Linear Dimensional Accuracy in Maxillomandibular Records: A Comparative Study of Scannable and Transparent Materials

Introduction

The mandible is connected to the cranium by 2 temporomandibular joints, while the natural dentitions, deciduous and permanent, serve as a dynamic functional connection once all the teeth assume their final position in occlusion. Bite registration, or maxillomandibular relationship recording, involves the clinical recording of any positional relationship of the mandible in reference to the maxillae, which can be made in any orientation, including vertical, horizontal, or lateral. The material used for recording such a relationship needs to be rigid and dimensionally stable to maintain accuracy over a time period, otherwise errors are induced in recordings that reflect directly in indirect restorations. Indirect restorations form the major bulk of prosthodontic and restorative treatment options, which are traditionally fabricated in a dental laboratory to be cemented later in place [1]. Inconsistencies between a prepared tooth or abutment and the restoration can have immediate, including pulpal and muscle co-contraction, and delayed effects, including secondary caries, pulpitis, periodontitis, loss of alveolar bone, and several other patient-related invisible effects. Pulpal consequences are mostly painful except in non-vital teeth and are typically as a result of inaccurate occlusion, including high points, slide in centric, and defective anterior guidance [2]. Biological, neuromuscular, and behavioral components of the stomatognathic system have refined primitive, solely mechanical occlusal concepts [3]. Fabrication of the occlusal surface in indirect restorations is one of the most complex laboratory procedures, for it requires the development of cusps and fossae anatomy that provides the path for the mandible to move in different directions. The accuracy of centric relation determination by the clinician is key to preventing any occlusal discrepancy [4], which has the potential to cause muscle dysfunction in craniomandibular/craniocervical areas and disc derangements in the temporomandibular joint [5]. Clinically, occlusal contacts for a restoration (crown, partial denture, onlays) that can work in harmony with stomatognathic components can be created by recording the accurate centric relation and conforming the artificial occlusion to the recorded centric relation, otherwise best described by centric occlusion coinciding with centric relation. Such restorations require minimal or no clinical adjustments, thereby saving precious clinical time. Most clinical adjustments of occlusion in indirect restorations that are subjective in nature are not accurate. However, to produce flawless restorations, the technician in the dental laboratory needs to fabricate occlusal surfaces on a programmed semi- and fully adjustable articulator, in which the static and dynamic influences on the patient’s mandibular movements, chiefly condylar and anterior guidance, are simulated [6]. These allow the dental technician to copy patients’ mandibular movements, including protrusion, lateral, opening/closing, and other eccentric movements, and adjust the artificial occlusion accordingly, preferably during restoration wax-up. Diagnosing existing occlusal problems, treatment planning, and fabrication of restoration occlusion all depend on the accuracy of transfer of patient-related data to the dental laboratory. The clinicians frequently document these patient influences using a variety of methods, including digital record transfers, functional generated path records, cephalometric imaging (in complete dentures), and graphic tracings (extra/intraoral) [7], but the more common, economical, and invariably reliable method is through making of physical occlusal records: interocclusal records or bite registration records (BRRs). BRRs are made in centric and eccentric (right and left lateral, protrusive) positions in most cases, to complete the transfer process [8].

