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Int Chin J Dent 2004; 4: 72-77.

Radiolucency of cast titanium for detection of internal porosity
Ikuya Watanabe, DDS, PhD,a P. Andrew Benson, BS,b and Khoi Nguyen, BSbaDepartment of Biomaterials Science, Baylor College of Dentistry, Texas A&M University SystembHealth Science Center, and Baylor College of Dentistry, Texas A&M University System Health
Science Center, Dallas, TX, USA
Purpose: The aim of this study was to examine the radiolucency of cast titanium discs having different
thicknesses using radiographs taken by a dental X-ray unit under various conditions to detect internal
porosity of the cast titanium metal frameworks.
Materials and Methods: Radiographs of eight cast discs (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, and 5.0 mm thick;
8 mm diameter) were taken on dental occlusal film using a dental X-ray unit under the following
conditions: tube voltage: 70 kV or 90 kV, exposure time: 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, or 4.0 s. The
density levels (0, black-255, white) of the radiographs were measured using computer graphics.
Results: The mean density levels of the radiographs taken at 90 kV dramatically decreased to zero (black,X-ray 100% penetrated) with shorter exposure time and greater disc thickness. When a tube voltage of 70
kV was applied, there was a wide range of exposure times to reach a "0" density level for each discthickness.
Conclusion: The use of lower voltage and a longer exposure time for dental X-ray unit is appropriate toclearly detect the internal porosity. (Int Chin J Dent 2004; 4: 72-77.)
Clinical Significance: Cast titanium frameworks must be examined radiographically before further steps tofabricate the prostheses in laboratory. On the basis of the results obtained in this study, the use of lower
voltage and a longer exposure time for dental X-ray unit is appropriate to clearly inspect the internal
porosity in cast titanium metal frameworks.
Key Words: cast defect, cast titanium, internal porosity, radiolucency.

Introduction
Cast titanium restorations have been increasingly applied for patient treatment, especially for patients
who are allergic to dental alloys,1 since titanium offers excellent corrosion resistance and biocompatibility.
Titanium and its alloys are relatively difficult to cast because of their high reactivity with gasses at hightemperatures and their light weight (4.5 g/cm3) as compared with other dental casting alloys (Co-Cr, 8-9g/cm3; Au alloys, 13-16 g/cm3). Gasses are easily incorporated into molten titanium during casting,resulting in undesirable internal porosity in the cast metal framework, which leads to a high risk of clinicalfailure. Metal frameworks must be examined carefully for internal porosity to obtain problem-free castings.
Radiography is one of the non-destructive methods to check the internal porosity in dental castings,2-9
especially, in cast titanium due to its low density. Radiographs on industrial or dental films were frequently
employed to detect internal porosity of the titanium framworks.10-19 Wang et al.10 introduced a simple
method of detecting porosity in titanium using a dental X-ray unit and occlusal film. They set the X-ray
unit at a tube (peak) voltage of 90 kV, tube current of 25 mA, and 30-s exposure time to take radiographs ofa titanium partial denture framework with a 1.2 mm-thick major connector.
Metal frameworks possess different thicknesses, varying from 0.3-0.5 mm for palatal plates to 1.2-2.072

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Int Chin J Dent 2004; 4: 72-77.

mm for lingual bars, major connectors, and minor connectors. In an extreme case, the pontic and its
connectors in a multi-unit fixed partial denture may be 2.0-5.0 mm thick. Therefore, it is necessary toinvestigate the radiolucency of cast titanium metal frameworks with a large range of metal thickness. In thepresent study, the radiolucency of different thicknesses of cast titanium was evaluated using radiographs(dental occlusal film) taken by a dental X-ray unit with 70 kV or 90 kV tube voltage and various exposure
time to detect internal porosity.

Materials and Methods
Eight acrylic discs of various thicknesses (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, and 5.0 mm thick; 8 mm
diameter) attached to sprue bars were connected on the sprue cone former and invested in a mold ring using
a MgO-based investment (Selevest CB, Selec Co., Osaka, Japan). After the investment had set, the moldwas burnt-out in a furnace (Accu-Therm III 6000, Jelenko, Armonk, NY, USA) according to the
manufacturer’s instruction, and was cast with commercially pure titanium (ASTM Grade 2, Titanium Ind.,
Grand Prairie, TX, USA) using a centrifugal casting machine (Ticast Super R, Selec Co.). The castingprocedure followed manufacturer’s instructions.

