Abstract
Objective
The aim of this laboratory study was to evaluate the horizontal and vertical effects of the polymerization shrinkage of three-unit temporary fixed dental prostheses (FDPs) on the position of the prepared teeth. In addition, the reduction of these effects by using different fabrication techniques was evaluated.
Methods
A total of 192 temporary FDPs were fabricated using one methacrylate (MA) and two dimethacrylate (DMA) materials. Each material group (n = 64) was divided into two groups according to the fabrication methods (M1: curing on the prepared teeth, M2: curing in a silicone mold). Each fabrication group was divided into four subgroups (n = 8) according to the relining method used (B: no relining, S: spacer foil 300 μm, DG: grinding-out with 500 μm cutting depth, and FG: free grinding). The experimental apparatus consisted of two abutment teeth lowered at right angles into a silicone mold. One prepared tooth was embedded in silicone to simulate the periodontium and permit slight horizontal tooth movement. The dimensional changes were recorded with an optical microscope. The test images were superimposed and measured using image analysis software.
Results
The statistical analysis showed that there were significantly higher horizontal changes for the MA than the DMA resins in M1, while there was none in M2. Regarding the vertical changes, there were significant differences between the baseline group and all relining and fabrication groups in all materials.
Significance
Relining of directly fabricated temporary FDPs significantly reduces the effect of polymerization shrinkage and thus secures the position of the prepared teeth.
1
Introduction
An essential step of the prosthetic treatment is the direct fabrication of provisional restorations . Teeth prepared for a fixed dental prosthesis (FDP) usually require a provisional restoration during the laboratory production of the final FDP. The main purpose of the provisional restoration is to protect the prepared teeth against thermal, mechanical and biological noxae, to stabilize the tooth position and to ensure masticatory function, phonetics and esthetics . To meet these requirements, the provisional materials must possess specific properties . Beside mechanical properties, a good marginal fit and dimensional stability are particularly important . The dimensional stability is important to ensure a stabile position of the abutment teeth during FDP fabrication.
A variety of provisional materials are available on the market . The majority of these materials can be divided into two main categories based on their compositions: unfilled methacrylate resins and filled dimethacrylate composite resins . During the polymerization of monomers a reduction of the intermolecular distances occurs, which is known as polymerization shrinkage . As a result of the lower molecular weight, higher polymerization shrinkage occurs with the use of monomethacrylates in comparison to dimethacrylate materials filled with inorganic fillers .
The polymerization shrinkage causes dimensional changes in the provisional materials , leading to internal stresses . This might result in decreased accuracy of the restoration and deterioration of the marginal fit , and subsequently leading also to inaccuracies in the occlusal area . Insertion of such poor fitting provisional restoration could result in migration of the abutment teeth causing subsequently a poor or misfitting final restoration. Therefore, to avoid the migration of abutment teeth after impression taking, the dimensions of temporary FDPs should be as accurate as that of final FDPs .
In one study report, it was found that the adaptation of the provisional crown improved significantly after the first and slightly after the second relining . Zwetchkenbaum et al., likewise described that after relining provisional restorations and subjecting them to thermal cycling and occlusal loading, marginal gaps were significantly reduced . Thus, relining can affect the accuracy of the provisional restorations positively.
It was also found that the marginal fit deteriorated with longer FDPs . Therefore, it can be assumed, that by increasing the provisional material volume between the abutment teeth, the polymerization shrinkage also increases and thus results in a greater dimensional change.
Many studies have examined the effect of polymerization shrinkage of the provisional restorations . However, the aforementioned studies investigated the shrinkage in regard to the marginal fit and occlusal interferences of the abutment teeth, but no studies investigating the correlation between polymerization shrinkage and tooth migration could be found in the dental literature.
Therefore, the aim of this laboratory study was to evaluate the effects of the horizontal and vertical polymerization shrinkage of temporary three-unit FDPs and to measure the position of the prepared teeth using optical methods. Additionally, it was examined whether these effects could be reduced by different fabrication techniques, which include different fabrication and relining methods.
The null-hypothesis of this study was, that there is no difference between the tested temporary resin materials and the fabrication techniques of the temporary three-unit FDPs regarding the positional stability of the abutment teeth.
2
Materials and methods
2.1
Study outline
All experiments were performed in laboratories under constant conditions (constant humidity of 40 ± 5% and an ambient temperature of 21 ± 1 °C).
Two prepared typodont teeth (KaVo Dental, Biberach/Riss, Germany) were slowly dropped at right angles into a mold made of silicone (Optosil Comfort Putty, Heraeus Kulzer, Hanau, Germany) with a weight of 1 kg secured by parallel guides. Each tooth was embedded separately. One tooth was fixed in resin and the other was embedded in silicone (Deguform, DeguDent, Hanau-Wolfgang, Germany) to simulate the periodontium and permit slight horizontal tooth movements. At the top of this movable tooth, a metallic cone was attached in central position using the centering gauge to ensure an identical starting position of the movable abutment tooth at the beginning of each measurement. A schematic drawing and an illustration of this experimental test set-up are shown in Fig. 1 a and b.