Walls et al [9] reported primarily transfer inaccuracies, which over a period of numerous studies are recognized to be due to material quality, clinical method/technique, stomatognathic system characteristics, and manipulation of bite registration material (BRM) [10]. Clinical variance and maximum tolerance in such transfers are calculated at 0.11 mm laterally and 0.07 mm anteroposteriorally [11]. However, studies have demonstrated values ranging from 0.95 to 3.5 mm laterally, 1.85 to 4.12 mm anteriorly, and 1.67 to 5.87 mm vertically [12], surpassing the maximum threshold. BRMs must have unique properties, including limited tooth displacement resistance [13], high post-setting rigidity, minimal dimensional changes [14], and high flow, to accurately record occlusal details [15]. They should also be clinically workable, inert on tissues, verifiable, and simple to disinfect [13,15]. BRMs include impression materials, including impression plaster, alginate, and zinc oxide eugenol, waxes or modified waxes, including modelling, corrected, and metallized waxes, elastomers, and others, including acrylic resin, T-Scan (Tekscan), pressure-sensitive films, typewriter ribbon, transparent acetate sheet, and occlusion sonography [16]. Most of the traditional BRMs have been, over time, superseded by elastomer-based (polyether and polyvinylsiloxane) registration materials, due to their superior properties. Their ability to undergo less dimensional changes over a period of time is seen as extremely convenient, since it overcomes the transport-related incorporation of inaccuracies from clinical to dental laboratories. The vinyl polysiloxane (VPS)-based BRMs have been found to be more dimensionally stable than other BRMs in clinical [17,18] and in vitro studies [19–26]. These studies also reported VPS BRMs to be more dimensionally stable than polyether-based BRMs [19–26]. However, Michalakis et al [14], Tejo et al [27], Pokale et al [28], and Sonkesriya et al [29] all reported polyether to be more dimensionally accurate than VPS at different time intervals (1, 24, 48, and 72 h). At the same time, Sharma et al [30] and Lozano et al [31] reported VPS BRMs to be more dimensionally stable than polyether. In assessing the dimensional correctness of several interocclusal records, including Aluwax, Godiva (thermoplastic bar), Occlufast Rock (VPS), and Futar D (injectable silicones), Lozano et al [31] found that Futar D was clinically viable for 22 days, although Occlufast silicone remained stable for as long as 7 days.

Digital dentistry has expanded vastly in the previous and current decade, with many material and technique-related drawbacks having been overcome. Likewise, jaw relations and BRRs, either virtual or physical, have overcome drawbacks related to the use of conventional materials and human-induced errors. In a single digital technique, a computer-assisted diagnosis, computer-assisted machining (CAD/CAM)-generated virtual dental cast can be mounted using algorithms based on the principle of best fit alignment, thus eliminating the need for a physical BRR [32]. Another common technical advance requires either intraoral scanning of the patient’s dentition or scanning of his cast/model/BRR to produce a virtual cast [33]. Physical casts can be made from these scans using CAD/CAM, which can then be mounted by scanning the buccal surfaces of the maxillary and mandibular teeth in maximum intercuspation or centric occlusion, which is then analyzed with software [32]. The occlusal contacts using both technologies have been reported to be not only accurate but to also provide objective data related to occlusion, like occlusal timing and contact sequences, and more importantly, the amount of occlusal forces [34]. Although virtual BRR (iTero Element Scanner) is more promising in terms of technical accuracy and duplicability than physical records, the most commonly used digital technique involves scanning of a VPS BRR in the intercuspal position [35]. Scanning of the physical BRR provides a 2-dimensional (2D) image, which upon analysis with image software, has been found to be highly reliable and valid for determining occlusal contacts [36–38]. This currently has been established as the standard method for digital jaw relations using BRR. Regardless of method, there are 3 steps involved in performing a digital static occlusal analysis: the patient closes in intercuspation on an indicator, such as a sensor, silicone material, or articulation indicator; the occlusal record is interpreted on a computer; and lastly, the BRR is stored and transferred [35,37]. All different indicators have been found to have high reliability and validity [38,39]. However, it has been observed that the digital articulator system needed to mount these recordings is less precise than the standard articulator system [40], with VPS BRM being more accurate than wax on both systems. When compared with occlusal registration, digital scans, namely the T-Scan/3D intraoral scan, were found to be less accurate in quantifying the occlusal contact area in another investigation [37,41]. The quantity of correction needed is also significantly influenced by the size and severity of the occlusal contact. The information concerning contact intensity that BRR can provide, regardless of the material used, has been found to be less accurate when virtual occlusal recordings are used [42]. Yazigi et al [43] studied the performance of conventional and scannable registration materials in recording maxillary-mandibular relationships and their dimensional stability after 2 different storage times, 1 h and 48 h. Results showed significant differences in vertical discrepancies, with scannable materials showing lower discrepancies. All materials showed clinically acceptable discrepancies, but increased after 48 h. The conventional BRMs have also seen advances in the form of providing the VPS BRM as transparent, which allows the clinicians to accept or reject the BRR depending upon the presence or absence of a void, bubble, or discrepancy. The transparency of these materials is due to the high percentage (30–50%) of quartz silica [43]. Many mounting errors are inherently due to these reasons and therefore can overcome the existing problems of delays due to rejections by dental laboratories. These errors are also significant when BRR is scanned for digital analysis, as they alter the image significantly, which in turn leads to errors in overlapping image fit, thereby causing an error [37,39]. When used with dual cure or light-cured temporary materials, the clear material also “cures through” quite well. The bite registration substance of transparent BRMs also sets quickly, is flexible, and has been claimed by manufacturers to be dimensionally stable. However, studies related to these materials are either minimal or nonexistent. The linear accuracy of BRMs has been reported to be more sensitive than the vertical accuracy, since any error in linear dimensions brings an entire change in the occlusal record. Such accuracy has been standardized to be measured according the specifications of American Dental Association (ADA).

