strength and Mode ofFailure of Single Implant

All-Ceramic AbutmentRestorations Under

Static Load

Aris Pelriii D. Tripodakis, DDS, MS, Dr OdonI'

lorg Rudolf Struii, DMD, PhD"

Heinz F. Kappert, Dr Rei Nal, PhD"'

Sleghert WMonski, CDT""

School of DentistryAlbert Ludn-igi UniversityFreiburg, Germany

The strength and mode of failure of three different designs of custom-madeall-ceramic implant abutments fabricated by milling of In-Ceram sinteredceramic blocks were compared with the conventional CeraOne system understatic load. Four test groups were formed with different locations of abutmentscrews. In three test groups, In-Ceram crowns were fabricated for placementon the all-ceramic abutments, and in one test group, a veneer porcelain wasfired directly on the abutment; crowns in the control group were fabricatedusing the CeraOne system. Ten-mm-lonp Brânemark implants were placedinto a brass block that ailowed loading at a 30-dcgree angle to the long axis.The test group in which the veneer porcelain was fired directly on the all-ceramic abutments was the weakest, and it showed fractures at a mean valueof 236 N. The fracture strength of the three other test groups was dependenton the extension of the crown margin relative to the location of the screwhead. Tbe test group that had the screw on the top compressing the entireceramic abutment showed a mean value of 422 N that was similar to theresults tbat were achieved witb the CeraOne system (427 N). Tbe weakestlink in tbe all-ceramic single implant restorations was the abutment screw inwhich tbe bending began at approximately 190 N. IntJ Prosihodont1995:8:265-272-

T he long-term prognosis of endosseous oralimplants has been extensively documented for

fully and partially edentulous patients.'"" Estheticsmay be compromised at the cervical third of thesuprastructure in the area where the abutment con-tacts the implant. The abutment must meet bio-logic, functional, and esthetic requirements.Biologically, the restorative material must be bio-

'Visiting Assistant Professor, üepurtmertt of Prosthodontics:

Currently. Assistant Professor, Department of Prosthodontics.

University of Athens, Greece.

"Professor and Chairman, Department of Prosthodontics.

•"Professor and Head, Section Dental Materisis.

""Head, Dental Technology.

Reprint requests: Dr Aris Petros D. Tripodaiiis. 92, Vas. Sophias

Ave. Athens 11528, Greece.

Awarded Best Tabie Clinic Presentation at the ¡0th Annual

Meeting of the Academy of Osseointegration, March 4^9. 1995,

Chicago. Illinois.

compatible and not promote plaque adherence.Functionally, the abutment must provide sufficientstrength to endure and transmit forces to theimplant and the supporting bone. Esthetically, itshould have the anatomically correct contours andreplicate the optical properties of a natural toothincluding ihe cervical area corresponding to thecrown-rool junction. The natural morphology ofthis area is not cylindrical and varies greatly. Thenatural tooth allows some light transmission andprovides the surrounding soft tissues with a fairlybright background that transmits light through afiberoptic effect.

On many occasions, conventional titanium abut-ments fail to meet esthetic requirements. Evenwhen placed subgingivally, a dull greyish back-ground may give the soft tissue an unnatural bluishappearance. Abutment components were modifiedto allow a more stable screw joint as well as toimprove the esthetic result."* Based on these modifi-cations, other abutment materials' and other tech-

265 The Internstionaf Journal uf Prosthodontic

Sireiiglii of Impbnl All-Ce : A bul mem Restorations Tri[]oilal<i^ el a I

Fig 1 Design oí single impiant ail-ceramic abutments.

niques to secure the abutment screw have beenpresented." The all-ceramic abutment CeramiGore(not commercially available) was developed forsingle and multiple units and allows good estbeticsand biocompatibility.-'" Knode and Sorensen" andMcGlumphy et al'- conducted studies comparingthe strength of single implant restorations usingCeramiGore abutments and traditional metal abut-ments. In contrast to the latter study, Knode andSorensen found significant differences between theperformance of test and controi abutments,although no attempts were made to describe tbemode of failure or discuss the relationship of failureto design. Because of tbe controversy in the litera-ture, further investigations are needed. Furthermore,the mode of failure must be evaluated to proposenew designs of all-ceramic abutments.

