Restoration of pulpless teeth: Application of traditional principles in present and future contexts

Restoration of pulpless teeth: Application principles in present and future contexts Steven Boston

M. Morgano, University,

of traditional

DMDa

Goldman

School

of Graduate

Dentistry,

Boston,

Mass.

Posts were recommended more than 100 years ago to retain artificial crowns. Recent studies suggest that posts can weaken teeth; therefore restorative procedures that help preserve pulpal vitality and eliminate the need for posts are desirable. If endodontic therapy is unavoidable, conservation of remaining tooth structure is most important. When a post is required to retain a core for an artificial crown, a custom cast post is the most effective means of conserving tooth structure. The length of the post should not be compromised, although 4 to 5 mm of apical gutta-percha must be maintained. The restorative prognosis is improved if the width of the post does not exceed one half the width of the root, and the cemented artificial crown should extend apical to the core to provide a 1.5 to 2 mm ferrule. Complex procedures have allowed the dentist to restore extensively damaged teeth. However, extraction and replacement with implant-supported prosthodontics may be more prudent with severely compromised teeth. (J PROSTHET DENT 1996;75:375-SO.)

M ethods of restoring pulpless teeth were described more than 100 years ago. In 1871 Harrisi recommended a post or “pivot” to retain an artificial crown in a root with an extirpated pulp, although endodontic techniques were crude at that time. Major advances in endodontic therapy that occurred in this century have significantly altered the practice of dentistry. Teeth that were once considered nonrestorable and extracted are commonly treated endodontically and restored to function. The post or dowel that was originally designed merely to retain the coronal restoration when there was inadequate remaining tooth structure2 was later viewed as a method of reinforcement of the pulpless tooth.3 The rigid post or dowel has been assumed to prevent horizontal fracture of the tooth by directing functional and parafunctional forces through the center of the root.4

CONCEPTS

OF REINFORCEMENT

Data from an in vitro investigation by Kantor and Pines5 reported that an intraradicular post doubled the fracture resistance of a root. Nevertheless, other studies have indicated that transmission of occlusal forces intraradicularly predisposed the root to vertical fracture.6, 7 Eissman and Radke3 recommended a cast restoration that extended at least 2 mm apical to the junction of the core and the remaining tooth structure and suggested that encirclement of the root with this “ferrule effect” would protect the pulpless tooth against fracture by counteracting spreading forces generated by the post.

Presented before the Annual Meeting dontics, Tucson, Ark., May 1995. “AssociateProfessor ofProsthodontics, Sciences. Copyright 0 1996 by the Editorial PROSTHETIC

0022-3913/96/$5.00

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+ 0 10/l/70207

of the Academy Department Council

of THE

of ProsthoofRestorative JOURNAL

OF

Fig. 1. Rigid post and core can prevent horizontal fracture of clinical crown; however, occlusal forces (OF) are transmitted to root as spreading forces (SF). If artificial crown extends 2 mm apical to junction of core and tooth, ferrule effect (FE) will resist spreading forces and help prevent vertical fracture. Post and core in combination with artificial crown provide coronoradicular stabilization. Others have advocated this cast ferrule,s, ’ and the combination of a post-core restoration with an overcasting to act as a ferrule is commonly accepted today as a rational approach to coronoradicular stabilization for a pulpless tooth (Fig. 1). Nevertheless, a recent nationwide survey of dentists’ philosophies and techniques of restoring pulpless teeth indicated major differences in treatment approaches and beliefs concerning reinforcement.rO Approximately 50% of the respondents to the survey believed that a post will reinforce an endodontically treated tooth.rO There are few in vivo reports of restored endodontically treated teeth,

