Comparative Evaluation of the Dimensional Accuracy of Silicone-Based Putty Reline Impressions with Different Spacer Acquisition Techniques in Fixed Partial Dentures

Introduction

Indirect restorations, fabricated in the dental laboratory and cemented on the targeted prepared tooth or abutment, play a significant role in restorative and prosthodontic treatment plans [1]. The most likely consequences of any disparity between prepared teeth and indirect restorations are periodontal and pulpal pathogeny, secondary caries, and bone loss [2]. Over-contoured or under-contoured restorations, marginal discrepancies (open or short margins), internal fit disparity, and interproximal and occlusal contacts of an indirect restoration all rely on the accuracy of the impression [3]. Despite the various advantages of digital impression technology, including rapid casting, 3-dimensional visualization, and quick clinic-to-dental laboratory transfer [4], conventional impressions using addition vinyl polysiloxane (VPS) are preferred, owing to their economic feasibility, dimensional stability, and higher levels of feature replication [5]. Studies have further substantiated their continued use in indirect restoration, demonstrating higher accuracy, compared with that of digital impressions [6]. Researchers found that adjacent gingival tissue covering or in close approximation to the prepared tooth margins reduced the accuracy of digital impressions [7]. Addition VPS is one of the American Dental Association’s specified elastomers, whose accuracy largely depends on its dimensional stability. Dimensional stability and accuracy of all elastomeric impression materials, including polyether, polysulfide, addition, and condensation silicones, are influenced by intrinsic influences – material composition, hydrophilicity, amount of polymerization shrinkage, product formation and elimination, elastic recovery, thickness – and extrinsic influences – thermal shrinkage, delayed pouring, mishandling, tray adhesive application, disinfectant used, and pouring time [8–12]. VPS and polyether impression materials currently enjoy clinical popularity, due to their superior properties – dimensional accuracy and stability, higher reproducibility, and elastic recovery – and clinical sustainability – easy handling and multiple accurate cast production [9,11–13]. The higher accuracy and dimensional stability of addition silicones over condensation silicones is well established [9] and is due to the elimination of by-products (hydrogen) via platinum and palladium catalysts [14]. Based on the viscosity, addition VPS is available commercially in 4 consistencies: putty, heavy body, medium body, and light body, with each consistency reflecting the amount of filler incorporated [7,11]. Changes in the polymer filler ratio [15] affect properties such as flow, dimensional stability (polymerisation shrinkage), stiffness, and elastic recovery. Increased filler content (putty) decreases flow but improves dimensional stability by decreasing polymerization shrinkage. Clinically, the light body VPS can reproduce intricate details (American Dental Association requirement of 25 μm) but undergoes higher shrinkage upon polymerization and cannot support itself, due to lower stiffness [16], while heavier consistencies, like putty, are more dimensionally stable but have less elasticity, flow, and fluidity, which hampers it from recording preparation details (can record 75 μm only) [17]. Chemically, the distortion of the elastomers is inevitable, which is why it is advised to minimize them through improvements in impression techniques [5,10,12]. When using VPS elastomers for indirect restorations, it is recommended to use a customized impression tray for the entire arch, as this allows for a homogeneous and minimal thickness of impression material. This results in the least amount of distortion during setting, which is caused by polymerization shrinkage [16]. However, 75% of clinicians have been reported to be using stock metal trays because they are convenient, less time-consuming, economical, and well accepted [17,18].

Using minimal elastomer not only makes the impression process cost-effective but also yields more predictable results. The putty relined technique with a light body (putty wash) VPS is a clinical technique that has shown similar results to impressions made with a custom tray [19], and with other advantages, including simplicity, using a stock metal or plastic tray, informal handling, accuracy, economy in terms of finance and time, and reduced laboratory procedures; it is therefore popular and widely accepted [20,21]. Its drawbacks are related to time spent relieving putty, extra and variable thickness, debris generation, and low adhesive strength between heavy and light bodies [17,21]. To overcome the drawbacks associated with the extra thickness of the putty and the controlled thickness of the spacer for the light body, various clinical techniques have been recommended. These vary on the basis of whether the impression can be made from the same material consistency (monophase), 2 different consistencies (dual-phase) or different material consistencies (dual-phase, single-phase, and 2-step) [22]. The single-step dual-phase putty reline technique involves placing a light body on the prepared tooth, over which putty material in the stock tray is placed, with both polymerizing together. Although earlier [21] and recent [23] studies have reported that the single-step putty reline technique has better accuracy than the 2-step putty reline technique, even more studies have reported that the 2-step putty reline technique is more accurate than the single-step technique [5,12,22,24–26]. A few studies have also concluded that both the single- and 2-step impressions provide accurate impressions [10,11,27,28].

