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Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature
Dental Materials (2006) 22, 397–404
www.intl.elsevierhealth.com/journals/dema
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature Jianxiang Taoa,*, Masanobu Yodaa, Kohei Kimuraa, Osamu Okunob a
Division of Fixed Prosthodontics, Department of Restorative Dentistry, Graduate School of Dentistry, Tohoku University, 4-1 Seiryou-machi, Aoba-ku, Sendai 980-8575, Japan b Division of Dental Biomaterials, Department of Restorative Dentistry, Graduate School of Dentistry, Tohoku University, Sendai, Japan Received 25 November 2004; received in revised form 25 February 2005; accepted 5 April 2005
KEYWORDS Fit; Metal ceramic crowns; Au-1.6 wt% Ti alloy; Finish line curvature
Abstract Objectives. The purpose of this study was to evaluate the fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy and investigate the effect of abutment finish line curvature on the fit of crowns. Methods: Three types of finish line curvature abutments were prepared (1, 3 and 5 mm-curvature). For each type of abutment, five metal ceramic crowns of the facial veneered type were fabricated, which were cast in Au-1.6 wt% Ti alloy. Used as controls, another fifteen specimens were made from a commercially available gold alloy. The fit was measured in the as-cast and after porcelain application. Results. In the as-cast specimens, the greater the finish line curvature was, the larger the gaps exhibited at the mesial and distal margins of copings, compared with labial and lingual margins. The distal margin of copings for 5 mm-curvature abutments showed the largest gap (35 (7) mm). After porcelain application, the greater was the finish line curvature, the larger the labial marginal gap became (mean 44, 34, 25 mm, respectively, for 5, 3, 1 mm-curvature). However, there was no significant difference on marginal gaps between specimens of Au-1.6 wt% Ti alloy and control gold alloy. Significance. This study indicated that the metal ceramic crowns cast in Au-1.6 wt% Ti alloy had equivalent accuracy to those that cast in control gold alloy, and the abutment finish line curvature had a significant effect on the marginal fit of metal ceramic crowns. Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction
* Corresponding author. Tel.: C81 22 717 8363; fax: C81 22 717 8367. E-mail address: [email protected] (J. Tao).
In the last decades, biocompatibility has become a great concern in biomaterials and demands for dental gold alloys with better biocompatibility have consequently increased. The excellent
0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.04.025
398 biocompatibility of titanium with human tissue is well established [1,2]. Recently the use of unalloyed titanium has extended over almost all fields of restorative dentistry based on improvements in dental casting technology and the introduction of innovative processing methods [3], but because oxide film is generated on a casting’s surface during the process of casting, it is laborious to grind the castings from pure titanium [4,5]. Fortunately it was proved that titanium has a small solid-solubility in gold, so a new type of gold alloy which combines with good biocompatibility and mechanical properties was expected to be developed by adding a small quantity of titanium to pure gold [6–8]. So far it has been proved that Au-1.6 wt%Ti alloy is excellent both in biocompatibility and physical properties such as tensile strength, hardness and metal– ceramic bond strength [9]. But the fit of metal ceramic crowns cast in Au-1.6 wt%Ti alloy which influences periodontal health, secondary caries and also the crown’s retention, is not clear. Metal substructures of metal ceramic crowns, verified to fit prepared abutments, do not always fit as well after porcelain application [10,11]. Deterioration of the initial fit of the metal coping has been observed after the porcelain firing cycle. Campbell and Faucher thought that potential causes of this distortion included (1) porcelain firing shrinkage, (2) differential between the porcelain and alloy thermal coefficient of expansion, (3) alloy mechanical properties, (4) substructure design, (5) tooth preparation, (6) metal oxide formation on the fitted surface of the alloy, (7) release of casting-induced stresses, (8) contamination of the casting, (9) grain growth of the alloy, and (10) contamination of the internal surface of the casting with porcelain etc [12,13]. Regarding finish line, the effect of margin shapes such as chamfer, shoulder, chamfer with bevel and shoulder with bevel has been discussed [11]. However, few data concerning the effect of the curvature of the abutment finish line are available. Compared with interdental gingival margin levels, the labial and lingual gingival margin levels are usually apical in the clinical situation. So after tooth preparation, the abutment marginal finish line exhibits some degree of curve. The abutment finish line curvature can change with different clinical situations. For example, elderly patients have a physiologically age-related gingival recession. Patients who have ever suffered periodontal disease may have pathological gingival recession. In such cases, abutment finish lines show a gradual curve, whereas in the cases of infra-labioversion teeth or canines which locate at the turning point of the dental arch, the labial gingival margin levels
J. Tao et al. become more apical than usual, abutment finish lines may show a sharper curve. This finish line curvature may have an effect on the fit of metal ceramic crowns. In this experiment, the accuracy of metal ceramic crowns cast from Au-1.6 wt% Ti alloy was examined. The effect of the curvature of the finish line on marginal gaps of the metal ceramic crowns was also investigated.