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but poorer successrates for pulpless teeth can be expected compared with teeth with vital pulps.rla la Endodontically treated teeth that serve as abutments to fixed or removable prostheses have been reported to be most prone to failure,i3, l4 and several retrospective studies of f=ed partial dentures (FPDs) with cantilever pontics suggested an inordinate failure rate for pulpless abutment teeth.15-i7 A comprehensive retrospective study by Sorensen and Martinoffrs rs of 1273 endodontically treated teeth in the practices of nine general dentists found the custom-tapered cast post and core more often associated with irreversible damage to the tooth, and post placement did not increase resistance to fracture of the tooth or dislodgment of the artificial crown. These investigators reported the highest success rate with the parallel-sided serrated post.ls, I9 FAILURES It is often assumed that the higher incidence of failure observed for restored endodontically treated abutments is primely related to the fragility of the sparse remaining tooth structure.2Q However, mechanical weakness may not be the only reason for the higher failure rate of pulpless teeth. An in vivo investigation of the pressoreceptive function of endodontically treated teeth has indicated reduced tactile sensation with these teeth.21 This altered pressoreceptive capacity may affect the patient’s ability to detect functional overload and could be a significant contributing factor to the higher fracture rate reported for pulpless teeth.

Numerous features of posts have been implicated as causes of failure. Tapered posts were claimed to produce wedging stresses in the root, which can predispose the root to fracture, and parallelism of the post has been suggested as a method to reduce unwanted stresses.22s23An additional advantage of parallelism is improved retention, so the post is less likely to be dislodged.24a25 Threaded posts that actively engage tooth structure provide maximal rem tention but have also been criticized for generating stresses within the radicular dentin. 26,27 Guidelines at this time are still confusing because of the lack of well-controlled in vivo studies and the conflicting results of many in vitro investigations.“” The studies of Sorensen and Martinoff18, I9 of 1273 endodontically treated teeth implied an inordinate failure rate with cast metal posts. However, the data indicated that almost one half the cast posts in their study were one half or less the length of the clinical crowns. When the length of the posts equaled the length of the clinical crowns, the success rate was 97.5%. It appears that a limitation of the investigations by Sorensen and Martinoffls ig was the lack of control of the clinical procedures performed by the nine dentists in the study. Preliminary data from a retrospective study in progress by Milot and Morgan0 conducted at the University of Montreal School of Dental Medicine have indicated an im376

pressive success rate for custom-cast metal posts and cores. More than 900 patients with pulpless teeth treated by predoctora1 dental students restored with cast posts were studied, and the overall success rate was approximately 96%. The one variable that appeared to have the greatest influence on success was the width of the post. When the radiographic width of the post exceeded one half the width of the root, the success rate was considerably poorer. RECOMMENDATIONS Indications

FOR THE

FUTURE

for posts

Posts do not reinforce endodontically treated teeth, but they are indicated when there is inadequate tooth structure to retain a core for a coronal restorationZg Frequently, pulpless anterior teeth can be conservatively restored with a bonded composite resin restoration rather than an artificial crown.3o When significant coronal tooth structure remains and preparation of the pulpless tooth is conservative, an artificial crown can often be placed without a post. Nevertheless, because of their small diameter, mandibular incisors and maxillary lateral incisors will commonly require a post and core if an artificial crown is indicated. If an anterior pulpless tooth will be restored with a complete crown and must serve as an abutment to a prosthesis; omission of the post may not be prudent regardless of the amount of remaining coronal tooth structure. Also, a post alone is not enough because the complete crown retainer must include a ferrule that is 1.5 to 2 mm long. The ferrule can improve resistance to dynamic occlusal loading, help maintain the integrity of the cement seal of the artificial crown retainer, and reduce the potential for stress concentration at the junction of the core and the post.31 When the ferrule is absent, occlusal forces must be resisted exclusively by the post, which may eventually fracture.17 Molars can often be restored with a coronoradicular silver amalgam build-up followed by a complete veneer crown or partial coverage cast restoration.32 This method is best applied to molars with slight-to-moderate destruction of coronal tooth structure where there is adequate depth to the pulpal chamber and can occasionally be used with premolars.33 The coronoradicular amalgam post and core is not as suitable when the molar has lost all tooth structure above the gingival crest and the roots are well spaced with a shallow pulpal chamber. Prefabricated posts are usually indicated to augment the retention of the amalgam core when the pulpa chamber is shallow. Dental amalgam bonding agents such as 4-methac~lo~ethyl~mellitate anhydride (4-META) resin can bond a silver amalgam core to the tooth structure and augment conventional mechanical retention. If prefabricated posts are used, a 4-META resin will also bond the silver amalgam to the posts. The longevity of the resinous bond to the silver amalgam in the oral cavity is upon, but this approach appears promising.

