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Shark fin test and impression quality: A correlation analysis
journal of dentistry 35 (2007) 409–415
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Shark fin test and impression quality: A correlation analysis Markus Balkenhol a,*, Bernd Wo¨stmann a, Masafumi Kanehira c, Werner J. Finger b a
Department of Prosthetic Dentistry, Justus-Liebig-University, Schlangenzahl 14, 35392 Giessen, Germany Department of Preclinical Dentistry, University of Cologne, Cologne, Germany c Tohoku University Graduate School of Dentistry, Sendai, Japan b
article info
abstract
Article history:
Objectives: To evaluate the correlation between the shark fin test and the dimensional
Received 14 September 2006
accuracy of impressions, surface detail reproduction of impressions and rheological proper-
Received in revised form
ties of impression materials within the manufacturer’s recommended working time.
11 November 2006
Methods: Four chemically different types of impression material (Flexitime: VPS; Fusion:
Accepted 21 November 2006
Polyether/VPS blend; Impregum: classical Polyether; P2: new Polyether) were subjected to the shark fin test as well as three other test regimes. Dimensional accuracy was determined as being the discrepancy in diameter between a steel master cone and stone dies poured
Keywords:
from impressions taken from the steel master cone at defined 30 s intervals after mixing
Shark fin test
within the manufacturer’s recommended working time. Surface detail reproduction was
Impression quality
calculated as being the difference in average arithmetic roughness (Ra) between a ground
Impression materials
dentin surface and the corresponding area of the impressions, taken at the same 30 s
Correlation analysis
intervals. Phase angle and storage modulus were measured using a rotational rheometer.
Dimensional accuracy
Spearman’s Rho was used for correlation analysis.
Surface detail reproduction
Results: With respect to the majority of impression materials used, significant correlations
Phase angle
mainly exist between shark fin test data, phase angle and storage modulus. No correlation
Storage modulus
was found between the results of the shark fin test versus dimensional accuracy, respec-
Clinical relevance
tively, surface detail reproduction.
Impression taking
Conclusions: Results obtained from the shark fin test within the manufacturer’s recommended working time do not allow predictions regarding the dimensional accuracy or surface detail reproduction of impressions as clinically relevant material characteristics. # 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
The shark fin test (SFT) was developed by 3M ESPEb several years ago as ‘‘a simulated application of impression material’’ for illustrating the flow properties of its Polyether product Impregum during impression taking.c In a couple of international conference contributions (IADR: 1997, 2004–2006; AADR:
2001 and 2006; CED-IADR: 2004; PEF-IADR: 2006) and two journal articles,1,2 the SFT was used to compare the flow properties of different impression materials. However, a medline database search using the terms ‘‘shark fin’’ and ‘‘shark fin test’’, respectively, revealed no hit in peer-reviewed literature. Summarizing the information from the available data sources as well as the marketing brochuresd of 3M ESPE, the SFT is
* Corresponding author. Tel.: +49 641 9946 144; fax: +49 641 9946 139. E-mail address: [email protected] (M. Balkenhol). b Impression Materials Update: Studies show clinical advantages in using innovative ‘‘soft’’ polyether versus vinyl polysiloxane. Leaflet 3M ESPE 2004. c http://www.hellmann.com/spotlight.cfm [homepage on the internet]. Solution Spotlight! [updated 15 August 2006]. d Impregum. Technical Product Profile 3M ESPE No. 70200947334/01 (01.05). 0300-5712/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2006.11.009
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journal of dentistry 35 (2007) 409–415
considered ‘‘an established method for analysing flow properties’’ of impression materials throughout the entire manufacturer’s recommended working time (MrWT). Good flow properties are supposed to result in high shark fins and the high fins obtained for the polyether products in turn are interpreted as a marker for ‘‘high clinical reliability’’ during impression taking, especially regarding flow in narrow sulcus areas. However, no peer-reviewed data are available correlating the SFT results with material characteristics, which are considered clinically relevant and consequently decisive for the quality of the final impression. Such parameters are: The impression’s dimensional accuracy (i.e. the ultimate clinically relevant proof), the impression material’s capability of reproducing surface detail and rheological properties (i.e. changes in phase angle and storage modulus during setting) within the MrWT.3–11 Therefore, the aim of this study was to test whether there is any significant correlation between those material characteristics and the results obtained from the SFT within the MrWT. The null hypothesis to be tested was four-fold: There is a correlation between the fin heights and dimensional accuracy of impressions (I), surface detail reproduction of impressions (II), phase angle (III) and storage modulus (IV) of the impression material, respectively, within the MrWT.
2.
Materials and methods
Four chemically different types of impression material were investigated (Table 1). The VPS impression material Flexitime and the VPS/Polyether blend Fusion cure by a Pt-catalysed hydrolisation reaction of vinyl end groups. Impregum is a conventional Polyether impression material that cures by ringopening polymerisation of the end groups of an ethylene-imine terminated polyether compound. According to its’ manufac-
turer, P2 is classified as a polyether impression material that cures via an acid catalysed cross-linking reaction of the end groups of a silane-terminated polyether compound.6,12,13 All materials were used in two different consistencies (types 2 and 3) according to their manufacturer’s instructions. The light body materials, Flex-MP and Fu-MP were extruded from double-chamber cartridges through static mixing tips, whereas P2-MP and Imp-PS were machine-mixed (Pentamix 2 automixing device; 3M ESPE, Seefeld, Germany) and delivered through dynamic mixing tips. All tests were carried out at 30 s intervals until MrWT after end of mixing (t-EoM) except the continuous measurement of the rheological properties and Imp-PS (last interval 15 s prior to MrWT). End of mixing was defined as the time, when the first portion of mixed impression material left the mixing tip. Start of mixing, which is the point of reference for determination of working time,14 was identical with end of mixing apart from a negligible time delay of 1–2 s due to the use of automixing systems. The following tests were performed at ambient laboratory atmosphere (23 8C/50% relative humidity) for each of the impression materials.
2.1.
Shark fin test
A shark fin device was used to determine the fin height of all materials at each 30 s interval after mixing. Fig. 1 shows a schematic cross-sectional drawing of the shark fin device. It basically consists of a split ring (SR; height: 14 mm; inner diameter: 25 mm; wall thickness: 2.5 mm) for incorporating approximately 7 ml of impression material. After the impression material (M) is dispensed in the ring and levelled, the housing (H) is placed on top. The housing contains a round, split cylinder (C) with a V-shaped slit of 1 mm (for type 3 impression materials, weight: 148.85 g) and 2 mm (for type 2 impression materials, weight: 148.25 g) in width at the cylinder periphery.
