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 Table of Contents  
Year : 2023  |  Volume : 23  |  Issue : 1  |  Page : 21-29

Comparison of accuracy of hexed and nonhexed pickup impression copings in a multiple variable impression setup for recording multiple straight and angulated implant positions: An in vitro study

Department of Prosthodontics and Crown and Bridge, Dr. Harvansh Singh Judge Institute of Dental Sciences, Chandigarh, India

Date of Submission29-Apr-2022
Date of Decision17-Jul-2022
Date of Acceptance25-Aug-2022
Date of Web Publication29-Dec-2022

Correspondence Address:
Tavleen Kaur
#3645, Sector 69, Mohali, Sahibzada Ajit Singh Nagar - 160 062, Punjab
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jips.jips_218_22

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Aim: The aim of this study was to evaluate and compare the accuracy of hexed and nonhexed pickup impression copings with and without splinting using polyether (PE) and polyvinyl siloxane (PVS) impression materials in open-tray technique in recording multiple straight and angulated implant positions.
Settings and Design: An accurate impression results in an accurate definitive cast, thus minimizing the incidence of prosthesis misfit. The critical aspect is to record the three-dimensional location of the implant in bone rather than reproducing fine surface details. Precise fit of a fixed implant-supported prosthesis depends on the accuracy of the implant analog location within the definitive cast. Factors which affect impression accuracy include implant angulation, impression material, impression copings, technique, and splinting.
Materials and Methods: A sample size of 80 study models fabricated from the impression of different groups was included. A reference master model based on All-on-4 implant concept with two parallel (implants 1 and 2) and two angulated (implant 3 at 17° and implant 4 at 30°) was fabricated using implant angulation guide. All impressions were recorded using open-tray impression technique. The groups were divided into two main groups of 40 samples each. Group A used hexed open-tray impression copings and Group B used nonhexed open-tray impression copings. Both the groups involved impression recording using splinted (Subgroup I) and nonsplinted impression copings (Subgroup II). Further, impressions in each subgroup were made using PE (Subsubgroups a) and PVS (Subsubgroup b). A total of eight subsubgroups with ten samples each were included. Impressions were recorded for each group and poured into Type IV die stone for fabrication of study models. After 24 h, the study models and reference master model were fitted with implant abutments for measurement with coordinate measuring machine.
Statistical Analysis Used: The mean differences of the interimplant distance R1 (1–2), R2 (1–3), R3 (2–4), and R4 (3–4) between the reference model and sample models in different subsubgroups were calculated and three-way analysis of variance test was applied with Tukey's post hoc tests.
Results: No significant difference was found in mean coronal deviations for distance R1, R2, and R3 (P > 0.05) between different study groups. P = 0.02 for R4 (distance between 17° and 30° implants) between impression materials subsubgroups suggested that significantly less distortion was created in location of highly angulated implants (>30°) using PVS impression material. Splinting and type of coping did not have a significant influence on impression accuracy. Increasing angulation decreased the accuracy.
Conclusion: PVS was found equivalent in accuracy to rigid PE for recording parallel or angulated implants. Impressions of implants with higher angulations were recorded more accurately with PVS. The study found no difference in accuracy with or without splinting. Furthermore, nonhexed impression copings facilitate easier and accurate recording of multiple angulated implant location in bone.

Keywords: Accuracy, hexed and nonhexed impression copings, implant angulation, implant impression, splinting

How to cite this article:
Kaur T, Singla S, Kumar L. Comparison of accuracy of hexed and nonhexed pickup impression copings in a multiple variable impression setup for recording multiple straight and angulated implant positions: An in vitro study. J Indian Prosthodont Soc 2023;23:21-9

How to cite this URL:
Kaur T, Singla S, Kumar L. Comparison of accuracy of hexed and nonhexed pickup impression copings in a multiple variable impression setup for recording multiple straight and angulated implant positions: An in vitro study. J Indian Prosthodont Soc [serial online] 2023 [cited 2023 Jan 30];23:21-9. Available from: https://www.j-ips.org/text.asp?2023/23/1/21/365943

  Introduction Top

Osseointegrated implants have been established as a successful alternative to conventional prosthesis in the replacement of missing teeth. The fixed dental prosthesis, the osseointegrated implants, and the bone act as a unified structure without any resiliency.

