Accuracy of Different Impression Techniques for

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Accuracy of Different Impression Techniques for Internal-Connection Implants Yun-Jung Lee, DDS, MSD1/Seong-Joo Heo, DDS, MSD, PhD2/Jai-Young Koak, DDS, MSD, PhD3/ Seong-Kyun Kim, DDS, MSD, PhD3 Purpose: This study evaluated the accuracy of four implant-level impression techniques with optical microscopy on two angulated conical internal-connection implants. Materials and Methods: A master cast with two internal-connection implant analogs angulated 10 degrees from each other and a master framework were fabricated. Four impression techniques were examined: octagonal transfer impression coping, nonoctagonal transfer impression coping, nonoctagonal pickup impression coping, and nonoctagonal pickup impression copings joined together with autopolymerizing acrylic resin. Experimental casts were fabricated from 40 polyether impressions divided into four groups. After the master framework was seated on each of the casts, one abutment screw was tightened and marginal gap measurements between analog and abutment were recorded on the other side with an optical microscope. Results: The casts produced from nonoctagonal pickup impression techniques were more accurate than those produced using transfer impression techniques (P < .05). However, there was no statistically significant difference in the accuracy between the unsplinted and splinted methods in pickup impression techniques (P > .05) or between the use of octagonal coping and nonoctagonal coping in transfer impression techniques (P > .05). Conclusion: The casts produced from nonoctagonal pickup impression techniques were more accurate than those produced by transfer impression techniques, regardless of whether they were splinted, for angulated conical internal-connection implants. INT J ORAL MAXILLOFAC IMPLANTS 2009;24:823–830 Key words: implant impression, implant-level impression, impression accuracy, internal connection

assive fit is one of the major concerns in implant dentistry. Although it is assumed that misfitting prostheses negatively impact the long-term success of implants, some evidence suggests that prosthetic misfit might not affect osseointegration.1–3 On the other hand, the absence of passive fit may lead to prosthetic failures, such as fracture and/or loosening of screws, and retention of biofilm caused by poorly fitting components.4–8

P

1Graduate

Student, Department of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, Seoul, South Korea. 2Professor, Department of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, Seoul, South Korea. 3Associate Professor, Department of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, Seoul, South Korea. Correspondence to: Dr Seong-Kyun Kim, Department of Prosthodontics and Dental Research Institute, School of Dentistry, Seoul National University, 28 Yeongun-dong, Chongro-gu, Seoul, 110-749, South Korea. Fax: +82-2-2072-3860. Email: [email protected]

The accuracy of the impression plays a major role in achieving a passive fit between the implants and the superstructure.9–12 The aim of implant impression is to transfer the implant or abutment positions from the oral cavity to the master cast. Factors affecting the accuracy of implant impressions include the transfer and splinting of the impression coping, angulation of the implants or abutments, the number of implants, the impression material, the impression tray, and prosthetic connection features. Most investigations of the accuracy of implant impression have evaluated the effects of the transfer and splinting of the impression coping. Some studies have found that splinting of pickup impression copings resulted in better accuracy of the impression.13–16 Among those studies, several showed that the splinting procedure was essential because unsplinted pickup impression copings exhibited no more accuracy than transfer impression copings.13,16 Other studies reported no significant difference in the accuracy of impressions resulting from splinted versus unsplinted copings. 17–20 Several authors reported that unsplinted copings exhibited less deviation from the master cast than splinted copings.21,22 The International Journal of Oral & Maxillofacial Implants 823

© 2009 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.

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Fig 1 (Left) Master cast with angulated analogs. Fig 2 (Right) Internal connection of Inplant System. 10 deg

