2Department of Adult Oral Health, Bart's and The London School of Medicine and Dentistry, Lefkada, Greece
Keywords: Provisional crowns; Marginal fit; Bis-acryl temporary; Temporary materials; Composite; Crown coping
Historically, autopolymerizing Poly-Methyl Methacrylate (PMMA) and Poly-Ethyl Methacrylate (PEMA) resins have been the materials of choice for the construction of provisional crowns and bridges. However, Bis-GMA acryl resin composites have become a popular alternative choice due to their ease of use. Research has shown Bis-GMA acryl resin composite to provide some advantages in physical properties including low polymerization shrinkage [7] and good marginal adaptation [8-10] low exothermic reaction [11,12] minimal pupil and soft tissue irritation [13], good surface hardness [14], and increased color stability [2,12].
Until recently, alterations for repairs and addition of Bis-GMA acryl resin to composite material was difficult, even though they are compatible with other composite materials [1,2]. Studies have found an 85% decrease in transverse strength after repair of a Bis-GMA acryl resin provisional material [15]. It has been suggested on the same studies that it might be more advantageous to make new provisional restorations than repair restorations made from Bis-GMA acryl resin provisional materials. However, clinical experience has shown that flowable composite resins can be used to repair Bis-GMA acryl resin provisional restorations with ease and success [16].
Numerous studies appear in the dental literature comparing the marginal fit of provisional restorations made from different types of materials [8-10,17] and different techniques [18-21] as well as the effect of environment's temperature [22] and storage conditions [23]. Dental implant companies, on the other hand, have all tried to make the temporization phase of treatment more predictable and many have produced stock templates for their own implant systems. The accuracy of these templates when modified to tooth shape has not been documented in the literature. The purpose of this investigation is to compare the accuracy of fit of three manufacturing methods under the test conditions in vitro and investigate the null hypothesis that there is no difference in the accuracy of fit of the three manufacturing methods under the test conditions in vitro.
Silicone putty (ExtrudeXP; Kerr Ltd, Peterborough, UK) was used to create a matrix which was used for the construction of the provisional crowns. In the surface of the putty matrix a notch was created with a number 10 scalpel, in order to allowthe excess of provisional material to be vented and secure the complete seating of the matrix over the analogue. Twenty five provisional crowns were made in each of the three groups tested.
The excess material at the margin was trimmed with polishing discs (Soflex; 3M ESPE) at x2.5 magnification (Rose Micro Solutions; x2.5 loupes). The crown margin was re-defined using the 'bead on' technique as described by Zwetchkenbaum [17]. The fresh provisional material was left to polymerize for 15 minutes. Once cured the acrylic resin provisional crown margin was further trimmed and then polished under x2.5 magnification loupes with medium, fine and super fine polishing discs (Soflex; 3M ESPE). A total of 25 provisional crowns were made in this manner with Trim II.
After the ITI Straumann temporary coping was clipped in place over the fixture head the putty matrix was filled with Bis- GMA acryl resin composite (Protemp 3 Garant; 3M ESPE), and fitted it. The procedure for polishing was repeated for this group except that no reline or margination procedure was used. A total of 25 provisional crowns were made in this manner with the temporary copping.
The light and medium body silicone was removed from the fit-surface of the provisional crowns and covered with low viscosity polyvinyl siloxane impression material of another company (Aquasil XLV Ultra; Dentsply, Weybridge, Surrey, UK) which has a different colour than the one used to record the fit surface of the provisional crown. The low viscosity impression material was left to set for a further 7 minutes and after setting sectioned down the center with a number 10scalpel. The sections were cut from the mesial to distal aspect as proposed by McLean and von Fraunhofer [24].
An Olympus BX60 optical microscope (Olympus Ltd, Watford, Hertfordshire, UK) was used to measure the thickness of the low viscosity Extrudeimpression material on all the sections of the rubber impression samples under x50 magnification. Seven measurements were made in total on each rubber impression sample (Figure 2d). The points of measurement for each sample are presented schematically in (Figure 2e).
Measurement A: The first measurement was made on the most external point of the margin on the mesial site (left hand side) of the rubber impression sample.
Measurement B: The second measurement was made 2 mm
Measurement C: The third measurement was made 4 mm occlusally from the restoration's mesial margin on the mesial axial wall of the rubber impression sample.
Measurement D: The fourth measurement was made at the middle of the occlusal surface of the rubber impression sample.
Measurement E: The fifth measurement was made 4 mm occlusally of the restoration's distal margin on the distal axial wall of the rubber impression sample.