While most of these studies are in vitro and use a die or typodont model to determine the accuracy of the putty reline technique, a clinical study compared the single-step and 2-step techniques on a metal framework fit in 92 patients and found that the single-step technique had the highest frequency of misfit (31.8%) [29]. Another study, which investigated the accuracy of the 2 techniques at different time intervals (1 h, 24 h, 48 h, and 1 week), reported the superior accuracy of the 2-step technique at all time intervals [25]. Extensive research has been conducted on the 2-step putty reline technique, leading to numerous variations, primarily related to the size and nature of the spacer used for the light body to achieve a putty wash. Franco et al conducted a comparison between single-step and 2-step putty reline techniques on stainless-steel dies for polyether (Impregum Soft) and vinyl polysiloxane (Perfectim Blue Velvet and Flexi-Velvet), with or without relief. They detected higher discrepancies for the 2-step impression technique without relief [30]. In a study about the relationship between spacer thickness and reproduction of sulcus depth in putty reline techniques, Shiozawa et al found that thick spacers are needed to make wide sulcus reproductions, while thin spacers can make deep sulcus reproductions [31]. A study by Gautam et al [32] tested the accuracy of 4 different putty wash techniques using a metal die: single-step, 2-step with polythene spacer, 2-step with 1-mm coping, and 2-step with 2-mm coping. They discovered that the 2-step technique with 1-mm spacer impressions made the most accurate casts, while the polyethene spacer made the biggest changes. Dugal et al conducted a comparison of the dimensional accuracy of 4 different putty wash impression techniques (single-step, 2-step with 0.5-mm, 1-mm, and 1.5-mm-thick metal spacer caps) for 7 different intra- and interabutment dimensions. They found significantly larger or smaller dimensions, compared with the control group (die), with the highest accuracy was achieved in the 2-step technique with 1-mm spacer thickness [5]. However, most recent studies have found that a 2-mm spacer for the light body in a 2-step putty reline technique is the most accurate [11,12,21,26], as previous studies have also recommended [33]. Chugh et al found 1-mm and 2-mm spacers to be more accurate than polythene spacers [10], while Manoj et al found the 2-step putty reline technique with cellophane spacers to be more precise [34]. The method to provide spacers for the light body depends on the choice of the clinician, which includes making a putty impression before tooth preparation [2], gouging of a post-preparation putty impression [35], use of spacers made of polythene [32], cellophane [36], aluminum foil [37], and wax adaptation close to abutments [36], compressing the prepared tooth on a partially set putty with an instrument handle [35], anterior posterior tray rocking during setting [37], temporary restoration [10,11,22,34], and tray relief [38]. While some of these methods tend to provide a uniform thickness of spacer, most methods tend to create space that is not uniform. A 2-stage technique, when used unspaced, generates hydrostatic pressure during the seating of the wash, which in turn leads to deformation and recoil of the putty upon removal [39]. Rebounding tray walls due to the recoil of the putty can lead to undersized buccolingual dies [39]. Moreover, the putty in such cases displaces the light body, which is similar to the single-step putty reline technique, thus reducing the chances of recording intricate details. Studies that investigate the accuracy of various impression techniques typically use either a metallic die with flat surfaces or a set of plastic jaws with typodont teeth, where the prepared areas meant for measurement are flattened out. This, however, does not truly represent the clinical picture of a prepared crown or abutments for a fixed partial denture. The prepared teeth present a wide range of flat and curved surfaces, along with a wider range of line and point angles [40]. The most favorable shrinkage pattern of VPS impression material is the one that is directed toward the walls of the impression tray, since such shrinkage results in larger dies, which will minimize framework misfit [41]. Since shrinkage occurs throughout the VPS elastomer, different geometrical features will result in different shrinkage patterns [42]. The influence of span lengths (short span and long span) is also bound to influence the shrinkage of the VPS impression material, which in turn will affect their accuracy. To the best of our knowledge, no one has ever studied how different line and point angles affect the accuracy of VPS impression material. Thus, this study was aimed at evaluating the dimensional accuracy of addition VPS impression material using different spacer acquisition methods for short- and long-span fixed partial dentures (FPD). The primary objective of the study was to evaluate the influence of various angles (line, point) and surfaces (flat, curved) on the accuracy of the 2-step putty reline technique. We hypothesized that there is a difference in the shrinkage of VPS elastomer between angular and flat surfaces and that different spacer acquisition techniques will have different levels of accuracy. Alternately, the null hypothesis stated that there is no difference in shrinkage of VPS elastomer between surface features and that the accuracy of the impression does not depend on the differences in the technique of spacer acquisition. The results of the study will improve the understanding of the clinician about the impact of line and point angles on the impression technique, which can assist in planning FPD impressions.

Material and Methods

ETHICS:

This research project was approved by the college/university Vice Deanship of Research and Ethics Committee (vide CODJU-2305I, October 22, 2023). The research project was administered and supervised by the staff of the Department of Prosthetic Dental Sciences.

