Effect of dentinal bonded resin post-core resistance to vertical root fracture
preparations
on
Richard T. Bex, DMD,a Michael W. Parker, DMD, MEd,b James T. Judkins, DMD,C and George B. Pelleu, Jr., PhDd Naval Dental School, National Naval Dental Center, Bethesda, Md. An in vitro study was conducted to compare the resistance to failure of two restorative protocols for endodontically treated teeth. Half of 24 specimens received cemented cast post-core restorations and the other half were restored with dentin-bonded composite resin using the ferric oxalate, NTG-GMA, and the resin post-core restoraPMDM system developed by Bowen. 2,3 The dentin-bonded tions provided significantly less resistance to failure than the cemented custom cast post-core. The dentin-bonded resin post-core fractured in every instance before the root fractured. A greater force was required to cause failure of the resin post as the cross-sectional area of the post increased. (J PROSTHET DENT 1992;67:768-72.)
D
espite efforts to reinforce endodontically treated teeth with internal posts and cores, fractures continue to
0ccur.l With recent improvements in the adherence of composite
resins to dentin, 2, 3 true internal
support
may
now be available. Pulpless canals have long been used to increase the retention of full coverage restorations. Pierre Fauchard used a wooden post in the 18th century, and G. V. Black employed a screw within a gold foil canal filling in the 19th century.4 More recently, the post-core has been used for
retention of crowns and as a reinforcement against root fracture.5-10 It has been generally accepted that the prime function of the post-core system is to provide resistance to fracture.8 Despite the findings of Guzy and Nicholls’ that no significant difference in the mean failure loads for central incisors and canines with and without posts could be
demonstrated, the assumption prevails that the prime function of a post-core is tooth reinforcementll, l2 There is general
agreement
that prosthodontic
abutments
should
be restored with a post-core and that almost all endodontically treated teeth should be reinforced.13 Clinical procedures should minimize weakening of teeth, and techniques for placement of post-cores have been developed that minimize the internal stresses that predispose
The opinions or assertions contained in this article are the private ones of the writers and are not to be construed as official or as reflecting the views of the Department of the Navy, the Department of Defense, or the 1J.S. Government. This project was supported under the Naval Medical Research and Development Command .Research Task No. MO09506-3014. aLieutenant Commander (DC) USN, Branch Dental Clinic, Naval Postgraduate School, Monterey, Calif. bCaptain (DC) USN; Chairman, Clinical Support Department. cCaptain (DC) USN; Commanding Officer, 22nd Dental Company, 2nd Dental Battalion, 2nd FSSG FMFLANT, Camp Lejune, N.C. dFormer Chairman, Research Department. 10/l/35789
768
pulpless teeth to fracture.* These include cast metal postcores, pin-retained composite resin cores, and stabilizing metallic rods5 Composite resins have been studied both as material for the post-core itself and for the cementing agent.2, 3,le21 Composite resins are superior to zinc phosphate cement in tensile strength when used to cement serrated dowels.16 Composite resin placed in a post preparation that includes undercuts in the walls of the canal without the use of a bonding agent will fracture in the core section before it dislodges.15 Chapman et al. l8 demonstrated clinically acceptable retention when cementing prefabricated posts with first-generation
agents that bond to dentin,
and res-
in-to-dentin bond strengths approximating 2000 psi have been reported.2 Composite resin has been studied as postcore material, as the luting agent with prefabricated posts, and most recently in a study by Trope et a1.22as the post material within an acid-etched post preparation. No study involving resin as both the post and core structure and as the bonding agent has been reported. This study determined whether dentin-bonded composite resin post-cores on anterior teeth would provide more resistance to vertical root fracture than cemented cast post-core restorations. MATERIALS
AND
METHODS
The crowns of 24 extracted mature human maxillary anterior teeth without caries, cervical abrasion, or fracture were removed, using a low-speed diamond saw (Isomet, Buehler Ltd., Evanston, Ill.) perpendicular to the long axis at a level reaching the most incisal extension of the cementoenamel junction, following the procedure of Johnson and Sakumura.17 Teeth of similar length, shape, girth, and diameter were paired. Half were arbitrarily assigned to an experimental dentin-bonded composite resin group and the other half were assigned to a control
(cemented
casting)
*References 4, 5, 7, 8, 10, 14, and 15.
JUNE
1992
VOLUME
67
NUMBER
6
POST-CORE
FRACTURE
STRENGTH
Fig.
1. Three-point
stress test design. CEJ, Cementoenamel junction.
group. All teeth were stored in 2% aqueous sodium azide solution before sectioning and between each manipulation.