In light of the foregoing context, in this study, we aimed to compare the dimensional stability of a scannable and a transparent BRM for the stipulated clinical time during which it is made (1 h). Scannable and transparent BRMs have been introduced lately for digital and conventional dentistry. To demonstrate that these BRMs are just as accurate as a standard (traditional) BRM, it is necessary to demonstrate that the dimensional changes of these BRMs are within the clinically acceptable ranges at the time of making these records. We also aimed to objectively determine whether there was a need for clinicians to switch from standard BRMs to scannable BRMs and whether transparent BRMs provide an alternative to a clinician, to overcome the errors induced in conventional BRMs. We hypothesized that there would be no differences between them, the basis of which is the compositional alterations that manufacturers make to provide appropriate properties to the material that has commercial value. Alternately, the null hypothesis stated that there would be differences between the different BRMs.

We aimed to answer 2 research questions: First, are scannable and transparent BRMs dimensionally as accurate as conventional BRMs? Second, are dimensional changes in scannable and transparent BRMs within the clinically acceptable limits?

Material and Methods

ETHICS:

As per the institute and university’s regulations, all research projects conducted on humans or animals require approval by the concerned institutional ethics committee, which was duly obtained for this study, with reference number CODJU-2326I, although no humans or animals were involved in the study. All materials used in this study are approved by international and national regulatory organizations.

STUDY DESIGN:

An observational comparative multiple-group experimental design (control and test group) was followed for this in vitro study. The investigation included different phases, namely fabricating a stainless steel die (standard sample size and thickness), assembling the mold, sample preparation and fabrication, and surface landmark measurement for dimensional accuracy.

OPERATIONAL DEFINITIONS:

An interocclusal record (synonymous with BRR) documents the positional relationship between opposing teeth, arches, or jaws [44]. An interocclusal record (jaw relationship impression) includes 3 components: a fixing component, stabilizing component, and guiding component. The fixing component replicates hard and soft tissues, and the stabilizing component fortifies the imprint material during transportation and trimming. The guiding component aligns the data with an anatomical horizontal plane [45]. The interocclusal record captures the position of opposing teeth, transmitting maxillomandibular relationships from the mouth to the articulator, to ensure horizontal stability and to avoid cast rotation or translation. CAD indicates computer-assisted diagnosis or material that is used during the process.

MATERIALS:

Presented in Table 1 are the interocclusal recording materials used in this study, together with details regarding their manufacturers, characteristics, and clinical guidelines (working time, setting time, dimensional stability, detail reproduction, and shore hardness). Six commercial brands, divided into scannable (CAD) and transparent (clear) experimental groups, as follows: scannable (CAD) BRMs: Occlufast CAD, Virtual CADBite, and Flexitime Bite; and transparent (clear): Maxill Bite, Charmflex Bite, and Defend ClearBite, were investigated in this study against a standard BRM: Occlufast Rock [46–51].