STUDY DESIGN:

This in vitro study followed a comparative study design that contained a control group and experimental groups. The control group provided the baseline values, while the intervention groups provided the experimental values. The independent variables (variables that were left unchanged) for the study included vinyl polysiloxane elastomeric impression material, spacer acquisition techniques, and span length (short and long span), while the dependent variables (variables that changed due to control) were the coordinates on various surfaces (flat or curved) and angles (line or point). Figure 1 presents the study flowchart, sequence, and variables. All sample preparations were conducted by a single person, while measurement recording was performed by operators blinded to sample identification and study outcome.

OPERATIONAL DEFINITIONS:

In the context of this study, the following terms are operationally defined, as follows (glossary of prosthodontic terms) [43]:

SAMPLE SIZE: To accomplish the research outcomes, the study required 1 control and 5 experimental groups. The control group was represented by a typodont model, and the experimental groups, by 5 different impression techniques and their respective dental stone models. For the experimental groups, the total number of samples was determined using the statistical formula [N = 2σ2× (Zα + Zβ) 2/2)] on a software programme (Nquery, V7, Informer Technologies, USA) [44], which came out to be 60, with 12 samples in each group: standards derived using type 1 error rate (a=0.05), power assumption (80%), and effect size (D2)=0.28. Two extra samples in each group were added, which represented substitutes in case any sample was defective.

SAMPLE PREPARATION AND STUDY SEQUENCE: Materials and instruments, along with their respective manufacturers and their recommendations and specifications, are tabulated in Table 1. The study sequence was as follows: pre-preparation putty impression, tooth preparations for short-span and long-span FPD on the typodont model, identification and measurement of coordinate dimensions on the prepared model, preparation of conventional and modified temporary restorations, putty impressions of various groups, putty relining with a light body, pouring of stone models, and, finally, measurement of stone models.

MASTER MODEL (TOOTH PREPARATION):

A full set of commercially available mounted and articulated typodont teeth (Practicon, Greenville, NC, USA) was used to represent the clinical scenario, master model, and control group (group C). The maxillary right second premolar was removed to represent a short-span partial edentulous arch. On the left side, 3 maxillary teeth (first, second premolar, and first molar) were removed to represent a long-span partial edentulous arch. The adjacent abutment teeth for short and long spans were then prepared for all ceramic tooth preparation by a single blinded operator using standard textbook specifications for occlusal, axial, and shoulder width preparation (Table 1) [13]. Routine preparation features for all ceramic restorations guided the preparation. The tooth preparations were performed using a high-speed air rotary handpiece that carried different types of tooth preparation dental burs. The preparation was performed under the magnifying loupe (2X, Yoctosun LED Head Magnifier), with overhead illuminating light in a preclinical laboratory setting. After preparing the abutment teeth, all line and point angles were finished by rounding off the sharp margins with hand cutting instruments. All concerned landmarks from which different angles and surface measurements would be made were identified with a fine-tip pen under a laboratory magnifying glass (10X, Hitti, USA) for short-span and long-span FPD (Figure 2). The various landmarks that represented coordinates for short-span and long-span FPD included point angles: P1 and P2 (for short span only) and P3, P4, P5, and P6 (for long span); line angles: L1, L2, L3, L4, L5, and L6; curved surfaces: C1, C2, C3, and C4; and flat surfaces (interabutment distance): F1, F2, F3, and F4 (Figure 2). The marked landmarks were then notched to a depth of 1 mm with a small round bur mounted on a slow-speed handpiece. All notches were placed on the smooth surfaces of the preparation features (occlusal surface and shoulder margin), avoiding contact with or notching the axial surfaces.

DEFINITIVE IMPRESSIONS, MODEL PREPARATION, AND GROUPING:

A perforated metal stock tray was selected over the typodont model based on the clinical criteria for impression tray selection for FPD. Similar types and sizes of stock trays were used for making impressions in all groups. A general protocol that was followed for all impression groups included immersing the master model in a water bath (37±1°C), simulating oral temperature, air drying of the master model after immersion, application of the manufacturer-recommended tray adhesive (3M ESPE, USA) as per recommendations, and room-temperature drying of the tray adhesive [45].

For each group, all impressions were made in the same setting on that particular day. Based on the spacer acquisition techniques, 5 experimental impression groups were made: (1) pre-preparation putty impression (group PP); (2) gouging putty (post-preparation putty; group GP); (3) polythene spacer (group PS); (4) conventional temporary (group CT); and (5) modified temporary (group MT).

The spacer acquisition techniques for the 5 groups were as follows:

For all the impressions in the respective groups, channels were created in the putty so that excess light body would flow away from prepared areas. For all putty impressions, the high-viscosity putty was hand-mixed by taking an equal amount of base and catalyst putty in the manufacturer’s provided dispenser (plastic spoons). The two were kneaded together until no streaks of original color were present. A standard 45-s mixing time and 3-min setting time were followed. All groups underwent relining of the putty impressions, adhering to standard textbook and clinical protocols. A dry retraction cord was placed in the gingival sulcus of the typodont teeth, which was removed just before placing the light body. The light body cartridges (base and catalyst) were loaded on an auto-mixing gun, and an auto-mixing tip was attached to the cartridge. A plastic microtip was further attached to the auto-mix tip to enhance the flow of the light body under pressure. The light body was flown around the margins of each prepared tooth on the master model and within the impression. The tray was repositioned on the arch and removed once the light body was set. All applicable devices were provided by the manufacturer and followed their recommended guidelines. All impressions were evaluated by a blinded 2-member team specialized in FPD treatment. Any impression that did not fulfil the criteria was discarded, and a new impression was made. All impressions were evaluated on the basis of a rubric that was designed for the study. This included the routine checklist that is used to evaluate the accuracy and completeness of impression during clinical procedures.

