Effe

abr

Nan Jiang, DDS,

Supported by grant No. 2007-3094

aGraduate student, School of StomabAssociate Professor, Department ofcAssociate Professor, Department ofdProfessor, Department of Prosthetic

Jiang et al

cts of clasp retention forces and

asion on different cast crowns

MSD,a Wei-min Gao, DDS, MDS, PhD,b

Hai Zhang, DMD, PhD,c and Dong-xiang Zheng, DDS, MSDd

Capital Medical University, Beijing, China; University of Washington,Seattle, Wash

Statement of problem. Dental alloys have different mechanical properties compared with enamel. However, few studies havebeen conducted to determine the effects of the retention forces of clasps when applied on different cast crowns.

Purpose. The purpose of this study was to evaluate the retention forces of cast circumferential clasps made ofcobalt-chromium alloy on complete cast crowns made of cobalt-chromium (CC group) and gold-silver-palladium(AC group) alloys, and to observe their abrasion patterns.

Material and methods. Two groups of specimens were fabricated (n¼5) and subjected to repeated insertion-removal tests(100 to 15 000 cycles). The mean values of removal forces at 100, 400, 800, 1500, 4500, 7500, 10 000, and 15 000 cycles,and their corresponding change rates compared with the initial 100 cycles’ retention were determined. The differencesbetween the 2 groups were analyzed by 2-way repeated measures analysis of variance at 100, 7500, and 15 000 cycles.The surfaces of specimens were observed with scanning electron microscopy.

Results. There were significant differences between the CC and AC groups in retention forces (P<.05). Clasp retentionshowed a descending trend for cobalt-chromium alloy crowns from the initial value, which decreased by 29.9% after15 000 insertion-removal cycles. A sharp increase in retention could be observed in the AC group, which rose by 99.7%ultimately. The worn surfaces of the gold-silver-palladium crowns showed different wear patterns compared with thecobalt-chromium alloy crowns.

Conclusions. The results indicate that cobalt-chromium alloy crowns and gold-silver-palladium alloy crowns performdifferently when cobalt-chromium alloy clasps are designed as retainers for partial removable dental prostheses. Crowndesigns should be changed, depending on the retainer and clasp materials for partial removable dental prosthesesabutment teeth. (J Prosthet Dent 2014;111:493-498)

Clinical Implications

Gold-silver-palladium crowns and cobalt-chromium alloy crowns affectthe retention force of cobalt-chromium alloy clasps in different ways.These results suggest that, when a tooth requires a surveyed crown for apartial removable dental prosthesis, it is important to consider matchingproperties of the clasp and crown.

The partial removable dental pros-thesis (PRDP) is a conventional ap-proach to replacement of missing teeth,and there are many situations in which

from Beiji

tology, CaProsthetiRestorativDentistry

abutment teeth are restored with fixedprostheses. For example, abutmentsmay require crown placement to ensureappropriate contour and strength. If an

ng Medicine Research and Development Fund.

pital Medical University.c Dentistry, School of Stomatology, Capital Mee Dentistry, School of Dentistry, University of W, School of Stomatology, Capital Medical Univ

abutment of a PRPD is damaged, it canbe restored with a crown to prolong theservice life of an existing PRPD. There-fore, it is necessary to investigate the

dical University.ashington.

ersity.

Table I. Alloys used for specimen fabrication

Material Composition (wt %) Manufacturer

Au-Ag-Pd alloy Au 40; Pd 3.8; Ag 47; other 9.2 BioZZ40, Beijing Ruilin Jinye Co Ltd

Co-Cr alloya Co 60;Cr 24;W 9; Mo 5 Shanghai Chang Ping Material Co

Co-Cr alloy Co 64; Cr 28.6; Mo 5; Si 1; Mn, C Wironit 5, BEGO

Au, gold; Ag, silver; Pd, palladium; Co, cobalt; Cr, chromium; W, tungsten;Mo, molybdenum; Mn, manganese.aCo-Cr alloy was used to fabricate crown specimens.

