The Success Of Implant

Published Date: 02 Nov 2017

Abstract

Introduction. The success of implant – supported prosthesis is dependent on the passive fitness of the framework.

Purpose. The purpose of this in vitro study was to compare the 3-dimensional accuracy of six implant impression techniques digitally.

Material and methods. The master model simulated two clinical conditions; first condition was a partial edentulous mandibular arch with an anterior modification (Dentate condition). Two implant analogs were inserted in bilateral canine sites. After elimination of the teeth, the model was converted to the second condition (Edentulous condition). 3 different impression techniques were taken (open splinted, open un-splinted, closed) from each condition. Six groups of casts (DOS,DOU,DC,EOS,EOU,EC) (n=8) with total of 48 casts were created. Two scan-bodies were secured on the master edentulous model, each test cast, digitized by an optical scanning system (12μm accuracy). The point clouds of the test casts and the reference model were superimposed and mean discrepancy for each cast was determined. The univariate analysis of variances was used to compare the 3-D discrepancies. The Kolmogorov- smirnov test was used for the analysis of normality.

Results. The statistical analysis showed no significant differences in the accuracy of casts as a function of model status (p=0.78) or impression technique (p=0.57) or the combination of them (p=0.29). The Kolmogorov- smirnov test showed the normality of data (p>0.05).

Conclusions. Model status and the impression technique didn’t influence the precision of casts. There is no difference between either impression techniques in both clinical simulated conditions.

Clinical relevance: the results of this study indicate that the accuracy of all implant- level direct impressions is equal regardless of impression technique or condition of dentition.

Key words: dental implants, impression techniques, three dimensional scanning, edentulous, partially edentulous

Introduction

Long term success of dental implant supported prostheses is dependent on the passive and precise fitness of the framework (1-2) .due to differences in the range of motion of natural teeth and osseointegrated implants ,lack of passivity will result in biological (3-4) and mechanical complications such as screw loosening, screw fracture, implant fracture, loss of osseointegration (1, 5-9) .Impression taking process is the first step that may potentially create misfit. There are two main techniques for taking impressions of dental implants, called direct and indirect techniques. In the direct technique the impression copings are picked up by the impression material whereas in the indirect technique the impression copings remain on the fixture or abutment, after that are removed, connected to the lab analogs and transferred to the impression (10-11).

Many studies have reported the direct technique to be the more accurate method (12-17). although the differences that were statistically significant may not be clinically different (18).

Implant level direct impression is originally an open tray technique. Some manufactures have fabricated closed tray snap-fit direct impression copings; there is little work in the literature to verify the accuracy of this type of impression copings (11, 19).

Several authors have investigated the effect of splinting the impression copings of direct technique on the master cast accuracy. The majority of them have reported better results with splinting (14, 16, 20-26).although some have demonstrated that dimensional change of splinting material is a concerning issue (27-28). Some found no significant difference between the results obtained by splinted and un-splinted impression copings (15, 29-32).although some authors reported more accurate results with the non splinted one(27, 33).One of the advantages of splinted techniques is the greater transfer precision because of the stability during the removal of the impression and analog connection (34).

Polyether and polyvinylsiloxane are commonly used as implant impression materials and it has been reported that they are of similar accuracy as far as taking impressions of dental implants are concerned (10).so we used a medium viscosity additional, elastomeric monophase impression material (Monopren® transfer, Kettenbach, Eschenburg,Germany) for this study.

The majority of research related to implant impression techniques has been carried out on the edentulous conditions and little done on partial edentulous conditions. Based on the knowledge of the authors of this article no study was found comparing the edentulous and partially edentulous conditions. It is hypothesized that the difference between edentulous and partially edentulous conditions may affect the accuracy of definitive cast; in dental implant impression taking the material of choice is generally a material with more stiffness. So the undercut areas related to dental and interdental tissues can complicate impression taking(35). As the stiffness of impression material is increased removing the impression from undercuts will be difficult and the force necessary for tray removal may affect the impression copings position in the material.

