The Three-DimensionalEffects of Adiustment

and Cementationon Crown Seating

Shane N. White, B Dent St, MS, MAAssistant ProfessorDepartment of Restorative DentistrySchool o Dentistry

Victnr Kipnis, PhDAssistant ProfessorSection of BiometryDepartment of Preventive MedicineSchool of Medicine

University of Southern California

This study investigated the three-dimensional nature of crownfit and the effects of adjustment and luting. Standardizedpreparations were made on extracted premolars andimpressions and dies made. Complete-coverage castings werefabricated using base metal alloy. The castings were placed ontheir respective teeth and measurements made. The castingswere then adjusted using a siiicone disclosing medium andremeasured. They were then assigned to one of five lutingagent groups and cemenled. A relative term describing tiltingwas defined. Casting adjustment decreased tilting andcementation increased tilting. Resin cements tilted castingssignificantly more than did zinc phosphate, glass-ionomer, orpolycarboxylate cements. lnl Prosthodont 1993;6:248-254.

M any studies have examined the fit of crowns asdetermined by vertical marginal opening.'-"Few studies have examined the three-dimensionalfit of crowns.'"" Several anecdotal references havebeen made regarding the three-dimensionalchanges in position that occur during crown place-ment and seating, but to the authors' knowledgeno study has yet attempted to quantify or qualifythese changes.

In a two-dimensional study on crown cementa-tion using zinc phosphate cement. Lange' notedthat all the crowns were tilted on the preparationsduring cementation (Fig 1). He suggested that thisoblique relationship of cemented crowns resultedfrom the collection of large cement particles incertain areas and the enhanced filtration of cementparticles in diametrically opposite areas. This phe-nomenon was also noted by |orgensen,^ who sug-gested that it could be related to cement film thick-ness and to preparation taper.

Reprint requests: Or Shane N. White, Assistant Professor, Depart-ment of Restorative Dentistry, use School of Dentistry, 925 WestThirty-fourth Street #4367, tos Angeles, California 90089-0I41.

White et al" studied the improvement in themarginal seating of cast restorations when a siii-cone disclosing medium was used. They noted lhatcasting adjustments made after the use of siiiconedisclosing medium improved fit and the castingstended to seat in more upright positions. Theystated that "Binding of the casting against the toothpreparation prevents seating on the binding sidewhile closing the margins on the opposite side.After adjustment, the binding side markedly im-

Fig 1 Diagram ot cemented crown.

lournal of Prosthodonlics 248 Volume 6,

Three-Dimensional Effeels of Adjuslmert and Cementation

proves, and the opposite side has a sligbtly greateropening."

It was bypotbesized tbat tbe initial fit of a crown,the adjustment, and the cement all may influencetbree-dimensional seating of ItJted restorations.The purpose ot this study was to determine tbeeffect of adjustment on crown tilting wben using asilicone disclosing medium and tbe influence offive luting agents on tbe tilting of cast restorationsduring placement.

Materials and Methods

Tbirty-five recently extracted intact human pre-molars were stored in 37C water. Standardizedcomplete crown preparations were performed us-ing a custom paralleling device. Tbe teeth wereancbored in a water-filled receptacle, and waterspray was used tbroughout tooth preparation. Amatched coarse diamond bur and 12 fluted carbidefinishing burs (RCBIIK24, Brasseler USA, Savannah,GA] were used. A total occlusal convergence ofapproximately 6 degrees, a preparation height of 4mm (measured at the midiacial surface of thetooth), and a chamfer finish line were used.

