388 Volume 104 Issue 6

The Journal of Prosthetic Dentistry Cheng et alChuang et al

28.Ho MH, Lee SY, Chen HH, Lee MC. Three-dimensional finite element analysis of the effects of posts on stress distribution in dentin. J Prosthet Dent 1994;72:367-72.

29.Ferrari M, Sorrentino R, Zarone F, Apicella D, Aversa R, Apicella A. Non-linear visco-elastic finite element analysis of the effect of the length of glass fiber posts on the bio-mechanical behaviour of directly restored incisors and surrounding alveolar bone. Dent Mater J 2008;27:485-98.

30.Peyton FA, Mahler DB, Hershenov B. Physical properties of dentin. J Dent Res 1952;31:366-70.

31.Yettram AL, Wright KW, Houston WJ. Cen-tre of rotation of a maxillary central incisor under orthodontic loading. Br J Orthod 1977;4:23-7.

32.Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface develop-ment. J Dent Res 1983;62:155-9.

33.Friedman CM, Sandrik JL, Heuer MA, Rapp GW. Composition and mechanical proper-ties of gutta-percha endodontic points. J Dent Res 1975;54:921-5.

34.Eraslan O, Aykent F, Yücel MT, Akman S. The finite element analysis of the effect of ferrule height on stress distribution at post-and-core-restored all-ceramic anterior crowns. Clin Oral Investig 2009;13:223-7.

35.Williams KR, Edmundson JT, Rees JS. Finite element stress analysis of restored teeth. Dent Mater 1987;3:200-6.

Corresponding author: Dr Shu-Fen ChuangInstitute of Oral MedicineNational Cheng Kung University College of Medicine and Hospital138 Sheng-Li RdTainan 70403TAIWANFax: +88662766626E-mail: sfchuang@mail.ncku.edu.tw

Copyright © 2010 by the Editorial Council for The Journal of Prosthetic Dentistry.

Statement of problem. Cast cobalt-chromium (Co-Cr) alloy clasps have been widely used for prosthodontic treat-ment, but long-term retentiveness of these clasps is not clear.

Purpose. The purpose of this study was to investigate the residual retentive force of 3 types of cast Co-Cr alloy clasps in relation to their proportional limits.

Material and methods. Thirty half-round straight clasps and 30 circumferential clasp assemblies were made from 3 Co-Cr alloys (Hardalloy, Regalloy, and Vera PDN) (n=10). All of the straight clasps were subjected to a compression-deformation test to obtain data for displacement/load at the proportional limit. Two undercut depths (0.25 mm and 0.50 mm, n=5) were included in a cyclic fatigue test. Each circumferential clasp assembly was subjected to an inser-tion/removal test on a metal abutment tooth for 7200 cycles to simulate 5 years of clinical use. The residual retentive force was measured after 360 cycles; thus, the retentive force of each clasp was recorded 21 times throughout the test. The data were subjected to repeated-measures ANOVA (α=.05) to compare the retentive force of each group of clasps for different periods of fatigue testing. The ANOVA. The Student t test (α=.05) was used for analysis of clasps fabricated with the same alloy but with different undercuts. A linear correlation analysis (α=.05) was used to test the correlation between retentive forces and cycling sequences.

Results. The mean values of displacement at the proportional limit for the 3 Co-Cr alloys varied from 0.30 mm to 0.36 mm. The mean retentive forces of each alloy group decreased dramatically after the first cycling phase (P<.05). Thereafter, the decrease was more gradual but was still significantly different compared to the original retentive forces (P<.05). Compared to clasps in the 0.25-mm undercut group, those in the 0.50-mm undercut group exhibited greater mean retentive forces before the cyclic fatigue test (P<.05), as well as a greater decrease in the retentive force at the end of the test (P<.05).

Conclusions. A negative correlation was found between the retentive force of cast Co-Cr alloy clasps and the loga-rithm of cycling sequences. After a test simulating 5 years of service, cast Co-Cr alloy clasps exhibited a residual reten-tive force to satisfy the requirements for clinical use. (J Prosthet Dent 2010;104:389-396)

Cyclic fatigue properties of cobalt-chromium alloy clasps for partial removable dental prostheses

Hui Cheng, BDS, MDS, PhD,a Minrui Xu, BDS, MDS,b Hai Zhang, DMD, PhD,c Weiqing Wu, BE, ME, PhD,d Ming Zheng, BDS, MDS,e and Xiurong Lif

School and Hospital of Stomatology, Fujian Medical University, Fuzhou, Fujian, China; Affiliated Hospital of Fujian Health College, Fuzhou, Fujian, China; School of Dentistry, University of Washington, Seattle, Wash; Fuzhou University, Fuzhou, Fujian, China

Supported by a Natural Science Grant of Fujian province (No. 2006J0340) and the Fujian Provincial Department of Science and Technology.

aProfessor and Chair, Department of Prosthodontics, School and Hospital of Stomatology, Fujian Medical University.bResident, Department of Prosthodontics, Affiliated Hospital of Fujian Health College. cAssistant Professor, Department of Restorative Dentistry, School of Dentistry, University of Washington.dResearch Fellow, Instrumentation Analysis and Measurement Center, Fuzhou University.eAssociate Professor and Deputy Director, Department of Prosthodontics, School and Hospital of Stomatology, Fujian Medical University.f Technician and Chair, Department of Prosthetic Technology, School and Hospital of Stomatology, Fujian Medical University.

Noteworthy Abstracts of the Current Literature

Altered vertical dimension of occlusion: A comparative retrospective pilot study of tooth- and implant-supported restorations

Ormianer Z, Palty A.Int J Oral Maxillofac Implants 2009;24:497-501.

Purpose. Altering the vertical dimension of occlusion (VDO) by increasing the interarch distance is common in oral rehabilitation, but little is known about the ability of implant patients, who lack sensory perception in implanted regions, to adapt to such changes. This study sought to evaluate the outcome of increasing VDO in patients restored with implant-supported fixed restorations opposed by restored natural teeth or implant-supported restorations.

Materials and Methods. VDO was increased by 3 to 5 mm to address the individual prosthetic needs of 30 patients. Group A (control) consisted of 10 patients with fixed restorations on natural dentition that opposed the natural dentition in a new VDO relationship. Two test groups consisted of 10 patients each, with fixed implant-supported restorations opposing either the restored natural dentition (group B) or fixed implant-supported restorations (group C). After an average follow-up of 66 months, marginal bone changes were calculated using standardized periapical radiographs, and mechanical prosthetic maintenance data were collected from patient files. The results were analyzed using Kruskal-Wallis one-way analysis of variance to identify significant differences between the groups.

Results. All patients successfully adapted to the new VDO. Two patients in group B and four in group C reported tooth clenching or grinding, which abated after 2 to 3 months (P < .05). More bone loss and tooth failures were observed in group A, and more mechanical complications, such as porcelain fractures, were observed in group C (P < .05).

Conclusion. Within the limitations of this study, alteration of VDO was an acceptable procedure in patients with implant-supported fixed restorations, but precautions should be taken to prevent mechanical problems.

Reprinted with permission of Quintessence Publishing.

390 Volume 104 Issue 6

The Journal of Prosthetic Dentistry

391December 2010

Cheng et alCheng et al

Clinical ImplicationsCast Co-Cr alloy clasps were determined to retain the clinically accept-able retentive force in undercuts of both 0.25 mm and 0.50 mm in a test that simulated 5 years of clinical use. These results suggest that the alloys tested in this study are suitable for long-term clinical use for partial removable dental prosthesis (PRDP) frameworks.