494 Volume 111 Issue 6

compatibility of clasps and crowns. It iscommonly known that the retentionforce of a clasp is related to both theabutment’s form, such as depth andangle of the undercut, and the clasp’smaterial and characteristics, such aslength, thickness, and cross-sectionalshape.1 It has been reported that noblemetal abutment crowns made of Type IVgold (Au) and high palladium (Pd) alloysshowed higher friction coefficient valuesthan nonmetal materials, such as humanenamel or porcelain, in contact withairborne-particle abraded clasps madeof cobalt (Co) chromium (Cr) alloys,which suggests that clasp designs shouldbe changed, depending on the abutmentcrown materials.2 However, most re-searchers conducted 3-point bendingtests to measure properties of clasps3-6

and have paid insufficient attention tometal crowns other than Co-Cr alloycrowns on the abutment teeth. Hence, itis necessary to investigate the retentionforce of a PRPD clasp and its variationwhen different alloys are used for castcrowns as abutments.

Various compositions and types ofcasting metal alloys are available fordental restorations. Noble alloys with 35to 50 wt% of Au content have becomean alternative to high-Au alloys for eco-nomic reasons.7 These alloys have amoderate elastic modulus but have in-creased hardness and yield strengthcompared with high-Au alloys. Ucaret al8 reported that there were no signif-icant differences in yield strength amongthe Au-Pd, Pd-silver (Ag), Pd-Ag-Au, andAu-Ag-Pd alloys, although wide variationwas found in percentage elongation.According to a study, Au-Pt-Pd dentalalloys released 10 mg/cm2 per week ofmetal ions or less and are the mostresistant to electrochemical and chemi-cal corrosion, more so than high-Au al-loys.9 Clinically, the Au-Ag-Pd alloy ismostly used to fabricate fixed prostheses.

Cr-containing base metal casting al-loys are the principal materials used in thefabrication of PRPD frameworks becauseof their strong mechanical propertiesand relatively low cost.10 There are severalavailable Cr alloy systems, includingCo-Cr alloy, Co-Cr-nickel (Ni) alloy, and

The Journal of Prosthetic Dentis

Ni-Cr alloy. Their hardness increases withhigher Cr content and decreases withhigher Ni content. Co-Cr-molybdenumalloys are popular because of theirhigher hardness and lower allergy riskthan Ni-based alloys.11,12 Although theygenerally do not contain carbon whenused for complete cast restorations, car-bon is frequently added to improvehardness and yield strength when they areto be used for PRPD frameworks.7 Thesesubstances are more rigid and fatigueresistant than noble alloys.13 The purposeof this in vitro study was to evaluate theretention forces of a frequently used castcircumferential clasps (Co-Cr alloy)applied to crowns made from Au-Ag-Pdand Co-Cr alloys, and to observe theabrasion between them. The null hy-pothesis was that there would be nodifference in the retentive forces andabrasion patterns of Co-Cr alloy castcircumferential clasps applied to Au-Ag-Pd and Co-Cr alloys crowns during fa-tigue cycles.

MATERIAL AND METHODS

The crown of a mandibular rightfirst molar typodont model tooth(500 A; NISSIN) was prepared for anabutment tooth with an occlusal restseat (spoon shaped, 2.5� 2.5� 1mm).A cylindrical wax sprue (4.0 mm; Den-taurum) was attached apically parallelto the long axis of the crown to serveas the retainer in the fatigue test. Themodel was copied with silicone rubber(Dublisil-15; Dreve-Dentamid GmbH)into wax patterns. Five crown-shapedspecimens were fabricated from Co-Cralloy, and 5 were made of Au-Ag-Pdalloy by lost-wax casting by following

try

the manufacturer’s recommendations(JN-A3 casting machine; Jianian FutongEquipment Co). Each specimen wasairborne-particleabraded(120#aluminiumoxide, 0.2 MPa, 45 seconds), ground(silicon carbide, 7000 rpm, light pres-sure, 90 seconds), and polished (rubberpoints, 4000 rpm, light pressure, 90seconds).