IMPRESSION POURING/MODEL FABRICATION:

Putty relined impressions of all groups were disinfected using spray (Zeta 7 solution/spray), after which they were wrapped for 10 min. The impressions were then cleaned with water and poured with die stone (Type IV, BegoStone Plus; BEGO Herbst GmbH & Co) that was mixed in a vacuum mixer (Mix-R, Dentalfarm, Torino, Italy) at 450-rpm speed for 30 s. The mixed die stone was then poured into each impression over a mechanical vibrator (GC, MeG Chemical Ltd, China). The poured impressions were then allowed to set at room temperature for time period recommended by manufacturers (3–5 h). Once dry, the stone models of each group were retrieved from the impressions manually and scrutinized for defects. All models were trimmed from excess, which included the provision of a flat base. Any defect that would compromise the measurement was discarded, and the procedure was repeated for that particular sample.

MEASUREMENTS:

All measurements of coordinates were made using a measuring microscope (AmScope, Irvine, CA, USA) that had a camera connected to a desktop computer and the measuring software program (version x64, 4.11, AmScope). Each sample was attached to the base the microscope, and then the lights were adjusted. The magnification knobs were then adjusted to give a clear view of the 2 areas on the specimen that were meant to be measured. Additionally, the measurements were validated and verified using a digital vernier calliper (Mitutoyo, Kawasaki, Japan). Prior to measurements, the microscope was calibrated using the manufacturer’s instructions, and intra-rater reliability was established by measuring 30 different coordinates [15 each on a typodont model and 15 on a single dental stone cast]. The coordinates for intra-rater reliability were measured at different time intervals using the same measuring device. For the study groups, a total of 18 coordinates were recorded for short span FPD, as follows: point angles: 1 ipsilateral (P1–P5) and 5 contralateral (P1–P2, P1–P4, P1–P6, P2–P3, P2–P5); line angles: 4 ipsilateral (L1–L4, L1–L5, L3–L4, L4–L5) and 2 contralateral (L1–L6, L2–L5); curved: 1 ipsilateral (C1–C3) and 2 contralateral (C1–C2, C1–C4); and flat (F1–F3, F1–F4, F2–F3) (Figure 3). For the long span, a total of 13 measurements were recorded, as follows: point angles: 2 ipsilateral (P1–P3, P1–P5), 2 contralateral (P1–P4, P1–P6); line angles: 2 ipsilateral (L1–L5, L3–L5), 2 contralateral (L1–L6, L3–L6); curved: ipsilateral (C1–C3) and contralateral (C1–C2, C1–C4); and flat (F1–F4, F2–F3) (Figure 3). For short-span and long-span FPD, the flat surface measurements represented the interabutment distance between the 2 prepared teeth. The choice of selecting the coordinates was random and included convenience to allow verification of manual measurements. Since certain coordinates fell on the same side of the preparation and were difficult to measure under the microscope due to overlapping, such measurements were therefore verified by manually measuring the concerned coordinates.

STATISTICAL ANALYSIS:

All measured coordinates were entered in a Microsoft Excel sheet, where the data was corrected, refined, and coded. The coded data was then entered into SPSS version 24.0 (IBM Corp, Armonk, NY, USA) for determining descriptive (mean, median) and inferential statistical tests. The mean and standard deviations for each coordinate in each group were calculated. For determining the intra-rater reliability of the measurements recorded, a Cohen’s kappa test was used. The normality tests showed the data to be normally distributed. The equality of variances was estimated by the Levene test. A one-way ANOVA test determined the differences between groups for each measured coordinate. Multiple group pairwise comparisons using the Tukey honest significant difference (HSD) test determined the differences between various group pairs. For all inferential statistics, the probability ‘P’ value equal to or less than 0.05 was considered to be significant.

Results

INTRA-RATER RELIABILITY:

For 30 measurements, the average intra-rater reliability for different coordinates was 0.899 (range 0.867 to 0.923; 95% confidence interval, 4 levels of precision), which was considered good on the scale. The reproducibility of the total measurements was analyzed, and the calculated intraclass correlation coefficient was high (0.807).