Before accepting modifications of implant abut-ments and implant restorations, it is important tocarefully evaluate tbe bioiogic and mechanicalperformance and the feasibility of the introducedalterations. The purpose of this study was to evalu-ate the fracture strength and the mode of failure ofdifferent designs of single implani all-ceramic abut-ment restorations under static load.

Materials and Methods

Twenty-four all-ceramic implant abutments withthree different designs were fabricated by millingsintered ALO, blocks (In-Geram, Vita Zahnfabrik,Bad Säckingen, Germany) prior to the glass infiltra-tion stage using a copy milling machine (Gelay,

Fig 2 Test abutments retained on implants pnor to glassinfiltration.

Mikrona Technologie, Spreitenbach, Switzerland),Titanium impression copings having an internalhexagonal form (DGA 099, Nobelpharma,Güteborg, Sweden) were copied to provide proto-types of identical designs of the cervical portion cfthe different all-ceramic implant abutments engag-ing the hexagonal head of the Branemarli implants(SDGA 001, Nobelpharma), The outer surface oftbe cervical parts diverged by an angle of 45degrees to reach a maximum diameter of 6 mm, 1mm above tbe base, Tbe prototypes of tbe tbreedifferent designs of ceramic implant abutmentscontain a crown portion with 3-degree inwardlysloping axial walls extending cervicaiiy to a 0,5-mm-deep rounded shoulder.

Figure 1 shows two different locations of theabutment screws and three different designs of all-ceramic impiant abutments. In design a and c thescrew channels house the screw heads, and in b

The Iniernatioral 266 Volume Ü, Niiml - ."..^_.:.

Tripod a kíi el Strenglti of Single Impbnt All-Ceramic Abuimeni Restorations

Test I Test

In-Ceram

Veneer porcelain (Vitadur Alpha)

Stainless steel p!ate

Test III Test IV Control

Titanium

Stainiess stee! or go!d

Sintered AijOg (CeraOne)

Fig 3 Design at single implant all-ceramic abtjtment restorations.

the screw head is p!aced on the top of the abut-ment. Downward tapering (design a and c) orupward tapering Idesign b) custom-made metaldiscs were placed between the screw heads andthe ai!-ceramic implant abutments. The main dif-ference between designs a and c was that on theexterna! surface of the all-ceramic implant abut-ments in design a, the external rounded sbou!derwas created at the level be!ow the internal shou!-der, whereas in design c the the rounded external5houlder was above the interna! shoulder.

Ceramic bloclcs were mi!led in the copy milüngmachine using a high-speed turbine in the wetcarving chamber to produce the a!!-ceramicimp!ant abutments." The stem areas weresmoothed using a fine diamond bur in a rotaryhandpiece prior to glass infiltration in an oven(Inceramat, Vita Zahnfabrik) at 1120 °C for 6 hours(Fig 2). The outer surface of the al!-ceramic imp!antabutments was covered with a mixture of glass anddistiüed water. After firing, the excess g!as5 wasremoved by airborne partic!e abrading using 50|jm A!;Oj powder at 3-bar air pressure. The fitbetween the aü-ceramic implant abutments andthe imp!ant head was reexamined and improvedby wet grinding the contact area using FEPA P

4000 siÜcon carbide paper mounted on a poüshingspinning whee! (K&B Grubbs, Dusseldorf,Germany).

Four test groups were formed containing sixspecimens each. !n test group !, six crowns werefabricated by firing the ceramic veneering materia!(Vitadur A!pha, Vita Zahnfabri!<) directly onto theceramic imp!ant abutments providing access forthe abutment screw (Fig 3-Test !). In each of theother test groups (Test ! l , l!l and IV), six impressionswere made using a po!yether impression materia!ümpregum, ESPE, Seefeld, Germany), and !n-Ceram crowns (Vita Zahnfabri!<) were fabricatedand !uted on the a!!-ceramic implant abutmentsusing ZnPOi cement (!Harvard R, Berlin, Germany)with finger pressure (Fig 3-Test 1!, ! ! l , and !V). Forthe control group, al!-ceramic crowns (VitadurAlpha, Vita Zahntabrik) were fabricated using theCeraOne system (Ceramic cap short, DCB 127.Nobelpharma). The crowns were !uted usingZnPOj on the titanium abutments (1-mm coüar,5DCA 121, Nobe!pharma) (Fig 3-Control). In a!!groups three specimens were retained to theimp!ants with go!d abutment screws (DCA 118,tNobe!pharma) and three with custom-made stain-iess steel screws using a controücd force of 32

267 I of Prostliodontii

of Sinük- ImRlont All-Cif Tripodakiselal

Fig 4 Completed test abutment retained with controiied(orceof 33Ncm.