Materials and methods In advance, Au-1.6 wt% Ti alloy was made in an inert argon gas environment. Pure gold of 99.99% (Tanaka Kikinzoku International, Tokyo, Japan) and sponge titanium of 99.8%(Sumitomo Titanium Corporation, Amagasaki-City, Hyogo, Japan) were weighed for titanium content of 1.6 wt% and gold content of 98.4 wt%. These were alloyed in an argon arcmelting furnace (TAM4-s, Tachibana Riko, Sendai, Japan) to manufacture the Au-1.6 wt% Ti alloy. KIK, a commercially available gold alloy for metal ceramic crowns (Au 85.5%, Ag 0.5%, Pt 4.0%, Pd 8.0%ite ISHIFUKU METAL INDUSTRY CO. LTD, Tokyo, Japan) was used as an appropriate control as it is used extensively for metal ceramic crown production in Japan. Three types of abutment finish line curvatures (1, 3, and 5 mm-curvature) were prepared (Fig. 1). A maxillary right central incisor Ivorine tooth (A1-500, NISSIN DENTAL CORPORATION, Tokyo, Japan) was prepared by an experienced prosthodontist with a circular 1 mm 90-degree shoulder finish line, a uniform 1.5 mm two-plane facial reduction, a 2 mm incisal reduction and 1 mm remaining axial reduction with 6-degree taper. The prepared tooth with a 1 mm vertical distance between proximal margin level and buccolingual margin level was defined as the 1 mmcurvature abutment, namely the abutment with a 1 mm-curvature finish line. The prepared tooth was reproduced in a gold-silver-palladium alloy (12% CASTWELL M.C., GC Corporation, Tokyo, Japan), and 1 mm-curvature metal abutment was made (Fig. 1, left). The additional 1 mm-curvature metal abutments were produced, then 3, 5 mm-curvature metal abutments were respectively fabricated by further apical reducing buccolingual margins of these 1 mm-curvature metal abutments and keeping the proximal margins untouched (Fig. 1, middle, right). The dimensions of 5 mm-curvature metal abutments are shown in Fig. 2. Wax patterns for metal ceramic crowns were fabricated directly on metal abutments with blue inlay wax (CROWN WAX, GC Corporation, Tokyo,
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature
1mm
Figure 1
3mm
8.20
7.60
6.20
5.60
1
1
4.85
5mm
Three types of finish line curvatures’ abutments (distal).
Japan). Thirty wax patterns were fabricated (ten for each type of abutment). Wax patterns were examined under a stereoscopic zoom microscope (SMZ-1, Nikon Corporation, Tokyo, Japan) to ensure that there was no gap between wax pattern and metal abutment margins. After storage at room temperature for 24 h, the wax patterns were vacuum invested in phosphate-bonded investment (CERAVEST G, GC Corporation, Tokyo, Japan) according to the manufacturer’s recommendation for the liquid: power ratio. The position relationship of sprue, wax pattern, and casting ring is illustrated in Fig. 3. Fifteen wax patterns (five for each type of abutment) were cast in Au-1.6 wt% Ti alloy with a titanium cast machine (Argon Caster T, SHOFU INC, Kyoto, Tokyo, Japan). The other fifteen were cast in ‘KIK’ alloy with a vacuum and pressure casting machine (CASPAC MK-3, Dentronics Co., Tokyo, Japan). The castings were recovered with the aid of a sandblaster using 50 microns Al2O3 under 0.2 MPa pressure, but internal surfaces were not sandblasted to avoid affecting fit. The investments inside copings were carefully removed by a wax carving knife and copings were steam
5.35
399
1
5.35
cleaned. Any metal nodules that might have prevented complete seating of the casting were removed with a round bur at low speed. The thickness of castings was measured by a measuring device and adjusted to ensure that the metal substructures were 0.3 mm thick at the labial and incisal surfaces, 1 mm at the lingual surface. The castings were seated on their respective metal abutments, using finger pressure of almost 1 N. No adjustments were performed on the internal surfaces of castings during seating. The marginal vertical gaps between the metal abutment and the casting were measured on a profile projector (V-16D, Nikon Corporation, Tokyo, Japan) under 20! power magnification. At labial and lingual sides, measurements were made at the point that was closest to the root apex. Contrarily, at the mesial and distal sides, measurements were made at the point that was closest to the incisal edge. The castings were treated with degassing and oxidation (Table 1). Subsequently, opaque porcelain
Casting ring
Ceramic liner
Wax pattern 1
Wax sprue 5
5
labial
distal
Figure 2 The dimensions of 5 mm-curvature metal abutment.
Cone
Figure 3 The position relationship of sprue, pattern and casting ring.
400 Table 1
Degassing Oxidation Opaque Dentin Glazing
J. Tao et al. Schedule of heat treatment thermal and porcelain firing. Start temp (8C)
Heat rate (8C/min)
Final temp (8C)
Vacuum (cm/Hg)
Holding time (min)
700 700 650 650 650
55 55 55 55 55
1000 1000 950 930 910
74 0 74 74 0
15 10 0 0 0
and dentin porcelain (Vintage, SHOFU INC, Kyoto, Japan) were applied and fired. Finally glazing was performed according to the manufacturer’s instructions (Table 1), and specimens of metal ceramic crowns of the facial-veneered type were completed. The porcelain was 1.2 mm thick at the labial surface, 1.7 mm at the incisal surface. After porcelain application, the marginal gaps were measured using the same method as for the as-cast specimens. For different marginal locations on the same crown, the data were analyzed by one-way ANOVA followed by Tukey’s test (alphaZ0.05) with the SPSS 11.0J for Windows software program to identify significant difference in location of measurement. For the same marginal location in different conditions, the data were analyzed by two-way ANOVA followed by Tukey’s test (alphaZ0.05) with the SPSS 11.0J for Windows software program to identify any significant difference in abutment finish line curvature and type of alloy. The correlation coefficient between the finish line curvature and the marginal gap was determined by Spearman’s correlation test.