POINT ANGLES: The longest distance between 2 coordinates was for contralateral coordinate P1–P6 (16.27), while the shortest distance was for contralateral coordinate P1–P2. Out of the 6 coordinates measured, 2 contralateral coordinates, P1–P4 and P2–P3, showed a decrease (contraction) in all the groups. The remaining coordinates showed increase (expansion) in all groups, except ipsilateral coordinate P1–P5 in group CT and contralateral coordinate P1–P6 in group PS and group MT, which showed lower values than baseline (contraction). No group differences existed for point-angle coordinates P1–P5 (ipsilateral) and P1–P2, P1–P4, and P2–P5 (contralateral). Significant group differences existed for 2 contralateral point angle coordinates P1–P6 and P2–P3 (P<0.05) (Table 2). These findings indicate that as the length between the 2 contralateral coordinate’s increases, the spacer acquisition technique and the thickness of the spacer for the light body becomes more critical for accuracy. Pairwise comparison tests (Tukey HSD) results for significant group differences in short-span FPD are presented in Table 3. The post hoc test showed that for contralateral coordinate P1–P6, the mean values of both group PP and group GP differed significantly from those of group PS and group MT, while those of group PS differed significantly from those of group CT. No differences existed between group PS and group MT. For contralateral coordinate P2–P3, group PP and group GP were found to differ significantly from the control group, group PS, and group MT. In addition, group GP showed a significant difference from group CT. No significant differences were observed for this coordinate between group PS, group CT, and group MT. These results when summarized indicate that PP and GP techniques are less accurate for recording contralateral coordinates, especially when they are situated at a distance (eg, between 2 abutments). In other words, PP and GP techniques are accurate for single-crown impressions only.

LINE ANGLES: The longest distance measured among the 6 line angles was for the contralateral coordinate L1–L6, at 19.71, whereas the shortest distance was for the ipsilateral coordinate L4–L5, at 5.90. Among the 4 ipsilateral line angle distances, the coordinate L1–L5 increased in distance from baseline (expansion), whereas the other 3 coordinates, L1–L4, L3–L4, and L4–L5, decreased in distance from baseline (contraction). Increased distances (expansion) were observed from baseline for contralateral angle coordinates L1–L6 and L2–L5. Differences between groups were observed in 2 ipsilateral line angles, L1–L5 and L3–L4, and 1 contralateral line angle, L1–L6 (P<0.05) (Table 2). The post hoc test results for significant line angles observed in short-span FPD are presented in Table 3. For coordinate L1–L5 (ipsilateral), only group GP differed significantly from group C (control). For coordinate L3–L4 (ipsilateral), only group MT differed significantly from group C. For contralateral line angle coordinate L1–L6, all groups differed from the group C except group MT. No significant difference for this coordinate was observed between groups.

SURFACES (CURVED AND FLAT): In short-span FPD, no significant differences were observed for any of the 3 coordinates that represented the linear interabutment distances. The flat surface that differed significantly was for coordinate F1–F4, which was the longest interabutment distance: mesial of the first premolar to distal of the first molar (Table 2). A post hoc test between groups showed that for coordinates F1–F4, the GP and CT techniques were less accurate than control (P<0.05) (Table 3). These results indicated that the technique of putty spacer acquisition became more critical if the distance between coordinates was greater. Both GP and CT tended to be less accurate if the distance was greater, irrespective of being on the same or opposite side. The results on comparison indicated the spacer acquisition technique that provided more thickness for light body (GP, CT) were less accurate than PS, which provided minimum thickness for light body.

IMPRESSION TECHNIQUE ACCURACY:

Out of 18 coordinates measured in a short-span FPD, when compared with the control, higher number of total coordinates were found to be less accurate in GP (4 coordinates) and PP (2 coordinates), both of which provided more thickness for light body. When compared with other techniques, GP and PP demonstrated higher numbers of total coordinates that were significantly different from the other 3 investigated techniques, thus further reiterating the significance of thickness of light body being critical for impression accuracy. The comparison with the control group reflects the accuracy of the technique, while the comparison with other groups does not reflect accuracy but merely indicates the degree of accuracy over other groups.

POINT ANGLES: Four point angle coordinates were measured, out of which contralateral coordinate P1–P6 (m=37.12) had the longest distance, while ipsilateral coordinate P1–P3 (m=29.49) had the shortest distance. Two ipsilateral coordinates, P1–P3 and P1–P5, had lower mean values, indicating contraction, while both contralateral coordinates, P1–P4 and P1–P6, had higher values, indicating expansion in the region. All point-angle coordinates, irrespective of being ipsilateral or contralateral, showed significant differences between the groups (Table 4), indicating their geometrical influence on the impression accuracy, since more angles and surfaces were involved. The post hoc test results for point angle coordinates in long-span FPD are presented in Table 5. For ipsilateral coordinate P1–P3, PP and GP techniques were less accurate than control, while for other ipsilateral coordinate P1–P5, GP and PS techniques were less accurate than control, thus indicating the influence of long distances on the accuracy of an impression made with elastomers. PP and GP techniques showed significant differences from the control group for both contralateral point angle coordinates, P1–P4 and P1–P6. Irrespective of being on same or opposite side, these results indicated that as the distance between landmarks increases, the technique and the spacer thickness becomes more critical for accurate transfer of point angles.