At this cone, a pyramid-shaped marking defined a specific point, which was recorded using a light optical microscope (M420 Macroskop 6:1: magnification 70×, Leica Microsystems, Heerbrugg, Switzerland) with a CMOS camera (EOS 60D, Canon, Tokyo, Japan) to measure later any horizontal changes. At the two ends of the experimental apparatus a plan stop was mounted, which was used to track the vertical discrepancy using a CMOS microscope camera (Digi Micro 2.0 Scale, DNT, Dietzenbach, Germany). Thus, both horizontal and vertical changes were recorded with optical methods.
A total of 192 temporary FDPs were fabricated using three materials. Each material group (n = 64) was divided into two groups (n = 32) according to the fabrication methods. Subsequently, each fabrication group was divided into four subgroups (n = 8) according to the relining method used. The study design is illustrated in Fig. 2 .
Three temporary restorative materials were tested. The first is handmix methacrylate (MA) with a mixing liquid to powder ratio of 1:2.3 (Trim, Bosworth, Skokie, USA). The other two materials are automix dimethacrylate (DMA) (DMA 1: Luxatemp Automix Plus, DMG, Hamburg, Germany and DMA 2: Protemp 4, 3M ESPE, St. Paul, USA). Additionally, a bonding agent (Luxatemp Glaze & Bond, DMG) was used with DMA 1, which was applied before relining for 20 s and then light-cured (Elipar™ 2500, 3M ESPE, Neuss, Germany). The materials used are listed in Table 1 .
| System | Main composition a | Manufacturer | Batch no. |
|---|---|---|---|
| Luxatemp Automix Plus | Dimethacrylate, stabilizer, catalyzer, additives | DMG, Hamburg, Germany | 672742 |
| Protemp 4 | Dimethacrylate, stabilizer, catalyzer, additives | 3M ESPE, St. Paul, USA | 469903 |
| 469930 | |||
| Trim | Powder: poly(ethyl methacrylate) | Bosworth, Skokie, USA | Powder: 1104-159 |
| Liquid: isobutylmethacrylate | Liquid: 1009-442 | ||
| Luxatemp Glaze & Bond | Methacrylate, stabilizer, catalyzer, additives | DMG, Hamburg, Germany | 671781 |
For each material, two different fabrication methods were investigated . For all groups, the provisional material was applied into the silicone mold. The excess was removed with a spatula to ensure that an identical resin volume was applied each time. Then the prepared teeth were slowly dropped into the silicon mold filled with provisional resin. Within 45 s after start of mixing, the centering gauge was removed. In the first fabrication method (M1) the temporary FDPs were left on the prepared teeth during the complete polymerization time of 10 min. In the second fabrication method (M2) the temporary restorations were removed from the prepared teeth at the onset of the initial polymerization (according to the manufacturers’ specifications). The polymerization was completed outside the abutment teeth in a silicone mold at room temperature.
In addition, four relining methods were:
B
Baseline: after fabrication of the temporary FDPs, no relining was done (n = 8).
S
The fabrication of the temporary FDPs was done on a 300 μm thick spacer foil (UZF, Dreve Dentamid, Unna, Germany) adapted to the prepared teeth. After 10 min of curing the spacer foil was removed and the crowns were relined (n = 8).
DG
Initially, temporary FDPs were fabricated directly on the prepared teeth with a curing time of 10 min. Then, defined grinding was done as the FDPs were ground out using a cross-toothed grinder (H79GSQ, Komet, Lemgo, Germany) after depth cuts of 500 μm were done with a depth cutting bur (834, Komet). After depth markings were no more seen, the temporary FDPs were relined (n = 8).
FG
Again, temporary FDPs were fabricated directly on the prepared teeth with a curing time of 10 min. Then, FDPs were ground out with a cross-toothed grinder (H79GSQ, Komet) without making depth cuts (free grinding) for 30 s at a rotary speed of 20,000 rpm and subsequently relined (n = 8).
2.2
Optical recording and measuring
All recordings of baseline subgroups (B) were done during fabrication of the FDPs, whereas for the relined FDPs (subgroups S, DG and FG) recordings were done during relining.
The digital images were recorded with two different digital cameras, a CMOS camera (EOS 60D, Canon) with a resolution of 5184 × 3456 pixels for the horizontal changes and a CMOS microscope camera (Digi Micro 2.0 Scale, DNT) with a resolution of 1600 × 1200 pixels for the vertical changes. The recordings were always strictly perpendicular to the central ray, also the focus was adjusted carefully so that the scaling in the later survey was correct. To ensure that, a connected monitor was used to control the focus.
First, the vertical and horizontal initial recordings (reference image) were made during the first 45 s after mixing and before removal of the central gauge, to document the initial situation. Afterwards, the recording started immediately with an interval of one picture every 30 s, to record the horizontal changes. For the first fabrication method (M1), recordings were done over 10 min during the polymerization period. While in the second fabrication method (M2), the temporary FDPs were removed from the prepared teeth, within their elastic phase, 2:30 min after start of mixing, and therefore, recordings were done during that time. After polymerization was complete, the final horizontal and vertical changes were recorded after repositioning the temporary FDP on the prepared teeth with a weight of 1 kg.
