Load Fatigue ofTeeth Restored With

Cast Posts and Cores andComplete Crowns

Warren I. Libman, DDS, MSD'

jack I. Nicholls, PhD*'

Department of Restorative DentistrySchool of DentistryUniversity of WashingtonSeattle, Washington

Twenty-five extracted human central incisors were divided into five groupsand prepared for complete cast crowns. Test teeth had cast dowel coresfabricated with the ferrule height varying from 0.5 to 2.0 mm in 0.5-mmincrements. The five control leeth did not have cast dowel cores. A 4.0-kgload was applied to each of the restored teeth at an angle of 135 degrees tothe long axis of each tooth. This load was applied cyclically at a rate of 72cycles per minute. The load application point was predetermined by awaxing jig that was used to wax all crowns. The primary variable was theferrule length. The independent variable was the number of load cyclesrequired to create preliminary failure. Preliminary failure was defined here asthe loss of the sealing cement layer between crown and tooth. An electricalresistance strain gauge was used to provide evidence of preliminary failure.The results of this study showed that the 0.5 mm and 1,0 mm ferrule lengthsfailed at a significantly lower number of cycles than the 1.5 mm and 2.0 mmferrule lengths and control teeth. Int J Proithodont I995;8:l 35-16!.

R esearch has clearly disputed the belief thatfoundation restorations reinforce endodonti-

cally treated teeth.'"* It has been suggested, how-ever, that they provide a replacement for losttootb structure to increase the retention andresistance form of a complete crown prepara-tion.'^ Various foundation materials and tech-niques have been directly tested for retention andresistance to fracture. Clinically, however, thedifferences are of l imited significance, sincefoundations are often covered by crowns thathave been shown to negate these differences.'-^The marginal area of a complete crown whenextended onto the tooth structure beyond thesefoundation materials creates a ferrule. The ferruleIs defined as a metal band or ring used to fit theroot or crown of a tooth.' Fatigue is tbe breaking

'Acting Assistant Professor."Professor.

Reprint requests: Dr ¡ack I. Nicholls, Department of RestorativeDentistry, 5choo! of Dentistry SM-56, University of Washington.Seattle, Washington 9B195.

or fracturing of a material caused by repeatedcyclic or applied loads below the yield limit.'Intraoral occlusal forces create this dynamicrepetitive loading; thus, instead of a monotoniestatic load to fracture, it would be more clinicallyrelevant to test specimens under a physiologicfatigue load.

Fatigue life is a term used explicitly in fatigue lit-erature to define the number of load cyclesrequired to fail a specimen for a given appliedload. In this research, fatigue life refers to a prelim-inary failure condition as opposed to completefracture of the specimen. This preliminary failure isdefined as the point at which a failure of the lutingcement occurs that c l in ical ly wi l l result inmicroleakage between the crown and the tooth,'"Such microleakage may eventually be manifestedas recurrent caries, loss of retention, fracturedposts, or complete dislodgment of the crown andfoundation restoration." "

This research investigated maxillary centralincisors restored with cast posts and cores andcomplete cast crowns. Four different ferrule lengthswere investigated: 0.5 mm, 1.0 mm, 1.5 mm and

^I.Í; B, Number 2, 1995 155 The Iniernaiional iournal of Prosthodonlii

Load Fstigue ol Cast Posts ^ Cores and Complete Caii Crowns

2.0 mm. Preliminary failure defined the comple-tion of each test. The controls were central incisorswith complete cast crowns, without posts andcores.

Materials and Methods

Test Croups

Twenty-five extracted human maxillary centralincisors were used in this study. These teeth wereselected on the basis of lack of cracks, fractures orcaries in the cervical area or root. The teeth werekept hydrated in room temperature distilled waterprior to the study, and were wrapped in a wetgauze during tooth preparation. All fatigue testingwas carried out with the teeth immersed in room-temperature water.

The teeth were randomly divided into fivegroups of five teeth each:

Control, No cast post and core, restored withcomplete cast crowns.

Croup 1. Restored with cast posts and cores andcomplete cast crowns with a 0.5-mm ferrule.

Group 2. Restored with cast posts and cores andcomplete cast crowns with a 1.0-mm ferrule.

Group 3. Restored with cast posts and cores andcomplete cast crowns with a 1.5-mm ferrule.

Group 4. Restored with cast posts and cores andcomplete cast crowns with a 2,0-mm ferrule.

Figure 1 shows these five groups diagrammatically.