LINE ANGLES:

Four line angle coordinates were measured, out of which contralateral coordinate L1–L6 (m=40.5) had the longest distance, while contralateral coordinate L3–L6 (m=32.5) had the shortest distance. Among all the line angle coordinates, only coordinate L3–L5 showed decreased distance (contraction) among all groups, while the other 3 showed an increase (expansion) in distance. Regardless of whether the groups were on the ipsilateral or contralateral side, there were notable variations between them for every line angle coordinate, thus indicating the influence of long distanced landmarks on the impression accuracy. For ipsilateral coordinate L1–L5, all groups differed significantly from the control group, which is due to the fact that these coordinates were located on the curvatures of respective abutments. The influence of curvature has added to the distance variable, thereby showing less accuracy. With PP and GP showing less accuracy than other techniques, the influence of spacer thickness became more critical when long distances were encountered. For the second ipsilateral line angle coordinate L3–L5, the PP technique was less accurate than control, while techniques using minimal light body thickness were more accurate. For contralateral line angle coordinate L1–L6, except for the MT technique, all other techniques were inaccurate, indicating the importance of providing minimal thickness if the line angle was situated at the curved surface of the tooth.

SURFACES (CURVED AND FLAT): The smallest distance between the curved surfaces was observed between surfaces C1 and C2 (m=5.92), which is contralateral and present within the same tooth (bucco-palatal). There were significant differences for all 3 coordinates between various groups (Table 4), irrespective of being on ipsilateral or contralateral sides, thereby implying that curved surfaces are critical for impression accuracy. However, if proper technique is used, then these inaccuracies can be minimized. All 3 spacer acquisition techniques that used minimal (PS) or appropriate (CT and MT) thickness of light body impression material showed no differences for all 3 coordinates (1 ipsilateral, 2 contralateral) from the control group (Table 5). The GP technique was the only technique that showed differences in all 3 coordinates from the control, indicating that this technique was not accurate to record curvatures of tooth surfaces.

For the 2 measured coordinates on a flat surface (interabutment axis), group differences were present for both coordinates (Table 4) F1–F4 was the longest distance between the 2 abutments, and all spacer acquisition techniques differed from control, including those that used minimum thickness. The effect of long distance and curved surface together could be the reason for inaccuracies in techniques that minimized light body impression material thickness. For coordinate F2–F3, the techniques that allowed minimal or appropriate light body thickness did not differ from control, which further substantiated the role of curved surface (F2 and F3 coordinate being on flat surface of abutments) (Table 5).

:

Among the 13 coordinates assessed for long-span FPD, the significant differences compared with the control were as follows: group GP (12 coordinates), group PP (10 coordinates), group PS (5 coordinates), and group CT and group MT (4 coordinates each). In comparison with other methodologies, the total number of significant coordinates in descending order was as follows: group PP (10 coordinates), group GP (8 coordinates), group MT (8 coordinates), group PS (5 coordinates), and group CT (3 coordinates). The comparison with the control group demonstrated the technique’s accuracy, but comparisons with other groups indicated just the relative degree of accuracy.

Discussion

CHOICE OF MATERIAL:

This study investigated VPS elastomeric impression material (Express XT, Putty Soft, and Light Body) using the putty reline impression technique, which can be used with addition and condensation silicones. We used addition silicone because it remains dimensionally stable after polymerization by-products are formed, as compared with condensation silicones, in which by-product evaporation leads to non-compensable shrinkage [47]. Mahmood et al evaluated the effect of viscosity and groove type (geometry U- and V-shaped grooves) on surface detail reproduction (1-mm and 2-mm groove depth) of elastomeric material using 2 polyvinylsiloxane and 2 polyether impression materials. They reported that putty and light-bodied materials produced the best surface details [48]. Therefore, it was a significant requirement for our study to record grooves in MT. Addition silicones have also been made hydrophilic (extrinsic addition of surfactant), have high tensile strength, which improves tear resistance, and have high elastic recovery, making additional silicone a more preferred choice over polyether in FPD impression [16,22,39,41].

PATTERN OF CHANGES IN THE IMPRESSION MATERIAL:

One of this study findings is that although the shrinkage occurs in VPS impressions, it does not necessarily reflect in increasing the distances between coordinates; it also decreases the distances between coordinates depending upon the location of the anatomical landmark of teeth for FPD. This is in agreement with a previous study that examined the accuracy of sulcus depth reproduction, focusing on a single putty reline technique (dual-phase 2-step) [31]. Our study, however, assessed the impact of VPS polymerization shrinkage on the dimensional accuracy of ipsilateral and contralateral coordinates across various putty reline techniques. Our study’s findings indicate that while shrinkage occurs in VPS impressions, it does not consistently result in increased distances between coordinates. Instead, it can also lead to decreased distances, contingent upon the anatomical landmark locations of the teeth for FPD. Our observations that shrinkage and expansion occur within an impression are not in agreement with the results reported by Vitti et al [9], who observed only one pattern in the form of shrinkage, while concluding no differences between different impression techniques.