Results A graphic plot which depicts the mean marginal gap and SD for metal ceramic crowns from three types of different curvature abutments in the as-cast specimens is shown in Fig. 4. Blocks connected by a bracket and * are statistically different at alphaZ0.05. For copings of metal ceramic crowns from 1 mmcurvature abutment group, labial, lingual, mesial, and distal marginal gaps were very similar, respectively, 19 (6), 20 (9), 19 (6), and 18 (7) mm. Compared with the 1 mm-curvature abutment, copings of metal ceramic crowns from the 3 mmcurvature abutment exhibited larger mesial and distal marginal gaps (respectively 25 (6), 25(9) mm) than labial and lingual marginal gaps (respectively 21 (5), 17 (4) mm). But there was no statistically
significant difference. For the copings of metal ceramic crowns from the 5 mm-curvature abutment group, mesial and distal marginal gap (respectively 30 (12), 35 (7) mm) were much larger than labial and lingual (respectively 22 (6), 20 (6) mm), and the distal margin showed significantly larger gap than labial and distal. There was a statistically significant correlation of 0.647 (p!0.05) between the finish line curvature and the mesiodistal marginal gaps. The greater the finish line curvature, the mesial and distal margins of copings exhibited larger gaps than the labial and lingual margins. The result of two-way ANOVA showed that abutment finish line curvature had a significant effect on mesial and distal marginal gaps of castings, but no effect on labial and lingual marginal gaps. The distal marginal gap for 5 mmcurvature abutments was significantly larger than that for 1 mm-curvature abutments. The mean marginal gaps for metal ceramic crowns of the ‘KIK’ alloy are also shown in Fig. 4. However, the type of alloys (Au-1.6 wt% Ti alloy and KIK) had no effect on either mesial and distal marginal gaps or labial and lingual marginal gaps. A graphic plot depicting the mean marginal gaps and SD for metal ceramic crowns after porcelain application is shown in Fig. 5. Following porcelain application, the labial marginal gap became larger than lingual, mesial, and distal. The metal ceramic crowns for a 1 mm-curvature abutment exhibited a larger labial marginal gap (25 (5) mm) than lingual, mesial, and distal marginal gap (respectively 22 (5), 22 (6), and 21 (9) mm). The labial marginal gap of metal ceramic crowns from the 3 mm-curvature abutment was 34 (10) mm, larger than lingual, mesial, distal (respectively 20 (12), 21 (6), and 25 (9) mm). Compared with 1 mm-curvature and 3 mmcurvature abutments, the metal ceramic crowns for a 5 mm-curvature abutment showed a much larger labial marginal gap (44 (18) mm) than lingual, mesial, distal (respectively 30 (12), 22 (9), and 22 (14) mm); labial marginal gap was significantly larger than mesial and distal. The result of two-way ANOVA revealed that finish line curvature had a significant effect on labial
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature
401
µm 1 mm-curvature
70
3 mm-curvature
60
5 mm-curvature
50
*
*
* *
*
*
*
*
*
40 30 20 10 0 labial
lingual mesial distal
labial
lingual mesial distal
Au-1.6wt%Ti alloy
Figure 4
KIK
The marginal gaps of metal ceramic crowns in the as-cast specimens. (* p!0.05).
marginal gaps of metal ceramic crowns. The coefficient of correlation between the finish line curvature and the labial marginal gap was 0.794 (p!0.05). The greater the finish line curvature, the larger the labial marginal gap became. The labial marginal gap of metal ceramic crowns from 5 mmcurvature abutments was significantly larger than that from 1 mm-curvature abutment. However, the type of alloys had no effect on marginal gaps of metal ceramic crowns.
Discussion The goal in the design of this in-vitro study was to compare the accuracy of Au-1.6 wt% Ti alloy metal ceramic crowns with that of control gold alloy KIK, and to investigate the effect of finish line curvature on the fit of metal ceramic crowns. Other possible
variable factors such as die spacer thickness [14], expansion of the die stone, and polymerization shrinkage of impression materials had been eliminated. For this reason, no spacer was used on the axial surfaces during wax pattern fabrication. The same metal abutment was used both for constructing the wax patterns and measuring the seating accuracy. The marginal gaps between crown and abutment were directly measured on the profile projector as this method is a valid non-invasive method, used extensively by clinicians and technicians. Although the replica technique is also a reliable non-invasive method of determining the adaptation of crown-totooth surface [15,16], different finish line curvatures of crowns may influence the results because the escape of replica materials from 5 mm-curvature crowns is more difficult than that form 1 mm-curvature crowns during measurement.
µm 1 mm-curvature
70
3 mm-curvature 5 mm-curvature
60 50
*
*
* *
*
*
*
*
*
40 30 20 10 0 labial
lingual mesial distal
Au-1.6wt%Ti alloy
Figure 5
labial
lingual mesial distal KIK
The marginal gaps of metal ceramic crowns after porcelain application. (* p!0.05).
