ceedings of an International Symposium on the acid etch technique. St Paul, Minn: North Central Publishing Co, 197550-62. 3. Mixson JM, Eick JD, Tim DE, Moore DL. The effects of variable wash times and techniques on enamel-composite resin bond strength. Quintessence Int 1988;19:279-85. Fuse K. Studies on bond strength of composite resin to the acid-etched enamel. Jpn J Conserv Dent 1978;21:326-49. Nakamichi I, Iwaku M, Fusayama T. Bovine teeth as possible substitutes in the adhesion test. J Dent Res 1983;62:1076-81. Yoshida T. The effect of environmental temperature and humidity on the adhesion of composite resins to the etched enamel surface. Jpn J Conserv Dent 1983;26:412-26. 7. Miyairi H, Fukuda H. A shear test method of dental adhesives. Jpn J Dent Mat 1987;6:614-20.
CONCLUSIONS This investigation examined the relationship between air-drying duration and the bond strength to bovine enamel and dentin for two composite resins and their bonding agents. There was (1) no statistical difference between the bond strength to the unetched and etched enamel versus the air-drying duration for NB and PB agents and (2) the bond strength to the unetched dentin depended upon the time of air drying, but the bond strength to the etched dentin was similar for NB and PB bonding agents.
1. Phillips RW. Advancements in adhesive restorative dental materials. J Dent Res 1966;45:1662-7. 2. Young KC, Hussey M, Gillespie FC, Stephen KW. In vitro studies of physical factors affecting adhesion of fissure sealant to enamel. Pro-
John A. Sorensen, D.M.D,* University
DR. KAZUYOSHI ICHIKI FUKUOKA DENTAL COLLEGE 700 TA, SAWARA-KU FUKUOKA JAPAN 814-01
and Michael J. Engelman, D.D.S.**
This study evaluated the fracture resistance of pulpless teeth with various ferrule designs and amounts of coronal tooth structure. One millimeter of coronal tooth structure above the crown margin substantially increased the fracture resistance of endodontieally treated teeth, whereas a contrabevel at either the tooth-core junction or the crown margin was ineffective. The thickness of axial tooth structure at the crown margin did not appreciably improve resistance to fracture. (J PROSTBET DENT 1990;63:529-36.)
n vitro and in vivo researchhasdemonstratedthat a dowel doesnot reinforce endodontically treated teeth.lm4 However, when there is sparseclinical crown remaining, a meansof replacing lost coronal tooth structure for crown retention is necessary.A cast dowel and core is onemethod of restoring a foundation for crown preparation.5-g The preparation design of pulpless teeth is a critical consideration in the restoration of endodontically treated teeth but has received only limited attention. A ferrule or encircling band of cast metal around the coronal surfaceof
meeting,Napa,Calif. This investigation was supported Research Support grant No. Institute of Dental Research, by the Jelenko Co., Armonk, *Assistant Professor, Director, **Assistant Professor, Section
in part by NIH as a Biomedical 2507RRO5304 from the National National Institutes of Health, and N.Y. Postgraduate Prosthodontics. of Removable Prosthodontics.
the tooth hasbeensuggestedto improve the integrity of the endodontically treated tooth (Fig. 1). Various ferrules have beenadvocated, suchasa two-plane preparation of the root surface1°-12 and a contrabevel around the occlusalsurface of the preparation of a pulplesstooth.13-16The ferrule as part of the core or the crown restoration is purported to prevent tooth fracture.5J1-1g The purpose of the ferrule is to improve the structural integrity of the pulpless tooth by counteracting (1) the functional lever forces,%(2) the wedging effect of tapered dowels,21 and (3) the lateral forcesexerted during insertion of the dowe1.22 Several authors have suggestedthat the crown should extend 2 mm beyond the tooth-core junction to ensure a protective ferrule effect.16-18 Although the concept of a ferrule appearslogical, its benefits have not beenconfirmed by researchnor has a superior ferrule designevolved. This in vitro study examinedthe effect of various ferrule designson fracture resistance of endodonticaliy treated anterior teeth.