Cobalt-chromium alloys have been used in prosthetic dentistry for partial removable dental prosthesis (PRDP) frameworks since 1929.1 Co-Cr alloys are popular because of their lower cost and lower density compared to gold alloys, but they exhibit a higher modu-lus of elasticity. It has been suggested that cast Co-Cr alloy clasps in a 0.25-mm undercut provide adequate reten-tion for a PRDP.2

Fatigue resistance is an important factor for clinical durability of dental materials. Clasps made of different materials engaging different undercuts have been reported to vary with respect to fatigue resistance.3,4 Clasp fatigue ad-versely affected the retentive properties of PRDPs using a constant deflection test, and the permanent deformation of clasps can cause loss of retention.4,5 Co-Cr alloy clasps were subjected to cyclic deflection of 0.25 mm, a preset value, for 106 cycles and did not fracture, but the clasp fatigue limit decreased dramatically when the preset value reached 0.50 and 0.75 mm.3

Clinically, clasp fatigue is based on the recurring deflection of clasps in the PRDP insertion and removal sequence; therefore, a repeated in-sertion/removal test is a good choice for a fatigue test. The retentive force of cast Co-Cr alloys clasps has been studied using repeated insertion/re-moval tests, but different outcomes have been reported. In a study by Rodrigues et al,6 the retentive force of Co-Cr alloy clasps increased over the course of the study; however, a decreasing trend in clasp force was recorded by both Bridgeman et al7 and Kim et al.8 The proportional limit is another important consideration in selecting metals for PRDP clasps.

Retentive clasps with greater propor-tional limits can be placed into deeper undercuts without exhibiting perma-nent deformation. Previous studies indicated that PRDP clasps made of more elastic materials demonstrated a higher resistance to retention loss.8,9 There have been studies that investi-gated the flexibility of wrought clasps, but few have examined the propor-tional limit of cast clasps.10-13

In the current study, the propor-tional limits of 3 commonly used cast Co-Cr alloy clasps were tested in vitro. Cast circumferential clasps were fabri-cated to evaluate changes in retentive force during repeated insertion and removal on abutment teeth, simulat-ing 5 years of clinical use. The objec-tive of this study was to investigate the changing pattern of clasp reten-tive force in a simulation test and its relationship to the proportional limit. The outcome of this study may help clinicians choose more appropriate alloys for their clinical practices. The null hypothesis was that there would be no difference in the retentive force of the cast Co-Cr alloy clasps before and after fatigue cycles.

MATERIAL AND METHODS

Dental Co-Cr alloys from 3 manu-facturers (Table I) were used in this study. Thirty straight half-round clasps were made using each of the 3 Co-Cr alloys (n=10). The wax patterns of pre-formed semicircular clasps (GEO molar clasps, self-adhesive; Renfert GmbH, Hilzingen, Germany) were invested ac-cording to the manufacturer’s instruc-tions. The Co-Cr alloys were melted and cast with a casting machine (Ac-toron; Dentply-Sankin KK, Tokyo, Ja-

pan) at the recommended centrifugal force. After being cast, the specimens were devested and subjected to air-borne-particle abrasion with 50-µm aluminum oxide for 30 seconds us-ing 0.4-MPa air pressure. Specimens were carefully examined using a ste-reomicroscope (SPZT-50IFM; Carton Optical Industries, Ltd, Tokyo, Japan) under x10 magnification to detect possible flaws (voids and blebs) on the clasps’ surfaces. To ensure unifor-mity, no polishing procedures were performed.

The clasps were marked on the flat side at the tip (loading area), and at the middle of the clasp (bending area), with a distance of 10.0 mm be-tween the 2 areas. The section of the clasp that was subjected to flexure was 0.97 ±0.026 mm thick and 1.77 ±0.03 mm wide, as measured with a digital micrometer (MDC-25PJ; Mi-tutoyo Corp, Kanagawa, Japan) with 0.01-mm accuracy.

A load-displacement test was con-ducted using a universal testing machine (model 8841; Instron Corp, Norwood, Mass), and the schematic diagram for this test is shown in Figure 1. The clasps were deflected 1.0 mm from the flat side at a crosshead speed of 1 mm/min. The crosshead stopped when the dis-placement reached 1.0 mm, and the load-displacement curves were record-ed automatically. A typical load-dis-placement curve is shown in Figure 2. A straight line from the “0” point was used to determine the straight portion of the curve. Point A was marked where the curve diverged from the straight line, indicating the proportional limit. The number on the abscissa represented the displacement, and the number on the ordinate represented the load

at the proportional limit (stress), re-spectively. Points B and C of the load-displacement curve were marked at 0.25 mm and 0.50 mm on the curve, and the ordinates of the points were their corresponding loads.

An acrylic resin mandibular first molar (A5-500; Nissin Dental Prod-ucts Co, Ltd, Kyoto, Japan) was used as the master abutment tooth for the study. An occlusal rest (2.5 mm long, 2.5 mm wide, and 2 mm deep) was prepared on the mesio-occlusal sur-face. Mesial and lingual guide planes, two thirds of the crown length, were prepared with a parallel milling ma-chine (APF450; Amann Girrbach

GmbH, Phorzheim, Germany) to de-termine the path of insertion. The prepared molar was duplicated using a silicone impression material (Rapid; Coltène/Whaledent AG, Altstätten, Switzerland). Wax patterns of the abutment tooth were fabricated by pouring inlay wax (Mighty wax/Hard; Shofu, Inc, Kyoto, Japan) into the impression and then cast into metal abutment teeth using 1 of the 3 Co-Cr alloys.

For cyclic fatigue testing, 2 metal abutment teeth for each alloy were cast (1 for each undercut, 0.25 mm and 0.50 mm, respectively), so that the cyclic fatigue tests of the clasps

could be performed on abutment teeth made of the same Co-Cr alloy. A separate metal abutment tooth, cast in Vera PDN alloy, was used as the master die for the fabrication of clasp frame-works and for the measurement of clasp retentive force. All 7 metal abutment teeth were polished by the same labo-ratory technician to ensure uniformity, and were later soldered to the middle of metal plates (30 mm x 30 mm x 3 mm) for the cyclic fatigue test.

The metal master abutment tooth was surveyed on the horizontal plane with a dental surveyor (Apex Industri-al Electronics, Ambala, India) to mark 0.25-mm and 0.50-mm undercut lo-

1 Schematic diagram for clasp proportional limit test. 2 Curved blue line in figure shows typical load-displace-ment curve recorded for each specimen. Straight line from 0 point was used to determine straight portion of curve. Point A represents proportional limit of clasp. Points B and C represent points at which clasp displacement was 0.25 mm and 0.50 mm, respectively.

Table I. Cobalt-chromium alloys used for clasp fabrication

Hardalloy

Regalloy

Vera PDN

*Composition provided by manufacturers

Shanghai Dental Material Co,Shanghai, China

Dentsply Trubyte,York, Pa

AalbaDent, Inc,Fairfield, Calif

Manufacturer

62.5 Co, 27.0 Cr, 5.5 Mo,2.5 Ni, 1.0 Fe

62.5 Co, 27.0 Cr, 5.5 Mo, 0.99 Ni, 2.0 Fe

63.5 Co, 27.0 Cr, 5.5 Mo, 2.0 Fe, 1.0 Ni

Composition* (wt %)Material

18

Load

(N

)

Displacement (mm)

12

14

16

10

8

4

2

0

6

0.10

Straight Edge

Proportional Limit C

A

B

Experimental Curve

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

390 Volume 104 Issue 6

The Journal of Prosthetic Dentistry

391December 2010

Cheng et alCheng et al

Clinical ImplicationsCast Co-Cr alloy clasps were determined to retain the clinically accept-able retentive force in undercuts of both 0.25 mm and 0.50 mm in a test that simulated 5 years of clinical use. These results suggest that the alloys tested in this study are suitable for long-term clinical use for partial removable dental prosthesis (PRDP) frameworks.

