The influence of mold temperature on the fit of cast crowns with commercially pure titanium

Commercially pure titanium (CP Ti) has been widely applied to fabricate cast devices because of its favorable properties. However, the mold temperature recommended for the manufacture of casts has been considered relatively low, causing inadequate castability and poor marginal fit of cast crowns. This study evaluated and compared the influence of mold temperature (430 degrees C--as control, 550 degrees C, 670 degrees C) on the marginal discrepancies of cast CP Ti crowns. Eight bovine teeth were prepared on a mechanical grinding device and impressions were used to duplicate each tooth and produce eight master dies. Twenty-four crowns were fabricated using CP Ti in three different groups of mold temperature (n = 8): 430 degrees C (as control), 550 degrees C and 670 degrees C. The gap between the crown and the bovine tooth was measured at 50 X magnification with a traveling microscope. The marginal fit values of the cast CP Ti crowns were submitted to the Kruskal-Wallis test (p = 0.03). The 550 degrees C group (95.0 microm) showed significantly better marginal fit than the crowns of the 430 degrees C group (203.4 microm) and 670 degrees C group (213.8 microm). Better marginal fit for cast CP Ti crowns was observed with the mold temperature of 550 degrees C, differing from the 430 degrees C recommended by the manufacturer.


INTRODUCTION
Commercially pure titanium (CP Ti) has come to be regarded as a promising material for dental prosthetic devices because of its excellent biocompatibility, radiographic radiopacity, high ductility, low thermal conductivity and relatively non-corrosive nature 5 .The possibility of fusing ceramics with titanium crowns has contributed to the rapid growing interest in CP Ti in dentistry.CP Ti has been extensively used to make crowns and fixed prostheses because of its biological and mechanical properties 8,18 .
However, in spite of its reliable mechanical properties, when titanium is used to fabricate cast devices, its high melting point and highly chemical reactivity at high temperatures make the manipulation of this material difficult 7 .Titanium has strong affinity to ambient elements and at high temperatures it can be easily oxidized, dissolving elements as oxygen, nitrogen and hydrogen 14,18 .Therefore, the reactive nature of titanium and uncontrolled atmospheres used in fabrication of denture frameworks might deleteriously alter the composition and microstructure of the products as provided by the manufacturers 4 .Cast machines have been developed to use inert atmospheric gas to avoid contamination during melting and casting.Titanium has been melted in arc-melting furnace or high frequency heating in argon or helium gas 14 .
The castability of titanium can be influenced by the investment material and mold temperature 17 .Investment materials used for casting titanium are refractory materials in which the reactivity with titanium is reduced by using components with low standard free energy of oxide, such as alumina, magnesia and zirconium oxide.However, the formation of the reaction layer or oxide layer is not inhibited by improvement of the investment 10 .
The mold temperature recommended by the manufacturer is low to protect the metal from reacting with the investment 15 .However, high thermal expansion for the compensation of the metal shrinkage does not occur at this low temperature 12 .Higher mold temperatures may affect fit of cast titanium crowns by increasing the fluidity of the melted metal during injection in the mold, improving the castability and yielding crowns with clinically acceptable levels of fit 7 .
Since the mold temperature recommended by manufacturers has been reported in the literature as relatively low to avoid chemical reactions, leading to lower castability and poor marginal fit of cast CP Ti crowns, the aim of this study was to evaluate and compare the influence of mold temperature of the investment (430°C -as control, 550°C, 670°C) in marginal fit of crowns made with CP Ti.

