Shimizu et al.

Int Chin J Dent 2005; 5: 61-64.

Fracture strengths of newly designed metal-based complete
maxillary dentures made from a cobalt-chromium alloy with highelastic modulus
Hiroshi Shimizu, DDS, PhD,a Fumitake Tsue, DDS,a Motofumi Obukuro, DDS,aHirofumi Kido, DDS, PhD,b Yutaka Takahashi, DDS, PhD,a and Hiroyuki Ohmura, DTc
abDivision of Removable Prosthodontics, Department of Oral Rehabilitation, Division of Oral
Implantology, Department of Oral Rehabilitation, Fukuoka Dental College, Fukuoka, Japan, andcFukuoka Branch Office, Dental Laboratory Aishi, Fukuoka, JapanPurpose: The purpose of the present study was to evaluate the fracture strengths of a metal-based complete
maxillary denture with a metal framework (that did not extend to the residual ridge crest), which was made froma cobalt-chromium alloy with high elastic modulus.
Materials and Methods: Sample newly designed metal frameworks of metal-based complete maxillary dentures
were cast in a cobalt-chromium alloy with high elastic modulus. The maximum fracture strengths of dentures
were measured using a universal testing machine. The mean value was statistically compared to that of aconventional cobalt-chromium alloy measured in a previous study.
Results: The fracture strength of the metal-based complete maxillary dentures made from a cobalt-chromium
alloy with high elastic modulus was significantly higher than that of the conventional cobalt-chromium alloy.
Conclusion: The results of this in vitro study indicate that a cobalt-chromium alloy with high elastic modulus
provides superior fracture strength for metal-based complete maxillary dentures with a newly designed metalframework. (Int Chin J Dent 2005; 5: 61-64.)
Key Words: cobalt-chromium alloy, elastic modulus, fracture strength, metal-based complete maxillary denture,
metal framework.

Introduction
The metal-based framework for a complete maxillary denture commonly covers the palate and residual ridges,
with the borders made of acrylic resin.1 In a previous study,2 a newly designed metal framework for metal-based
complete maxillary dentures that did not extend to the residual ridge crest was devised in order to facilitate
arranging the artificial teeth when there is not enough space between the upper residual ridge crest and theantagonist. The previous study showed that the fracture strength of the metal-based complete maxillary denturewith the newly designed metal framework made from a conventional cobalt-chromium alloy was approximately
two times greater than that of the resin-based denture, but was lower than the strength of a conventionallydesigned metal-based denture.
It was hypothesized that the increased mechanical properties of the metal framework material should improve
the fracture strength of the metal-based denture. Therefore, the purpose of the present study was to attempt toachieve better fracture strengths of a metal-based complete maxillary denture with a newly designed metalframework using a cobalt-chromium alloy with high elastic modulus, which has superior mechanical properties
compared with a conventional cobalt-chromium alloy.

Materials and Methods
The cobalt-chromium alloys used in this study and the previous study,2 their trade names, manufacturers,
batch numbers, chemical compositions, and technical data are listed in Table 1. Stone casts (Fujirock, GC Corp.,

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Int Chin J Dent 2005; 5: 61-64.

Tokyo, Japan) and also duplicate and refractory casts (Rema Exact, Dentaurum Inc., Newtown, PA, USA) of themaxillary edentulous model (G1-402, Nissin Dental Products Inc., Kyoto, Japan) were made.

The metal

2framework was constructed with the new design cast from a cobalt-chromium alloy with high elastic modulus
using the refractory casts. The cast metal frameworks were airborne-particle abraded with 50 µm grain-sizedaluminum oxide particles using a grit blaster (Micro Blaster MB102, Comco Inc., Burbank, CA, USA) and thenelectrochemically polished.

A heat-polymerized poly(methyl methacrylate) (Vertex Rapid Simplified,Vertex-Dental B.V., Oldenbarneveltlaan, Netherlands) was applied to the denture base material on the stone
casts. The thickness of the denture bases was adjusted to 2.5 mm according to the method described previously.2
All the dentures were stored in 37°C distilled water for 50 hours before testing.
Table 1.