Fig 5 impiant placed in a brass biock bearing a test sampie ofgroup I ready tor loading at a 30 degree angle to the iong axis.

Ncm (Torque Controller, Nobelpharma) (Fig 4¡.The use of stainless steel screws was introduced toexamine in vitro the bending properties of astronger material, Ti and Au being approximatelyequally weak. The 10-mm-Iong Brânemarkimplants were placed in a brass block that allowedloading at a 30-degree angle to the long axis. Theincisai edges of the crowns were covered by 0.4-mm-thick tin foil, tripie folded, to ensure transmis-sion of the appiied static loading forces (Zwick1445, Ulm, Germany) through their entire iengths(Fig 5). The tests were performed using a crossheadspeed of 1 mm per minute (according to ISO 6872),whereby the applied force was graphically recordedon an x-t-recorder (Linseis, Selb, Germany). Thegraphs show the initial elastic bending of the all-ceramic abutment restorations and Ihe beginning ofthe irreversible plastic screw bending under theapplied forces. The critical force for the beginning ofthe plastic screw deformation is defined as bendingforce in this context. Means and standard deviationsof the recorded forces for the plastic bending of theabutment screws and the fractures of the ceramicsingle implant restorations were calculated. TheStudent's t-test was used to evaluate the differencesbetween test and control groups.

After testing, the specimens were invested intoacrylic resin blocks (Technovit 5000-Technovit4071, Heraeus-Kulzer, Wehrheim, Germany), andthey were sectioned buccolinguaiiy using a dia-mond microsaw (Capeo, London, England) formicroscopic examination.

Resuits

The results with means and standard deviationsof the strength and mode of failure of the differentsingle impiant restorations under static load areiisted in Tabie 1 and presented in Fig 6. The statis-tical analysis revealed differences between the testand control groups for the bending forces of theabutment screws and the fracture strength of thesingle implant restorations (Table 2).

For the bending forces of the abutment screws.Student's ; test revealed no significant differencesbetween gold and stainless steel abutment screwsin all groups (mean values 195 N and 205 N, P>.05). There were some differences between themean values of the bending forces of the differentgroups. The mean value of the bending forces oftest group III (243 N) was higher than that of testgroups I and IV (177 N, 178 N; P< .05).

The mean values of the fracture forces in testgroups li and III and the control group (373 .N,422 N, and 417 N) are significantly higher thanthat in test group I (236 N, P < .05). There was asignificant difference between the fracture force oftest groups 111 and IV.

Discussion

In this study, the strength and mode of failure ofdifferent single implant all-ceramic abutmentrestorations was analyzed and compared with theCeraOne system under static load. The samples of

The International lournal of Prosrhodoniic^ 268

Tripod a fcii et a Strength of Singie Implan! Ali.Cersmic Abulment Reslorarii

Table 1 Strength (N) and Mode of Faiiure of Single implant All-Ceramic AbutmentRestorations Under Static Load

Samples

Test group 1

Test group II

Test group III

Test group IV

Controi group

gSS

gSS

gSS

gSS

sSS

BF

170180180180200300190170130260

F

210255365340400500-215360225455-

BF

180180190170330210180170230230

F

240180390340450360470215470-460-

BF

180170200210210210180180180250

F

240290470330400440250250470420-

BF = tjending torce; F = aOutment fracture; g = gold screw; ss = stainless steei'No fracture implant bend.

