Foundation Restorations in Fixed Prosthodontics: Current Knowledge and Future Needs

Foundation restorations in fixed prosthodontics: Current knowledge and future needs

Steven M. Morgano, DMD,a and Susan E. Brackett, DDS, MSb Faculty of Dentistry, Kuwait University, Safat, Kuwait, and Oklahoma City, Okla.

Purpose. The Ad Hoc Committee on Research in Fixed Prosthodontics established by the Academy of Fixed Prosthodontics publishes a yearly comprehensive literature review on a selected topic. The subject for this year is foundation restorations. Methods. Literature of various in vitro and in vivo investigations that included technical and clinical articles was reviewed to provide clinical guidelines for the dentist when selecting methods and materials for restoration of structurally compromised teeth. Topics discussed and critically reviewed include: (1) desirable features of foundation restorations, (2) foundations for pulpless teeth, (3) historic perspectives, (4) cast posts and cores, (5) role of the ferrule effect, (6) prefabricated posts, (7) direct cores, (8) foundation restorations for severely compromised teeth, (9) problems and limitations, (10) future needs, and (11) directions for future research. Conclusion. This comprehensive review brings together literature from a variety of in vitro and in vivo studies, along with technique articles and clinical reports to provide meaningful guidelines for the dentist when selecting methods and materials for the restoration of structurally compromised teeth. (J Prosthet Dent 1999;82:643-57.)

CLINICAL IMPLICATIONS The topic of foundation restorations involves many materials and techniques used in everyday dental practice. This article comprehensively reviews the dental literature on this subject to provide clinically relevant guidelines for the dentist. Limitations in knowledge are discussed, and suggestions for future research to improve the profession’s understanding of the clinical performance of foundation restorations are made.

The Ad Hoc Committee on Research in Fixed lodgment: (1) design of the tooth preparation, (2) fit Prosthodontics established by the Academy of Fixed of the casting, and (3) nature of the cement. This Prosthodontics is dedicated to sustaining academic review will concentrate on methods to improve the excellence and interest in fixed prosthodontics. The design of tooth preparations for structurally compro- goal of the committee is to disseminate knowledge mised teeth by using foundation restorations. relevant to fixed prosthodontics with a yearly publica- Tooth preparations must possess retentive and resis- tion of a comprehensive literature review on a select- tance form to ensure long-term serviceability of fixed ed topic. The subject for this year is foundation prosthodontic restorations. Retention will prevent dis- restorations. lodgment of a casting along a path parallel to the path of insertion of a restoration and resistance will prevent PURPOSES OF FOUNDATION dislodgment along any other path (Fig. 1). The denti- RESTORATIONS tion is subjected to a 180-degree field of force vectors Successful fixed prosthodontic treatment depends (Fig. 2), so resistance form is considered to be more on the ability of cemented cast restorations to resist dis- critical than retentive form. However, it is impossible to lodgment from tooth preparations. The interaction of separate these 2 features.1,2 3 primary factors appears to influence potential for dis- Resistance has been associated with the degree of taper of a tooth preparation.3 Wiskott et al2 suggested a linear relationship between height or diameter of the preparation and resistance. Grooves have also been Presented to the American Academy of Fixed Prosthodontics Ad Hoc shown to enhance the resistance form especially in Committee, Chicago, Ill., February 1999. 4 aAssociate Professor of Prosthetic Dentistry and Dental Materials Sci- molars, and a recent clinical study of cast restorations ence, Faculty of Dentistry, Kuwait University. indicated a lack of resistance form to the tooth prepa- bPrivate practice, Oklahoma City, Okla. rations of dislodged castings.5

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Fig. 2. During function, artificial crown is subjected to 180- degree field of force vectors.2

