|Year : 2014 | Volume
| Issue : 5 | Page : 635-640
|Strain gauges's analysis on implant-retained prosthesis' cast accuracy
Mariana A Rodrigues1, Leonardo F Luthi1, Jessica MFK Takahashi2, Mauro AA Nobilo1, Guilherme EP Henriques1
1 Department of Periodontology and Prosthodontics, Piracicaba Dental School, Campinas State University (UNICAMP), Piracicaba, São Paulo, Brazil
2 Amazonas State University, Manaus, Amazonas, Brazil, Brazil
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|Date of Submission||14-May-2013|
|Date of Decision||03-Oct-2013|
|Date of Acceptance||14-Jul-2014|
|Date of Web Publication||16-Dec-2014|
| Abstract|| |
Introduction: A proper cast is essential for a successful rehabilitation with implant prostheses, in order to produce better structures and induce less strain on the implants.
Aims: The aim of this study was to evaluate the precision of four different mold filling techniques and verify an accurate methodology to evaluate these techniques.
Materials and Methods: A total of 40 casts were obtained from a metallic matrix simulating three unit implant-retained prostheses. The molds were filled using four different techniques in four groups (n = 10): Group 1 - Single-portion filling technique; Group 2 - Two-step filling technique; Group 3 - Latex cylinder technique; Group 4 - Joining the implant analogs previously to the mold filling. A titanium framework was obtained and used as a reference to evaluate the marginal misfit and tension forces in each cast. Vertical misfit was measured with an optical microscope with an increase of 120 times following the single-screw test protocol. Strain was quantified using strain gauges. Data were analyzed using one-way ANOVA (Tukey's test) (α =0.05). The correlation between strain and vertical misfit was evaluated by Pearson test.
Results: The misfit values did not present statistical difference (P = 0.979), while the strain results showed statistical difference between Groups 3 and 4 (P = 0.027).
Conclusions: The splinting technique was considered to be as efficient as the conventional technique. The strain gauge methodology was accurate for strain measurements and cast distortion evaluation. There was no correlation between strain and marginal misfit.
Keywords: Biomechanics, dental models, implant-supported dental prosthesis
|How to cite this article:|
Rodrigues MA, Luthi LF, Takahashi JM, Nobilo MA, Henriques GE. Strain gauges's analysis on implant-retained prosthesis' cast accuracy
. Indian J Dent Res 2014;25:635-40
The success of oral rehabilitation with osseointegrated implants linked to an accurate fit between prosthetics' components and implants.  Many injuries to the implants can be mitigated by increasing the accuracy of prosthesis fabrication techniques.
|How to cite this URL:|
Rodrigues MA, Luthi LF, Takahashi JM, Nobilo MA, Henriques GE. Strain gauges's analysis on implant-retained prosthesis' cast accuracy
. Indian J Dent Res [serial online] 2014 [cited 2021 Dec 5];25:635-40. Available from: https://www.ijdr.in/text.asp?2014/25/5/635/147113
An adequate mold is essential to implant rehabilitation success. It should reproduce the same implant characteristics presented clinically. Beyond an accurate mold, its maintenance is expected when the mold is filled with plaster, due to the gypsum's crystallization, because of hygroscopic expansion, the cast accuracy could be impaired.
It can be considered that the gypsum expansion is related to its crystallization volume, so one possibility is to fill the mold in two stages. A second-alternative technique advocates that implant analogs should be involved by a latex cylinder and filled in two stages (mitt technique).  Another possibility involves gathering the implant analogs using metallic bars.
Nevertheless, irrespective of the technique, to ensure its efficacy, the casts must be evaluated and compared with the original situation. The comparison is based on linear measurements in most studies. ,, However, these measurements can be incomplete, leading to precipitated conclusions. Thus, it was necessary to evaluate different techniques by an accurate methodology to judge the techniques' accuracy.
| Materials and methods|| |
A stainless steel master cast [Figure 1] was based on a mandibular gypsum cast, and it contained two tapered mini pillars 3.75 mm standard platform (Conexão Sistemas de Próteses, Sao Paulo, Brazil). The two implant analogs were fixed by cyanoacrylate adhesive (Loctite Super Bonder, Henkel KGaA, Sao Paulo, Brazil) into the first premolar and first molar positions, simulating a three unit fixed prosthesis.
A custom tray was made to ensure uniform thickness of the impression material. This tray was made from the polymethylmethacrylate resin (Jet, Classico Odontological Goods Ltd., Sao Paulo, Brazil), with an orifice corresponding to the place where the square impression transfers were positioned. Three cylinders were placed on a tray like a tripod to ensure stability, whereas the cast was poured. The base was made with notches to fit the custom tray using vinyl polysiloxane (VPS), so a uniform pattern model was obtained according to the master cast.
