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The Relationship ofPeriotest Values,Biom aterial, and Torque toFailure in Ad ult Baboons

Alan B.Carr DMD, MS*Efstratios PapazogtouDDS, MS**Peterf. Larsen, DDS'**TheOhio State UniversityCollege of D entistryColumbus, Ohio

A quantitative measure of implant stability wo uld be beneficial to decision-making regarding the status of the bone anchorage around an implant. ThePeriotest device has been reported to provide such a measure, but the in vivodata are incomplete and inconclusive in support of this claim . The purposeof this study was to determine in an in vivo model the relationship betweenPeriotest values (PTV), three different implant bioma terials, and torque tofailure. A total of 79 screw-shape implants made of comm ericially puretitanium , titanium alloy, and HA-coated alloy were evaluated in the posteriormax illary and mandibular quadrants of six female baboons follo win g healingfor 3 to 4 months. At uncovering, the implants were tested with the Periotestdevice and torqued to failure using an electronic torque driver. Analysis ofthe PTV-biomaterial relationship revealed no significant difference amongthe implants based on the biomaterial [P= .7453). Analysis of PTV-torquerevealed a significant relationshipP^ 0117), however, the associationbetween PTV and torque was not strong and does not explain 92 % of thevariation between the parameters (Pearson correlation .2823,R^=.08),nl i Prosthodont 1995;8:15-20.

unctional stab ility of the de ntition is a prerequi-site to normal mastication, and periodic mea-sures of tbis stability are important to assign a prog-nosis for function over time. Historically, tbis sta-bility measure bas largely been categorized intomobility grades based on subjective assessments oftooth movement under either static or dynamicforce.' Increasing mobility, most often associated

Assistant Professor DepartmentofRestorativean dProstheticDentistry; Director of Ma xillofaciai Prosthetics. Coilege ofDent5try and AG lames Ca ncer Hospilai at^d ResearchInstitute.AssistantProfessor DepartmentofRestorativean dProstheticDenlistry.' Associate Professor. Department of Oral & MaxillofacialSurgery; Director GraduateOral and MaxillofacialSurgery.

print requests Or Alan B.Carr Department ofRestorativean d Prosthetic Dentistry, College of Dentistry, The Ohio StateUniversity, College of Dentistry, 305 West }2th Avenue,Columbus Ohio 43210-1241.

with an environment of chronic inflammatory peri-odontal disease, is evidence of a poor prognosis,while mobility in conjunction with a bealtby peri-odontium tbat does not adversely effect function isnot deemed to be indicative of a poor prognosis.'From animal studies, the reported relationship ofmobility, bone loss, and periodontal disease helpeddevelop an appreciation of tbe interaction of thesefactors and suggested appropriate treatments for thenatural tootb nnodel.' Although m obility is not con -sidered favorable to the prognosis of implants, asimilar description of the same factors interactingwitb an implant-tissue interface model that couldprovide a better understanding of the respectiveroles each factor has on success has not beenshown, although some data are appearing.^''

The functional stability inherent in the healedendosseous implant is remarkably different fromthat o f the natura l tootb. Tbe d i f ference isdescribed by tbe qualitative and quantitative man-ner witb which healed alveolar bone (ie, bone

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aterial,and Torqire to F ailjrt

extracellular matrix, mineral crystals and relatedcells) encasing ihe biomaterial provides the func-tional response to load that previously was providedby the periodontal ligament. Detailed in vivo infor-mation comparing and contrasting the functionaltootb and implant support is not available'; conse-quently, functional parameters involving tooth sup-port that have prognostic value for teeth cannot beassumed io have value for implants without experi-mental documenlation. Some data that describe lat-eral displacement of implants and compare thecharacter of displacement to natural teeth has beenreported, but how this relates to functional supportdifferences is unknown,A device that could provide a quantifiable andprognostic parameter associated with the function-al stabil ity of teeth and implants would be benefi-cial. Such a device would need to identify a char-acteristic for both tooth and implant support sys-tems that has a high correlation to stability, Lukasand Sch ulte' described the deve lopment and useof such a device for teeth (Periotest, BloResearcb,Milwaukee, Wisconsin], This device uses the con-tact time of the tapping rod with the tooth duringdece leration to provide a biophys ical parameter(Periotest value or PTV] that correlates with a c l in i-cally determined tooth mobili ly classification.For the natural toolh, the amount of movementcreated by the device has been shown to repre-sent only a portion of the movement capabi l i ty,Consequently , m obi l i ty , wb ich connotes move-ment capacity, may be an inappropriate term forthe phenomenon occurr ing wi th teeth. ' Whethermove me nt induce d by the Periotest device is apo r t i on o f the to ta l movemen t capab i l i t y f o rendosseous implants is not known. The amount ofmovement created by the impact of the devicemay be helpful to provide some understanding ofthe s ign i f i cance o f the PTV (even when i t i sknown that the actual characteristic of the impactdeceleration signal used for establishing the value