One of the critical factors in its success is a passive fit of the prosthesis. Lack of passive fit may lead to strain and movement between components which acts as a precursor to many biological and mechanical complications.[1] In case of multiple implant-supported prostheses, apart from accurate record of individual implant position, precise recording of their three-dimensional inter-relation is critical. Other factors such as type of impression coping, splinting technique, and splinting material also influence the impression. Vigolo et al. and Akalin et al. reported that the most important criterion for accuracy of impression is the magnitude of angulation of implants.[2],[3] To minimize the errors of multiple angulated implants, direct transfer technique for impression has been advocated.[4] The choice of impression material depends on the presence of undercuts, state of edentulous arch, number of implants, angulation, and impression copings of implant system. The most widely used materials are polyvinyl siloxane (PVS) and polyether (PE).[5] Impression material in open-tray technique is required to be sufficiently rigid to maintain position and prevent movement of impression copings during the procedure. Splinting (S) of impression copings has been advocated to prevent positional distortion of copings.[6],[7],[8]

In internal hexed implants, the hexed (H) pickup impression copings are difficult to draw from multiple divergent implants, especially when rigidly splinted together. Shallow nonhexed (NH) internal connection impression copings require less maneuvering while drawing, thus minimizing distortion of impression. The risk of distortion during removal increases when rigid impression material along with splinted long connection impression copings is used.[8],[9],[10] Studies by Lee et al. and Richi et al. have reported superior accuracy with nonhexed splinted impression copings in multiple angulated implant impressions.[8],[11]

Numerous studies have compared the influence of impression variables on accuracy of implant-level impression with varying results. Available literature indicates a need to study the influence of factor variation simultaneously, especially on multiple straight and angulated implants.[12] The aim of this study was to evaluate the effect of hexed and nonhexed open impression coping, splinting, and impression material on accuracy of recording by multiple straight and angulated implants in an edentulous situation. Null hypothesis was that there was no significant difference in impression accuracy of multiple angulated implants with different variables including impression material, splinting, and coping type.

  Materials and Methods Top

The comparison of accuracy of the study model obtained with direct impression technique using hexed and nonhexed impression copings (variable G1), with and without splinting (variable G2), utilizing two different impression materials, PVS and PE (variable G3), was done in this study. Direct/open-tray/pickup impression technique was used in all groups. This study approved by Institute RDC. IRB number - HSJ/19/291 DATED 6/2/19.

Study groups

Ten models were made for each subsubgroup (n = 10). The division of the groups is shown in [Figure 1].
Figure 1: Study groups

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Model preparation

A vertical milling machine and an implant angulation guide (Institut Straumann AG, Switzerland) were used to place two parallel implants (BioHorizonsTM Tapered Internal, 3.5 mm × 12 mm, USA) in canine region bilaterally, one at 17° distal and another at 30° distal angulation in premolar region in an acrylic resin maxillary edentulous model [Figure 2] and [Figure 3]. The implants were secured with self-cure acrylic resin (DPI RR, Dental Products of India Ltd.), and the region around the implants was substituted with a 2 mm thickness gingival mask (Gingifast Elastic, Zhermack) to simulate oral mucosa.
Figure 2: Implant angulation guide used for placing implants in reference model

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Figure 3: Reference model with implants

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Custom tray fabrication

Reference model was duplicated after adaptation of 3 mm wax spacer to accommodate open-tray impression copings. This duplicated spaced model was used to fabricate open impression custom trays of light-cure resin (Plaque Photo®, WP Dental, Willmann and Pein GmbH). Two types of custom open impression trays were prepared. Forty impression trays were fabricated with four openings/windows for individual implants [Figure 4] for recording impressions in Subgroup II (NS impression copings) The other forty open custom impression trays were prepared with continuous window extending from the distal-most implant on one side to the other for Subgroup I to accommodate splinted copings [Figure 5]. The opening was covered with wax sheet to prevent the extrusion of impression material during impression making. The trays were left at room temperature for 24 h before use.[13]
Figure 4: Four-holed custom tray for nonsplinted impression subgroups

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Figure 5: Continuous window custom tray for the splinted impression subgroups

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Impression procedure

A standard procedure was followed for all impressions. Respective hexed (Group A) and nonhexed (Group B) impression copings were tightened to the implants with the help of a hex driver at 10 N cm torque. Tray adhesive (3M ESPE™) was painted on all the trays for 15 min prior to each procedure to obtain adequate tensile bonding strength, before recording the impressions. For splinting, impression copings were tied with dental floss and pattern resin (GC Corporation, Japan) was applied in 2 mm thickness [Figure 6] and [Figure 7]. After 17 min, the splint was sectioned into four pieces with a diamond disk and resplinted, to minimize polymerization shrinkage.[14] There were total eight subsubgroups with ten study models each [Figure 1]. Twenty impressions were recorded for each subgroup, i.e., splinted (Subgroup I) and nonsplinted (Subgroup II) with respective impression material in each subsubgroup (a and b) [Figure 8] and [Figure 9]. PE (Monophase, 3M ESPE) auto mixed with Pentamix Lite and PVS putty and light body consistencies (President, Coltene/Whaledent) were used for Subsubgroups a and b, respectively.
Figure 6: Open-tray impression copings splinted with pattern resin