MT SPACER ACQUISITION:

For MT techniques, the results showed it to be most accurate to record all surfaces (flat/curved) and angles (line/point), irrespective of being either on the ipsilateral or contralateral side in either short- or long-span FPD. This was the only technique that exhibited no discrepancies from the control in coordinate L1–L6 in either short and long spans, which represented the maximum distance between 2 locations in each span. Compared with the next most accurate spacer acquisition techniques (CT) of this study, the accuracy of the MT technique over CT can be explained by the reduction of spacer thickness in both regions of coordinate L1–L6, thereby minimizing the changes that occurred due to high polymerization shrinkage of light body elastomer. In CT, the thickness remains that of the temporary restoration that replaced the lost tooth structure. For short span, the MT also showed more accuracy than CT for coordinate F1–F4, which represented the interabutment axis. Recording of the accurate interabutment axis is crucial for the final fit of the FPD [5,17,26]. Our results do not match those of Sayed et al [37], who reported that a modified technique, anterio-posterior rocking during setting, and aluminium foil spacer were more accurate than the temporary crown technique. However, their study exhibited methodological differences, including the use of only 1 ipsilateral coordinate measurement via a vertical groove positioned at the center of the premolar and molar, sectional plastic tray, full-arch aluminium spacer, automatic mixing of putty, and absence of tray adhesive, which could be the reason for differences. More importantly, the temporary crown used in their study was constructed prior to the tooth preparation (indirect-direct approach), resulting in a thicker spacer than the actual preparation, because it was fabricated on a cast that differed from the original preparation.

CT SPACER ACQUISITION:

For short-span and long-span FPD, CT showed more accuracy than PP and GP techniques but less accuracy than MT for recording long-distance coordinates. These results are in agreement with those of previous studies [5,9,10,12,49] that reported that the appropriate thickness for spacer by using a temporary crown or other suitable method that provides uniform space was more accurate. The accuracy of the conventional and modified temporary techniques in this study can be attributed to the spacer design that is associated with these 2 techniques. Both techniques use spacer thickness, which is equivalent to the amount of tooth reduction on the prepared tooth. All ceramic tooth preparations made on the master model provided an occlusal space of 2 mm, while on the axial surface, the spacer thickness was 1.2 mm. Levartovsky et al studied the dimensional stability of VPS in relation to gingival sulcus area and recommended 1-mm and 1.5-mm spacer thickness to be equally accurate [46].

For short-span and long-span FPD, the MT and CT techniques showed more accuracy in areas that have more than one burden to overcome. A line angle or a point angle presents different challenges when present on a flat surface as opposed to a curved surface. For all restorative purposes, they need to be rounded off to avoid concentration of stresses within the restoration. Likewise, during impression making, they undergo a polymerization shrinkage process that differs from other surfaces. For short-span FPD, MT and CT were the only techniques that were more accurate when dual influences (long distance and curved surface) were present in a tooth preparation.

PP AND GP RELINE TECHNIQUES:

Between the PP and GP techniques, the PP technique showed more accuracy than the GP technique in short-span (L1–L5, F1–F4) and long-span (P1–P5, C1–C2) FPD. Both techniques, however, did not show accuracy for long distances and coordinates present on curvatures in either short- or long-span FPD. The techniques differ in the amount of spacer acquisitions, with the GP technique being more subjective in nature, in that it does not provide accurate space thickness for a light body impression material, since the putty is being scooped out with a hand instrument. The loss of accuracy in the PP technique can be explained on the basis of the amount of polymerization shrinkage of the putty VPS that occurs during the time when the putty impression is made and when it is relined with a light body impression material (preparation finishing time). Although the clinician may anticipate that the spacer thickness will be exactly equivalent to the amount of tooth reduction, the shrinkage of the PP increases the spacer thickness, which results in an excessive amount of light body impression material during reline. The lack of research on the PP approach is likely due to the fact that it is not used in clinical practice. Tooth preparation, gingival retraction, temporization, and definitive impressions are all clinically required to be finished in a single patient visit, making the method quite demanding for the practitioner. Our findings on the GP technique corroborate the earlier results of Messias et al [23], who indicated mean values in GP that were further removed from their baseline. The GP approach cannot be regulated regarding uniform spacer thickness, applied pressure, or quantity of light body impression material used [22]. In their study, however, Messias et al used a bur to eliminate putty, resulting in minute rubber particles that adhered to the impression and induced inaccuracies.