is the contact time]. Some reports have providedinformation regarding impact movement; however,it is difficult to reconcile data that report a 10 to 20j jm movement created by the Periotest devicewhen impacting a healthy tooth with a report ofin vitro data suggesting approximately three timesth i s mo v e me n t f o l l o w i n g PTV i mp a c t f o r a nimp lan t r ig id ly encased in s te e l . - In sp ite o ftbese seemingly contrasting reports, some autborsare claiming prognostic usefulness for the Periotestdevice when assessing implant stabi l i ty, '^ Suchclaims can only be substantiated with diagnosticutil i ty investigations. In vitro studies of reliabil ityand repeatabi l i ty suggest good performance by

the Periotest device, al though variabi l i ty in therange of 3 to 4 PTVs was demonstrated to frea son ab le,' ' ' , , , , ., ,,,|The d iagnost ic u t i l i ty shou ld be deterrmr. 'abased on an understanding of the sensitivity anaspecif icity of Periotest performance ^m pa re cl oit^portant clinical parameters and a gold standardof histologie data addressing the biologic suppo^for endosseous implants. This paper provides datatoward that end, reporting important prel i t^inaryrelat ionships betwee n PTVs and other c l i n i ca lparameters described, A future report wil l providedata relating histologie data and PTVs,The purpose of this study was twofold. Tbe firstwas to determine the relat ionship between tbePTVs and varying biomaterials. The second was torelate PTVs to failure torque. The working hypothe-sis of the study was that tbere is no differencebetween PTVs obtained from different biomateri-als, and PTVs are inversely proportional to failuretorque values.

Materials and MethodsThe animal model used for this study was theadult female baboon. Animal management proce-dures were approved by and fol lowed the guide-lines of the Institutional Laboratory Animal Careand Use Committee, Six animals, ranging in agefrom 10 to 15 years and weighing from 14 to 20

kg, had all posterior teeth extracted and 6 implantsper quadrant placed fol lowing 2 months of heal-ing. Seventy-nine of the total 144 implants placedwere used for tbe Periotest data collected for thisstudy. Al l surg ical procedures fo l lowed c l in icalprotocols specif ic to buman implant appl ication.Alternate biom aterial placem ent, wh i le not a ran-dom allocation of the samples, did assure differentlocations of the biomaterials wilhin tbe arches foreach animal and resulted in each quadrant having2 implants of the same type, Eor this report, datawere collected on 23 commercially pure (Cp) tita-nium implanis and 28 of both ti tanium al loy (Ti-6AI-4V) and hydroxypati te-coated (HA] implants.The implants used were endosseous screw-shapeddevices with the same geometry, 3,8 m m in diame-ter and 10,0 mm long (Steri-Oss, Yorba Linda, CA),

Fol lowing a 3- to 4- montb heal ing period, theanimals were k i l led, specimens were harvestedand hemisectioned, and quadrants were embeddedin plaster blocks and rigidly stabilized in a vice (Eig1 , Periotest values were then collected before andafter torque to failure. The torque to failure testingme thod and resu lts are des c r i bed i n ano the rrep ott. Data col lection was accom plished with in

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f,3rsdTorqu t lo F

Fig 1 Maxillary quadrant embedded m plaster orienling the implants perpendicular to the floor left),and the plaster embeddedsegment (a separate mandibular specimen) stabilized in the vice tor testing rigf>t).Testing was accomplished within 30 minutes ofkilling of animals. Orientation m arks on the bone and implants were for torque-move ment observations. Perpendicular impact loca-tion for these im plants was (rom the side opposite this view.