Cobalt-chromium alloys have been used in prosthetic dentistry for partial removable dental prosthesis (PRDP) frameworks since 1929.1 Co-Cr alloys are popular because of their lower cost and lower density compared to gold alloys, but they exhibit a higher modu-lus of elasticity. It has been suggested that cast Co-Cr alloy clasps in a 0.25-mm undercut provide adequate reten-tion for a PRDP.2

Fatigue resistance is an important factor for clinical durability of dental materials. Clasps made of different materials engaging different undercuts have been reported to vary with respect to fatigue resistance.3,4 Clasp fatigue ad-versely affected the retentive properties of PRDPs using a constant deflection test, and the permanent deformation of clasps can cause loss of retention.4,5 Co-Cr alloy clasps were subjected to cyclic deflection of 0.25 mm, a preset value, for 106 cycles and did not fracture, but the clasp fatigue limit decreased dramatically when the preset value reached 0.50 and 0.75 mm.3

Clinically, clasp fatigue is based on the recurring deflection of clasps in the PRDP insertion and removal sequence; therefore, a repeated in-sertion/removal test is a good choice for a fatigue test. The retentive force of cast Co-Cr alloys clasps has been studied using repeated insertion/re-moval tests, but different outcomes have been reported. In a study by Rodrigues et al,6 the retentive force of Co-Cr alloy clasps increased over the course of the study; however, a decreasing trend in clasp force was recorded by both Bridgeman et al7 and Kim et al.8 The proportional limit is another important consideration in selecting metals for PRDP clasps.

Retentive clasps with greater propor-tional limits can be placed into deeper undercuts without exhibiting perma-nent deformation. Previous studies indicated that PRDP clasps made of more elastic materials demonstrated a higher resistance to retention loss.8,9 There have been studies that investi-gated the flexibility of wrought clasps, but few have examined the propor-tional limit of cast clasps.10-13

In the current study, the propor-tional limits of 3 commonly used cast Co-Cr alloy clasps were tested in vitro. Cast circumferential clasps were fabri-cated to evaluate changes in retentive force during repeated insertion and removal on abutment teeth, simulat-ing 5 years of clinical use. The objec-tive of this study was to investigate the changing pattern of clasp reten-tive force in a simulation test and its relationship to the proportional limit. The outcome of this study may help clinicians choose more appropriate alloys for their clinical practices. The null hypothesis was that there would be no difference in the retentive force of the cast Co-Cr alloy clasps before and after fatigue cycles.

MATERIAL AND METHODS

Dental Co-Cr alloys from 3 manu-facturers (Table I) were used in this study. Thirty straight half-round clasps were made using each of the 3 Co-Cr alloys (n=10). The wax patterns of pre-formed semicircular clasps (GEO molar clasps, self-adhesive; Renfert GmbH, Hilzingen, Germany) were invested ac-cording to the manufacturer’s instruc-tions. The Co-Cr alloys were melted and cast with a casting machine (Ac-toron; Dentply-Sankin KK, Tokyo, Ja-

pan) at the recommended centrifugal force. After being cast, the specimens were devested and subjected to air-borne-particle abrasion with 50-µm aluminum oxide for 30 seconds us-ing 0.4-MPa air pressure. Specimens were carefully examined using a ste-reomicroscope (SPZT-50IFM; Carton Optical Industries, Ltd, Tokyo, Japan) under x10 magnification to detect possible flaws (voids and blebs) on the clasps’ surfaces. To ensure unifor-mity, no polishing procedures were performed.

The clasps were marked on the flat side at the tip (loading area), and at the middle of the clasp (bending area), with a distance of 10.0 mm be-tween the 2 areas. The section of the clasp that was subjected to flexure was 0.97 ±0.026 mm thick and 1.77 ±0.03 mm wide, as measured with a digital micrometer (MDC-25PJ; Mi-tutoyo Corp, Kanagawa, Japan) with 0.01-mm accuracy.

A load-displacement test was con-ducted using a universal testing machine (model 8841; Instron Corp, Norwood, Mass), and the schematic diagram for this test is shown in Figure 1. The clasps were deflected 1.0 mm from the flat side at a crosshead speed of 1 mm/min. The crosshead stopped when the dis-placement reached 1.0 mm, and the load-displacement curves were record-ed automatically. A typical load-dis-placement curve is shown in Figure 2. A straight line from the “0” point was used to determine the straight portion of the curve. Point A was marked where the curve diverged from the straight line, indicating the proportional limit. The number on the abscissa represented the displacement, and the number on the ordinate represented the load

at the proportional limit (stress), re-spectively. Points B and C of the load-displacement curve were marked at 0.25 mm and 0.50 mm on the curve, and the ordinates of the points were their corresponding loads.

An acrylic resin mandibular first molar (A5-500; Nissin Dental Prod-ucts Co, Ltd, Kyoto, Japan) was used as the master abutment tooth for the study. An occlusal rest (2.5 mm long, 2.5 mm wide, and 2 mm deep) was prepared on the mesio-occlusal sur-face. Mesial and lingual guide planes, two thirds of the crown length, were prepared with a parallel milling ma-chine (APF450; Amann Girrbach

GmbH, Phorzheim, Germany) to de-termine the path of insertion. The prepared molar was duplicated using a silicone impression material (Rapid; Coltène/Whaledent AG, Altstätten, Switzerland). Wax patterns of the abutment tooth were fabricated by pouring inlay wax (Mighty wax/Hard; Shofu, Inc, Kyoto, Japan) into the impression and then cast into metal abutment teeth using 1 of the 3 Co-Cr alloys.

For cyclic fatigue testing, 2 metal abutment teeth for each alloy were cast (1 for each undercut, 0.25 mm and 0.50 mm, respectively), so that the cyclic fatigue tests of the clasps

could be performed on abutment teeth made of the same Co-Cr alloy. A separate metal abutment tooth, cast in Vera PDN alloy, was used as the master die for the fabrication of clasp frame-works and for the measurement of clasp retentive force. All 7 metal abutment teeth were polished by the same labo-ratory technician to ensure uniformity, and were later soldered to the middle of metal plates (30 mm x 30 mm x 3 mm) for the cyclic fatigue test.

The metal master abutment tooth was surveyed on the horizontal plane with a dental surveyor (Apex Industri-al Electronics, Ambala, India) to mark 0.25-mm and 0.50-mm undercut lo-

1 Schematic diagram for clasp proportional limit test. 2 Curved blue line in figure shows typical load-displace-ment curve recorded for each specimen. Straight line from 0 point was used to determine straight portion of curve. Point A represents proportional limit of clasp. Points B and C represent points at which clasp displacement was 0.25 mm and 0.50 mm, respectively.

Table I. Cobalt-chromium alloys used for clasp fabrication

Hardalloy

Regalloy

Vera PDN

*Composition provided by manufacturers

Shanghai Dental Material Co,Shanghai, China

Dentsply Trubyte,York, Pa

AalbaDent, Inc,Fairfield, Calif

Manufacturer

62.5 Co, 27.0 Cr, 5.5 Mo,2.5 Ni, 1.0 Fe

62.5 Co, 27.0 Cr, 5.5 Mo, 0.99 Ni, 2.0 Fe

63.5 Co, 27.0 Cr, 5.5 Mo, 2.0 Fe, 1.0 Ni

Composition* (wt %)Material

18

Load

(N

)

Displacement (mm)

12

14

16

10

8

4

2

0

6

0.10

Straight Edge

Proportional Limit C

A

B

Experimental Curve

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

392 Volume 104 Issue 6

The Journal of Prosthetic Dentistry

393December 2010

Cheng et al Cheng et al

cations on the distobuccal surfaces. The retentive arm, from the clasp tip to the minor connector, was 10 mm in length, with the terminal one third of the clasp in the retentive undercut.14 Tripod marks were added to index casts for subsequent repositioning. The metal master abutment tooth marked with 2 undercuts was dupli-cated using silicone impression mate-rial (Rapid; Coltène Whaledent AG). Impressions were poured to make re-fractory dies using phosphate-bond-ed investment (Univest Nonprecious; Shofu, Inc). An individual refractory die was poured for each clasp.

