Dimensional accuracy of impression techniques for the endosteal implants (An in vivo study): Part II

Aims: To determine the most accurate impression techniques and materials to transfer the exact position of a single implant to stone die to construct the prosthesis for selected clinical cases. Materials and methods: Five clinical cases were selected with special criteria to construct implant (Frialit–2) prosthesis on a stone die with high accuracy according to the results of study. Four impression techniques were used (direct and indirect, each with one and two steps) using condensation, addition (heavy, medium and light consistencies) silicone impression materials. Ten impressions were taken for each patient to produce a total number of 50 stone casts. The direct and indirect measurements were performed by using digital vernier. Results: The significant differences between the impression techniques at the three axes of dimensions (p ≤0.05) were applied clinically to construct the fixed prosthesis for each patient. The results of this study showed that the high dimensional change of laboratory cast was related to indirect one step impression technique to transform the implant position from patient mouth to laboratory cast. Addition curing (medium consistency) silicone impression material produced the lowest accurate stone die. Conclusions: The results of this study showed that the direct two steps impression technique was the accurate impression technique for transformation of implant position from patient mouth to laboratory cast. Additional curing (light and heavy consistency) silicone impression material produced the most accurate stone die and gave a successful treatment to the patient.


INTRODUCTION
Several implant impression techniques have been advocated for transfer of implant position before construction of the prosthesis. In relation to screw-type titanium implants, impression transfer copings may be retained either within the impression as it is removed from the mouth (direct technique) or retained on the implant and later removed, and replaced in the impresssion which is called indirect technique. (1) These techniques have been investigated with varying result. (2)(3)(4) Several authors reported that the direct method is more accurate. (5)(6)(7) One study observed better fitting castings is produced by the indirect impression technique (8) where as other studies found no difference in the accuracy of master casts when either methods were used. (9,10) Many factors are important in sele-cting an impression material, some of which are accuracy, dimensional stability, working time, storage time, shelf life and taste. (11,12) Aims of this study were to detect the most accurate impression technique, and types of impression material used to transfer single or multiple implants position from the master model to the stone dies by two methods of measurement (An in vitro study).