Fig. 1. Illustration interface.
of ferrule effect at tooth-restoration
Sixty intact maxillary central incisors were randomly assigned to six groups of 10 teeth and stored in saline solution during all procedures. The teeth were decoronatedgroups 1 and 2 to 15 mm and groups 3 through 6 to 17 mm lengths. Endodontic therapy was completed for the 60 teeth. Each tooth was instrumented to a size No. 45 file and to a size No. 55 file 1 mm short of the apex. During instrumentation, irrigation was performed with saline. The canals were then dried with air and Kerr medium absorbent paper points (Kerr Mfg. Co., Romulus, Mich.). A No. 40 gutta-percha master cone was fit to the apex and Kerr pulp canal sealer was mixed and applied to the master cone. The master cone was laterally condensed with No. 2 finger pluggers (R. Chige-Switzerland). Three medium-fine gutta-percha accessory points were laterally condensed, the excess was removed with a heated instrument, and the teeth were returned to the saline solution. The post space was prepared with a No. 3 Peeso reamer (Union Broach Co., Long Island, N.Y.) to within 4 mm of the apex. Next, a No. 4 stainless steel Parapost (Whaledent International, New York, N.Y.) post was fitted to within 4 mm of the apex. The six groups of teeth were prepared according to the guidelines in Fig. 2. The crown preparations followed the cementoenamel junction to simulate the clinical situation. The teeth were prepared with high-speed water-cooled instrumentation to 1.5 mm of axial reduction at the margin. The definition of axial tooth structure is the amount of tooth structure between the inside wall of the canal and the exterior surface of the tooth at the preparation margin.
Coronal dentinal extension is the tooth structure occlusal to the shoulder preparation. Group 1 was prepared with a go-degree shoulder and no coronal dentinal extension. Axial structure was removed with a cone-shaped carbide bur (Miltex Instruments Co., Lake Success, N.Y) at slow speed under water irrigation, maintaining 1 mm of axial tooth structure at the shoulder. Group 2 was prepared with a go-degree shoulder without coronal dentinal extension. Group 3 was prepared with a 130-degree angle forming a sloped shoulder from the base of the core to the margin and all tooth structure above the axial-gingival line angle was removed. Group 4 was prepared with a go-degree shoulder and a 1 mm wide go-degree bevel finish line with no coronal dentinal extension. Group 5 was prepared with a go-degree shoulder, a 1 mm wide 60-degree bevel finish line, and a 1 mm coronal dentinal extension. A butt-joint configuration was prepared at the tooth-core junction. Group 6 was prepared with a go-degree shoulder, a 1 mm wide 60-degrees bevel finish line, and a 2 mm coronal dentinal extension. A 1 mm wide 60-degree contrabevel was prepared at the tooth-core junction. The coronal tooth structure was measured at the points indicated in Fig. 3, and the width of the root was recorded at several levels (Fig. 4). The length of the prepared canal was confirmed and a plastic No. 4 Parapost burnout pattern (Whaledent International) was fitted to the canal. A preformed anterior core pattern No. 1 (Parkell, Farmingdale, N.Y.) was adjusted to contact the proximal surfaces and 7 mm in length from the facial margin to the incisal edge. The core pattern was filled with acrylic resin (Duralay, Reliance Dental Mfg. Co, Worth, Ill.) and seated on a wet tooth along the long axis viewed from the proximal surface. In group 1 the acrylic resin extended 2 mm into the prepared canal, reflecting the tapered shape of the canal, with identification numbers on the facial surface of the core. After complete polymerization, the core was prepared to 1.5 mm axial reduction with water-cooled high-speed instrumentation. The dowel and core patterns were invested with a phosphate-bonded investment (Cera-Fina, Whip-Mix Corp., Louisville, Ky.) and five patterns were placed in each 4.45 cm ring and cast in Albacast silver-palladium metal (Jelenko, Armonk, N.Y.). The cast post and cores were refined, finished, and abraded with 50 um aluminum oxide under 2.6 kg/cm2 pressure. The canals were irrigated with water and dried with an air syringe and paper points. Zinc phosphate cement (Flecks, Mizzy, Inc., Clifton Forge, Va.) was mixed and introduced into the canal with a Lentulo spiral drill (Pulpdent Corp., of America, Brookline Village, Mass.) until the canal appeared full. Cement was placed on the dowel and
Fig. 2. Six experimental tooth preparation designs.