402 The evaluation of crown-tooth discrepancies by replication also has its limitations and inherent errors such as recognition of crown margins and finish lines or rupture of the elastomeric film on removal from crowns. Taggart first described the lost wax casting process in dentistry in 1907 and observed that the resultant restorations were undersized because of shrinkage of the alloy on solidifying [17–19]. It is necessary to achieve compensation for the shrinkage of the solidifying alloy by investment expansion. Today, three methods of compensation for the alloy shrinkage on cooling are in regular use: (1) setting expansion of the investment, (2) hygroscopic expansion of the investment, and (3) thermal expansion of the investment. Although a select number of phosphate-bonded investments can be used in the ringless casting technique [20,21], most phosphate-bonded investments do not have sufficient compressive strength, therefore, use of a metal casting ring is necessary to protect the investment during casting forces [22,23]. However, the metal casting ring restricts the thermal expansion of the investment as the thermal expansion of the ring is less than that of the investment. To compensate for this limitation, a paper ceramic liner is recommended [24,25]. Because the ceramic liner can compensate for most casting ring restriction, the metal ring technique is clinically acceptable and allows for the fabrication of accurate castings [26]. As a standard process, the conventional metal ring casting technique was used in this study, however, due to the casting ring’s restriction, the investments expand more in an axial direction than horizontal. So the axial surface of the casting becomes longer than the original length of the wax pattern. The longer the axial surface of the wax pattern, the greater the increase in length of the casting’s axial surface is. This explains why the finish line curvature was greater, the larger the gap the mesial and distal margins of copings exhibited compared with labial and lingual margins in the ascast specimens. Length difference between buccolingual axial surfaces and mesiodistal axial surfaces of wax patterns was small for the 1 mm-curvature abutment, moderate for the 3 mm-curvature abutment and large for the 5 mm-curvature abutment, so after casting, length increment difference between buccolingual axial surfaces and mesiodistal axial surfaces was small for the 1 mm-curvature abutment, moderate for the 3 mm-curvature abutment and large for the 5 mm-curvature abutment. This is why mesiodistal marginal gaps were similar to buccolingual marginal gaps for 1 mm-curvature abutments, whereas mesiodistal marginal gaps
J. Tao et al. were much larger than buccolingual for 5 mmcurvature abutments. Deterioration of the initial fit of the metal coping was observed after the porcelain firing cycle. After porcelain application, the porcelain firing shrinkage and difference between porcelain and metal thermal coefficient of expansion caused substructure metal to bend towards the porcelain side. For the 1 mm-curvature abutment, the length of labial axial surface and proximal axial surfaces was similar, bending stress among labial axial surface and proximal axial surfaces almost redressed the balance. For the 5 mm-curvature abutment, because the labial axial surface was much longer than proximal axial surfaces, the cervical part of the labial axial surface acted like a cantilever. Bend in distortion towards the porcelain side at the cervical part of the labial axial surface was rarely restricted by proximal axial surfaces. Conversely, bending distortion of the proximal axial surfaces was restricted by buccolingual axial surfaces. The labial margin was the point that seemed to be most subjected to the bending distortion. So after porcelain application, the labial marginal gap became larger than the lingual and proximal margins. The greater the finish line curvature, the larger the labial marginal gap became (mean 44, 34, 25 mm, respectively, for 5, 3, 1 mm-curvature abutments). Usually the casting shrinkage of an alloy is related to its melting point. The melting point of Au-1.6 wt% Ti alloy is about 1100 8C and is close to that of KIK (1200 8C), so in the as-cast specimens there is no significant difference between specimens from Au-1.6 wt% Ti alloy and KIK. There is a little difference between the mechanical properties of Au-1.6 wt% Ti alloy and KIK under than experimental conditions [9]. The micro Vicker’s hardness of Au-1.6 wt% Ti alloy was 172 and that of KIK was 191. The metal’s thermal expansion coefficient in the 50–500 8C range was 14.4G0.3!10K6/8C for Au-1.6 wt% Ti alloy and was 14.1G0.2!10K6/8C for KIK [9]. The modulus of elasticity was 67 GPa for Au-1.6 wt% Ti alloy, was 84 GPa for KIK. These differences may have had an effect on accuracy of the metal ceramic crowns, but after porcelain application, no significant difference was found between metal ceramic crowns from Au-1.6 wt% Ti alloy and KIK, so it was concluded that the difference in mechanical properties have no statistically significant effect on the fit of metal ceramic crowns. Accuracy of fit has been extensively investigated in the dental literature, but the size of marginal gap that is clinically acceptable is difficult to document. Although marginal gaps of 100 mm are considered
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature clinically acceptable with regard to longevity [27], theoretical requirements of marginal gaps should be lower than 40 mm [28]. In this in-vitro study, for 5 mm-curvature abutments, there were specimens with marginal gaps over 50 mm in the as-cast specimens or after porcelain application. However, in daily practice other factors such as percentage completion of tooth preparation, expansion of model stone, impression materials and skill of technicians also have an effect on the accuracy of crowns. The greater the finish line curvature, the more difficult the tooth preparation and the creation of the impression in the marginal area. If these factors are taken into account, it seems that crowns with sharply curved finish lines more often lead to poorly fitting margins that may be clinically unacceptable. From this study, it is clear that finish line curvature has a significant effect on the marginal fit of crowns, so clinicians should be aware of the effects of finish line curvature in their daily tooth preparations. In cases of canines and infralabioversion teeth or periodontally diseased teeth, labial or lingual surfaces of roots may be exposed. If the design of subgingival margin is adopted, the metal ceramic crowns cover all exposed surfaces of roots, a sharply curved finish line is produced. Labial subgingival margins that have a poor fit may incite or aggravate periodontal disease. Tooth preparation with sharply curved finish lines should be avoided by using the design of a supragingival margin if possible. The porcelain butt margins technique could be used to improve labial marginal fit if sharp curved finish lines could not be avoided. The marginal gap of metal ceramic crowns with porcelain butt margins is rarely influenced by the distortion of substructure metal [29]. For 5 mm-curvature abutments, using a direct lift technique, the authors fabricated five metal ceramic crowns with porcelain butt margins, and found the mean labial marginal gap was 30 mm, smaller than conventional metal ceramic crowns (44 mm).
Conclusions Data obtained in this in-vitro study allowed the following conclusions to be made. † The metal ceramic crowns cast in Au-1.6 wt% Ti alloy had accuracy as good those as-cast in control gold alloy KIK. † In the as-cast speciments, the greater the finish line curvature, the mesial and distal margins of
403
copings exhibited larger gaps than labial and lingual margins. † After porcelain application, for metal ceramic crowns of the facial-veneered type, the greater the finish line curvature, the larger the labial marginal gap became.
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404 [19] Hollenback GM. A brief history of cast restoration. J South Calif State Dent Assoc 1962;30:8–18. [20] Morey EF. Dimensional accuracy of gold alloy castings. Part 1. A brief history and the behaviour on inlay waxes. Aust Dent J 1991;36:302–9. [21] Morey EF, Earnshaw R. The fit of gold-alloy full-crown castings made with pre-wetted casting ring liners. J Dent Res 1992;71:1858–64. [22] Chew CL, Land MF, Thomas CC, Norman RD. Investment strength as a function of time and temperature. J Dent 1999;27:297–302. [23] Luk HW, Darvell BW. Strength of phosphate-bonded investments at high temperature. Dent Mater 1991;7:99–102. [24] Earnshaw R. The effect of casting ring liners on the potential expansion of a gypsum-bonded investment. J Dent Res 1988;67:1366–70.