Crown specimens were duplicatedwith refractory dies (Wirovest; BEGO)Each die was surveyed, and the desiredundercut areas (0.25-mm depth) weredetermined when assuming that theinsertion path of the clasp was parallelto the long axis of the crown. Waxcircumferential clasps were fabricatedwith occlusal rests by using preformedsemicircular wax clasp profiles (MK;Dentaurum). The lower edge of the waxclasp profile was securely placed alongthe undercut line of each die to ensure thesame length and identical location ofclasps. Cylindrical wax sprues (4.0 mm;Dentaurum), which served as retainers inthe fatigue test, were connected to theupper surfaces of the rests, parallel to thelong axes of the crowns, by using a sur-veyor. Each specimen was airborne-particle abraded (120# Al2O3, 0.2 MPa,45 seconds) and electrolytically polished(sulfuric acid 53%, glycerol 25%, water13%, ethanol 8%, Co dichloride 0.001 toapproximately 0.01%, 2 A, 6 minutes).The composition and manufacturers ofmetals used for specimen fabrication inthis study are listed in Table I.

The internal porosity of the clasps wasexamined with dental radiographs (70kV, 25 mAs; Instrumentarium Imaging).The external defects were detected by vi-sual observation with the aid of dentalloupes (�4; HEINE Optotechnik). The

Jiang et al

1 Clasps placed on abutment toothfor insertion-removal test.

June 2014 495

accuracy of the clasps was determined byusing silicone rubber (Affinis Preciousregular body; Coltène/Whaledent) underthe same dental loupes. A specimen

AC20.00

18.00

16.00

14.00

12.00

10.00

8.00

6.00

4.00

2.00

10002000

3000.00

0

Ret

enti

on F

orce

(N

)

2 Mean changes in retentio800, 1500, 4500, 7500, and

Jiang et al

would be excluded if the key parts (clasparm, outer surface of abutment, and thejoint of abutment and its retainer) weredefective or if the silicone rubber remainedat the location where the clasp and crowncontacted.

Specimens were divided into 2groupsaccording to crown type:CC (Co-Cr alloy crownseCo-Cr alloy clasps) andAC (Au-Ag-Pd alloy crownseCo-Cr alloyclasps). Each group contained 5 pairs ofspecimens. The fatigue test was con-ducted by using a universal testing ma-chine (Electroforce 3100; Bose Corp)under dry-air condition. The clasps wereplaced on the abutment teeth, whichensured that the occlusal rests werecompletely in contact with the rest seatsand attached to the testingmachinewiththe help of a clamping apparatus(Fig. 1). Each cycle consisted of the claspbeing moved on and off the abutment,parallel to the long axis of the crown. Thetest was performed at 1 Hz for 15 000cycles, which simulated 10 years of clin-ical use (inserting and removing thePRPD 4 times a day). The maximum

CC

40005000

60007000

80009000

10000

1Cycle Number

n forces required to dislodge clasps of CC a15 000 cycles (bars indicate standard dev

values of the displacement forces wererecorded in N. The test was terminated ifa specimen fractured. The mean (stan-dard deviation) values of force mea-surements at 100, 400, 800, 1500,4500, 7500, 10 000, and 15 000 cycleswere recorded. The differences of reten-tion values between both groups wereexamined by using 2-way repeatedmeasures ANOVA at 3 time points:initial 100 cycles, 7500 cycles, and15 000 cycles (which represented base-line, middle, and final measurements).The change rates of retention values atkey cycles compared with the initial 100cycles’ retention were calculated by us-ing the formula PY¼(YieY0)�100%/Y0,where PY is the change rate, Yi is theretention force and Y0 is the meanretention value in the first 100 cycles.Statistical significance was set at a¼.05.The surfaces of retention arm tips(0.2 cm in diameter) and the corre-sponding parts of the abutments wereobserved with scanning electron micro-scopy (�1000, S-4800; Hitachi) beforeand after the insertion-removal test.