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Length Every effort should be made to ensure maximal length of the post while maintaining 4 to 5 mm of apical guttapercha sea1.35 When roots are unusually short or curved, the post must often be shorter than desired and retention will be less predictable. 36 In addition, greater leverage is exerted when the post is shorter than the clinical crown length, and this unfavorable leverage can also predispose to fracture of the root.36 Commonly the dentist will resort to a threaded post that actively engages radicular dentin to compensate for the limited retention of a short post. However, an active post is least desirable. A 4-META resinous cement can provide retention that rivals a threaded post but does not produce the unfavorable stresses that occur when tooth structure is actively engaged.37 There are no long-term clinical trials on the effects of repeated mechanical loading and thermocycling on the integrity of the bond of this resinous cement. Therefore the in vitro retention reported with a 4-META cement may fatigue with time as a result of plastic deformation from normal intraoral forces.38 When the design of the post follows traditional guidelines, conventional cements are probably the preferred luting agents. Most resinous cements are relatively expensive, technique sensitive, and difficult to manipulate,3g whereas traditional cements are less complicated, more cost effective, and have a long history of success. Also, unlimited retention is not necessarily the goal with any post. If a post fractures, it must be removed from the root to permit retreatment. 4o The remnant of a post that is “bonded” to the root may require mechanical removal with a high-speed handpiece and surgical-length round bur-an arduous task that can result in perforation and loss of the tooth. Furthermore, it is preferable that a post and core dislodge because of cement failure rather than fracture the root if the restoration is subjected to excessive force.

Conservation

of tooth

structure

Conservation appears to be the most critical factor for success, although the dentist should also strive for suffcient length of the post. 41 Because any prefabricated post system requires the dentist to instrument the root to fit the stock post, a custom-fitted cast post is potentially more conservative of tooth structure. With narrow single-rooted teeth such as mandibular incisors preservation of tooth structure is especially important, and custom-cast posts have been reported to offer better retention and resistance to fracture compared with parallel-sided serrated posts.42 Not all cast posts are made conservatively. Strict adherence to the guideline of parallelism of the post space may result in overpreparation of the apical one third of the post channel. There will also be a sharp line angle at the apical termination of the post preparation that can concentrate stresses where the radicular dentin is thinned and weakened (Fig. 2).43

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Fig. 2. A, From facial view it appears that there is adequate thickness of dentin in apical one third of root for parallel-sided post (arrows). B, Proximal view of root reveals sparse remaining facial and lingual dentin. Sharp line angles (arrows) predispose to stress concentration at apical termination of post. Slightly tapered posts are easier to prepare and more conservative because most roots are tapered. The dentist can remove the gutta-percha to the desired depth and then eliminate any undercuts or residual endodontic sealer on the walls of the root canal. The post can then be designed to “fit the available space.” The resultant slightly tapered post could theoretically generate stresses at the coronal one third of the root22; however, these stresses can be effectively counteracted with a ferrule design to the overcasting.3 A tapered post is also less retentive than a perfectly parallel post is, 25 but sufficient length should provide adequate retention so long as the taper is kept to a minimum.44

Enhancing

the ferrule

effect

A beveled finish line for the overcasting can enhance the ferrule effect and increase the fracture resistance of the root. Bevels were once considered desirable for all cast restorations by many prosthodontists because of the geometric advantage or “slip effect” that facilitated marginal closure.45 Numerous studies have challenged this assumption that a cavosurface bevel will improve marginal adaptation.46-4g The contemporary approach to tooth preparations for complete crowns has tended to eliminate cavosurface bevels, which require a potentially unesthetic

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graph may curve dramatically in the third plane, and rigid engine reamers can perforate the root when carried apitally to remove gutta-percha (Fig. 3). Shillingburg and Kesslers5 also advocated extremely large Peeso reamers for canal preparation, and a No. 6 reamer has been suggested for maxillary central incisors. Because of the inherent risk of failure with excessively wide posts, these recommendations should be revised.