After casting, the mold was bench-cooled to room

temperature. The cast titanium discs were retrieved, and the surfaces of the cast discs were air-abradedwith 50 µm Al2O3 particles.
Radiographs of the eight cast titanium discs were taken on dental occlusal film (57x76 mm, Ultra-speed,Kodak, Rochester, NY, USA) using a dental X-ray unit (GX-900, Gendex, Des Plaines, IL, USA) under thefollowing conditions: tube voltage, 70 kV or 90 kV; exposure time, 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 3.0, or 4.0
s. The tube current and target-film distance were set at constants of 15 mA and 50 cm. Three radiographsof the cast discs were taken for each tube voltage and exposure time. The occlusal films were developed ina dental X-ray film processor (A/T2000 XR, Air Techniques, Hicksville, NY, USA).
The developed radiographs were scanned into a computer using a scanner (hp scanjet 5470c,Hewlett-Packard, Palo Alto, CA, USA) at a resolution of 1,200 dpi, and the density level of radiographs foreach disc was measured using computer graphics (Adobe Photoshop 6.0, Adobe Systems Incorporated, SanJose, CA, USA) (density level; 0, black-255, white). The density levels of five areas (each square; 0.5x0.5mm) for each disc were averaged. The density levels of three radiographs were averaged, totalling 15
measurements for each disc. The areas where small internal porosity was found in the disc were notselected for density level measurements. The data were statistically analyzed using three-way ANOVA
with the factors of tube voltage, exposure time, and disc thickness at a significance level of α=0.05.

Results
Fig. 1 shows the radiographs taken by the dental X-ray unit set at 70 kV and 90 kV. The density levelsof the occlusal film taken at 70 kV and 90 kV are presented in Figs. 2 and 3, respectively. Compared to thevalues at 70 kV, the mean density levels of the radiographs taken at 90 kV dramatically decreased to zero
(black; 100% X-ray penetrated) with shorter exposure time and greater disc thickness. For example, when73

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a tube voltage of 90 kV was applied, the exposure time to reach a density level of "0" was 1.5 s for 3.0 mm
thickness and 2.0 s for 4.0 mm thickness. When a tube voltage of 70 kV was applied, depending of metal
thickness the exposure time needed to reach a density level 0 has a wide range and varied between 0.75 s
and more than 4 s. The results of the statistical analysis are presented in Table 1. Although statisticalsignificances (p<0.05) were found for each factor (tube voltage, exposure time, and disc thickness) andthree-way interaction, there were no significances for two-way interactions between tube voltage andexposure time or disc thickness.

Fig. 1. Radiographs of cast titanium discs with various thickness using a dental X-ray unit.
Arrows indicate internal pores of the 3 mm thick disc taken at different conditions(70 kV-1.5 s vs. 90 kV-0.75 s).

Fig. 2. Film density level taken at 70 kV.

Fig. 3. Film density level taken at 90 kV.

Discussion
When the X-ray beam passes through the different thicknesses of cast titanium discs, its intensity is74

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Table 1. The results of the three-way ANOVA.

Tube voltage (a)Exposure time (b)Disc thickness (c)axbaxcbxc
axbxc
Error

Sum of squaredf

Mean square

F valuep valueSig.

306,153.758
3,801,149.904
957,181.298
89,942.883
56,972.044
553,823.284
295,961.381
8,922.068

17
7
77
49
49
512

306,153.758
543,021.415
136,740.185
12,848.983
8,138.863
11,302.516
6,040.028
17.42620.48229.982
10.203
2.127
1.347
1.871
346.612

0.003
0.000
0.000
0.058
0.249
0.015
0.000

SSSNSNSNSS

S, significant; NS, not significant at α=0.05.