Endodontic
procedures
Each canal was prepared to within 1 mm of the radiographic apex. Each master apical file was three sizeslarger than the initial instrument used. With the step-back technique, the canals were progressively enlarged and flared to accommodate a No. 70 file to within 6 mm of the established working length. All samples were obturated using a low-temperature injectable gutta-percha filling system (Ultrafil, Hygenic Corp., Akron, Ohio) to lessen the risk of creating internal stresses during lateral condensation and to eliminate the need for a sealer containing eugenol. Eugenol may compromise the dentin-resin bond or the polymerization of resin if it is incompletely removed from the canal. Gauze soaked in physiologic saline was used to hold the tooth during instrumentation and obturation to prevent dehydration of specimens. Post preparations were made with a No. 2 Peeso reamer (Union Broach Corp., Long Island City, N.Y.) to within 4 mm of the fill length.
Cast post-core
fabrication
(control
group)
A custom cast post-core was the control because of its wide acceptance as a means of restoring endodontically treated teethz3 and as a method that conserves tooth structure.7*3s lo, l2 Resin post-core patterns (Duralay, Reliance Dental Mfg. Co., Chicago, Ill.) were obtained for 12 of the specimens. A micrometer caliper was used to prescribe the outline of a standard core. Four marks were made on
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
the cut surfaces of the teeth 1.5 mm from the pulp chamber in lingual, mesial, and distal directions and 2 mm from the chamber in the facial direction. These marks were joined by an ovoid outline that prescribed the extent of the standard core. The patterns were sprued, invested in a phosphate-bonded material in rings without liners, and cast in Pentillium (Pentron Corp., Wallingford, Conn.). The restorations were cemented with zinc phosphate cement (Flecks, Mizzy, Inc., Clifton Forge, Va.) under digital pressure.
Resin bonded (experimental
post-core group)
fabrication
The dentin surfaces of the remaining 12 specimens were treated with an eight-step process prescribed by Bowen et a1.2,3 The internal and occlusal surfaces were wet for 60 seconds with a 6.8% aqueous solution of ferric oxalate delivered with a tuberculin syringe. They were then washed with distilled water for 10 seconds with an irrigating syringe, dried with absorbent paper points, and blown with an air syringe for 10 seconds. A 10% acetone solution of NTG-GMA (the adduct of N[p-tolyl] glycine and glycidyl methacrylate) was then injected with a tuberculin syringe and allowed to stand for 60 seconds. The unbound NTGGMA was eliminated with a lo-second bath of pure acetone. Internal and occlusal surfaces were again dried with paper points and compressed air. A 5% acetone solution of PMDM (the addition reaction product of pyromellitic dianhydride and 2-hydroxyethyl methacrylate) was then applied for 60 seconds with another tuberculin sy769
BEX ET AL
With
A 6
Failure Mode
Cast
F
D
With Dentin Bonded Resin Post-Core
Post-Core
P
I
I
I
1’ 0
P
0 Flexed & Extruded
Facial Vw.w
Number
F
4
5
3
6
-T-i
2. Failure modes and distribution with cast post-core restorations (left) dentin-bonded resin post-core restorations (right).
Fig.
I. Mean failure loads
Table
Restorative
Number of Samples
method
Cemented cast post-core Dentin-bonded resin
load
Failure (N) Mean
12
848 * 334
12
280 ic 132
+ SD *
post-core *Significant
dilkence
(Student’s
t test, p < 0.001).
II. Relation between failure load and cross-sectional area of resin posts
Table
Cross-sectional area (mm2) Mean f SD
Failure (N) Mean
13.3 k 4.6
280 f
*Positive correlation for a sample the two-tailed t test was used).