1000

12000

13000

14000

15000

nd AC groups at 100, 400,iation).

496 Volume 111 Issue 6

RESULTS

No specimens fractured during thetest. Clasp retention altered with re-peated insertion and removal (Fig. 2).Mean values of removal forces at 100,400, 800, 1500, 4500, 7500, 10000, and15000 cycles are recorded in Table II, andtheir corresponding change rates arecompared with the initial force. In the CCgroup, mean force values decreased withcycles, whereas they increased in theAC group. Two-way repeated measuresANOVA revealed significant differences inretention values among different groups(Table III).

Representative scanning electronmicrographs of clasps and crowns areshown in Figures 3, 4. Before the fatiguetests, the inner faces of Co-Cr alloy

Table II. Retention force values and chacycle numbers

No. Cycles

CC G

RetentionForce, N,mean (SD

100 (approx. 1 mo of use) 7.02 �1.36

400 (approx 3 mo of use) 6.72 �1.44

800 (approx. 0.5 y of use) 6.58 � 1.34

1500 (approx. 1 y of use) 6.37 �1.44

4500 (approx. 3 y of use) 6.06 �1.32

7500 (approx. 5 y of use) 5.69 �1.31

10 000 (approx. 7 y of use) 5.52 �1.31

15 000 (approx. 10 y of use) 4.92 �1.31

SD, standard deviation; approx, approximately.

Table III. Two-way repeated measures Agroups at 100, 7500, and 15 000 cycles

Degrees ofFreedom

Group 1

Cycle 2

Group � cycle 2

Between-subject error 8

Within-subject error 16

Total 5

The Journal of Prosthetic Dentis

clasps were relatively smooth, except forsome pits and protuberances (Figs. 3A,4A). After 15 000 cycles of insertion andremoval, Co-Cr alloy clasps exhibitedgrooves that were predominantly paral-lel to the sliding direction (Fig. 3B). Ovalwhite patches were visible on the Co-Crclasps in the AC group (see arrow inFig. 4B). Irregular scratches wereobserved on the crowns (Figs. 3C, 4C).Shallow grooves as well as irregular-shaped pits appeared on Au-Ag-Pdalloy crowns after the fatigue test (seearrow in Fig. 4D).

DISCUSSION

Based on the results, the null hy-pothesis was rejected because differ-ences were found in the retentive forces

nge rates of different groups at different

roup AC Group

)ChangeRate (%)

RetentionForce, N,mean (SD)

ChangeRate, (%)

7.74 �1.16

�4.3 10.92 �2.07 41.1

�6.3 11.72 �2.63 51.4

�9.3 13.57 �3.44 75.3

�13.7 14.83 �2.27 91.6

�18.9 15.60 �3.71 101.6

�21.4 15.70 �3.67 102.8

�29.9 15.46 �3.58 99.7

NOVA of retention forces in AC and CC

Sum ofSquares

MeanSquare F P

373.6 373.7 33.5 <.001

62.4 31.2 11.4 .001

150.9 75.5 27.7 <.001

89.2 11.152

43.5. 2.722

try

and abrasion patterns of Co-Cr alloycast circumferential clasps applied toAu-Ag-Pd and Co-Cr alloys crownsduring fatigue cycles.

Molars are ideal abutment teeth forPRPDs with good support and reten-tion ability. Ahmad et al14 found that70% of PRPD retention is provided bymolar abutments. Cast metal crownsare used extensively for molar restora-tion. In this study, the mandibular rightfirst molar was chosen for the experi-ment. Given that the mesial contour ofthe crown is more concave at the cer-vical third than the distal,15 the occlusalrest seat distally and clasp tips mesiallywere designed to ensure clasp tips awayfrom gingival margins, which benefitthe periodontal health of abutments.When the occlusal rest was fully seated,the clasp was closely adapted to theabutment crown with restricted path ofinsertion.