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www.intl.elsevierhealth.com/journals/dema
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature Jianxiang Taoa,*, Masanobu Yodaa, Kohei Kimuraa, Osamu Okunob a
Division of Fixed Prosthodontics, Department of Restorative Dentistry, Graduate School of Dentistry, Tohoku University, 4-1 Seiryou-machi, Aoba-ku, Sendai 980-8575, Japan b Division of Dental Biomaterials, Department of Restorative Dentistry, Graduate School of Dentistry, Tohoku University, Sendai, Japan Received 25 November 2004; received in revised form 25 February 2005; accepted 5 April 2005
KEYWORDS Fit; Metal ceramic crowns; Au-1.6 wt% Ti alloy; Finish line curvature
Abstract Objectives. The purpose of this study was to evaluate the fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy and investigate the effect of abutment finish line curvature on the fit of crowns. Methods: Three types of finish line curvature abutments were prepared (1, 3 and 5 mm-curvature). For each type of abutment, five metal ceramic crowns of the facial veneered type were fabricated, which were cast in Au-1.6 wt% Ti alloy. Used as controls, another fifteen specimens were made from a commercially available gold alloy. The fit was measured in the as-cast and after porcelain application. Results. In the as-cast specimens, the greater the finish line curvature was, the larger the gaps exhibited at the mesial and distal margins of copings, compared with labial and lingual margins. The distal margin of copings for 5 mm-curvature abutments showed the largest gap (35 (7) mm). After porcelain application, the greater was the finish line curvature, the larger the labial marginal gap became (mean 44, 34, 25 mm, respectively, for 5, 3, 1 mm-curvature). However, there was no significant difference on marginal gaps between specimens of Au-1.6 wt% Ti alloy and control gold alloy. Significance. This study indicated that the metal ceramic crowns cast in Au-1.6 wt% Ti alloy had equivalent accuracy to those that cast in control gold alloy, and the abutment finish line curvature had a significant effect on the marginal fit of metal ceramic crowns. Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction
* Corresponding author. Tel.: C81 22 717 8363; fax: C81 22 717 8367. E-mail address: [email protected] (J. Tao).
In the last decades, biocompatibility has become a great concern in biomaterials and demands for dental gold alloys with better biocompatibility have consequently increased. The excellent
0109-5641/$ - see front matter Q 2005 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2005.04.025
398 biocompatibility of titanium with human tissue is well established [1,2]. Recently the use of unalloyed titanium has extended over almost all fields of restorative dentistry based on improvements in dental casting technology and the introduction of innovative processing methods [3], but because oxide film is generated on a casting’s surface during the process of casting, it is laborious to grind the castings from pure titanium [4,5]. Fortunately it was proved that titanium has a small solid-solubility in gold, so a new type of gold alloy which combines with good biocompatibility and mechanical properties was expected to be developed by adding a small quantity of titanium to pure gold [6–8]. So far it has been proved that Au-1.6 wt%Ti alloy is excellent both in biocompatibility and physical properties such as tensile strength, hardness and metal– ceramic bond strength [9]. But the fit of metal ceramic crowns cast in Au-1.6 wt%Ti alloy which influences periodontal health, secondary caries and also the crown’s retention, is not clear. Metal substructures of metal ceramic crowns, verified to fit prepared abutments, do not always fit as well after porcelain application [10,11]. Deterioration of the initial fit of the metal coping has been observed after the porcelain firing cycle. Campbell and Faucher thought that potential causes of this distortion included (1) porcelain firing shrinkage, (2) differential between the porcelain and alloy thermal coefficient of expansion, (3) alloy mechanical properties, (4) substructure design, (5) tooth preparation, (6) metal oxide formation on the fitted surface of the alloy, (7) release of casting-induced stresses, (8) contamination of the casting, (9) grain growth of the alloy, and (10) contamination of the internal surface of the casting with porcelain etc [12,13]. Regarding finish line, the effect of margin shapes such as chamfer, shoulder, chamfer with bevel and shoulder with bevel has been discussed [11]. However, few data concerning the effect of the curvature of the abutment finish line are available. Compared with interdental gingival margin levels, the labial and lingual gingival margin levels are usually apical in the clinical situation. So after tooth preparation, the abutment marginal finish line exhibits some degree of curve. The abutment finish line curvature can change with different clinical situations. For example, elderly patients have a physiologically age-related gingival recession. Patients who have ever suffered periodontal disease may have pathological gingival recession. In such cases, abutment finish lines show a gradual curve, whereas in the cases of infra-labioversion teeth or canines which locate at the turning point of the dental arch, the labial gingival margin levels
J. Tao et al. become more apical than usual, abutment finish lines may show a sharper curve. This finish line curvature may have an effect on the fit of metal ceramic crowns. In this experiment, the accuracy of metal ceramic crowns cast from Au-1.6 wt% Ti alloy was examined. The effect of the curvature of the finish line on marginal gaps of the metal ceramic crowns was also investigated.