Table 1 – Impression materials investigated Material
Code
Manufacturer
Viscosity type
Lot
Chemical type
MrWTa [s]
Flexitime Correct Flow
Flex-CF
Heraeus Kulzer (Hanau, Germany)
3
210473
Vinyl polysiloxane (VPS)
150
Fusion Light Body
Fu-LB
GC Dental Products Corp. (Toriimatsu-cho, Japan)
3
0412271
Hybrid polyether/vinyl polysiloxane
120
Impregum Garant L DuoSoft
Imp-GDS
3M ESPE (Seefeld, Germany)
3
B: 202183; C: 201970
Polyether
120
P2 Light
P2-L
Heraeus Kulzer (Hanau, Germany)
3
230213
Polyether
120
Flexitime Mono Phase
Flex-MP
Heraeus Kulzer (Hanau, Germany)
2
230018
Vinyl polysiloxane (VPS)
150
Fusion Monophase
Fu-MP
GC Dental Products Corp. (Toriimatsu-cho, Japan)
2
0501181
Hybrid polyether/vinyl polysiloxane
120
Impregum Penta Soft
Imp-PS
3M ESPE (Seefeld, Germany)
2
B: 203407; C: 203910
Polyether
165
P2 Monophase
P2-MP
Heraeus Kulzer (Hanau, Germany)
2
230329
Polyether
120
a
Manufacturer’s recommended working time.
journal of dentistry 35 (2007) 409–415
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Dies were poured with Moldastone type IV die stone (free linear expansion: 0.09 0.01%, Heraeus Kulzer, Germany) at a 21/100 liquid/powder mixing ratio. The die accuracy, an indirect measure for the dimensional accuracy of the impression, was determined as the occlusal discrepancy at four places 908 apart between the stone model and a seated steel ring, fitting the original steel master cone with less than 3 mm occlusal discrepancy. The occlusal discrepancy was recorded with a displacement transducer (MT 1201, Heidenhain, Germany; measuring accuracy 0.5 mm). The mean value of the 4 readings was used to calculate the deviation from the master cone’s diameter (Dd [mm]).
2.3.
Fig. 1 – Left: Schematic drawing of the shark fin device. SR, split ring; M, impression material; H, housing; C, split cylinder, R, metal rod for releasing the split cylinder. Right: horizontal cross-section of the split cylinder with the Vshaped slit.
After placing the housing, the split cylinder was held in an upper position by a metal rod (R) 1 mm above the surface of the freshly mixed impression material. The split cylinder was released by pulling the rod at the defined 30 s intervals after mixing, allowing the split cylinder to move down and dip inside the impression material. The impression material flowed around the cylinder and entered the slit, resulting in a shark fin like layer of impression material on top. After setting, specimens (n = 6 per material and time interval) were removed from the mould and the maximum height of the fin determined using a Digimatic caliper (Mitotoyo, Japan; measuring accuracy 0.02 mm).
2.2.
Surface detail reproduction
Human molars with at least one proximal surface free of caries and with no restoration were immersed in 1% chloramine solution immediately after extraction and stored at ambient laboratory temperature for a maximum of 6 months before use. The teeth were cleaned with pumice prior to placing them, with a sound peripheral surface facing the bottom of cylindrical rubber moulds (diameter 25 mm), in slow setting epoxy resin (Lekutherm X20, Bayer, Germany). The specimens were then ground in one direction only using wet SiC paper (HandiMet, Buehler, Lake Bluff, USA) grits 240 through 320, until a flat area was exposed in peripheral dentin. After rinsing and gently air-drying the surface, a 20 mm wide adhesive transparent strip (tesaFilm clear, tesa AG, Hamburg, Germany) with a 5.5 mm wide cylindrical hole was placed in the middle of the exposed dentin surface to demarcate the measurement area of the ground dentin. The specimen surface was adjusted horizontally with the grinding scratches aligned at 908 to the direction of measurement. The surface roughness was determined in the marked dentin area using a Perthen 55P surface roughness analyser (Mahr, Go¨ttingen, Germany) equipped with a Focodyn laser scanner (1 mm focus diameter, maximum linear horizontal deviation <1%, Mahr, Go¨ttingen, Germany). Seven line profiles
Dimensional accuracy
Impressions were taken of a truncated steel master cone (8 mm in height and base diameter, 108 convergence angle) in circular perforated steel impression trays (thickness 0.75 mm, inner diameter 16 mm). The trays were clamped in a stand to achieve a uniform impression material thickness of 4 mm between the cone base and tray wall and 2 mm thickness between the top of the cone and tray. The mixed impression material was filled into the tray. Then the steel master cone was placed into the filled impression tray. Impressions were taken at the defined 30 s intervals until MrWT and removed 10 min after mixing. For each material and time interval six impressions were taken and stored dry at ambient laboratory atmosphere for 24 h prior to die casting.
Fig. 2 – Schematic drawing of the acrylic mould (AM) fixed on a flat ground embedded tooth (ET) with demarcated ground dentin area (GDA). Scratches in the GDA aligned perpendicular to the long side of the notch (LSN). Object slide (SL) prior attaching to mould.
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journal of dentistry 35 (2007) 409–415
Table 2 – Fin height in mm (median values, n = 6) including interquartile range determined at the various 30 s intervals in parentheses Material
Fin height [mm] 30 s
60 s
90 s
SRo 120 s
150 s 5.0 (4.9 + 5.0)
Flex-CF
8.8 (8.2 + 8.9)
8.1 (7.9 + 8.3)
8.5 (8.4 + 8.6)
7.1 (7.0 + 7.6)
Fu-LB
9.0 (8.6 + 9.2)
7.3 (7.2 + 7.5)
5.7 (5.5 + 5.8)
5.0 (4.9 + 5.1)
0.97**
16.1 (16.0 + 16.3)
15.6 (15.3 + 15.8)
14.4 (14.2 + 14.7)
13.8 (3.7 + 14.0)
0.97**
P2-L
8.8 (8.1 + 9.9)
7.6 (7.2 + 8.7)
5.9 (5.8 + 6.2)
2.1 (1.9 + 2.7)
0.94**
Flex-MP
2.8 (2.6 + 3.0)
2.1 (2.0 + 2.2)
1.9 (1.9 + 1.9)
1.4 (1.3 + 1.4)
Fu-MP
5.2 (5.0 + 5.4)
4.5 (4.3 + 4.8)
3.4 (3.4 + 3.6)
2.0 (1.9 + 2.0)
Imp-PS
9.9 (9.7 + 9.9)
9.4 (9.3 + 9.5)
9.1 (9.0 + 9.1)
7.4 (7.2 + 7.6)
P2-MP
5.3 (5.0 + 5.7)
3.9 (3.6 + 4.1)
3.0 (2.8 + 3.1)
1.4 (1.2 + 1.6)
Imp-GDS
1.3 (1.3 + 1.3)
0.81**
0.98** 0.95**
4.2 (4.1 + 4.3)
0.98** 0.97**
Spearman’s r values added (SRo) in the last column. *p < 0.05, **p < 0.01 based on the two-sided analysis of significance.
(tracing length 1.5 mm, cut-off 0.25 mm) 100 mm apart were determined per specimen. Mean values of average arithmetic roughness (Ra) were calculated from the individual measurements and used as references (Ra-REF). One dentin specimen was prepared for each impression material. Hereafter, a rectangular acrylic mould (width 35 mm 35 mm, base plate thickness 2 mm) was attached on top of the dentin surface using a double-sided adhesive tape. The acrylic mould had a central 10 mm wide U-shaped notch (length 20 mm) at one side and ended in a half circle (radius 5 mm). A 5 mm wide and 6 mm high rim on three sides of the acrylic mould acted as an objective slide holder. The long side of the U-shaped notch in the middle of the acrylic mould was aligned perpendicular to the grinding scratches in the marked dentin area (Fig. 2). Finally a microscope object slide coated with impression material adhesive was clamped on the base plate within the rims, resulting in a U-shaped slit 2 mm in height and simulating an impression tray. The mixed impression materials were extruded from their respective automixing systems into a Penta elastomer syringe (3M ESPE, Seefeld, Germany), prior to injection into the slit, at the same 30 s intervals after end of mixing within the MrWT as for the
other tests or until injection became impossible due to increased viscosity. After setting, object slides were detached from the acrylic mould exposing the cylindrical impression of the ground dentin surface. The surface roughness of the impression materials (Ra-IMP) in the demarcated area was determined using the same equipment and procedure as described before. For all materials, except the two P2 materials, five impressions were taken. P2 was excluded because of apparent surface layer inhibition after contact with dentin, which rendered determination of surface roughness impossible. The discrepancy between Ra-REF and corresponding Ra-IMP was calculated (DRa [mm]) for all 30 s intervals as a measure of surface detail reproduction of the impression material.
