In Vitro Assessment of Linear Dimensional Accuracy of Polyvinyl Siloxane-Based Computer-Aided Design and Transparent Interocclusal Record Materials at 3 Different Storage Times

Introduction

A primitive mechanical concept of occlusion has been completely replaced by a biomechanical one as a result of its influence on stomatognathic system neuromuscular control, biological effects, and behavioral consequences [1]. The centric relation has taken the spotlight as the primary focus by way of comprehending its function and significance in dental occlusion. Centric relation is a bony relation between the mandible and maxilla that can be influenced by natural teeth and surrounding musculature. One of the key factors in effective mastication is the biologically correct occlusal contact area, which determines the bite force in mastication, and is completely reliant on the ability to coincide with centric relation [2]. This relation is key to generating optimum masticatory force and is very important in maintaining neuromuscular balance of both the agonist and antagonist masticatory muscle groups. Recording these relations has always been considered an arduous task, especially in completely edentulous patients [3], while in fixed prosthodontic treatments, they have been termed as complex and time consuming [4]. To establish biocompatibility of artificial occlusion, the accurate proximity of centric occlusion to centric relation is crucial, for such failures can lead to neuromuscular dysfunction not only of craniomandibular but also of the craniocervical musculature (muscle co-contraction) [5]. Incorporating an artificial occlusion in a restoration is not merely placing cusps and fossae, but more importantly, it must provide the paths for the mandible to move without causing interference to its normal function. All indirect restorations are constructed in the dental laboratory without the patient present; consequently, dental professionals use a programmed articulator that simulates the patient’s masticatory dynamics, including condylar and anterior guidance [6]. These patient values are commonly recorded by the clinicians using various techniques, including graphic tracings (extra-/intraoral), functional path records, cephalometric imaging (in complete dentures), and digital [7], but are more common in the form of physical interocclusal records (IORs), which are transferred to the dental laboratory. IORs are made clinically and represent the static and/or functional relation of maxillary and mandibular teeth, depending upon their type. Depending upon the articulator used, the IORs take the form of a face bow record, centric and eccentric (protrusive, right and left lateral). In addition to planning comprehensive dental treatments, all occlusal problems and disorders are diagnosed on a programmed articulator. Since the original reports of inaccuracies in these IOR transfers, researchers have determined the causes to be either due to material manipulation and properties, clinical procedures, or even patient’s characteristics [8]. The maximum variance (clinical tolerance limits) allowed in such transfers has also been established as 0.07 mm anteroposteriorly and 0.11 mm laterally [9,10], but studies have found higher ranges than acceptable limits: laterally up to 3.5 mm, anteriorly up to 4.12 mm, and vertically up to 5.87 mm [11]. Since 1756, when Philip Pfaff introduced the IOR using natural waxes [12], a wide range of materials have been used, including dental (impression plaster), resin acrylics (self-cure), impression pastes (zinc-containing), theremoplastic waxes and resins, and elastomers [13]. Elastomers were introduced in 1961 and patented in 1969 [14], and since then, have been subjected to extensive research due to continuous refining of the material composition, handling, and storage. Digitization has provided technical advances in the form of T-scan (Tekscan), pressure-sensitive films, and occlusion sonography, which is reported to be essentially more beneficial in complex case designs, such as pier abutment [15]. The qualities of an IOR material must differ from those of their other dental uses. In addition to being clinically workable, innocuous on tissues, verifiable, and easy to clean, they must accurately record occlusal features, not resist tooth displacement, have high post setting rigidity, and undergo the least dimensional changes [15,16]. Limited jaw resistance is required to prevent deviation of the mandible from the typical recording position (centric relation, centric occlusion).

