Digital Dynanometer for Cementation and its Influence on the Film Thickness of Ceramic Restorations
Marcelo Taveira Barbosa*1, Mauro A. Caldeira de Andrada2, Luiz Clovis Cardoso Vieira2, Luiz Narciso Baratieri2
1Federal University of Alfenas, Brazil.
2Federal University of Santa Catarina, Brazil.
The aim of this study was to develop a standardization technique of cementation force. Firstly, a digital dynamometer for clinical application was developed for standardization of the cementation force. Next, the accuracy of the dynamometer was verified through a comparative evaluation of the internal adaptation of all-ceramic crowns cemented under digital force or using the device. Ten human extracted molars were prepared for full crowns, and received all-ceramic crowns fabricated using an injection-molded system (IPS Empress 2) veneered by IPS e.max Ceram. A manikin containing the adapted teeth was mounted to an artificial head with a mask to simulate lips and cheeks. At the moment of the test of simulated cementation, each crown was filled by a freshly mixed, PVS material specifically designed for this purpose, and immediately seated at the final position on the respective tooth. In the Mechanical Group, the fork of the dynamometer was inserted into the oral cavity, and the tip protected by the silicone was placed over the ceramic crown. The patient was asked to stabilize the bite force for 3 minutes when the dynamometer’s digital display indicated ~20.0N. Three graduate students performed the procedures for the Manual Cementation Group. Cross-sections of replicas were obtained, after serial cuts in the labial-lingual and mesiodistal directions. Three local measurements were taken (cervical, axial, and occlusal). Mean final film thicknesses were 86.48 μm for the Mechanical group, and 103.58μm, 103.05μm, and 104.49μm for the Manual Cementation group. In spite the results revealed no statistically significant differences, the cementation using the dynamometer standardized the cementation force. In addition, its use reduces the need of the aid by a dental assistant, and takes the hands free to remove excess material, for checking accuracy fit, and stabilization of the indirect restoration, including at the light-curing step.
Keywords: Ceramics; Dental Marginal Adaptation; Bite Force
The development of stronger and more versatile ceramics systems, together with the adhesive technique, has provided the application of metal-free ceramic restorations in teeth in both anterior and posterior with esthetics, preservation of periodontal health and restoration of function. However, regardless of the system, the success of metal-free ceramic restorations is related to a series of clinical and laboratory steps that must be performed with high standards and with a consistent quality control . In this sense, considering that there was success in planning, preparation, impression-taking and fabrication of the restoration, the outcome of the whole procedure will only be successful if the luting is suitable. It requires from the dentist and assistant, exceptional training and coordination, in addition to knowledge on the materials and the technique. It should be noted that what makes the cementation so important is the fact that it cannot be remade without prejudice to the restoration .
During the process of cementation, some factors such as the force applied on the ceramic restorations for cementation, can affect significantly the marginal adaptation [3,4]. If the force is excessive, it may cause fracture of the margins or even the entire ceramic restoration . On the other hand, if the force is insufficient or even off the path of insertion, it can result in misadapted restorations, influencing significantly the occlusal adjustment, the retention of the restoration, the premature staining of the margins and marginal leakage .
For a long time the force of cementation is evaluated and controlled in vitro studies [4,7-14]. However, the control of the force is difficult in clinical evaluations and routine professional activity, and it is up to each professional to manually establish the force of cementation he/she believes to be appropriate.
To consider that the importance of the force of cementation can be reduced by the use of adhesive cementation technique is currently premature and unreliable. Although resin cements are less susceptible to dissolution, one must consider that they are not completely wear resistant, even in minor marginal discrepancies .
In addition to the conventional resin cements, adhesive cementation can be accomplished with adhesive systems filled or unfilled, self-adhesive resin cements, resins of low viscosity (flowable) ; heated composites [17,18], under different modes of activation. Despite the wide range of materials, the cementation technique has changed little. The force and form of application of pressure are also similar to those performed for the centennial zinc phosphate cement.
A new device that would allow for higher time for seating, with stable and continuous force, allowing the professional to release
both hands for checking the margins, for removal of excesses and the proper light activation of the luting material can provide better marginal adaptation and resulting decreased labor in the following steps, ie, the occlusal adjustment, finishing and polishing the margins.
The aim of this study was to propose standardizing the force of cementation technique in the cementation. For this, the procedures were divided into two phases: first, the development of a digital dynamometer for clinical use which allows standardization of the cementation force, and the second stage, a comparative evaluation of the internal adaptation of cemented crowns with manual force and the developed dynamometer.