Some investigators found that the transfer impression technique produced a more accurate master cast than pickup impression techniques.17,21 Such controversial results have also been reported in studies investigating pickup versus transfer impression techniques and the effects of splinting in nonparallel conditions of multiple abutments.12,20,23 Those studies were limited in that they used standard abutments with their impression copings. Assunção et al evaluated the effect of various implant angulations on implant-level pickup impression techniques of external-hex implants.24 Although greater deviations were caused by increased implant angulation, the splinting of impression copings resulted in more accurate impressions, especially when greater angulations were involved. Several recent studies have evaluated various implant-level impression techniques for internal-connection implants. The impression techniques for internal slip-fit connection systems were analyzed with transfer cap and acrylic resin splinting under parallel conditions. 25,26 In conical-connection systems, snap-on techniques and splinting methods were evaluated for multiple parallel and nonparallel implants.27,28 Despite the similar connection mechanisms, different methods were recommended by the authors of these studies and the implant manufacturers. Because of these discrepancies, further experiments with more variables and standardized methods are necessary to determine which implant impression techniques are optimal. The varying results among studies of external- and internal-connection implants are the result of employing different prosthetic connection mechanisms and measurement methods. To evaluate the accuracy of the impression, researchers measured the positional change of an abutment or implant analog from the reference plane; the change of interabutment or interimplant distance or interimplant angle; prosthetic strain; and changes in the prosthetic gap. Regardless of measurement level (abutment, analog, or prosthetic), those measurements were difficult to translate

into clinically meaningful values. The one-screw test has been widely used for a long time in multiple-unit implant prostheses effectively.29 It has also been used in experimental studies about the accuracy of implant impression techniques.14,30 Therefore, the gap in prosthetic-abutment or abutment-implant assembly could be examined with the one-screw test. The purpose of this study was to evaluate the accuracy of four implant-level impression techniques with microscopic gap measurement in the use of two angulated conical internal-connection implants.

MATERIALS AND METHODS Fabrication of Master Cast A stone block was fabricated from type III dental stone (New Plastone; GC Company). Two implant analogs (Inplant System, 4.3 mm in diameter; Warantec) were positioned 4 mm apart, edge to edge, in a 10-degree convergent relationship within the stone block (Fig 1). About 2 mm of each platform were left exposed. The prosthetic connection of this implant system had a 14-degree internal taper and parallel octagonal walls (Fig 2). Two implant-level nonoctagonal two-piece castable abutments were fabricated with gold alloy and a plastic sleeve that covered the outer margin of the implant platform. A master metal framework (15 ⫻ 6 ⫻ 6 mm) simulating a porcelain-fused-to-metal fixed partial denture was patterned and cast in goldpalladium alloy (Cerapall 2, Metalor). To modify the master cast, one analog was removed. A new analog and the remaining analog of the cast were screwed to the master framework, and the new analog was fixed in the hole with type III dental stone. This method was used to obtain a master cast with passive fit between the framework and the implant analogs. To mimic the clinical situation, a soft tissue analog was made on an analog head with light-consistency polyvinylsiloxane impression material (Exafine, GC).

824 Volume 24, Number 5, 2009 © 2009 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.

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Fig 3 Impression copings used in the four experimental groups.

OT

Fabrication of Custom Trays Two wax spacers with a total of 3 mm thickness were placed on the cast, which had the pickup and the transfer impression coping for the fabrication of their respective custom trays.31,32 The wax-spaced master cast was duplicated with polyvinylsiloxane impression material (Express STD, 3M ESPE). Custom resin trays, 2 mm in thickness, were made with self-curing acrylic tray resin (SR Ivolen, Ivoclar Vivadent) for two duplicated patterns. For pickup impression copings, a 2-mm hole was made on top of the trays.The tray was removed after complete polymerization and stored at room temperature for 24 hours before the impression was made.

Impression Copings Forty polyether impressions of this model were made with custom acrylic resin trays. Four groups of 10 specimens each were made with different impression techniques: in the first group, octagonal transfer impression copings and closed trays were used (OT group); in the second group, nonoctagonal transfer impression copings and closed trays were used (NOT group); in the third group, nonoctagonal pickup impression copings and open trays were used (NSP group); and in the fourth group, nonoctagonal pickup impression copings joined together with autopolymerizing acrylic resin and open trays were used (SP group) (Fig 3). For the splinted group (SP), autopolymerizing acrylic resin (Pattern Resin, GC Company) was applied around the pickup impression copings using an incremental application technique with a brush. The amount of acrylic resin was assumed to be satisfactory when the square surfaces of the copings were fully covered with a layer about 2 mm in thickness. After splinted impression copings were removed from the master cast, a thin section was made in the middle of the acrylic resin splinting, and the excess

NOT

NSP

SP

resin was trimmed until the diameter of the joints of resin splinting was 3 mm.33 The modified pickup copings were screwed to the master cast and the acrylic resin was added to the sectioned area. A 15-minute interval was allowed after splinting to guarantee complete resin polymerization.