Post and Cores Fabrication

All teeth in the four experimental groups had thecoronal section removed level with the proximalCFJ, using a supercoarse diamond instrument(Brasseler USA, Savannah, GA) and copious irriga-tion. This prepared surface was further refined usingwet 600 grit abrasive paper creating a planar sur-face perpendicular to the long axis ofthe root. Thecanal space was then prepared to a depth of 8 mmfrom the occlusal flat prepared surface to a finaldiameter of 0.05 inches (1.25 mm) using twist drills(Goitene/Whaledent, Mahwah, N|). An antirota-tional notch measuring 1.0 mm occlusoapicallyand 1.0 mm buccolingually was placed in the topflat surface on the lingual of the canal space prepa-ration. The sharp junction between the top flat sur-face and the canal preparation was beveled with agreen stone (Shofu, Menlo Park, CA). Three reten-tive notches were placed in the external root sur-face of each tooth to hold the tooth firmly in theholder during fatigue testing. Each tooth was

Libmjn/Nicholls

attached to a surveyor rod and lowered into thecenter of a machined aluminum cylinder whichacted as the holder. The center of the post spacewas parallel to the axis of the machined cylinder.All teeth were embedded to a specific depth in theirindividual aluminum cylinders and retained withautopolymerizing resin (Duralay, Reliance, Worth,IL). Teeth were positioned with the crown margin2.5 mm above the resin surface.

A 1.25-mm plastic burnout pattern (Goltene/Whaledent) was inserted into the 8-mm preparedcanal space and a custom post and core patternwas constructed using autopolymerizing resin (GCPattern Resin, GC America, Scottsdale, AZ). Theteeth with the foundation restorations in placewere then prepared for complete crowns using acoarse diamond instrument (Brasseler 6847/016).The preparation was refined under lOX magnifica-tion using a Brasseler 8847/016 fine diamond. Thecontrol teeth which had no foundation restoration,had a preparation height of 6.0 mm. The core por-tion of the post and core restoration on all experi-rnental teeth was 6.0 mm in height. On the experi-mental teeth the preparation finish lines werelocated 0.5 mm, 1.0 mm, 1.5 mm, and 2,0 mmapical to the core, giving preparation heights of 6.5mm, 7.0 mm, 7.5 mm, and 8.0 mm, respectively. Itshould be noted here that providing a ferrulelength increase for the tooth with a questionableferrule may be achieved in one of two ways: (1> byperiodontai crown lengthening, or (2) by orthodon-tic extrusion. Since crown lengthening seems to bethe more commonly used procedure, this was usedin this study. In doing so, however, the cast crownsincreased in occlusocervical dimension withincreasing ferrule length.

The finish lines were circumferential shoulders1.0 mm in width. The ferrule height was measuredat eight locations around the periphery of thepreparation during the refining procedure. All mea-surements were made with a dial caliper having anaccuracy of 0.005 mm and measured using 10Xmagnification. The ferrule was measured at eightlocations around the preparation, and an experi-mental accuracy of ± 0.05 mm obtained. The pat-terns were invested (Complete, JF Jelenko. Armonk,NY) and cast (Midigold Hard, Ivoclar/Williams,Buffalo, NY). The castings were then inspectedunder 20X magnification and adjusted to assure apassive fit. The cast dowel cores were then air-borne particle abraded using 25 \irr\ aluminumoxide under 3 kg/cm' pressure and luted under fin-ger pressure using zinc phosphate cement (Fleck's,Mizzy, Cherry Hill, NJ) mixed according to manu-facturer's directions.

The International ¡oiirnal of Prosthodontics 1 5 6 Volume B, Nu.

übmaiVN icholl s Load Fatigue oí Cssl Posts & Cotes a ntl Complete Cast Crowns

Control Group 1 Group 2 Group 3 Group 4

Fig 1 Diagrammatic representation of the four test groups and the control group.

Cast Crown Fabrication

Impressions of the final tooth preparations weremade using a polyether impression material(Impregum, ESPE/Premier. Norristown, PA) andpoured in Type IV dental stone (Fujirock, CCAmerica), Two coats of die spacer (PDQ, WhipMix, Louisville, KY) were applied to the dies, 'vVaxcopings (Maves #3 Inlay wax, Maves, Cleveland,OH) were fabricated on the dies and were trans-ferred to the corresponding test specimens, whichwere positioned in a custom waxing jig (Fig 2).This waxing jig provided a standardized locationor notch for the gold crown where the fatigue loadwas to be applied. This notch was located 1,0 mmincisai to the incisai edge of the core (see Fig 1).After waxing the notch location, each pattern wastransferred back to the appropriate die, and thecrown contours were completed, invested, andcast. Following casting and devesting, the crownswere inspected under 20x magnification for fit.When fit had been ascertained, the internal of thecrowns was airborne particle abraded using 25 |jmaluminum oxide under 3.0 kg/cm- pressure. Thecrowns were cemented to the teeth wilh zinc phos-phate cement, mixed according to manufacturer'sdirections. During tbe cementing procedure, eachcrown was held in place for 10 minutes under a10- kg load.