30 minutes of the kill ing of the animals. All datawere col lected by the same operator who hadestablished method reliability prior to data collec-tion on an in vitro model following instrumentcali-brat ion. Recalibration was accomplished prior toeach measurement session which was on three dif-ferent days. Data were accepted only when threeconsecutive values were obtained that differed byno more than 1 PTV unit, and all three values wereused to provide a mean PTV score. The PTV hand-piece placement was standardized on all implantsto impac t a t a l oca t i on cons is ten t be tweenimplants and at the implant collar base closest tothe bone support (Fig 2). Handpiece-to-implant ori-entation was standardized for midimplant orthora-dial impact.The torque data were obtained using a counter-c l o c k w i s e c o m p u t e r i z e d t o r q u e d r i v e r(Compudriver-Computorque, Model 501-CI, Con-solidated Devices, City of Industry, CA), Stan-dardization measures were taken to decrease biasin the data recording procedure. The instrumentused was calibrated by the manufacturer for accu-racy 1 ) and precision (0.5 ). Local calib ratio n,using a Norbon Digital torque calibrator (Model400017, no. 13141), determined the accuracy tobe 2 for a torqu e transducer with a range of 30 to560 Ncm. An attempt was made to standardizeoperator torque methodology using an in v i t romo d e l wh e r e i mp l a n t s we r e a t t a c h e d t o ama ndibular model w ith acrylic resin. Torque driverorientation and rate were standardized by repeti-tive trial tests. Because the biomechanical valuemeasured is dependent upon strain rate,'' the rateof torque application was established at 90 per

Fig 2 Placement of Periotest devioe relative to an impiantshowing impact location, distance of handpiece from theimplant (approximately 1 5 mm), and handpiece orientation.The actual value of Periotest data is reflective of the proximityOf impact location to the crestal bone .

second to m im ic the clinic al screw-fasten ing rateas delivered by common manual screw drivers atthe point of final torque.Statistical analysis of PTV-biomaterial and PTV-torque was by ANOVA and linear regression.

sults

Table 1 presents the implant allocation by ani-mal and reveals that 23 CP Ti, 28 HA-coated, and28 T-6AI-4V implants were used for this study.The implant al location by arch reveals that 16mand ibu la r and 63 max i l l a r y imp lan ts we reinvolved in the data gathering. The mean implantPTV for the maxillae (-6.2|.9)} and the mandible

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Perioteii Values, Biorra tcrial, and Torque to Fa

Table Implant Al locat ion by Animal and ArehAnim al C P T i T-6AI-4V HA-coated Maxillae Mandible123456T o t a l

3563242 3

d774242B

4774242 8

111112116126 3

08800016A total ot79 implants distribuled among 6 animals with a 4:1 maxiiiae-to-mandiBie sartipiing ratio. Previous data aniaysis revealed no ditter-ence in groups based on time intervais 12 ttirough 16 weeks; ttiere-tore,dala ana iysis considered all groups as one time interv al.

Table2 Periotest Values (PTVs) for Each BiomaterialWith ANOVA TableBio m ate rialC P T iHA-coatedT-6A-4VANOVASourceModelErrorC total

MeanPTV SD-5 .6-5 .6-5 .4

df27 67 8

1.00.91.1

Su mot squares0.71220283.00931783.721519

Samplesize2 32 82 8

Meansquare0.356101.09223

F ratio03260Prob>F0.7228

Table 3 Linear Regression ol Torqu e antj PTV, PearsonCorrelation, and R StatisticsLinear regressionSource ofModel 1Error 77C total 73Pearson correlation-.2823

Sum otsquares226,31102612.77392839.0849.079713

Mean square 6 31133.932

F ratio6.6695Prob>F0.0117

The linear regression suggests a linear relationship between PTV andtorqje at the P = .0117 ievel. However, the R statistic reveis thatonly8%of the variability in PTV isaccountedlorbythetonquedata.

Table 4 Relationship of PTV and Mean Torque forAll Implants Within Spcifie PTV GroupsP TV- 7- 65-A- 3

n162 42 513

1

Mean torque(Nom)142.3111.7115.277.9

37,3

S D77,965.562.159.8

(-5.311.01) did not represent a difference that waclinical ly signif icant, especial ly considering treported acceptable varia bility of the dev ice,' a^therefore the data were grouped for a nalysis Pranalysis of data from this investigation revealed not i t^e interval d i f ferences ^^^^^^^^^ ' ' ^ % ' ; ,^ J ,m onth mea sures; co ns eq ue nt ly , 1 fgrouped relative to time for all ^-^'V^e s At theL e of uncover ing, a l l implants were c l in ical lyosseointegrated. , , . ^ . .Table 2 describes the PTVs for each btom ale al.The PTV-biomaterial relationship shows no differ-ence due lo material (AN O VA , only P = .7228)with the tTiean PTV lor each biomaterial being -5.4o r - 5 . 6 .Linear regression of PTV and torque reveals asignificant linear relationship (P = .0117, Table 3].The Pearson correlation coefficient describes tberelationship as inverse and not strong (-0.28231, = .08]. The relationship is shown in Table 4,where PTVs from -3 to -7 are seen to ref lecttorque values from 37.3 to 142,3 Ncm.