2.4.
Rheological properties
The phase angle d [8] and storage modulus G0 [Pa] were continuously determined for all materials (n = 3) with a rotational rheometer RheoStress 1 (Thermo/Haake, Germany) equipped with a serrated 20 mm base plate and serrated 20 mm sensor with a ceramic shaft (222-1370, Thermo/Haake,
Table 3 – Differences in diameter (Dd [mm]) between stone die and master cone (median values, n = 6) including interquartile range at the 30 s intervals tested in parentheses Material
Dd [mm]
SRo
30 s
60 s
90 s
120 s
150 s
Flex-CF
19.2 (16.2–24.6)
23.5 (21.8–33.7)
28.0 (23.4–31.5)
28.7 (22.2–33.2)
27.4 (16.3–41.0)
Fu-LB
52.1 (47.4–55.0)
52.1 (50.6–59.2)
57.8 (44.5–65.0)
64.0 (53.5–74.0)
0.45*
Imp-GDS
34.2 (28.0–41.1)
34.2 (25.6–39.5)
35.3 (31.7–41.4)
33.2 (28.7–43.0)
0.10
P2-L
73.7 (67.8–80.2)
70.5 (64.6–76.9)
86.9 (76.6–93.5)
66.6 (59.2–81.0)
0.00
Flex-MP
21.0 (18.8–26.9)
24.0 (20.9–34.0)
21.4 (19.3–37.3)
31.6 (26.2–34.5)
Fu-MP
44.1 (41.2–51.1)
45.2 (40.2–49.0)
41.1 (39.5–47.6)
43.1 (40.3–53.7)
Imp-PS
27.9 (16.0–44.7)
21.2 (15.4–23.3)
26.8 (24.6–33.3)
20.1 (18.2–25.0)
P2-MP
69.5 (64.4–72.3)
69.6 (68.6–80.3)
78.4 (71.2–88.6)
77.5 (72.9–88.2)
44.1 (41.1–49.3)
0.34
0.59** 0.05
32.6 (26.4–46.5)
Spearman’s Rho values added (SRo) in the last column. *p < 0.05, **p < 0.01 based on the two-sided analysis of significance.
0.26 0.58**
journal of dentistry 35 (2007) 409–415
413
tions ( p = 0.05) between the SFT on the one hand and the alternative tests on the other. All statistical tests were carried out using SPSS Win 12.0 (SPSS Inc., Chicago, USA).
3.
Fig. 3 – Box-Whisker diagrams representing median values (horizontal bar) of DRa [mm] calculated within the MrWT (n = 5) for the type 3 (a) and type 2 (b) viscosity impression materials. The ends of the boxes represent the 25th (lower) and 75th percentile (upper). Whiskers denote maximum and minimum values excluding extremes (*) and outliers (*).
Results
Fin height decreased for all materials with increasing t-EoM (Table 2). A significant negative correlation between fin height and t-EoM was observed ( p < 0.001) for all eight materials. The differences in diameter (Dd [mm]) between the stone dies and the steel master cone (Table 3) were significantly correlated with t-EoM for Fu-LB ( p < 0.05), P2-MP ( p < 0.01) and Flex-MP ( p < 0.01) only, whereas it was insignificant for all other materials tested ( p > 0.05). During MrWT all materials showed constant DRa values (Fig. 3) as reflected by a lack of significant correlation (Spearman’s Rho p > 0.05) between DRa and t-EoM. It was impossible to determine the Ra-IMP values of P2-L and P2-MP due to a distinct inhibition layer. Imp-PS could not be extruded from the syringe 150 s after end of mixing due to increased viscosity. The results of the rheological analysis (Fig. 4) revealed a significant negative correlation ( p < 0.05) between d(t) and tEoM for all materials except Imp-GDS and Imp-PS ( p > 0.05) and a significant positive correlation between G0 (t) and t-EoM for all materials ( p < 0.05) except Imp-PS ( p > 0.05).
Germany), at a distance of 0.5 mm and an impression material volume of 0.5 ml, loaded on the lower plate. In pretests the serrated base plate and sensor have proven to be effective means for this purpose as they prevent the impression material slipping and, consequently, acquiring flawed data sets. The tests were carried out at 1 Hz oscillation frequency8 in the CS mode (controlled stress) using t = 800 Pa for type 3 materials and t = 2000 Pa for type 2 materials. d(t) and G0 (t) were calculated with the Software RheoWin 3.21 (Thermo/Haake, Germany). The measurement was initiated 3–5 s after mixing and continued until phase angle and storage modulus reached a constant level. For statistical analysis, d(t) and G0 (t) were calculated as means of three measurements at the same 30 s intervals as for the other tests.
2.5.
Statistical analysis
All data sets were subjected to the Kolmogorov–Smirnov test to check for normal distribution ( p = 0.05). Since not all groups were normally distributed, non-parametric statistics were applied (Spearman’s Rho) to identify significant correlations between the respective test results and t-EoM within the MrWT ( p = 0.05). For each regime median values for every 30 s interval and material were aggregated and tested for significant correla-
Fig. 4 – Line diagrams representing the median values (n = 3) of the phase angle (d [8] = upper lines) and storage modulus (G0 [Pa] = lower lines) for the type 3 (a) and type 2 (b) impression materials.
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journal of dentistry 35 (2007) 409–415
Table 4 – Coefficient of correlation (Spearman’s Rho) between fin height and all other tests performed Material
Dd
DRa
d(t)
G0 (t)
Flex-CF Fu-LB Imp-GDS P2-L
0.5 1.0** 0.4 0.4
0.4 0.4 1.0** n.a.
0.9* 1.0** 0.3 1.0**
0.9* 1.0** 0.8 1.0**
Flex-MP Fu-MP Imp-PS P2-MP
0.9* 0.6 0.1 0.8
0.7 0.8 0.2 n.a.
1.0** 1.0** 0.4 1.0**
1.0** 1.0** 0.3 1.0**
*p < 0.05, **p < 0.01 based on the two-sided analysis of significance. Dd, difference in diameter between master cone and stone die; DRa, difference in average surface roughness between ground dentin surface and corresponding impression area; d(t), phase angle; G0 (t), storage modulus.
The overall correlation analysis (Table 4) was significant ( p < 0.05) primarily for the correlation between fin height and d(t) as well as G0 (t) except for Imp-GDS and Imp-PS ( p > 0.05). With regard to all other paired groups, a significant correlation was only observed between fin height and Dd for Fu-LB ( p < 0.01) and Flex-MP ( p < 0.05).
4.