MOLD DESIGN (STANDARDIZING OF SPECIMEN): To ensure consistency in the specimens, a mold for producing the samples was initially made from stainless steel in accordance with the requirements of American Dental Association (ADA) specification no. 19 (non-aqueous elastic dental impression) [52]. The die was designed in the form of a cylinder on a computer sketch, with known dimensions. A 3-component mold (cylindrical ruled block [A], material mold [B], rostrum) when assembled together formed a standardized mold (Figure 1). The top of the cylinder surface of the block was engraved with 3 horizontal lines (X, Y, Z) and 2 vertical lines (CD, C’D’). The surface was raised 3 mm from the body circumferentially, leaving a shoulder-like shelf that could accept a 6-mm-high material mold. The exterior surface of the material mold determined that, when placed on the cylinder, the space for the BRM would be around 3 mm. The ruled block has 3 distinct widths of horizontal lines: medium (X=55 μm), small (Y=24 μm), and thick (Z=85 μm). Two 85-μm horizontal lines, 1 on each side of the cylinder surface, passed through the 2 vertical lines. Approximately 2.5 cm separated the 2 horizontal lines. For the purpose of releasing the interocclusal record material from the mold after it had set, a rostrum was manufactured with a 30-mm diameter and a 4-mm thickness. Alternatively, it served as a guide to trim the excess material from the top of the mold.

MANIPULATION OF INDIVIDUAL VPS BRMS:

The common commercial supply for all BRR materials used in this investigation was in the form of cartridges, which were automixed by a tip upon plunging the contents with the aid of a gun-shaped dispenser. The manufacturer supplies individual dispensers for all BRMs, which underwent the manufacturer’s recommended sterilization (prevaccum, 132°C, 3 min of exposure, and 20 min of cooling time) and disinfection (solution of isopropyl alcohol 15% and quarternary ammonium salt 0.3%) procedures. To replicate the conditions that would be encountered in a clinical setting, the dispenser was disinfected after the conclusion or beginning of each mix, and the dispenser sterilization process was conducted before the material cartridge was replaced. In accordance with the manufacturer’s recommendations, all of the BRMs were supplied in a 2-cartridge system, consisting of a base and catalyst. These materials were stored in controlled temperature circumstances, as guided by each manufacturer’s recommendations.

SAMPLE PREPARATION AND GROUPING: The bite registration cartridges of 7 BRMS – Occlufast Rock, Occlufast CAD, Virtual CADBite, Flexitime Bite, Maxill Bite, Charmflex Bite, and Defend ClearBite – were attached to the dispenser (automix gun), and the automix tip was used to mix the base and catalyst cartridge contents for each BRR sample. The cartridges were loaded and locked in the dispenser, with each press on the handle pushing the plunger forward to push equal amounts of base and catalyst into the automix tip. The automix tip in turn released a uniformly mixed material through the tip, which was directed at the site of concern. After the released material was evenly distributed throughout the whole surface of the mold, a laminated glass slab (polyethylene) was used to compress the material into the mold. To simulate occlusal pressure, a load of 500 g was applied over the top, until each material set as per their respective setting times [14]. The same load was also sufficient enough to counter material closure resistance (average 0.5 to 13.8 N) [10]. At this point, the mold was submerged in a water bath that was maintained at a temperature of 37°C, which was designed to simulate the conditions of the oral cavity. Table 1 displays the setting time of each material, which ranged from 4 to 5 min. This time served as a guide for the immersion process. Once set, the sample specimen was forced out of the mold using the rostrum, and excess set material was cut to yield an individual sample of predetermined size (30 mm in diameter, 3 mm in thickness) with inscribed lines X, Y, Z, CD, and C’D’. To imitate the clinical protocol, each specimen was disinfected by being submerged in a 0.5% glutaraldehyde disinfectant for 10 min [53]. After that, it was washed in water for 15 s, dried, and then stored at a relative humidity of 100% for 1 h. All of the specimens were kept in a sealed environment that was free of humidity (a polyethylene bag) at room temperature (between 26 and 28°C, with a margin of error of ±2°C) until their measurements were recorded [27]. A total of 105 specimens, 15 control and 90 experimental, were thus prepared, with 2 surplus samples for each subgroup as replacements. After examining surfaces for each specimen, the specimens were subgrouped under 3 main groups of control, scannable, and transparent, as follows: (1) control: Occlufast Rock (group C); (2) scannable (CAD): Occlufast CAD (group CadO), Virtual CADBite (group CadV), Flexitime Bite (group CadF); and (3) transparent (clear): Maxill Bite (group ClM), Charmflex Bite (group ClCh), and Defend ClearBite (group ClD) (Figure 2).