8 7 6 I I I ’
Facial Fig. 3. Occlusal view of tooth structure through 6.
delicately seated by finger pressure, but during cementation hydraulic pressure was released and the core was reseated. The teeth were placed in a static loading device for 15 minutes with 2.72 kg of pressure. The excess cement was removed and the preparations refined. The preparation was lubricated with saline and a crown was directly waxed by use of 3M size 15 polycarbonate crown (3M, St. Paul, Minn.) and inlay wax (Maves Co., Cleveland, Ohio). The wax crown was removed, sprued, and invested with a phosphate-bonded investment. The crowns
Proximal Fig. 4. Proximal view of buccolingual through 9.
were induction-cast in base metal alloy (Super 14, Dental Alloy Products, Inc., Compton, Calif.) refined under a X10 microscope and abraded with 50 micron aluminum oxide under 2.8 kg/cm2 pressure. The crown preparations were rinsed with Cavity and Crown Preparation cleaner (CPC) (Lavoric Corp., St. Louis, MO.) and dried with an air syringe. The crowns were luted with zinc phosphate cement mixed by using the ce-
4mm _I 9mm
6. Buccal cross-sectionalview of specifications for endodontically treated teeth restored with post, core, and crown.
7. Schematic of loading apparatus.
I. Mean failure loads Mean
1 2 3 4 5 6 *95% Confidence
6. Proximalcross-sectionalviewofradiographicmeasurements10 and 11. Fig.
mentation jig at 2.72kg of pressurefor 15minutes. The excesscement was removed after complete setting and the teeth were placed in physiologic saline for 24 hours. Fig. 5 depicts the dimensions of the restored endodontically treated tooth. The post length wasmadeequal to the crown length in accord with researchindicating superior clinical prognosiswith this ratio.23 The specimenswere then radiographed from the proximal aspect and the width of the labial and lingual tooth
29.5 29.0 35.0 36.3 65.3 69.4
6.0 9.5 10.4 10.9 21.8 7.7
structure wasmeasuredat the apex of the dowel as in Fig. 6. The teeth were examinedwith a X20 microscopeand defects were recorded. The teeth were embedded in autopolymerizing acrylic resin with a mold that had a flat surface 2 mm below the facial cementoenameljunction. This designsimulated the natural biologic width.24 An Instron testing machine (Instron Corp., Canton, Mass.) applied controlled loadsto the teeth at a crosshead speedof 2.54 mm per minute. A device wasmade that allowed loading of the tooth at an angle of 130degreesto its long axis (Fig. 7). This angle of loading waschosento simulate a contact angle in classI occlusionsbetween maxillary and mandibular anterior teeth.25*26The failure threshold wasdefined asthe maximum load that a samplecould withstand, and catastrophic failure occurred asa result of crown displacement, post displacement, root fracture, or post fracture. Failure loads, modesof failure, and tooth preparation design were recorded and statistically anaMAY
8. Mean failure loads of six preparation designs.
Mode of failure Group
Cementfailure Postfracture Tooth fracture Each’
3 4* 4*
one sample in more than one category.
lyzed for significant correlations between designand failure loads.