B all height of the final cast restoration.6 A foundation restoration made from a restorative material is often indicated when inadequate coronal tooth structure remains to permit development of retentive and resistance form. Shillingburg et al7 advocated the placement of a core reconstruction or foundation restoration when one half or more of the coronal tooth structure is missing, and pins can be used to augment the retention of the foundation restoration.8 After the restorative core material is secured, the tooth with its core reconstruction can be prepared following accepted guidelines for tooth preparations of intact teeth.9 DESIRABLE FEATURES OF FOUNDATION RESTORATIONS Foundation restorations replace coronal tooth structure that was lost as a result of dental caries, previous restorations or tooth fracture and may be fabricated from various restorative materials. The desirable features of the foundation restoration vary depending on Fig. 1. A, Retention prevents dislodgment of restoration clinical conditions. along path of insertion. B, Resistance prevents dislodgment Minute depressions or undercuts may be present in a of restoration by forces directed in apical, oblique or hori- tooth preparation. If adequate retentive and resistance zontal direction. form can be developed from natural tooth structure, strength of the foundation restoration is less critical, and these minor irregularities can be restored with the adhe- It is the responsibility of the dentist to incorporate sive restorative materials such as glass ionomer, resin- retention and resistance form in the design of the modified glass ionomer, or compomer cements.10 A tooth preparation that will receive a cast restoration. foundation restoration that does not contribute to the Nevertheless, cast restorations are commonly placed overall retention and resistance form of the tooth prepa- on damaged teeth with substantial loss of tooth struc- ration is commonly described as a base. When the foun- ture. Retention and resistance form became compro- dation restoration augments the retention and resistance mised as the height of the prepared tooth is reduced provided by the remaining tooth structure, it is usually in relation to the width of the preparations and over- described as a core reconstruction.

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Physical properties of a core reconstruction become crowns became unpopular because they were not more important as residual intact tooth structure practical. This was evident when divergent paths of decreases.10 Some desirable features of a core material insertion of the post-space and remaining tooth struc- include adequate compressive strength to resist intrao- ture existed, especially for abutments to fixed partial ral forces,11 sufficient flexural strength to prevent flex- dentures (FPDs). One-piece dowel-crown restorations ure of the core during normal intraoral functions,11 also presented problems when the crown or FPD biocompatibility,12 resistance to leakage of oral fluids at required removal and replacement. These difficulties the core/tooth interface,13,14 ease of manipulation,15 led to development of a post-and-core restoration as a ability to bond to remaining tooth structure,16-18 ther- separate entity with an artificial crown cemented over a mal coefficient of expansion and contraction similar to core and remaining tooth structure. tooth structure,13 dimensional stability,19 minimal With the advent of scientific endodontic therapy in potential for water absorption,20-22 and inhibition of the 1950s, the challenges increased for restorative den- dental caries.23 tistry. Teeth that were commonly extracted without When retentive and resistance features are derived hesitation were successfully treated with predictable primarily from the core material, the strength of a foun- endodontic therapy, and a satisfactory restorative solu- dation restoration and the retention of a core can tion was necessary, especially for teeth with severe directly influence survival of the artificial crown. Cer- damage. Cast posts and cores became routine methods tain core materials may lack the inherent strength to for restoration of endodontically treated teeth. support a complete crown. A tooth that must serve as CAST POSTS AND CORES an abutment to a fixed or removable prosthesis is subjected to increased stress, and the overall mechanical The development of cast dowel cores was a logical properties of the core must be adequate to resist these evolution from the Richmond crown. For endodonti- forces. Posterior teeth will be exposed to higher force cally treated anterior teeth with moderate to severe thresholds than anterior teeth and the direction of the destruction, cast posts and cores have been described as force differs. Therefore, required compressive and flex- the restorative method of choice.31 Conversely, molars ural strength may differ, depending on the location of often perform satisfactorily with direct cores retained the tooth in the dental arch. In addition, a foundation by engaging the pulpal chamber and a portion of the restoration that supports a translucent all-ceramic root canals,32,33 and retention of the core can be crown should not adversely effect the esthetic qualities augmented by placement of 1 or more prefabricated of the final restoration.24 intraradicular posts. Premolars may be restored with either custom cast posts and cores or prefabricated FOUNDATION RESTORATIONS FOR post(s) with direct cores. PULPLESS TEETH Historical perspectives Methods of fabricating cast posts and cores The concept of using the root of a tooth for reten- A reliable method for fabricating a custom dowel core tion of a crown is not new.25 In the 1700s Fauchard is direct fabrication of the pattern.34 The tooth is pre- inserted wooden dowels in canals of teeth to aid in pared for the crown after the existing restorations, den- crown retention.26 Over time the wood would expand tal caries, and weakened tooth structure are removed; in the moist environment to enhance retention of the the post space is then prepared. Guidelines for the dowel until, unfortunately, the root would often frac- length of the post include a length equal to the length ture vertically.25 Additional efforts to develop crowns of the clinical crown of the final restoration,35 and two retained with posts or dowels in the 1800s were limit- thirds or three quarters the length of the root in bone.36 ed by the failure of the “endodontic” therapy of the In vivo studies have suggested that clinical success of era. Several of the 19th century versions of dowels also posts is directly proportional to their lengths; so it is used wooden “pivots,” but some dentists reported the rational to prepare a post channel as long as it is consis- use of metal posts favored by Black27 in which a porce- tent with anatomic limitations while maintaining lain-faced crown was secured by a screw passing into a 4 to 5 mm of apical gutta percha seal.37-40 A shorter post gold-lined root canal. A device developed by Clark in is undesirable because it is less retentive and can produce the mid-1800s was extremely practical for its time unfavorable leverage and shear stresses within the root because it included a tube that allowed drainage from canal that may predispose the root to fracture.41,42 The the apical area or the canal.28 width of the post is also an important consideration The Richmond crown was introduced in 1878 and because arbitrarily widening the diameter of the post will incorporated a threaded tube in the canal with a screw- reduce the thickness and strength of the radicular retained crown. The Richmond crown was later modi- dentin.43 Thickness of remaining dentin is critical. fied to eliminate the threaded tube and was redesigned The post space should provide resistance to rotation as a 1-piece dowel and crown.29,30 One-piece dowel- of the dowel core. If the configuration of the prepared