Wax circumferential clasps with mesial occlusal rests and minor con-nectors were fabricated according to the marks on the teeth using pre-formed semicircular clasp patterns (GEO molar clasps, self-adhesive; Renfert GmbH). A round plastic sprue (3 mm in diameter and 30 mm in length) was positioned on the oc-clusal rest parallel to the path of inser-tion using a surveyor (Apex Industrial Electronics). This sprue was later used to attach the clasp test specimen to the universal testing machine. Each as-sembly (die and pattern) was invested with the same investment material

(Univest Nonprecious; Shofu, Inc) and then cast according to the man-ufacturer’s instructions. Ten clasps were made of each Co-Cr alloy (n=5 for each undercut). In a pilot study, a 2-sided t test with a significance level of .05 was performed to determine the sample size. The effect size was set at one third of the average of the retention, and the power of the test was set at 0.8. The results indicated a sample size of 5 would be adequate for this study. After being cast and re-moved from the die, each clasp was airborne-particle abraded with 50-µm aluminum oxide for 30 seconds using 0.4-MPa air pressure, and then carefully examined to detect possible flaws (voids and blebs) in the clasp surface under x10 magnification using the stereomicroscope (SPZT-50IFM; Carton Optical Industries, Ltd). To ensure uniformity, no polishing pro-cedures were performed.

The fatigue test was conducted using the universal testing machine (model 8841; Instron Corp). The clasps were seated on the metal abut-ment teeth and attached to the testing machine (Fig. 3), then moved vertical-ly on and off the metal abutment teeth parallel to the guide planes. The initial

retentive force for each clasp was measured on the master abutment tooth with a crosshead speed of 10 mm/min. After the initial retentive forces were measured, the clasps were attached to the corresponding abut-ment tooth and cycled on and off the abutment tooth for a sequence of 360 times, simulating 3 months of clinical use of a PRDP (estimating 4 complete cycles per day). The cyclic fatigue test was performed at a speed of 45 cycles/min. After the cycling test, each clasp was reattached to the master abutment tooth to measure the residual retentive force, followed by another sequence of cyclic fatigue testing. The sequences were repeated 20 times, each clasp undergoing a to-tal of 7200 cycles, simulating 5 years of clinical use.6 Because the residual retentive force was measured after each 360-cycle sequence, the reten-tive force of each clasp was measured 21 times over the course of testing.

The data were subjected to re-peated-measures analysis of variance (ANOVA) to compare the reten-tive force of each group of clasps at different points in the fatigue test. The ANOVA was used to compare the retentive forces of clasps of different alloys, and the Student t test was used for analysis of clasps fabricated with the same alloy but with different un-dercuts. A linear correlation analysis was used to test the correlation be-tween the clasp retentive forces and the cycling sequences. Statistical sig-nificance was set at α=.05.

RESULTS

The outcomes of the load-dis-placement test (Table II) indicated that the mean values of displacement at the proportional limit of the 3 Co-Cr alloys varied from 0.30 mm to 0.36 mm. The flexibility of Regalloy and Vera PDN alloys was similar (P=.179) and was greater than that of the Har-dalloy group (P<.001). The mean val-ues of load at the proportional limit varied from 7.86 N to 10.32 N, with that of the Vera PDN group being the

3 Clasps were placed on metal abutment tooth for insertion and removal test. A, Clasp on abutment. B, Clasp off abutment.

A B

greatest (P<.001). No significant dif-ference was found between the Regal-loy and Hardalloy groups (P=.142). Similar loads were found among the 3 groups when the displacement reached 0.25 mm (P=.062), and the highest load was for Vera PDN when the displacement reached 0.50 mm (P=.005). No significant difference was found between the Regalloy and Hardalloy groups (P=.457).

Figures 4 and 5 show the changes in retentive force required to remove clasps from the 0.25- and 0.50-mm undercuts. The retentive forces of all clasps tended to decrease simi-larly throughout the cyclic fatigue test. For the 0.25-mm undercut, the initial retentive forces of all 3 Co-Cr alloy cast clasps were almost equal

to one another before the cyclic fa-tigue test (P=.802). Results of the repeated-measures ANOVA indicated that retentive forces for each group decreased dramatically after the first cycling sequence (P<.05). Thereafter, the decrease lessened gradually but still remained statistically significant (P<.05). The retentive force of the Vera PDN group was less stable com-pared with that of the other 2 groups. However, the mean retentive force of the Vera PDN group remained the highest throughout the entire cyclic fa-tigue test (P<.001). A similar trend was found in the clasp groups engaging the 0.50-mm undercut. The retentive force of the Hardalloy and Regalloy groups decreased dramatically from the first to the second cycling sequence. At

the completion of testing, the Vera PDN group had the highest mean re-tentive force (P<.001) and the least amount of decrease in retentive force (P=.022).

The mean initial retentive force of the clasps in the 0.50-mm undercut was greater than that of the clasps in the 0.25-mm undercut (P<.001), and the decrease in the retentive force of the clasps in the 0.50-mm undercut was also greater after the cyclic fatigue test (P<.001). As a result, similar re-tentive forces of clasps in the 2 under-cuts were observed at the end of the cyclic fatigue test (P=.635). Figures 4 and 5 demonstrate that the changing patterns in the clasp retentive forces in the 2 undercuts approximated a logarithmic curve. The outcomes of

4 Changes in retentive force required to remove 3 cast Co-Cr clasps engaged in 0.25-mm undercut (mean ±SD). (Each measurement was made at end of 360 fatigue cycles. First measurement was made prior to any fatigue cycling. Twenty-first measurement corresponds to 7200 fatigue cycles.)

5 Changes in retentive force required to remove 3 cast Co-Cr clasps engaged in 0.50-mm undercut (mean ±SD). (Each measurement was made at end of 360 fatigue cycles. First measurement was made prior to any fatigue cycling. Twenty-first measurement corresponds to 7200 fatigue cycles.)

Table II. Mean (SD) displacement (mm) and load at proportional limit (N) of 3 cast Co-Cr alloy clasps in N (n=10)

Hardalloy

Regalloy

Vera PDN

0.30 (0.02)

0.35 (0.02)

0.36 (0.04)

Proportional LimitDisplacement at

7.86 (0.65)

8.66 (0.45)

10.32 (1.90)

Proportional LimitLoad at

6.62 (0.58)

6.34 (0.52)

7.18 (1.10)

DisplacementLoad at 0.25-mm

12.15 (0.96)

11.75 (0.71)

13.79 (1.67)

DisplacementLoad at 0.50-mm

Material

10

Ret

enti

ve F

orce

(N

)

Measuring Times

7

8

9

6

5

3

2

1

0

4

31 5 7 9 11 13 15 17 19 21

HardalloyRegalloyVera PDN

10

Ret

enti

ve F

orce

(N

)

Measuring Times

7

8

9

6

5

3

2

1

0

4

31 5 7 9 11 13 15 17 19 21

HardalloyRegalloyVera PDN

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cations on the distobuccal surfaces. The retentive arm, from the clasp tip to the minor connector, was 10 mm in length, with the terminal one third of the clasp in the retentive undercut.14 Tripod marks were added to index casts for subsequent repositioning. The metal master abutment tooth marked with 2 undercuts was dupli-cated using silicone impression mate-rial (Rapid; Coltène Whaledent AG). Impressions were poured to make re-fractory dies using phosphate-bond-ed investment (Univest Nonprecious; Shofu, Inc). An individual refractory die was poured for each clasp.