Need for realistic

Fig. 3. From facial view palatal root of maxillary molar appears relatively straight (A); however, root curves dramatically in third plane (E) and engine reamer such as Pees0 reamer can perforate root. metal m~~n.50 Nevertheless, for endodontically treated teeth it appears that the beveled finish line is beneficial and desirable when esthetics will permit. Milot and Stein51 have reported the results of an in vitro study that used standardized plastic analogs to investigate the influence of the cavosurface bevel on the fracture resistance of the root. These investigators found a statistically significant increase in resistance to root fracture with three different post systems for a beveled preparation compared with a nonbeveled preparation. Shillingburg et a1.52have advocated a contrabevel in the preparation for the cast core to act as an additional ferrule that is independent of the cast crown; however, Sorensen and Engleman53 found no advantage to this contrabevel. Also, a 1 mm beveled finish line for a complete crown preparation without additional tooth structure coronal to the bevel did not improve fracture resistance of the root.53

Conservative, controlled removal of gutta-percha

approach

to

A safe, rapid technique for removal of the gutta-percha is the use of a heated instrument.54 The Touch h Heat controlled heat inst~ent (Analytic Technology Corp., Redmond, Wash.) is convenient for this purpose and virtually eliminates the potential to perforate the root. Peeso reamers can then be used to remove the endodontic sealer only. Rarely is a size larger than a No. 2 Peeso reamer necessary. Shillingburg and Kessler55 have recommended the Peeso reamer for removal of the gutta-percha. However, a root that appears straight on the two-dimensional radio-

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The philosophy of retaining severely compromised teeth regardless ofthe cost of treatment or the prognosis also requires reevaluation. Commonly teeth that are carious to the gingival margin are treated with surgical crown len~he~ng or o~hodontic extrusion. A core is then retained with an intraradicular post, and the tooth is restored with an artificial crown. The literature has described numerous approaches to restoring compromised pulpless molars with resected roots.56 Complex state-ofthe-art dental procedures have allowed the dentist to restore teeth that are technically nonrestorable, but what is the prognosis of these resurrected teeth? In the past these relatively fragile teeth were treated similar to sound, vital teeth. Often they were expected to serve as abutments to cantilever FPDs, but failure was inordinately high.i5 Endodontically treated teeth with resected roots were also commonly used as key abutments to FPDs, often as a method to avoid a removable partial prosthesis.56 Nevertheless, a failure rate ranging from 32% to 38% has been reported for resected pulpless teeth.57J 58 Root resective procedures are technically demanding, and a high percentage of failures appears to be the result of faulty resections.5g

New treatment

options

Advances in endodontics altered the practice of dentistry in the 1950s and provided the dentist with many more treatment options. The reported predictability of implant dentistry has further enhanced the dentist’s treatment alternatives.60‘65 It is now difficult for the contemporary dentist to justify the use of questionable teeth as abutments to complex prosthetic restorations. Severely compromised teeth can often be extracted and replaced with implant-supported FPDs or single crowns for comparable costs and with a better prognosis. Finally, the best approach to avoiding the problem of the endodontically treated tooth is to avoid the need for endodontics. Wee-mean~g recommendations of very aggressive tooth preparations for artificial crowns that provide the laboratory technician with “more than enough room” to ensure favorable esthetics can jeopardize the vitality of healthy pulps. A more conservative tooth preparation might challenge the ingenuity of the laboratory technician who is striving for optimal esthetics, but preservation of sound, vital tooth structure will improve the long-term

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prognosis of the tooth. Traditional guidelines for tooth preparations should be reemphasized where tooth reduction is conservative but adequate for the planned restorative material and esthetic concerns are realistic. A fragile masterpiece with a guarded prognosis is not necessarily in the best interest of the patient or the dentist.@

SUMMARY Endodontically treated teeth present unique problems; therefore conservative restorative techniques that avoid endodontic treatment are highly desirable. If endodontic therapy is unavoidable, conservation of tooth structure becomes even more important. A post may be required to support a core, but the length of the post should not be compromised and the width should be minimal. Slightly tapered posts are more conservative and easier to prepare than parallel-sided posts. The dentist must also develop a ferrule for teeth restored with posts. When the length ofthe post must be compromised because of anatomic limitations, a 4-META resinous cement can be used and is probably preferable to an active post. For severely compromised teeth extraction and replacement with conventional or implant-supported prosthodontics should be considered because this treatment approach may be more predictable.