Over time, many conventional IOR materials have been replaced by elastomer-based registration materials, such as polyether and polyvinyl siloxane (PVS), owing to their enhanced properties. Their capacity to experience minimal dimensional alterations over time is regarded as highly advantageous, as it mitigates the potential for inaccuracies during the transfer from clinical settings to dental laboratories. Clinical [17,18] and in vitro [19–27] studies have potentiated their dominance over other materials in terms of dimensional stability in vertical and lateral dimensions. While these studies found PVS-based IOR materials to have higher dimensional accuracy than polyether, a few studies reported the reverse [12,28,29]. However, they also found PVS to be more dimensionally accurate than polyether over a long period of time. Lozano et al [30] investigated the dimensional accuracy of Aluwax, Godiva (thermoplastic bar), Occlufast Rock (PVS), and Futar D (injectable silicones). Futar D was clinically viable for 22 days, while Occlufast silicone was stable for 7 days. Other studies also support the long-term dimensional accuracy of PVS-based commercial IOR materials [17,31,32]. With the elimination of numerous material and technique-related limitations, digital dentistry has grown substantially during the previous few decades. Virtual or physical jaw relations and IORs have also solved problems associated with using traditional materials and iatrogenic errors. There is no longer any requirement for a real IOR, thanks to one digital technology that mounts a CAD/CAM-generated virtual dental cast, using algorithms based on the notion of best-fit alignment [33]. Scanning the patient’s cast, model, or IOR to create a virtual cast is another typical technological advancement that involves scanning the patient’s dentition intraorally [34]. After scanning the buccal surfaces of the upper and lower jaws in either maximum intercuspation or centric occlusion, the data can be used to create numerous physical casts using CAD/CAM [33]. Both systems have been found to produce precise occlusal contacts, which in turn offer objective data about occlusion, such as the quantity of occlusal forces, sequence of contacts, and timing of occlusal openings [35]. The most popular digital technology includes scanning a PVS IOR in the intercuspal location [36]. However, the virtual IOR (iTero Element scanner) shows promise over physical records in terms of technical accuracy and duplicability. According to research [37–39], the 2-dimensional image produced by scanning the physical bite registration record is a valid and reliable method for determining occlusal contacts when analyzed with imaging software. As far as IOR-based digital jaw relations are concerned, this is presently the criterion standard. When performing a digital static occlusal analysis, there are 3 steps involved, regardless of the method that is used: the patient closes their mouth in intercuspation on an indicator (such as a sensor, silicone material, or articulation indicator); the occlusal record is interpreted on a computer; and, finally, the IOR is stored and transferred [36,38]. It was discovered that each and every one of the many indicators have a high level of reliability and validity [39,40]. Recently, Yazigi et al [41] investigated scannable and conventional PVS materials for recording maxillary-mandibular relationships and vertical dimensional stability after 1 and 48 h. Compared with typical materials, scannable materials have significantly fewer vertical discrepancies. The median vertical difference at 1 h was between 11 μm (O-Bite) and −2 μm (FS). According to these results, PVS IOR materials are more suited because they are stable vertically. However, because the linear accuracy depends entirely on material intrinsic properties, the linear accuracy of BRRs is less affected by the clinical method. During registrations, the IOR makes very close contact with the cusps and fossae of natural teeth. The engagement of every tooth cast involved in the IOR is affected by even a small linear shrinkage that occurs after the record is removed, causing horizontal and vertical inconsistencies. As a result, linear shrinkage leads to major errors in both the lateral and vertical directions and causes the opposing casts to mismatch. Clinically, linear shrinkage causes numerous occlusal abnormalities that take a significant amount of time to identify and fix. Keeping in pace with the digital advances in IOR materials, the traditional IOR material manufacturers have also advanced completely transparent IOR materials, which can highlight a small incorporated error (void, bubble) in the IOR. These materials are chemically different from scannable materials in that they contain higher quantity of silica (quartz). Irregularity on the surface of an IOR creates mounting errors, irrespective of it being used as a physical or a scannable IOR. To the best of our knowledge and according to our search, the linear accuracy of either scannable/CAD or transparent IOR materials have not been studied till date. Therefore, this in vitro study aimed to evaluate the dimensional changes of 3 time intervals (1, 3, 7 days) on the linear accuracy of PVS-based CAD and transparent IOR materials. The study was based on the hypothesis that PVS-based IOR materials would not show any differences in linear accuracy over the investigated time periods. Alternately, the null hypothesis stated that there would be differences between scannable/CAD and transparent IOR materials.

Material and Methods

ETHICS:

Written ethics approval for conducting this study was obtained according to the organization policies (ref. No. CODJU-2415F). All investigated materials are biocompatible to humans and have approval by international and local drug organizations.