DECEMBER 1999 645 THE JOURNAL OF PROSTHETIC DENTISTRY MORGANO AND BRACKETT canal is circular in cross section, it will not provide this gold standard that resulted in a fixed, inexpensive price resistance to rotation. A keyway should be placed with- for gold. When this regulation for the price of gold was in the canal.44 A positive seat for the core at the open- removed, the cost of gold elevated dramatically in the ing of the post-space is desirable to prevent overseating 1970s. With the cost of gold at unprecedented levels, of the dowel, which may wedge the root and cause ver- there was an incentive to develop alternative alloys for tical fracture.44 cast restorations, and included dowel cores. Base metal Numerous materials have been described for fabrica- alloys traditionally used to cast frameworks for remov- tion of the dowel-core pattern. These materials includ- able partial dentures (RPDs) were suggested as logical ed: wax with a plastic rod as a carrier and sup- alternatives to gold alloys, and their use for dowel cores port,34,36,45 wax with a dental bur,44 and acrylic resin was advocated.47,60 A major disadvantage of base metal with a solid plastic sprue.35,46-49 Another method alloys was their hardness because these castings were developed a core of acrylic resin with an endodontic file ground and contoured chairside. Alternative alloys coated with wax that adapted to the prepared canal.50 were later introduced to resolve the problems of con- A variation of the direct custom dowel core incorporat- touring and finishing posts and cores fabricated from ed a prefabricated plastic pattern manufactured to cor- base metal alloys. Dowel cores made from silver-palla- respond to the diameter and configuration of a specific dium alloys were more easily adjusted chairside and reamer. With this method, the desired reamer was used were suitable castings.61 Many properties of these sil- to instrument the canal, and the matching plastic pat- ver-palladium alloys are similar to those of gold casting tern was inserted into the post channel. Acrylic resin alloys, and they offer an economical and satisfactory was then adapted to the coronal surface of the post pat- alternative for custom-cast posts and cores. tern and contoured to the desired form. These prefab- Cast posts and cores as a method of restoring ricated plastic patterns can be divided into 2 types: pulpless teeth (1) precision parallel dowels and (2) precision tapered dowels. Cast dowel cores have been reported to provide Custom cast dowel cores require 2 visits. A primary excellent service for endodontically treated teeth with disadvantage of the direct method of fabricating posts moderate-to-severe damage. A 6-year retrospective and cores is the chair time to fabricate the pattern. The study of 96 endodontically treated teeth with extensive indirect method conserves chair time by delegating the loss of tooth structure and restoration with the use of pattern for the post and core to a dental laboratory cast dowel cores indicated a 90.6% success rate.62 Cast technician. Nevertheless, an accurate impression of the posts are best applied to single-rooted teeth, especially prepared post space that extends deeply in the canal of incisors and canines; and the use of custom cast dowel an endodontically treated tooth is a challenge. Success cores, fabricated directly or indirectly, remains an inte- of the indirect method depends on the accuracy of the gral component of prosthodontic treatment. A recent- impression replicating the internal surface of the pre- ly reported national survey investigated dentists’ pared root canal. Impression material may be injected philosophies and techniques of restoring endodontical- in the post space and distributed by a spiral paste filler ly treated teeth. The results indicated that the majority to capture the internal morphology of the canal.51 A of dentists in the United States used either cast posts rigid object is inserted in the canal before the initial set exclusively or both cast posts and prefabricated posts in of impression material to strengthen this impression their practices.63 and minimize potential for distortion. Suggested rein- ROLE OF THE FERRULE EFFECT forcement mechanisms include toothpicks,52 wire,53,54 paper clips,55,56 and plastic sprues.57 A post and core in a pulpless tooth can transfer Prefabricated precision metal posts58 and fit-sized occlusal forces intraradicularly with resultant predispo- plastic patterns59 offer an alternative approach that uses sition to vertical fracture of the root.64,65 The role of a pick-up impression. The post space is prepared with the final cast restoration in protection of the dowel- the appropriate instruments to conform to the preselect- restored pulpless tooth has been discussed for decades. ed pattern, and the pattern is inserted in the canal with In 1959 Frank66 indicated the importance of protective a substantial extension beyond the coronal tooth finish coronal coverage of pulpless teeth, and Rosen44 sug- line. An impression is made that picks up the pattern gested that the “hugging action” of a subgingival col- that is transferred to the working cast, and the dowel lar of cast metal provided extracoronal bracing that core can be fabricated by a dental laboratory technician. could prevent fracture of tooth structure. Eissman and Radke67 used the term ferrule effect to describe this Alloys for cast posts and cores 360-degree ring of cast metal and recommended exten- Traditionally, custom dowel cores were cast in a sion of the definitive cast restoration at least 2 mm api- gold alloy comparable to the alloys used for complete cal to junction of the core and remaining tooth struc- crowns. For decades the US government maintained a ture (Fig. 3).