Transfers were adapted on the level of the attachment (Conexão Sistemas de Protese, Sao Paulo, Brazil) and then were used to make the working casts. The impression transfers were splinted to reduce possible distortion of the working casts due to movement when removing the mold. The transfers were placed over the master cast and linked using dental floss covered with methylmethacrylate pattern resin (Pattern Resin; GC America Inc., Alsip, United States). After polymerization, the joint was sectioned with a diamond wheel and linked again using methylmethacrylate pattern resin. Next, the impressions were performed using high- and low-viscosity VPS (Flextime, HaerusKulzer, Hanau, Germany) following a double mixture pattern. To stabilize and standardize the insertion force, 1000 g device was placed over the custom tray.
The custom tray was removed from the metallic cast after the impression material set (10 min) and the implant analogs were bolted to the mold transfers. Forty (n = 10) gypsum working casts were made using this technique. Type IV gypsum was manipulated following the manufacturer's instructions (100 mg: 20 ml) under mechanical mixing (Multivac 4 Degussa, Germany). These casts were distributed over four groups following its different treatments [Diagram 1]:
- Group 1 (control): 10 working casts were made through conventional mold filling
- Group 2: 10 casts were made through two-step mold filling
- Group 3: 10 casts were made using the latex cylinder technique
- Group 4: 10 casts were made by joining the implant analogs prior to the mold filling.
Thus, for Group 1, the molds were poured slowly in small amounts with type IV gypsum (Herostone, Vigodent, Rio de Janeiro, Brazil) following the conventional poured technique. After 60 min, the cast was removed and stored at a controlled temperature.
For Group 2, gypsum was poured into the mold up to half of the analogues' length. After 30 min, more gypsum was poured to complete the mold filling. Sixty minutes later, the cast was removed from the mold.
The casts from Group 3 were made using a latex cylinder around the analogues. After its placement, the gypsum was poured into the mold, so that it did not touch the analogs. The gypsum crystallized, and the latex cylinders were removed. A second gypsum portion was poured into the space left around the implants' analogues [Figure 2].
|Figure 2: Group 3 pouring technique - space around the analogues from the latex cylinders removal|
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For Group 4, 10 metallic bars were first obtained from cobalt-chromium casting. The implant analogs on the mold were then joined by a metallic bar placed perpendicularly with methylmetacrylate (Pattern Resin; GC America Inc., Alsip, United States) [Figure 3]. Then, the mold was filled with gypsum following the conventional technique.
After all the casts had been made, the metallic infrastructure was fabricated using an implant position index. Prosthetic pillars were settled over the analogues and a titanium bar was laser-welded to the prosthetic abutments, so the measurements could be performed. 
The vertical misfit was measured with an optical microscope with a magnification of 120 times and 1 μm accuracy (UHL VMM 100 BT; Reino Unido, UK), equipped with a digital camera (KC-512NT; Kodo BR Eletrτnica Ltda, São Paulo, SP, Brazil) and an analyzer unit (QC 220-HH; Quadra-Check 200, Metronics Inc., Canada), following the single screw test protocol to test the implant structures' passivity.  One screw was tightened, and the misfit evaluation measured on the other retainer. ,, The measurements were performed at the tagged point in the buccal region between the prosthetic cylinder base and the implant analogue platform [Figure 4]. The measurements were repeated three times, and the means corresponded to each cast evaluation.
|Figure 3: Group 4 pouring technique; metallic bar joined to the analogues|
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|Figure 4: Vertical misfit between implant platform to prosthetic component surface (microscopic image)|
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Subsequently, two-strain gauges were placed on the infrastructure surface; one was placed at the top and the other at the bottom.  The screws were tightened in the same sequence with 10 Ncm. The strain was quantified by the infrastructure's elastic deformation at the strain gauges area, during 3 min since the screw's tightening [Figure 5]. The elastic deformation was identified at the strain gauges by electric signals that were captured by a computer program (ADS0500; Lynx Tecnologia Eletrτnica Ltda, São Paulo, SP, Brazil) and processed by the AqDados 7 software (Lynx Tecnologia Eletrτnica, São Paulo, SP, Brazil), , which allowed the strain quantifying. The data obtained were presented by the software in microstrains (με). This process was repeated for three times, and the strain means corresponded to the cast' evaluation.