DiscussionThe two stated aims of this study were to deter-mine both the relationship between Periotest dataand three different implant biomaterials. and therelationship between Periotest data and fai luretorque. The relationship among tbe various bioma-

terials supports the hypothesis that PTVs are similarwith different biomaterial surfaces. When evaluat-ing the clinical literature that reports PTVs for aspecif ic type of implant, i t wi l l be important toknow if the data are specific to the implant bioma-terial or some other factor(s). The data from thisreport suggest that for unloaded implants assessedat 3 to 4 months postplacement, the PTV data areno d i f fe ren t be tween the th ree most commonimplant biomaterials (Cp t i tanium, t i tanium alloy,and HA-coated titanium alloy). These data do notsupport the inference that this relationship wouldnecessarily retnain following functional loading. Inotber words, though the PTVs start at the samelevel, the response of the supporting tissues mayhave b iomater ia l -spec i f ic in te rac t ions tha t a rerevealed by differing PTVs at later points in time.

The PTV should provide tbe clinician with infor-mation about tbe implant support that can be com-bined with other appropriate clinical data to arriveat a diagnosis. The Periotest value assigned to theimplant must be meaningfu l in a predict ive orprognostic sense. It has been implicated that valuesof torque failure may be considered predictive ofimp lant perform ance .' Fo llowing such reasoning^

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fcnolesl Viilucs, Bioinaierial, ^nd Torque lo Failure

higher torque failure values are considered indica-tive of favorable and predictive implant-bone con-ditions. Assuming this to be true, the relationshipof PTV to torque would be expected to be inverseand strong (a high torque related to a low PTV forthe majority of observations]- This relationship wasnot found for the implants measured in Ihis study,and Ihe data do not support the hypothesis relatedto torque. The linear regression of torque and PTVrevealed a significant difference. However, with anR-of on ly 8 % , this differen ce was not a strongassociation. Examples il lustrating this are takenfrom three different implants with torque values of0.00 Ncm that exhibited PTVs of -A -4 and -3.Eor another implant w ith the same PTV of -4 , thecorresponding torque value was 173.5 Ncm. Thesensitivity and specificity of the diagnostic devicefor these observations are poor, and they serve toillustrate the statistical finding. In light of the weakassociation between torque and PTV, a significantcorrelation has been shown between torque andimplant type. '

Both the PTV and torque value are consideredindicators of biome chanical perfo rm anc e,' Thatthese indicators are predictive of functional perfor-man ce has not been demonstrated, Torque resis-tance is not the gold standard for functional perfor-mance, but it has been implicated as a means ofestablishing an appropriate healing response priorto deciding to connect abutments- '^ I t would bemost helpful to correlate the PTV and torque val-ues to a measure of the implant-bone response, orwhat has been referred to as percentage integra-t ion, to determine whether the parameters are pre-dictors of bone anchorage, ideally, this comparisonshould be made longitudinal ly using functioningimplants; however, this is difficult io do with sucha destruc tive p aram eter as torqu e- This placesgreater importance on data such as that providedby the Periotest device.The value of the Periotest data should be estab-lished by gathering data on a broad spectrum ofimplant-tissue responses, which include conditionsof health and disease, that are validated against agold standard. This would enable correlations tobe established that demonstrate the level of sensi-tivity and specificity of the diagnostic device. Themost im porta nt pred ict ions that must be estab-lished are those associated with tbe marginal orintermediate stability values. Diagnoses are moreeasily assigned to values at the extremes of thespectrum of stability or health. As has been statedbefore, the range from a clinica lly firm implant tojust tangible mobility represents the actual problemzone for diagnosis.- ' A fol low-up report to this

study wi l l present data corre lat ions for PTV,torque , and percentage integration. The study sam-ple does not include a wide spectrum of clinicallydetermined disease/health conditions, and there-fore may not allow the full spectrum of correlationsto be made; however, the correlations that will bemade will gain strength from the histologie goldstandard of reference.

