Comparison between open and closed-tray impression techniques on the implant transfer accuracy

Objetivos: O presente estudo in vitro objetiva determinar e comparar a precisão dimensional das técnicas de transferência de implantes direta e indireta para próteses implantossuportadas. Material e Métodos: a partir de um modelo mestre mandibular desdentado, foram instalados paralelamente quatro implantes hexágonos externos associados a quatro componentes multi unit e quatro capas multi unit rotacionais. Confeccionou-se uma infraestrutura mestre metálica unindo todos os elementos, considerando o conjunto como grupo controle (GC). Ao todo, fabricaram-se cinco moldeiras individuais para realizar 10 moldagens mediante as técnicas direta e indireta (n= 5), sendo os modelos obtidos submetidos à análise dimensional e estatística. Com uma lupa estereoscópica binocular com ampliação de 60 x, estabeleceram-se três pontos no centro da face vestibular de cada um dos implantes (A, B, C e D) por amostra. Em cada ponto, mensurou-se o desajuste vertical entre a estrutura metálica e os análogos. Os resultados obtidos foram submetidos ao teste de normalidade de Dixon e teste de KruskalWallis para amostras independentes (p< 0,05) com o auxílio do ABsTRACT

A natural tooth can move above 100 µm inside the periodontal ligament and the tooth displacement is compensated by installing a fixed partial prosthesis.On the other hand, the osseointegrated implant movement is limited to 10 µm in average and its lack of intrusion generates a force on the restauration, which if not relieved, cause all the complications cited above [5-6, 9,18].Thus, it is necessary to evaluate the factors accounting for relieving the lack of flexibility of the implants that can directly influence on the transfer impression as well as the reliability with the original control geometry [2,[9][10]23].
The implant transfer impression technique largely influences on the accuracy of the working casts obtained for the construction of the implant-supported prosthesis [2][3][4][5][8][9]11,20,22,26] because the transfer impression technique aims to record the tridimensional implant position [2,8] and to copy details as deepness, angulation, and position in relation to other implants, adjacent teeth, and antagonist arch [8].
To sum up, two impression techniques have been used to transfer the implant position in mouth to the study cast: direct (open tray) and indirect (closed tray) techniques.The direct technique (open tray) [5,8,12,17,20,22,27,28], with or without splinting, requires the construction of a customized tray with a perforation on the area of the implants to make the insertion of the transfer copings easy.After the impression, the transfer copings are loosening to be removed together with the impression.The open-tray technique is specially indicated for impression of more than three implants to reduce the effects caused by angulation, to decrease the impression material deformation, and to eliminate the caution in repositioning the transfer coping in the respective space in the impression.The direct impression drawbacks are the difficult and the need of expertise to remove all the impression+transfer coping set from the mouth [3][4][5]12,18,19,21].
In the indirect technique (closed tray) [3,5,8,12,[17][18][19][20][21][22]27,28], the transfer copings are fixed to the implant and splinted to the implant after the impression removal.Then, the transfer coping is loosened, screwed to the analogue, and inserted in the impression manually.This is a simpler and faster, but less accurate procedure that is indicated in cases with one/two implants in individuals with limited mouth opening and/or marked vomiting reflex; or in situations in which not enough space is available to access the transfer copings; or in the presence of angulated implants [3,5,8,12,[17][18][19][20][21][22]27,28].

Master metallic structure
The control group (CG) comprises the master metallic superstructure.Initially four implants were associated to four multi unit transfer copings (Inp; Sistema de Implantes Nacional e de Próteses Comércio Ltda, São Paulo, Brazil) and four multi unit rotational cobalto-chromium caps (Inp; Sistema de Implantes Nacional e de Próteses Comércio Ltda, São Paulo, Brazil).These were fixed to the master model with the aid of threads Mix (Mult Unit M 1.4 x 3.5) under torque of 20 Ncm on each implant.The torque was calibrated with the aid of torque wrench (Kit Protético, INP, SP, Brazil) according to the manufacturer's instructions.

