, the diagonal distance for B1A2 was 81±219 mm and for C1D2 it was 80±112 mm. The mean difference and standard deviation for the bench-top scans for the master casts fabricated using Pindex system for the diagonal distances was 135±259 mm for B1A2 and 129±155 mm for C1D2.

The P value for mean horizontal linear distances A1B1 (P<0.039), A2B2 (P<0.041), C1D1 (P<0.033), C2D2 (P<0.031), and B1C1 (P<0.024), and mean vertical distances between B1A2 (P<0.037), C1D2 (P<0.024), B1B2 (P<0.047), and C1C2 (P<0.018) were more significant in the Pindex system than with intra-oral scan. Similarly, the P value for mean horizontal linear distances A1B1 (P<0.020), A2B2 (P<0.030), C1D1 (P <0.017), C2D2 (P<0.027), and B1C1 (P<0.010), and mean vertical distances between B1A2 (P<0.024), C1D2 (P <0.013), B1B2 (P<0.032), and C1C2 (P<0.008) were more significant in the Di-Lok tray system than with intra-oral scan (Table 1). However, between the bench-top scans performed on the master casts fabricated using Di-Lok tray system and Pindex system, the P value for mean horizontal linear distances A1B1 (P<0.270), A2B2 (P<0.152), C1D1 (P<0.196), C2D2 (P<0.251), and B1C1 (P<0.356), and mean vertical distances between B1A2 (P<0.309), C1D2 (P<0.196), B1B2 (P<0.298), and C1C2 (P<0.204) were not significant. The study results showed that the intra-oral scan was the most accurate for the horizontal, vertical, and diagonal measurements, compared to bench-top extra-oral scans with partial digital workflow using master casts fabricated with Pindex and Di-Lok tray systems. However, the difference between Pindex and Di-Lok tray systems was not significant (Figures 4, 5). Data analysis results are summarized in Table 1.

Discussion

In the present study, the accuracy of full digital flow using intra-oral digital impressions was compared to that of partial digital workflow with conventional impressions using the Pindex and Di-Lok tray systems, which are the 2 most commonly used die systems. The mean horizontal linear distances between A1B1 (P<0.039), A2B2 (P <0.041), C1D1 (P <0.033), C2D2 (P <0.031), and B1C1 (P <0.024), and mean vertical distances between B1A2 (P <0.037), C1D2 (P <0.024), B1B2 (P <0.047), and C1C2 (P <0.018) were significantly longer with the Pindex system and Di-Lok tray system than with intra-oral scanning. However, the difference in linear dimension between the Pindex and Di-Lok tray systems was insignificant. The discrepancies along the horizontal, vertical, and diagonal linear dimensions were compared between the 3 groups (the intraoral scan group, the Pindex system group, and the Di-Lok tray system group). We rejected the null hypothesis that the digital impressions made by the intra-oral scanner (IOS) and extra-oral scanner used for conventional impression for the casts generated with the Pindex system and Di-Lok tray system would generate similar accuracy. Intra-oral scans were more accurate for linear horizontal and vertical dimensions than extra-oral scans for the Pindex system and Di-Lok tray system.

Conventional impressions often require the selection of appropriate trays based on the patient’s dental anatomy. Choosing the wrong tray size or type can lead to inaccuracies in the obtained impression. The choice of impression material is crucial. Variations in viscosities and setting times among the impression materials might impact the accuracy. The process of taking conventional regular impressions can be uncomfortable for patients, especially those with a strong gag reflex. Cooperation and proper technique are necessary for a successful impression. Sterilization and disinfection of impressions are critical to prevent the transmission of infections. Conventional impressions may be more challenging to disinfect thoroughly compared to digital impressions. Storing physical cast models generated from conventional regular impressions can be space-consuming. Over time, there is also a risk of damage or deterioration, requiring careful storage and maintenance. Greater predictability, fewer and shorter treatment sessions, less patient discomfort, and elimination of plaster models are advantages of digital workflow with computer-aided designing and computer-aided manufacturing (CAD-CAM) over the conventional workflow [45,46]. The ability to combine and superimpose three-dimensional (3D) meshes from various images to generate a virtual patient model is another essential component of digital workflow [47]. This ability improves virtual treatment planning and patient communication.

Intra-oral scanning (IOS) has been shown to be more accurate than conventional impressions for generating final outcomes of computer-aided designing and computer-aided manufacturing (CAD-CAM) crowns and short-span fixed partial dentures [48–51]. It has been reported that marginal gaps for computer-aided designing and computer-aided manufacturing (CAD-CAM) dental crowns made using IOS are less than 60 μm. However, the marginal gaps for crowns made with conventional impressions can reach 183 μm [48]. Traditional impressions can also be digitized to support the adoption of digital workflows using desktop optical scanners. Intra-oral scans generate considerably smaller marginal gaps than desktop scans; these gaps are estimated to be between 50 to 60 μm [52]. According to a study by Katsoulis (2017), to ensure the passivity of the framework in implant-supported prostheses, an average distance deviation of 50 to 100 μm was required [53,54]. According to our findings, the intra-oral digital impressions’ distance variation ranged from 25 to 44 μm for horizontal distances and from 25 to 35 μm for vertical distances. As a result, intra-oral digital impression images appear to be more precise and consistent with the framework’s passivity in an implant-supported prosthesis. The linear horizontal and vertical distances of the conventional impression deviated more from the acceptable values (147 μm for horizontal distances and 135 μm for vertical distances) despite exceeding the norm. This could result in non-passivity of the framework. However, other variable could also have an influence on final framework characteristics such as material hardness, roughness, flexural strength, and staining [55–58]. Therefore, future studies are needed involving these important factors.

