Endodontics

Comparison of carbon fiber and stainless steel root canal postsD. G. Purton*/J- A. Payne*

Abstract This in viiro suidy compared phyxical properlies of root canal pnsts made of carbonfiber-reinforced epoxy resin wUiuhose of slain iess sled posts. Three-poinI bending¡esls were used lo derive the transverse modiiiiis ofeiasiiciiy ofihe posis. Resincomposite cores on the posts nere siihjecied lo tensile forces lo test the bondsbenveen llie cores and posts. Carbon fiber posts appeared to have adequate rigidityfor their designed purpose. The bond strength of the resin composite cores to thecarbon fiber posts was significantly iess than ¡hai to the stainless steel posts.(Quintessence lnt }99ñ;27:93-97.)

Clinical relevance

Carbon fiber posts have the potential to replacemetal posts in many clinical situations. They areadequately rigid, fatigue and corrosion resistant,and reversible. If conilgurations such as serrationscan improve their bond to cores, they could becomeas universal in application as metal posts.

Introduction

The scientific literature supports the use of root canalposts to retain prosthetic cores' - but lends no supportto the use of posts to strengthen teeth. Considerablevariation exists in the type of posts systems available.This invariably results in confusion in the selection forclinical usage.

Many studies have reported the failure rate ofrestorations with posts and cores to be higher than thatof restorarions on vital teeth.'"' Failure can result fromfracture or bending of posts, loss of retention, corefracture, or root fracture. Corrosion of metalhc posts

' Senior Lecturer. DepanjnenI of Restorative Demjstri', Univerbily ofOmgo. Faculty of Dentistry. DunedJn. New Zealand.

Reprint reqjests: Dr D. G. Purton, Senior Leeturer. Department ufRestorative Dentistry. Llniversity of Otago. Faculty of Dentislry. PO Box647. Dunedin. New Zealand.

has been proposed as a cause of fractures.̂ Researchinto posts and cores continues in efforts to developsystems that are biocompatible, preserve root dentin,do not stress the root, and are strong, retentive todental cements, corrosion resistant, and compatiblewith restorative materials.

Carbon fiber-reinforced resins are considered viablealternatives to metals in many fields, when strength,stiffness, lightness, and resistance to corrosion andfatigue are required. Carbon compounds in differentforms have been studied in various dental and surgicalapplications,^' where they have proved to be bio-compatible and mechanically satisfactorj' for manypurposes.

Composipost (RTD) is a system of root canal postsmade from carbon fiber-reinforced epoxy resin. End-odontic drills of matching diameter, resin lutingcement, and resin composite core material comple-ment the system. These root canal posts are fabricatedfrom continuous, unidirectional carbon fibers. 8 \im indiameter, uniformly embedded in an epoxy matrix. Thefibers constitute 64% by weight of the post.' The coreresin and the resin luting cement resin are bis-GMA-based materials.'

The aims of this study were to investigate thefiexural stiffness of the carbon ñber root canal postsand compare it with the stiffness of stainless steel postsand to compare the retention of a resin composite corematerial to the carbon fiber and the stainless steelposts.

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Fig 1 Carbon fiber (left) and stainless steel (right) posts.

Fig 2 Stainless steel post undergoing the ihree-pointbending test.

FJg 3 Specimen undergoing line tensiie strength teat.

Method and materials

Bending test

Composipost carbon fiber posts are cylindrical withtapered apical portions. Ten i,40-mm-diameter postswere tested with a three-point bending test in auniversal testing machine (Instron), The crossheadspeed was 10.0 mm/min and the load was applied untilthe eiastic limit ofthe posts was reached. Ten stainlesssteel posts (Parapost, Whaiedent}, 1,25 mm in diame-ter, were tested in the same way. Examples ofthe posts

are shown in Fig I and the testing arrangement isshown in Fig 2,

Recorded data were analyzed in an Apple Macintoshcomputer by using MacLab Chart V3,0 software. Plotsof stress versus defiection were made for the posts, andthe elastic modulus (E) was derived for each materialwith the following formula:

where P= load applied in Newtons; L - length ofthespan in mm; rf= the deflection in mm: and /= momentof inertia. The moment of inertia was calculated withthe following formula:

I = TidV64where d^ diameter ofthe post in milhmeters.