Discussion
The shark fin test was introduced by 3M ESPE several years ago to illustrate the flow properties of their polyether impression materials in relation to clinically reliable impression taking. However, the interpretation of the test results lacks peerreviewed evidence regarding correlation to other clinical and physical material parameters. Consequently, our study aimed to identify possible correlations between results obtained with the SFT on the one hand and material properties considered clinically relevant on the other. All experiments were carried out at 23 8C, since determination of working time according to ISO 482314 as well as the SFT is carried out at this temperature. The results thus reflect the properties of the tested materials at room temperature. However, in clinical situations time for further manipulation is reduced as soon as the impression material is applied into the oral cavity due to increased temperature.4 The ISO 4823 recommendation for start of mixing was obeyed for all measurements, since this is the point of reference for manufacturers to determine the working time and to comply with the standard. For determination of dimensional accuracy a truncated steel master cone was selected as a master abutment simulating a tooth after crown preparation. Undercuts at the tapered master cone were omitted for the following reason: When removed from undercuts, a set impression material is deformed in the undercut areas, which in turn may lead to permanent deformation. This deformation might then hide inaccuracies, caused by elastic rebound as a consequence of progressed cross-linking and chain elongation at late stages after end of mixing. The same test set-up has been used successfully in previous studies.6,15–17 Contact-free measurement of the surface roughness9 of the two substrates (impression surface versus dentin surface)
might have influenced the DRa values due to different optical properties. However, since the DRa did not exceed 0.5 mm, it is believed that potential influence of the different optical properties was negligible in the present study. In addition, the surfaces of all specimens were inspected for irregularities (e.g. inhibition layer) that might have influenced the results. Due to the selected 30 s intervals, the last point of observation was in between two 30 s intervals for Imp-PS (15 s prior to MrWT). Therefore we checked whether this might have affected the test results. Taking into consideration the last point of observation at 180 s for Imp-PS (15 s beyond the MrWT) no difference in significance for the correlation analysis between each of the test parameters and t-EoM was observed, except for the correlation between G0 (t) and tEoM ( p < 0.05). However, the significance levels of the overall analysis were not affected. Consequently, the conclusions drawn from the results are valid. After mixing, the rheological properties of impression materials change during polymerisation from a more fluid-like to a more elastic behaviour upon setting.4,8,18,19 The phase angle d [8] and storage modulus G0 [Pa] are two parameters that are considered appropriate measures to describe such changes during setting.4,8,18 As the SFT is said to represent such material characteristics,2 these two parameters were determined. For all materials tested, a decrease in fin height was noted in the SFT caused by a change in flowability representing chain elongation and cross-linking reaction within the impression material after mixing.19,20 As the cross-linking reaction can be traced by quantifying the phase angle and storage modulus,4,8 the significant correlation observed between fin height and d(t) as well as fin height and G0 (t) for most materials was not surprising. However, the lack of correlation for Imp-GDS und Imp-PS lead us to conclude, that change in fin height is not fully reflected by d(t) and G0 (t). Hence fin height may possibly be related to flow characteristics caused by e.g. filler type and shape and/or monomer properties rather than the change in flow properties caused by the cross-linking reaction during setting. Based on this assumption it is hypothesized that other rheological properties (e.g. shear viscosity, thixotropy, yield point) might be more suitable for explaining the results of the shark fin test. Determination of, e.g. shear viscosity allows conclusions regarding the intermolecular shear forces within a fluid, which are mainly influenced by the interaction between the monomer molecules. Therefore the chemical composition of a classical Polyether might be the key parameter, which influences the shear viscosity and potentially the results of the SFT most. Our protocol in contrast was adapted to quantify d(t) and G0 (t) and the use of serrated plates made determination of further rheological properties, especially shear viscosity, impossible. As most materials investigated showed no significant correlation between dimensional accuracy and t-EoM it is concluded, that it is possible to produce accurate impressions, even during late stages within the MrWT, with the majority of impression materials. Consequently dimensional accuracy of impressions is not necessarily affected by the change of flow properties of an impression material during setting, as long as the impression is taken within the MrWT.
journal of dentistry 35 (2007) 409–415
The surface detail reproduction of the impression materials tested was constant during MrWT, representing independence of t-EoM. Therefore SFT results do not reflect the ability of an impression material to precisely reproduce surface details. This was finally proven by a lack of significant correlation between SFT and surface detail reproduction. In summary, the results of all four parts (I–IV) of the null hypothesis have to be rejected. Results of the SFT neither correlate with the dimensional accuracy nor with the surface detail reproduction, phase angle, and storage modulus of impression materials determined after mixing within the MrWT. Consequently the SFT has to be seen as a test in its own right, which does not reflect the results of the accepted and clinically relevant materials’ properties tests for impression materials used in this study. Good flow of an impression material – which might be expressed by high shark fins obtained in the SFT – is not necessarily the pre-requisite for high dimensional accuracy and good surface detail reproduction of the impressions, respectively.21,22 In addition it has to be considered, that good flow of an impression material might be advantageous for distinct clinical situations (e.g. inlay and onlay preparations), whereas other clinical situations (i.e. infragingival finishing lines) definitely require a more firm and less flowable viscosity for successful impression taking.21,22 Taking into account this clinical experience, it is speculated that the SFT may represent advantageous flow properties that, in turn, may correlate to void-free impression taking of complex shaped bodies, e.g. implant impression posts and onlay or inlay preparations. This question, however, has to be addressed in further laboratory and clinical studies. It is suggested that, for unequivocal interpretation of test results with regard to their clinical relevance, future materials’ properties tests should be subjected to scientific scrutiny prior to introduction.
5.
Conclusions
Results obtained from the SFT do not allow predictions regarding dimensional accuracy or surface detail reproduction of impressions taken within or at the end of the working time. These parameters are consequently not affected by the flow properties determined with the SFT. The SFT only partially reflects the cross-linking reaction of a material during setting, as no correlation between fin height and phase angle was observed for the Impregum products.
Acknowledgements We would like to thank Dr. Manfred Hollenhorst of the Department for Documentation, Justus-Liebig-University Computer Centre, Giessen, for his assistance with the statistical analysis. In addition, the authors appreciate the materials donated by the respective manufacturers. We would also like to thank Heinrich Bethge of the Dental Clinic Giessen for creating the drawings.