Gypsum materials have some expansion during setting; the more the bulk of gypsum the more the expansion. It seems that the difference between the volumes (bulk) of gypsum material in the impressions taken from the master model in the two conditions (edentulous and partial edentulous) could affect the position of implant analogs in the poured cast (7, 15).

Several measuring tools have been used by authors for assessment of distortion values in different investigations, such as digital micrometer (7, 17), optical microscope (12, 29, 32, 36). Strain gauges (7, 37-39), coordinate measuring machine (CMM) (40-42), photogrammetry (43-44) laser videography(45). Recently 3-dimensional optical digitization systems have become available in dental technology and many CAD/CAM dental systems have been introduced, some authors have used these systems for measuring distortion in the assessment of implant definitive casts (16, 18, 46-47).

The purpose of this study was to compare the dimensional accuracy of six impression techniques created by the combination of three direct impression techniques (closed, open splinted, open un-splinted) and two clinical simulated conditions (edentulous, partially edentulous) by 3-dimensional optical scanning.

The null hypothesis was that there is no difference between the accuracy of casts obtained from either impression technique and/or model condition.

Material and methods

In this experimental study two simulated clinical conditions were assessed; partially edentulous and edentulous (overdenture) conditions.

I) partially edentulous condition (dentate condition; D condition)

1- Model preparation

The model simulated a partial edentulous condition (kennedy CL-IV-I) with two stainless steel tissue level regular neck implant analogs (ITI Dental Implant System ,Straumann ,Waldenburg, Switzerland) positioned 1 mm supragingivally and parallel to each other. These analogs served as replicas of the fixtures inserted in bilateral mandibular canine sites. The center to center distance was 24 mm. The model was mounted on impression system. The system was composed of a base with four vertical bars connected to it and an upper plate connected to the vertical bars. A hole was made into the center of upper plate for the passage of vertical handle of trays. This system provided a single path for tray removal (fig 1).

2- Tray fabrication

Two open tray impression copings (ITI Dental Implant System, Straumann, Waldenburg, Switzerland) were secured on the model and a primary impression was taken by a condensation silicon (Speedex,Coltene,Germany), the lab analogs were connected ,the impressions were poured with type IV dental stone (GC Fujirock EP; GC Europe,Leuven, Belgium) .the created cast was used for making special trays and splint bars. For each impression technique the related impression coping was positioned on the cast and 3 customized trays were made with an autopolymerizing resin (Acropars,Tehran,Iran).these trays were casted and converted to metal trays (Al alloy), perforated uniformly. Each tray had a vertical handle connected to center of its lingual portion. The handle passed through the upper plate of the impression system. A metal plate was screwed to the tray handle. The roles of this plate was; 1- site of seating force applying, 2-removal force handle 3-stable base for trays during impression pouring.

3- Impression taking

Three direct impression techniques were performed (n=8).

Splinted technique(dentate open splint group; DOS group)

Splint bars fabrication

A 3mm thick bar was used. 16 bars with length of 15mm (0.5 mm shorter than the inter impression coping distance were cut.

Two Open tray square impression copings were secured to the model (hand tightened). One day before impression taking splint bar was connected to one of the impression copings with autopolymerization acrylic resin (pattern resin, GC Corporation, Tokyo, Japan). The other impression coping was connected to the bar 5min before impression taking .A silicon adhesive (Panasil Adhesive, KETTENBACH, Germany) was applied to the internal surface and borders of metal tray and waited until they get dried. A medium body polyvinylsiloxane impression materiel (Monopren® transfer, Kettenbach, Eschenburg,Germany) was dispensed with an automix cartridge . Into the metal tray and around the impression copings and the tray was seated on the model. A 1.5 kg weight was put on top of the metal plate during the setting of impression material (5mins) (fig4). After complete polymerization of material the positioning screws were loosened and the tray was removed in vertical direction.

Then the lab analogs (ITI Dental Implant System, Straumann, and Waldenburg, Switzerland) were screwed to the impression copings while the analogs were held with a hemostate and hand tightened.

Un-splint open-tray technique(dentate open un-splint group ;DOU group)

Except for splinting, the protocol was the same as DOS group.