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Fig. 3. Occlusal forces (OF) are transmitted to center of root Fig. 4. Pulpless maxillary first premolar with post in buccal through post as spreading forces (SF) that can cause vertical root. Occlusal force (large arrow) can produce tensile stress- fracture of root. If artificial crown extends 2 mm apical to es at lingual aspect of crown margin (small arrow) that may junction of core and tooth, ferrule effect (FE) will resist these jeopardize integrity of marginal seal of crown. spreading forces. Post and core in combination with artificial crown provide coronoradicular stabilization. cement seal of the artificial crown occurred first on the tension side of the tooth, especially when the ferrule In vitro studies by Barkhorder et al68 and Hemming was small and the post was off-center (Fig. 4).73 Loss et al69 reported an improved resistance to fracture when of the cement seal of the coronal restoration is insidi- encircling collars or ferrules were used with posts. Assif ous and clinically undetectable initially. Nevertheless, et al70 examined in vitro the effect of post design on the leakage will occur between the crown margin and the fracture resistance of pulpless premolars restored with tooth surface and may extend into the post space, cast crowns. Their results indicated that the design of which could lead to dental caries and potential loss of the post did not influence resistance to fracture if the the tooth.74 A clinical study by Torbjörner et al75 ret- core was covered with a complete cast crown that rospectively evaluated the survival and failure charac- extended 2 mm apical to the finish line of the core. An teristics of teeth restored with posts and artificial in vitro study by Isidor et al71 evaluated the effects of crowns, and their results indicated a higher potential post length and ferrule length on the resistance to for fracture of the post when the cemented crowns did dynamic loading of bovine teeth restored with artificial not provide a ferrule effect (Fig. 5). crowns. Resistance to failure was greatest for the group Cementation of a post with a dentinal bonding sys- restored with a combination of the longest posts (10 tem could theoretically provide internal bracing of mm) and the longest ferrules (2.5 mm). the root that substitutes for the extracoronal ferrule. Libman and Nicholls72 evaluated in vitro the effects Two recent in vitro studies have suggested this possi- of ferrules on the integrity of the cement seal of cast bility.76,77 Clinical studies to corroborate the internal crowns, and reported improved resistance to fatigue reinforcement of roots with dentinal bonding systems failure of the cement seal of a crown when the crown are lacking. Thus, there is no compelling evidence margin extended at least 1.5 mm apical to the margin to suggest abandonment of the classic extracoronal of the core. Another study indicated that failure of the ferrule.