Fig. 4.
Radiograph of a cast titanium
partial denture framework taken
at 70 kV-1.5 s.
Arrows indicate internal pores ofthe left-side I-bur and the left
molar lingual clasp.
reduced by absorption or scatter in the cast titanium. As a result, the different beam intensities change theoptical density of the radiographs. When one considers the number of photons in an X-ray beam, n, as it
passes through a thin layer of attenuating material, a relatively small number, Δn, are absorbed or scatteredin the layer (thickness of the layer; Δx)20
Δn = – µ · n · Δx
where, µ is the linear attenuation coefficient. The minus sign is used because n refers to the photons
remaining in the beam that are unaffected by matter. µ · n · Δx photons are removed from the beam by thelayer, so the change Δn in the total number still in it is negative. The linear attenuation coefficient is afunction of the density, ρ, the atomic number of the irradiated medium, Z, and the energy of the photons
that constitute the x-ray beam, hν (h; Planck’s constant, 6.626x10-34 J·s: ν; frequency of the radiation):µ = µ (ρ, Z, hν)
The same argument applies to the X-ray beam’s energy fluence and to its intensity, I.
ΔI = – µ · I · ΔxCompared to other elements included in dental casting alloys, titanium has relatively low density, ρ (4.5g/cm3) and atomic number, Z (22), resulting in a relatively higher radiolucency (low ΔI) as observed on the75

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occlusal film employed for dental use.9,10
When low tube voltage (70 kV) was used to inspect the cast titanium discs for internal porosity, theradiographs had wide ranges of film density level and exposure time for various metal thicknesses. The useof low-energy X-rays enhances the photoelectric contrast.20

If lower-energy X-rays are used, subtle

differences in the amounts of photoelectric attenuation occurring in different substances (because ofdifferences in their densities and atomic numbers) may be enhanced, and may result in greater contrast in aradiograph. Therefore, internal porosity in cast titanium may be more clearly detected by applying lower
tube voltages and longer exposure times. This finding can be confirmed by examining the two radiographsthat show similar density levels for titanium discs with the same thickness (3.0 mm) under two different
conditions: 70 kV-1.5 s vs. 90 kV-0.75 s (as shown by arrows in Fig. 1). The internal pores in theradiograph taken at 70 kV-1.5 s are more apparent compared with those found in radiographs taken at 90
kV-0.75 s.
When one considers the optimal conditions (tube voltage and exposure time) for taking radiographs todetect the internal porosity in metal frameworks of different thicknesses, it is necessary either to choose a
condition covering a certain range of film density levels for different metal thicknesses, or to take several
radiographs under different conditions. Based on the radiographs and the data obtained in this study,density levels between 50 and 200 seem to be appropriate for detecting internal porosity. Compared to thevalues at 90 kV, there was a wide range of exposure times at 70 kV that produced a density level between
50-200 for different metal thicknesses. Therefore, within the limits of this study, radiographs taken at atube voltage of 70 kV were suitable for checking internal porosity of cast titanium with different thickness.
Fig. 4 shows a radiograph of a cast titanium metal framework taken at 70 kV and 1.5 s. A small pore thatmight be a source of fracture during habitual use (insertion and removal of dentures) can be seen in the leftI-bur. The end of the left molar lingual clasp also contained some tiny pores that might lead to a fracturewith lengthy usage. These bars and clasps are approximately 2.0-3.5 mm thick. The radiograph also
indicated that the major connector has several pores which might not be critical for the breakage of metalframework during long term usage.
Another issue that should be considered is the different density levels caused by the shape and angle ofthe complicated metal framework as it is placed on the occlusal film. The cast titanium discs (Fig. 1) usedin this study were placed directly on the occlusal film, leaving no space between the titanium disc and film.
Therefore, the attenuated X-rays passing through the discs exposed the film immediately after penetration.
When taking a photograph of cast titanium framework, some spaces (air gaps) are present between theframework and occlusal film (subject-film distance) due to the shape of the framework (a large space under
the palatal plate exists in Fig. 4). The subject-film distance (air gap) enlarges the radiographic image of thesubject and affects its density level and contrast due to the scattered radiation.21,22 The shape of metalframework also leads to geometric distortion on the radiograph. Thus, it might take several radiographs toobtain one optimal radiograph that will detect internal porosity in different portions of the cast titanium
framework. In any case, most internal porosity can be found if the radiograph is inspected carefully.

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Conclusions
The results of this study indicated that the use of dental X-ray unit and occlusal film are useful for theinspection of internal porosity in titanium castings. The use of lower tube voltage and a longer exposure
time is appropriate to clearly detect the internal porosity. Specifically, the results showed that a tube
voltage of 70 kV covered a wide range of exposure times to produce certain density levels for eachthickness of cast titanium disc.
Acknowledgment
This investigation was supported in part by a grant Y2001-Z from the Baylor College of Dentistry Faculty Intramural
Grant Program. Editorial assistance by Mrs. Jeanne Santa Cruz is also appreciated.