load + SD
Pearson correlation coefficient
132
size of 12 (r was significant,
I
(r)
0.62* p < 0.05, when
and with
ogous to the forces on maxillary anterior teeth caused by their mandibular opponents in function. Each specimen rested on a platform with two supports 12 mm apart. One support contacted the root 8 mm from the tooth-core junction; the other contacted the core 4 mm from the tooth-core junction (Fig. 1). Specimens in both groups had the palatal portion of the core section flattened slightly to stabilize the resting position. A load cell advancing at 1 mm/min contacted the specimens 2 mm below the tooth-core interface on the facial aspect of the root. The force exerted by the load cell was recorded by an Omnigraphic 2000 (Bausch & Lomb Corp., Rochester, N.Y.). The failure threshold was defined as the point at which the loading force ceased to increase, either as a result of fracture or because of displacement were examined with the aid of 5 power magnification head loupes. The location and direction of fracture lines in roots with cast post-cores were documented, and resin adaptation, density, and tag formation were evaluated in teeth with resin post-cores. RESULTS
ringe. After drying, the dentin surfaces were ready to receive the post-core resin. The post-core preparations were immediately injected with composite resin (Adaptic, Johnson & Johnson, East Windsor, N.J.) in a C-R syringe (Cavitron Corp., Cleveland, Ohio). Adaptic was chosen for the resin group because of its superior wetting and spreading qualities, as well as its excellent compressive, tensile, and shear strengths.ls The resin was injected at the base of the post preparation with a 27-gauge needle to vent trapped air. A core former, such as that used for the Duralay posts, was filled with Adaptic from the same mix. A No. 11 scalpel blade was used to remove excess resin flash. Testing All 24 specimens were load-tested for shear strength with a testing machine (FM-lo, United Calibration Corp., Garden Grove, Calif.). The load test was designed to be anal-
770
The mean failure loads are given in Table I. The types and numbers of failure are shown in Fig. 2. There were distinct differences in failure patterns between teeth with cemented cast post-cores and dentin-bonded resin postcores. Of the 12 teeth with cemented cast post-core restorations, 75% sustained vertical root fracture. Four of these fractured vertically through the midfacial section of the root, and five teeth fractured in a vertical-oblique pattern through the root, originating midinterproximal and ending in the facial mid third of the root. The three remaining teeth (25%) experienced a flexing of the casting at the post-core junction, with extrusion of the casting from the root. By inspection, these three posts had the smallest cross-sectional area at the junction of the post to the core. The mean failure load for these 12 teeth was 848 k 334 N. Of the 12 teeth with the dentin-bonded resin post-cores,
JUNE
1992
VOLUME
67
NUMBER
6
POST-CORE
FRACTURE
STRENGTH
none had root fractures. Half of the composite resin cores exhibited cohesive fractures obliquely from the palatal aspect of the tooth-core interface, progressing incisofacially. Half of the resin cores separated cleanly from the tooth, along with a millimeter or less of the resin post. There was no visual evidence of resin tags at any of the core interfaces. The mean failure load for the dentin bonded resin group was 280 rt 132 N. When the mean failure loads for the two groups were compared statistically using Student’s t test, the difference was significant (p < 0.001). The failure loads in the resin post-core group were compared with the cross-sectional surface of the posts at the orifice using Pearson’s product-moment correlation coefficient. A positive correlation (r = +0.62) was found between the force required to cause failure and the post diameter (Table II). The results of a two-tailed Student’s t test for significance of r was significant (p < 0.05). When sectioned, the teeth in the dentin-bonded resin group had densely filled resin in intimate contact with canal walls. DISCUSSION In the dentin-bonded resin group, the failure was at the core-root interface and within the core itself. Although the resin post-cores failed under less stress than the castings, the integrity of the root was preserved in every instance. The core failed before the root fractured, with the resin post-core acting as a “shear pin” under excessive loading. In the group with cemented cast post-cores, the predominant failure mode (75%) was vertical root fracture, rendering the teeth unrestorable. The mean failure load of the casting group was almost identical to that found by Guzy and Nicholls,l who more nearly simulated physiologic conditions. It was interesting to note that there was no apparent bonding between the resin core and the root truncation, possibly because of the method by which the dentin was managed. The processes of sectioning, storing in sodium azide, and endodontically treating dentin may have compromised the dentin smear layer essential for optimal dentin bonding,24 which could account for the lack of tag formation observed in the resin group. A positive correlation was found between the failure load and the cross-sectional areas of the resin posts, suggesting the need for additional studies to determine which teeth (anterior/posterior; maxillary/mandibular) would be best suited for resin post-cores and at what stage of maturity the post-cores should be placed. Considering the strength that this treatment imparts to the tooth and the acceptable “shear pin” type of failure sustained, there is a strong possibility that restoring an endodontically treated tooth in this manner may be clinically indicated. Ferric oxalate, NTG-GMA, and PMDM experimental solutions within the pulp canal space raise questions about their effect on the periodontium and on gutta-percha. Studies by Stanley et a1.25and by Chohayeb et a1.26suggest