The means were calculated, and the data were statistically analyzed using variance analysis (one-way ANOVA and Tukey's test) using the SAS 9.1 statistics program (SAS Institute, Cary, NC, United States).
| Results|| |
When the vertical misfit values were compared between different treatment groups, it was observed that there was no significant difference between the four groups (P = 0.979). The strain analysis data presented a significant difference between Groups 3 and 4 (P = 0.027). However, there was no significant difference either between Groups 1, 2, and 3 or between Groups 1, 2, and 4 (P > 0.05) [Table 1].
|Table 1: Mean values and SD for misfit (ìm) and strain (ìƐ) according the different techniques |
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The comparison between the two studied variables using the Pearson correlation test presented no correspondence among the strain and misfit data (r = −0.0403) [Table 2].
| Discussion|| |
Despite the fact that the strain values induced on the infrastructure presented minor values on the metallic bar group compared with the other groups, the vertical misfit did not present a difference compared with the conventional pouring technique group. However, the four pouring techniques could be satisfactory if only the misfit data were considered because the average values were lower than 150 μm, which is an acceptable value. , However, it is important to note that the latex cylinder technique is not as accurate as the rigid joined analogues technique, probably because the second gypsum portion poured into the mold could induce strain on the first one that has already crystallized because it involves two masses that would suffer linear expansion in different times and circumstances, which could lead to damage to the dimensional stability. Unlike Del'Acqua et al.,  who proposed that the first crystallized gypsum portion would be a limiting factor through the second portion expansion poured into a mold, and so the casts were more accurate. 
The strain data comparison among the conventional pouring technique and the two-step pouring techniques in this study yielded a different result from the data presented by Castilho et al.,  in which the latex cylinder and conventional pouring techniques were compared, and the latex cylinder technique was considered to be more accurate. There was no difference between Groups 1, 2, and 4, which could be explained by the fact that the impression transfers are rigidly joined;  the analogs' rigid joints could provide major stability related to the analogs' position, preventing their possible displacement when the plaster is poured into the mold. Thus, this technique could induce less strain on the structure.
The expansion of the gypsum compensates for the impression material's contraction; however, this property could also damage the casts' dimensional accuracy. In implant dentistry, type IV gypsum is used, according to ADA's 25 th specifications, due to its high resistance and hardness. , Thus, the maximum type IV gypsum expansion is about 0.1%.  Furthermore, some pouring techniques were developed to contour this dimensional alteration, such as a minor gypsum crystallization.  The cast accuracy can be evaluated by linear measurements or by other comparisons such as vertical misfit and strain related to a metallic structure.
In accordance with the study presented by Del'Acqua et al.,  the results demonstrated that, independent of the pouring technique, the cast distortion was not directly influenced if the impression transfers were joined previously. Unlike in the study conducted by McCartney and Pearson,  in which a small portion of dental plaster was poured around the analogs to obtain better accuracy; however, the transfers were not joined before the impression procedure, which could explain the differing results. In the literature consulted, the papers that reported different results among the pouring techniques did not join the transfers before the impression procedure. ,, This observation suggests that, since the gypsum manipulation is correct, the transfer union previously the impression making, ,, become more relevant than the pouring technique.
The methodology using strain gauges is a complementary method to linear measurements, which are insufficient to make conclusions about any cast technique's accuracy. There was no difference between the groups when the vertical misfit measurements were evaluated, in spite of the fact that the comparison of the strain values demonstrated differences between Groups 3 and 4. This data confirms that, if only linear measurements are performed, such as the vertical misfit, the conclusion about the pouring technique's accuracy could be imprecise. Thus, the evaluation of distortions in other directions such as the strain values represented a complementary analysis, which can lead to more precise conclusions. The strain gauge data is reliable, and quantitative assays have proven its accurate results. ,,,
There was no correlation between vertical misfit and strain along the infrastructure. According to Watanabe et al.,  the strain occurred when the prosthesis' screw was tightened; therefore, it is possible that an infrastructure presenting high-misfit value would induce lower strains when it is settled due to an adequate distribution of forces. Nevertheless, other papers have reported that the strains are correlated to the misfit. , However, there was no proportionality among these variables in either study. This lack of linearity suggests that the misfit could be a factor that induces strains; however, a direct relation with increasing strain could not be confirmed.  It is also important to consider that the clinical misfit values are lower than those evaluated through the passivity test following the one-screw protocol, due to all the screw-tightening; this could lead to forced positioning and thus, strain induction. 
| Conclusion|| |
Despite the study's limitations, it is possible to conclude that the rigid analogue union before the gypsum pouring can provide casts that are as accurate as the conventional pouring technique. And the strain gauge analysis is an adequate method to evaluate cast accuracy through infrastructure deformations. Furthermore, there is no direct correlation between vertical misfit and structure deflexion.
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Mariana A Rodrigues
Department of Periodontology and Prosthodontics, Piracicaba Dental School, Campinas State University (UNICAMP), Piracicaba, São Paulo
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]
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