Tensile bond strength test

In the second part of the study, cylindrical cores ofresin composite (Ti-Core, Essential Dental Systems)were fonned on 1.80-mm-diameter carbon fiber postsand on I,50-mm-diameter stainless steel posts with atwo-part silicone putty mold. The posts were cleanedwith 70% methanol, dried in air, and placed into thecore resin carefully to avoid any contamination oftheirsurfaces. The posts and cores were removed from themold after the manufacturer's recotnmended settingtime. Square sections of resin composite were appliedto the opposite end of the carbon fiber posts tofacilitate gripping in the jaws of the testing tnachine.They were arranged in the testing machine as shown inEig 3 and a tensile force was applied. The crossheadspeed was 10,0 mm/min, and the force was applieduntil the specimen fractured. Data were recorded withthe computer.

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Table I Tensile bond strength between resin com-posite cores and posts

Post type Diameter (mm) Mean force (kg) SD

Stainless steel*Stainless steelCarbon fiber'̂Carbon Fiber

' Paraposl,t Composipost,

1.251,501,401,80

/I,6534^S

4,6,49

29.715,66.7

12,1

Results

The three-point bending tests resulted in the followingvalues forthe transverse modulus of elasticity: stainlesssteel (Parapost) 213 GPa; carbon fiber (Composi-post) 319 GPa,

Results of the tensile testing of the resin compositecores and posts combinations are shown in Table 1.Appraisal with an analysis of variance revealed statisti-cally significant differences in core retention betweenthe 1.25-mm Parapost and 1,40-mm Composipost{P< ,05), the 1.50-mm Parapost and 1,40-mmCom-posipost {F< ,01). and the i,50-mm Parapost and1.80-nim Composiposte {P < ,05). Other combina-tions were not significantly different.

Discussion

The three-point bending test provides a basis forcomparison between posts made of different materials.The tested carbon fiber posts exhibited greater rigiditythan did the tested stainless steel posts. Becausecarbon fiber material is structurally anisotropic, themodulus of elasticity would be different if the postswere tested at different angles. According to themanufacturer's technical document.^ the modulus ofelasticity is at its lowest value at right angles to the iongaxis of the post. This was the mode of testing in thestudy.

Stiffness, or high modulus of elasticity, is acceptedas a prime requirement of restorative materials,'" Asrestorative materials deflect under fianctional load,luting cements are subjected to tensiie and shearingforces. Cements are much weaker in tension and shearthan in compression.'" A restoration with a highermodulus of elasticity is therefore less likely to suffercementation failure in function, all other things beingequal.

The higher the modulus of elasticity of the materialused for a post, the smaller the diameter can be forequivalent strength. This allows a more conservativepreparation for the post space, Consei-vation of dentinreduces the likelihood of tooth fracture in fianciion andin the event of trauma,"

Resin composite was chosen as the core material inthis study for several reasons:

1, The manufacturers of the posts recommend resincomposite and report that the epoxy resin matrix ischemically compatible with bis-GMA resins,*̂

2, The use of resin composite for cores is clinicallysimple and is common among dental practitioners.

3, Studies suggest that teeth restored with resincomposite cores of the type used in the presentstudy are more likely to be retrievable or retreatablein the event of mechanical failure, than are teethrestored with cast metal cores,'- because the root isless likely to be fractured, (In this context, one ofthe advantages ciaimed by the manufacturer of thecarbon fiber posts is that reentry of the canal ispossible by the use of solvents and engine-drivenreamers,**)

The tensile mode of testing the posts and cores wasused for this study because it is a simple test thatremoves many of the variables associated with the useof extracted teeth and because comparabie data areavailable. The bond strengths between resin compositecores and stainless sieel posts reported by Millstein etal '•' are the same as those obtained in the present study.

The tensile bond strength between cores and theposts is reported to be strongly dependent on theconfiguration of the post, but not on the thickness ofthe core material. '•• The influence of configuration wasrefiected in the present study by the way in which theresin composite cores failed. Tensile loading of thestainless steel posts, with their retentive heads, re-sulted in shattering of the resin composite cores. Whenthe carbon fiber posts were loaded, the cores werepulled off without breakage (Fig 4), The lack of aretentive feature on the head of the posts appears to bea weakness of the Composipost system. This accountsfor the iower tensile values attained for these posts.The manufacturer appears to have acknowledged thisweakness by recently introducing serrated versions ofthe posts.