MEASUREMENTS FOR LINEAR ACCURACY:

Each sample was positioned on the stage plate of a stereomicroscope equipped with a USB CCD camera (Amscope, Irvine, CA, USA). Illumination was regulated for both the upper and lower lights using control mechanisms. The specimen was examined using the eyepiece diopter, and the image was adjusted and focused using the focus knob (10× magnification). For each sample, the distance between lines CD and C’D’ was measured at 3 predetermined points of intersection (p1–p2, p3–p4, p5–p6) between horizontal (CC’, DD’) and vertical lines (X,Y,Z), yielding 3 individual analyses for each sample (Figure 1). Thus, 3 readings were taken for each sample in each group, and the average of the 3 was considered for statistical analysis. For measurement of the ruled block on the die, 3 repeated measures were taken by the same calibrated operator, and the average of 3 was taken as the final value.

STATISTICAL ANALYSIS:

Data for each sample was input into Microsoft Excel (version 20H2), where it was refined, standardized, and coded for subsequent analysis using the Statistical Package for the Social Sciences (SPSS, Version 25, IBM Corp, Armonk, NY, USA). The dimensional changes occurring in the BRMs were estimated using the mathematical equation D (dimensional change)%=[(X-Y)/X×100], where X represents the initial standard measurement of coordinates in the die/control samples, and Y denotes the observed measurements of coordinates on the sample within a specific group. All changes were expressed in millimeters and percentages, and the summary was expressed as either increased or decreased. The normality test for data distribution was performed using the Shapiro-Wilk test. The differences between the group mean and median values for distances were assessed using one-way ANOVA on ranks (Kruskal-Wallis test), while the differences within each subgroup were statistically evaluated using the post hoc Dunn test following Bonferroni correction (corrected α=α/m where m=number of tests/pairs) for the P value. Differences between the groups were considered to be significant at a P value ≤0.05, and differences within the groups were significant at a P value ≤0.0023 (0.05/21).

Results

LINEAR DIMENSIONAL ACCURACY BETWEEN GROUPS:

Table 2 presents the median values, interquartile ranges, and mean rank scores for linear dimensional accuracy of the control (Occlufast Rock) and experimental groups (scannable/CAD and transparent/clear BRM) and respective subgroups: groups CadO, CadV, CadF, ClM, ClCh, and ClD. The spread of the second and third quartiles for each subgroup is represented by the interquartile range along with maximum and minimum values. Group CadF (median [IQR], 24.89 [0.09]) was the only subgroup similar to group C (24.89 [0.09]), while the highest changes were observed in group CadV (24.94 [0.03]), group ClM (24.94 [0.30]), and group ClD (24.94 [0.50]). The one-way ANOVA Kruskal-Wallis rank test results showed that the differences between the studied subgroups were statistically significant (P≤0.05). The mean rank scores showed that, except group CadF (25.67), all other groups had higher ranks than the control group.

LINEAR DIMENSIONAL ACCURACY WITHIN SUBGROUPS:

The post hoc test results for multiple group comparisons showing the differences in the mean ranks between the subgroups and their significance levels are presented in Table 3. When comparing the mean rank of groups with that of the control group, only group CadV showed significant differences (P≤0.0023), while the differences in the median ranks of all other subgroups did not show any difference from the control group. Within the scannable group, group CadV differed significantly from group CadF (mean rank difference, 45.57, P≤0.0023). Three subgroups, groups CadO, ClCh, and ClD, did not show any significant differences between them, indicating they were closer to the values of the control group and therefore more accurate at a 1-h time period. Overall, 2 brands of transparent BRM (group ClCh, group ClD) and 1 brand of scannable BRM (group CadO) had median ranks closer to those of the control group.