Wax circumferential clasps with mesial occlusal rests and minor con-nectors were fabricated according to the marks on the teeth using pre-formed semicircular clasp patterns (GEO molar clasps, self-adhesive; Renfert GmbH). A round plastic sprue (3 mm in diameter and 30 mm in length) was positioned on the oc-clusal rest parallel to the path of inser-tion using a surveyor (Apex Industrial Electronics). This sprue was later used to attach the clasp test specimen to the universal testing machine. Each as-sembly (die and pattern) was invested with the same investment material

(Univest Nonprecious; Shofu, Inc) and then cast according to the man-ufacturer’s instructions. Ten clasps were made of each Co-Cr alloy (n=5 for each undercut). In a pilot study, a 2-sided t test with a significance level of .05 was performed to determine the sample size. The effect size was set at one third of the average of the retention, and the power of the test was set at 0.8. The results indicated a sample size of 5 would be adequate for this study. After being cast and re-moved from the die, each clasp was airborne-particle abraded with 50-µm aluminum oxide for 30 seconds using 0.4-MPa air pressure, and then carefully examined to detect possible flaws (voids and blebs) in the clasp surface under x10 magnification using the stereomicroscope (SPZT-50IFM; Carton Optical Industries, Ltd). To ensure uniformity, no polishing pro-cedures were performed.

The fatigue test was conducted using the universal testing machine (model 8841; Instron Corp). The clasps were seated on the metal abut-ment teeth and attached to the testing machine (Fig. 3), then moved vertical-ly on and off the metal abutment teeth parallel to the guide planes. The initial

retentive force for each clasp was measured on the master abutment tooth with a crosshead speed of 10 mm/min. After the initial retentive forces were measured, the clasps were attached to the corresponding abut-ment tooth and cycled on and off the abutment tooth for a sequence of 360 times, simulating 3 months of clinical use of a PRDP (estimating 4 complete cycles per day). The cyclic fatigue test was performed at a speed of 45 cycles/min. After the cycling test, each clasp was reattached to the master abutment tooth to measure the residual retentive force, followed by another sequence of cyclic fatigue testing. The sequences were repeated 20 times, each clasp undergoing a to-tal of 7200 cycles, simulating 5 years of clinical use.6 Because the residual retentive force was measured after each 360-cycle sequence, the reten-tive force of each clasp was measured 21 times over the course of testing.

The data were subjected to re-peated-measures analysis of variance (ANOVA) to compare the reten-tive force of each group of clasps at different points in the fatigue test. The ANOVA was used to compare the retentive forces of clasps of different alloys, and the Student t test was used for analysis of clasps fabricated with the same alloy but with different un-dercuts. A linear correlation analysis was used to test the correlation be-tween the clasp retentive forces and the cycling sequences. Statistical sig-nificance was set at α=.05.

RESULTS

The outcomes of the load-dis-placement test (Table II) indicated that the mean values of displacement at the proportional limit of the 3 Co-Cr alloys varied from 0.30 mm to 0.36 mm. The flexibility of Regalloy and Vera PDN alloys was similar (P=.179) and was greater than that of the Har-dalloy group (P<.001). The mean val-ues of load at the proportional limit varied from 7.86 N to 10.32 N, with that of the Vera PDN group being the

3 Clasps were placed on metal abutment tooth for insertion and removal test. A, Clasp on abutment. B, Clasp off abutment.

A B

greatest (P<.001). No significant dif-ference was found between the Regal-loy and Hardalloy groups (P=.142). Similar loads were found among the 3 groups when the displacement reached 0.25 mm (P=.062), and the highest load was for Vera PDN when the displacement reached 0.50 mm (P=.005). No significant difference was found between the Regalloy and Hardalloy groups (P=.457).

Figures 4 and 5 show the changes in retentive force required to remove clasps from the 0.25- and 0.50-mm undercuts. The retentive forces of all clasps tended to decrease simi-larly throughout the cyclic fatigue test. For the 0.25-mm undercut, the initial retentive forces of all 3 Co-Cr alloy cast clasps were almost equal

to one another before the cyclic fa-tigue test (P=.802). Results of the repeated-measures ANOVA indicated that retentive forces for each group decreased dramatically after the first cycling sequence (P<.05). Thereafter, the decrease lessened gradually but still remained statistically significant (P<.05). The retentive force of the Vera PDN group was less stable com-pared with that of the other 2 groups. However, the mean retentive force of the Vera PDN group remained the highest throughout the entire cyclic fa-tigue test (P<.001). A similar trend was found in the clasp groups engaging the 0.50-mm undercut. The retentive force of the Hardalloy and Regalloy groups decreased dramatically from the first to the second cycling sequence. At

the completion of testing, the Vera PDN group had the highest mean re-tentive force (P<.001) and the least amount of decrease in retentive force (P=.022).

The mean initial retentive force of the clasps in the 0.50-mm undercut was greater than that of the clasps in the 0.25-mm undercut (P<.001), and the decrease in the retentive force of the clasps in the 0.50-mm undercut was also greater after the cyclic fatigue test (P<.001). As a result, similar re-tentive forces of clasps in the 2 under-cuts were observed at the end of the cyclic fatigue test (P=.635). Figures 4 and 5 demonstrate that the changing patterns in the clasp retentive forces in the 2 undercuts approximated a logarithmic curve. The outcomes of

4 Changes in retentive force required to remove 3 cast Co-Cr clasps engaged in 0.25-mm undercut (mean ±SD). (Each measurement was made at end of 360 fatigue cycles. First measurement was made prior to any fatigue cycling. Twenty-first measurement corresponds to 7200 fatigue cycles.)

5 Changes in retentive force required to remove 3 cast Co-Cr clasps engaged in 0.50-mm undercut (mean ±SD). (Each measurement was made at end of 360 fatigue cycles. First measurement was made prior to any fatigue cycling. Twenty-first measurement corresponds to 7200 fatigue cycles.)

Table II. Mean (SD) displacement (mm) and load at proportional limit (N) of 3 cast Co-Cr alloy clasps in N (n=10)

Hardalloy

Regalloy

Vera PDN

0.30 (0.02)

0.35 (0.02)

0.36 (0.04)

Proportional LimitDisplacement at

7.86 (0.65)

8.66 (0.45)

10.32 (1.90)

Proportional LimitLoad at

6.62 (0.58)

6.34 (0.52)

7.18 (1.10)

DisplacementLoad at 0.25-mm

12.15 (0.96)

11.75 (0.71)

13.79 (1.67)

DisplacementLoad at 0.50-mm

Material

10

Ret

enti

ve F

orce

(N

)

Measuring Times

7

8

9

6

5

3

2

1

0

4

31 5 7 9 11 13 15 17 19 21

HardalloyRegalloyVera PDN

10

Ret

enti

ve F

orce

(N

)

Measuring Times

7

8

9

6

5

3

2

1

0

4

31 5 7 9 11 13 15 17 19 21

HardalloyRegalloyVera PDN

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Cheng et alCheng et al

linear correlation analysis (Table III) indicated that there was a linear cor-relation between the clasp retentive force and the logarithm of cycling se-quences in each group. No clasp frac-ture occurred in this test, but there was some visible wear on the inner surface of each clasp tip when the first cycling sequence was completed (Fig. 6). However, no visible perma-nent deformation of the clasps was found when they were reattached to the master abutment tooth after the cyclic fatigue test (Fig. 7).

DISCUSSION

Based on the results, the null hy-pothesis was rejected, as differences were found in the retentive force of Co-Cr alloy cast clasps before and after fa-tigue cycling. The decrease in retentive force in the 3 clasp groups varied after the cyclic fatigue test. There are some factors contributing to the differences. One is the change in the coefficient of friction of the clasp against the abut-

ment tooth. Coefficient of friction is one of the dominant factors for clasp retentive force.15 In this study, the clasps were airborne-particle abrad-ed, and the abraded surface had a much higher coefficient of friction. However, the inner surface of the clasp tip exhibited wear, resulting in a smooth surface during the insertion/removal test. As the clasp’s coeffi-cient of friction against the abutment tooth diminished, the clasp’s retentive force decreased as well. The change in coefficient of friction could cause a dramatic decrease in the clasp reten-tive force from the first to the second cycling sequence. A similar phenom-enon has been reported in previous studies.6,7

Permanent deformation of clasps has occurred during retention tests.8,16,17 In the current study, the 3 Co-Cr alloy cast clasps demonstrated a maximum elastic deflection at the proportional limit, greater than 0.25 mm, but clasp curvature may decrease the flexibility of the clasp.5 Therefore, permanent defor-

mation would occur even in the clasps engaged in a 0.25-mm undercut. The displacement of the clasps engaged in the 0.50-mm undercut was 0.50 mm, which greatly exceeded their maxi-mum elastic deflection. Thus, perma-nent deformation must have occurred during the cyclic fatigue test. The re-tentive force of the clasps engaged in the 0.50-mm undercut was greater than that of the clasps engaged in the 0.25-mm undercut before the cyclic fatigue test; thus, the difference in the changes in retentive force of the clasp groups could be attributed to the dif-ference in the magnitude of perma-nent deformation of the clasp arms.9

In this study, the displacement of the 3 Co-Cr alloy clasps at their pro-portional limits was larger than 0.25 mm but smaller than 0.50 mm. In the-ory, deformation occurring within the proportional limit can be completely recovered. These results indicate that these Co-Cr alloy cast clasps can en-gage a 0.25-mm undercut without elastic deformation, but when they are

6 Clasp in Regalloy 0.25-mm group is shown as example of evidence of wear (indicated by arrow) on inner surface of clasp tip.