The results of the present study were sufficientlyencouraging to proceed with other tests of this newproduct. Proposed tests include the effect of surfacetreatments of the posts on the tensile bond strengths of

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i%Fig 4 Cores pulled off a carbon fiber post and shatteredoff a stainless steel post.

Fig 5 Serrated carbon fiber post in a maxillary canine.

Fig 6 Resin composite core over a carbon fiber post in ama!<illary canine

cores; the bond strengths of the posts to difterent corematerials; the eftects of surface configuration; the bondstrength of the posts to teeth; and the influence of thecarbon fiber posts on the fracture resistance of teeth.

Carbon fiber posts have the potential to replacestainless steel and other metal posts in many clinicalsituations. They are more rigid than are metal posts,allowing smaller diameters to be used for equivalentstrength. This has the potential for conservation ofroot dentin. If surface treatments or alterations toconfiguration such as serrations (Fig 5) improve thebond strength to resin composite cores ( Fig 6 ), carbonfiber posts couid become as universal in appiication asstainless steel posts are at present. The claimedadvantages of higher fatigue strength and corrosionresistance add to the universal appUcability of the

material. This combination of properties should con-vey greater clinical durability to carbon fiber posts thanis achieved by present materiais. A clinical disadvan-tage of carbon fiber posts, however, is their lack ofradi opacity.

Summary

1. The carbon fiber material was stiffer under trans-verse loading than was stainless steel. It appeared tobe adequately rigid for its purpose.

2. The tested resin composite core material wasretained more strongly to the stainless steel poststhan to the carbon fiber posts in tensile testing.

3. The configuration of the posts significantly affectedthe retention of the resin composite cores and themode of fracture on tensile loading,

4. The diameter of the posts did not infiuence theretention of the cores for either type of post.

References1. Shiliineburg HT, Kessler JC Restoration of the Endodontically

Trcalod Tooth. Chicago: Quintessence, 1982.

2. Rosenstiel SF, Land MF, FujimoloJ. Resloration of tlie endodontic-ally treated tooth ln. Contemporary Fixed Prosthodotitics, ed 2. SiLouis: Mosby, i99S:23B-267.

3. Roberts DH. Tlie failure of retaitiers in bridge prostheses. Ananalysis of 2,000 retainers. BrDentJ 1970; 128: i 17-Í24.

4. Turner CH. Post-retained crown failure: A survey. Dent Update19S2;May:221-234.

5. Silness J, Gustavseti F, Hunsbeth J. Distribution of corrosionproducts in teeth restored with metal crowns retained by stainlesssteel posts. Acta Odontoi Scand I979;37:317-32i.

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6, Malqtiartj G, Berruet RG, Bois D, Prosthetie use of carbon fiber-reinforced eposy resin for esthetic erowns and fixed partial dentures,J Prosthet Dent 1990:63;25i-257,

7. Adams D, Williams DF, Carbon fiber-reinforced tarbon as apotential implant material, J Biomed Mater Res l978;t2:35-42.

8, Composiposte, Technicat Document. Meylan Cedex, France: RTD,1994.

9. Duret PB, Reynaiid M, Duret F, Un nouveau concept de recon-stitution corono-radiculaire. Le Composiposte U), Chirurg DentFrance i990:54O; 131-141

10, Croig RG, Restorative Dental Materials, ed 5. St, LouiS: Mosby,19S0,

11, Trabeit KC, Caputo AA, Abou-Rass M, Tooth fiacture-A compari-son of enilodontic and restorative treatments, J Endod 1978,4:341-345,

t2, Ptasmans PJJM, Weite PR, Vrijhoef MMA. tn vitro resistance ofcomposite resin dowel artd cores, J Endod t9Sií;14.3Ü0-Í04,

13, Mitistein PL, Ho J, Nathanson D, Retention between a serrated steeldowet and different eore tnateriats. J Prosthet Dent t99t,65:480-482,

t4. Chang W-C, Mitlstein PL, Effect of design of pretabricated postheadb on core materiats, J Prosttiet Dent t993:69;475-482. D

Quintessence International VolumelZ^Number 2/t996

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