Dental Materials Journal 23(4): 497-503, 2004

Deformation Properties of Ti-6Al-7Nb Alloy Castings for Removable Partial

Denture Frameworks

Viritpon SRIMANEEPONG1, Takayuki YONEYAMA1,2, Noriyuki WAKABAYASHI3, Equo KOBAYASHI1, Takao HANAWA1 and Hisashi DOI11Institute of Biomaterials and Bioengineering

, Tokyo Medical and Dental University, 2-3-10 Kanda-Surugadai, Chiyoda-ku,

Tokyo 101-0062, Japan2Department of Materials Engineering

, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan3Faculty of Dentistry

, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan

Corresponding author, E-mail:yoneyama.met@tmd.ac.jp

Received August 26, 2004/Accepted September 30, 2004

Ti-6Al-7Nb alloy was cast into three differently designed, removable partial denture frameworks: Palatal strap (PS), Ante-rior-posterior bar (AP), and Horseshoe-shaped bar (HS). The vertical displacement and local strain of Ti-6Al-7Nb alloy frameworks were investigated to compare against those of Co-Cr alloy frameworks. Vertical loading force of 19.6N was ap-

plied at two locations, 10 and 20mm, from the distal end of the framework. Although higher vertical displacement and local strain were observed for Ti-6Al-7Nb alloy frameworks than those for Co-Cr alloy frameworks, the PS framework ap-peared to show the least deformation. In addition, the strain at 10-mm location was higher than that at 20-mm location for AP and HS frameworks. This study thus proved that design had a significant influence on the deformation properties of denture frameworks. The PS design was evaluated to be a suitable design for the removable denture framework with Ti-6Al-7Nb alloy.

Key words: Ti-6Al-7Nb alloy, Deformation, Denture framework

INTRODUCTION

Many kinds of metallic material have been used to fabricate varieties of restorations and appliances for dental treatment. Due to the optimum rigidity re-

quirement for most dental prostheses such as remov-able denture frameworks and long-spanned fixed restorations1,2), base metal alloy is one of the com-monly used casting alloys. This is because in addi-tion to being a cost-effective material, base metal alloy shows high elastic modulus and low density3). Co-Cr and Ni-Cr alloys are popular base metal alloys which have been widely used in dentistry, especially for removable dentures4,5). However, from bio-compatibility viewpoint, there remains the possibility of adverse effects such as toxicity and hypersensitiv-ity6-10).

Against this background of increasing concern over toxicity and hypersensitivity, commercially pure titanium (CP Ti) and various kinds of titanium alloy have been studied and introduced because of their su-

perior biocompatibility11-18). However, they have not yet become popular casting materials because of the processing method that requires special casting sys-tem and mold materials19-26). Recently, several disad-vantages of CP Ti have been highlighted. The first weakness lies in its lack of mechanical strength for some dental applications such as removable partial denture framework13). The second drawback arises from its poor wear resistance, which makes it

difficult to be polished27-31). Then, there is also a concern about the questionable cytotoxicity of vana-dium (V) in Ti-6Al-4V alloy. Consequently vana-dium was replaced by niobium (Nb) for biomedical α+β Ti-6Al-7Nb alloys32,33).

Due to its high biocompatibility and mechanical strength, Ti-6Al-7Nb alloy is a viable alternative to the traditional base metal alloys or CP Ti for remov-able dentures or other dental applications12,29,34,35). Al-though a number of its properties have been investigated, the deformation properties of Ti-6Al-7Nb alloy - which are important factors for clinical application - have not been studied for dental pros-theses. Therefore, the deformation properties of Ti-6Al-7Nb alloy castings for clinically simulated, removable partial denture frameworks were investi-

gated in this study.

MATERIALS AND METHODS

Specimen preparationA maxillary model (EI-569, Nissin, Japan) with missing teeth on both sides (#15-#17 and #26-#27) of dental arch was selected to be used as the master model in this study. Two different casting alloys

(Ti-6Al-7Nb and Co-Cr) and three conventional de-signs of denture frameworks, i.e., Palatal strap (PS), Anterior-posterior bar (AP), and Horseshoe-shaped bar (HS), were compared. The master model was

partly modified for testing purpose by leveling teeth

498 Ti-6Al-7Nb ALLOY CAST DENTURE FRAMEWORKS

#14, #24, and #25 to half height. 24-gauge sheet waxes, 0.55mm in thickness (GC, Japan), were used for wax constructing.