7 No visible permanent deformation of clasps (indicated by arrow) was found after cyclic fatigue test.

Table III. Linear correlation outcome of 3 cast Co-Cr alloy clasps

r

P

r: correlation coefficient

0.85

<.001

0.25 mmHardalloy

0.88

<.001

0.25 mmRegalloy

0.69

<.001

0.25 mmVera PDN

0.91

<.001

0.50 mmHardalloy

0.83

<.001

0.50 mmRegalloy

0.98

<.001

0.50 mmVera PDN

Group

placed in a 0.50-mm undercut, elas-tic deformation will inevitably occur. Therefore, the retentive forces of the 3 Co-Cr cast clasps were similar when engaging a 0.25-mm undercut, and this might be explained by the similar loads at 0.25-mm deformation.

In previous, similar studies, clasp retentive forces were measured and cyclic fatigue tests were performed on a single abutment tooth.6-8 This mis-match of materials caused increased surface wear on the abutment tooth and changes to the surface coefficient of friction, and, thus, may have affect-ed the experimental outcomes. In the current study, the cyclic fatigue test-ing for each group was performed on an abutment tooth made of the same alloy, but the retentive forces of all clasps in the 3 groups were measured using a single abutment tooth. This experimental method has 2 advan-tages. First, the hardness of the clasps and that of the corresponding abut-ment tooth were the same. Thus, the surface wear between the clasp and the abutment were similar among the groups during the cyclic fatigue test-ing. Secondly, since the retentive forces were measured in a separate abutment tooth, the abutment surface wear may have been reduced, and the abutment surface coefficient of friction may have been changed as well. Thus, the im-pact on the clasp retentive forces was lessened and the clasp retentive forces could be measured more accurately. Although the previous method6-8 bet-ter simulated the clinical situation, the current method may be a better way to isolate the effects caused by different alloys in in vitro testing.

Clasp fatigue occurred during the cyclic fatigue test. Previous stud-ies indicated that repeated deflection of the clasps during insertion/remov-al affects the clasp by causing metal fatigue.3,5 In this cyclic fatigue test, clasp retentive forces decreased gradu-ally, indicating the presence of metal fatigue over the entire experiment. Clasps made of different materials and engaged in different undercuts have been reported to vary in fatigue resis-

tance,3,4 which may explain why the retentive forces differed in the 3 Co-Cr alloy clasps engaged in different undercuts in this cyclic fatigue test. The retentive force was the greatest and the loss of retention was least in the Vera PDN group clasps engaged in both undercuts, indicating that clasps made of this brand of Co-Cr alloy had the best fatigue resistance. The com-position of the 3 Co-Cr alloys used in this study was similar, as a previous study suggested that the differences in fatigue resistance of different commercial Co-Cr alloys may be due to the different casting moment of the melted alloy recommended by the manufacturers.4 Since the moduli of elasticity of the 3 alloys are 26.1 x 106 MPa for Vera PDN, 23.3 x 106 MPa for Regalloy, and 21.8 x 106 MPa for Hardalloy, the higher modulus of elas-ticity of the Vera PDN alloy may have contributed to a higher resistance to retention loss.8,9

It has been suggested that a reten-tive force of 5 N is required for ad-equate function of a PRDP.18 In the current study, the retentive force of 3 Co-Cr alloy cast clasps decreased significantly in both the 0.25-mm and 0.50-mm undercuts, yet the residual retentive forces were maintained at about 5 N, indicating that the clasp’s retentive capacity was still adequate to satisfy the need for retentive force after 5 years of simulated use. No clasp fractures occurred after 7200 insertion/removal cycles. As reported in previous studies, a 7200-cycle in-sertion/removal test has been dem-onstrated to be well below the fatigue limit of Co-Cr alloy cast clasps.3-5 However, the test in the present study was performed with a rigid system, the insertion/removal test was done in vitro under dry conditions, and the sample size was not large. The out-comes may not be the same in the actual oral environment due to wet conditions and the physiological mo-bility of natural teeth. These factors may have increased the retentive force values for the test compared to actual clinical usage due to greater friction-

al resistance.7 Therefore, the actual clasp retention under clinical condi-tions may be less than that found in this experiment.

CONCLUSIONS

Within the limitations of this in vi-tro study, a negative correlation was found between the retentive force of Co-Cr alloy cast clasps and the loga-rithm of cycling sequences. Although the retentive forces decreased signifi-cantly in all groups, adequate reten-tion was maintained according to a 5-N standard.

REFERENCES

1. O’Brien WJ, ed. Dental materials and their selection. 4th ed. Chicago: Quintessence; 2008. p. 230-42.

2. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P. Clasp design. Br Dent J 2001;190:71-81.

3. Mahmoud A, Wakabayashi N, Takahashi H, Ohyama T. Deflection fatigue of Ti-6Al-7Nb, Co-Cr, and gold alloy cast clasps. J Prosthet Dent 2005;93:183-8.

4. Vallittu PK, Miettinen T. Duration of induc-tion melting of cobalt-chromium alloy and its effect on resistance to deflection fatigue of cast denture clasps. J Prosthet Dent 1996;75:322-6.

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. Rodrigues RC, Ribeiro RF, de Mattos Mda G, Bezzon OL. Comparative study of circumferential clasp retention force for titanium and cobalt-chromium remov-able partial dentures. J Prosthet Dent 2002;88:290-6.

7. 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.

8. Kim D, Park C, Yi Y, Cho L. Comparison of cast Ti-Ni alloy clasp retention with con-ventional removable partial denture clasps. J Prosthet Dent 2004;91:374-82.

9. Kotake M, Wakabayashi N, Ai M, Yoneyama T, Hamanaka H. Fatigue resistance of titanium-nickel alloy cast clasps. Int J Prosthodont 1997;10:547-52.

10.Frank RP, Nicholls JI. A study of the flexibil-ity of wrought wire clasps. J Prosthet Dent 1981;45:259-67.

11.Frank RP, Brudvik JS, Nicholls JI. A com-parison of the flexibility of wrought wire and cast circumferential clasps. J Prosthet Dent 1983;49:471-6.

394 Volume 104 Issue 6

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Cheng et alCheng et al

linear correlation analysis (Table III) indicated that there was a linear cor-relation between the clasp retentive force and the logarithm of cycling se-quences in each group. No clasp frac-ture occurred in this test, but there was some visible wear on the inner surface of each clasp tip when the first cycling sequence was completed (Fig. 6). However, no visible perma-nent deformation of the clasps was found when they were reattached to the master abutment tooth after the cyclic fatigue test (Fig. 7).