Fig 6 Strength and mode offailure of single impiant ail-ceramie abutment restorationsunder static load (means andstandard deviations]. 500

300

200

100

Screw-bending forcesID Fracture-failure Iorces

Test i Test II Test iii Test IV Control group

Table 2 Student's íTest Analysis

Pairs

Group lil-BF - Group I-BFGroup lil-BG - Group iV-BFGroup l i -F - Group i-FGroup lil-F-Group I-FCortroi Group - Group i-FGroup Ill-F-Group IV-F

[test value

2.882.805.187.424.312.79

P value

P<.05

P<.001P<.001P< 01P<.05

BF = bending force; F = failure force.

test group I, consisting of an all-ceramic abutmentwith a direct porcelain veneer and an occiusalscrew hole, provided the weakest restoration. Thenature of fractures in this group revealed the pres-ence of tensile stresses around the screw head.Cracks originated from the metal disc outwardly.

probabfy because the incisai part of the crownshifted away from the screw head during loading.Fractures must be expected, because ceramicmateriais are weak under tension. The presence ofan occiusai screw hole weakened the singleimpiant all-ceramic abutment restorations. The factthat the veneering materiai and the all-ceramicabutment were fused together increased the risk ofsurface microcraci<s that can propagate and resultin fracture.

Testing the three other all-ceramic abutmentgroups was designed to specificaiiy evaiuate thepotentiai reinforcement provided by a luted crownover the abutment and the influence of the loca-tion of the head of the stabilizing screw.

In test group il, the involved abutments are iden-

, Number 3, 1995 269

Strengtii ol Siiii;lp Impl.int Aii-Ct-

Fig 7 In groups li and iil the nature of fracture involved chip-ping of tbe ceramic material in tbe marginai area of tbe sideopposite to tbe appiieO force after a smali gap betweenimplant bead and abutment had formed foiiowing bending oftine retaining screw.

tical to group I but with a cemented crown insteadof a porcelain veneer fired directly on the all-ceramic abutment. The strength of these restora-tions was significantly higher (373 N] than that oftest group I (236 N), revealing the major impor-tance of the cemented crown in reinforcing theabutment.

In test group IN, in which the screw head waslocated on top of the all-ceramic abutment, thestrength of the restorations was improved (422 N).One can speculate that the tensile stress aroundthe screw head was eliminated and only compres-sive forces were applied from the screw headthrough the disc to the abutment and its margin. Inboth test groups (II and III), fractures occurred aftera gap had formed between the abutment marginand ihe implant head as a result of the the screwbending (Eig 7). In one test sampie in group III, theceramic margin managed to move away from thehexagonal head without fracturing, and no otherfracture occurred before the destruction of theinternal threads began.

Test group IV was designed with the margin ofthe all-ceramic crown located at a higher levelthan the head of the screw, reducing the strength-ening influence. The average fracture strength ofthis group (293 N] was not significantly greaterthan that in group I, but significantly less than ingroups II and III. Eractures occurring in this groupwere similar to those in groups II and 111 (Fig 8a). Inone specimen a horizontal fracture occurred, initi-ated from tbe screw head outwardly. This probablyresulted from the tensile stress not being neutral-ized by the embracing crown margin that waslocated at a higher level.

In most of the control group no fracture occurredbefore implant failure (Eig 8b). In two specimens,however, the ceramic crowns fractured at 225 N and470 N- The fraclure lines involved both the veneerceramic material and the sintered ceramic core.These incidents indicate that a weak feature of theCeraOne system is that the reinforcing short ceramiccore does not follow the external anatomy of thecrown. The long ceramic cap that can be modified tothe required anatomy would be preferable.

McClumphy et al'- compared the amount of sta-tic force necessary to cause failures of sintered alu-mina core abutments (CeramiCore), gold palla-dium abutments (UCLA type, Implant Innovations,West Palm Beach, ELI, and CeraOne. They foundthat the resistance to failure of all-ceramic abut-ments may approach that of conventional metal-ceramic abutments.

Knode and Sorensen" found in a similar studythat all-ceramic single implant restorations usingall-ceramic CeramiCore abulmenls are significantlyweaker than metal ceramic restorations (UCLAtype, Implant Innovations) and the CeraOne(Nobelpbarma).

Andersson et a l " investigated the mechanicalstrength of three different types of crowns that wereplaced on CeraOne single tooth abutments (metalceramic crowns, all-ceramic crowns in whichporcelain was fired to prefabricated ceramic cop-ings, all-ceramic crowns made without any copings)and found that the weakest link was the abutmentscrew. The failure loads of the screws were lowerthan the failure loads of the crowns (exception of theall-ceramic crowns without any copings).