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references
1. Wirz J, Schmidli F. Moderne Werkstoffe zur Abformung und Bissregistrierung. ZWR 2004;113:126–36. 2. Klettke T, Dauelsberg H-J, Zawta C. Entscheidende Eigenschaften von Pra¨zisionsabformmaterialien fu¨r den klinischen Erfolg. Quintessenz Zahntechnik 2005;31:414–30. 3. Tan E, Chai J. Working times of elastomeric impression materials according to dimensional stability and detail reproduction. International Journal of Prosthodontics 1995;8:541–7. 4. Berg JC, Johnson GH, Lepe X, Plaza SA. Temperature effects on the rheological properties of current polyether and polysiloxane impression materials during setting. Journal of Prosthetic Dentistry 2003;90:150–61. 5. Chai J, Pang IC, Leong D, Kim JE, Torres P. An investigation of the rheological properties of 10 vinyl-polysiloxane impression materials. Journal of Prosthodontics 1993;2:167–73. 6. Endo T, Finger WJ. Dimensional accuracy of a new polyether impression material. Quintessence International 2006;37:47–51. 7. Finger W, Ohsawa M. Accuracy of cast restorations produced by a refractory die-investing technique. Journal of Prosthetic Dentistry 1984;52:800–3. 8. McCabe JF, Carrick TE. Rheological properties of elastomers during setting. Journal of Dental Research 1989;68:1218–22. 9. Takahashi H, Finger WJ. Dentin surface reproduction with hydrophilic and hydrophobic impression materials. Dental Materials 1991;7:197–201. 10. Johnson GH, Lepe X, Aw TC. The effect of surface moisture on detail reproduction of elastomeric impressions. Journal of Prosthetic Dentistry 2003;90:354–64. 11. Petrie CS, Walker MP, O’Mahony AM, Spencer P. Dimensional accuracy and surface detail reproduction of two hydrophilic vinyl polysiloxane impression materials tested under dry, moist, and wet conditions. Journal of Prosthetic Dentistry 2003;90:365–72. 12. Kanehira M, Finger WJ, Endo T. Volatilization of components from and water absorption of polyether impressions. Journal of Dentistry 2006;34:134–8. 13. Endo T, Finger WJ. Evaluation of the elastic recovery of polyether impression materials. American Journal of Dentistry 2005;18:355–60. 14. ISO 4823: Dentistry—elastomeric impression materials. 3rd ed. International Standardization Organization; 2000. 15. Ohsawa M, Finger W. Working time of elastomeric impression materials. Dental Materials 1986;2:179–82. 16. Finger W, Ohsawa M. Accuracy of stone-casts produced from selected addition-type silicone impressions. Scandinavian Journal of Dental Research 1983;91:61–5. 17. Takahashi H, Finger WJ. Effects of the setting stage on the accuracy of double-mix impressions made with addition-curing silicone. Journal of Prosthetic Dentistry 1994;72:78–84. 18. McCabe JF, Carrick TE. Onset of elasticity in setting elastomers. Journal of Dental Research 1990;69:1573–5. 19. Metzger S T.. Das Rheologie-Handbuch. 1st ed. Hannover: Curt R. Vincentz; 2000. 20. Chiayi S. Impression materials. In: Anusavice KJ, editor. Phillips’ science of dental materials. 11th ed. St. Louis: Saunders; 2003. p. 205–54. 21. Wo¨stmann B. Klinische Bestimmungsvariablen bei Abformung pra¨parierter Za¨hne. Stomatologie 1996;93: 51–7. 22. Wo¨stmann B. Clinical parameters of impression techniques in dentistry. Stomatologie 1996;93:531–2.
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Shark fin test and impression quality: A correlation analysis Markus Balkenhol a,*, Bernd Wo¨stmann a, Masafumi Kanehira c, Werner J. Finger b a
Department of Prosthetic Dentistry, Justus-Liebig-University, Schlangenzahl 14, 35392 Giessen, Germany Department of Preclinical Dentistry, University of Cologne, Cologne, Germany c Tohoku University Graduate School of Dentistry, Sendai, Japan b
article info
abstract
Article history:
Objectives: To evaluate the correlation between the shark fin test and the dimensional
Received 14 September 2006
accuracy of impressions, surface detail reproduction of impressions and rheological proper-
Received in revised form
ties of impression materials within the manufacturer’s recommended working time.
11 November 2006
Methods: Four chemically different types of impression material (Flexitime: VPS; Fusion:
Accepted 21 November 2006
Polyether/VPS blend; Impregum: classical Polyether; P2: new Polyether) were subjected to the shark fin test as well as three other test regimes. Dimensional accuracy was determined as being the discrepancy in diameter between a steel master cone and stone dies poured
Keywords:
from impressions taken from the steel master cone at defined 30 s intervals after mixing
Shark fin test
within the manufacturer’s recommended working time. Surface detail reproduction was
Impression quality
calculated as being the difference in average arithmetic roughness (Ra) between a ground
Impression materials
dentin surface and the corresponding area of the impressions, taken at the same 30 s
Correlation analysis
intervals. Phase angle and storage modulus were measured using a rotational rheometer.
Dimensional accuracy
Spearman’s Rho was used for correlation analysis.
Surface detail reproduction
Results: With respect to the majority of impression materials used, significant correlations
Phase angle
mainly exist between shark fin test data, phase angle and storage modulus. No correlation
Storage modulus
was found between the results of the shark fin test versus dimensional accuracy, respec-
Clinical relevance
tively, surface detail reproduction.
Impression taking
Conclusions: Results obtained from the shark fin test within the manufacturer’s recommended working time do not allow predictions regarding the dimensional accuracy or surface detail reproduction of impressions as clinically relevant material characteristics. # 2006 Elsevier Ltd. All rights reserved.
1.
Introduction
The shark fin test (SFT) was developed by 3M ESPEb several years ago as ‘‘a simulated application of impression material’’ for illustrating the flow properties of its Polyether product Impregum during impression taking.c In a couple of international conference contributions (IADR: 1997, 2004–2006; AADR:
2001 and 2006; CED-IADR: 2004; PEF-IADR: 2006) and two journal articles,1,2 the SFT was used to compare the flow properties of different impression materials. However, a medline database search using the terms ‘‘shark fin’’ and ‘‘shark fin test’’, respectively, revealed no hit in peer-reviewed literature. Summarizing the information from the available data sources as well as the marketing brochuresd of 3M ESPE, the SFT is
* Corresponding author. Tel.: +49 641 9946 144; fax: +49 641 9946 139. E-mail address: [email protected] (M. Balkenhol). b Impression Materials Update: Studies show clinical advantages in using innovative ‘‘soft’’ polyether versus vinyl polysiloxane. Leaflet 3M ESPE 2004. c http://www.hellmann.com/spotlight.cfm [homepage on the internet]. Solution Spotlight! [updated 15 August 2006]. d Impregum. Technical Product Profile 3M ESPE No. 70200947334/01 (01.05). 0300-5712/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2006.11.009
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journal of dentistry 35 (2007) 409–415
considered ‘‘an established method for analysing flow properties’’ of impression materials throughout the entire manufacturer’s recommended working time (MrWT). Good flow properties are supposed to result in high shark fins and the high fins obtained for the polyether products in turn are interpreted as a marker for ‘‘high clinical reliability’’ during impression taking, especially regarding flow in narrow sulcus areas. However, no peer-reviewed data are available correlating the SFT results with material characteristics, which are considered clinically relevant and consequently decisive for the quality of the final impression. Such parameters are: The impression’s dimensional accuracy (i.e. the ultimate clinically relevant proof), the impression material’s capability of reproducing surface detail and rheological properties (i.e. changes in phase angle and storage modulus during setting) within the MrWT.3–11 Therefore, the aim of this study was to test whether there is any significant correlation between those material characteristics and the results obtained from the SFT within the MrWT. The null hypothesis to be tested was four-fold: There is a correlation between the fin heights and dimensional accuracy of impressions (I), surface detail reproduction of impressions (II), phase angle (III) and storage modulus (IV) of the impression material, respectively, within the MrWT.
2.
Materials and methods
Four chemically different types of impression material were investigated (Table 1). The VPS impression material Flexitime and the VPS/Polyether blend Fusion cure by a Pt-catalysed hydrolisation reaction of vinyl end groups. Impregum is a conventional Polyether impression material that cures by ringopening polymerisation of the end groups of an ethylene-imine terminated polyether compound. According to its’ manufac-
turer, P2 is classified as a polyether impression material that cures via an acid catalysed cross-linking reaction of the end groups of a silane-terminated polyether compound.6,12,13 All materials were used in two different consistencies (types 2 and 3) according to their manufacturer’s instructions. The light body materials, Flex-MP and Fu-MP were extruded from double-chamber cartridges through static mixing tips, whereas P2-MP and Imp-PS were machine-mixed (Pentamix 2 automixing device; 3M ESPE, Seefeld, Germany) and delivered through dynamic mixing tips. All tests were carried out at 30 s intervals until MrWT after end of mixing (t-EoM) except the continuous measurement of the rheological properties and Imp-PS (last interval 15 s prior to MrWT). End of mixing was defined as the time, when the first portion of mixed impression material left the mixing tip. Start of mixing, which is the point of reference for determination of working time,14 was identical with end of mixing apart from a negligible time delay of 1–2 s due to the use of automixing systems. The following tests were performed at ambient laboratory atmosphere (23 8C/50% relative humidity) for each of the impression materials.