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Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHET DENT 1978;40:157-62. Pascoe DF. An evaluation of the marginal adaptation of extracoronal restorations during cementation. J PROSTHET DENT 1983;49:657-62. Byrne G. Influence of finish-line form on crown cementation. Int J Prosthodont 1992;5:137-44. Goldman M, Laosonthorn P, White RR. Microleakagefull crowns and the dental pulp. J Endodont 1992;18:473-5. Shillingburg HT Jr, Jacobi R, Brackett SE. Fundamentals of tooth preparations for cast metal and porcelain restorations. Chicago: Quintessence, 1987:259-78. Milot P, Stein RS. Root fracture ip endodontically treated teeth related to post selection and crown design. J PROSTHET DENT 1992;68:428-35. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fEed prosthodontics. 2nd ed. Chicago: Quintessence, 1981:150-l. Sorensen JA, Engleman MJ. Ferrule design and kacture resistance of endodontically treated teeth. J PROSTHET DENT 1990;63:529-36. Haddix JE, Mattison GD, Shulman CA, Pink FE. Post preparation techniques and their effect on the apical seal. J PROSTHET DENT 1990; 64515-g. Shillingburg HT Jr, Kessler JC. Restoration of the endodontically treated tooth. Chicago: Quintessence, 1982:46. Newell DH, Morgan0 SM, Baima RF. Fixed prosthodontics with periodontally compromised dentitions. In: Malone WF, Koth DL, Cavazos E Jr, Kaiser DA, Morgan0 SM, eds. Tylman’s theory and practice of fixed prosthodontics. 8th ed. St Louis: Ishiyaku EuroAmerica, 1989:87111. Langer B, Stein SD, Wagenberg B. An evaluation of root resections: a ten-year study. J Periodontol 1981;52:719-22. Biihler H. Evaluation of root-resected teeth: results after 10 years. J Periodontol 1988;59:805-10. Newell DH. The role of the prosthodontist in restoring root-resected

Bound

volumes

available

molars: a study of 70 molar root resections. J PROSTHET DENT 1991;65: 7-15. 60. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseoin@rated dental implants: the Toronto study, I: surgical results. J PROSTHET DENT 1990;63:451-7.

61. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseoin&grated dental implants: the Toronto study, II: the prosthetic results. J PROSTHET DENT 1990;64:53-61. 62. Zarb GA, Schmitt A. The longitudinal clinical effectiveness of osseointegrated dental implants: the Toronto study, III: problems and complications encountered. J PROSTHET DENT 1990;64:185-94. 63. van Steenberghe D, Lekholm U, Bolender C, et al. Applicability of osseointegrated oral implants in the rehabilitation of partial edentulism: a prospective multicenter study on 558 fixtures. Int J Oral Maxillofac Implants 1990;5:272-81. 64. Bahat 0. Treatment planning and placement of implants in the posterior maxillae: report of 732 consecutive Nobelpharma implants. Int J Oral Maxillofac Implants 1993;8:151-61. 65. Cordioli G, Castagna S, Consolati E. Single-tooth implant rehabilitation: a retrospective study of 67 implants. Int JProsthodont 1994;7:52531. 66. Gordon SR, Lloyd PM. Fixed prosthodontics in the elderly population: life expectancy offixed restorations, failures, and retreatment methods. Dent Clin North Am 1992:36:783-95. Reprint requests to: DR. STEVEN M. MORGANO BOSTON UNIVERSITY GOLDMAN SCHOOL OF GRADUATE DENTISTRY DEPARTMENT OF RESTORATIVE SCIENCES, ROOM 612 100 E. NEWTON ST. BOSTON, MA 02118

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