Cobalt-chromium alloys used in this study and the reference 2, their chemical composition(mass %), and technical data.*

Trade name
Manufacturer
Batch numberComposition
Yield strength Rp 0.2Tensile strength Rm
Hardness (HV 10)
Elongation range A5Modulus of elasticity E

Co-Cr alloy with high elastic modulus

Conventional Co-Cr alloy

Remanium GM800
Dentaurum Inc., Newtown, PA, USA
340301
Co 63.3%, Cr 30.0%, Mo 5.0%,
Si 1.0%, others 0.7%
720 MPa
960 MPa370
6.0%
230 GPa

Remanium GM380
Dentaurum Inc.
9930
Co 64.6%, Cr 29.0%, Mo 4.5%,
others 1.9%
640 MPa
900 MPa360
6.5%
220 GPa*The data were obtained from the literature on manufacturer’s product information.

The maximum fracture strengths were measured using a universal testing machine (Autograph AGS-J,Shimadzu Corp., Kyoto, Japan) at a crosshead speed of 5.0 mm/minute with an 11.5 mm ball attachment. The
mean fracture strength of 10 dentures was statistically compared with that of the identically designed dentures
made with a conventional cobalt-chromium alloy obtained in the previous study2 using the Student t-test(p<0.01). The types of fracture were categorized after testing.

Results
The comparison between the present and previous results revealed that the fracture strength of the metal-based
complete maxillary dentures cast in a cobalt-chromium alloy with high elastic modulus was significantly higher
than that cast in a conventional alloy2 (Table 2). The types of specimen failure after testing are presented inTable 3.
Table 2.
Group

Fracture strengths for each alloy.
Fracture strength (kN)

Co-Cr alloy with high elastic modulus
Conventional Co-Cr alloy

MeanSD

2.01
1.49

0.50
0.24

SD, standard deviation. The data shown for the conventional Co-Cr alloy were obtained from the reference 2.

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Table 3.

Int Chin J Dent 2005; 5: 61-64.

Type of fracture.

Type of fracture

Co-Cr alloy with high elastic modulus

Fracture or deformation of metal framework
Fracture at metal framework outline
Fracture of base resin

Conventional Co-Cr alloy

20%
40%60%

60%
40%
20%

The data shown for the conventional Co-Cr alloy were obtained from the reference 2.

Discussion
The fracture strengths of the metal-based complete maxillary dentures made of a conventional
cobalt-chromium alloy2 were superior to the maximal bite forces of the complete denture wearers.3,4 This result
suggested that the application of this new metal framework is acceptable. However, when a conventional
cobalt-chromium alloy was used, the fracture strength of the new metal framework was significantly lower thanthat of the conventional design,2 but the fracture strength of the new design should be at least the same as theconventional one. Furthermore, even if the yield strength or the proportional limit is within the acceptable range
for the denture, repetitive loads may finally lead to denture fracture. Therefore, a cobalt-chromium alloy withhigh elastic modulus was used to obtain higher fracture strength for the complete maxillary dentures.
The results of this study indicated that the fracture strength of metal-based complete maxillary dentures madeof a cobalt-chromium alloy with high elastic modulus was significantly higher than that of the dentures made ofa conventional alloy. This finding is probably due to the superior mechanical properties, especially the higher
elastic modulus of the alloy (Table 1). The fracture strength of a cobalt-chromium alloy with high elastic
modulus may be satisfactory for metal-based complete maxillary dentures because additional evaluation revealed
no significant differences between the present experimental framework and the metal framework using a
conventional alloy (2.01 kN)2 (p>0.01). Cobalt-chromium alloys with a much higher elastic modulus are used tomake clasps, which result in very rigid frameworks, and have a high fatigue fracture resistance.5 This property is
one of the advantages of using a cobalt-chromium alloy with high elastic modulus.
Fracture at the metal framework outline without fracture of the base resin occurred in 40% of the specimens.
Such a fracture indicated that the stress concentration against the compressive load in the center of the plate
tended to occur along this line. The frequency of fracture or deformation of the metal frameworks decreased,
and the fracture of the base resin increased using a cobalt-chromium alloy with high elastic modulus compared
to conventional alloy.