2.1.
Shark fin test
A shark fin device was used to determine the fin height of all materials at each 30 s interval after mixing. Fig. 1 shows a schematic cross-sectional drawing of the shark fin device. It basically consists of a split ring (SR; height: 14 mm; inner diameter: 25 mm; wall thickness: 2.5 mm) for incorporating approximately 7 ml of impression material. After the impression material (M) is dispensed in the ring and levelled, the housing (H) is placed on top. The housing contains a round, split cylinder (C) with a V-shaped slit of 1 mm (for type 3 impression materials, weight: 148.85 g) and 2 mm (for type 2 impression materials, weight: 148.25 g) in width at the cylinder periphery.
Table 1 – Impression materials investigated Material
Code
Manufacturer
Viscosity type
Lot
Chemical type
MrWTa [s]
Flexitime Correct Flow
Flex-CF
Heraeus Kulzer (Hanau, Germany)
3
210473
Vinyl polysiloxane (VPS)
150
Fusion Light Body
Fu-LB
GC Dental Products Corp. (Toriimatsu-cho, Japan)
3
0412271
Hybrid polyether/vinyl polysiloxane
120
Impregum Garant L DuoSoft
Imp-GDS
3M ESPE (Seefeld, Germany)
3
B: 202183; C: 201970
Polyether
120
P2 Light
P2-L
Heraeus Kulzer (Hanau, Germany)
3
230213
Polyether
120
Flexitime Mono Phase
Flex-MP
Heraeus Kulzer (Hanau, Germany)
2
230018
Vinyl polysiloxane (VPS)
150
Fusion Monophase
Fu-MP
GC Dental Products Corp. (Toriimatsu-cho, Japan)
2
0501181
Hybrid polyether/vinyl polysiloxane
120
Impregum Penta Soft
Imp-PS
3M ESPE (Seefeld, Germany)
2
B: 203407; C: 203910
Polyether
165
P2 Monophase
P2-MP
Heraeus Kulzer (Hanau, Germany)
2
230329
Polyether
120
a
Manufacturer’s recommended working time.
journal of dentistry 35 (2007) 409–415
411
Dies were poured with Moldastone type IV die stone (free linear expansion: 0.09 0.01%, Heraeus Kulzer, Germany) at a 21/100 liquid/powder mixing ratio. The die accuracy, an indirect measure for the dimensional accuracy of the impression, was determined as the occlusal discrepancy at four places 908 apart between the stone model and a seated steel ring, fitting the original steel master cone with less than 3 mm occlusal discrepancy. The occlusal discrepancy was recorded with a displacement transducer (MT 1201, Heidenhain, Germany; measuring accuracy 0.5 mm). The mean value of the 4 readings was used to calculate the deviation from the master cone’s diameter (Dd [mm]).
2.3.
Fig. 1 – Left: Schematic drawing of the shark fin device. SR, split ring; M, impression material; H, housing; C, split cylinder, R, metal rod for releasing the split cylinder. Right: horizontal cross-section of the split cylinder with the Vshaped slit.
After placing the housing, the split cylinder was held in an upper position by a metal rod (R) 1 mm above the surface of the freshly mixed impression material. The split cylinder was released by pulling the rod at the defined 30 s intervals after mixing, allowing the split cylinder to move down and dip inside the impression material. The impression material flowed around the cylinder and entered the slit, resulting in a shark fin like layer of impression material on top. After setting, specimens (n = 6 per material and time interval) were removed from the mould and the maximum height of the fin determined using a Digimatic caliper (Mitotoyo, Japan; measuring accuracy 0.02 mm).
2.2.
Surface detail reproduction
Human molars with at least one proximal surface free of caries and with no restoration were immersed in 1% chloramine solution immediately after extraction and stored at ambient laboratory temperature for a maximum of 6 months before use. The teeth were cleaned with pumice prior to placing them, with a sound peripheral surface facing the bottom of cylindrical rubber moulds (diameter 25 mm), in slow setting epoxy resin (Lekutherm X20, Bayer, Germany). The specimens were then ground in one direction only using wet SiC paper (HandiMet, Buehler, Lake Bluff, USA) grits 240 through 320, until a flat area was exposed in peripheral dentin. After rinsing and gently air-drying the surface, a 20 mm wide adhesive transparent strip (tesaFilm clear, tesa AG, Hamburg, Germany) with a 5.5 mm wide cylindrical hole was placed in the middle of the exposed dentin surface to demarcate the measurement area of the ground dentin. The specimen surface was adjusted horizontally with the grinding scratches aligned at 908 to the direction of measurement. The surface roughness was determined in the marked dentin area using a Perthen 55P surface roughness analyser (Mahr, Go¨ttingen, Germany) equipped with a Focodyn laser scanner (1 mm focus diameter, maximum linear horizontal deviation <1%, Mahr, Go¨ttingen, Germany). Seven line profiles
Dimensional accuracy
Impressions were taken of a truncated steel master cone (8 mm in height and base diameter, 108 convergence angle) in circular perforated steel impression trays (thickness 0.75 mm, inner diameter 16 mm). The trays were clamped in a stand to achieve a uniform impression material thickness of 4 mm between the cone base and tray wall and 2 mm thickness between the top of the cone and tray. The mixed impression material was filled into the tray. Then the steel master cone was placed into the filled impression tray. Impressions were taken at the defined 30 s intervals until MrWT and removed 10 min after mixing. For each material and time interval six impressions were taken and stored dry at ambient laboratory atmosphere for 24 h prior to die casting.
Fig. 2 – Schematic drawing of the acrylic mould (AM) fixed on a flat ground embedded tooth (ET) with demarcated ground dentin area (GDA). Scratches in the GDA aligned perpendicular to the long side of the notch (LSN). Object slide (SL) prior attaching to mould.
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journal of dentistry 35 (2007) 409–415
Table 2 – Fin height in mm (median values, n = 6) including interquartile range determined at the various 30 s intervals in parentheses Material
Fin height [mm] 30 s
60 s
90 s
SRo 120 s
150 s 5.0 (4.9 + 5.0)
Flex-CF
8.8 (8.2 + 8.9)
8.1 (7.9 + 8.3)
8.5 (8.4 + 8.6)
7.1 (7.0 + 7.6)
Fu-LB
9.0 (8.6 + 9.2)
7.3 (7.2 + 7.5)
5.7 (5.5 + 5.8)
5.0 (4.9 + 5.1)
0.97**
16.1 (16.0 + 16.3)
15.6 (15.3 + 15.8)
14.4 (14.2 + 14.7)
13.8 (3.7 + 14.0)
0.97**
P2-L
8.8 (8.1 + 9.9)
7.6 (7.2 + 8.7)
5.9 (5.8 + 6.2)
2.1 (1.9 + 2.7)
0.94**
Flex-MP
2.8 (2.6 + 3.0)
2.1 (2.0 + 2.2)
1.9 (1.9 + 1.9)
1.4 (1.3 + 1.4)
Fu-MP
5.2 (5.0 + 5.4)
4.5 (4.3 + 4.8)
3.4 (3.4 + 3.6)
2.0 (1.9 + 2.0)
Imp-PS
9.9 (9.7 + 9.9)
9.4 (9.3 + 9.5)
9.1 (9.0 + 9.1)
7.4 (7.2 + 7.6)
P2-MP
5.3 (5.0 + 5.7)
3.9 (3.6 + 4.1)
3.0 (2.8 + 3.1)
1.4 (1.2 + 1.6)
Imp-GDS
1.3 (1.3 + 1.3)
0.81**
0.98** 0.95**
4.2 (4.1 + 4.3)
0.98** 0.97**
Spearman’s r values added (SRo) in the last column. *p < 0.05, **p < 0.01 based on the two-sided analysis of significance.