Thewet tootb preparations were burnisbed usinga damp cotton cioth prior to making the impressionand before each seating evaluation to remove anyplaque that might have formed during storage inwater. Impressions were made using a polyivinylsiloxane) material (Reprosil, Dentsply, Milford, DE).Light body impression material was injected aroundthe teeth, wbicb were tben placed in trays contain-ing heavy body material. Dies were cast in improvedstone (Die Keen, Columbus Dentai, St Louis, MO)and trimmed under x 10 magnification. The sur-faces apical to tbe finishing line were coated with acyanoacrylate adbesive (910 Adbesive, NationalStarcb and Chemicai, Engiewood, N|). The dieswere painted witb tbree coats of die spacer (Cementspacer, Beiie de St Ciaire, Cbatsworth, CA) to witbin1 mm of the finisbing iine, wbere no spacer wasadded. Tbe dies were iigbtiy iubricated (Die-Lube.IM Ney, Bioomfieid, CT|, copings were formed us-ing a dip wax tecbnique (Dura Dip Wax, Belle de StClaire], and the margins were waxed using biueiniay wax (Kerr Manufacturing, Romuius, Ml) usingX 10 magnification. Reference marks were placed inthewax pattern at tbe midfacial, iinguai, mesial, anddistai surfaces of each coping. Tbe copings weresprued and invested in a phosphate bonded invest-ment (Cera-Eina, Whipmix, Louisviiie, KY). A pilotstudy using a stainiess steei die' determined anoptimai special iiquid to powder ratio of 14.5 cciiquid : 50 g powder. Wax was eiiminated in an oven

(|M Ney) using a temperature riseof30E per minutefrom room temperature to feOO^E wbere it was beidfor 1 hour, then raised to 1,350E and heid for anadditionai bour. The mold was cast using base metaiaiioy (Rexiiiium III, Jeneric/Pentron, Wallingford,CT] melted in an induction casting machine (ECMiil , Howmedica, Chicago, IL).

After tbe castings were divested, the sprues wereremoved using siiicon carbide disks (Highspeedseparating disks, )elenko, Armonk, NY) and fin-ished externaliy using biue, green, and brown rub-ber wbeeis (Dedeco, Long island, NY]. Internalpositive defects were removed using a balf-roundbur under x 10 magnification. Tbe castings werethen initiaiiy adapted to their dies. The internalsurfaces were air abraded using 50-fi.m M0, andareas of hypercontact were identified by seatingtbe casting on its die coated with a thin iayer of dieiubricant. Wet, dark, shiny contact areas were ad-justed using a baif-round bur. Tbis procedure wascarried out twice.

Each casting was piaced on the naturai preparedtootb in a piston device and axiaiiy ioaded with aforce of 49N'-' provided by an eiastic band. Thisforce was checked after measuring to verify tbat nostress reiaxation had occurred. Marginai openingwas measured at the four reference marks on eacbcasting and was determined as the verticai openingin micrometers between the most apicai part of tbeprepared tooth margin and the most apicai portionof the casting margin (Eig 1]. Tbree independentmeasurements were made at eacb point by differ-ent operators. Eine focus, a tungsten iight source,mirrors, and f iiters were used to visualize tbe mar-gins. Tbe measurements were made at x 500 mag-nification using a metaiiurgic microscope (BHM],Olympus Opticai,Tokyo, lapan] andadigital travei-ing micrometer with an accuracy of 4 p.m (MitutoyoMFG, Tokyo, Japan). InterOperator reliability wascaicuiated.

Einal fitting of tbe castings to their naturai teetbwas tben achieved using a siiicone disclosing me-dium (Fit-Ghecker, GC Corp, Tokyo, japan); areasin wbicb the metal showed tbrougb the siiiconewere adjusted using a haif-round bur." This proce-dure was carried out twice, Tbe marginai openingswere then remeasured as described above.