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
that the protocol of Bowen et a1.2,3 causes no adverse pulpal response. They surmise that insoluble products block the tubules and thereby protect the pulp externally. A similar mechanism of action may seal them from within and protect the periodontium when post spaces are being treated. Acetone plays an integral part in this protocol, but its solvent action might affect the remaining guttapercha and subsequently jeopardize the integrity of the apical sea1.27This is another subject that needs to be examined. The steps presently required for this oxalate bonding system are numerous and cumbersome. This detailed process might be unnecessary if similar results could be obtained from simple acid etching of the canal space, as reported by Trope et a1.22 CONCLUSIONS An in vitro study was conducted to compare the resistance to failure of two restorative protocols for endodontitally treated teeth. Half of 24 specimens received cemented cast post-core restorations and the other half were restored with dentin-bonded composite resin using the ferric oxalate, NTG-GMA, and the PMDM system developed by Bowen et aL2T3The following conclusions were drawn: 1. The dentin-bonded resin post-core restorations provided significantly less resistance to failure than the cemented custom cast post-core. 2. The dentin-bonded resin post-core fractured in every instance before the root fractured. 3. A greater force was required to cause failure of the resin post as the cross-sectional area of the post increased. REFERENCES 1. Guzy GE, Nicholls JI. In vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. J PROSTHET DENT 1979;42:39-44. 2. Bowen RL, Cobb EN. A method for bonding to dentin and enamel. J Am Dent Assoc 1983;107:734-6. 3. Bowen RL, Cobb EN, Rapson JE. Adhesive bonding of various materials to hard tooth tissues: improvement in bond strength to dentin. J Dent Res 1982;61:1070-6. 4. Shillingburg HT, Kessler JC. Restoration of the endodontically treated tooth. Chicago: Quintessence Publishing Co, 1982:45-73. 5. Kantor ME, Pines MS. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;38:405-12. 6. Newburg RE, Pameijer CH. Retentive properties of post and core systems. J PROSTHET DENT 197636636-43. ‘7. Perel ML, Muroff FI. Clinical criteria for posts and cores. J PROSTHET DENT 1972;28:405-11. 8. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effects of cement, dowel diameter, and design. J PROSTHET DENT 1978;39:400-5. 9. Stern N, Hirshfeld Z. Principles of preparing endodontically treated teeth for dowel and core restorations. J PROSTHET DENT 1973;30:162-5. 10. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. J Endodont 1978;4:341-5. 11. Chan RW, Bryant RW. Post-core foundations for endodontically treated posterior teeth. J PROSTHET DENT 1982;48:401-6. 12. Tjan AH, Whang SB. Resistance to root fracture of dowel channels with various thicknesses of buccal dentin walls. J PROSTHET DENT 1985; 53:496-500. 13. Sokol DJ. Effective use of current core and post concepts. J PROSTHET DENT 1984;52:231-4.
771
BEXETAL
14. Landwerlen JR, Berry HH. The composite resin post and core. J PROSTHETDENT 1972;28:500-3. 15. Stahl GJ, O’Neal RB. The composite resin dowel and core. J PROSTHET D~~~1975;33:642-8. 16. Assif D, Ferber A. Retention of dowels using a composite resin as a cementing medium. J PROSTIIET DENT 1982;48:292-6. 17. Johnson JK, Sakumura JS. Dowel form and tensile force. J PROSTHET DENT 197&40:645-g. 18. Chapman KW, Worley JL, van Fraunhofer JA. Effect of bonding agents on retention of posts. Gen Dent 1985;33:128-30. 19. Chan DC, Reinhardt JW, Schulein TM. Bond strengths of restorative materials to dentin. Gen Dent 1985;33:236-8. 20. Doukoudakis S, Doukoudakis A, Rasmussen S, et al. A shear test evaluation of a new, modified, unfilled resin. J PROSTHET DENT 1985; 53:586-91. 21. Morin DJ, DeLong R, Douglas WH. Cusp reinforcement by the acid-etch technique. J Dent Res 1984;63:1075-8. 22. Trope M, Maltz DO, Tronstad L. Resistance to fracture of restored endodontically treated teeth. Endodont Dent Traumatol 1985;1:108-11.
23. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design.J PROSTHET D~~~1984;52:28-35. 24. Strassler HE, Litkowski LJ. Dentin bonding with composite resin: an update on materials and techniques. Compend Contin Educ Dent 1987;8:318, 320, 322. 25. Stanley HR, Bowen RL, Cobb EN. Pulp responses to experimental treatments of dentin for bonding composites [Abstract]. J Dent Res 1985;64:222. 26. Chohayeb AA, Bowen RL, Rupp NW, Adrian J. Pulpal response to a new dentin and enamel system [Abstract]. J Dent Res 1985;64:222. 27. Oliva RA, Lowe JA. Dimensional stability of composite used as a core material. J PROSTHET DENT 1986;56:554-61.
Reprint requests to: CAPT STEVE ARTHUR,DC,USN CHAIRMAN,RESEARCHDEPARTMENT NAVALDENTAL SCHOOL NATIONAI.NAVAL DENTAL CENTER BETHESDA,MD 20814-5077
Guides for authors
available
The Guide to Preparing Articles for THE JOURNAL OF PROSTHETIC DENTISTRY, revised by Professor Paul Barton, Editorial Consultant to the JOURNAL, and the editors, is available to prospective authors. The guide provides the format for developing different types of scientific manuscripts, a checklist for effective writing, and detailed instructions for preparing manuscripts in the style acceptable by the JOURNAL. Also available are the Guidelines for Reporting Statistical Results and an Author’s Guide to Controlling the Photograph. Guides can be obtained from the office of the Editor (Dr. Glen P. McGivney, The Journal of Prosthetic Dentistry, State University of New York at Buffalo, School of Dental Medicine, 345 Squire Hall, Buffalo, NY 14214).