Impression Procedures Regular-viscosity polyether impression material (Impregum Penta, 3M ESPE) was used for all transfer procedures. An automix machine (Pentamix 2, 3M ESPE) was used to standardize all mixtures. The appropriate adhesive (polyether adhesive, 3M ESPE) was applied to the custom trays. After 15 minutes of adhesive application, impressions were made. Impression copings were hand-tightened with guide pins onto the analogs in the master cast and then the impression was made. Polyether impression material was injected around the impression copings and placed inside the custom tray using the dispenser. The custom trays were seated over the master cast with finger pressure. After 15 minutes, the custom trays were removed. The same dentist manually attached analogs to the impression copings. For the transfer impression techniques (OT, NOT), the transfer copings connected to their analogs were replaced in their corresponding holes. The soft tissue analog was made around the abutment-analog connection with light-consistency polyvinylsiloxane impression material, and the impression was poured with vacuummixed type IV dental stone (Fujirock EP, GC Company) in accordance with the manufacturers’ instructions to obtain an experimental cast.

Assessment Accuracy After the soft tissue analog was removed from the experimental cast, the master metal framework was seated. One abutment screw (Warantec) was tightened to 10 Ncm using a torque controller (Warantec), The International Journal of Oral & Maxillofacial Implants 825


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Abutment 10 Ncm

Fig 4 One-screw test and marginal gap. Analog Fig 5 Microscopic image of gap and least distance between abutment and analog.

and marginal gap measurements between analog and abutment were recorded on the other side of the framework (Fig 4). The marginal fit was evaluated by measuring the gap between the edge of the abutment and the analog using a light microscope with image processing (Accura 2000, Intek-plus) at 240⫻ magnification. The accuracy of this light microscope technique was ± 0.1 µm. Images of the abutment-analog complex were obtained, and the gap was measured at the nearest point of the object lens of the light microscope part to minimize image distortion.34 The gap was measured as the minimum distance from one point of the abutment to a line determined by the least squares of points at the analog. All measurements and the least squared lines were computed by a programmed macro in the Accura 2000 Software System (Fig 5). The marginal gap of the middle, buccal, and lingual side of each abutment was measured three times at the nearest point to the object lens of the light microscope, and the mean value of the three measurements was calculated. After the measurement of one abutment, the same procedure was repeated at the other abutment. Therefore, four mean values for the marginal gaps were produced for each cast, two for each abutment. To guarantee the same degree of screw wear, a new abutment screw was used for each cast. The procedure was repeated for all experimental casts and for the master cast. Four mean values of the master cast represented the baseline error of this experimental design. The difference in mean values between the experimental casts and the master cast at the same site was defined as the deviation. The maximum value among the four deviations of each experimental cast was selected to represent the accuracy of each impression technique.

The means of the maximum deviation per group were calculated, and statistical inferences among the groups were made using analysis of variance and the Tukey test at the .05 level of significance.

RESULTS The results are presented in Tables 1 and 2 and Fig 6. Table 1 shows that the mean marginal gaps of the master cast ranged from 29.1 to 37.6 µm. Table 2 represents the mean values and standard deviations of the maximum deviations seen among the experimental groups. Analysis of variance revealed significant differences among the experimental groups (P < .05). The four experimental groups were analyzed with a post hoc Tukey test. A significant difference (P < .05) was found between the transfer (OT, NOT) and the pickup (NSP, SP) impression techniques. No significant differences were found between the OT group and the NOT group or between the NSP group and the SP group (P > .05).