Materials and methods In advance, Au-1.6 wt% Ti alloy was made in an inert argon gas environment. Pure gold of 99.99% (Tanaka Kikinzoku International, Tokyo, Japan) and sponge titanium of 99.8%(Sumitomo Titanium Corporation, Amagasaki-City, Hyogo, Japan) were weighed for titanium content of 1.6 wt% and gold content of 98.4 wt%. These were alloyed in an argon arcmelting furnace (TAM4-s, Tachibana Riko, Sendai, Japan) to manufacture the Au-1.6 wt% Ti alloy. KIK, a commercially available gold alloy for metal ceramic crowns (Au 85.5%, Ag 0.5%, Pt 4.0%, Pd 8.0%ite ISHIFUKU METAL INDUSTRY CO. LTD, Tokyo, Japan) was used as an appropriate control as it is used extensively for metal ceramic crown production in Japan. Three types of abutment finish line curvatures (1, 3, and 5 mm-curvature) were prepared (Fig. 1). A maxillary right central incisor Ivorine tooth (A1-500, NISSIN DENTAL CORPORATION, Tokyo, Japan) was prepared by an experienced prosthodontist with a circular 1 mm 90-degree shoulder finish line, a uniform 1.5 mm two-plane facial reduction, a 2 mm incisal reduction and 1 mm remaining axial reduction with 6-degree taper. The prepared tooth with a 1 mm vertical distance between proximal margin level and buccolingual margin level was defined as the 1 mmcurvature abutment, namely the abutment with a 1 mm-curvature finish line. The prepared tooth was reproduced in a gold-silver-palladium alloy (12% CASTWELL M.C., GC Corporation, Tokyo, Japan), and 1 mm-curvature metal abutment was made (Fig. 1, left). The additional 1 mm-curvature metal abutments were produced, then 3, 5 mm-curvature metal abutments were respectively fabricated by further apical reducing buccolingual margins of these 1 mm-curvature metal abutments and keeping the proximal margins untouched (Fig. 1, middle, right). The dimensions of 5 mm-curvature metal abutments are shown in Fig. 2. Wax patterns for metal ceramic crowns were fabricated directly on metal abutments with blue inlay wax (CROWN WAX, GC Corporation, Tokyo,
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature
1mm
Figure 1
3mm
8.20
7.60
6.20
5.60
1
1
4.85
5mm
Three types of finish line curvatures’ abutments (distal).
Japan). Thirty wax patterns were fabricated (ten for each type of abutment). Wax patterns were examined under a stereoscopic zoom microscope (SMZ-1, Nikon Corporation, Tokyo, Japan) to ensure that there was no gap between wax pattern and metal abutment margins. After storage at room temperature for 24 h, the wax patterns were vacuum invested in phosphate-bonded investment (CERAVEST G, GC Corporation, Tokyo, Japan) according to the manufacturer’s recommendation for the liquid: power ratio. The position relationship of sprue, wax pattern, and casting ring is illustrated in Fig. 3. Fifteen wax patterns (five for each type of abutment) were cast in Au-1.6 wt% Ti alloy with a titanium cast machine (Argon Caster T, SHOFU INC, Kyoto, Tokyo, Japan). The other fifteen were cast in ‘KIK’ alloy with a vacuum and pressure casting machine (CASPAC MK-3, Dentronics Co., Tokyo, Japan). The castings were recovered with the aid of a sandblaster using 50 microns Al2O3 under 0.2 MPa pressure, but internal surfaces were not sandblasted to avoid affecting fit. The investments inside copings were carefully removed by a wax carving knife and copings were steam
5.35
399
1
5.35
cleaned. Any metal nodules that might have prevented complete seating of the casting were removed with a round bur at low speed. The thickness of castings was measured by a measuring device and adjusted to ensure that the metal substructures were 0.3 mm thick at the labial and incisal surfaces, 1 mm at the lingual surface. The castings were seated on their respective metal abutments, using finger pressure of almost 1 N. No adjustments were performed on the internal surfaces of castings during seating. The marginal vertical gaps between the metal abutment and the casting were measured on a profile projector (V-16D, Nikon Corporation, Tokyo, Japan) under 20! power magnification. At labial and lingual sides, measurements were made at the point that was closest to the root apex. Contrarily, at the mesial and distal sides, measurements were made at the point that was closest to the incisal edge. The castings were treated with degassing and oxidation (Table 1). Subsequently, opaque porcelain
Casting ring
Ceramic liner
Wax pattern 1
Wax sprue 5
5
labial
distal
Figure 2 The dimensions of 5 mm-curvature metal abutment.
Cone
Figure 3 The position relationship of sprue, pattern and casting ring.
400 Table 1
Degassing Oxidation Opaque Dentin Glazing
J. Tao et al. Schedule of heat treatment thermal and porcelain firing. Start temp (8C)
Heat rate (8C/min)
Final temp (8C)
Vacuum (cm/Hg)
Holding time (min)
700 700 650 650 650
55 55 55 55 55
1000 1000 950 930 910
74 0 74 74 0
15 10 0 0 0
and dentin porcelain (Vintage, SHOFU INC, Kyoto, Japan) were applied and fired. Finally glazing was performed according to the manufacturer’s instructions (Table 1), and specimens of metal ceramic crowns of the facial-veneered type were completed. The porcelain was 1.2 mm thick at the labial surface, 1.7 mm at the incisal surface. After porcelain application, the marginal gaps were measured using the same method as for the as-cast specimens. For different marginal locations on the same crown, the data were analyzed by one-way ANOVA followed by Tukey’s test (alphaZ0.05) with the SPSS 11.0J for Windows software program to identify significant difference in location of measurement. For the same marginal location in different conditions, the data were analyzed by two-way ANOVA followed by Tukey’s test (alphaZ0.05) with the SPSS 11.0J for Windows software program to identify any significant difference in abutment finish line curvature and type of alloy. The correlation coefficient between the finish line curvature and the marginal gap was determined by Spearman’s correlation test.