The copings were tben randomiy assigned to oneof five luting agent groups: glass-ionomer (Gl](Ketac-Cem, ESPE, Seefeld/Oberbay, Germany];polycarboxyiate (PG) (Durelon, ESPE); zinc phos-pbate (ZP) (Eiecks zinc pbospbate. Keystone,Cberry iHiii, N|]; microfilied bisphenoi glycidyimethacrylate resin witb an oxaiate dentin bondingagent (GMA-I-NPG] (Thin Eiim Cement and Tenure,

249 atioral Journal of Prothodontics

-fhree-Oimensional Effects of Adiustment and Cementatii White/KipniE

Den-Mat, Santa Maria, CA); and microfilled bis-phenol giycidyl methacrylafe/phosphate ester resin(GMA/PF) (Panavia Fx, Kuraray, Okayama, Japan),Thecastingswerelufed to their teeth and placed ina custom paralleling jig. The samples were sup-ported in a vertically upright position and a flatplunger applied a vertical force of 49N'^ ' against thedomed occiusai surfaces,-After 7 minutes fhe spec-imens were removed from the jig, and the excessluting agent was removed. The marginal openingswere remeasured after the specimens were storedat37Cim0O% humidity for 24 hours,

A relative term to describe tilting was defined inthe following way. Each of the four individual mea-surements on a single specimen were subtractedfrom its mean, giving four individual deviationsfrom the mean. These deviations were squared,added together, divided by fhe number of degreesof freedom (3), and the square root derived. Thisrelative term describes the relationship of the indi-vidual measurements to their mean. If there is notilting the relative term will be zero, and as tiltingincreases the differences between the four individ-ual measurements will increase, as will the relativeterm. This procedure is the same as that for deter-mining standard deviation, which is used to mea-sure how much individual terms differ from theirmean.

Analysis of variance (ANOVA) was applied to eval-uate fhe differences in tilting resulting from theinitial fitting, adjusted, and cemented status of thecastings, ANOVA was based on a split-plot factorialdesign with individual crowns as blocks becausemeasurements of tilting in the initial, adjusted, andcemented conditions were from the same crownsand the five groups with different cement types

were independent. Therefore, each specimenserved as its own control because it was subjectedto the three treatment states. This pairing tech-nique also removed the effects of idiosyncrasiesassociated with particular castings or teeth from theanalysis. The different luting agent groups repre-sented five treatment levels of the first (between-biock) factor, and differences resulting from statusrepresented three treatment levels of the second(within-block) factor. Overall significance arisingfrom each factor and the interactions was estab-lished at the 5% level. As significant interactionsbetween the factors preclude an assessment of theindividual factors, follow-up simple main effectANOVA tests were performed if interactions werefound. The same experiment-wise significancelevel as used ior the main effect ANOVA was main-fained by adjusting fhe significance levels for thesefollow-up tests according to Bonferroni's inequal-ity, A significant difference resulting from any fac-tor was followed by a multiple pair-wise compari-son using the Tukey's Studentized range methodwith the experiment-wise level the same as for thecorresponding simple main effect test.

Table 1 Interclass Correlation Coefficients ofReliability for Marginal Openings With Different LtitingAgents and Casting Status (n = 140)

Casting status

InitialAdjustedCemented

ZP

.994

.998

.997

Gl

.984

.983,997

Luting agent

PC GMA-I-NPG

ID

ID

IDID

ID

ID

CD

CO

Ul

.989997.999

: GMA/PE

,990,995,998

Table 2 The Means, Standard Errors, and Relative Coefficients of Variation (RCV)of the Relative Tilting Terms of Different Crowns (n - 7)-uting agent

ZP

Gl

PC

GMAflMPG

GMA/PE

The Irternatiora

Casting status

InitialAdjusted

CementedInitial

AdjustedCemented

InitialAdjusted

CementedInitial

AdjustedCemented

InitialAdjusted

Cemented

lournal of Proslhodontics

Mean

50.4044.3257,2736,43t9.7057.5450,844t,8467,3850,4045.S0

160,4t35.7921.77

203.21

250 Volu

Standard error

8,868,54

14.608.876.22

14 4115.7812.4430,5012,489.73

30,068.785,76

68.07

me G, Nurr' "'""'^

RCV %

17.5819,2625,4924.3531.5825,0431,0329.7445.2724.7621.261B,7424.5426,4733.50

While/Kipni r h rcc-Dimensional Effects of Adjuslment and Cementation

Results

The interexaminer reliability analysis revealedvery high reliability among three observers for allconsidered groups and casting status (Table 1). Thecorresponding ntraclass correlation coefficientsvaried from .997 (Gl as cemented) to .999(CMA+NTC as cemented), allowing the ANOVAtests that followed to be based on the averages ofthe three respective measurements on marginalopenings.