POLYTHENE SPACER:

The PS showed accuracy in all coordinates for short-span FPD, except the longest distance contralateral coordinate (L1–L6). For long-span FPD, the PS showed more accuracy in coordinates that were closer to each other, while longer distance coordinates were not accurate. This was true for coordinates on either ipsilateral or contralateral sides. Line and point angles showed equal accuracy when this technique was used; however, when long-distance coordinates on curved surfaces were present, the technique showed less accuracy. Our results for the PS technique are in agreement with those reported in previous studies that reported spacer thickness for relining putty [11,12,21,26]. The dimensional accuracy of the putty wash VPS impression technique was reported by Omar et al [50] to decrease if the volume of wash material was larger and was not vented out. Stress induction and recoil of VPS wash upon removal cause rebound, leading to inaccurate dies. Our results on the PS technique also fall in line with those reported by Nissan et al [49], who reported that, other than the lack of proximal adaptation of the cellophane spacer, the discrepancy range from the master model was small (0.773% to 0.482%). Other studies that have reported the accuracy of foil as a spacer using the dual-phase 2-step technique on a stock tray include those of Sayed et al [37] and Mann et al [51]. These results, along with supporting findings from other studies, suggest that the traditional approach of uniform spacer thickness may not be the most effective way to address VPS shrinkage.

These findings further validate the foundation of the MT approach used in the present study. The shrinkage pattern at line and point angles differed from that of flat and curved surfaces. In every point angle, the shrinkage pattern manifests in 3 dimensions, but in a line angle, it happens in 2 dimensions. The shrinkage pattern will vary based on the location of the point angle; the point angle positioned proximally will exhibit a different shrinkage pattern than that located at the center of the occlusal surface. The shrinkage pattern will differ for pits and fissures on the occlusal surface. Tray adhesive also plays a significant role in the shrinkage pattern, since it allows the shrinkage to occur in the mould space [12,22], which is advantageous because it produces dies with a larger diameter, which is compensated by metal shrinkage during casting. On flat and curved surfaces, the contraction is distributed across the wash impression area, leading to a diffuse contraction of a singular surface. The updated temporary approach reduces the thickness of the temporary restoration at all line and point angles, hence decreasing the volume of wash material in those regions and resulting in diminished polymerization shrinkage. The shrinkage in these areas continues to remain that of inherent VPS, which is due to the addition polymerization reaction (between vinyl siloxane of the base and hydrogen siloxane through a platinum catalyst) [31]. The overall results of the present study do not agree with the study by Fenske [52], who did not find any significant difference in a dual-phase, 2-step putty light body technique using a plastic sheet (1-mm space) and cut-out (gouging, 2-mm space]. As Fenske’s study fails to specify the type of tray used, it is consequently challenging to deduce the discrepancies. Generally, this study also supports the view that impression techniques influence the outcome of a putty reline impression technique in terms of dimensional accuracy for both short- and long-span FPD, which has been coined by various authors [11,12,18,24–26,28,49]. Contrary to this, authors in the past and present do not accept the view that the impression technique influences dimensional accuracy in putty reline impressions [53,54].

STRENGTHS AND LIMITATIONS:

The main strength of this study, as compared with other studies, is that only one technique with different spacer acquisition techniques was investigated, thus decreasing the influence of confounding variables when 2 different techniques are compared. The study also measured a wide array of linear dimensions that are related to angles (line and point) and surfaces (curved or flat), where impression accuracy is going to be most affected. The study’s limitations include the inaccuracies in the impression technique for the FPD, whereas clinically, other factors associated with the laboratory process, such as mould expansion and casting shrinkage, also play a role. Other limitations are that short span and long span were investigated on one model, which could influence the results; however, in doing so, the standardization of tooth preparation would affect the results.

Conclusions

Within the disclosed limitations of this study, the following conclusions can be drawn. The spacer thickness for relining the putty impression in the putty reline technique affects the accuracy of the casts produced, with increased thickness for light body impression material resulting in significant changes in line and point angles on either ipsilateral and contralateral or short-span and long-span FPD. Various angles (line and point) and surfaces (curved and flat) demonstrated different shrinkage patterns that influenced all putty spacer acquisition techniques, with longer distances between point angles resulting in increased risk of errors, especially shrinkage of elastomer. The coordinates associated with these demonstrate not only different effects, namely contraction and expansion, but also vary according to the span length, short and long. Contraction was seen between adjacent line and point angles, while expansion was seen between opposing line and point angles. In a short-span FPD, with the exception of the GP technique, with which 4 coordinates in short span and 12 coordinates in long span were significantly different from control, all other techniques showed less influence and can be routinely used in clinical practice. In a long-span FPD, the CT and MT techniques, each with only 4 coordinates that differed from the control, were equally accurate. Spacer designs with less thickness for a light body impression material tended to be more accurate than spacer designs with extra spacer thickness, with short-span FPD having the following number of coordinates different from control: PS (1), CT (2), and MT (1); and long-span FPD having the following number of coordinates different from control: PS (5), CT (4), MT (4). Further studies are needed to investigate the influence of occlusal surface anatomy on the accuracy of the putty reline technique. Such study designs should consider the different types of cuspal angles that are clinically encountered, such as between young permanent teeth, which have prominent cusps with deep fossa, and older teeth, in which cuspal anatomy is absent due to attrition. Similar studies could also be directed toward those individuals who loose cuspal anatomy due to parafunctional habits.