Closed tray technique(dentate closed group; DC group)

The impression caps and the positioning cylinders (ITI Dental Implant System, Straumann, and Waldenburg, Switzerland) were snapped onto the necks of implant analogs, synOcta plastic positioning cylinders were placed in the impression caps. The procedure was the same as for DOU group. After the material was set the lab analogs (ITI Dental Implant System, Straumann, and Waldenburg, Switzerland) were snapped on the impression copings.

Edentulous (overdenture) condition

Model preparation

The teeth on the dentate model were eliminated and the model was converted to an edentulous condition, simulating a clinical implant supported overdenture condition with two implants in mandibular canine sites (fig 2).

The same protocol was performed as partially edentulous condition (steps 2, 3) .The groups created were(fig 3);

Splinted technique(EOS group)(n=8)

Un-splinted open tray technique(EOU)(n=8)

Closed tray technique(EC group)(n=8)

After analog connection the impressions were poured.

Pouring the impressions

The procedure was performed as two step pouring technique with a die stone (GC Fujirock EP; GC Europe,Leuven, Belgium) ) in a vacuum mixing system (Multivac 4; Degussa GmbH, Hanau, Germany)according to the manufacturers recommendations(45 seconds mixing time, powder/water ratio 1:5).after 1 hour the positioning screws were loosened and the trays were separated from the casts.

All clinical and laboratory procedures were performed by the same clinician.

Scanning

The cover screws were fastened on the model implant analogs and an opaquing medium (Spotcheck® SKD-S2 Non-Halogenated Solvent Developer, Magnaflux, Sweden, UK) was sprayed on the model. after unscrewing the cover screws two scan-bodies (custom made) were secured on the model , the model was positioned in an industrial white light scanner (REXCAN III,SOLUTIONX, Seoul, Korea).after completion of the process the data were saved as .STL in a computer file. For each test cast each scan-body was accurately transferred to the same specified orientation as on the model. The scanning process was repeated for all test casts.

Superimposition

This process was accomplished with an industrial software (geomagic qualify v12; GEOMAGIC, Morrisville, USA).The .STL data related to edentulous master model and each test cast were imported into the software and specified as reference, test respectively. Only areas related to scan-bodies were used for superimposition and other portions were deleted. Point clouds of test casts were superimposed to point clouds related to the master model by best-fit algorithm. 25000 points were considered in each superimposition. Before applying the best-fit the scans were fitted by 3-2-1 algorithm and then fine adjustments were performed. After each superimposition a report was taken and the calculated mean error was recorded and the 3-dimensional color maps were saved (fig 5). In addition, for calculation of base line (systematic) error of scanning and superimposition the point clouds pertaining to the left scan-bodies of 10 scans were superimposed to the scanned model.

Data analysis

The calculated mean errors were imported into statistical software (SPSS 16.0; INC, Chicago, III) for data comparison. Univariate ANOVA was used for analyzing the relationship between the variables. The level of statistical significance was set at 5% .for analysis of normality Kolmogorov- smirnov test was performed.

Results

Six groups with 8 casts each were created, for a total of 48 casts.

The descriptive statistics (mean, SD, range) are presented in table 1.

For the edentulous groups the calculated mean discrepancy was 25.7(SD 6.4), the lowest value was 12μ (EOS group) and the highest was 37μ (EOS group).

For the partially edentulous groups (D groups) the calculated mean discrepancy was 26.3(SD7.9), the lowest value was 15μ (DOU group) and the highest was 43μ (DOS group).

Superimposition of left scan bodies demonstrated a discrepancy of 12(SD 1μm).

Thus the baseline error (systematic error) was 12 μm.

The kolmogrov-smirnov test showed the normal distribution of data (p>0.05).

Neither the reference model status, impression technique nor the combination of them had a significant effect on the mean 3D discrepancy (table2).