MATERIALS AND METHODS
Eight bovine teeth with their roots embedded in acrylic resin (Clássico Ltda., Campo Limpo Paulista, Brazil) blocks were prepared based on the protocol reported by Blackman et al. 3 (1992): 1.5 mm shoulder finish line and an 8° angle of axial convergence (Figure 1).Preparations were standardized on a mechanical grinding device (TR 600, Riosulense, Rio do Sul, SC, Brazil) with constant irrigation with distilled water at room temperature.Approximately 1 mm below the cervical preparation line, four diametrically opposed reference points (Figure 1) for marginal fit measuring procedures were made using a round bur (KG Sorensen Ind. e Com.Ltda., São Paulo, Brazil) in a rotary instrument (Dabi Atlante, Ribeirão Preto, Brazil) 6 .
The 0.7 mm-thickness patterns were waxed (Dentaurum JP Winkelstroeter KG, Pforzheim, Germany) on the master dies through a wax dipping unit (Hotty, Renfert GmbH, Hilzingen, Germany) and a sprue was waxed in the labial cusp areas with 3.0 mm-diameter wax 13 .The investment material for titanium casts was used (Rematitan Plus, Dentaurum JP Winkelstroeter KG, Pforzheim, Germany) with a ratio of 250 g of powder to 40 ml of liquid.The investment material was dispensed into a mixing vessel and the powder was fully incorporated.The mixing was carried out under vacuum at a speed of 425 rpm for 30 seconds using a mechanical mixer (Degussa, Frankfurt, Germany).The patterns were invested in casting rings (Dentaurum JP Winkelstroeter KG, Pforzheim, Germany) through mechanical vibration 12 .
After setting, the investment blocks were put in an electric furnace (Vulcan 3.550, Ney Dental Inc., Yucaipa, USA) at a heating rate of 7°C/minute, following the cycle: 250°C/60 minutes, 1,000°C/90 minutes, and then, cooling in the furnace at a rate of 7°C/minute for the target temperature of each group (430°C, 550°C and 670°C), which was maintained for two hours.The CP Ti crowns were cast in a titanium-casting machine (Dentaurum JP Winkelstroeter KG, Pforzheim, Germany) with the use of 31 g ingots and 0.95 bar argon pressure 1,6,19 .
After casting, the investment blocks were immediately quenched in cold water and castings were covered with the aid of a sandblaster (Odontolarcon Ind. Bras.Ltda., Maringá, Brazil) using 100 µm-aluminum oxide (N.Martins, São Paulo, Brazil) for 5 seconds at 5.5 bars 3,6,19 .Sprues were removed with a separating disk (Dentaurum JP Winkelstroeter KG, Pforzheim, Germany) and external surfaces of the crowns were finished with tungsten burs (Dentaurum JP Winkelstroeter KG, Pforzheim, Germany).Any nodules that might prevent complete seating of copings were removed with a round bur at low speed.
Each crown of groups 1, 2 and 3 was positioned on their respective bovine teeth and axially loaded at 8.82 N/m 2 to avoid uncontrolled displacement or seating error by application of finger pressure.The gap between the crown and the bovine tooth was measured at 50 X magnification with a traveling microscope (STM, Olympus Optical Co, Tokyo, Japan), accurate to 0.5 µm.An acrylic resin device (Figure 2) was used to align the crown and the tooth margins in the same optic reading plane.The measurements of marginal gap were recorded three times for each predetermined set of reference point.The mean of 12 measurements was considered for each tooth.
The statistical significances were evaluated by the nonparametric Kruskal-Wallis test at the 3% significance level.The identification of the groups that showed significant differences was carried out by the multiple comparison test at the 5% significance level.

RESULTS
The marginal fit values (and standard deviations -SD) of the cast CP Ti crowns fabricated at 430°C, 550°C and 670°C investment mold temperatures are shown in Table 1.The Kruskal-Wallis test revealed statistically significant differences among groups (p = 0.03).The multiple comparison test showed that there were no significant differences between group 1 (203.4µm) and group 3