Removable dies for working models are a valuable aid in the laboratory phase of fabrication of implant-supported fixed partial dentures. The advantage of using a removable die is that it is convenient to use for complete three-dimensional scanning, and various drawbacks of working with separate dies can be overcome. If the die does not fit exactly in the working cast, there is a chance that defects will be introduced into the wax pattern. Therefore, the fundamental requirements of any die system should be that the dies can return to their initial positions, maintain their stability even when inverted, and be simple to mount on an articulator for casts that incorporate dies. Among the various die systems, the Di-Lok tray system was chosen for the present study because of its positional accuracy compared with the Model Tray, Sterdo Split Model Tray, and Tricodent One-Cast die tray systems. Richardson et al reported that the Di-Lok tray system was easy to use, sawing out of the dies and die replacement into the tray was not difficult, and it could be mounted directly to the articulator [59]. With both the Sterdo Split and Tricodent One-Cast systems it was difficult to saw out the dies because the stone blocks to be cut were very large and a special power saw had to be used to cut the dies supplied by the manufacturer. The Pindex system was chosen for the current investigation because Serrano et al found it had the least amount of horizontal movement when compared to DVA system and conventional brass dowel systems [10].

Our experimental study demonstrated that the Di-Lok tray system had minimal distortion compared to the Pindex die system because the Di-Lok tray die system uses a small plastic tray shaped to receive a trimmed master cast. The inner surfaces of these trays are provided with index grooves to facilitate insertion of individual dies into their original locations after removal. In contrast to the Di-Lok tray system, in the Pindex system the resultant improper fit after 30 cycles of removal and insertion may prevent full seating of dies. The inherent setting expansion of the type IV stone used in the production of dies is also a variable that can influence the quality of working casts. For the initial impression pour in the Di-Lok tray and Pindex systems, the type IV die stone was utilized. It is impossible to precisely replicate the original tooth position in the working cast in the Pindex technique because the type III dental stone used for the base pour has a larger inherent setting expansion than type IV die stone. The elimination of the second pour in the “double-pour” system due to advancements in die systems has decreased the influence of expanding gypsum depending on inter-die position accuracy. The efficiency of the die fabrication process has also been enhanced by the advanced technologies, which require a prefabricated plastic base frequently referred to as a die tray.

The significant difference detected among the intra-oral scan, Di-Lok tray system, and Pindex system groups indicated that the methodology of this investigation was appropriately sensitive to achieving the established objectives. Advancements in dental technology, such as digital impressions and 3D printing, have addressed some of these challenges. Digital impressions offer increased precision, quicker turnaround times, and improved patient comfort. Additionally, 3D printing allows for efficient and space-saving storage of digital models. It is important to note that partial digital workflow with conventional impressions and full digital workflow with digital impression methods both have advantages and disadvantages. The choice typically depends on factors such as the specific case, clinician preference, and available technology. A similar study was conducted by Chuang Bi et al in 2022, which found that digital and traditional implant impressions were equally accurate for short-span scanning, and digital impressions were much less precise than traditional impressions in long-span scanning [60]. A study by Waldecker at al stated that the partially edentulous maxilla with prepared teeth had its impression accuracy affected by the impression technique. Digital impression is advised for scans up to a quadrant. Due to the large scatter of accuracy values, larger scan volumes are not yet appropriate for producing a fixed partial denture [61].

The objective of the current investigation was to assess the precision of digital impressions on partially edentulous arches. However, the intra-oral scanner for completely edentulous arches does not achieve the same precision as conventional impressions, as the scanning accuracy decreases with increasing numbers of missing teeth. Various factors such as the nature of the scanned surface, oral health conditions, and the patient’s movements can reduce the impression quality. The intra-oral scanner may also be limited by anatomical abnormalities including a high-arched palate, large edentulous gaps, or patient ability to open the mouth [62]. Further clinical is needed on completely edentulous arches using Triswiss (Matrix, Switzerland), the first approved dental implant system of its kind specifically developed for digital manufacturing technologies (computer-aided designing and computer-aided milling CAD/CAM milling and 3D printing) incorporated with a new concept in which prostheses can be directly fixed to the implant without the use of an abutment. It is essential to fabricate accurate master casts to eliminate discrepancies in the fit, even though they might not be visually apparent. Of the 3 approaches analyzed in this study, only the complete digital flow with an intra-oral scan system provides this degree of precision, which can eliminate the need for unnecessary soldering or re-fabrication procedures.

Conclusions

The result of the study suggests that complete digital workflow with intra-oral scans were more accurate than partial digital workflow with extra-oral scans for the Pindex system and Di-Lok tray systems. Intra-oral scans can be recommended to obtain the digital impression for the best possible fit to avoid bone strain resulting from the uncontrolled loading of implants through the superstructure. For the partial digital workflow with bench-top scans, due to the better-locking mechanism of the individual dies, Di-Lok trays can be preferred to the Pindex system.

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