Results A graphic plot which depicts the mean marginal gap and SD for metal ceramic crowns from three types of different curvature abutments in the as-cast specimens is shown in Fig. 4. Blocks connected by a bracket and * are statistically different at alphaZ0.05. For copings of metal ceramic crowns from 1 mmcurvature abutment group, labial, lingual, mesial, and distal marginal gaps were very similar, respectively, 19 (6), 20 (9), 19 (6), and 18 (7) mm. Compared with the 1 mm-curvature abutment, copings of metal ceramic crowns from the 3 mmcurvature abutment exhibited larger mesial and distal marginal gaps (respectively 25 (6), 25(9) mm) than labial and lingual marginal gaps (respectively 21 (5), 17 (4) mm). But there was no statistically
significant difference. For the copings of metal ceramic crowns from the 5 mm-curvature abutment group, mesial and distal marginal gap (respectively 30 (12), 35 (7) mm) were much larger than labial and lingual (respectively 22 (6), 20 (6) mm), and the distal margin showed significantly larger gap than labial and distal. There was a statistically significant correlation of 0.647 (p!0.05) between the finish line curvature and the mesiodistal marginal gaps. The greater the finish line curvature, the mesial and distal margins of copings exhibited larger gaps than the labial and lingual margins. The result of two-way ANOVA showed that abutment finish line curvature had a significant effect on mesial and distal marginal gaps of castings, but no effect on labial and lingual marginal gaps. The distal marginal gap for 5 mmcurvature abutments was significantly larger than that for 1 mm-curvature abutments. The mean marginal gaps for metal ceramic crowns of the ‘KIK’ alloy are also shown in Fig. 4. However, the type of alloys (Au-1.6 wt% Ti alloy and KIK) had no effect on either mesial and distal marginal gaps or labial and lingual marginal gaps. A graphic plot depicting the mean marginal gaps and SD for metal ceramic crowns after porcelain application is shown in Fig. 5. Following porcelain application, the labial marginal gap became larger than lingual, mesial, and distal. The metal ceramic crowns for a 1 mm-curvature abutment exhibited a larger labial marginal gap (25 (5) mm) than lingual, mesial, and distal marginal gap (respectively 22 (5), 22 (6), and 21 (9) mm). The labial marginal gap of metal ceramic crowns from the 3 mm-curvature abutment was 34 (10) mm, larger than lingual, mesial, distal (respectively 20 (12), 21 (6), and 25 (9) mm). Compared with 1 mm-curvature and 3 mmcurvature abutments, the metal ceramic crowns for a 5 mm-curvature abutment showed a much larger labial marginal gap (44 (18) mm) than lingual, mesial, distal (respectively 30 (12), 22 (9), and 22 (14) mm); labial marginal gap was significantly larger than mesial and distal. The result of two-way ANOVA revealed that finish line curvature had a significant effect on labial
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature
401
µm 1 mm-curvature
70
3 mm-curvature
60
5 mm-curvature
50
*
*
* *
*
*
*
*
*
40 30 20 10 0 labial
lingual mesial distal
labial
lingual mesial distal
Au-1.6wt%Ti alloy
Figure 4
KIK
The marginal gaps of metal ceramic crowns in the as-cast specimens. (* p!0.05).
marginal gaps of metal ceramic crowns. The coefficient of correlation between the finish line curvature and the labial marginal gap was 0.794 (p!0.05). The greater the finish line curvature, the larger the labial marginal gap became. The labial marginal gap of metal ceramic crowns from 5 mmcurvature abutments was significantly larger than that from 1 mm-curvature abutment. However, the type of alloys had no effect on marginal gaps of metal ceramic crowns.
Discussion The goal in the design of this in-vitro study was to compare the accuracy of Au-1.6 wt% Ti alloy metal ceramic crowns with that of control gold alloy KIK, and to investigate the effect of finish line curvature on the fit of metal ceramic crowns. Other possible
variable factors such as die spacer thickness [14], expansion of the die stone, and polymerization shrinkage of impression materials had been eliminated. For this reason, no spacer was used on the axial surfaces during wax pattern fabrication. The same metal abutment was used both for constructing the wax patterns and measuring the seating accuracy. The marginal gaps between crown and abutment were directly measured on the profile projector as this method is a valid non-invasive method, used extensively by clinicians and technicians. Although the replica technique is also a reliable non-invasive method of determining the adaptation of crown-totooth surface [15,16], different finish line curvatures of crowns may influence the results because the escape of replica materials from 5 mm-curvature crowns is more difficult than that form 1 mm-curvature crowns during measurement.
µm 1 mm-curvature
70
3 mm-curvature 5 mm-curvature
60 50
*
*
* *
*
*
*
*
*
40 30 20 10 0 labial
lingual mesial distal
Au-1.6wt%Ti alloy
Figure 5
labial
lingual mesial distal KIK
The marginal gaps of metal ceramic crowns after porcelain application. (* p!0.05).