RESULTS The meanfailure loadswere: group 1,29.5 kg + 6; group 2,29 kg f 9.5; group 3,35 kg -+10.4;group 4,36.3 kg rt 10.9; group $65.3 kg -+21.8; group 6,69.4 kg 17.7 (Table I and Fig. 8). ANOVA basedon rank revealed differencesamong groups (F [5,543= 20.175,p < 0.0001).Multiple rangetest revealed that groups 5 and 6 were significantly different than groups 1 through 4. ANOVA was used to determine whether there was a substantial difference in tooth thicknessbetweengroupsat eleven points. ANOVA demonstrated that, for measurements 1 through 4, the thickness of the tooth structure of group 1 was different than groups 2 through 6 (measurement No. 1 F[5,54] = 56.76, p < 0.0001; No. 2 F[5,54] = 29.96, p < 0.0001~No. 3 F[5,54] = 37.70, p < 0.0001; and No. 4 F[5,54] = 21.90,p l4 The most critical step in the tooth preparation is the parallel walls of the dentin coronal to the shoulder of the preparation (Fig. 9). One of the problems encountered in making the post and core is the conflicting objectives in casting expansion and contraction. Typically, in casting a crown, provisions are made in the casting procedure for expansion of the coping to compensate for alloy shrinkage.33l 34Conversely, technicians underexpand the cast post to avoid binding and fracture of the tooth during cementation.14 With the design of group 6, the two diametrically opposed casting goals are impossible. The design of group 6 makes delivery and cementation arduous for the clinician. Based on this clinical study, the goal should be preservation of maximum residual coronal tooth with a butt-joint margin between core and tooth structure. The ferrule is defined as an encircling band of cast metal.16vl8 This study demonstrated that it was not the contrabevel or metal collar of the cast core, but the resistance form of the artificial crown against the residual coronal dentin that is crucial in the design. The ferrule is provided by parallel walls of dentin coronal to the shoulder of the preparation that elevates resistance form. A modification of the definition of “ferrule effect” is suggested. The ferrule effect is a 360-degree metal collar of the crown surrounding the parallel walls of the dentin extending coronal to the shoulder of the preparation (Fig. 9). The result is an elevation in resistance form of the crown from the extension of dentinal tooth structure. The critical design in endodontically restored teeth is the support of the crown against the reciprocating walls of residual dentin coronal to the shoulder. This follows accepted principles of tooth preparation.36-37 Other in vitro studies on restoration of endodontically treated teeth corroborate these findings. ‘Haag and Dwyer38 discovered that the method of coronal radicular stabilization was not as important as complete crowns and finish lines beyond the buildup restoration for mandibular molars. Similarly, Gelfand et a1.3gfound that the type of core for molar teeth was
not critical if covered by a complete crown, nor was the type or length of the post important. Recent research indicated that in vitro studies on restoration of pulpless teeth should include cementation of crowns. The support provided by the coronal extension of dentinal tooth structures above the crown shoulder emphasizes that clinical crown length is crucial to the success of the restored endodontically treated tooth. Increased clinical crown length can be achieved by surgical crownlengthening or by orthodontic extrusion of the root. Coronal tooth extension can then provide for greater resistance form of the endodontically treated tooth. The dentist must evaluate the root length. If the root is too short or the crown-to-root ratio unfavorable, the tooth may be unsuitable as an abutment. If the osseous support and the root length are inadequate, the dentist should relate to the patient that the prognosis is poor. Factors contributing to a poor prognosis are (1) lack of coronal tooth structure, (2) poor crown-to-root ratio, and (3) inadequate root length for extrusion. These factors may render the prognosis of the tooth so poor that the tooth is extracted and an alternate tooth selected as the strategic abutment. Endodontically treated teeth used as removable partial denture (RPD) abutments have a five times greater failure than single teeth. 4o These considerations, in addition to minimal coronal tooth structure, make the prognosis questionable for a compromised tooth. Pulpless teeth are commonly avoided as abutments for an RPD, especially if the terminal abutment is for a distal extension.41 As with all in vitro research, this study’s application may be limited in clinical situations. The elasticity of the periodontal ligament, bone, and tooth structure, including the difference in functional forces in the stomatognathic system, were not simulated in this study.
CONCLUSIONS The following conclusions were drawn from this in vitro study on endodontically treated teeth: 1. One millimeter of coronal dentin above the shoulder significantly increased the failure threshold. 2. The preparation of the coronal walls should be parallel for maximum resistance form. 3. The contrabevel design at either the tooth-core junction or the crown margin did not improve the failure threshold. 4. The axial width of the tooth at the crown margin did not significantly increase the fracture resistance or alter the failure threshold. We thank Mr. Wayne Mito for technical Jack Lee for stat,istical analysis.