The mean relative tilting terms, standard errors,and relative coefficients of variation of the relativetilting term for each group (initial, adjusted, andcemented) are displayed in Table 2 and Fig 2. Allgroups demonstrated the same trend in mean rela-tive tilt. Tilting decreased after adjustment and thenincreased beyond the initial level after luting.

The main effect ANOVA demonstrated a highlysignificant interaction (P = .0018) between thechange in relative tilting resulting from casting sta-tus and cement group (Table 3), Therefore, follow-up simple main effect ANOVA tests were per-formed and the results are provided in Table 4.Crown tilting was not significantly different amongcement groups in the initial fitting and adjustedstates (P > .5). This indicates that bias resultingfrom orientation or other facfors had not beenintroduced, confirming that the treatment groupshad all been treated equally fotbis point. However,

Table 3 Main Effect ANOVA (a =Source of variation

Luting agentCasting statusInteraction between iuting agent

and casting status

.05) (n = 35)P

.0812< .0001

-0018

Table 4 Simple Main Effect ANOVA(adjusted a = .0188)Testing for:

ZPGlPCGMA+NPGGMA'PEInitialAdjustedCemented

Source of variation

Casting status'

Cement group"

P

> .5.25 < P < ,50

> 5.0007

< .0001>.5> .5< .0001

N = 7." N = 35.

there was ahighly significant difference (P< .0001)in crown tilting among different luting agents aftercementation. Significant differences in crowntilting in the initial fitting, adjusted, and cementedstates were found in only two individual cementgroups: CMA + NPG (P = .0007) and GMA/PEiP< .0001),

Fig 2 The means and standarderrors of tbe relative tilting mea-sures of ditferent crowns.

2nc Glass PolycarboKylate GMA- NGTPtiospbate lonomer resin

Luting Agents

GMA/PEresin

. Mumber3,t993 251 The International loiirnal o( Prosthodontics

Three-Dimensional Effects of Adiustment and Cemenlatii

The Tukey's multiple comparisons test for differ-ent status of castings (Tabie 5) showed that thedifference between GMA-fNPG and MA/Pli re-sulted trom the significantly less upright positionsof the castings after cementation, while tilting afteradjustment was not significantly iess than for theinitiai condition. The same test applied lo differentcement groups (Table 6) demonstrated that thechoice of cement influenced the tilting of ce-mented castings. Tilting was the least for ZP, fol-lowed by Gl and PC, although these three groupswere not significantly different from each otber.The GMA+ NPG and GMA/PE groups tilted crownssigniticantiy more than ZP, Gl, or PC, but were notsignificantly different from each other.

Discussion

The extremely high interoperator reliability coef-ficients show that interoperator differences, andhence intraoperator differences, had no effect onthis study. The high interoperator reliability coeffi-cients suggest that the digital travelling micrometermay be more accurate than its manufacturer statesand that the individual examiners were in goodagreement with one another.

The condition of the casting, ie, initial vs adjustedvs cemented fit, significantly affected tilting (Tables4 and 51- The same trends were found in all lutingagents (Table 2, Fig 2), Adjustment using a siliconedisclosing medium significantly decreased overalltilting and cementation increased overall tilting(n = 35); however, the sample sizes were too smallto demonstrate statistically significant changes forall the luting agents separately (n ^ 7) (Table 4|,Significant changes were demonstrated for theresin groups GMA-i-NPG and GMA/PE whichcaused more tilting of the castings (n = 7) (Table 5).Retrospective calculations suggested that signifi-cant changes might have been demonstrated forluting agent groups ZP, Gl, and PC if a sample sizeof 20 had been used for each.