Fig 2 Waxing ¡ig showing notch where fatigue load wasapplied.

Tooth Dimensions

Prior to iuting of the post and core, the bucco-lingual dimensions of each tooth at the crown fin-ish lines were measured and recorded.

Fatigue Loading Device

The fatigue loading device was designed andfabricated at the University of Washington, and hasbeen previously described,'' In keeping with the

157 The Inlemalional [ournal of ProBl

Losd Fatigue of Cast Po is and Complete Cast

Fig 3 Strain gauge on lingual surfaceof a test tooth.

loading conditions applied in vivo, the teeth usedin this study vi'ere subjected to a fatigue loadapplied at an angle of 135 degrees to the long axisof the tooth," A 4-kg load was applied to the load-ing notch on each of the test crowns at a frequencyof 72 cycles per minute." This load is within therange of physiological masticatory forces,"" Alltest and control teeth were immersed in a room-temperature water bath during fatigue loading.

Strain Gauges

The strain gauges (EA-06-062AP 120, Micro-Measurements Division of Measurements Group,Raleigh, NC) had an internal electrical resistanceof 120 ohms, and had a grid size of 0,062 inches{1,6 mm).

The lingual crown/tooth interface was lightlyground with a diamond disk to create a flat surfaceand airborne particle abraded using 25 \¡rr\ alu-minum oxide at a pressure of 3 kg/cm'. This sur-face was then cleaned with acetone, and a straingauge was cemented to place using M-Bond straingauge cement IMicro-MeasurementsI ensuring thatthe strain gauge grid was cemented over thetooth/crown interface. Following cementation,wires were soldered to the strain gauge terminals{Fig 3), and the strain gauge was completely cov-ered with impression tray adhesive {Mirror 3, KerrManufacturing, Romulus, Mil to ensure exclusionof water during fatigue testing.

Definition of Preliminary Failure

Preliminary failure was defined as the propaga-tion of a crack in or around the luting cementlayer. This crack was initiated at the lingual marginof tbe crown and propagated up tbe lingual axialwall. From a feasibility study, it was found that theexistence of a crack and the micromovement of thecast crown relative to the margin was not dis-cernible to the unaided eye. Propagation of thiscrack was visually observable in terms of the out-put on a model 455V Gould Chart Recorder{Clevite Corporation, Cleveland, OH) monitoringthe strain gauge output.

Measurement of Preliminary Failure

The strain gauge on the lingual margin of thetooth/crown interface was connected as one arm ofa Wheatstone Bridge circuit. According to themathematics of the Wheatstone Bridge, tbe voltageoutput of this bridge is proportional to the move-ment of the lingual margin of the crown withrespect to the finish line. This output voltage wasrecorded on a Gould Chart Recorder {CleviteCorporation), The tracing on this chart recorderdepicted the relative micro-movement between thelingual margin of the crown and the finish line.Initially, the recorded amplitude was small andregular, with the tracing returning to zero when thetooth was unloaded. This indicated that the move-ment of the cast crown was elastic and that nocrack had formed in the cement. Once a crack inthe cement layer had formed, the amplitude of thetracing on the Gould chart recorder increased.Finally, the movement of the crown marginreached a magnitude that was beyond the range ofthe strain gauge, A diagrammatic representation ofthe strain gauge output at preliminary failure isshown in Fig 4,

Verification of a Failure Crack in the CementLayer

The failed teeth were examined under 20X mag-nification, but no visible evidence of the existenceof a crack was seen. Verification of this crack wasobvious when a drop of water was placed on thelingual margin and intermittent pressure wasapplied to the crown while stabilizing the alu-minum cylinder holder. Water was seen to pumpin and out between the tooth finish line and lingualcrown margin,

A visual method of indicating the extent of thecrack in the zinc phosphate cement was provided

The Interrational Journal of Prostliodontk 158 Volume e,N

Load FaligLe of Cast Posts & Cores and Complete Cast Cri

Strain gauge output

Crack formation in cement

Cyclic load application

Fig 4 Ttieoretical diagram ot strain gauge output at preliminary failure.

by immersing the test teeth in black ink ¡PelikanDrawing Ink A, Pelikan AC, Germany) for 12hours, then allowing the ink to dry for 6 hours.Following this, the crowns were sectioned in themesiodistal plane with a carborundum disk(National Keystone, Cherry Hill, NJ|. The crownwas then separated from the tooth, and themicroleakage pattern observed but not quantified.