Table 1. Descriptive data of the discrepancy of groups

N

Mean

Std. Deviation

Std. Error

95% Confidence Interval for Mean

Minimum

Maximum

Lower Bound

Upper Bound

EOS

8

25

8.86405

3.13392

17.5895

32.4105

12.00

37.00

EOU

8

24.6

4.30739

1.52289

21.0239

28.2261

19.00

30.00

EC

8

27.6

5.78020

2.04361

22.7926

32.4574

19.00

34.00

DOS

8

29.8

7.19995

2.54557

23.8557

35.8943

17.00

43.00

DOU

8

24.7

8.46421

2.99255

17.6737

31.8263

15.00

39.00

DC

8

24.3

7.87287

2.78348

17.7931

30.9569

16.00

41.00

Total

48

26

7.16064

1.03355

23.9624

28.1209

12.00

43.00

EOS: Edentulous Open Splint EOU: Edentulous Open Un-splint EC: Edentulous Closed

DOS: Dentate Open Splint DOU: Dentate Open UN-splint DC: Dentate Closed

TABLE 2. Univariate Analysis of Variance table. Dependent variable: mean 3D discrepancy. Independent variables: reference model status, impression technique

Source

Type III Sum of Squares

df

Mean Square

F

Sig.

Corrected Model

197.917(a)

5

39.583

.752

.590

Intercept

32552.083

1

32552.083

618.078

.000

STATUS

4.083

1

4.083

.078

.782

TECHNIQU

60.542

2

30.271

.575

.567

STATUS * TECHNIQU

133.292

2

66.646

1.265

.293

Error

2212.000

42

52.667

Total

34962.000

48

Corrected Total

2409.917

47

Discussion

This study found no significant effect of impression technique and/or model status (condition) on the accuracy of definitive casts. Therefore, the null hypothesis wasn’t rejected. It was hypothesized that there was no difference in the accuracy of impressions between the edentulous and partial edentulous conditions based on the difference in undercuts and difference in the bulk of gypsum material. The dental and interdental undercut areas could complicate the impression procedure(35). Locking stiff impression material into the large undercuts necessitates increased force for removing the tray that might be in any direction .Thus permanent deformation of impression material or movement of the impression copings inside the impression material would be possible. However this was not confirmed by our results .This can be partly attributed to the fact that the undercut areas in the master model were not as deep as in a real clinical situation. Moreover the strain tolerance of polyvinylsiloxane impression materials makes distortion of impressions lower than the polyether impression materials(35). There are situations in which the condition will be more complicated such as gingival recessions, drifted/extruded/tilted teeth. It seems that the use of a stiffer impression material such as a polyether would result in less accurate impressions in the partial edentulous status(48).

The difference in the bulk of gypsum also was not effective on the cast accuracy in all experimental groups. This could be explained by very low setting expansion (0.018%) of the selected die stone and / or the high stiffness of impression material and/or splinting material that would resist against setting expansion forces.

In addition the study conditions were standardized, although in routine clinical condition these may not be considered precisely or not be possible to be done completely. Moreover the in vitro conditions differ from in vivo environment. For example inability to control saliva may prevent the impression material from close contact to the impression copings and this potentially affects the impression accuracy especially for the non-splinted techniques.

The removal of trays was in vertical direction. However in clinical situation this is difficult to achieve and the exerted lateral forces would create lateral stresses on the impression material, affect the positions of the impression copings. Perforations and the applied adhesive provided the retention required for the impression material from pulling off the tray.

Researchers have employed different devices (methods) for assessing the accuracy of impressions or fitness of frameworks in their studies. However the results may be dependent on the type of technique used(49). The different results reported in the studies could be explained to some extent by the different machining tolerances of components, by the differences in methods and materials used for accuracy measurements.

The accuracy of scans was adequate. According to the manufacturers statement the scanning error is less than 10 microns in their scanning system (Rexcan III,solution,). In our study by superimposition (best fit) of the left scan-bodies the base line error (systematic error) was 12(SD=1) microns that was the sum of scanning system error and software error. In diverse optical scanning studies the stated precision was ≤20 microns (18, 46, 50). Although the configuration and the height, design of scan-bodies has been different.