The value of the relative tilting term of an individ-ual specimen is dependent on the orientation ofthe four measurements to the maximum and mini-mum opening. However, the entire population ofspecimens was treated equally and assigned ran-domly to treatment groups, ANOVA compares Ihemeans of groups, not their individual members, sodifferences in orientation are averaged out withineach group and do not affect the analysis. Theabsence of bias among groups was confirmed bysimple main effect ANOVA (Table 4], Not only didthe directions o the original tiltings occur at ran-dom, but changes in direction resulting from the

Table 5 The Tukey's Multiple Comparison of RelativeTilting Tertns Between Different Casting Status(adjusted a = ,0188) (n = 7)Luting agent Casting status Mean skewed ness

GMA-i-NPG

GMA,'PE

InitialAdjustedCementedInitialAdjustedCemented

45.8050,40

160,4121,7735.79

203,21

Casting states connected by vertical lines are rot significantiv different.

Table 6 The Tukey s Multiple Comparison of RelativeTilting Measures Among Different Luting Agents asCemented (adjusted a = 0,188) (n = 7)Luting agent Mean relative tilting

ZPGlPCGMA-FNPGGMA,'PE

57,2757,5467,38

160,41203,21

Luling agents cornecled by vertical lines are not significantly different.

two levels of treatment also occurred randomly.The value of the relative tilting term of an individ-

ual specimen is dependent on the number of mea-surements made. Three measurements are theminimum necessary to describe a plane. For ease ofstandardization, placement of reference marks,and measurement, all specimens in this study weretreated equally and were measured at four refer-ence points.

Units have not been assigned to the tilting terms,because they are relative, not absolute, measuresof tilting. If a unit were to be associated with therelative term it would be (j.m,as this was the originalunit of measurement.

Alignment in engineering is usually determinedby linear measurements that can be trigonometri-cally converted to angles. Direct angular measure-ments are difficult to make unless large distancesare involved. If angular measurements are to bemade to describe a similar relative angular tiltingterm, three measurements would be necessary. Ifan absolute measurement of displacement were tobe determined, six measurements, with five de-grees of freedom, would be necessary.

The mean marginal opening and the standarddeviation of the entire population of castings priorto cementation were 34 ixm and 17 j^.m, respec-tively. This group of castings was within the range ofclinical acceptability.

The Interiiational lojrnsi of Prosthodontics 252

Whit/Kipnis iional [Itecls ol Ad|Lslment and Ce

The initial castings were tilted. This misorienta-tion could be partly accounted for by differences inmarginal configuration of castings caused by labo-ratory procedure variables. As the castings wereunable to rotate independently of the teeth onwhich they were placed, these differences in mar-ginal configuration remained constant throughoutthe study and did not affect the "paired" split plotanalysis. The interexaminer variability was so smallas to be insignificant (Table 1).

The exact clinical significance of tilting is notknown. However, it is reasonable to assume thattilting is undesirable, and that clinicians shouldattempt to minimize it through choice ot techniqueor luting agent. Greatly tilted cemented castingswill have some margin areas much more open thanothers. These widely open areas are more prone tocement failure and subsequent disease. Internalthicknesses of cement also vary, exposing someareas to greater stresses.

The tilting of the castings luted with resin ce-ments was so great that occlusal contacts could besignificantly deranged horizontally. Resin cementsalso have greaterfilm thicknesses and much poorermarginal seating than other cements, causing sig-nificant vertical changes.'-'^ Therefore, castingsluted using resin cements will require careful oc-clusal examination and adjustment following ce-mentation.

Different methods of applying seating forcemight produce different amounts of tilting. Mostcurrent knowledge of casting seating and cementa-tion is derived from quasi-static axial loading situa-tions,'" " so the quasi-static axial method was usedin this study. When the simplest situations havebeen fully understood more complex variationsshould be investigated. The dynamic loadingmethod might increase seating and decreasetilting." This method should be further investigatedwith reference to tilting and a wide variety of lutingagents.