STUDY DESIGN:

This in vitro study used a comparative multiple-group experimental design approach, consisting of control and test specimens distributed among various groups. The independent variables included CAD and transparent IOR materials and the time intervals 1 h and 1, 3, and 7 days, while the dependent variable was the linear accuracy in millimeter and percentage, which was time-dependent. The investigation comprised several stages: machining a standardized 3-component die (stainless steel), mold assembly, specimen preparation, and, finally, measurement of surface markings that depicted linear accuracy.

:

When 2 teeth, arches, or jaws are in opposing positions, the record of their static or dynamic relationship is called an IOR (synonym bite registration record, bite record, occlusal record) [42]. A guide, stabilizing component, and fixing component constitute an interocclusal record. The stabilizing element strengthens the impression material during transportation and trimming, while the fixing component duplicates hard and soft tissues. A horizontal plane representing the human anatomy is used by the guiding component to align the data. The IOR captures the position of opposing teeth, conveying maxillomandibular relationships from the mouth to the articulator, to provide horizontal stability and to avoid cast rotation or translation.

MATERIALS:

Table 1 presents in brief the 6 commercially available IOR materials investigated in this study, with 3 of them being scannable/CAD IOR and 3 being transparent. All IOR materials are based chemically on PVS elastomers with differences being in the compositions and their respective percentages. Clinical guidelines in terms of the time used for mixing, manipulation, working, and setting were followed for each brand, as per their respective manufacturer’s recommendations.

SAMPLE SIZE:

The study was designed to have 2 main experimental groups, with each group represented by 3 different commercial IOR materials. Each commercial IOR material was further divided into 4 subgroups based on time interval: 1 h=control; 1, 3, and 7 days=experimental. Based on this criterion, the total sample size estimated was 360 specimens (power assumption 80%, type 1 error rate 0.05 and effect size 0.28), with a minimum of 15 specimens in each subgroup. The calculations were derived from Nquery software (v7.0; Informer Technologies, CA, USA), which calculates sample size according to mathematical formula [(N = 2 σ2× (Z α + Z β) 2/2)] [43].

SPECIMEN DESIGN AND STANDARDIZATION:

Figure 1 presents a graphic flowchart of the preparation, measurement, and grouping of the specimens. The recommended thickness of an IOR is considered to be 3 mm, according to various clinical guidelines. To ensure the same thickness of specimens across all control and experimental groups, a stainless steel die was machined, with its dimensions guided by the American Dental Association (ADA) specification number 19, meant for determining dimensional accuracy of elastic dental impression materials [44]. A multi-unit assembling mold, consisting of a lined cylinder, mold former, and riser, was machined. When these components are joined, they create a mold suitable for pouring material, which can then set. After the material has solidified, the mold can be disassembled to retrieve the set material (Figure 1). Three horizontal (X,Y,Z) and 2 vertical (CC′, DD′) lines were imprinted on the cylinder top, with the distance between the intersecting lines of each coordinate (1–2, 3–4, 5–6) being 25 mm from the intersection by vertical line. The top of the cylinder surface was also elevated than the body by about 3 mm, leaving a 3-mm circumferential shelf on the outside of the cylinder. The 3-mm difference between the top of the lined cylinder and the top of the mold former provided the thickness of the specimen to be approximately 3 mm.

SPECIMEN PREPARATION (MANIPULATION OF THE IOR MATERIALS):

All investigated IOR materials are commercially available in the form of a standard 100-mL dual-conjoined cartridge system containing a base and a catalyst, that is mounted on a readily sterilizable dispenser. The dispenser is in the shape of and works like a gun on the principles of hydraulics. The dispenser can be easily dissembled into many sections, which can then be sterilized using an autoclave cycle (prevaccum, 3 min contact 132°C heat, 20 min cool time), or disinfected (15% isopropyl alcohol, 0.3% ammonium salt). For each cartridge change, sterilization of the dispenser was used, while after every specimen, disinfection was performed to simulate the clinical method. Individual brands were stored in the respective manufacturers’ recommended conditions that ranged generally in mild temperature differences. Individual cartridges containing the base and catalyst PVS pastes are joined together at the front by placement of an automix plastic tip that is fastened to the cartridge system by a keyway lock mechanism. The dispenser also carries 2 individual plungers that fit into the backside of each cartridge. To dispense equal amounts, the gun is activated by moving the plungers into forward motion by squeezing the handle (trigger) which forces the material through the automix tip. The automix tip mixes the 2 materials by using its length, and a clean mix is expelled through the tip, which can be directly placed on the site of concern.