For each design group, four framework speci-mens were cast with Ti-6A1-7Nb alloy (T-Alloy Tough, GC, Japan) by an argon-gas centrifugal cast-ing machine (Ticast Super R, Selec, Japan), and a magnesia-based investment (Selevest(R) DM & D, Selec, Japan) was used as the mold material. Addi-tionally for the same design group, four more frame-work specimens were cast with Co-Cr alloy (Cobaltan Clasp, Shofu, Japan) by a centrifugal casting ma-chine (Denko Auto Sensor MD-201, Denko, Japan) with a phosphate-bonded investment (Wiroplus(R) N, Bego, Germany).

All framework specimens were prepared on the same model by an author in order to ensure frame-work uniformity and to minimize as much as possi-ble any variation during specimens preparation. After casting and careful polishing, all framework specimens were examined by measuring thickness at specific points (Fig. 1), and being weighed and in-spected under nondestructive X-ray instrument (model DCX-100, Asahi Roentogen, Japan) to detect any internal defects. The results are presented in Table 1.

Measurement procedureDeformation properties of the frameworks were tested using a hydraulic testing machine (Servopulser EHF-F1, Shimadzu, Japan) with one side (teeth #24,

#25) of each framework fixed to the device. Verti-cal loading force of 19.6N was applied on two loca-

tions - one location at a time - 10 and 20mm

from the distal end of the retentive framework,

which was opposite to the fixed side. The loading

force was applied using a load cell (Type SSL-01,

Showa, Japan) at a cross-head speed of 0.4N/s.

Two biaxial strain gauges (KFG-1-120-D16-

11L1M2S, Kyowa, Japan) were used for each speci-

men to measure the local strain. Each gauge con-

sisted of two linear strain gauge components with

120.4•}0.4ƒ¶ resistance and 2.07•}1.0% gauge factor.

The two biaxial strain gauges were positioned using

gauge cement (CC-33A, Kyowa, Japan) on two same

locations at the palatal part. These two locations

were namely between teeth #13 and #14 and be-

tween teeth #16 and #17 for all framework speci-

mens. For both biaxial strain gauges, one strain

gauge component was parallel to the long axis

(antero-posterior) of the framework while the other

strain gauge component was perpendicular to it (Fig.

2). The strain gauges and the load cell were con-

nected to a controller (Servopulser model 4826,

Shimadzu, Japan) through bridge circuits.

In each framework specimen, each loading point

was loaded five times. Taking into consideration

that the clamping condition might introduce some

variations, the framework was reconnected to the

loading machine for each measurement. During load-

ing, the vertical displacement at the loading point

and the local strain at each gauge location were

Fig. 1 Three designed framework specimens with specific measuring points of thickness.

Table 1 Thickness and weight measured at the specific points* of six framework specimen groups

*see Fig. 1.

SRIMANEEPONG et al. 499

Fig. 2 Schematic diagram of testing specimen with strain gauges attached.

measured. Repeated-measurement ANOVA and post-hoc LSD test were performed to analyze the data with significance level at 95%.

RESULTS

Vertical displacementFig. 3 shows the vertical displacement results meas-ured at two loading locations for all specimen

groups. Comparisons were made across different framework designs, alloys, and loading locations. Within each framework design group, the vertical displacement of Co-Cr alloy frameworks was less than that of Ti-6Al-7Nb alloy frameworks for all three design groups (p<0.05).

With respect to framework design, the PS frame-work deformed less vertically than AP framework (p<0.05) and the vertical displacement of HS frame-work was the largest amongst the three designs for both casting alloy groups (p<0.05). However, there was a different tendency on the design groups which showed no significant difference in the vertical dis-

placement. For Ti-6Al-7Nb alloy group, the vertical displacements of AP and HS frameworks were not significantly different when the loading force was ap-

plied at the 20-mm location (p=0.2). On the other hand, for Co-Cr alloy group, the vertical displace-ments of AP and PS frameworks did not show sig-nificant differences either at the 10-mm (p=0.08) or 20-mm (p=0.125) location.

With respect to loading location, the vertical dis-

placement at the 10-mm location was significantly higher than that at the 20-mm location in all condi-tion groups (p<0.05).