DISCUSSION

Based on the results, the null hy-pothesis was rejected, as differences were found in the retentive force of Co-Cr alloy cast clasps before and after fa-tigue cycling. The decrease in retentive force in the 3 clasp groups varied after the cyclic fatigue test. There are some factors contributing to the differences. One is the change in the coefficient of friction of the clasp against the abut-

ment tooth. Coefficient of friction is one of the dominant factors for clasp retentive force.15 In this study, the clasps were airborne-particle abrad-ed, and the abraded surface had a much higher coefficient of friction. However, the inner surface of the clasp tip exhibited wear, resulting in a smooth surface during the insertion/removal test. As the clasp’s coeffi-cient of friction against the abutment tooth diminished, the clasp’s retentive force decreased as well. The change in coefficient of friction could cause a dramatic decrease in the clasp reten-tive force from the first to the second cycling sequence. A similar phenom-enon has been reported in previous studies.6,7

Permanent deformation of clasps has occurred during retention tests.8,16,17 In the current study, the 3 Co-Cr alloy cast clasps demonstrated a maximum elastic deflection at the proportional limit, greater than 0.25 mm, but clasp curvature may decrease the flexibility of the clasp.5 Therefore, permanent defor-

mation would occur even in the clasps engaged in a 0.25-mm undercut. The displacement of the clasps engaged in the 0.50-mm undercut was 0.50 mm, which greatly exceeded their maxi-mum elastic deflection. Thus, perma-nent deformation must have occurred during the cyclic fatigue test. The re-tentive force of the clasps engaged in the 0.50-mm undercut was greater than that of the clasps engaged in the 0.25-mm undercut before the cyclic fatigue test; thus, the difference in the changes in retentive force of the clasp groups could be attributed to the dif-ference in the magnitude of perma-nent deformation of the clasp arms.9

In this study, the displacement of the 3 Co-Cr alloy clasps at their pro-portional limits was larger than 0.25 mm but smaller than 0.50 mm. In the-ory, deformation occurring within the proportional limit can be completely recovered. These results indicate that these Co-Cr alloy cast clasps can en-gage a 0.25-mm undercut without elastic deformation, but when they are

6 Clasp in Regalloy 0.25-mm group is shown as example of evidence of wear (indicated by arrow) on inner surface of clasp tip.

7 No visible permanent deformation of clasps (indicated by arrow) was found after cyclic fatigue test.

Table III. Linear correlation outcome of 3 cast Co-Cr alloy clasps

r

P

r: correlation coefficient

0.85

<.001

0.25 mmHardalloy

0.88

<.001

0.25 mmRegalloy

0.69

<.001

0.25 mmVera PDN

0.91

<.001

0.50 mmHardalloy

0.83

<.001

0.50 mmRegalloy

0.98

<.001

0.50 mmVera PDN

Group

placed in a 0.50-mm undercut, elas-tic deformation will inevitably occur. Therefore, the retentive forces of the 3 Co-Cr cast clasps were similar when engaging a 0.25-mm undercut, and this might be explained by the similar loads at 0.25-mm deformation.

In previous, similar studies, clasp retentive forces were measured and cyclic fatigue tests were performed on a single abutment tooth.6-8 This mis-match of materials caused increased surface wear on the abutment tooth and changes to the surface coefficient of friction, and, thus, may have affect-ed the experimental outcomes. In the current study, the cyclic fatigue test-ing for each group was performed on an abutment tooth made of the same alloy, but the retentive forces of all clasps in the 3 groups were measured using a single abutment tooth. This experimental method has 2 advan-tages. First, the hardness of the clasps and that of the corresponding abut-ment tooth were the same. Thus, the surface wear between the clasp and the abutment were similar among the groups during the cyclic fatigue test-ing. Secondly, since the retentive forces were measured in a separate abutment tooth, the abutment surface wear may have been reduced, and the abutment surface coefficient of friction may have been changed as well. Thus, the im-pact on the clasp retentive forces was lessened and the clasp retentive forces could be measured more accurately. Although the previous method6-8 bet-ter simulated the clinical situation, the current method may be a better way to isolate the effects caused by different alloys in in vitro testing.

Clasp fatigue occurred during the cyclic fatigue test. Previous stud-ies indicated that repeated deflection of the clasps during insertion/remov-al affects the clasp by causing metal fatigue.3,5 In this cyclic fatigue test, clasp retentive forces decreased gradu-ally, indicating the presence of metal fatigue over the entire experiment. Clasps made of different materials and engaged in different undercuts have been reported to vary in fatigue resis-

tance,3,4 which may explain why the retentive forces differed in the 3 Co-Cr alloy clasps engaged in different undercuts in this cyclic fatigue test. The retentive force was the greatest and the loss of retention was least in the Vera PDN group clasps engaged in both undercuts, indicating that clasps made of this brand of Co-Cr alloy had the best fatigue resistance. The com-position of the 3 Co-Cr alloys used in this study was similar, as a previous study suggested that the differences in fatigue resistance of different commercial Co-Cr alloys may be due to the different casting moment of the melted alloy recommended by the manufacturers.4 Since the moduli of elasticity of the 3 alloys are 26.1 x 106 MPa for Vera PDN, 23.3 x 106 MPa for Regalloy, and 21.8 x 106 MPa for Hardalloy, the higher modulus of elas-ticity of the Vera PDN alloy may have contributed to a higher resistance to retention loss.8,9

It has been suggested that a reten-tive force of 5 N is required for ad-equate function of a PRDP.18 In the current study, the retentive force of 3 Co-Cr alloy cast clasps decreased significantly in both the 0.25-mm and 0.50-mm undercuts, yet the residual retentive forces were maintained at about 5 N, indicating that the clasp’s retentive capacity was still adequate to satisfy the need for retentive force after 5 years of simulated use. No clasp fractures occurred after 7200 insertion/removal cycles. As reported in previous studies, a 7200-cycle in-sertion/removal test has been dem-onstrated to be well below the fatigue limit of Co-Cr alloy cast clasps.3-5 However, the test in the present study was performed with a rigid system, the insertion/removal test was done in vitro under dry conditions, and the sample size was not large. The out-comes may not be the same in the actual oral environment due to wet conditions and the physiological mo-bility of natural teeth. These factors may have increased the retentive force values for the test compared to actual clinical usage due to greater friction-

al resistance.7 Therefore, the actual clasp retention under clinical condi-tions may be less than that found in this experiment.

CONCLUSIONS

Within the limitations of this in vi-tro study, a negative correlation was found between the retentive force of Co-Cr alloy cast clasps and the loga-rithm of cycling sequences. Although the retentive forces decreased signifi-cantly in all groups, adequate reten-tion was maintained according to a 5-N standard.

REFERENCES

1. O’Brien WJ, ed. Dental materials and their selection. 4th ed. Chicago: Quintessence; 2008. p. 230-42.

2. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P. Clasp design. Br Dent J 2001;190:71-81.

3. Mahmoud A, Wakabayashi N, Takahashi H, Ohyama T. Deflection fatigue of Ti-6Al-7Nb, Co-Cr, and gold alloy cast clasps. J Prosthet Dent 2005;93:183-8.

4. Vallittu PK, Miettinen T. Duration of induc-tion melting of cobalt-chromium alloy and its effect on resistance to deflection fatigue of cast denture clasps. J Prosthet Dent 1996;75:322-6.

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. Rodrigues RC, Ribeiro RF, de Mattos Mda G, Bezzon OL. Comparative study of circumferential clasp retention force for titanium and cobalt-chromium remov-able partial dentures. J Prosthet Dent 2002;88:290-6.

7. 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.

8. Kim D, Park C, Yi Y, Cho L. Comparison of cast Ti-Ni alloy clasp retention with con-ventional removable partial denture clasps. J Prosthet Dent 2004;91:374-82.

9. Kotake M, Wakabayashi N, Ai M, Yoneyama T, Hamanaka H. Fatigue resistance of titanium-nickel alloy cast clasps. Int J Prosthodont 1997;10:547-52.

10.Frank RP, Nicholls JI. A study of the flexibil-ity of wrought wire clasps. J Prosthet Dent 1981;45:259-67.