Siiicone disclosing media improve the marginaladaptation of castings."" They also have the addi-tional advantage of decreasing the tilt of full cover-age castings.

The results of this study are in agreement withthe observations of Lange' and orgensen-, whonoted that cementation with zinc phosphatecaused castings to become tilted, and with those ofWhite et al", who noted that adjustment using asiiicone disclosing medium resulted in a partialuprighting of tilted castings.

Prior studies on film thickness and on marginalseating showed that resin cements, CMA/PE andGMA-i-NPC, had significantly greater film thick-

nesses and poorer marginal seating than lutingagents."" Tilting is undoubtedly related to filmthickness and marginal seating.

This study used natural teeth and simulated rou-tine clinical and laboratory procedures. Despite theeffects of all the possible variables, clear trendswere found.

Conclusions

Complete crown restorations were adjusted us-ing a siiicone disclosing material and subsequentlyluted to natural teeth using one of five differentluting materials. Within the design of this study andthe limitation of the number of castings evaluated,the following conclusions may be made:

1. A relative term capable of describing three-dimensional tilting of castings was defined.

2. The three-dimensional tilt of a routinely madecrown was affected by its initial fit, internal ad-justment, and luting.

3. Adjustment with a siiicone disclosing mediumtended to upright crowns.

4. Luting tended to tilt crowns.5. Resin cements tilted crowns significantly more

than glass-ionomer, zinc phosphate, or polycar-boxylate luting agent5.

References

1. Lange F. Forsog over cementering af finerkroner (Experi-ments on cementing of crowns). Tandlaegebladet 1955;59:181-186.

2. lorgensen KD. Factors affecting the film thickness of zincphosphate cements. Acta Odontol Scand 1960;18:479-490.

3. Fusayama T, Ide K, Hosada H, Relief of resistance of cementof full cast crowns. | Prosthet Dent 1964; 14:95-105.

4. Iones MD, Dykema RVV, Klein Al. Television micromeasure-ment ot vents and non vented cast crown margin adaption.Dent Clin North Am 197l;15:663-678.

5. Beiser UC, Mactntee Ml, Richter WA. Fit of three porcelainfused to metal marginal designs in vivo: A scanning electronmicroscope study. | Proslhet Dent 1985; 53:24-29.

S. Cooney |P, Richter WA, MacEntee Ml. Evaluation of ceramicmargins for metal-ceramic restorations. | Prosthet Dent1985; 54:1-5.

7. Hung SH, Hung KS, Eick |D, Chappeli RP. Marginal fit ofpoicelain fused to metal and two lypes ol ceramic crown. |Prosthet Dent 199n;63:26-31.

8. Byrne C. Influence of finish-line form on crown cementa-tion, Intl Prosthodont1992;5.137-144,

9. McLean |W, Von Fraunhofer |A. The estimation of cementfilm thickness hy an in vivo technique. Br Dent ] 1971;131:107-111.

10. Kelly |R, Davis SH, Campbeii SD. Non destructive threedimensional internal fil mapping of fixed prostheses. | Pros-thel Dent 1989;61:368-373,

f 6, Number 3,1993 253 The International Journal of Prosthodonlii

Thrcc-Dimensional Etteas of Adjuiinent and Cementation Wliite/Kipris

11. While SN, Sorensen |A, Kang SK. Improved marginal sealingo cast restorations using a silicone disclosing medium. Int JProsthodonl 1991 ;4:333-326.

12. While SN, Yu Z. Fiim Ihiciiness of new adhesive lutingagenls. | Proslhet Deni 1992;67:782-785.

13. While SN.YuZ, KipnisV.Theelfect o adhesive lLlingagenl5on (he marginal seating ol casi restorations. J Prasthet Deni1993;69:28-l.

14. Rosenstiel SF, Gegauff AC. Improving the cementation ofcomplete cast crowns: A comparison of sealing methods. JAm Dent Assoc 198B;117:B4S-e48.