Local strain

By using two biaxial strain gauges, the strains exist-

ing on the anterior and posterior parts of the frame-

work specimen were measured through Channels 1/2

and Channels 3/4 (Fig. 2). The schematic illustra-

tions of the mean values and directions of local

strain for each framework design are shown in Figs.

4-7. These results revealed that under the same de-

sign and testing condition, the strain observed for

Co-Cr alloy frameworks was lower than that for Ti-

6Al-7Nb alloy frameworks (p<0.05). The degree of

local strain observed in this investigation ranged

from 2.56ƒÊƒÃ in HS/Co-Cr framework to 1,460ƒÊƒÃ in

AP/Ti-6Al-7Nb framework.

Regardless of alloy or loading location, the same

directions of local strain were found in AP and PS

frameworks: compressive strain in antero-posterior

(A-P) direction and tensile strain in bucco-palatal (B-

P) direction. Conversely, the opposite directions

were found in HS frameworks. The local strain ex-

isting on the anterior part was higher than the pos-

terior part in all framework designs. Especially for

HS and PS frameworks, they exhibited very small

amounts of local strain on the posterior part, regard-

less of alloy or loading location.

When loading location was changed from 20mm

to 10mm, the B-P strain on the anterior part of

both AP and HS frameworks significantly increased,

while the B-P strain on the posterior part of AP

frameworks simultaneously decreased (p<0.05).

However, PS frameworks showed no statistical differ-

ences in local strain between the two loading loca-

tions (p>0.1) for both Ti-6Al-7Nb and Co-Cr alloys.

500 Ti-6Al-7Nb ALLOY CAST DENTURE FRAMEWORKS

Fig. 3 Vertical displacement of six framework specimen groups under maximum load-

ing force of 19.6N at 10-mm or 20-mm loading locations. PS: Palatal strap;

AP: Anterior-posterior bar; HS: Horseshoe-shaped bar; Ti67: Ti-6Al-7Nb; CoCr:

Co-Cr. Underlines above the bars indicate no statistically significant differences

between the same alloy group.

Fig. 4 Schematic illustrations of local strains of three de-signed Ti-6Al-7Nb frameworks at 20-mm loading location (F: loading force).

Fig. 5 Schematic illustrations of local strains of three de-signed Co-Cr frameworks at 20-mm loading loca-tion (F: loading force).

SRIMANEEPONG et al. 501

Fig. 6 Schematic illustrations of local strains of three de-

signed Ti-6Al-7Nb frameworks at 10-mm loading location (F: loading force).

Fig. 7 Schematic illustrations of local strains of three de-signed Co-Cr frameworks at 10-mm loading loca-tion (F: loading force).

DISCUSSION

The deformation properties of denture frameworks are of great importance in clinical applications. Op-timum rigidity of denture frameworks is one of the

principal considerations for long-term success of prosthodontic restorations and treatments1,2). Ade-quate rigidity of denture framework provides cross-arch stability and distributes the occlusal stress through dental arch and supporting tissue. On the other hand, adequate flexibility is equally necessary for retention in order not to harm abutment teeth2).

Many researchers have conducted studies on CP Ti/Ti alloy removable dentures13,36-39) The casting

problems of Ti alloys19-21,23) and some properties of Ti-6Al-7Nb alloy have likewise been investi-

gated12,29,34,40) However, no studies on the deforma-tion characteristics of CP Ti/Ti alloy prostheses have been conducted. In a report by Au et al., the high flexibility of Ti denture frameworks was believed to be related to the high incidence of acrylic lifting from the metal retentive framework13). Nevertheless, the rigidity of denture frameworks depends on many factors such as the mechanical properties of alloy and the configuration of the framework conforming to the physiological anatomy41).

In the present study, the deformation properties of removable partial dentures cast with Ti-6Al-7Nb alloy were evaluated by two parameters: vertical dis-

placement and local strain. It was found that Ti-6Al-7Nb alloy frameworks showed higher vertical displacement and local strain than Co-Cr alloy frame-works, mainly due to the lower elastic modulus of Ti-6Al-7Nb alloy.

With respect to framework design on vertical displacement, the PS frameworks emerged as the

most rigid framework among the three design groups for both casting alloys. This may not corre-spond to previous studies which claimed the AP framework to be the most rigid denture frame-work42,43). One possible reason could be difference in the framework configuration among the experimental conditions.