ConclusionsGiven the limitations of sample size, period ofintegration, and possible physiologic variation withthe present model, the following conclusions aremade:

1, Per iotest data for unloaded, screw-shapedimplants of comm ercially pure titanium, titaniumalloy, and hydroxyapatite-coated titanium alloyare similar after a i- to 4-month healing period.2. The re lat ionship between torque and PTV,though significant, is not strong enough to useone parameter to predict another.

cknowledgmentsThe authors would like lo express their apprecialion (o Steri-Oss for Iheir gracious support of this ongoing project,BioRe5earch for the loan of the Periotest device, and Dr BobRashid for his assistance w ith the statistical analysis.

eferences1. Laster L, Laudenback KW, Stoller NH . An evaluation ofclinical looth mob ility, ] Periodonl 197S;46:603-607-2. Linde I, Nyman S, Occlusal therapy. In: Linde J led].Textbook of Cl in ical Periodontology, Copenhagen:Munksgaard, 1983:451-465.3. Poison AM , Meitner SW, Zander HA, Trauma and progres-sion o marg inal pe riodo nlilis in squirrel m onkeys 111Adaption of interprosimal alveolar bone lo repetitiveinjury. ) Periodont Res 976; ;279-289,4. Poison AM, Meitner SW, Zander HA. Trauma and progres-

sion of marginal periodonlilis in squirrel monkeys, IV.Reversibility of bone loss due to trauma alone and traumasuperimposed upon periodont i l is . | Periodonl Res1976;11:290-298,5. Lindhe ], Berglundh T, Ericsson I, Liljenbetg 6 , MarinelloC. Experimental breakdown of peri-implant and periodon-lal tissues. Clin Oral ImpI Res 1992;3:9-16.6. Berglundh T, Lindhe |, Marine llo C, Ericsson I, LiljenbergB. Soft tissue reaction to de novo plaque formation onimplants and teeth Clin Oral ImpI Res l992 ;3:1-87. Kohn DH . Overview of factors important in implantdesign.J Oral Implantol 1992:18:204-219.8. Sekine H, Komiyama H, Hotta H, Voshida K. Mob ilitycharacteristics and tactile sensitivity of osseointegratedfisture-supporting systems. In: van Steenberghe D (edi.Tissue Integration in Oral and Maxil lofacial Recon-struction,Amstsrdam:Excerpla Med ica, 1986 :326-33 2.

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9. Br i insk i IB . B io m e c h a n i c i l f a c l o r s a f f e c t in g th e b o n e -denta l imp lan i in te r face , CNn Mater 1 992;1 0 :153 -20 1.10. Lukas D , Schulte W , Fer iotestA dynamic procedure fo rt h e d ia g n o s i s o f l h e h um a n p e r i o d o n t i u m . C l i n P hy sPh y i io l Meas I 590 :1 1:65-7 5 .i 1, Lukas D,Schulte W , Knig M , Reim M . High-speed f i lm-in g o f the Pe r io te s t me a s u re me n t . JC l i n P e r i o d o n t o l19 9 2 ;19 :3 8 8 - .19 I .12 , Chavez H , O r t r r a n LF , DeFranco R L, Med ige J. Assess-

m e n t o f o r a l i m p l a n t n i o b i l i l y , P r o s l h e t D e n t1 9 9 3 ; 7 0 : 4 2 l - 4 2 6 ,13 . Ol ive J, Apa r ic io C. The Periotest method 3 i a nieasure o fo s s e o i n t e g r a t e d o r a l i m p l a n t s t a b i l i t y . In t | O r a lMax i f lo fac Imp lan ts 1990:5 :390-400.14 , Webber RL, Some ca ndid observations an d reflections o nthe results o f an in-vitro test o f the Siemens Periotest dev ice,Compen Cont in Educ Dent 1988; 2(supp[) :4 iaS-44]S.15 . M a n z M C ,M o r r is H F , O c h i S A n e v a l u a l i o n o f l h ePeriotest system: Examiner re i jabi l i ly a nd repeatabi l i ty o fread ings . Imp l jn l Dent 1992:1 :142-146.

