the effect of impression technique and implant angulation on the impression accuracy of external-...

1422 Volume 27, Number 6, 2012

Dental implants have become the treatment of choice in many situations where missing teeth

require functional and esthetic replacements. Repro-duction of the position and orientation of intraoral implants by means of an accurate impression in the definitive cast is the first step in achieving a passively fitting implant-supported prosthesis.1–3

Several studies have examined the clinical variables affecting the accuracy of the implant impression, such as differing impression techniques,4,5 the use of differ-ent impression materials and trays,6,7 splinting or not splinting the implants,8 the relative implant angula-tions,9–11 and the lengths of impression coping con-nections.11 The relevant scientific literature reveals many controversial issues regarding the accuracy of impressions using open-tray (pickup) and closed-tray (transfer) techniques in situations where three or more implants were placed.4,12–20 Most researchers have re-ported that the open-tray technique is more accurate and predictable than the closed-tray technique using

1 Research Associate, Department of Fixed Prosthesis and Implant Prosthodontics, School of Dentistry, Aristotle University of Thessaloniki.

2 Assistant Professor, Department of Fixed Prosthesis and Implant Prosthodontics, School of Dentistry, Aristotle University of Thessaloniki.

3 Research Associate, Section of Mechanical Design and Control Systems, School of Mechanical Engineering, National Technical University of Athens.

4 Professor and Chairman, Department of Fixed Prosthesis and Implant Prosthodontics, School of Dentistry, Aristotle University of Thessaloniki.

Presented at the 33rd Annual Congress of European Prosthodontic Association, Innsbruck, Austria, October 1–3, 2009.

Submitted in partial fulfillment of the requirements for a PhD degree.

Correspondence to: Prof Petros Koidis, Department of Fixed Prosthesis and Implant Prosthodontics, School of Dentistry, Aristotle University of Thessaloniki, University Campus, Dentistry Building, GR 54124, Thessaloniki, Greece. Fax: +30-2310-999676. Email: [email protected]

The Effect of Impression Technique and Implant Angulation on the Impression Accuracy of

External- and Internal-Connection ImplantsPavlos Mpikos, DDS, PhD1/Dimitrios Tortopidis, DDS, PhD2/Christos Galanis, PhD3/

George Kaisarlis, PhD3/Petros Koidis, DDS, MSc, PhD4

Purpose: The purpose of this in vitro study was to investigate the effect of impression technique and

implant angulation on the impression accuracy of external- and internal-connection implants using a novel

experimental device. Materials and Methods: An experimental device was designed and fabricated to make

in vitro impressions by means of open- and closed-tray techniques. Impressions of eight implants with two

different connections (four external-hex and four internal-hex) at three angulations (0, 15, and 25 degrees)

were made using a medium-consistency polyether material. Evaluation of implant impression accuracy was

carried out by directly measuring the difference in coordinate values between the implant body/impression

coping positioned on the base and the impression coping/laboratory analog positioned in the impression

using a touch-probe coordinate measuring machine. Experimental data were analyzed by two-way analysis of

variance. The significance level of all hypothesis testing procedures was set at P < .05. Results: The results

showed that: (1) for implants with external connections, impression accuracy is not significantly affected

by the impression technique, implant angulation, or their interaction; and (2) for implants with internal

connections, impression accuracy is significantly affected only by implant angulation: Impression inaccuracy

was greater at the 25-degree implant angulation. Conclusions: Within the limitations of this in vitro study,

the open- and closed-tray techniques had no effect on the accuracy of multiple implant impressions. The

interaction between impression technique and implant angulation was also not significant. However, implant

angulation significantly affected the impression accuracy when implants with internal connections were

used. Int J Oral MaxIllOfac IMplants 2012;27:1422–1428

Key words: external-connection implants, implant angulation, impression accuracy, impressions, internal-connection implants

© 2012 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.

Mpikos et al

The International Journal of Oral & Maxillofacial Implants 1423

repositionable copings.12–16 Other authors, however, either found no significant differences between the two techniques or concluded that the closed-tray tech-nique produced a more accurate definitive cast than the open-tray impression technique.10,17–20

With regard to implants at various angulations, pre-vious studies have found that impressions made in the presence of angulated implants were less accurate than those made with parallel implants.9,11,21,22 Inter-estingly, only a few investigations have compared the accuracy of implant impression techniques performed for external- and internal-connection implants, and these few have produced varying results.16,23–27

In previous studies, the accuracy of the implant im-pressions was evaluated by measuring the differences in the relative positions and orientations of implants on the resulting definitive casts in vitro, without taking into account possible inaccuracies arising from dimen-sional changes in the dental stone during setting, the implant definitive cast technique, and the laboratory analog placement. Thus, the aim of the current in vitro study was to investigate the effect of impression tech-nique, implant angulation, and their interaction on the accuracy of impressions of external-and internal-connection implants by using a novel experimental device that allowed direct measurement of the defini-tive impression. The research hypothesis was that the impression technique, implant angulation, and their interaction would have a significant effect on the ac-curacy of impressions made from either external- or internal-connection implants.