(tracing length 1.5 mm, cut-off 0.25 mm) 100 mm apart were determined per specimen. Mean values of average arithmetic roughness (Ra) were calculated from the individual measurements and used as references (Ra-REF). One dentin specimen was prepared for each impression material. Hereafter, a rectangular acrylic mould (width 35 mm 35 mm, base plate thickness 2 mm) was attached on top of the dentin surface using a double-sided adhesive tape. The acrylic mould had a central 10 mm wide U-shaped notch (length 20 mm) at one side and ended in a half circle (radius 5 mm). A 5 mm wide and 6 mm high rim on three sides of the acrylic mould acted as an objective slide holder. The long side of the U-shaped notch in the middle of the acrylic mould was aligned perpendicular to the grinding scratches in the marked dentin area (Fig. 2). Finally a microscope object slide coated with impression material adhesive was clamped on the base plate within the rims, resulting in a U-shaped slit 2 mm in height and simulating an impression tray. The mixed impression materials were extruded from their respective automixing systems into a Penta elastomer syringe (3M ESPE, Seefeld, Germany), prior to injection into the slit, at the same 30 s intervals after end of mixing within the MrWT as for the
other tests or until injection became impossible due to increased viscosity. After setting, object slides were detached from the acrylic mould exposing the cylindrical impression of the ground dentin surface. The surface roughness of the impression materials (Ra-IMP) in the demarcated area was determined using the same equipment and procedure as described before. For all materials, except the two P2 materials, five impressions were taken. P2 was excluded because of apparent surface layer inhibition after contact with dentin, which rendered determination of surface roughness impossible. The discrepancy between Ra-REF and corresponding Ra-IMP was calculated (DRa [mm]) for all 30 s intervals as a measure of surface detail reproduction of the impression material.
2.4.
Rheological properties
The phase angle d [8] and storage modulus G0 [Pa] were continuously determined for all materials (n = 3) with a rotational rheometer RheoStress 1 (Thermo/Haake, Germany) equipped with a serrated 20 mm base plate and serrated 20 mm sensor with a ceramic shaft (222-1370, Thermo/Haake,
Table 3 – Differences in diameter (Dd [mm]) between stone die and master cone (median values, n = 6) including interquartile range at the 30 s intervals tested in parentheses Material
Dd [mm]
SRo
30 s
60 s
90 s
120 s
150 s
Flex-CF
19.2 (16.2–24.6)
23.5 (21.8–33.7)
28.0 (23.4–31.5)
28.7 (22.2–33.2)
27.4 (16.3–41.0)
Fu-LB
52.1 (47.4–55.0)
52.1 (50.6–59.2)
57.8 (44.5–65.0)
64.0 (53.5–74.0)
0.45*
Imp-GDS
34.2 (28.0–41.1)
34.2 (25.6–39.5)
35.3 (31.7–41.4)
33.2 (28.7–43.0)
0.10
P2-L
73.7 (67.8–80.2)
70.5 (64.6–76.9)
86.9 (76.6–93.5)
66.6 (59.2–81.0)
0.00
Flex-MP
21.0 (18.8–26.9)
24.0 (20.9–34.0)
21.4 (19.3–37.3)
31.6 (26.2–34.5)
Fu-MP
44.1 (41.2–51.1)
45.2 (40.2–49.0)
41.1 (39.5–47.6)
43.1 (40.3–53.7)
Imp-PS
27.9 (16.0–44.7)
21.2 (15.4–23.3)
26.8 (24.6–33.3)
20.1 (18.2–25.0)
P2-MP
69.5 (64.4–72.3)
69.6 (68.6–80.3)
78.4 (71.2–88.6)
77.5 (72.9–88.2)
44.1 (41.1–49.3)
0.34
0.59** 0.05
32.6 (26.4–46.5)
Spearman’s Rho values added (SRo) in the last column. *p < 0.05, **p < 0.01 based on the two-sided analysis of significance.
0.26 0.58**
journal of dentistry 35 (2007) 409–415
413
tions ( p = 0.05) between the SFT on the one hand and the alternative tests on the other. All statistical tests were carried out using SPSS Win 12.0 (SPSS Inc., Chicago, USA).
3.
Fig. 3 – Box-Whisker diagrams representing median values (horizontal bar) of DRa [mm] calculated within the MrWT (n = 5) for the type 3 (a) and type 2 (b) viscosity impression materials. The ends of the boxes represent the 25th (lower) and 75th percentile (upper). Whiskers denote maximum and minimum values excluding extremes (*) and outliers (*).
Results
Fin height decreased for all materials with increasing t-EoM (Table 2). A significant negative correlation between fin height and t-EoM was observed ( p < 0.001) for all eight materials. The differences in diameter (Dd [mm]) between the stone dies and the steel master cone (Table 3) were significantly correlated with t-EoM for Fu-LB ( p < 0.05), P2-MP ( p < 0.01) and Flex-MP ( p < 0.01) only, whereas it was insignificant for all other materials tested ( p > 0.05). During MrWT all materials showed constant DRa values (Fig. 3) as reflected by a lack of significant correlation (Spearman’s Rho p > 0.05) between DRa and t-EoM. It was impossible to determine the Ra-IMP values of P2-L and P2-MP due to a distinct inhibition layer. Imp-PS could not be extruded from the syringe 150 s after end of mixing due to increased viscosity. The results of the rheological analysis (Fig. 4) revealed a significant negative correlation ( p < 0.05) between d(t) and tEoM for all materials except Imp-GDS and Imp-PS ( p > 0.05) and a significant positive correlation between G0 (t) and t-EoM for all materials ( p < 0.05) except Imp-PS ( p > 0.05).
Germany), at a distance of 0.5 mm and an impression material volume of 0.5 ml, loaded on the lower plate. In pretests the serrated base plate and sensor have proven to be effective means for this purpose as they prevent the impression material slipping and, consequently, acquiring flawed data sets. The tests were carried out at 1 Hz oscillation frequency8 in the CS mode (controlled stress) using t = 800 Pa for type 3 materials and t = 2000 Pa for type 2 materials. d(t) and G0 (t) were calculated with the Software RheoWin 3.21 (Thermo/Haake, Germany). The measurement was initiated 3–5 s after mixing and continued until phase angle and storage modulus reached a constant level. For statistical analysis, d(t) and G0 (t) were calculated as means of three measurements at the same 30 s intervals as for the other tests.
2.5.
Statistical analysis
All data sets were subjected to the Kolmogorov–Smirnov test to check for normal distribution ( p = 0.05). Since not all groups were normally distributed, non-parametric statistics were applied (Spearman’s Rho) to identify significant correlations between the respective test results and t-EoM within the MrWT ( p = 0.05). For each regime median values for every 30 s interval and material were aggregated and tested for significant correla-
Fig. 4 – Line diagrams representing the median values (n = 3) of the phase angle (d [8] = upper lines) and storage modulus (G0 [Pa] = lower lines) for the type 3 (a) and type 2 (b) impression materials.