This finding indicates that it is more difficult for frameworks made with acobalt-chromium alloy with high elastic modulus to deform. Further in vitro studies are necessary to evaluate
the effect of the bonding system on the fracture strength of the metal-based denture.6Conclusion
The cobalt-chromium alloy with high elastic modulus provides superior fracture strength of metal-based
complete maxillary dentures with a newly designed metal framework.
Acknowledgment
The authors gratefully acknowledge the editorial assistance of Mrs. Jeanne Santa Cruz.

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References
1. Anusavice KJ. Phillips’ science of dental materials. 10th ed. Philadelphia: W.B. Saunders Co.; 1996. p. 237-72.
2. Shimizu H, Tsue F, Obukuro M, Kido H, Takahashi Y, Ohmura H. Fracture strength of metal-based complete maxillarydentures with a newly designed metal framework. Int Chin J Dent 2005; 5: 33-8.
3. Eerikainen E, Kononen M. Forces required by complete dentures for penetrating food in simulated function. J Oral Rehabil
1987; 14: 607-13.
4. Ogata K, Satoh M. Centre and magnitude of vertical forces in complete denture wearers. J Oral Rehabil 1995; 22: 113-9.
5. Vallittu PK, Kokkonen M. Deflection fatigue of cobalt-chromium, titanium, and gold alloy cast denture clasp. J Prosthet Dent
1995; 74: 412-9.
6. Shimizu H, Kurtz KS, Tachii Y, Takahashi Y. Use of metal conditioners to improve bond strengths of autopolymerizing
denture base resin to cast Ti-6Al-7Nb and Co-Cr. J Dent 2005; 30: (in press).

Reprint request to:Dr. Hiroshi Shimizu
Division of Removable Prosthodontics, Department of Oral Rehabilitation, Fukuoka Dental College2-15-1, Tamura, Sawaraku, Fukuoka 814-0193, Japan
FAX: +81-92-801-0513 E-mail: simizuh1@college.fdcnet.ac.jp
Received July 25, 2005. Revised August 5, 2005. Accepted August 9, 2005.
Copyright ©2005 by the Editorial Council of the International Chinese Journal of Dentistry.

International Chinese Journal of Dentistry 2005 Outstanding Article Award(J. P. Geng Award)
International Chinese Journal of Dentistry announces the fourth Outstanding Article Award.
Award:A cash prize of US$ 500 and a certificate of commendation are awarded for the most outstanding article published in theInternational Chinese Journal of Dentistry.
Criteria:
Original articles, clinical reports, and dental technology articles published in the Journal between issue 1 and issue 4 areconsidered for the award. The winning article will be selected by a committee on the basis of scientific impact on the dental
professional communities, and interest to the readers. The 2005 Award is sponsored by the International Chinese Journal ofDentistry and the following award sponsors.
Award Sponsors:
GC Corporation, Tokyo, Japan, http://www.gcdental.co.jp
Kuraray Medical Inc., Tokyo, Japan, http://www.kuraray.co.jp/dental
Shofu Inc., Kyoto, Japan, http://www.shofu.co.jp
Sun Medical Co., Ltd., Moriyama, Japan, http://www.sunmedical.co.jp
Toho Dental Products, Saitama, Japan
Tokuyama Dental Corp., Tokyo, Japan, http://www.tokuyama-dental.co.jp
3M Health Care Limited, Sagamihara, Japan, http://www.3m.com/espe/
2004 Award winner
Hiroshi Shimizu, Kouichi Watanabe, Toru Okabe, and Yutaka Takahashi
Mold filling of titanium castings using a 45° marginal edge angle
Int Chin J Dent 2004; 4(3): 67-71.

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