Retention forces for cast clasps forPRPD should be approximately 5 N,16

provided by an undercut depth for Co-Cr clasps of 0.25 mm.4 Mahmoud et al5

reported that Co-Cr clasps could survivemore than 106 loading cycles withoutfracture under a 0.25-mm deflection. Inthe present study, there was no claspfracture for either group. The initialretention forces were slightly higher than5 N. The present test was conducted ina dry condition outside the oral environ-ment, which may have contributed to theobserved results because the lubricantaction of saliva will decrease the co-efficients of friction.17

With repeated insertion andremoval, different crowns affected claspretention in different ways. According tothe 2-way repeated measures ANOVA,clasp retention values in different testcycles were significantly influenced bygroups (Table III). Co-Cr clasps losetheir retention compared with the initialvalues on Co-Cr alloy crowns (Fig. 2),which decreased by 29.9% after 15 000insertion-removal cycles. It did not meetclinical requirements at the end oftesting when considering that theretention force was smaller than 5 N at15 000 cycles (Table II). In contrast withthe above result for the CC groups, an

Jiang et al

3 Scanning electron microscopy of contact surfaces of CC group. A, Cobalt (Co)echromium (Cr) alloy clasp before test. B, Co-Cr alloy clasp after test. C, Co-Cr alloycrown before test. D, Co-Cr alloy crown after test (original magnification �1000).

4 Scanning electron microscopy of contact surfaces of AC group. A, Cobalt (Co)echromium(Cr) alloy clasp before test. B, Co-Cr alloy clasp after test (arrow: oval white patches).C, Gold-silver-palladium alloy crown before test. D, Gold-silver-palladium alloy crown aftertest (arrow: exfoliative pits) (original magnification �1000).

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498 Volume 111 Issue 6

increase in force values was observed inthe AC group (Fig. 2). The mean reten-tion values doubled after the middleperiod of the test (Table II). The stan-dard deviation values were greater in ACgroup specimens compared with CCgroup specimens (Fig. 2). This probablycame from the surface finishing processof crown specimens. The percentageelongation and hardness represent theburnishability of a material. Au-Ag-Pdalloy, with lower hardness and higherductility than Co-Cr alloy,18 is moreprone to uneven dimensional changesduring the finishing process. Other re-searchers also reported similar findings.Rodrigues et al3 postulated that the in-creases in force values were caused byprolonged cold working of the claspsand the strictly defined insertion path.Bridgeman et al19 hypothesized that theincrease in delta values was due to workhardening of the metal. Nevertheless, nosolid explanation of this phenomenonhas yet been published. The differencesbetween retention force change curves ofthe 2 groups also may be derived fromsome other factors, such as microstruc-tural phases or mechanical properties aswell as the wear and fatigue behaviors ofthese 2 casting alloys. The scanning elec-tron microscopy showed the irregularlyshaped pits on the Au-Ag-Pd alloy crown(arrow in Fig. 4D) and the oval whitepatches on the clasp (Fig. 4B), whichwere rarely observed in the CC group(Fig. 3). X-ray diffraction and metallo-graphic testing should be performed in thefuture to determine the actual reasons ofthis difference.

It should be noted that this study wasconducted in a dry-air condition, whichwas different from the oral environment.Saliva, hard metal debris, and electro-chemical reactions between different me-tals could affect the outcomes of attritionand retention force values.20 Moreover,this study only involved certain alloycompositions. Different proportions of

The Journal of Prosthetic Dentis

elements may affect mechanical proper-ties of alloys and, consequently, theirfriction behavior. Furthermore, this studyis limited to macroscopic property ofthese dental materials. Further researchis needed to study the clinical conditionsof clasps applied on different metalcrowns and to explore the element ex-change between the clasps and crowns,for example, by using energy dispersive x-ray spectroscopy to reveal microscopicfriction characteristics.