All techniques were similar in accuracy (C, OS, and OU) regardless of model status (E or D). Previous in vitro studies comparing splinted and non-splinted impression techniques have reported diverse results. Several studies have shown better results with splint technique (14, 16, 20-24, 26, 51).Liou AD(52) and also Rashidan(53) showed that the impression copings with different designs have different accuracies. Humphries et al(54), Hsu et al(55), and Herbst et al (29) found no significant differences between the acrylic resin-splinted groups versus un-splinted copings. Inturregui et al(33) and Burawi et al(27) reported that the splinted technique was less accurate than the un-splinted technique. In a study performed by Naconecy MM (37) the accuracy of non–splinted impression copings was similar to the indirect method (37). It may be due to the low retentive form of the open – tray impression copings(21). Our study showed that the design of impression coping could provide the required stability in the impression material (Fig). The open-tray impression copings used in this study had a retentive configuration that could help engagement of the impression coping firmly in the stiff impression material. This might be the cause of similarity of results in splinted and non-splinted groups.

B

C

C

B

AC:\Documents and Settings\majid\Desktop\capture1.jpgC:\Documents and Settings\majid\Desktop\capture2.jpg

A

Fig 6. The inaccuracy of impressions takes place in three sites. (A) in the connection site , (B) junction of impression coping and splint or impression material, (C) impression / splint material.

Also Lee H(56) in his systematic review stated that in cases with limited number of implants there is not any difference between splinted and non-splinted techniques (56).

The concerns about splinting include shrinkage of splinting material during setting, fracture of the material between impression coping and the splinting material and the increased chair time.

The greatest discrepancy calculated in this article was 43 microns (DOS group).this value is the mean of misfits of all points from connection to the most coronal portion of scan-bodies.the greatest values are related to the most coronal points and the smallest values are related to points adjacent to the connection. According to Lee H(56) the maximum acceptable discrepancy was 0.6-136 microns calculated at the connection level. Jemt T(57) believed that a level of biological tolerance exists. He demonstrated that a vertical gap of 50-100 microns is acceptable. Other reports indicated that a tolerance exists between implant and abutment due to a machining tolerance and rotational freedom between the machining components (58-60).Ma et al(58) reported that the measured tolerances ranges from 22 to 100 μm. So all test groups in this study were at acceptable clinical range. The lack of any reference value for defining misfit makes the comparison between results of different researches difficult.

There are situation that necessitate the use of the closed tray techniques such as limited mouth opening, tendency to gag, difficulty to access the posterior region of the mouth (52).until now little research has been performed on snap-fit impression copings .Burawi G(27) ,Cehreli MC (11) have showed that the use of snap-fit impression copings is more precise than the open-tray impression copings. Daoudi MF (61) stated that the snap-fit impression taking is more accurate than transfer technique. Our results is in accordance with the findings of Akca K(11) that has worked on the same implant system (ITI Dental Implant System ,Straumann ,Waldenburg, Switzerland).One advantage of snap-fit impression copings is that the rotation of impression copings during analog insertion will be decreased because there is no screw fastening. However the cost will be increased with this type of copings because it is disposable.

Authors have used different tools to assess the 2 or 3 dimensional misfits, although by use of methods such as light microscope or digital micrometer some information was lost, because assessment was performed in only 2 dimensions. The advantages of 3-dimensional scanning include high accuracy, reliability, low operator error and 3 dimensional data production. In the present study only mean values of 3D discrepancies was reported although the discrepancies might be in different directions.

For recommending the best impression technique that is accurate clinically a passive reference framework is required to be tested on each produced cast for evaluation of fitness. Although because of machining tolerance (58-59) and rotational misfit of paired implant components some amount of misfit related to impression techniques would be hidden.

The implants (implant analogs) inserted in the master model were nearly parallel and as tissue level. However the angle or the depth of implants has shown to be effective on the accuracy of impressions (62-63). Although some studies reported no effect (30, 64). So the effect of this variables and the extent of impact on each status (edentulous vs. partially edentulous) needs to be explored.

Conclusions.

Cast accuracy was not affected by model status. The difference in undercuts and the bulk of type IV die material didn’t change the accuracy of casts.

Cast accuracy was not affected by impression technique. The open tray splinted, open tray un-splinted and closed tray techniques have the same accuracy. There is no preference to use any of these techniques for implant impression taking.

There is no difference to use any of the techniques in any clinical simulated conditions.

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