Statistical Analysis

The independent variable recorded was thenumber of load cycles to preliminary failure. Thisvariable was subjected to a one-way ANOVA, withthe Student-Newman-Keuls test used to define sig-nificant subsets at the 95% confidence level.

Results

Table 1 shows the number of load fatigue cyclesrequired to create preliminary failure for each testgroup- This table also contains the results of thestatistical analysis, with statistical subsets definedby the vertical bars. These results are also shown inbar graph form in Fig 5. The one-way ANOVA testrevealed that Croups 1 and 2 were significantlydifferent from Groups 3, 4, and tbe control group(P< .05). It should be noted here that one samplein the control group exhibited root fracture prior topreliminary failure. Thus, only four samples for thisgroup were evaluated in the statistical analysis.

Table 1 Number of Cycles to FailureGroupnumber

1234

Control

Ferrulelength (mm)

0.51.01520-

Mean

1131140

71,65160,04591,208

Standarddeviation

83665

53,59026,60449,891

Statisticalsubsets""

'Five samples in eacii group.'•Rool fractured occurred prior to preliminary faiijre in one sample.'"Vertical lines indicate subsets ttiat are significantly differentlP<.05|.

o

r 1,. • • i

Ferrule length (mm¡

Fig 5 Number of load cycles to create preliminary failure.

159 a I of Prostliodontics

Load FatiËUe ol Casi Posts S, Cores and Complète Cnt Cr.

Fig 6 Microleakage between the crown ferrule and tooth. Fig 7 ivlicroleakage pattern showing leakage along the post.

Microleakage Evaluation

In all test groups, ink penetration occurredbetween the crown margin and lingual tooth finishline, up the ferrule, and into the tooth-core inter-face (Fig 61. In some specimens there was evidenceof microleakage in the coronal portion of the postspace (Fig 7). In no instance was ink observedbetween the crown and core.

Discussion

Earlier studies adopted a 2-mm ferrule length asbeing necessary for long-term clinical success ofendodontically treated teeth.'"" For the conditionsapplied in this study, the results suggest that theminimum ferrule length for a central incisor shouldbe 1.5 mm, since there was a significant increasein the number of load cycles to preliminary failurewhen the ferrule length was increased from 1.0mm to 1.5 (Table 11.

The number of cycles to preliminary failure wascorrelated with buccolingual dimensions of the testteeth. No significant correlation was found. Therewas an unexpected drop in the number of cycles topreiiminary failure when the ferrule length wasincreased from 1.5 to 2.0 mm. Although not signifi-cant, this drop may be explained by the greatercrown height associated with the 2.0 mm ferrulethat introdLtced a greater bending moment or dis-lodging moment on the crown.

Preliminary failure did occur for the controlgroup even though the preparations were entirelyon tooth structure. The complete crown preparationof the central incisor provides minimal resistanceform when loaded from the lingual (Fig 1|. Thus,crown disiodgment is possible for this restoration.

Tbe microleakage evaluation showed leakagepatterns invading the ferrule, and progressing intothe tooth-core junction and the post space. Theseobservations are consistent with clinical findings ofcaries at the tooth-core junction, and fracturedposts with the core and crown being dislodged as asingle unit."""

As with many in vitro studies, it is difficult toextrapolate these results directly to the clinical situa-tion. The elasticity of the periodontal ligament wasnot duplicated, and the ferrule height was a constantheight around the periphery of the teeth. Teeth aregenerally prepared, however, with their finish linesfollowing the coronal extension of the gingival tissuelevel interproximally. Also, the ferrule height usuallyvaries around the circumference of the tooth.

In light of the results of this study, one shouldcertainly consider some modification in treatmentplan when a tooth presents with a ferrule of lessthan 1.5 mm. This becomes even more important ifthe final restoration will be subjected to a possibleload increase, as a fixed partial denture abutment,for example, or an abutment for a distal extensionbase removable partial denture. Periodontal crownlengthening, or orthodontic extrusion, may be indi-cated to increase the ferrule height.