MATERIALS AND METHODS

Fabrication of the Experimental Device For this study, a novel experimental device was de-signed and fabricated from solid aluminum, which was subsequently anodized. In the device, eight implants (four with external-hex connections and four with internal-hex connections; Dr Ihde Dental AG), each 4.1 mm in diameter and 13 mm in length, were mount-ed at angles of 0, 15, or 25 degrees relative to the hori-zontal matrix surface. The experimental device was designed to allow a clinical simulation of impression making by means of open- and closed-tray techniques in a standardized and reproducible manner. It com-prised a base and an upper part.

The base of the device was a rectangular block with eight blind cylindric holes, each 5 mm in diameter and 15 mm deep, in its flat upper surface for mount-ing the eight implants (Fig 1). The eight implants were arranged in a semicircular formation on the base with angulations typical of teeth in the maxilla and sequen-tially numbered 1 to 8 (Fig 1). The external-connection

implants were placed in positions 1 to 4 on the left side, and the internal-connection implants were placed in positions 5 to 8 on the right side (all at 0, 15, or 25 degrees of angulation). The implant bodies were secured in the base component with autopolymer-izing acrylic resin (Pattern Resin LS, GC Corporation). The base of the device had three plane surfaces, which were used as reference geometry (datum) features for the determination of the base reference system. These plane surfaces were finished to a high quality (Inter-national Organization for Standardization #4287:1997 surface roughness [Ra] = 1.8 µm). The base reference system allows the accurate determination of points in three-dimensional (3D) space and thus the position of all points in the impression.

The upper part of the device consisted of three separate components: an impression frame, a second component that fit inside the frame and allowed the simulation of an impression tray, and a cover plate that could be attached or removed according to the im-pression technique (open or closed tray). The compo-nents were connected together by set screws (Fig 1). The outer surfaces forming the upper reference planes were also finished to the same standard as the base (Ra = 1.8 µm) to form reference geometric (datum) features for the coordinate measuring machine (CMM) during the second series of measurements concerning the impression material itself.

Fig 1 Schematic diagram of the experimental device with im-plant bodies secured to the base.


Mpikos et al


Impression Procedures

In both experiments, the minimum required sample size was determined by means of an a priori power analysis using the GPower software (version 3.1, Franz Faul, Uni-versität Kiel). Four impressions with impression copings were made with each of the two different techniques in a room with controlled temperature (23°C ± 2°C) and a relative humidity of 50% ± 10%. Polyether im-pression material (Impregum, medium consistency, 3M ESPE) was used in accordance with the manufac-turer’s instructions. An automix machine (Pentamix 2, Automatic Mixing Unit, 3M ESPE) was used to standard-ize all mixtures. If the impression was made by means of the closed-tray technique, the impression copings were retained on the implants upon removal of the impression and had to be repositioned on their labo-ratory analogs in the corresponding holes. When the impression was made using the open-tray technique, the impression copings, which remained in the impres-sion, had to be unscrewed before the impression could be removed from the base. The same torque setting (20 Ncm, standardized with a torque wrench) was used to tighten all the screws throughout the study. All pro-cedures were performed by the same operator.

CMM MeasurementsFor the processing of the measurements, the CMM soft-ware (PC-DMIS, version 4.2, Wilcox Associates Inc) cre-ated a cylinder in 3D space of best fit of the measured cylindric parts. The coordinates of the long axis of this cylinder of any circular plane along this axis and the co-ordinates of the points on this axis that met the plane constituted the reference system for measurements of position and angulation. The relative difference in co-ordinate values of the cylinder before the impression (comprising the implant body and the impression cop-ing on the base of the device) and the cylinder after the impression was made (comprising the impression

coping and the laboratory analog within the setting impression material) was assessed in degrees by means of a Mistral 070705 (DEA, Brown & Sharpe) direct com-puter-controlled 3D CMM (Figs 2a and 2b).