DISCUSSION Generally, the abutment-level impression technique has been the favored technique for intraoral connection of abutments for internal-connection implant systems. However, selection of abutments can be difficult under conditions of extensive rehabilitation where vertical space or angulation of implants is inappropriate. Laboratory evaluation of the master cast produced from implant-level impressions facilitates the selection and correction of abutments and prostheses. Because of this, the implant-level impression technique is indis-


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Table 1 Mean Gap (µm) in Master Cast Left Abutment After Tightening Right One Buccal Lingual Mean SD

37.6 1.7

32.3 2.9

Right Abutment After Tightening Left One Buccal Lingual 36.0 1.0

29.1 1.3

Table 2 Maximum Deviations (in µm) in Experimental Groups Experimental group Octagonal transfer method Nonoctagonal transfer method Unsplinted nonoctagonal pickup method Splinted nonoctagonal pickup method

Mean

SD

Significance*

180.4

87.7

A

127.3

65.4

A

44.8

15.8

B

33.5

11.0

B

*Tukey test: (1) significant difference between groups A and B (P < .05), (2) no significant difference within group A, B (P > .05).

300 *

*

200 Dmax (µm)

pensable not only for external-connection implant systems, but also for internal-connection implant systems. Deep internal-connection systems allow the least room for the removal of implant-level impression copings from implants during the impression procedure. Therefore, deep internal-connection systems give rise to different considerations than external-connection systems with regard to impression procedures. For this study, the Inplant System ( Warantec), which is characterized by a tapered cone and an octagonal antirotational feature consisting of eight parallel walls, was selected. The internal taper allows a conical seal or friction fit, which results in a rigid connection between the abutment and implant. Internal octagonal parallel walls permit the abutments to be seated into place repeatedly on an implant. The accuracy of implant impressions is affected by various factors that are related to the condition of the implants (eg, number, angulation, connection type) and the impression technique (eg, type of impression copings, splinting, impression level, impression material, and impression tray). In this study, two angulated internal-connection implants were selected and tested with implant-level pickup or transfer impression copings. The necessity of splinting of the impression copings has been controversial in external-connection implant systems. A number of studies of impression accuracy of external-connection systems used standard abutments. Previous studies of impressions of internal-connection implants yielded varying results. Lorenzoni et al showed that the use of a transfer cap resulted in a more accurate impression in the pickup technique of the Frialit-2 System (Dentsply Friadent).26 Vigolo et al reported that the splinting of impression copings with acrylic resin produced less

100 ** **

0

OT

NOT NSP Impression technique

SP

Fig 6 Maximum deviations (Dmax) of the four experimental groups.

deviation of implant positions than unsplinted copings in the 3i Certain System (Biomet 3i). 25 Some studies have investigated implant systems with a conical seal connection. Akca and Cehreli reported no significant difference between pickup impression copings and snap-on plastic copings in the conical joint of the ITI Implant System (Straumann).28 Choi found no significant difference between splinted copings and unsplinted copings, regardless of the parallelism of implants, in the conical joint of the Astra Implant System (Astra Tech).27 Comparison of results from previous studies is difficult because different impression techniques and evaluation methods have been used. Standardized test variables for impression techniques and quantitative measurements are necessary to facilitate study comparisons and improve impression accuracy. The International Journal of Oral & Maxillofacial Implants 827


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In previous studies, a number of different methods were used to assess the accuracy of fabricated master casts. Most studies evaluated the positional change of implant analogs by measuring interimplant distance, distance from the reference plane, or the distance between reference lines with profilometers or a coordinate measuring machine. 12,16–18,20,23,25,26,28 Although these devices have resolution on the scale of micrometers, repeated measurement of a point or line is difficult to do. Several investigators measured prosthetic strain in a metal framework with a strain gauge after tightening of the abutment or gold screw. 13,15,22,27 Although strain gauges enable the measurement of deformation in multiple directions, the strain magnitude does not directly reflect the degree of misfit. Some experiments used microscopy to measure the displacement of analogs in test specimens compared to a master cast at selected points. 14,29 Unfortunately, manual inspection and measurements have limitations. In this study, the fit between a metal framework and master casts was evaluated. Because the gap distance between the margin of a restoration and the abutment has clinically well-proven implications, it has been measured microscopically in most studies analyzing the accuracy of fixed restorations on natural abutments. Accurate measurements have been achieved using a combination of a reflex light microscope and twodimensional geometric measurement software. Most commercial implant systems with conical seal connections have an abutment design that covers the implant platform to allow the abutment to seat itself into the implant. For the gap measurement, the abutment was fabricated to cover the implant platform and enable the outsides of both components to meet end to end. Because the implant platform had the outer bevel on the top, the baseline gap between the abutment and the implant was considered to represent complete seating of the abutment into the implant. This gap was considered as baseline error and measured from the master cast to calculate the actual deviation of the analog in experimental casts. The master cast was fabricated with type III dental stone and analogs and modified to achieve the best fit. Therefore the position of analogs in the master cast was stable. Possible error might be a result following wear of the analogs. Therefore, the authors made an effort to minimize this error. In this study the impression coping was hand-tightened, since handtightening torque has been reported to be about 10 Ncm.35 Four impression techniques were used one after another to minimize the accumulation of error in any specific technique. Jemt suggested the one-screw test for evaluating the framework fit of implant prostheses.29 This tech-