11.Frank RP, Brudvik JS, Nicholls JI. A com-parison of the flexibility of wrought wire and cast circumferential clasps. J Prosthet Dent 1983;49:471-6.

396 Volume 104 Issue 6

The Journal of Prosthetic Dentistry Orenstein et alCheng et al

12.Morris HF, Asgar K, Brudvik JS, Winkler S, Roberts EP. Stress relaxation testing. Part IV: Clasp pattern dimensions and their influence on clasp behavior. J Prosthet Dent 1983;50:319-26.

13.Waldmeier MD, Grasso JE, Norberg GJ, Nowak MD. Bend testing of wrought wire removable partial denture alloys. J Prosthet Dent 1996;76:559-65.

14.Phoenix RD, Stewart KL, Cagna DR, DeFre-est CF. Stewart’s clinical removable partial prosthodontics.4th ed. Beijing: Quintes-sence; 2003. p. 58.

15.Sato Y, Abe Y, Yuasa Y, Akagawa Y. Effect of friction coefficient on Akers clasp retention. J Prosthet Dent 1997;78:22-7.

16.Ghani F, Mahood M. A laboratory examina-tion of the behaviour of cast cobalt-chromi-um clasps. J Oral Rehabil 1990;17:229-37.

17.Bates JF. Retention of partial dentures. Br Dent J 1980;149:171-4.

18.Sato Y, Tsuga K, Abe Y, Asahara S, Akagawa Y. Finite element analysis on preferable I-bar clasp shape. J Oral Rehabil 2001;28:413-7.

Corresponding author:Dr Hui ChengDepartment of ProsthodonticsSchool and Hospital of StomatologyFujian Medical University246 Yangqiao Middle StFuzhou, Fujian, 350001PR CHINAFax: 86 0591 83700838E-mail: ch.fj@163.com

AcknowledgementsThe authors thank Drs James Brudvik and John Wataha in the Department of Restorative Dentistry, University of Washington School of Dentistry, for their constructive comments. The authors also thank Ms Eugenie Fyfe from the University of Washington for her editing assistance.

Copyright © 2010 by the Editorial Council for The Journal of Prosthetic Dentistry.

Prosthesis fracture is a common complication associated with implant-retained overdentures. A new overdenture can be strengthened by incorporating a cast metal reinforcement during processing. The authors describe a technique for converting an existing conventional nonreinforced serviceable denture into an overdenture that includes a cast metal reinforcement and its attachments. (J Prosthet Dent 2010;104:397-400)

A technique for converting an existing denture into a cast metal-reinforced implant-retained overdenture

Ira H. Orenstein, DDS,a Bradley F. Weinstein, DMD,b Asher S. Gelman, DMD,c Serge Fiks,d and John W. McCartney, DDSe

James J. Peters Veterans Affairs Medical Center, Bronx, NY; Columbia University College of Dental Medicine, New York, NY; Mount Sinai School of Medicine, New York, NY; Department of Verterans Affairs Central Dental Laboratory, Washington, DC

aStaff, James J. Peters Veterans Affairs Medical Center; Assistant Clinical Professor, Columbia University College of Dental Medicine. bGeneral Practice Dental Resident, Mount Sinai School of Medicine/James J. Peters Veterans Affairs Medical Center. cGeneral Practice Dental Resident, Mount Sinai School of Medicine/James J. Peters Veterans Affairs Medical Center.dDental Laboratory Technician, Supervisor: Removable Prosthetics Division, Central Dental Laboratory.eAssociate Director, Central Dental Laboratory.

The implant-retained overdenture has been proposed as the standard of care for the restoration of the eden-tulous mandibular arch.1 The Mc-Gill Consensus statement of 20022 and the York consensus statement of 20093 discussed the advantages of the mandibular implant-retained overdenture when compared to the conventional complete denture pros-thesis. The benefits include improved stability, retention, function, and es-thetics, reduced ridge resorption, simplicity of fabrication, and the abil-ity to convert an existing denture into an overdenture.4

Acrylic resin denture base fracture is a complication associated with im-plant-retained overdentures.5,6 Frac-ture of the prosthesis is less likely to occur when there is sufficient space for the attachment system and den-ture teeth. Solutions for the patient with limited interarch space have been proposed, including performing alveoloplasty at the time of implant surgery and the use of a low-profile attachment system.7 To further re-

duce the risk of denture fracture, met-al reinforcement can be incorporated into the denture base.4,8-13 This rein-forcement is particularly helpful when the implant-retained overdenture opposes natural teeth or an implant-supported prosthesis.14

Previous reports described vari-ous methods for incorporating a metal framework into a newly fabri-cated implant-retained overdenture using conventional denture fabrica-tion procedures.14-16 The conversion of an existing mandibular prosthe-sis into an implant-supported over-denture (without incorporation of a cast framework) has also been de-scribed.17-21

To date, the authors have identi-fied no published report that combines these techniques into a convenient and predictable method for converting an existing denture into a metal-reinforced implant-retained overdenture. The au-thors present a technique for converting a serviceable complete denture into a cast metal-reinforced implant-retained overdenture with attachments; this

method requires only one additional office visit.

During the surgical phase of im-plant treatment, the intaglio surface of the patient’s denture is usually re-lieved and relined several times with tissue conditioner. This procedure may cause gradual deterioration of the denture base acrylic resin and weak-ening of the denture prosthesis. The technique presented replaces a sig-nificant portion of the denture with newly processed acrylic resin.

Incorporating a framework into an existing denture necessitates that the prosthesis be surrendered by the pa-tient. In the technique described, it is necessary for the dentist to duplicate the existing denture prior to its con-version to provide the patient with an interim prosthesis. This additional procedure can, however, serve other functions. The duplicated denture can also be used as a scanning template during 3-dimensional imaging (bari-um sulfate can be incorporated into the acrylic resin at the time of du-plicate denture fabrication, thereby

Noteworthy Abstracts of the Current Literature

Bone changes around early loaded chemically modified sandblasted and acid-etched surfaced implants with and without a machined collar: A radiographic and resonance frequency analysis in the canine mandible

Valderrama P, Jones AA, Wilson TG Jr, Higginbottom H, Schoolfield JD, Jung RE, Marcel Noujeim M, Cochran DL.Int J Oral Maxillofac Implants 2010;25:548-57.

Purpose. The purpose of this study was to evaluate the radiographic bone level and stability changes around early loaded chemically modified sandblasted and acid-etched implants with and without a machined collar.

Materials and Methods. Seventy-two chemically modified sandblasted and acid-etched implants 4.1 mm in diameter and 8 mm in length were placed in six dogs. Thirty-six implants had no machined collar (NMC) and 36 had a 2.8-mm machined collar (MC). Resonance frequency measurements were obtained at placement and weekly for 3 weeks. All implants were loaded 21 days after surgery. Standardized periapical radiographs were obtained at baseline, at 3 weeks, and at 3, 6, 9, and 12 months. The radiographs were randomized and digitized, and linear measurements of the distance from the first bone-to-implant contact to the shoulder of the implant were performed at the mesial and distal aspects of each implant. For statistical analysis, mixed-model repeated-measures analysis of variance was used.

Results. All implants achieved hard and soft tissue integration clinically and radiographically and were clinically im-mobile. From placement to week 3, the mean implant stability increased for MC implants by more than 5 ISQs and for NMC implants by more than 7 ISQs. Radiographically, there were significant differences between treatment groups beginning at 3 months. After 12 months of loading, the MC implants presented a mean bone loss of 1.00 mm and the NMC implants presented a mean bone gain of 0.11 mm.

Conclusions. Chemically modified sandblasted and acid-etched implants without a machined collar presented bone gain, and implants with a machined collar showed bone loss after a 1 year following early (21-day) loading. The ten-dency toward a coronal apposition of bone observed under these conditions may be attributed to the osteoconduc-tive properties of the chemically modified surfaces of these implants and to the absence of the machined collar.

Reprinted with permission of Quintessence Publishing.