JUNE1992
VOLUME67
NUMBER6
preparations
on
Richard T. Bex, DMD,a Michael W. Parker, DMD, MEd,b James T. Judkins, DMD,C and George B. Pelleu, Jr., PhDd Naval Dental School, National Naval Dental Center, Bethesda, Md. An in vitro study was conducted to compare the resistance to failure of two restorative protocols for endodontically treated teeth. Half of 24 specimens received cemented cast post-core restorations and the other half were restored with dentin-bonded composite resin using the ferric oxalate, NTG-GMA, and the resin post-core restoraPMDM system developed by Bowen. 2,3 The dentin-bonded tions provided significantly less resistance to failure than the cemented custom cast post-core. The dentin-bonded resin post-core fractured in every instance before the root fractured. A greater force was required to cause failure of the resin post as the cross-sectional area of the post increased. (J PROSTHET DENT 1992;67:768-72.)
D
espite efforts to reinforce endodontically treated teeth with internal posts and cores, fractures continue to
0ccur.l With recent improvements in the adherence of composite
resins to dentin, 2, 3 true internal
support
may
now be available. Pulpless canals have long been used to increase the retention of full coverage restorations. Pierre Fauchard used a wooden post in the 18th century, and G. V. Black employed a screw within a gold foil canal filling in the 19th century.4 More recently, the post-core has been used for
retention of crowns and as a reinforcement against root fracture.5-10 It has been generally accepted that the prime function of the post-core system is to provide resistance to fracture.8 Despite the findings of Guzy and Nicholls’ that no significant difference in the mean failure loads for central incisors and canines with and without posts could be
demonstrated, the assumption prevails that the prime function of a post-core is tooth reinforcementll, l2 There is general
agreement
that prosthodontic
abutments
should
be restored with a post-core and that almost all endodontically treated teeth should be reinforced.13 Clinical procedures should minimize weakening of teeth, and techniques for placement of post-cores have been developed that minimize the internal stresses that predispose
The opinions or assertions contained in this article are the private ones of the writers and are not to be construed as official or as reflecting the views of the Department of the Navy, the Department of Defense, or the 1J.S. Government. This project was supported under the Naval Medical Research and Development Command .Research Task No. MO09506-3014. aLieutenant Commander (DC) USN, Branch Dental Clinic, Naval Postgraduate School, Monterey, Calif. bCaptain (DC) USN; Chairman, Clinical Support Department. cCaptain (DC) USN; Commanding Officer, 22nd Dental Company, 2nd Dental Battalion, 2nd FSSG FMFLANT, Camp Lejune, N.C. dFormer Chairman, Research Department. 10/l/35789
768
pulpless teeth to fracture.* These include cast metal postcores, pin-retained composite resin cores, and stabilizing metallic rods5 Composite resins have been studied both as material for the post-core itself and for the cementing agent.2, 3,le21 Composite resins are superior to zinc phosphate cement in tensile strength when used to cement serrated dowels.16 Composite resin placed in a post preparation that includes undercuts in the walls of the canal without the use of a bonding agent will fracture in the core section before it dislodges.15 Chapman et al. l8 demonstrated clinically acceptable retention when cementing prefabricated posts with first-generation
agents that bond to dentin,
and res-
in-to-dentin bond strengths approximating 2000 psi have been reported.2 Composite resin has been studied as postcore material, as the luting agent with prefabricated posts, and most recently in a study by Trope et a1.22as the post material within an acid-etched post preparation. No study involving resin as both the post and core structure and as the bonding agent has been reported. This study determined whether dentin-bonded composite resin post-cores on anterior teeth would provide more resistance to vertical root fracture than cemented cast post-core restorations. MATERIALS
AND
METHODS
The crowns of 24 extracted mature human maxillary anterior teeth without caries, cervical abrasion, or fracture were removed, using a low-speed diamond saw (Isomet, Buehler Ltd., Evanston, Ill.) perpendicular to the long axis at a level reaching the most incisal extension of the cementoenamel junction, following the procedure of Johnson and Sakumura.17 Teeth of similar length, shape, girth, and diameter were paired. Half were arbitrarily assigned to an experimental dentin-bonded composite resin group and the other half were assigned to a control
(cemented
casting)
*References 4, 5, 7, 8, 10, 14, and 15.
JUNE
1992
VOLUME
67
NUMBER
6
POST-CORE
FRACTURE
STRENGTH
Fig.
1. Three-point
stress test design. CEJ, Cementoenamel junction.
group. All teeth were stored in 2% aqueous sodium azide solution before sectioning and between each manipulation.