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Figure 7: Continuous window tray placed on the resin-splinted impression copings in a study model

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Figure 8: PE impression with open-tray impression copings. PE: Polyether

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Figure 9: PVS impression with gingival silicone being placed around the impression copings. PVS: Polyvinyl siloxane

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Cast production

Once the impression material had been set, the impression copings were loosened with the aid of a hex driver and the recorded impression was retrieved with impression copings embedded within the impression material. The implant analogs were tightened onto the impression copings with a hex driver manually. Gingival mask (Gingifast Elastic, Zhermack SpA, Italy) was applied around the impression copings and analogs, and once set, the impression was poured in a vacuum-mixed Type IV dental stone (Kalrock, Kalabhai, Mumbai). The impressions were separated from the cast after 1 h. All the casts were stored at room temperature for 24 h.


BioHorizonsTM (3.5 diameter, regular) standard abutments were screwed onto the implants in reference model to get reference measurements for each distance R1, R2, R3, and R4 [Figure 10]. In each study model also, BioHorizonsTM (3.5 diameter, regular) standard abutments were screwed onto the analogs. A single examiner measured the distance between abutment heads (R1, R2, R3, and R4) using a coordinate measuring machine (CMM) (Mitutoyo, Japan) in reference and study models [Figure 11]. The flowchart for measurement procedure is depicted in [Figure 12]. The formula for Euclidean distance for each measurement in three axes was R = √(Δx2 + Δy2 + Δz2). Differences of mean distances in each subsubgroup from respective distance on reference model were taken as coronal deviation from accuracy and compared.
Figure 10: Schematic diagram of the measurements made. R1 denotes anterior interpositional difference of parallel implants in anterior region, R2 denotes interpositional difference of the straight (implant 1) and 17° angulated implant (implant 3), R3 denotes interpositional difference of a straight (implant 2) and 30° angulated implant (implant 4), R4 denotes interpositional difference of a 17° (implant 3) and a 30° angulated implant (implant 4)

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Figure 11: Stone cast being measured with standard abutments in place on the CMM machine. CMM: Coordinate measuring machine

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Figure 12: Flowchart for measurement calculations

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  Results Top

The mean differences of Euclidean distances R1, R2, R3, and R4 between the reference model and each study cast in every subsubgroup, along with the standard deviations, was statistically analysed by three-way analysis of variance (ANOVA) as depicted in [Table 1] and [Table 2].
Table 1: Descriptive Statistics for R1, R2, R3, and R4

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Table 2: Three-way ANOVA P values

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Statistical analysis revealed that implant angulation and impression material had a significant effect on mean coronal deviations of copings [Table 2]. [Figure 13] depicts the mean box plots for all groups.
Figure 13: Boxplot showing final results where G1 is the impression coping variable, G2 is the splinting technique variable, and G3 is the impression material variable. CI: Confidence interval

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Mean deviations in each study group increased with increasing angulation, i.e., from R1 (two parallel implants) to R4 (implants angulated at 17° and 30°). R3 with one posterior 30° distal angulation implant gave maximum coronal deviation in each subsubgroup [Figure 13]. Furthermore, mean deviations in each subgroup were significantly lesser with PVS impression material [Table 1]. In the present study, subgroup in nonhexed impression coping group (Group B) revealed lesser mean deviation across all subsubgroup as compared to hexed impression coping group (Group A) except in nonhexed nonsplinted PVS subsubgroup (Group BIb) which showed higher coronal deviation for all angulations in comparison to corresponding hexed coping group (Group AIb) [Figure 13].

Maximum mean deviation in parallel implant distance (R1) was reported with hexed nonsplinted PE impression (Group AIa), whereas maximum mean deviation in angulated implants (R3 and R4) was reported with hexed splinted PE group (Group AIIa).

Impressions made with nonhexed splinted PVS group (Group BIIb) showed the least coronal deviation for all angulation groups, followed by hexed splinted PVS group, but the difference was not found statistically significant.