Customized trays
On the installed implants, a 3-mm relief with condensation silicone (Optosil ®; Heraeus Kulzer) was made aiming to achieve a uniform thickness of the impression material and an adequate positioning of the trays during the procedure [Figure 2a].
Then, a base structure for the construction of all trays were obtained.
The general dimensions (base, height, width, deepness, and relief contour) were marked with the aid of pink wax #7 (Lysanda ® -Produtos Odontológicos Ltd.) [Figure 2b and 2c].Next, the base structure was filled with condensation silicone (Xantopren ®, Heraeus Kulzer) and catalyzer (Activator, Heraeus Kulzer) [Figure 2d].In Group 1, after the transfer coping installation and the application of an universal adhesive for silicone impressions (Universal Tray Adhesive, Zhermack) on the customized tray, the impression material was inserted and an impression of the master mold was taken.After the impression material setting, the opentray digital multi unit threads (Inp; Sistema de Implantes Nacional e de Próteses Comércio Ltda, São Paulo, Brasil) were loosened [Figure 3a] and the transfer copings removed together with the impression [Figure 3b].Then the transfer copings were screwed on the respective analogues (Inp; Sistema de Implantes Nacional e de Próteses Comércio Ltda, São Paulo, Brazil) [Figure 3c].
In Group 2, the transfer copings were screwed with the aid of multi unit digital threads (Inp; Sistema de Implantes Nacional e de Próteses Comércio Ltda, São Paulo, Brazil) for closed trays.The labial surfaces of these threads were marked with permanent pen.Then, an universal adhesive for silicone impressions (Universal Tray Adhesive, Zhermack) was applied on the tray and the impression material was inserted and an impression of the master mold was taken.After the impression material setting [Figure 4a], the impression was removed, the transfer copings loosened, screwed to their respective analogues (Inp; Sistema de Implantes Nacional e de Próteses Comércio Ltda, São Paulo, Brazil) [Figure 4b], and inserted in the impression manually [Figure 4c].Finally, all impressions were immediately poured with type IV dental plaster (Durone®) and elapsed one hour, the impressions were removed and the working casts obtained [Figures 5a and 5b] to be dimensionally compared to the control group (GC).
Passive adaptation is defined as the adequate positioning of all surfaces of both the implant and fixed (cemented or screwed) prosthesis, without application of forces.In these conditions, previous studies reported the vertical misfit between the metallic structure and the implants should respect the limits of distortion between 100-150 µm to be considered as acceptable [2,[4][5][6]9,[11][12]19,21].Values lower than 30 µm are not detectable to the naked eye [2].Thus, according to the mean results obtained in the control group (22.445 µm ± 7.7106), G1 (24.6474 µm ± 14.8883), and G2 (26.2257 µm ± 9.7421), the adaptation of all studied

Dimensional Analysis
All samples and the master model received the master metallic structured fixed with threads Mix (Mult Unit M 1.4 x 3.5) under 20 Ncm torque on each implant, calibrated with the aid of a torque wrench (Kit Protético, INP, SP, Brazil), following the manufacturer's instructions.
The marginal adaptation and the dimensional analysis of all groups were performed with the aid of a Stereoscopic Magnifying Glass (Physis; S/N: K300202053 3.0MP) with x60 magnification, on three points on the labial surface of each implant, named implant A, B, C, and D.
The vertical misfit of each implant was calculated as the distance between the metallic structure and the analogue on each point.An arithmetic mean of each implant was obtained.Then, the median of each sample for each group was obtained with the mean of the four implants A, B, C, and D.Then, the mean values of the samples was the total mean of the group [Figure 6].The means of group 1 (open tray) and 2 (closed tray) were compared to the control group (master model) and submitted to the statistical analysis with the aid of Bioestat 5.0 software.

Statistical analysis
Data was submitted to Dixon's normality test to verify extreme values.Due to unequal sample sizes, data normality and homogeneity was not confirmed and a non-parametric test was applied.Kruskal-Wallis test for independent samples was applied with level of significance of (p < 0. 05).