1. Lovdahl PE, Nicholls Jl. Pin retained amalgam cores vs. cast gold dowel and cores. J PROSTHET DENT 1977;38:507-14. 2. Guxy GE, Nicholls JI. In vitro comparison of intact endodonticahy
teeth with and without
3. Trope M, Maltz DO, Tronstad L. Resistance to fracture of retored endodontically treated teeth. Endod Dent Traumatol 1985$108-11. 4. Sorensen JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J PROSTHET DENT 1984;51:780-4. 5. Rosen H. Operative procedures on mutilated endodontically teeth. J PROSTHET DENT 1961;11:973-86. 6. Tylman SD. Theory and practice of crown and fixed partial
prosthodontics. Philadelphia: WB Saunders Co, 1970:580. I. Johnston JF, Phillips RW, Dykema RW. Modern practice of crown and bridge fixed partial prosthodontics. Philadelphia: WB Saunders Co, 1971:608. FL Clinical criteria for posts and cores. J PROSTHET 8. Perel M, Muroff DENT 9. Lorey
RE. Abutment considerations. Dent Clin North Am 1980;24:6379. 10. Sheets CE. Dowel and core foundations. J PROSTHET DENT 1970;23:5865. 11. Kornfeld 12. 13. 14. 15.
M. Mouth rehabilitation: clinical and laboratory procedures. Vol 1, 2nd ed. St Louis: CV Mosby Co, 1974389. Shelby DS. Anterior restoration, fixed bridgework, and esthetics. Springfield, 111: Charles C Thomas Publishers, 1976107. Rosner D. Function, placement, and reproduction of bevels for gold castings. J PROSTHET DENT 1963;13:1160-99. Shillingburg HT, Hobo S, Whitsett LO. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Pub1 Co Inc, 1982:150-5. Weine FS, Herschman J, Strauss S. Restoration of the endodontically treated tooth and bleaching. In: Weine FS, ed. Endodontic therapy. 3rd ed. St Louis: CV Mosby Co, 1982:600. Eissmann HF, Radke RA. Postendodontic restoration. In: Cohen S, Burns RC, eds. Pathways of the pulp. St Louis: CV Mosby Co, 1987:
17. Trabert KC, Cooney JP. The endodontically treated tooth: restorative concepts and techniques. Dent Clin North Am 1984;28:923-51. 18. Trabert KC, Cooney JP, Cap&o AA, Standlee JP, Tee1 S, Wands DM, Ingle JI. Restoration of endodontically treated teeth and preparation for overdentures. In: Ingle JI, ed. Endodontics. 3rd ed. Philadelphia: Lea & Fehiger, 1985:810-59. 19. Staffanou R. Preparation design for crowns and retainers. In: Rhoads JE, Rudd KD, Morrow RW, eds. Dental laboratory procedures: fixed partial dentures. vol. 2. 2nd ed. St Louis: CV Mosby Co, 1986:165-6. 20. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. Chicago: Quintessence Publishing Co, Inc, 1987:185-203. 21. Standlee JP, Caputo, AA, Collard EW, Pollack MH. Analysis of stress distributions by endodontic posts. Oral Surg 1972;33:952-60. 22. Standlee JP, Caputo AA. Interaction of endodontic posts with tooth structure. In: Kurer P, ed. Kurer anchor system. Chicago: Quintessence Publishing Co, Inc, 1984:160-4. 23. Sorensen JA, Martinoff JT. Clinically significant factors in dowel design. J PROSTHET DENT 1984;52:28-35. 24. Garguilo AW, Wentz FM, Orban B. Dimensions and relations of the dentogingival junction in humans. J Periodontol 1961;32:261-7. 25. Wheeler RC. Dental anatomy, physiology and occlusion. 5th ed. Philadelphia: WB Saunders Co, 1974:436. 26. Meyers RE. Handbook of orthodontics. 3rd ed. Chicago: Year Book Medical Publishing Inc, 1977:411. 21. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. J Endodont 1978;4:341-5. 28. Hock DA. Impact resistance of posts and cores [Master’s thesis]. Ann Arbor, Micb: University of Michigan, 1978:28. 29. Mattison GD. Photoelastic stress analysis of cast-gold endodontic posts. J PROSTHET
30. 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. 31. Henry PJ. Photoelastic
analysis of post core restorations. Aust Dent J 1978;22:157-9. 32. Plasmans PJ, Visseren LG, Vrijhoef MM, Kayser AF. In vitro comparison of dowel and core techniques for endodontically treated molars. J Endodont 1986;12:382-7. 33. Craig RG. Restorative dental materials. 7th ed. St Louis: CV Mosby Co, 1985:412-3.