In the present study, the abutment screw bend-ing force started at 190 N, Between the two typesof abutment screws (gold and stainless steel), nosignificant differences of bending forces werefound. Elastic bending of the screw during staticloading does not necessarily mean failure of theentire restoration, but the plastic deformationwhich occurred under loads above 190 N must beconsidered as a failure.

lörneus et al'^ analyzed different screw designsin bench situations, and ihe results were comparedwith clinical situations. They suggested the use ofgold alloy screws with a flat head and high tighten-ing torque (35 N/cm).

The results of the present study show that properdesign of the all-ceramic abutments can improvethe strength of all-ceramic single implant restora-tions although ceramics do not possess the flexuralstrength of metals. It is shown that all-ceramic sin-gle implant restorations with a custom-made all-ceramit abutment can reach the level of the

Tiie Inter rial I or a i iournii 270 Volume n, Nuniljf ' .-'-'.'-'—"

Tripodakis el .il Strength of Singie

Figs 8a and 8b Sample cross sections, (a) Test Group !• The fracture originated from the metai disk outward reveaiing the pres-ence ol tensiie stresses around the screw head, and (b) Controi' Extreme bending ot the occiusai screw caused impiant faiiure.

Strength of the GeraOne system. It must be consid-ered that tbe weakest link of tbe system is the abut-ment screw that is vulnerable to bending." Theauthors suggest that these types of restorations onlybe placed in the anterior part of the maxillae andthe mandible inasmuch as tbe average biting forcesof canines and incisors are 208 and 135 N.'"Further data are needed before single implant all-ceramic abutment restorations can be recom-mended for clinical use.

Conclusions

Four test designs of all-ceramic restorations werecompared to a commercially available all-ceramicrestorative system. Tbree screw retention designsusing both gold and stainless steel screws wereevaluated using a static load to failure, Witbin thedesign parameters of the study, the following con-clusions may be made:

1, Single implant all-ceramic abutment restora-tions using custom-made in-Geram abutmentveneered porcelain were sbown to be theweakest of all tbe designs tested,

2, The fracture strength of single implant all-ceramic abutment restorations is dependent onthe extension of the crown margin relative tothe location of the retaining screw head.

3, If the stabilizing abutment screw is located ontop of the ceramic, abutment fracture resistancecan be improved.

4, No difference was recorded in tbe bendingproperties oí stainless steel and gold stabilizingscrews.

5, The weakest link of all-ceramic single implantrestorations is the abutment screw.

Acknowledgment

The authors express Iheir appreciation to MikronaTechnologie, Spreitenbach, Switzerland, for the preparation ofthe samples and to Nobelpharma, Göteborg, Sweden, and VitaZahnfabrik, Bad Säckingen, Germany, for their generous dona-tion of products.

References

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271 The Internatiunal lournal ol Prosthodontics

Shenj;th of Single Impi.inl Aii-Ce Tripodakis el M

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Literature Abstract-

Implant-Suppotied Prostheses for the Treatment of Adults With Cleft Pa!ate

Posterior pharyngeal fiap procedures for repair of olett palate were not widely performed in the

United Stales until the late 195O's. Therefore, there is a popuiation of adults 40 years of age

ana oider with unrepaired or nonfunctional repaired paiates. Ttnis articie described the use of

self-tapping titanium implants for the retention of maxiiiary prostheses, some with a pharyn-

geal section addcid for speech enhancement. Six adult patients (ages 47 to 78 years) with cleft

palates were treated with impiants Twenty-three implants were placed, with 21 achieving

osseointegration. Ali patients were treated using a maxillary complete denture or overdenture.

Five patients (83%) required the addition of a pharyngeal section to decrease hypernasal

speech. The use ol implants in Ihe treatment of persons with cleft palate can provide

adequate retention and slability tor the prosthetic restoration of oropharyngeal struc-

tures, thus itnproving speecti, deglutition, mastication, and esthetics.

Arcurl MR, LaVelle WE, Higuchi KW, Svec BR. J Prosttist DenM994;71:375-37B. References: 5.Reprints: M.R. An:uri. Department ot Otolaryngoiogy. University oí iowa tHospitals and Clinics, iawa City,iA 52242.—tilarma Sauce, DDS. Prosttiodontic resident. New York yeterans Administration Medical Center,New York

The International loiirnal ot Prosihodonlii 272 Voiume B, Number