Endodontic
procedures
Each canal was prepared to within 1 mm of the radiographic apex. Each master apical file was three sizeslarger than the initial instrument used. With the step-back technique, the canals were progressively enlarged and flared to accommodate a No. 70 file to within 6 mm of the established working length. All samples were obturated using a low-temperature injectable gutta-percha filling system (Ultrafil, Hygenic Corp., Akron, Ohio) to lessen the risk of creating internal stresses during lateral condensation and to eliminate the need for a sealer containing eugenol. Eugenol may compromise the dentin-resin bond or the polymerization of resin if it is incompletely removed from the canal. Gauze soaked in physiologic saline was used to hold the tooth during instrumentation and obturation to prevent dehydration of specimens. Post preparations were made with a No. 2 Peeso reamer (Union Broach Corp., Long Island City, N.Y.) to within 4 mm of the fill length.
Cast post-core
fabrication
(control
group)
A custom cast post-core was the control because of its wide acceptance as a means of restoring endodontically treated teethz3 and as a method that conserves tooth structure.7*3s lo, l2 Resin post-core patterns (Duralay, Reliance Dental Mfg. Co., Chicago, Ill.) were obtained for 12 of the specimens. A micrometer caliper was used to prescribe the outline of a standard core. Four marks were made on
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
the cut surfaces of the teeth 1.5 mm from the pulp chamber in lingual, mesial, and distal directions and 2 mm from the chamber in the facial direction. These marks were joined by an ovoid outline that prescribed the extent of the standard core. The patterns were sprued, invested in a phosphate-bonded material in rings without liners, and cast in Pentillium (Pentron Corp., Wallingford, Conn.). The restorations were cemented with zinc phosphate cement (Flecks, Mizzy, Inc., Clifton Forge, Va.) under digital pressure.
Resin bonded (experimental
post-core group)
fabrication
The dentin surfaces of the remaining 12 specimens were treated with an eight-step process prescribed by Bowen et a1.2,3 The internal and occlusal surfaces were wet for 60 seconds with a 6.8% aqueous solution of ferric oxalate delivered with a tuberculin syringe. They were then washed with distilled water for 10 seconds with an irrigating syringe, dried with absorbent paper points, and blown with an air syringe for 10 seconds. A 10% acetone solution of NTG-GMA (the adduct of N[p-tolyl] glycine and glycidyl methacrylate) was then injected with a tuberculin syringe and allowed to stand for 60 seconds. The unbound NTGGMA was eliminated with a lo-second bath of pure acetone. Internal and occlusal surfaces were again dried with paper points and compressed air. A 5% acetone solution of PMDM (the addition reaction product of pyromellitic dianhydride and 2-hydroxyethyl methacrylate) was then applied for 60 seconds with another tuberculin sy769
BEX ET AL
With
A 6
Failure Mode
Cast
F
D
With Dentin Bonded Resin Post-Core
Post-Core
P
I
I
I
1’ 0
P
0 Flexed & Extruded
Facial Vw.w
Number
F
4
5
3
6
-T-i
2. Failure modes and distribution with cast post-core restorations (left) dentin-bonded resin post-core restorations (right).
Fig.
I. Mean failure loads
Table
Restorative
Number of Samples
method
Cemented cast post-core Dentin-bonded resin
load
Failure (N) Mean
12
848 * 334
12
280 ic 132
+ SD *
post-core *Significant
dilkence
(Student’s
t test, p < 0.001).
II. Relation between failure load and cross-sectional area of resin posts
Table
Cross-sectional area (mm2) Mean f SD
Failure (N) Mean
13.3 k 4.6
280 f
*Positive correlation for a sample the two-tailed t test was used).