  Discussion Top

Accurate implant-level impression is a critical step to achieve success in multiple implant prosthesis. Variables such as impression material, impression techniques, type of impression coping, splinting/nonsplinting of impression copings, and number and angle of implants influence the accuracy of implant impression.[15],[16] Very few studies have reported cumulative influence of these variables on implant impression. The present study evaluated multiple parallel and angulated implant impression accuracy in multiple variable impression setup and observed that increasing angulation inversely affects the accuracy of impression [Table 1]. PVS was reported to have significantly higher accuracy in multiple angulated implants. Splinting did not have any significant effect on impression accuracy. NH impression copings were found to provide an easy and viable alternative for recording multiple angulated implants with no significant difference from H impression copings [Table 2].

In multiple implant impressions, impression copings are aligned at different angles and there can be pronounced rotational movement of copings leading to inaccuracy. Further, deep wide connection area (hex) in internal connection implants is more engaging and may cause movement of impression copings and hence distortion within impression necessitating the use of nonhexed copings.[6],[8]

The impression procedure in our study was standardized by using light-cure custom trays of uniform thickness fabricated on the same duplicated cast and with same-sized stops for the accurate positioning of the tray on the reference model each time an impression was made. The pickup impression copings were hand tightened with a hex driver by the same operator, eliminating the difference in force used for tightening to simulate a clinical situation. The PE impressions were separated from the cast after more than 6 min and PVS impressions were separated more than 4 min to compensate the difference in room and oral cavity temperature. For pouring the stone casts, die stone was vacuum mixed to ensure standard W: P ratio and avoid the incorporation of air bubbles. The stone casts were separated from the impressions after 1 h to allow the stone to reach a certain strength, and measurements were made after 24 h to allow the die stone to reach its full dry strength. The accuracy of the readings measured by the CMM was up to 0.001 mm. The operator of the machine was blinded to avoid observation bias.

In the present study, NH impression coping groups revealed lesser mean deviation across all groups as compared to H impression coping groups except nonhexed, nonsplinted PVS group (Group BIb) which showed higher coronal deviation for all angulation in comparison to corresponding hexed coping group (Group AIb) [Figure 13].

Long internal connection of hexed implant and impression coping makes withdrawal of impression difficult, especially in multiple and angulated implants.[8],[17],[18] A nonhexed pickup impression coping that has a shorter connection area permits a greater angle of draw during impression withdrawal reducing the withdrawal stress and also eases retrievability from mouth.[6],[8] In the present study, nonhexed group B reported a lower mean deviation for parallel implants (R1) and angulated implants, thus recommending nonhexed copings as a viable and easier alternative for recording multiple nonparallel implant positions of up to 30° angulations. Richi et al., in a recent study, have also favored the use of nonhexed copings for multiple angulated implants up to 20° angulation.[8] Nonsplinted nonhexed copings recorded with PVS (Group Bib) impression material gave higher coronal deviation because of more maneuverability and movement allowed with nonsplinted copings in resilient PVS impression material. Although hexed impression copings provide better positional accuracy than nonhexed impression copings, especially in fewer implants with straight angulation, this study found that NH impression copings are more accurate and easier alternatives for multiple angulated implants.

Splinting the impression copings did not cause a significant lowering influence on mean deviations in any of the subgroups for parallel as well as angulated implant positions [Table 2]. To manage shrinkage or fracture of splint material, in this study, pattern resin used for splinting of impression copings was sectioned from midline with diamond disc. The sections were rejoined after 17 min to compensate polymerization shrinkage.[9],[14] Our results are in accordance with studies by Hsu et al., Phillips, Chang et al., and Buzayan et al.[10],[19],[20],[21] Del'Acqua and Baig et al. and Tsagkalidis et al. also reported no significant advantage of splinting the impression copings.[22],[23],[24]