CONCLUSIONS

Under the experimental conditionsin this study, Co-Cr alloy crowns and Au-Ag-Pd alloy crowns performed in differentways when Co-Cr alloy circumferentialclasps were applied on them. Crown de-signs should be changed, depending onthe PRDP retainer (clasp) materials.

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1. Carr AB, McGivney GP, Brown DT.Mc Cracken’s removable partialprothodontics. 11th ed. St Louis: Mosby/Elsevier; 2005. p. 156.

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

3. Rodrigues RC, Ribeiro RF, de Mattos Mda G,Bezzon OL. Comparative study ofcircumferential clasp retention force fortitanium and cobalt-chromium removable par-tial dentures. J Prosthet Dent 2002;88:290-6.

4. Cheng H, Xu M, Zhang H, Wu W, Zheng M,Li X. Cyclic fatigue properties of cobaltchromium alloy clasps for partial removabledental prostheses. J Prosthet Dent 2010;104:389-96.

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

6. Vallittu PK, Kokkonen M. Deflection fatigue ofcobalt-chromium, titanium, and gold alloy castdenture clasp. J Prosthet Dent 1995;74:412-9.

7. Wataha JC, Messer RL. Casting alloys. DentClin N Am 2004;48:499-512.

8. Ucar Y, Brantley WA, Johnston WM,Dasgupta T. Mechanical properties, fracturesurface characterization, and microstructuralanalysis of six noble dental casting alloys.J Prosthet Dent 2011;105:394-402.

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9. Manaranchea C, Hornbergerb H. A proposalfor the classification of dental alloysaccording to their resistance to corrosion.Dent Mater 2007;23:1428-37.

10. O’Brien WJ. Dental materials and theirselection. 3rd ed.Chicago: Quintessence;2002. p. 225-38.

11. Matkovi�c T, Matkovi�c P, Malina J. Effects ofNi and Mo on the microstructure and someother properties of CoeCr dental alloys.J Alloy Comp 2004;366:293-7.

12. Wataha JC. Alloys for prosthodonticrestorations. J Prosthet Dent 2002;87:351-63.

13. Gapido CG, Kobayashi H, Miyakawa O,Kohno S. Fatigue resistance of cast occlusalrests using Co-Cr and Ag-Pd-Cu-Au alloys.J Prosthet Dent 2003;90:261-9.

14. Ahmad I, Sherriff M, Waters NE. The effect ofreducing the number of clasps on removablepartial denture retention. J Prosthet Dent1992;68:928-33.

15. Ash MM, Nelson SJ. Wheeler’s dentalanatomy, physiology and occlusion. 9th ed.Philadelphia: Saunders; 2009. p. 194.

16. Sato Y. Clinical methods for adjustingretention force of cast clasps. J Prosthet Dent1999;82:557-61.

17. Schipper RG, Silletti E, Vingerhoeds MH.Saliva as research material: biochemical,physicochemical and practical aspects. ArchOral Biol 2007;52:1114-35.

18. Sakaguchi RL, Powers JM. Craig’s restorativedental materials. 13th ed.St. Louis: Mosby;2012. p. 221.

19. Bridgeman JT, Marker VA, Hummel SK,Benson BW, Pace LL. Comparison oftitanium and cobalt-chromium removablepartial denture clasps. J Prosthet Dent1997;78:187-93.

20. Lippo VJ, Lassila LV, Pekka KV. Effect ofwater and artificial saliva on the low cyclefatigue resistance of cobalt-chromiumdental alloy. J Prosthet Dent 1998;80:708-13.

Corresponding author:Dr Dong-xiang ZhengSchool of StomatologyCapital Medical UniversityNo. 4 Tiantan Xili, Dongcheng DistrictBeijing, 100050P.R. CHINAE-mail: dxzheng123@hotmail.com

AcknowledgmentsThe authors thank Ms Octavia Swanson(Predoctoral dental student, University ofWashington) for her kind help in editing andrevising the manuscript.

Copyright ª 2014 by the Editorial Council forThe Journal of Prosthetic Dentistry.

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