All images were recorded in predetermined succession and were measured using an image analysis software (Photoshop CS3 Extended, Adobe Systems Incorporated, San José, USA) . Before measurements, calibration was done using a slide (Leitz, Wetzlar, Germany) with standardized scale (2 mm interval 0.01 mm). This measurement scale was focused and recorded with the same respective camera used for the experimental images. On the image of the scale, the defined length (2 mm) was assigned to a specific number of pixels. Therefore, in the experimental images taken with the same focus and magnification, with the software the distance between two points through determining the number of pixels, i.e., the length, could be determined.
For the measurement of horizontal changes, the images were grouped into layers and placed exactly above the corresponding reference image, one above the other (matching), according to the group in which they were recorded. The pyramid-shaped marks were marked using the brush tool (pixel size 1) using an image which had the same focus as the calibration slide. That was achieved by reducing the pixels on the reference image, so that differences in focus of the successive images could be neglected. Therefore, the markings were done on the reference image, and the results were shown as a travel path. So it was possible to measure the effects of the horizontal shrinkage with the analysis function of the used software ( Fig. 3 a).
For the vertical changes measurements, two images were recorded, i.e., initial (reference) and finial images, as described above. Then the changes were calculated from the differences between the initial and final images ( Fig. 3 b and c).
2.3
Statistical analysis
Results are expressed as median values with quartiles since it could not be assumed that the data were normally distributed (Kolmogorov–Smirnov test). Therefore, statistical comparisons were performed non-parametrically by Kruskal–Wallis followed by Wilcoxon rank sum test for multiple pair-wise tests of all groups. The level of significance was set at 5%, corrected according to the procedure of Bonferroni–Holm for multiple comparisons. SPSS for Windows (release 12.01, SPSS Inc., Chicago, USA) was used for all statistical analyses.
2
Materials and methods
2.1
Study outline
All experiments were performed in laboratories under constant conditions (constant humidity of 40 ± 5% and an ambient temperature of 21 ± 1 °C).
Two prepared typodont teeth (KaVo Dental, Biberach/Riss, Germany) were slowly dropped at right angles into a mold made of silicone (Optosil Comfort Putty, Heraeus Kulzer, Hanau, Germany) with a weight of 1 kg secured by parallel guides. Each tooth was embedded separately. One tooth was fixed in resin and the other was embedded in silicone (Deguform, DeguDent, Hanau-Wolfgang, Germany) to simulate the periodontium and permit slight horizontal tooth movements. At the top of this movable tooth, a metallic cone was attached in central position using the centering gauge to ensure an identical starting position of the movable abutment tooth at the beginning of each measurement. A schematic drawing and an illustration of this experimental test set-up are shown in Fig. 1 a and b.
At this cone, a pyramid-shaped marking defined a specific point, which was recorded using a light optical microscope (M420 Macroskop 6:1: magnification 70×, Leica Microsystems, Heerbrugg, Switzerland) with a CMOS camera (EOS 60D, Canon, Tokyo, Japan) to measure later any horizontal changes. At the two ends of the experimental apparatus a plan stop was mounted, which was used to track the vertical discrepancy using a CMOS microscope camera (Digi Micro 2.0 Scale, DNT, Dietzenbach, Germany). Thus, both horizontal and vertical changes were recorded with optical methods.
A total of 192 temporary FDPs were fabricated using three materials. Each material group (n = 64) was divided into two groups (n = 32) according to the fabrication methods. Subsequently, each fabrication group was divided into four subgroups (n = 8) according to the relining method used. The study design is illustrated in Fig. 2 .
Three temporary restorative materials were tested. The first is handmix methacrylate (MA) with a mixing liquid to powder ratio of 1:2.3 (Trim, Bosworth, Skokie, USA). The other two materials are automix dimethacrylate (DMA) (DMA 1: Luxatemp Automix Plus, DMG, Hamburg, Germany and DMA 2: Protemp 4, 3M ESPE, St. Paul, USA). Additionally, a bonding agent (Luxatemp Glaze & Bond, DMG) was used with DMA 1, which was applied before relining for 20 s and then light-cured (Elipar™ 2500, 3M ESPE, Neuss, Germany). The materials used are listed in Table 1 .
| System | Main composition a | Manufacturer | Batch no. |
|---|---|---|---|
| Luxatemp Automix Plus | Dimethacrylate, stabilizer, catalyzer, additives | DMG, Hamburg, Germany | 672742 |
| Protemp 4 | Dimethacrylate, stabilizer, catalyzer, additives | 3M ESPE, St. Paul, USA | 469903 |
| 469930 | |||
| Trim | Powder: poly(ethyl methacrylate) | Bosworth, Skokie, USA | Powder: 1104-159 |
| Liquid: isobutylmethacrylate | Liquid: 1009-442 | ||
| Luxatemp Glaze & Bond | Methacrylate, stabilizer, catalyzer, additives | DMG, Hamburg, Germany | 671781 |
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