The CMM accuracy performance specification, per In-ternational Organization for Standardization #10360-2, is 3.5 µm (maximum permissible volumetric error) for the computational manipulation and mathematical fitting of the 3D coordinate of selected points into geometric features (planes, cylinders, etc). The industry-standard Renishaw PH10M motorized head, in conjunction with a TP200 probe of 20 mm in length and a spherical tip with diameter of 1 mm, were used for the capture of the 3D coordinates of the required contact points. Measure-ments were made on eight different points of the outer surface of each implant system part according to the British Standards Institution #7172:1989. The deviation of the position and angulation of a solid body, such as an implant body/impression coping or laboratory analog/impression coping, was defined as the magnitude of its total translation and rotation from its initial state in 3D space to its final one, as an impression coping/laboratory analog.28,29 To calculate the total translation and the an-gular deviation of a specific reference point and axis re-spectively, computer-aided mechanical design software (SolidWorks, Dassault Systèmes SolidWorks) was used.

Measurement Procedure The accurate measurements were made in a specially modulated laboratory space with a stable temperature (20°C ± 1.5°C/12 hours; recorders of control: TESTO 175H2, s/n 20038973/408).

In the initial phase of the experiment, measurements were made to determine the machining tolerances of the components of the specific implant system, since these constituted a significant factor affecting the final impression accuracy and contributed to the total resul-tant distortion in implant impression. Mechanical parts

Z

Y

X

PL NB_X ASXB_X

PNTB_X

CYLB_X

CYLA_X

PNTA_XAXSA_X

Fig 2a Measurement of coordinates of the impression copings and implant bodies using the CMM.

Fig 2b Schematic diagram showing the cylinders created from the CMM x, y, and z coordinate system adjusted to the base reference system.


Mpikos et al


had to undergo machining and milling. The high preci-sion of this procedure allowed the authors to consider as common the central axis of the connected cylindric bod-ies. During this phase, the implant bodies were attached in their position in the base component and secured.

The CMM was then calibrated. This was followed by the measurement of the coordinates CYLA_X (repre-senting the implant body) and CYLB_X (representing the impression coping) and by the setup of the base reference system. The first measurement set con-cerned the measurement of the coordinate values of the eight assembled implants and impression copings fixed in the base of the device. The surfaces available for measurement were the planar face of the top of the impression coping PLNB_X and the cylindric surface of CYLA_X and CYLB_X (implant body and impression coping). At least six contact points were captured on each planar face and at least eight points were cap-tured on each cylindric surface (Figs 2a and 2b).

After polymerization of the polyether, the upper part of the experimental device was manually removed from the base carrying the impression material (like a custom impression tray). The upper part of the device was reinstalled and reset, reversed, and repositioned on the measurement table of the CMM.

The second set of measurements concerned the posi-tions of the eight assembled laboratory analogs CYLC_X and impression copings CYLB_X within the impression material. In this case, the surfaces available for measure-ment were the planar face at the bottom of the cylindric surface of the implant laboratory analog, PLNC_X, which protruded from the impression (Figs 3a and 3b).

In a computer-aided design environment, the coor-dinates of the cylinders were mathematically reversed so that they represented the new position of the im-pression coping following the procedure of impression taking. Thus, a third set of measurements was created involving the measurement of the relative changes in

position between the geometric features (planar and cylindric surfaces) of the implant bodies and those of its assembled coping or lab analog. This was per-formed for each of the eight assembled pairs used dur-ing the in vitro experimental procedure.

Using computer-aided design, the authors calculated the difference between the different implant positions (initial – final); this indicated the resultant positional distortion (x-, y-, and z-axes) that can be attributed to different impression techniques for the different im-plant connection geometries (external or internal) and the different axial angulations. After that, the computer mathematically turned the two cylinders to the degree of their highest congruence, having as a reference the common point of their axial intersection. The degree of divergence of congruence of the two cylinders gave the total deformation occurring in each case.

Statistical AnalysisCoordinate values in both types of implants (external and internal connections) were analyzed by two-way analysis of variance (ANOVA) according to the linear model, which included two factors between experi-mental units with interaction.

Specifically, the model involved the main effect of impression technique (factor A) with two levels (open or closed tray); the main effect of implant angulation (factor B) with three different orientations (0, 15, or 25 degrees) in four positions (θ1, θ2, θ3, θ4), respec-tively; and the interaction between the two factors (A × B). For each of the eight factor combinations (two impression techniques × four implant positions), four replications (impression cycles) were performed.