nique has been used for a long time for external-connection systems, especially for long-span multipleunit prostheses, and is effective in clinical and laboratory procedures. In this experimental study, the gaps between abutments and analogs were reduced on the tightened side while being increased on the other side as one abutment screw was being tightened. After two abutment screws were tightened, the gaps were reduced. The reduction of gaps caused strain to develop in the metal framework and other components. In the present study, one abutment screw was tightened with 10 Ncm torque so that the abutment could be minimally seated into the analog and the implant-abutment gap at the other abutment could be clearly expressed. Nonparallelism of implants is a common clinical problem because of anatomic limitations and esthetic considerations. Despite this, most previous studies only tested situations involving parallel implants. In one previous study, the Astra implant system allowed the pickup impression coping to diverge up to 8 degrees from the implant, and splinting of the pickup impression coping was possible without modification.27 Octagonal pickup impression was excluded from the present study. In a pilot phase of the study, excessive friction was necessary to remove the abutmentimplant complex from the master cast after the impression material had set. Wear of components and distortion or breakage of the impression and tray occurred as results. Therefore three impression copings were fabricated for four impression techniques in this study. The octagonal transfer coping and nonoctagonal transfer coping were fabricated for the transfer technique. A nonoctagonal coping was made by removal of octagonal parallel walls because the resultant mating surface of the abutment and analog was mainly confined to the conical area. Nonoctagonal pickup copings were made for splinted and unsplinted pickup impression techniques. Greater error was observed in the transfer technique groups during repositioning of the impression coping into polyether impression. Clinically, greater force may be necessary to remove the impression from the master cast in the transfer impression technique, and the impression material might be distorted as a result. It is assumed that this result is partially attributable to the angulated condition of the implant analogs. A limit of the results of this study is that it is not known whether the implant connection type can affect distortion of the impression coping in impression material upon removal of the impression material from the master cast. The presence of the octagonal feature in the impression coping did not affect the accuracy of the


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experimental casts. The octagonal feature might have no effect in friction between the impression coping and the analog. In this study, with the pickup method, splinting of the impression coping resulted in no more accurate experimental casts versus the unsplinted method. Because all pickup impression copings were modified with the nonoctagonal design, removal of impression copings after the impression material had set required minimal force and thus caused minimal stress to develop. Therefore it is assumed that the rigidity of the impression tray and material might ensure that the coping does not become distorted during removal of the impression. This result is consistent with a previous study of conical-connection implants.26 Although a clinically well-defined marginal gap was measured in this study, comparison of these results with those of other studies was difficult. Further studies will be required to evaluate the clinical relevance of three-dimensional movements of impression copings inside impression material and their effects on the marginal gap.

CONCLUSION Within the limitations of this study, it was concluded that: 1. The master casts created using nonoctagonal pickup impression techniques were more accurate than those created using transfer impression techniques (P < .05). 2. There was no statistically significant difference in the accuracy between the nonsplinted and splinted methods in pickup impression techniques (P > .05). 3. There was no statistically significant difference in the accuracy of the octagonal coping and the nonoctagonal coping in transfer impression techniques (P > .05).

ACKNOWLEDGMENT This work was supported by a grant from the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (02PJ3-PG6-EV11-002).

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The International Journal of Oral & Maxillofacial Implants 829 © 2009 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART OF THIS ARTICLE MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.

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