ResulTs
G1 (open tray) and G2 (closed tray) means and standard deviations were compared to the means of control group (master model) and described the vertical misfit between the metallic structure and the implants (Table I).The comparison between groups showed no statistically significant differences (p = 0.3465).
Table I -Mean and standard deviation of the studied groups samples was classified as acceptable because in most part of the analyses, the vertical misfit was not observed by naked eye.
The studies on the mechanical and/or biological complications due to poorly executed impressions that resulted on maladapted prostheses report consequences ranging from the possibility of implant component loss or fracture, porcelain fracture, structure fracture, occlusal alterations, pain, biofilm accumulation, peri-implant mucositis, which may evolve to peri-implantitis consequently leading to the loss of the osseointegration and implants.Thus, implant transfer impression techniques and innovative materials should aim at overcoming these problems clinically [2][3][4][5][7][8]14,[17][18][19][20][21][22]26].Thus, the ideal implant transfer impression technique should provide greater comfort to the patient with greater practicality during the procedure, shorter working time, and lower cost.All these variables would influence on the accuracy and quality of the constructed transfer working casts [9,12,23].
The literature lacks consensus on the best technique: some studies advocate the use of open tray (direct) technique [3,5,8,13,18,22,24,27] and other report no statistically significant differences between the techniques [19][20], which corroborates the results of this study (Kruskal-Wallis test; p= 0.3465).Further studies are necessary to complement these opinions expressed over the years.The literature review of 59 studies published between 1990 and 2012 retrieved 56 in vitro studies and three case reports.Of these, 25 studies examined the open and closedtray implant transfer impression techniques.Twelve studies concluded that the open tray technique was the best and 11 studies found no statistically significant differences between the techniques.The authors concluded that the open tray techniques behave better, especially when more implants were involved [10].A study surveyed 32 researches published between 2009 and 2013 to identify the most accurate implant transfer impression technique and the main factors accounting for affecting the accuracy and observed that in cases with four implants or more, six studies advocated the use of the open tray technique and four studies found similar results between the techniques.In the installation of three implants or lesser, two studies pointed out no differences between the techniques and two studies reported that the open tray technique had the greatest accuracy [17].
Both techniques have advantages, disadvantages, and clinical applications.The literature report that the open tray technique is more accurate than the closed tray technique and indicated for the impression of four implants or more.The rationale behind this indication is that the removal of the transfer copings together with the impression even considering the minimum movement of loosening avoids the implant angulation and internal movements in the impression material keeping the precision.The open tray technique drawbacks are longer working time and necessity of experience to construct the orifices in the trays with adequate location and diameter, which may interfere in the impression accuracy to numbers similar to that of closed tray technique [2][3][4][5]12,18,20,22].
The closed tray technique has satisfactory results for the impression of up to three implants with a simpler and faster method.However, the disadvantages are the displacement of transfer copings and analogues in the impression material during the manual placement by the dentist, which distorts the impression and alters the accuracy of the transfer cast mainly in the case of multiple implants [3,5,18,20,22].
The group 1 (open tray) showed the smaller distance (mean of 10.9567 µm -sample #1, implant #4) and the greatest distance (mean of 63.77 µm -sample #3, implant #3) between the master structure and implant.This was an in vitro study, similar to most of the scientific literature researched.In vitro studies do not mimic the clinical conditions such as the interference of the texture of the teeth and gingiva, bite force, humidity, and temperature.Therefore, further in vivo studies are necessary to mimic the clinical situation through more accurate impressions demanding the dentist's expertise [3][4][8][9][10]14,16,20,22,25,[27][28].

CoNClusIoN
Both the open and closed-tray impression technique showed similar accuracy in the transfer of the implants.Considering the indications, contraindications, and clinical applications, the dentist should choose the most suitable technique for daily practice.
This in vitro study aimed to determine and compare the dimensional accuracy of open and closed-tray implant transfer impression technique for implant-supported prosthesis.The hypotheses are: (1) the open-tray technique is more accurate than closed-tray technique; (2) the closed-tray technique is more accurate than open-tray technique; and (3) both techniques have similar accuracy in implant transfer technique.

Figure 1 -
Figure 1 -Master model and master metallic structure.

Five 3 -
mm open trays were constructed with chemically-activated acrylic resin (Jet; Artigos Odontológicos Clássico Ltd, São Paulo, Brasil).To obtain the indirect implant transfer impression technique and the closed trays, five acrylic plates were cut (1.8 x 3.8 cm) and fixed on the open trays with the aid of chemically-activated acrylic resin (Jet; Artigos Odontológicos Clássico Ltd, São Paulo, Brazil) [Figure 2e].

Figure 2 -
Figure 2a) Implant relief with Optosil ®; b) construction of the customized tray with dental wax #7; c) dental wax mold to duplicate the customized trays; d) Filling with Xantopren ®; e) chemically-activated acrylic resin (Jet ®) customized trays.

Figure 3 -
Figure 3 -Open tray technique).a) filling of the customized tray with Xantopren ®, impression, and loosening of the transfer copings; b) impression with Xantopren ® and transfer copings; c) insertion of analogues.

Figure 4 -
Figure 4 -Closed tray technique.a) filling of the customized tray with Xantopren ® and impression; b) loosening of the transfer coping and manually screwing to the analogues; c) manual insertion of the transfer coping+analogue set into the mold.

Figure 5 -
Figure 5a) Working casts obtained with open tray technique; b) Working casts obtained with closed tray technique.

Figure 6 -
Figure 6 -Dimensional analysis (open tray, sample #4, implant #3) of the magnified image obtained with the Stereoscopic Magnifying Glass on the three points at the center of the labial surface of the implant to measure the vertical misfit between the metallic structure and the analogue.The three measurements chosen are marked with red circle.

A
systematic review aimed to evaluate and compare the results obtained by 50 studies published between 1990 and 2013 on implant transfer impression techniques based on the respective techniques and methods used, advantages, and disadvantages.Of the 17 studies comparing the accuracy between the open and closed-tray techniques, 10 studies advocated the use of open tray technique and only one advocated the use of closed tray technique; eight studies reported no dimensional differences between the techniques [7].
These data taken together with the mean of each group revealed greater variations in the results of group 1 (open tray).The median values for control group (20.605 µm), G1 (18.8833 µm), and G2 (23.635 µm) show the smallest values for the open tray technique.