However, it is of interest to note that the verti-cal displacements of AP/Co-Cr and PS/Co-Cr frame-works were not significantly different (p=0.08 for 10-mm location and p=0.125 for 20-mm location). On the other hand, the AP/Ti-6Al-7Nb framework showed significantly higher vertical displacement than the PS/Ti-6Al-7Nb framework. Moreover, at 20-mm loading location, the AP framework exhibited significantly less vertical displacement than HS framework when Co-Cr alloy was used, while the ver-tical displacements of these two framework designs of Ti-6Al-7Nb alloy were not significantly different

(p=0.2). These seemed to imply that framework de-sign had an influence over the mechanical properties of alloy, especially on the degree of denture framework's deformation when different casting al-loys were used.

When comparing the two loading locations, the vertical displacements at 10-mm loading location were higher than those at 20-mm loading location. This could be attributed to the increase in distance be-tween the loading point and the fixed part41,44).

Besides vertical displacement, strain was another indicator for the deformation characteristics of Ti-6Al-7Nb alloy frameworks. The amount and direc-tion of strain varied, depending on the configuration of the clinically simulated specimen. When force was applied on the framework specimen, the latter de-formed in three dimensions - including torsional

502 Ti-6A1-7Nb ALLOY CAST DENTURE FRAMEWORKS

deformation - instead of mere simple bending. Owing to the reverse relation between strain and elastic modulus, less strain was observed for Co-Cr alloy frameworks than for Ti-6A1-7Nb frameworks.

In this study, strain was observed in A-P and B-P directions at the gauge locations on the anterior and posterior parts of the framework. A higher strain was observed on the anterior part than the

posterior part for all framework designs. This could be due to the way the experiment was set up. One side of the anterior part of each specimen was fixed, which was opposite to the loading location (Fig. 2). This indicated that the retentive locations of remov-able partial denture - such as clasp, occlusal rest, and proximal plate - which restrict the movement of denture under functional occlusal force would also affect def ormation45>

According to the general deformation properties of metal46~, when force is imposed to induce elastic elongation in the B-P direction on a framework, com-

pressive strain will simultaneously occur perpendicu-larly to the tensile stress in the A-P direction, and vice versa. This explains the same directions of local strain found on AP and PS frameworks, but con-trary directions of local strain on HS frameworks. This could be attributed to the different 3-D configu-rations of each framework design. For HS and PS frameworks, a very small degree of strain was ob-served on the posterior part of the framework, re-

gardless of alloy or loading location. This could be due to lack of posterior bar in HS and PS designs -unlike the AP design. As a result, the posterior part of both HS and PS frameworks was less restricted in structure, hence leading to small amount of strain at this area.

When loading point was changed from 20 mm to 10 mm, the strain observed on the anterior part of all framework designs tended to increase. The in-crease in distance between the fixed part and the loading point seemed to be related to the increase not only in terms of vertical displacement, but also in local strain. However, this effect was hardly ob-served in PS frameworks which showed no signifi-cant differences in local strain between the two loading locations. This coincided with the low verti-cal displacement results for PS design, which exhib-ited high rigidity. In addition, though it be the same design, the inevitable variations in thickness of the denture framework in general dental practice would also have different effects on def ormation44,47)

CONCLUSIONS

The deformation properties of biomedical Ti-6A1-7Nb alloy denture frameworks were evaluated in terms of vertical displacement and local strain. The results showed that the vertical displacement and local strain of Ti-6A1-7Nb alloy denture frameworks were

significantly higher than those for Co-Cr alloy den-

ture frameworks, while the least vertical displace-ment and local strain were observed on PS framework for both casting alloys. The variation in

vertical displacement values due to framework design further differed by the alloy used - a factor closely

related to the clinical performance of removable par-tial dentures. In addition, the change in vertical dis-

placement and local strain due to loading location was also influenced by framework design.

It can be concluded that design has a consider-

able influence on the deformation characteristics of

denture frameworks, apart from the mechanical

properties of the casting alloy. Under the testing conditions of this study, the PS design proved to pro-

vide the highest rigidity to the removable denture framework. According to the results of the present

investigation, the PS design should be considered if Ti-6Al-7Nb alloy were to be cast for removable par-

tial denture framework.

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