MATERIALS AND METHODS
Two maxillary partially edentulous models (Frascaco,Franz Scachs and Co. GMBH, Germany, Class I, and Class III Kennedy classification ) were used to construct the master models. Three implants (Friatic system ,Friadent Co.,Germany ) were used to be fixed to the master mode-Dimensional accuracy of impression techniques for the endosteal implants (An in vitro study): Part I ls: one implant fixture (15mm length and 3.8 mm diameter) was fixed to the bounded case, and two implants fixture (13mm length and 3.8mm diameter ) were fixed to the free end saddle case. A computerized micro motor hand piece (Elcomed 100, microprocessor controlled, W&H, Austria) with different drilling burs was used for drilling the sockets that receive the implants fixtures.
One type impression stock tray was used for all steps to take an impression by using three types of silicone impression materials( Condensation Ormadent Major Italy, light and patty, Addition Perfexil Septodont France Medium Body and President Colten Switzerland Low Body, and Putty Type). The impression was made by using mechanical apparatus (13) that secure a consistent master model position within the impression tray, providing the desirable thickness of impression materials, identical direction of insertion and removal of the upper metal plate with the tray that contains the impression material. A certain modification was performed to this apparatus to facilitate using it in the open tray technique.
Four impression techniques were used in this study which are direct (open tray) one step (14) , direct two steps, indirect (close tray) one step, and indirect two steps. For the medium body, two techniques only used (direct and indirect one step). Dental stone was used to pour each impression. Five impressions were taken with each technique for every master model of single, and multiple implants to produce hundred impressions that when poured will produce a hundred stone dies: a. Indirect (one step)and (two steps) techniques: Plastic transfer caps were fited to the metal transfer coping, which were screwed to the implant fixture on the master cast with short screw No.4305 ( Figure 1). Stock tray with test apparatus was used for holding a silicone rubber base impression material. Impression along with the plastic caps was removed from the master cast. The metal copings were unscrewed from the abutments and fixed with connecting screw No.4305 to laboratory analog. The assembled metal copingsanalogs were then pressed into keyed position (flat surfaces) in the plastic transfer cap within the impression. The circumferential groove provides a vertical stop and will hold the coping in place while the impression was being poured (Figure 2).
In putty-wash (one-step) impression technique, both phases of the impression material were placed in the tray at the same time and the light body material was injected around the abutments, followed by immediate placement of the tray loaded with the heavy body impression material. The mixing, and setting time were controled by a timer, and the method was kept almost constant for all the trials.
The double-impression procedure (two-steps) was used in which a prelimnary impression was taken in putty like consistency material. The standard spacer (3M Dental products, USA) was placed over the master model to provide enough space for the light body material. The impression materials were allowed to set for 15 minutes from the start of mixing; the manufacturers setting time was doubled to compensate for a delayed polymerization reaction at room temperature. (15) b. Direct (one step)and (two steps) techniques: A connecting screw No.1615 (long) was used to connect the transfer coping to the implant fixture ( Figure 1). The stock tray was modified by creating window openings over the position of the transfer copings to continue with the openings that are prefabricated in the upper part of the test apparatus. The stock tray was loaded with silicone impression material (in one step technique) and pressed gently over the master model. The excess of the impression material bulging out through these opening was removed to clear the slot head of the connecting screw. After the impression material has been set, the connecting screw was disconnected through the openings and the disconnection should be continued until two or three clicks are heared to insure that the transfer coping was completely separated from the implant fixture.
At this step the impression would be separated from the master model gently. The transfer copings would be retained inside the impression (pick up) and separated from the implant fixture ( Figure 2). Then the laboratory analogs would be screwed to the transfer copings. While they are still in their place inside the impresssion, careful screwing should be taken place not to over torque the screw but to prevent the rotation of the transfer copings inside the impression material.
Steps of direct Two step technique was followed as described previously. The impressions then were left on the bench for about 15 min. before pouring in order to get maximum precision according to manufacturers' instructions.
All impressions were poured with Silky Rock die stone. The water/powder ratio was 100gm of powder added to 23ml of distilled water. All stone dies were separated from impression after 1 hour. (15) Stone dies were based with stone using a tripod-method on a surveyor, this technique facilitated the orientation of the reference surfaces on the same plane, and aided in measurement. All stone dies were allowed 24 hours before they were tested for accuracy, in order to obtain maximum dryness, hardness and strength. (16) Two methods for measurements were applied, one by using the digital caliber of 0,001 mm accuracy (using surveyor), and the second measurement was done by using the optical traveling microscope (accuracy of 0,001mm at magnification power x10) to obtain the x, y and z axes on the master model and stone dies (   Descriptive, One sample t-test, Analysis of variance (ANOVA), and Duncan's Multiple Range Test were used in order to analyze, and assess the results.

RESULTS
One sample t-test was used to compare between the means of the three axes of the master models, and the means, and sta-ndard deviation of 100 stone dies resulted from using three brands of silicone impression material and four impression techniques, and to show if there was any signifycant difference between the levels of variable in this study; the result showed that there was highly significant difference on cast accuracy between these variables up to p 0.001 (Tables 1-4).     Analysis of variance (ANOVA) for the dimensional accuracy of stone casts resulted from using of four impression techniques showed that there were significant differences between most of the variable levels, Tables (5 and 6). Duncan's multiple range test showed that open tray two steps impression techniques were the most perfect impression techniques (Figures 4-7).   Analysis of variance for the dimensional accuracy of stone casts resulted from using of three brands of silicone impression materials showed that there was a significant difference between most of the vari-able levels Tables (7 and8). Duncan's multiple range test showed that the addition curing (two phase) impression materials was the best impression materials used in dental implant (Figures 8-11).      Paired t-test was used to compare between the accuracy of measurements obtained from digital caliber and optical microscope methods as shown in Table ( 9). This table showed that there was a significant difference between these two methods of measurements in (Z) axis for both single, and double implants case, where as there was no significant difference in the other axes.

DISCUSSION
Three-dimensional measurements (X, Y and Z) of stone cast produced from direct (open tray) two steps technique was the most accurate technique for transferring implant position to the laboratory cast as shown in Figure (4-7). This can be explained in that the transfer coping in this tech-nique is still inside the impression materials during its separation from master model, and when connecting the implant analog to the implant fixture. By this way the distortion in the impression materials at the site of transfer coping was avoided, unlike the indirect (close tray) impression technique, in which the transfer coping was separated from the impression material during the separation of impression from the master model and reseated again after connecting it to the implant analog to its place inside the impression material. By this way distortion of the impression material at the site of transfer coping during removal, and reseat again cannot be avoided, and this will affect the accuracy of transferring the implant position especially in (Z) axis. cast accuracy than the one step especially with addition curing silicone impression material. The numbers of dental implants have no significant effect on the accuracy of stone cast.