SPECIMEN PREPARATION (PREPARATION AND GROUPING OF THE SPECIMENS):

For each IOR material, the specimens were prepared by releasing material from the dispenser on the top of the assembled mold (Figure 1). Once filled, the excess was removed by placing a polythene-laminated glass slab, which also served to simulate the compression pressure of occlusion (500 g load) [24,28]. The load was applied for each material according to their respective setting times (Table 1). The mold with the load was then submerged in a thermostatically controlled water bath (37°C) to simulate the oral cavity condition, till each material was set. Each specimen was then removed from the mold and the excess was trimmed or finished. Each sample was thus standardized, with diameter of 30 mm, thickness of 3 mm, and inscribed lines (X, Y, Z, CD, C′D′, and coordinates 1–2, 3–4, 5–6). To follow the same steps as in the clinical study, each tissue was cleaned by putting it in a 0.5% glutaraldehyde solution for 10 min [45]. This was followed by washing, drying, and storing, as per the designated groups, depending upon the time intervals. For all subgroups, the specimens were stored in a sealed polythene bag that was free of humidity, and were stored at room temperature until their respective measurements were recorded. A total of 360 specimens were thus divided into 2 main groups: CAD and transparent, with 180 samples each. The CAD group was further subdivided into 3 subgroups depending upon the commercial IOR (Occlufast CAD, Virtual CADBite, Flexitime CAD Bite) and depending upon the time interval (1 h 1 day, 3 days, 7 days) to yield a total of 16 subgroups with 15 specimens each. Similarly, the transparent IOR material group was subdivided into 3 subgroups based on commercial type (Maxill Bite, CharmFlex Bite, Defend ClearBite) and time interval for measurement (1 h and 1, 3, and 7 days), yielding 16 subgroups with 15 specimen each (Figure 2).

MEASUREMENTS:

A stereoscope (Amscope, Irvine, CA, USA) was used to make linear measurements of the 3 horizontal lines from the point of intersection at one side (CC′) to other side (DD′) in 3 coordinates, namely 1–2, 3–4, and 5–6, thus providing 3 readings for each specimen. All observations were measured by a single operator who was first calibrated to the use and measurement of the stereoscope. For each specimen, the stage plate was used to position its flat surface, followed by illumination by upper/lower lights and controlling the camera, which connected by a USB charge-coupled device. All specimens were examined with standard settings (10× magnification) through the eyepiece diopter after regular adjustment and focusing, till the marking lines (edges) of the line were clearly visible. For each specimen, the 3 measurements of 1–2, 3–4, and 5–6 were averaged to yield the mean of that specimen.

STATISTICAL ANALYSIS:

All the raw data for each sample under each subgroup and respective groups were transferred to Microsoft Excel (version 20H2) for refining, standardizing, and coding, before the statistical analysis was done with Statistical Package for the Social Sciences (SPSS, Version 25, IBM Corp, Armonk, NY, USA). Mean values and standard deviations were used to express the measurements recorded, while distances (in millimeters) and percentages expressed the dimensional changes from the original dimensions on the die. Data were first tested for distribution using a normality test (Shapiro-Wilk). For determining the influence of time over the accuracy of IOR materials, the means at various time intervals (1, 3, 7 days) were compared with the control (1 h) using a one-way ANOVA test that determined the differences between CAD and transparent IOR materials. For each material, CAD and transparent, the differences within groups (pairwise comparison) were determined using the Tukey-Kramer post hoc test. For calculating the physical changes in the IOR materials (CAD and transparent), the change in dimensions was derived from simple subtraction (X−Y), where X was the original measurement of the coordinate on the die and Y was the mean of each group. A positive (+) indicated increase in dimensions, while a negative (−) indicated decrease in the dimensions expressed in millimeters. The overall dimensional change was expressed in percentage, which was determined by using mathematical equation D (dimensional change)% = [(X−Y)/X×100], where X and Y values represented the same as for above. For all statistical interpretation, the probability value of P was considered to be significant if the differences between 2 measurements were equal to or less than 0.05 (P≤0.05).