414
journal of dentistry 35 (2007) 409–415
Table 4 – Coefficient of correlation (Spearman’s Rho) between fin height and all other tests performed Material
Dd
DRa
d(t)
G0 (t)
Flex-CF Fu-LB Imp-GDS P2-L
0.5 1.0** 0.4 0.4
0.4 0.4 1.0** n.a.
0.9* 1.0** 0.3 1.0**
0.9* 1.0** 0.8 1.0**
Flex-MP Fu-MP Imp-PS P2-MP
0.9* 0.6 0.1 0.8
0.7 0.8 0.2 n.a.
1.0** 1.0** 0.4 1.0**
1.0** 1.0** 0.3 1.0**
*p < 0.05, **p < 0.01 based on the two-sided analysis of significance. Dd, difference in diameter between master cone and stone die; DRa, difference in average surface roughness between ground dentin surface and corresponding impression area; d(t), phase angle; G0 (t), storage modulus.
The overall correlation analysis (Table 4) was significant ( p < 0.05) primarily for the correlation between fin height and d(t) as well as G0 (t) except for Imp-GDS and Imp-PS ( p > 0.05). With regard to all other paired groups, a significant correlation was only observed between fin height and Dd for Fu-LB ( p < 0.01) and Flex-MP ( p < 0.05).
4.
Discussion
The shark fin test was introduced by 3M ESPE several years ago to illustrate the flow properties of their polyether impression materials in relation to clinically reliable impression taking. However, the interpretation of the test results lacks peerreviewed evidence regarding correlation to other clinical and physical material parameters. Consequently, our study aimed to identify possible correlations between results obtained with the SFT on the one hand and material properties considered clinically relevant on the other. All experiments were carried out at 23 8C, since determination of working time according to ISO 482314 as well as the SFT is carried out at this temperature. The results thus reflect the properties of the tested materials at room temperature. However, in clinical situations time for further manipulation is reduced as soon as the impression material is applied into the oral cavity due to increased temperature.4 The ISO 4823 recommendation for start of mixing was obeyed for all measurements, since this is the point of reference for manufacturers to determine the working time and to comply with the standard. For determination of dimensional accuracy a truncated steel master cone was selected as a master abutment simulating a tooth after crown preparation. Undercuts at the tapered master cone were omitted for the following reason: When removed from undercuts, a set impression material is deformed in the undercut areas, which in turn may lead to permanent deformation. This deformation might then hide inaccuracies, caused by elastic rebound as a consequence of progressed cross-linking and chain elongation at late stages after end of mixing. The same test set-up has been used successfully in previous studies.6,15–17 Contact-free measurement of the surface roughness9 of the two substrates (impression surface versus dentin surface)
might have influenced the DRa values due to different optical properties. However, since the DRa did not exceed 0.5 mm, it is believed that potential influence of the different optical properties was negligible in the present study. In addition, the surfaces of all specimens were inspected for irregularities (e.g. inhibition layer) that might have influenced the results. Due to the selected 30 s intervals, the last point of observation was in between two 30 s intervals for Imp-PS (15 s prior to MrWT). Therefore we checked whether this might have affected the test results. Taking into consideration the last point of observation at 180 s for Imp-PS (15 s beyond the MrWT) no difference in significance for the correlation analysis between each of the test parameters and t-EoM was observed, except for the correlation between G0 (t) and tEoM ( p < 0.05). However, the significance levels of the overall analysis were not affected. Consequently, the conclusions drawn from the results are valid. After mixing, the rheological properties of impression materials change during polymerisation from a more fluid-like to a more elastic behaviour upon setting.4,8,18,19 The phase angle d [8] and storage modulus G0 [Pa] are two parameters that are considered appropriate measures to describe such changes during setting.4,8,18 As the SFT is said to represent such material characteristics,2 these two parameters were determined. For all materials tested, a decrease in fin height was noted in the SFT caused by a change in flowability representing chain elongation and cross-linking reaction within the impression material after mixing.19,20 As the cross-linking reaction can be traced by quantifying the phase angle and storage modulus,4,8 the significant correlation observed between fin height and d(t) as well as fin height and G0 (t) for most materials was not surprising. However, the lack of correlation for Imp-GDS und Imp-PS lead us to conclude, that change in fin height is not fully reflected by d(t) and G0 (t). Hence fin height may possibly be related to flow characteristics caused by e.g. filler type and shape and/or monomer properties rather than the change in flow properties caused by the cross-linking reaction during setting. Based on this assumption it is hypothesized that other rheological properties (e.g. shear viscosity, thixotropy, yield point) might be more suitable for explaining the results of the shark fin test. Determination of, e.g. shear viscosity allows conclusions regarding the intermolecular shear forces within a fluid, which are mainly influenced by the interaction between the monomer molecules. Therefore the chemical composition of a classical Polyether might be the key parameter, which influences the shear viscosity and potentially the results of the SFT most. Our protocol in contrast was adapted to quantify d(t) and G0 (t) and the use of serrated plates made determination of further rheological properties, especially shear viscosity, impossible. As most materials investigated showed no significant correlation between dimensional accuracy and t-EoM it is concluded, that it is possible to produce accurate impressions, even during late stages within the MrWT, with the majority of impression materials. Consequently dimensional accuracy of impressions is not necessarily affected by the change of flow properties of an impression material during setting, as long as the impression is taken within the MrWT.
journal of dentistry 35 (2007) 409–415
The surface detail reproduction of the impression materials tested was constant during MrWT, representing independence of t-EoM. Therefore SFT results do not reflect the ability of an impression material to precisely reproduce surface details. This was finally proven by a lack of significant correlation between SFT and surface detail reproduction. In summary, the results of all four parts (I–IV) of the null hypothesis have to be rejected. Results of the SFT neither correlate with the dimensional accuracy nor with the surface detail reproduction, phase angle, and storage modulus of impression materials determined after mixing within the MrWT. Consequently the SFT has to be seen as a test in its own right, which does not reflect the results of the accepted and clinically relevant materials’ properties tests for impression materials used in this study. Good flow of an impression material – which might be expressed by high shark fins obtained in the SFT – is not necessarily the pre-requisite for high dimensional accuracy and good surface detail reproduction of the impressions, respectively.21,22 In addition it has to be considered, that good flow of an impression material might be advantageous for distinct clinical situations (e.g. inlay and onlay preparations), whereas other clinical situations (i.e. infragingival finishing lines) definitely require a more firm and less flowable viscosity for successful impression taking.21,22 Taking into account this clinical experience, it is speculated that the SFT may represent advantageous flow properties that, in turn, may correlate to void-free impression taking of complex shaped bodies, e.g. implant impression posts and onlay or inlay preparations. This question, however, has to be addressed in further laboratory and clinical studies. It is suggested that, for unequivocal interpretation of test results with regard to their clinical relevance, future materials’ properties tests should be subjected to scientific scrutiny prior to introduction.
5.
Conclusions
Results obtained from the SFT do not allow predictions regarding dimensional accuracy or surface detail reproduction of impressions taken within or at the end of the working time. These parameters are consequently not affected by the flow properties determined with the SFT. The SFT only partially reflects the cross-linking reaction of a material during setting, as no correlation between fin height and phase angle was observed for the Impregum products.
Acknowledgements We would like to thank Dr. Manfred Hollenhorst of the Department for Documentation, Justus-Liebig-University Computer Centre, Giessen, for his assistance with the statistical analysis. In addition, the authors appreciate the materials donated by the respective manufacturers. We would also like to thank Heinrich Bethge of the Dental Clinic Giessen for creating the drawings.
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references
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