Comparisons of means were performed by the least significant difference (LSD) criterion.30 The significance level for all hypothesis testing procedures was set at P < .05. All statistical analyses were conducted with SPSS software (version 17.0, IBM).

Z

Y

X

PL NC_XASXC_XPNTC_X CYLC_X

CYLB_XPNTB_X

Fig 3a The base of the device was reversed on the measure-ment table of the CMM after the impression making, with the im-plant laboratory analogs projected from the impression material.

Fig 3b Diagram of the measured elements with the frame ref-erence system.


Mpikos et al


RESULTS

For external-connection implant coordinate val-ues, ANOVA revealed that: (1) the main effect of im-pression technique was not statistically significant (F[1,24] = 3.43, P = .076) (Table 1); (2) the main effect of implant angulation was not statistically significant (F[3,24] = 2.60, P = .075) (Table 2); and (3) the interac-tion between the two factors was not statistically sig-nificant (F[3,24] = 0.18, P = .909) (Table 3).

For internal-connection implant coordinate values, ANOVA revealed that: (1) the main effect of impression

technique was not statistically significant (F[1,24] = 0.96, P = .337) (Table 1); (2) the main effect of implant angula-tion was statistically significant (F[3,24] = 58.13, P < .001) (Table 2); and (3) the interaction between the two factors was not statistically significant (F[3,24] = 0.57, P = .640) (Table 4). For internal-connection implants, the impres-sion inaccuracy was greater at 25 degrees of implant angulation, position θ4, compared with the other three positions (θ1, θ2, and θ3) with both impression tech-niques (Tables 2 and 4). In both cases, the differences between the corresponding mean values (θ4 versus θ1, θ2, or θ3) were greater than the critical LSD value.

Table 1 Main Effect for Impression Technique

Impression technique n

External-connection implants Internal-connection implants

Mean SD Mean SD

Open tray 16 0.370 0.150 1.588 1.275

Closed tray 16 0.557 0.392 1.402 1.486

P value (LSD) .209 .391

SD = standard deviation.

Table 2 Main Effect for Implant Angulation

Implant angulation n

External-connection implants Internal-connection implants

Mean SD Mean SD

Θ1 8 0.521 0.196 0.714 0.368

Θ2 8 0.438 0.255 0.739 0.409

Θ3 8 0.643 0.445 0.868 0.262

Θ4 8 0.354 0.158 3.660 0.851

P value (LSD) .296 .553


Table 3 Interaction Effect for Impression Technique × Implant Angulation for External-Connection Implants

Impression technique/implant angulation n Mean SD

Open tray

Θ1 4 0.390 0.147

Θ2 4 0.355 0.068

Θ3 4 0.518 0.034

Θ4 4 0.317 0.157

Closed tray

Θ1 4 0.651 0.150

Θ2 4 0.521 0.359

Θ3 4 0.767 0.648

Θ4 4 0.391 0.173

P value (LSD) .468


Table 4 Interaction Effect for Impression Technique × Implant Angulation for Internal-Connection Implants

Impression technique/implant angulation n Mean SD

Open tray

Θ1 4 0.923 0.370

Θ2 4 0.964 0.286

Θ3 4 0.827 0.314

Θ4 4 3.640 0.563

Closed tray

Θ1 4 0.505 0.251

Θ2 4 0.514 0.416

Θ3 4 0.908 0.239

Θ4 4 3.681 1.171

P value (LSD) .782



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DISCUSSION

The findings do not support the research hypothesis for external-connection implants; the impression tech-nique, implant angulation, and their interaction had no effect on the accuracy of impressions (P > .05 for all ef-fects) (Tables 1 to 3). In contrast, for internal-connection implants, the implant angulation had a significant effect on the impression accuracy (P < .001) (Tables 2 and 4).

Passive fit of an implant-retained prosthesis de-pends on the accuracy of the impression made.3,14 The accuracy of implant impressions is affected by various clinical variables related to the implants (eg, angula-tion, connection type) and the impression technique (eg, different direct or indirect techniques, impression tray, and splinting or no splinting of impression cop-ings).4–16 In most previous studies, the accuracy of the implant impression was evaluated indirectly by mea-suring differences in the relative positions and orien-tations of implants on a typodont or a definitive cast in relation to the diagnostic cast.5,7, 9,11,16 The novel experimental device used in this study allowed im-pression making with either open-tray or closed-tray techniques. The relative difference in the positional ac-curacy between the implant body/impression coping on the base of the device and the impression coping/laboratory analog within the impression was therefore measured directly, using a CMM, without the use of a dental cast, thereby eliminating the possible inherent additional distortion caused by the laboratory fabri-cation of a cast. The pouring procedure can alter the positional relationship of the copings because of the expansion involved in the setting of the dental stone.