Results

CAD IOR MATERIALS:

Table 2 presents the mean values of the linear dimensions measured for CAD IOR materials at various time intervals, and their one-way ANOVA test results for determining the differences between groups. At 1 h, the Virtual CADBite group showed highest accuracy when compared to the mean values of the die (m=25.0), followed by Occlufast CAD and Flexitime Bite. The changes in dimensions at 1 day were equal in all 3 groups [0.01] when compared to their respective controls (1 h). All materials showed subsequent changes at various time intervals (1, 3, 7 days) by a decrease in the linear dimensions, with the highest changes observed in the Virtual CADBite group. The changes in linear dimensions among all groups were only significant for Virtual CADBite, which demonstrated the least changes at 1 h. The changes from 1 h to 7 days in other CAD IOR materials were non-significant.

TRANSPARENT IOR MATERIALS:

The change in mean linear dimensions over different time intervals for transparent IOR materials is presented in Table 2. At 1 h the least changes were observed in Maxill Bite (0.06), compared with the CharmFlex Bite and Defend ClearBite groups (0.08) when compared with the die (25.0). However, the changes in mean linear dimensions from 1 h to 7 days were observed to be higher in the Maxill Bite group than the other 2 groups, with changes being statistically significant. The changes in the other 2 groups were not significant.

INFLUENCE OF TIME:

Tables 3 and 4 present the post hoc test results showing comparative differences at various time intervals and their probability values for CAD and transparent IOR materials, respectively. For the CAD group, only the Virtual CADBite group showed statistically significant differences at day 7 compared with the values obtained at 1 h (control) (Table 3). There were no significant changes in the Virtual CADBite group at days 1 and 3. Although other groups had changes in linear dimensions at different time intervals, the changes were not significant. For different types of transparent IOR materials, only the Maxill Bite group showed significant differences at day 7 compared with control (1 h), while no significant changes were observed at days 1 or 3. The other 2 subgroups did not show any significant differences at any time interval in this group.

DIMENSIONAL ACCURACY OVER VARIOUS TIME INTERVALS:

Table 5 presents the overall dimensional variations in terms of the changes observed in metrics (millimeters) and percentages among CAD and transparent IOR at various time intervals when compared against the original measurements of the die. The clinical threshold of linear changes in IOR is 0.11 (positive or negative), which was observed in only 1 subgroup (Flexitime Bite=0.13 to 0.15, −0.52% to 0.60%). This suggest that Flexitime Bite did not meet the clinical requirements even at 1 h and continued to fall below the clinical threshold at all time intervals. All other materials showed linear accuracy maintained at 7 days, with the least changes observed in the Virtual CADBite (−0.24 to −0.40%) and Maxill Bite (−0.24% to −0.36%) groups. Between the 2 material groups (CAD and transparent), the overall changes in linear dimensions were observed to be lower in transparent-based IOR (range −0.06 to −0.10 mms, 0.24% to 0.40%) than CAD-based IOR (range −0.06 to −0.15 mms, 0.24% to 0.60%) when compared with the original die dimensions. All subgroups showed overall decreases in dimensions; however, no change in dimensions occurred for Flexitime Bite (between days 3 and 7), CharmFlex Bite, and Defend ClearBite (between days 1 and 3).

Discussion

STRENGTH AND LIMITATIONS:

Few studies have investigated single IOR materials, and most of them compared chemically and physically different materials, which introduces confounding effect on the research results. In the present study, we investigated different types of structurally and chemically similar PVS materials that have established records of dimensional accuracy when used for IOR uses. The study has limitations in that it is an in vitro design; therefore, many of the clinical factors were not simulated in the experiment. However, at the same time, these very influences do have a confounding effect and do not basically determine the individual capabilities of a particular material. Other limitations include that results are pertinent to the conditions under which experiments were conducted and cannot be generalized. Also, there are additional types of IOR materials that demand an investigation. Finally, vertical accuracy was not investigated in this study.

Conclusions

Within the in vitro conditions, the following can be concluded about the CAD and transparent IOR materials. (1) Transparent IOR materials showed less deviations in linear dimensions than CAD IOR type materials. (2) All materials showed a decrease in linear dimensions at all time intervals, with differences being significant for Virtual CADBite and Maxill Bite at 7 days. (3) All materials except Flexitime Bite showed linear changes well within the clinically acceptable limits. (4) Flexitime Bite, which is a CAD IOR, was the only material that showed linear changes beyond the clinically acceptable limit. (5) Finally, all IOR materials investigated in this study were accurate enough to allow dental cast mounting up to 7 days.