DIMENSIONAL CHANGES:

Dimensional changes were evaluated by comparing the observed group medians from the actual measurement on the die and the median values of the control group. The observed dimensional variations of unit length and percentage are presented in Table 4. All BRMs studied when compared with original die median values, including the control group, showed a decrease in linear dimensions, with the lowest being that of group CadV, group ClM, and group ClD (−0.06 mms, 0.24%). The range of change for all BRMs was between −0.06 and −0.11 mm, which fell between 0.24% and 0.44%. When compared with the control group (Occlufast Rock), all BRMs except group CadF showed an increase in linear dimensions at 1 h (0.04 to 0.05 mm, 0.16% to 0.20%), with group CadF showing no change in linear dimensions from control.

Discussion

RATIONALE FOR THIS STUDY:

Scannable and transparent BRMs can be termed recent and advanced types of VPS BRM, the knowledge and use of which is both lacking and deficient among general dental practitioners and specialists, including prosthodontists [54]. Maru et al [55] even found that most dental practitioners still use some form of interocclusal wax as BRM, despite the availability of better and more economical VPS-based materials. They attributed their findings to a lack of knowledge, awareness, and, in some cases, resources (academic-based).

WHY LINEAR ACCURACY IS MORE SIGNIFICANT THAN VERTICAL ACCURACY:

The vertical accuracy of different BRMs has been studied in many clinical [18] and in vitro studies [18,20,24]. Using a 3D-coordinate measuring machine, Dwivedi et al [20] compared the accuracy and stability of 3 interocclusal recording materials, polyether (Ramitec), polyvinyl siloxane (Imprint), and wax (Maarc), at 3 time intervals, 0, 1, and 24 h, for 3 axes, vertical, lateral, and anteroposterior. With statistically non-significant variations in the lateral axis, polyvinylsiloxane was found to be the most stable; the other 2 axes showed notable departures. Vergos et al [56] tested polyether, polyvinyl siloxane, acrylic resin, and wax for accurately recording and reproducing vertical interocclusal connections, using metallic arches. Polyvinyl siloxane had the narrowest disparity (101 μm), followed by polyether (107 μm) and wax (168 μm). Transferring records onto castings caused a 0.5-mm disparity, which was over threshold levels. Clinically, however, the vertical discrepancies in BRMs are not bound to cause occlusal errors provided the hinge axis is located and the maxillary dental casts have been correctly oriented on the articulator in the way that is present in the patient [57]. While making interocclusal records in centric relation [58], one needs to adjust the articulator for the thickness of the BRR, which is accomplished by adjusting the vertical pin on the articulator. Such vertical adjustments, despite being subjective, do not induce occlusal errors [4], since both casts are oriented to the hinge axis of the mandible. Studies investigating the vertical accuracy of BRMs, irrespective of being clinical or in vitro, do not mention the use of a facebow to transfer the maxillary casts, which could be the most likely reason for higher vertical discrepancies using BRRs in these studies. Also, while using such study methods, the confounding effects of the clinical technique are also induced. Recording and transferring of the hinge axis are compulsory in fixed and partial removable prosthodontics [4]. The use of multiple records for a single registration is recommended by Erkisson et al [45] on the basis of their findings that clinical technique is the main factor for occlusal discrepancies, and not materials. Yazigi et al [43] recently tested conventional (Registrado X-tra, Futar D Fast, and O-Bite) and scannable (Registrado Scan, Futar Cut & Trim Fast, and O-Bite Scan) VPS BRMs for maxillary-mandibular relationship recording and dimensional stability after 1 h and 48 h. Vertical disparities were substantially more reduced in scannable materials than in traditional ones. The median vertical difference at 1 h was −2 μm (FS) to 11 μm (O-Bite). These findings indicate that VPS BRMs are stable vertically, also making them a more ideal BRM. On the other hand, linear accuracy of BRRs is less influenced by clinical technique, since it is completely dependent on the inherent property of a BRM. The BRR engages the cusps and fossaes of natural teeth during registrations, making it a very close contact procedure. Minor linear shrinkage after removal of the record results in horizontal and vertical discrepancies and affects the engagement of all the dental casts involved in the BRR. Thus, linear shrinkage results in the misfit of the opposing casts and causes significant errors in lateral and vertical directions. Clinically, linear shrinkage results in multiple occlusal errors, which are difficult to locate and correct and result in significant time loss.