16 . Carr AB , Larsen P, Fapazoglu E, M tG lum ph y EA.^ | ^ j,ns .t o r q u e f a i l u r e o f s c r e w - s l i a p e i n i p ' a n l s in ^^^ ^ ^^ ,Baseline data fo r a b ii tme n l t o rq u e a p p l iw t i o " -Maxi l lofac Implants 1995 ( inpress), ^ .^^ , . M a r t i n7 . Ma r t i n K B ,Bur r Df . M e c h a n i c a l P ^^P ^ ^ j A d a P ' J t i o nRG, Bur r DB [eds) . Sl r i i f tu re , ' ' ' j ^ f ' , e i > , 1 9 8 9 :o f C o m p a c t B o n e . N e w YorK.K7 5 - 7 7 . Lp ^ r r o rp h o me t r i c a nd18. Sennerby L, Thomsen P, E--' ^, n rabbi t

b iomechan ic compar ison o f t iia M ax i l lo f acc o rt ic a l a n d c a n c e l lo u s b o n e , i m i u r aImplants 1992;7 :62-71 . n la n f A p rac tica l p r i -

Periotest instrument, I Prosthod 1993;3.y IA2 1 , Teer l inck I, Q u i r y n e n M , Dar ius P, van S eenberghe D,Feriotest: A n ob jec t ive c l in ica l d iagnos is o f bone appos i -t i o n t o w a r d i m p l a n t s . i iU I O r a l M a x i U o f a c I m p l a n ts1 9 9 1 ; 6 : 5 5 - 6 1 .

Literature Abstrscts-C o m b in e d G u id ed T i ssu e R e g e n e r a t io n , R o o t C o n d i t io n in g , a n d F ib r in -F rb ronect in Sys tem App l ica t ion in the Trea tment o f G ing iva l Recess ion ,A 15-Ca5e Repor tThis paper descr ibes a me t i i o d of g j i d e d t is s u e re g e n e ra ti o n l o ach ieve roo t coverage f o rpat ien ts hav ing g ing iva i reoess ion ,4 t o 6 mm d e e p , w i t h u n d e r ly i n g b o n e d e h i s c e n c e itinvolves If ie u s e o l a n ePTFE membrane tha i re ta rds ep i the i ia l migra t ion a n d p ro mo te s c o n -nec t ive t i ssue a t tachment combined wi t f i te t racyc l ine H C icond i t ion ing o f the roo t sur face a n dth e use o f a f ibnn- f ib ronec t in sys tem app l ica t ion to p rom ote a t tachme nt . F i f teen pa t ien ts w i t i is imi la r i so la ted mucog ing iva l de fec ts were se lec ted fo r th is s tudy a n d we re f o l l o we d up to 6mo nths pos topera t ive iy Resu l ts show ed a d e c re a s e in d e p th o f g ing iva l recess ion f rom am e a n o f 4 .7 mm to 1 .1 mm, wi th roo t coverage averag ing a t 77 4 % a n d me a n w id th o f k e ra -t in ized t issue ga in f rom 1 .8 m m to 2 .9 mm These f ind ings show cons is ten t and favorab lees the t ic resu l ts wi l f i min ima l pos topera t ive pa in a n d root sensi t iv i ty .Trombel l i L, Schincagl ia G, Checchi L, Calura G,J Penot/oio 1994:65:796-803. Reierence s 34Reprints. Dr Leonardo Trombe iii. Departmentof Periodontology, Sctioolof Denlistry. University o( Ferrara.Via Savonaro la 9. 44100 Fer rara. I ta ly.Mar ta C. Kmg, DDS, New York V.A, Medical Center.Ne wYork.We w York

An in V it ro inves t iga t ion in to t i ie Wear E f fec ts o f Ung lazed , G lazed , andPo l ish e d P o r ce la in o n H u m a n En a m e lAbras ive v^ear was tested on unglazed, g laze d, and poi ished pcrcela in spe c imens wi th humanenam el as t i ie opposing p late spec imen s. The tests were conducted on a n apparatus spec ia l lydes igned to s imuiate the m ast icatory cyc ie The amount of wear ot enamel caused byglazed and unglazed porcelain is similar. However polished porce lain produces sub-stantially less enamel wear.This study a lso found Ihat the g lazed sur faces w ere removedafter a relatively short per iod of wear (2 hours). The results of this study confirm the potentialdamage porcela in can in f lic t on tooth ename l and reaff irms the need for caut ion when us ingporcelain for restoration of occlusal surfaces.J a g g e r , O C .J Prosthet Denl 1994 :72 :320-323 . Re te rences . 14 . Repr in t s : D . C . Jagger . Depar t r r en t o fPros t i iodont ics and Pe r i odon to logy, t jn iversi ty of Br isto l Denta l Scnoo l . Lower Ma udl in Street . Br isto l . BS 12LV, Un i ted K ingdom.Ha in i Wang, DDS, New York Veterans Adminislration MedicalCenter New York

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