Measurement E: The fifth measurement was made 4 mm occlusally of the restoration's distal margin on the distal axial wall of the rubber impression sample.
Measurement G: The seventh measurement was made on the most external point of the margin on the distal site (right hand side) of the rubber impression sample.
A total of 75 measurements were made at each point in all 3 groups tested.
Analysis of the data in the study involved two parts. The first part assessed the reliability of the measures used in this study. The method described by Bland and Altman [25], was used to demonstrate the precision of measurements taken.
The second part tested the hypotheses of the study, which requires analytical statistics. Non-parametric tests were selected, because the sample's size was small, thus the assumptions for parametric tests were not met. The Mann-Whitney test was chosen to compare the differences in the calculated distances of the samples produced by the three provisional materials under investigation. The level of statistical significance was set at 0.05.
Descriptive statistics are presented at (Table 1). Analytical statistics resulted at significant differences between the isobutyl methacrylate and Bis-GMA acryl resin composite groups in almost all points of measurement. Results on the accuracy of fit indicate that Bis-GMA acryl resin composite produced better fitting provisional crowns than Isobutyl methacrylate. Only at points E and F the differences was not of statistical significance (p > 0.05) although Bis-GMA acryl resin composite exhibited better fitting restorations than Isobutyl methacrylate. The results of this comparison are presented at (Table 2).
Overall, highly statistical significant differences were found between the Isobutyl methacrylate and the ITI Straumanntemporary coping modified on tooth form with Bis-GMA acryl resin composite resin groups in all points of measurement. Results on the accuracy of fit indicate that Isobutyl methacrylate produced the best fitting provisional crowns over the ITI Straumann temporary coping modified to tooth form with Bis-GMA acryl resin composite. Detailed comparison of
Material |
Point of measurement |
N |
Mean |
Standard Deviation |
Isobutyl methacrylate |
Measurement A |
25 |
83.61 |
12.22 |
Measurement B |
25 |
29.30 |
20.00 |
|
Measurement C |
25 |
34.36 |
21.99 |
|
Measurement D |
25 |
184.52 |
97.28 |
|
Measurement E |
25 |
19.91 |
19.75 |
|
Measurement F |
25 |
19.81 |
15.71 |
|
Measurement G |
25 |
84.46 |
14.85 |
|
Bis-GMA acryl resin composite |
Measurement A |
25 |
69.97 |
16.21 |
Measurement B |
25 |
17.49 |
11.37 |
|
Measurement C |
25 |
14.67 |
10.52 |
|
Measurement D |
25 |
35.94 |
18.68 |
|
Measurement E |
25 |
15.83 |
7.82 |
|
Measurement F |
25 |
16.90 |
9.45 |
|
Measurement G |
25 |
71.12 |
18.07 |
|
Modified stock templates |
Measurement A |
25 |
203.00 |
31.67 |
Measurement B |
25 |
72.30 |
22.77 |
|
Measurement C |
25 |
74.72 |
18.33 |
|
Measurement D |
25 |
392.15 |
126.36 |
|
|
|
|
|
|
Measurement E |
25 |
63.51 |
22.71 |
|
Measurement F |
25 |
61.43 |
23.70 |
|
Measurement G |
25 |
240.70 |
67.91 |
Point of measurement |
Group |
Mean Rank |
P value |
Measurement A |
Group 1 |
31.50 |
0.004 |
Group 2 |
19.50 |
||
Measurement B |
Group 1 |
30.80 |
0.010 |
Group 2 |
20.20 |
||
Measurement C |
Group 1 |
32.48 |
0.001 |
Group 2 |
18.52 |
||
Measurement D |
Group 1 |
38.00 |
0.000 |
Group 2 |
13.00 |
||
Measurement E |
Group 1 |
25.08 |
0.839 |
Group 2 |
25.92 |
||
Measurement G |
Group 1 |
26.20 |
0.734 |
Group 2 |
24.80 |
||
Measurement F |
Group 1 |
31.28 |
0.005 |
Group 2 |
19.72 |
All groups in this experiment exhibited better fit in the axial walls than at the restorations margins or under the occlusal surfaces. This observation can be explained from the fact that polymerization shrinkage occurs towards the bulk of the material forcing the material to shrink away from the restoration's margins. The increased space under the restoration's occlusal
Point of measurement |
Group |
Mean Rank |
P value |
Measurement A |
Group 1 |
13.00 |
0.000 |
Group 3 |
38.00 |
||
Measurement B |
Group 1 |
15.24 |
0.000 |
Group 3 |
35.76 |
||
Measurement C |
Group 1 |
15.48 |
0.000 |
Group 3 |
35.52 |
||
Measurement D |
Group 1 |
15.40 |
0.000 |
Group 3 |
35.60 |
||
Measurement E |
Group 1 |
14.68 |
0.000 |
Group 3 |
36.32 |
||
Measurement G |
Group 1 |
14.72 |
0.000 |
Group 3 |
36.28 |
||
Measurement F |
Group 1 |
13.00 |
0.000 |
For Group 3 the significantly increased space under the occlusal surface, which reached the 585 µm in one restoration, can be explained by the fact that the temporary coping is a hollow cylinder 1.5 mm higher than the ITI Straumann Solid abutment and that space was not completely filled with the provisional material during its modification in tooth form. Even the use of low viscosity silicone impression material to record the space between the provisional crowns and the analogue used in this in vitro investigation might have caused incomplete seating of the crowns resulting in increased marginal discrepancies and increased space under the occlusal surfaces of the restorations. The use of cement lute in clinical practice can have the same effect on the restorations, thus the technique employed can result in a fit similar to that produced in clinical practice [24].