Conclusion

Twenty-five central incisors were restored withcast posts and cores and complete crowns, andsubjected to a cyclically varying (fatigue) 4-kgload. These twenty-five teeth were divided intofour test groups and one control group accordingto the ferrule length of the crown. The test groupshad ferrule lengths of 0,5 mm, 1.0 mm, 1.5 mm

The Intemalinral loiirnal of Frost hod on ties 160

Libman/Nichoils l.o.id F.KiKiie ct" Cast Posts ë, Cores and Complete Casi Crowns

and 2.0 mm. The control group had no cast dowelcores. Loading of all specimens was continueduntil preliminary failure occurred, when movementbetween the crown and the tooth increased as aresult of crack formation either la> at thecement/crown interface, (b) at the cement/toothinterface, or (c) in the cement. From the results ofthis study the following conclusion may be made.When the ferrule length was greater than or equalto 1.5 mm, a significant increase in the number ofload cycles to preliminary failure was recordedwhen compared to crowns having a ferrule lengthless than 1.5 mm.

References

1. Lovdahl PE, Nicholls II. Pin retained amsigani cores vs.cast gold dowel-cores. ] Prosthet Dent 1977;38:507-514.

2. Guzy CE, Nicholis II . In vitro comparison of intactendodontically treated teeth with and without endo-postreinforcement. | Prosthet Dent 1979^42:39-44.

3. Trope M, Maltz DO, Tronslad L. Resistance to fracture otrestored endo-dontically treated teeth. Endodont DentTraumatcl t985:1:1 08 -1 I I .

4. Sorensen |A, Martinoff IT. Intracoronal reinrorcement andcoronal coverage: A study of endodontically treated teeth.I Prosthet Dent 1984:51:780-784.

5. Rosen H. Operative procedures on mutilated endodonti-cally treated teeth. I Prosthet Dent 1961 ;l 1:976-986.

6. Shillingburg HT Jr, Hobo S, Whitsett LD. Fundamentals ofFised Prosthodontics. Chicago: Quintessence, 1981:147.

7. Hoag P, Dwyer T. A comparative evaluation oi three postand core techniques. | Prosthet Dent I 982;47:1 77-.181.

8. Celtand M, Goldman M, Sunderman EJ. Effect of completeveneer crowns on the compiessive strength of endodonti-cally treated posterior teeth, i Prosthet Dent 1964;52:635-638.

9. The Glossary of Prosthodontic Terms, ed 6. i ProsthetDent 1994;71[l):72.

10. Freeman MA, Nicholls | l , Kydd W, Harrington GW.Leakage associated with load fatigue induced preliminaryfailure of full crowns over three post and core systems. |Endodont |in press).

11. Pashley DH. Clinical considerations of microleakage. IEndodont 1990:1 6|2):70-7?

12. Lewis R, Smith BC A clinical survey of failed postretained crowns. Br Dent ¡ 1 988;l65(3):95-97.

I 3. Sorensen |, Engelman M. Ferrule design and fracture resis-tance of endodonticaliy treated teeth. | Prosthet Dent1990;63:529-536.

14. Fissore B, Nicholls | l , Yuodelis RA. Load fatigue of teethrestored by a dentin bonding agenl and a posterioi com-posite i-esin. I Prosihet Dent 1992;6S:!iO-e5.

15. Reitz PV, Aoki H, Yoshloka M, Uehara |, Kuhota Y. Acephalometric study of tooth position as related to facialstructure in profiles oi human beings: A comparison ofJapanese (Oriental! and American [Caucasian] adults. |Prosthet Dent 1972:29:157-166.

16. Bates JF, Stafford GO, Harrison A. Masticatory function—A review oi the literature. II. Speed of movement oi themandible, rate of chewing and forces developed in chew-ing. I Oral Rehab 1975,2.349-361.

17. Helkimo £, Ingervall B. Bite force and functional state ofthe masticatory system m young men. Swed Dent J1978:2:167-175.

18. Anderson D]. Measurement of stress in mastication. 1. |Dent Res 1956;35:664-670.

19. Anderson DJ. Measurement of stress in mastication. II. JDenlRes 1956:35:671-673.

20. Trabert KC, Cooney IP. The endodontically treated tooth:Restorative concepts and techniques. Dent Clin North Am1984:28:923-951.

21. Rosen H, Part I da-Rivera M. latrogenic fracture of rootsreinforced with a cervical collar. Oper Dent1986;n(2):46-50.

22. Barkhordar R, Radke R, Abbasi |. Effect of metal collars onresistance of endodontically treated teeth to root fracture.J Prosthet Dent 1989,61:676-678.

Number 2, 1 161 The International lournal oi ProsthodontK