CHOICE OF MATERIALS FOR THE STUDY:

The results of our study showed that the BRM used as a control (Occlufast Rock) showed dimensional changes of 0.44% at 1 h, which amounted to a decrease (shrinkage) by 0.11 mm. These are similar to those obtained by Lozano et al [31] after 1 h (m=0.12), who also reported Occlufast silicone to be stable up to 7 days (m=0.12). In a recent study, Rovira-Lastra et al found that Occlufast Rock achieves 85% to 95% agreement in occlusal contact location between sessions, while Occlufast CAD, 200-μm articulating film, and T-Scan offer 79% to 86%, 68% to 75%, and 65% to 75%, respectively [59]. Transparent BRMs have high ultimate tensile strength and high elastic moduli, as compared with other BRMs, both properties being essential requirements for BRRs.

WHY VPS ARE BETTER BRMS:

The results from our study showed that scannable and transparent VPS BRMs were dimensionally accurate when compared with a conventional BRM, indicating that the dimensional accuracy of these materials was not affected by the compositional changes that render them scannable or transparent. The results obtained for Occlufast Rock are in general agreement with results of many studies that have shown the superior dimensional accuracy of VPS BRMs over waxes [17,20–23,25,26,45,56], resin [13,16,57], zinc oxide eugenol paste [18,27,30], impression plaster [17], and polyether [19–21,60]. However, while our study did not compare these materials, the dimensional changes we obtained were analogous to those found in previous studies. Our results on dimensional change percentage for Occlufast Rock differ from the results of 2 previous studies [14,27] and 2 recent studies that found polyether to have lesser dimensional changes at 1 h than VPS. The differences from the previous 2 studies are explained on the basis of differences in materials, methods, improvements, and refinements of both BRMs. These studies also reported that VPS BRMs showed greater dimensional stability over a period of time than did polyether [27]. A recent study by Pokale et al also reported polyether to have superior dimensional stability at 1 h. Our study results contradict the findings of Narde et al [61], who did not find any significant differences between addition silicones AvueBite and Aluwax (USA) on an E4 scanner. However, the differences could be because of the study design, which was clinical, and measurements in the form of distances were taken from intraoral landmarks.

Scannable and transparent VPS BRMs are chemically similar to conventional VPS BRMs; however, certain components are replaced and/or altered in composition to have desired effects. VPS BRMs are generally also similar to VPS impression materials in that they basically are a composite of the elements Si, C, O, Al, Ti, Ca, Na, Mg, with Si, C, and O in higher content. The set material becomes a particle reinforced composite within an organic matrix (VPS). This multiphase system has differences in phases that are influenced by mean atomic number distributions and contrast [62]. The high C, Si, and O content is attributed to SiO2, or silicate-glass, filler particles as well as the vinyl-polysilane matrix. Higher TiO2 content of all scannable BRMs adds to the high reflectivity of the material, therefore fulfilling the requirements for CAD/CAM optical scanning [63]. Together with Al, Mg, and Zn oxides or salts, CaCO3 is used as non-reinforcing filler to improve dispersion, processing, extrudability, viscosity, and sagging of the final product [64]. Furthermore, the matrix produces hydrogen gas, which the alkaline CaCO3 [65] absorbs. When determining a material’s scannability, the contrast and brightness, both of which should ideally have a value of 100%, are the 2 most crucial criteria to consider. Intraoral recording materials are made more scannable by adding pigments that increase brightness and contrast. As a result, the material’s dynamic scannability value increases [43]. Brightness of scannable VPS BRM is enhanced by adding oxides, hydrates, or carbonates of metal (15–80% weight), while contrast is optimized by adding pigments (0.01–0.0001% weight) [43]. The transparent BRMs, on the other hand, contain a large percentage of quartz silica, which naturally is transparent in different light ranges, acid resistant, inert, and resistant to thermal changes up to a certain temperature.