34. Phillips RW. Skinner’s science of dental materials. 8thed. Philadelphia: WB Saunders, 1982425. 35. Kaufman EG, Coelho AB, Colin L. Factors influencing the retention of cemented gold castings. J PROSTHET DENT 1961;11:487-502. 36. Gilboe DB, Teteruck WR. Fundamentals of extracoronal tooth preparation: part 1: retention and resistance form. J PROSTHET DENT 1974; 32~651-6.
37. Potts RG, Shillingburg HT, Duncanson MG. Retention and resistance of preparations for cast restorations. J PROSTHET DENT 1980;43:303-8. 38. Hoag ED, Dwyer TG. A comparative evaluation of three post and core techniques. J PROSTHET DENT 1982;47:177-81. 39. Uelfand M, Goldman M, Sunderman EJ. Effect of complete veneer crowns on the compressive strength of endodontically treated teeth. J PROSTHET
40. Sorensen JA, Martinoff J PROSTHET 41. Kratochvil
FJ. Partial Co, 1988:lOl.
DR. JOHN A. SORENSEN CHS 33-041 SCHOOL OF DENTISTRY UNIVERSITY OF CALIFORNIA Los ANGELES, CA 90024
of the relationship between anterior and posterior disclusive angles. Part II: Study of a popultition
Lionel B. Pelletier, D.D.S., M.Med.Sc.,* and Stephen D. Campbell, D.D.S., M.Med.Sc.** Harvard School of Dental Medicine, Boston, Mass. The lateral and protrusive anterior and posterior disclusive angles were recorded and measured by using a newly developed method (part I) on a popuiation of 50 subjects (mean age 2’7.2 years). All of the subjects had natural anterior teeth, 46 had a clinically acceptable anterior guidance, and the remaining five had a partial (unilateral) or total lack of anterior guidance. A statistically significant correlation between the anterior and posterior disclusive angles was not found either in the entire population (n = SO) or in the stratum of the population that consisted of subjects with a clinically acceptable anterior guidance (n = 45). The Angle class of occlusion or anterior deep bite malocclusion were not identified as possible confounders or effect modifiers. This study suggested that the intra-articular tissues of the temporomandibular joint played a significant role in the relationship of their osseous components. Practical guidelines to determine anterior guidance could be established by using population means. (J PROSTHET DENT 1990;63:636-40.)
he determinants of the morphology of human temporomandibular joints (TMJ) have beenthe subject of numerousinvestigations’but remain poorly understood. It has been hypothesized that the lingual surfaces of the maxillary anterior teeth influence the growth, development, and shape of these joints.l There is disagreement
From a thesis submiked in partial fulfillment of the requirements for the degree Master of Medical Sciences in Oral Biology, Harvard University. Supported in part by an award of the Northeastern Gnathological Society. Third place, Stanley D. Tylman Research Award, American Academy of Crown and Bridge Prosthodontics. *Faculty member, Prosthetic Dentistry. **Assistant Professor of Prosthetic Dentistry, Co-Director of Postgraduate Prosthodontics. 10/l/13379
among various authors correlating the existence of an anatomofunctional interrelationship betweencondylar and anterior guidancesof the mandible in humans.The current literature, consists of unproved st&ements1‘4or studies omitting the articulrir disk becausebiometric measurements were performed on dry bone specimen&l3 or with radiographic techniques.14* l5 This study first correlated in humansthe lingual guiding inclines of the maxillary incisorswith the posterior disclusive angles.It alsocorrelated the lingual guiding inclinesof the maxillary caninesand maxillary incisorsfunctioning in lateral excursionsof the mandible, with the lateral posterior disclusive angle of the corresponding nonworking condyles, acknowledging all componentsof the TMJ.
This project required the design of a system of instrumentation obtained by modifying existing armamentarium