McLean and von Fraunhofer [24] reported that the use of polyether rubber to mimic the cement lute can reproduce films as thin as 10 µm quite accurately. It was observed in our investigation that the use of modern polyvinylsiloxane impression materials can reproduce film thickness less than 10 µm. Films as thin as 5 µm were recorded on the axial walls of provisional restorations made from isobutyl methacrylate and Bis-GMA acryl resin composite. As the ISO standard for acceptance as a cement lute is 25 µm such a close adaptation of the provisional restorations to the axial walls of the analogue may cause incomplete sitting of the restorations during clinical practice.
A number of studies have compared acrylic methacrylate based provisional materials with Bis-GMA acryl resin composite materials reporting the superiority of Bis-GMA acryl resin composite in terms of marginal fit, but the restoration's fit on the axial walls and under the restoration's occlusal surfaces has not been evaluated. Also, in our knowledge there are no studies comparing those materials with temporary copings provided from implant manufacturers to simplify provisionalization phase over implant abutments.
A study which compared 6 different provisional materials including those used in our study reported similar findings in terms of marginal integrity [12]. The mean marginal discrepancies reported were 110 µm for Isobutyl methacrylate and 95 µm for Bis-GMA acryl resin composite. No information is given in that study whether the restorations were left to complete polymerization over the analogue or whether they were removed. No margination technique was performed in that study which may explain the small differences with our study.
The materials used on our experiment were also compared in terms of marginal adaptation in another study which concluded that both materials result in mean marginal discrepancies of 60 µm [18]. In that study the restorations were only removed once from the analogue for a short period of time, and then they were reseated on the analogue and polymerization completed by immersing them in a water bath at 37°C. No reline or margination technique was performed.
The same methodology was used in a number of other studies. Restorations were placed in a water bath at 37°C to complete polymerization and a spring loading holding device was used to secure the interim crowns over the analogue during measurements were performed with a measuring microscope. Tjan et al. [9] reported marginal discrepancies of 40.1 µm for Bis- GMA acryl resin composite and 23.6 µm for isobutyl methacrylate. The same group of investigators in a more recent study reported an improved marginal fit for provisional crowns made from Bis- GMA composite resin provisional materials [8] using again the same methodology. Bis-GMA acryl resin composite provisional material resulted in 22 µm of marginal opening, and a visible light cured provisional material 29 µm.
The difference in findings between our study and the previous studies might be explained by the fact that provisional restorations were placed on a water bath to complete polymerization at 37°C in an attempt to mimic body temperature but the mouth is often not subject to such high temperatures especially when open during treatment. It has been shown that curing of acrylic provisional materials in water baths significantly improves marginal fit of produced restorations [22].
In this investigation the materials were left to complete polymerization on room temperature which was significantly lower than 37°C, which resulted in increased polymerization time and probably increased polymerization shrinkage. The clinically unrealistic use of the spring loaded device to secure the restorations over the analogue and the direct measuring of the marginal opening in just one axis might have also influenced the results due to compression of the provisional restorations over the analogue.
The differences in marginal discrepancies can also be the result of increased polymerization shrinkage of provisional restorations in our investigation as they were stored on dry air for one hour prior to the application of McLean and von Fraunhofer [24] technique. Polymeric materials exhibit continuous polymerization even after one week storage in dry air resulting in increased marginal discrepancies [23].
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