REASONS FOR DIMENSIONAL ACCURACY:

The results of our study indicated that both the scannable and transparent BRMs showed linear accuracy within the clinically acceptable limits: 0.06 for Virtual CADBite, to 0.11 for Flexitime Bite, when compared with the original die dimensions of 0.00 for Flexitime Bite, to 0.06 for Virtual CADBite and Maxill Bite, and when compared with control (Occlufast Rock) at 1 h. All BRMs showed an increase in length when compared with the control and a decrease when compared with the die. A similar range of values was obtained at 1 h by Dua et al [19], Dwivedi et al [20], Pokale et al [28], Lozano et al [31], and Anup et al [60]. According to Tejo et al [27], polyether had smaller changes in size than did VPS BRMs, polyether (0.011%) and polyvinylsiloxane (0.012%), after 1 h, which were not significant, while ours had changes of 0.24% to 0.44%. Their study used a different formula to calculate the percentage, which could be the reason for the difference. When compared with the die, all materials showed a decrease in length, indicating shrinkage of the specimens. The VPS matrix expands during initial setting, which is countered later by setting shrinkage [30]. This is caused by the formation rate of H radicals and the consumption rate of VPS monomers, with high-molecular-weight monomers having a lower reaction capacity. This neutralizes initial changes due to expansion [65]. Another feature of VPS BRMs that enhances dimensional stability is the production of early phase setting exothermic heat, which triggers swift setting reactions and influences the rate of shrinkage conversion. Extended post-setting conversion is minimized; thus, it could adversely lower dimensional stability [66]. Also, elastomers are generally formed through continuous crosslinking polymerization, resulting in macroscopic shrinkage and dimensional changes. These materials can undergo polymerization even after 30 min, with negative changes after 24 h [67]. The shrinkage-strain, though, is lower than that of monomethacrylate. Polyether and VPS exhibit low setting shrinkage-strain, making them suitable for use as BRMs [68]. High viscosity of the elastomer also contributes to low post-polymerization shrinkage strain. Chun et al [66] studied the shrinkage strain of interocclusal recording materials, finding that dimethacrylate-based materials had the highest shrinkage, followed by polyether-based and polyvinylsiloxane-based materials. Researchers have investigated the influence of BRRs on VPS BRMs, given that all patients require routine disinfection [69]. The results indicated a linear dimensional change of 0.5% or less over a period of 14 days, even following disinfection, which complies with ANSI/ADA specification no. 19, 2004 version, maximum limit of 1.5% for all elastomeric impression materials.

CLINICAL SIGNIFICANCE:

All transparent BRMs investigated in this study were more accurate than control when tested against the original dimensions of the die. Scannable BRMs, except Flexitime, were also found to be clinically accurate.

STRENGTHS AND LIMITATIONS OF THE STUDY:

Linear accuracy of scannable and transparent BRMs has not been explored, despite the wide range of materials and applications in digital dentistry, thus making this a novel study. The use of multiple commercial BRM types, the study design of comparison with dies, and a standard BRM are other strengths of this study. The study has limitations, however, which include the in vitro design, which eliminates many of the clinically related factors that can affect materials. The study was also limited by a single-axis measurement and the exclusion of other commercially available BRMs.

Conclusions

From this study’s results, we conclude that all of the BRMs, control and experimental, exhibited a reduction in linear dimensions, which was indicative of shrinkage occurring with the BRM during the first hour. Also, all of the BRMs that were investigated exhibited linear differences from the original dimensions, which were, however, clinically acceptable. The transparent BRMs used in this study exhibited less linear dimensional changes than did the scannable and standard BRMs.