402 The evaluation of crown-tooth discrepancies by replication also has its limitations and inherent errors such as recognition of crown margins and finish lines or rupture of the elastomeric film on removal from crowns. Taggart first described the lost wax casting process in dentistry in 1907 and observed that the resultant restorations were undersized because of shrinkage of the alloy on solidifying [17–19]. It is necessary to achieve compensation for the shrinkage of the solidifying alloy by investment expansion. Today, three methods of compensation for the alloy shrinkage on cooling are in regular use: (1) setting expansion of the investment, (2) hygroscopic expansion of the investment, and (3) thermal expansion of the investment. Although a select number of phosphate-bonded investments can be used in the ringless casting technique [20,21], most phosphate-bonded investments do not have sufficient compressive strength, therefore, use of a metal casting ring is necessary to protect the investment during casting forces [22,23]. However, the metal casting ring restricts the thermal expansion of the investment as the thermal expansion of the ring is less than that of the investment. To compensate for this limitation, a paper ceramic liner is recommended [24,25]. Because the ceramic liner can compensate for most casting ring restriction, the metal ring technique is clinically acceptable and allows for the fabrication of accurate castings [26]. As a standard process, the conventional metal ring casting technique was used in this study, however, due to the casting ring’s restriction, the investments expand more in an axial direction than horizontal. So the axial surface of the casting becomes longer than the original length of the wax pattern. The longer the axial surface of the wax pattern, the greater the increase in length of the casting’s axial surface is. This explains why the finish line curvature was greater, the larger the gap the mesial and distal margins of copings exhibited compared with labial and lingual margins in the ascast specimens. Length difference between buccolingual axial surfaces and mesiodistal axial surfaces of wax patterns was small for the 1 mm-curvature abutment, moderate for the 3 mm-curvature abutment and large for the 5 mm-curvature abutment, so after casting, length increment difference between buccolingual axial surfaces and mesiodistal axial surfaces was small for the 1 mm-curvature abutment, moderate for the 3 mm-curvature abutment and large for the 5 mm-curvature abutment. This is why mesiodistal marginal gaps were similar to buccolingual marginal gaps for 1 mm-curvature abutments, whereas mesiodistal marginal gaps
J. Tao et al. were much larger than buccolingual for 5 mmcurvature abutments. Deterioration of the initial fit of the metal coping was observed after the porcelain firing cycle. After porcelain application, the porcelain firing shrinkage and difference between porcelain and metal thermal coefficient of expansion caused substructure metal to bend towards the porcelain side. For the 1 mm-curvature abutment, the length of labial axial surface and proximal axial surfaces was similar, bending stress among labial axial surface and proximal axial surfaces almost redressed the balance. For the 5 mm-curvature abutment, because the labial axial surface was much longer than proximal axial surfaces, the cervical part of the labial axial surface acted like a cantilever. Bend in distortion towards the porcelain side at the cervical part of the labial axial surface was rarely restricted by proximal axial surfaces. Conversely, bending distortion of the proximal axial surfaces was restricted by buccolingual axial surfaces. The labial margin was the point that seemed to be most subjected to the bending distortion. So after porcelain application, the labial marginal gap became larger than the lingual and proximal margins. The greater the finish line curvature, the larger the labial marginal gap became (mean 44, 34, 25 mm, respectively, for 5, 3, 1 mm-curvature abutments). Usually the casting shrinkage of an alloy is related to its melting point. The melting point of Au-1.6 wt% Ti alloy is about 1100 8C and is close to that of KIK (1200 8C), so in the as-cast specimens there is no significant difference between specimens from Au-1.6 wt% Ti alloy and KIK. There is a little difference between the mechanical properties of Au-1.6 wt% Ti alloy and KIK under than experimental conditions [9]. The micro Vicker’s hardness of Au-1.6 wt% Ti alloy was 172 and that of KIK was 191. The metal’s thermal expansion coefficient in the 50–500 8C range was 14.4G0.3!10K6/8C for Au-1.6 wt% Ti alloy and was 14.1G0.2!10K6/8C for KIK [9]. The modulus of elasticity was 67 GPa for Au-1.6 wt% Ti alloy, was 84 GPa for KIK. These differences may have had an effect on accuracy of the metal ceramic crowns, but after porcelain application, no significant difference was found between metal ceramic crowns from Au-1.6 wt% Ti alloy and KIK, so it was concluded that the difference in mechanical properties have no statistically significant effect on the fit of metal ceramic crowns. Accuracy of fit has been extensively investigated in the dental literature, but the size of marginal gap that is clinically acceptable is difficult to document. Although marginal gaps of 100 mm are considered
Fit of metal ceramic crowns cast in Au-1.6 wt% Ti alloy for different abutment finish line curvature clinically acceptable with regard to longevity [27], theoretical requirements of marginal gaps should be lower than 40 mm [28]. In this in-vitro study, for 5 mm-curvature abutments, there were specimens with marginal gaps over 50 mm in the as-cast specimens or after porcelain application. However, in daily practice other factors such as percentage completion of tooth preparation, expansion of model stone, impression materials and skill of technicians also have an effect on the accuracy of crowns. The greater the finish line curvature, the more difficult the tooth preparation and the creation of the impression in the marginal area. If these factors are taken into account, it seems that crowns with sharply curved finish lines more often lead to poorly fitting margins that may be clinically unacceptable. From this study, it is clear that finish line curvature has a significant effect on the marginal fit of crowns, so clinicians should be aware of the effects of finish line curvature in their daily tooth preparations. In cases of canines and infralabioversion teeth or periodontally diseased teeth, labial or lingual surfaces of roots may be exposed. If the design of subgingival margin is adopted, the metal ceramic crowns cover all exposed surfaces of roots, a sharply curved finish line is produced. Labial subgingival margins that have a poor fit may incite or aggravate periodontal disease. Tooth preparation with sharply curved finish lines should be avoided by using the design of a supragingival margin if possible. The porcelain butt margins technique could be used to improve labial marginal fit if sharp curved finish lines could not be avoided. The marginal gap of metal ceramic crowns with porcelain butt margins is rarely influenced by the distortion of substructure metal [29]. For 5 mm-curvature abutments, using a direct lift technique, the authors fabricated five metal ceramic crowns with porcelain butt margins, and found the mean labial marginal gap was 30 mm, smaller than conventional metal ceramic crowns (44 mm).
Conclusions Data obtained in this in-vitro study allowed the following conclusions to be made. † The metal ceramic crowns cast in Au-1.6 wt% Ti alloy had accuracy as good those as-cast in control gold alloy KIK. † In the as-cast speciments, the greater the finish line curvature, the mesial and distal margins of
403
copings exhibited larger gaps than labial and lingual margins. † After porcelain application, for metal ceramic crowns of the facial-veneered type, the greater the finish line curvature, the larger the labial marginal gap became.
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