References
1. Suzuki N. Metal allergy in dentistry: detection of allergen metals with X-ray fluorescence spectroscope and its
applicationtoward allergen elimination. Int J Proshodont 1995; 8: 351-9.
2. Pascoe DF, MWimmer J. A radiographic technique for detection of internal defects in dental castings. J Prosthet Dent
1978; 39: 150-7.
3. Lewis AJ. Radiographic evaluation of porosities in removable partial denture castings. J Prosthet Dent 1978; 39:278-81.
4. Wictorin L, Jullin P, Mollersten L. Rentogenological detection of casting defects in cobalt-chromium alloy framework. J
Oral Rehabil 1979; 6: 137-46.
5. Wise HB, Kaiser DA. A radiographic technique for examination of internal defects in metal frameworks. J Prosthet Dent
1979; 42: 594-5.
6. Elarbi EA, Ismail YH, Azarbal M, Saini TS. Radiographic detection of porosities in removable partial denture castings. JProsthet Dent 1985; 54: 674-7.
7. Dharmar S, Rathnasamy RJ, Swaminathan TN. Radiographic and metallographic evaluation of porosity defects andgrain structure of cast chromium cobalt removable partial dentures. J Prosthet Dent 1993; 69: 369-73.
8. Eisenburger M, Tschernitschek H. Radiographic inspection of dental castings. Clin Oral Invest 1998; 2: 11-4.
9. Eisenburger M, Addy M. Radiological examination of dental castings - a review of the method and comparisons of theequipment. J Oral Rehabil 2002; 29: 609-14.
10. Wang RR, Boyle AM. A simple method for inspection of porosity in titanium castings. J Prosthet Dent 1993; 70: 275-6.
11. Hero H, Syverud M, Waarli M. Mold filling and porosity in castings of titanium. Dent Mater 1993; 9: 15-8.
12. Syverud M, Okabe T, Hero H. Casting of Ti-6Al-4V alloy compared with pure Ti in an Ar-arc casting machine. Eur J OralSci 1995; 103: 327-30.
13. Chai TI, Stein RS. Porosity and accuracy of multi-unit titanium castings. J Prosthet Dent 1995; 73: 534-41.
14. Chan D, Guillory V, Blackman R, Chung K. The effect of sprue design on the roughness and porosity of titanium
castings. J Prosthet Dent 1997; 78: 400-4.
15. Bridgeman JT, Marker VA, Hummel SK, Benson BW, Pace LL. Comparison of titanium and cobalt-chromium
removable partial denture clasps. J Prosthet Dent 1997; 78: 187-93.
16. Al-Mesmar H, Morgano S, Mark RE. Investigation of the effect of three sprue designs on the porosity and thecompleteness of titanium cast removable partial denture frameworks. J Prosthet Dent 1999; 82: 15-21.
17. Zinelis S. Effect of pressure of helium, argon, krypton, and xenon on the porosity, microstructure, and mechanicalproperties of commercially pure titanium castings. J Prosthet Dent 2000; 84: 575-82.
18. Baltag I, Watanabe K, Kusakari H, Miyakawa O. Internal porosity of cast titanium removable partial dentures: Influenceof sprue direction on porosity in circumferential clasps of a clinical framework design. J Prosthet Dent 2002; 88: 151-8.
19. Cecconi BT, Koeppen RG, Phoenix RD, Cecconi ML. Casting titanium partial denture frameworks: A radiographicevaluation. J Prosthet Dent 2002; 87: 277-80.
20. Wolbarst AB. Physics of radiology. Norwalk CT: Appleton & Lange Publisher; 1993. p. 107-31.
21. Hendee WR. Radiologic physics, equipment and quality control. Chicago IL: Year Book Medical Publisher; 1977. p.
152-62.
22. Selman J. The fundamentals of X-ray and radium physics. Springfield IL: Charles C. Thomas Publisher; 1994. p. 384-5.

Reprint request to:
Dr. Ikuya Watanabe
Department of Biomaterials Science, Baylor College of Dentistry
Texas A&M University System Health Science Center
3302 Gaston Ave., Dallas, TX 75246, USA
Fax: +1-214-828-8458 E-mail: iwatanabe@bcd.tamhsc.edu
Received August 20, 2004. Revised August 28, 2004. Accepted September 1, 2004.
Copyright ©2004 by the Editorial Council of the International Chinese Journal of Dentistry.

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