The findings of the present study contradict the re-sults of previous studies that reported the superiority of the open-tray technique.12–16 In this investigation, no statistically significant differences concerning impres-sion accuracy were detected between the open- and closed-tray techniques. These results are in accordance with the few studies focused on this subject.8,10,24,27 It should be emphasized that the cited studies used dif-ferent evaluation methods and measurement devices for the positional changes of implant analogs, including a reflex microscope,8 a measuring stylus,10 strain gaug-es,24,27 or a profile projector,11 and all used a definitive cast as the final reference point. This difference in find-ings is highly likely to be a result of the use of different components and study designs, all of which involved the use of a definitive dental cast, as opposed to the di-rect measurement technique used in the current study.

The different connection geometry between and within commercial implant systems may also affect the accuracy of impressions. Several studies1,2,23 have evaluated the accuracy of impression techniques with external-connection implants, and few studies have

examined the impression accuracy with internal-con-nection implants.16,27 The varying results among stud-ies of external- and internal-connection implants are the consequence of employing different prosthetic connection mechanisms and measurement methods. Additionally, the results can probably be explained by the higher level of stress between impression mate-rial and impression copings that is created when an impression with impression copings is removed from internal-connection implants.

Angulated implants in the maxillary arch are com-mon clinically because of anatomic limitations and es-thetic considerations. It must be noted that impression accuracy measurements in the present study included eight implants dispersed in an arch similar to a max-illa. The results of this study showed that the main ef-fect of implant angulation on impression accuracy was significant only for internal-connection implants. The effect was a result of the high relative differences in coordinate values between implant body/impression coping and impression coping/laboratory analog for an implant angulation of 25 degrees.

The results are in line with previous findings that stated that angulations of the implants may cause dis-tortion of impressions, possibly because of the higher forces required for removal of the impression.9,11,21,22 Furthermore, it seems that the presence of angulated implants with internal connections had a significant effect on the accuracy of the experimental casts com-pared with the definitive casts owing to the distortion of the impression material.11 However, other studies have shown that the axial angulation of two or three implants was not associated with inaccuracy in the im-pression.10,27 In cases of three or fewer implants, the angulation effect may be compensated by the elastic recovery of the impression material. If multiple im-plants dispersed along the dental arch are examined, the difference in angulation may be added to the linear distortion, resulting in overall increased distor-tion. The angulation effect on the accuracy of impres-sions may be heightened by an increased number of internal-connection implants because of the higher forces required to remove the impression tray after the impression material has set. Finally, the varying re-sults among previous investigations of angulated im-plants may be a result of the employment of different numbers of implants, different prosthetic connection mechanisms, and different evaluation methods.

A possible limitation of the present study design was that the removal forces, and the consequent impression distortions in clinical practice, were con-sidered to be different from those applied in the ex-perimental conditions because of the different extent of dental undercuts in the novel experimental device and the resulting differences in removal forces. Further


Mpikos et al


studies addressing the much greater angulations com-monly encountered in implant prosthodontics are re-quired to evaluate the effect of connection geometry on implant impression accuracy. In addition, a com-parison of dimensional differences in coordinate val-ues between implant bodies/impression copings and impression copings/laboratory analogs should be also evaluated on definitive casts using the CMM.

CONCLUSIONS

Within the possible limitations of this study, the follow-ing conclusions were drawn:

1. The findings of this study suggested that, for ex-ternal-connection implants, impression accuracy is not affected by the impression technique and implant angulations.

2. For internal-connection implants, although im-pression accuracy is not affected by the impression technique, it is significantly affected by implant angulations.

3. The interaction between impression technique and implant angulation was not significant in the accuracy of impressions for either external- or in internal-connection implants.

ACKNOwLEDgMENTS

This study was partially supported by a grant from the General Secretariat for Research and Technology, Greek Ministry for Devel-opment (PENED 01-197). The authors express special thanks to Professor Emeritus N. Kafantaris for helpful assistance in aspects of the project including planning, support, and helpful comments. The authors also thank Professor Emeritus M. Sfantsikopoulos for providing laboratory equipment and helpful comments. The authors reported no conflicts of interest related to this study.

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