Material and Methods
Development of the Dynamometer
In order to develop the dynamometer, it was required to design an arch. Using computer-aided tools (CAD, i.e., Computer Aided
Design) and FEA (Finite Element Analysis) was essential for both the three-dimensional visualization of the product as for the calculation and simulation of the device (Figure 1).
Figure 1. Graphic representation of the elastic strain accordingto the finite element analysis of the dental contact arch prior to its fabrication.
After the initial design of the bow, the most suitable materialfor its production has been selected. To meet the basic requirements,we chose to use stainless steel to make the arc ofdental contact, more specifically, ABNT 420 steel, for the goodmechanical properties and because it is the metal used forapplication in hospital, surgical, and dental instruments, andtherefore, capable of sterilization.
After fabrication of the stainless steel arch, the componentswere selected to make up the dynamometer. To mount the dynamometer,devices such as strain gauges, transducers, loadcell, multi-display panel and switching power supply havebeen used.
The extensometers were selected according to the materialto be deformed - in this case, ABNT 420 steel- and connected following the scheme of a full Wheatstone bridge. Four straingauges are connected to the arc of stainless steel.
During the stages of development of the dynamometer it wasdesigned an aluminum cable for ease of handling and grip for using of the stainless steel arch. This cable was designedto be hollow, allowing for the input and output of the wiresfrom the four gages. A plastic box was used (PB 220/140, Patola,Sao Paulo, Brazil) to receive the load cell transducer, themulti-panel and switching power supply. In order to facilitatethe sterilization it was developed two stainless steel removableadapters for the dental contact area. Each adapter has receiveda detachable disposable silicone cover for the contactarea (Figure 2).
Figure 2. Frontal view of the dynamometer ready to perform the tests.
Calibration and Maximum error of the Dynamometer
System calibration was performed using nine standard massesof 0.5 kg up to 4.5kg, with intervals of 0.5kg. For eachload three measurements were carried out, and displayed bythe multi-panel indicator. The acceleration of gravity was g =9.79117 m/s2. The value used as a constant correction was obtainedby the average of three measurements. The average valueof 0.0815 was inserted into the multi-indicator panel. Theestimated maximum error was ± 0.5 N.
In Vitro Test
Collection and selection of teeth
This research project was submitted to the Committee of Ethics(Nº 1090). To prepare this study we selected 10 healthyhuman third molars, with shape and size with first or secondmolars. The teeth were stored in a 0.1% thymol -containingsolution, to prevent bacterial growth.
The root portion of teeth was scraped off with the help of peri odontal curettes (Hu-Friedy, Chicago, IL, USA) and cleanedwith rubber cups impregnated with a slurry of pumice andwater. After cleaning, the teeth were inspected with a stereomicroscope(Carl. Zeiss, Göttingen, Niedersachsen, Germany),with 20x magnification, in order to exclude teeth with cracks,defects, and structural changes, which could compromise theresults.
The cavity preparations were made for full crown with chamfermargins, with 1.5mm axial occlusal reduction and 2mm. Thesewere performed with tapered round end diamond points (#4137, 4137 F and 4137 FF, KG Sorensen, Barueri, SP, Brazil).The points were replaced every five preparations by new ones,to maintain the efficiency of cutting. All preparations were carriedout with slight pressure and copious air / water coolant toavoid heating of the dental structure.
For polishing the preparation,, silicone, rubber and siliconcarbide mounted points were used (Vigodent SA Industry andCommerce, Rio de Janeiro, Brazil, Batch 002/09).
Then, each tooth was submitted to impression technique withsilicon single step addition of putty consistency (Virtual, Ivoclar Vivadent, Schaan, Liechtenstein, Batch JL 4169) and lightbodyconsistency silicone (Virtual, Ivoclar Vivadent, Schaan,Liechtenstein, Batch HL 4133).
Manufacturing of ceramic crowns
The clinical and laboratory steps for the fabrication of the crowns, using the injection system (IPS Empress 2), are explained in Table 1.
Table 1. Main laboratorial steps for fabrication of the ceramic crowns.
Table 2. Distribution of the teeth by anatomy and position in the mannikin.