load + SD
Pearson correlation coefficient
132
size of 12 (r was significant,
I
(r)
0.62* p < 0.05, when
and with
ogous to the forces on maxillary anterior teeth caused by their mandibular opponents in function. Each specimen rested on a platform with two supports 12 mm apart. One support contacted the root 8 mm from the tooth-core junction; the other contacted the core 4 mm from the tooth-core junction (Fig. 1). Specimens in both groups had the palatal portion of the core section flattened slightly to stabilize the resting position. A load cell advancing at 1 mm/min contacted the specimens 2 mm below the tooth-core interface on the facial aspect of the root. The force exerted by the load cell was recorded by an Omnigraphic 2000 (Bausch & Lomb Corp., Rochester, N.Y.). The failure threshold was defined as the point at which the loading force ceased to increase, either as a result of fracture or because of displacement were examined with the aid of 5 power magnification head loupes. The location and direction of fracture lines in roots with cast post-cores were documented, and resin adaptation, density, and tag formation were evaluated in teeth with resin post-cores. RESULTS
ringe. After drying, the dentin surfaces were ready to receive the post-core resin. The post-core preparations were immediately injected with composite resin (Adaptic, Johnson & Johnson, East Windsor, N.J.) in a C-R syringe (Cavitron Corp., Cleveland, Ohio). Adaptic was chosen for the resin group because of its superior wetting and spreading qualities, as well as its excellent compressive, tensile, and shear strengths.ls The resin was injected at the base of the post preparation with a 27-gauge needle to vent trapped air. A core former, such as that used for the Duralay posts, was filled with Adaptic from the same mix. A No. 11 scalpel blade was used to remove excess resin flash. Testing All 24 specimens were load-tested for shear strength with a testing machine (FM-lo, United Calibration Corp., Garden Grove, Calif.). The load test was designed to be anal-
770
The mean failure loads are given in Table I. The types and numbers of failure are shown in Fig. 2. There were distinct differences in failure patterns between teeth with cemented cast post-cores and dentin-bonded resin postcores. Of the 12 teeth with cemented cast post-core restorations, 75% sustained vertical root fracture. Four of these fractured vertically through the midfacial section of the root, and five teeth fractured in a vertical-oblique pattern through the root, originating midinterproximal and ending in the facial mid third of the root. The three remaining teeth (25%) experienced a flexing of the casting at the post-core junction, with extrusion of the casting from the root. By inspection, these three posts had the smallest cross-sectional area at the junction of the post to the core. The mean failure load for these 12 teeth was 848 k 334 N. Of the 12 teeth with the dentin-bonded resin post-cores,
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none had root fractures. Half of the composite resin cores exhibited cohesive fractures obliquely from the palatal aspect of the tooth-core interface, progressing incisofacially. Half of the resin cores separated cleanly from the tooth, along with a millimeter or less of the resin post. There was no visual evidence of resin tags at any of the core interfaces. The mean failure load for the dentin bonded resin group was 280 rt 132 N. When the mean failure loads for the two groups were compared statistically using Student’s t test, the difference was significant (p < 0.001). The failure loads in the resin post-core group were compared with the cross-sectional surface of the posts at the orifice using Pearson’s product-moment correlation coefficient. A positive correlation (r = +0.62) was found between the force required to cause failure and the post diameter (Table II). The results of a two-tailed Student’s t test for significance of r was significant (p < 0.05). When sectioned, the teeth in the dentin-bonded resin group had densely filled resin in intimate contact with canal walls. DISCUSSION In the dentin-bonded resin group, the failure was at the core-root interface and within the core itself. Although the resin post-cores failed under less stress than the castings, the integrity of the root was preserved in every instance. The core failed before the root fractured, with the resin post-core acting as a “shear pin” under excessive loading. In the group with cemented cast post-cores, the predominant failure mode (75%) was vertical root fracture, rendering the teeth unrestorable. The mean failure load of the casting group was almost identical to that found by Guzy and Nicholls,l who more nearly simulated physiologic conditions. It was interesting to note that there was no apparent bonding between the resin core and the root truncation, possibly because of the method by which the dentin was managed. The processes of sectioning, storing in sodium azide, and endodontically treating dentin may have compromised the dentin smear layer essential for optimal dentin bonding,24 which could account for the lack of tag formation observed in the resin group. A positive correlation was found between the failure load and the cross-sectional areas of the resin posts, suggesting the need for additional studies to determine which teeth (anterior/posterior; maxillary/mandibular) would be best suited for resin post-cores and at what stage of maturity the post-cores should be placed. Considering the strength that this treatment imparts to the tooth and the acceptable “shear pin” type of failure sustained, there is a strong possibility that restoring an endodontically treated tooth in this manner may be clinically indicated. Ferric oxalate, NTG-GMA, and PMDM experimental solutions within the pulp canal space raise questions about their effect on the periodontium and on gutta-percha. Studies by Stanley et a1.25and by Chohayeb et a1.26suggest