15. Davis SH, Kelly |R, Campbell SD. Use ol an elastomerii:material lo improve the occlusal seat and marginal seal orcast restorations. | Prosthel Dent 1989;62:288-291.

Ulerature Abstracts

Swaliowing Activity of Lip Muscles in Patients With aComplete Upper and a Partial Lower Deniure

Previous prosthodontic literature has dealt with facial muscle activity in subjects provided withcomplete dentures. The present investigation used electromyography to study patients provided withimmediale complete upper and parlial lower distal extension dentures. The swallowing activity of theorbicularis oris and menlalis muscles in relation to the anterior temporal muscle was investigated 1)before extraction of the residual maxillary anterior dentition, 12) after denture placement, and O)during 2 years of denture use. Muscle activity was studied during the swallowing of saliva and water. Ingeneral, dominant eleciromyographic activity was recorded in lower lip and mentaiis muscles incomparison with upper lip and anterior temporal muscles at all stages of the investigation. The strengthof muscle activity showed no significant changes during the 2-vear ohservation period. This result is likelyrelated to preservation of the residual anterior denlilion in Ihe mandible of the present sample. Thisfinding differs from previous reports in which the rnuscle activity of complete denture wearerssignificantly increased over time and was presumably related to the need for stabilisation of themandibular denture due to markedly decreased retention. tHowever, the duration of swallows in thepresenl sample showed a marked increase following the first year of denture use. This increase is likelyassociated with the need to stabilize the maxillary complete denture due to continuing rsorption ofthe maxillary ridge.

Taligren A. Tryde, C/O'a'Reiaoi/1992^9:329-341. Heference: 14. Reprints: nr Antje Tallgren, Deparrment ot DentalProBthetics, School ol Dentistry, University of Copenhagen, 20 Norre Alle, DK 2200, Copenhagen N.Denmarii. David R Cagna, DMD, Depsrlmenr of Prosrhodontics, The Unnenity of Tenas Health Science Center si SsnAntonio, San Antonio, Texas

Bone Formation Over Partially Exposed implants UsingGuided Tissue Generation

The purpose of this pilot study was to test, using the dog tibia, whether covering the exposed cervicalportion of both titanium implants and hydrcixylapatite IMA)-coated implants with an expandedpolytetrafluoroelhylene le-PTFCl membrane would result in new bone formation on fhe exposedsurface. Twenty threaded titanium and ten HA-coated implants were placed in the tibias of fivemongrel dogs. The implants were placed in a manner that left the cervical 2 to i mm exposed. Fifteenimplants were used as controls and Ihe rest were covered with an e-PTFE membrane. Dogs weresacrificed after 6, 8, and 12 weeks. Histologie ewmination of the implants showed a progressiveformation of new bone over the exposed surfaces of those implants covered by the e-PTFE membrane.The HA-coated control implants, in contrast lo the titanium controls, also showed a progressivegrowth of new bone. No significant difference in the quantity of new bone was observed beiweenmembrane-covered and control HA-coated implants. The aulhors conclude lhat guided tissue generationusing an e-PTFE membrane induces new bone formalion over the exposed portion of an implant, that theamounl of new hone formed is influenced by the width of the space between Ihe membrane and the implantsurface, and that new bone forms on an exposed HA-coaled implanl even in Ihe ahsence of a guidingmem bran e-

Arora BK, Worley CM, Guttu RL, Laskin DM, ; OrsI Mnxillofac Surg 1392;50(1111;106-10bS. References: 24. Reprints; DrDaniel M. Laskm, Deparlment of Oral and Masillolacial Surgery, Box 566 MCV Station, Medical College of Virginia,Riebmona, ^A 23295. Hichid K. Seals, Ir, DDS, MEd, MS, Department of Prosthodontics, The University of TexasHealth Science Center at San Antonio, 5an Antonio, Texas

The Inlernational lournal of Prosthodontii 254