Conflicting results exist in the literature regarding cast accuracy using PE and PVS impression materials. PE has been reported to cause lesser deviation in impression recording because of its rigidity and dimensional stability.[3],[12],[22],[25],[26] Some studies reported PVS to be better due to its high elastic recovery, hence reduced permanent distortion, maybe due to decreased stress between impression copings and impression material.[11],[17],[27],[28] In the present study, one-way multivariant ANOVA gave an insignificant P value for difference in performance of PVS and PE in various study groups for straight as well as angulated implants [Table 1]. However, application of three-way ANOVA gave a significant P value (0.00129) for parallel as well as angulated implant [Table 2]. This result points to a better accuracy with PVS, especially in multiple implants with higher angulations. Between straight and up to 30° angulation, a significant difference was found between PVS and PE in open-tray impression technique. Assuncao et al. had reported an insignificant difference between PE and PVS in recording implant angulation up to 25°.[4] Many authors have reported an insignificant difference in impression accuracy with PVS and PE for straight and varying angulated implants.[23],[29],[30],[31] Our study is in accordance with Vojdani et al. (2015) who reported the advantage of PVS over PE as impression material for implants angulated up to 20° mesially/distally.[13] However, the present study is the first to establish the superiority of PVS for recording up to 30° of divergence between two implants with minimal deviation, thus partially rejecting the null hypothesis, i.e., significantly lower mean coronal deviation was found with PVS impression for multiple parallel and angulated implants.

It was observed that during tightening, open-tray impression copings in the PE material did not rotate as easily it did in PVS impressions accounting for higher accuracy in PE group with nonhexed nonsplinted impression copings (Group BIa) than hexed nonsplinted PE group (Group AIa). In the latter group, H copings in combination with rigid PE material generated higher stresses during withdrawal leading to higher coronal deviation. The splinting material broke in a few samples as the withdrawal force had to be increased to separate the impression from the master model generating stresses. In a few cases of hexed splinted impression groups, the custom tray fractured as well, indicating the requirement of higher degree of maneuverability as compared to nonhexed splinted impression copings. Higher mean coronal deviations in the present study were comparable to those in a recent study by Richi et al. comparing the accuracy of hexed and nonhexed impression copings for implant angulation up to 20°.[8] Mean coronal deviation for R3 was higher in all groups maybe because 30° distal tilted posterior implant, compared to straight anterior implant, altered their planes of withdrawal to the maximum. The study also made a unique observation that with a greater number of rigid impression variables, manipulation and stresses increase during tray withdrawal leading to increased coronal deviation in recording multiple angulated implant positions. Group BIIb using splinted NH impression copings recorded with PVS material gave minimal coronal deviation for all measured distances, but splinting in combination with other rigid variables such as H impression coping and PE material gave higher coronal deviation for angulated implants.

In vitro setup is the possible limitation of this study since oral cavity conditions including humidity, temperature, and presence of saliva were not reproduced in this case. Furthermore, only linear measurements were made, rotational discrepancy was not tested. Machining tolerance in mismatch of implant component (22–100 μm) was not taken into consideration.[16] There is a need for in vivo studies as the angle of removal of impression varies in mouth compared to that of an in vitro model. Impression technique with digital scanning needs to be explored and assessed for accuracy so as to avoid errors in conventional techniques and enhance the chances of prosthesis success.

  Conclusion Top

The present study concluded that edentulous arches with parallel implants can utilize PE on account of its low strain of compression, but PVS is more accurate in areas of undercuts or multiple angulated implants. No significant difference was found between the use of hexed and nonhexed impression copings with or without splinting in multiple parallel and angulated implants.

Within the limitations of this study, it can be concluded that:

  1. Nonhexed impression copings are an easy and viable alternative for recording multiple angulated implants
  2. Both PE and PVS have acceptable accuracy for impression in multiple implants, but PVS is recommended for higher angulations (>30°), as the withdrawal of PE impression gets difficult in higher angulated implants and increased undercuts
  3. Splinting does not significantly affect the impression accuracy
  4. Increasing angulation inversely affects the accuracy of impression.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Sahin S, Cehreli MC. The significance of passive framework fit in implant prosthodontics: Current status. Implant Dent 2001;10:85-92.  Back to cited text no. 1
Vigolo P, Fonzi F, Majzoub Z, Cordioli G. An evaluation of impression techniques for multiple internal connection implant prostheses. J Prosthet Dent 2004;92:470-6.  Back to cited text no. 2
Akalin ZF, Ozkan YK, Ekerim A. Effects of implant angulation, impression material, and variation in arch curvature width on implant transfer model accuracy. Int J Oral Maxillofac Implants 2013;28:149-57.  Back to cited text no. 3
Assuncao WG, Filho HG, Zaniquelli O. Evaluation of transfer impressions for osseointegrated implants at various angulations. Implant Dent 2004;13:358-66.  Back to cited text no. 4
Reddy S, Prasad K, Vakil H, Jain A, Chowdhary R. Accuracy of impressions with different impression materials in angulated implants. Niger J Clin Pract 2013;16:279-84.  Back to cited text no. 5
[PUBMED]  [Full text]  
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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]

  [Table 1], [Table 2]


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