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that the protocol of Bowen et a1.2,3 causes no adverse pulpal response. They surmise that insoluble products block the tubules and thereby protect the pulp externally. A similar mechanism of action may seal them from within and protect the periodontium when post spaces are being treated. Acetone plays an integral part in this protocol, but its solvent action might affect the remaining guttapercha and subsequently jeopardize the integrity of the apical sea1.27This is another subject that needs to be examined. The steps presently required for this oxalate bonding system are numerous and cumbersome. This detailed process might be unnecessary if similar results could be obtained from simple acid etching of the canal space, as reported by Trope et a1.22 CONCLUSIONS An in vitro study was conducted to compare the resistance to failure of two restorative protocols for endodontitally treated teeth. Half of 24 specimens received cemented cast post-core restorations and the other half were restored with dentin-bonded composite resin using the ferric oxalate, NTG-GMA, and the PMDM system developed by Bowen et aL2T3The following conclusions were drawn: 1. The dentin-bonded resin post-core restorations provided significantly less resistance to failure than the cemented custom cast post-core. 2. The dentin-bonded resin post-core fractured in every instance before the root fractured. 3. A greater force was required to cause failure of the resin post as the cross-sectional area of the post increased. REFERENCES 1. Guzy GE, Nicholls JI. In vitro comparison of intact endodontically treated teeth with and without endo-post reinforcement. J PROSTHET DENT 1979;42:39-44. 2. Bowen RL, Cobb EN. A method for bonding to dentin and enamel. J Am Dent Assoc 1983;107:734-6. 3. Bowen RL, Cobb EN, Rapson JE. Adhesive bonding of various materials to hard tooth tissues: improvement in bond strength to dentin. J Dent Res 1982;61:1070-6. 4. Shillingburg HT, Kessler JC. Restoration of the endodontically treated tooth. Chicago: Quintessence Publishing Co, 1982:45-73. 5. Kantor ME, Pines MS. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;38:405-12. 6. Newburg RE, Pameijer CH. Retentive properties of post and core systems. J PROSTHET DENT 197636636-43. ‘7. Perel ML, Muroff FI. Clinical criteria for posts and cores. J PROSTHET DENT 1972;28:405-11. 8. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effects of cement, dowel diameter, and design. J PROSTHET DENT 1978;39:400-5. 9. Stern N, Hirshfeld Z. Principles of preparing endodontically treated teeth for dowel and core restorations. J PROSTHET DENT 1973;30:162-5. 10. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. J Endodont 1978;4:341-5. 11. Chan RW, Bryant RW. Post-core foundations for endodontically treated posterior teeth. J PROSTHET DENT 1982;48:401-6. 12. Tjan AH, Whang SB. Resistance to root fracture of dowel channels with various thicknesses of buccal dentin walls. J PROSTHET DENT 1985; 53:496-500. 13. Sokol DJ. Effective use of current core and post concepts. J PROSTHET DENT 1984;52:231-4.
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14. Landwerlen JR, Berry HH. The composite resin post and core. J PROSTHETDENT 1972;28:500-3. 15. Stahl GJ, O’Neal RB. The composite resin dowel and core. J PROSTHET D~~~1975;33:642-8. 16. Assif D, Ferber A. Retention of dowels using a composite resin as a cementing medium. J PROSTIIET DENT 1982;48:292-6. 17. Johnson JK, Sakumura JS. Dowel form and tensile force. J PROSTHET DENT 197&40:645-g. 18. Chapman KW, Worley JL, van Fraunhofer JA. Effect of bonding agents on retention of posts. Gen Dent 1985;33:128-30. 19. Chan DC, Reinhardt JW, Schulein TM. Bond strengths of restorative materials to dentin. Gen Dent 1985;33:236-8. 20. Doukoudakis S, Doukoudakis A, Rasmussen S, et al. A shear test evaluation of a new, modified, unfilled resin. J PROSTHET DENT 1985; 53:586-91. 21. Morin DJ, DeLong R, Douglas WH. Cusp reinforcement by the acid-etch technique. J Dent Res 1984;63:1075-8. 22. Trope M, Maltz DO, Tronstad L. Resistance to fracture of restored endodontically treated teeth. Endodont Dent Traumatol 1985;1:108-11.
23. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design.J PROSTHET D~~~1984;52:28-35. 24. Strassler HE, Litkowski LJ. Dentin bonding with composite resin: an update on materials and techniques. Compend Contin Educ Dent 1987;8:318, 320, 322. 25. Stanley HR, Bowen RL, Cobb EN. Pulp responses to experimental treatments of dentin for bonding composites [Abstract]. J Dent Res 1985;64:222. 26. Chohayeb AA, Bowen RL, Rupp NW, Adrian J. Pulpal response to a new dentin and enamel system [Abstract]. J Dent Res 1985;64:222. 27. Oliva RA, Lowe JA. Dimensional stability of composite used as a core material. J PROSTHET DENT 1986;56:554-61.
Reprint requests to: CAPT STEVE ARTHUR,DC,USN CHAIRMAN,RESEARCHDEPARTMENT NAVALDENTAL SCHOOL NATIONAI.NAVAL DENTAL CENTER BETHESDA,MD 20814-5077
Guides for authors
available
The Guide to Preparing Articles for THE JOURNAL OF PROSTHETIC DENTISTRY, revised by Professor Paul Barton, Editorial Consultant to the JOURNAL, and the editors, is available to prospective authors. The guide provides the format for developing different types of scientific manuscripts, a checklist for effective writing, and detailed instructions for preparing manuscripts in the style acceptable by the JOURNAL. Also available are the Guidelines for Reporting Statistical Results and an Author’s Guide to Controlling the Photograph. Guides can be obtained from the office of the Editor (Dr. Glen P. McGivney, The Journal of Prosthetic Dentistry, State University of New York at Buffalo, School of Dental Medicine, 345 Squire Hall, Buffalo, NY 14214).
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VOLUME67
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