The impression of tooth root portion of the plastic was carried out with silicone addition medium consistency (Virtual, Monophase Fast Set, Ivoclar Vivadent, Schaan, Liechtenstein, batch NL4008), injected into a plastic bottle. Then the root portion of the manikin tooth was inserted at the center of the vial. After polymerization of the silicone, the tooth was removed from the impression and it was filled with acrylic resin (Dencrilon, Dencril Commerce of Plastics Ltda., Pirassununga, Sao Paulo, Brazil) (Figure 3).
Figure 3. After the cavity preparations, the natural teeth were embedded in self-curing acrylic resin to simulate the root portion.
Figure 4. Buccal view of tooth “B” placed in the manikin.
Cementation with controlled force
Figure 5. Buccal view of the ceramic crown and tooth “B” placed in the mannikin.
Next, the manikin was attached to the patient simulator, and the flexible mask has been adapted. For the cementation simulation, it has been selected a specific addition silicone (Tokuso Fit Tester, Tokuyama Dental Corp., Tokyo, Japan, Batch 012087). Equal parts of the base and catalyst pastes of the silicone were dispensed. A digital chronometer was used to tracking the time of handling, insertion and polymerization of the silicone. The PVS material was mixed for 20s using a #24 spatula (DUFLEX, SSWhite, Rio de Janeiro, Brazil). The crown was filled with freshly manipulated PVS material and was immediately adapted to the respective tooth Following, the tip of the dynamometer was inserted into the oral cavity, and the tip with silicone protection was placed on the ceramic crown.
Cementation with manual force
Figure 6. The chin screw of the mannikin was tightened until reaching a value next to 20 N. This step was monitoramento at the digital multiindicator panel of the dynamometer.
Figure 7. Upon reaching a 20N (+0,5N) value, tightening was ceased and excess of silicone was removed with the aid of a brush. The force was maintained stable for 3 minutes.
Figure 8. In Group B, the ceramic crown was maintained under manual (digital) force for 3 min.
Because of the short time of polymerization of the silicone any excess of the interproximal has not been removed and, therefore
Figure 9. A regular body consistency silicone (Virtual Monophase Fast Set, Ivoclar Vivadent) was injected firstly inside the metal device, then inside the crown.
Figure 10. Sectioning the replica in the mesio-distal direction.
Figure 11. View of the replica after the removal of lateral excess.
Measurement of film thickness
An image analysis software (Image Tool 3.0 for Windows, University of Texas, Health Science Center San Antonio, Texas, USA) was used for measurement of the film thickness.
For each section of obtained from cutting in the buccolingual direction, three measurements were made at five different places of the film length. Three more measurements at five different locations were made for sections obtained from mesio-distal sectioning, at the same locations of measurement. Thus, 30 measurements were performed for each crown. The mean of three measurements was considered as the final value for each location.
Figure 12. Film thickness (mean and standard desviations), according to measurement sites (cervical, axial and occlusal) and cementation techniques (dynamometer and manual). Vertical lines represent the data dispersion.
Cementation is the crowning moment of all the previous steps and what makes it so important is that it cannot be repeated without prejudice to the restoration. This importance justifies the several studies that led to the development of new materials and techniques of cementation. Nevertheless, the cementation continues to be a critical and stressful step for the dental team .
In the conventional cementation, after the insertion of the restoration previously filled with the resin cement, the professional should: a) carry out and maintain the force of cementation with a hand while, with the other hand, must perform the removal of excess with disposable applicators and / or manual tools; b) remove the interproximal excess with dental floss c) check the adjustment of margins, and d) perform the light curing. Generally, the professional needs an assistant to perform the tasks, and regardless of the skill, he/she lacks the professional training to detect and solve possible complications. Further, this would demand an unreachable stability hands-free the application of force during cementation by the operator. Thus, using the dynamometer facilitated this procedure, which is one of most important – the adhesive cementation. In addition to reducing or even eliminating the dependence on the assistant, the instrument releases both hands of the dentist so he/she can remove excess cement, evaluating the marginal adaptation, remove the retraction cords and also perform light curing, because the handle of the dynamometer tip can be operated by the patient based on previous instructions.
One feature of the adhesive cementation is the great variability of film thickness of the materials used [16,30]. Another complicating factor in relation to the film thickness is the variability of presentation of these materials: filled and unfilled adhesive systems; conventional, and self-adhesive resin cements, low viscosity composite resins, heated composite resins, and
Cite this article: Marcelo T B. Digital Dynanometer for Cementation and its Influence on the Film Thickness of Ceramic Restorations. J J Dent Res. 2014, 1(1): 007.