DISCUSSION
The technique for accurate CP Ti castings is not yet fully optimized to prevent reactions between the molten metal and investment material 2 .Molten titanium reacts with investment materials, particularly with SiO 2 -based or phosphatebonded investments.This reactivity results in the formation of hard, brittle reaction layers on the cast surfaces, called α-case oxide layer 14,18 .This layer has been removed during the cleaning of cast CP Ti crowns 12 , and although the formation of an oxide layer cannot be completely inhibited, low mold temperatures have been used to accelerate the solidification of molten titanium, reducing the reactivity with the investment.As a result, manufacturers have recommended low mold temperatures to minimize the formation of the reaction layer.Nevertheless, this procedure has reduced metal castability and consequently produced dimensional changes in the castings 15 .
The castability of CP Ti is modified by factors such as: mold temperature, mold permeability, viscosity and fluidity of metal.Pure titanium can produce more casting defects than other dental alloys, mainly when the investment mold does not allow adequate fluidity of the metal 17 .These factors are under control and CP Ti has been used for fixed partial denture frameworks 18 .However, many studies have still shown that the casting temperature of investment mold presented unsatisfactory results 7,12 .This study analyzed different mold temperatures to find an appropriate investment temperature to cast CP Ti crowns with clinically acceptable marginal gaps, and consequently, the low mold temperature of 430°C (recommended by the manufacturer) was found to show high values of marginal gap (203.4 µm).
Increased gaps when using low mold temperatures were explained by Kikuchi et al. 10 (2001), who reported accelerated solidification of molten titanium and reduced castability.In addition, Takahashi et al. 15 (1990) reported undersized cast CP Ti crowns for mold temperatures of 350°C.Conversely, Blackman et al. 3 (1992) showed satisfactory mean marginal gap (50 µm) using the mold temperature suggested by the manufacturers, although adopting an investment material that allowed control of thermal expansion.
With high mold temperatures, Takahashi et al. 16 (1993a) obtained high thermal expansion that allowed compensation for the casting shrinkage of titanium, but greater reaction with investment was produced.Mori et al. 12 (1994) and Watanabe et al. 18 (1999) reported that the reaction layer is thicker when using high mold temperatures.Air-abrasion with aluminum powder is effective in removing this oxide layer, but the marginal gap of crowns can be enlarged.Supporting this hypothesis, the present study also found greater values of marginal gap (213.8 µm) with a mold temperature of 670°C.
In the present study, the 550°C-mold temperature (different from the manufacturer's instructions) produced 95 µm of mean marginal gap for CP Ti crowns, which was statistically better than that produced with 430°C (recommended temperature) and 670°C.At this intermediate temperature, the fluidity of the molten metal was maintained and marginal gaps were kept to a minimum, even after sandblasting.
In addition, following the manufacturer's instructions and using 430°C as the final casting temperature, the heating cycle for the titanium investment material lasts about 10 hours.After reaching the maximum temperature (1,000°C), with the period of cooling for the target temperature changed to 550°C, the heat cycle can be reduced in about 1 hour, reducing costs in commercial laboratories.
In the related literature, one frequently finds controversial opinions on the clinical relevance of the size of marginal gap.McLean, von Fraunhofer 11 (1971) reported that restorations with a gap extending over a range of 10 to 160 µm at the cervical margin might be tolerable.In the present study, only the 550°C-mold temperature crowns could be considered adequate for clinical application.These results are similar to others that advocated an acceptable clinically marginal fit in the order of 100 µm 6,9,11 .

CONCLUSIONS
Within the limitations of this study, it can be concluded that: 1. Commercially pure titanium crowns demonstrated marginal fit clinically acceptable when castings were performed with a mold temperature of 550°C.2. The fit of CP titanium crowns with the mold temperature of 430°C -as recommended by the manufacturer -and 670°C was considered clinically unacceptable.

FIGURE 1 -
FIGURE 1 -Bovine tooth prepared for full crown restoration embedded in resin blocks and reference points below cervical line for marginal fit measuring procedures.

FIGURE 2 -
FIGURE 2 -Device to align the crown and the tooth margins for marginal gap measurements.A:1 -Screw for vertical alignment of the tooth; 2 -Acrylic ring to support the embedded blocks; 3 -fixation screw for the embedded blocks; 4 -titanium crown; B: Lateral view.

TABLE 1 -
Means (in micrometers) and standard deviations (SD) of marginal gap for cast CP Ti crowns fabricated at 430°C, 550°C and 670°C mold temperatures.
Means followed by the same letter do not present statistical difference by the Kruskal-Wallis test at the 3% significance level.Standard deviations in parentheses.