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of plastic tooth analogs restored with three different post systems. Substantial tooth structure remained coronal to a rounded-shoulder finish line for complete crowns, and a 1-mm bevel was added to half the speci- mens. The results of their in vitro study indicated an appreciable increase in mean failure thresholds when the length of the ferrule was increased by the addition of a 1-mm bevel to the finish line.81 Despite the well-intended recommendation to develop a cast ferrule as a component of the cast core, there is little evidence to support the contrabevel and ferrule as integral components of a cast core.39,82 Preparation of a contrabevel for the core requires the removal of sound coronal tooth structure and may compromise the ferrule effect from the cemented artificial crown. This design to the post-and-core preparation also results in a final casting with both intracoronal and extracoronal components, which complicates compensation for thermal contraction of the alloy during investing and casting. Finally, this ferrule as a part of the cast core cannot be developed with any of the direct core materials. However, failure of teeth restored with direct core reconstructions has not been associated with the lack of a ferrule effect from the core material when the cemented artificial crown provides a ferrule. Fig. 5. When ferrule is absent, occlusal forces are concen- Current knowledge has confirmed that the dentist trated at junction of post and core, and post may fracture. should retain as much coronal tooth structure as possi- ble when preparing pulpless teeth for complete crowns to maximize the ferrule effect. A minimal height of 1.5 to 2 mm of intact tooth structure above the crown Shillingburg et al78 have advocated a contrabevel in margin for 360 degrees around the circumference of the tooth preparation for a cast post and core to pro- the tooth preparation appears to be a rational guideline duce a core with a collar that serves as a secondary fer- for this ferrule effect. Surgical crown lengthening83 or rule, independent of the ferrule provided by the cast orthodontic extrusion84 should be considered with crown. However, Sorensen and Engleman79 reported severely damaged teeth to expose additional tooth no advantage to this contrabevel and collar when a structure to establish a ferrule. If these provisions for crown was cemented over the core. Their results indi- developing a ferrule are impractical, extraction of the cated that the ferrule effect was obtained from nearly tooth and replacement with conventional or implant- parallel walls of intact tooth structure coronal to the supported prosthodontics should be considered. finish line for the artificial crown and not from the con- PREFABRICATED POSTS trabevel on the core preparation.79 They also reported that a 1-mm beveled finish line for a complete crown Prefabricated posts have become more popular, and preparation without additional tooth structure coronal there is a variety of systems available. A recent nation- to the bevel did not improve the fracture resistance of wide survey of dentists indicated that 40% of general the root.79 Loney et al80 conducted an analysis of dentists used prefabricated posts most of the time, and stresses developed in photoelastic-resin models of max- the most popular prefabricated post was the parallel- illary canines restored with cast cores. Half the speci- sided serrated post.63 The use of prefabricated posts mens contained cores with a 1.5-mm collar to provide with a direct core reconstruction is often regarded as a ferrule as a component of the core itself, and half the restorative method of choice for restoration of omitted this collar. Their results indicated substantially pulpless molars with substantial loss of tooth struc- higher mean stresses with the collared cores and sug- ture.31 These commercially available posts are supplied gested that incorporation of a ferrule with a cast core in various shapes with numerous surface configura- may be undesirable. tions. They may be parallel-sided or tapered. Some par- Milot and Stein81 investigated the ability of beveled allel-sided posts are tiered, whereby parallelism is main- tooth preparations to improve the resistance to fracture tained but their diameters are narrowed in their apical

648 VOLUME 82 NUMBER 6 MORGANO AND BRACKETT THE JOURNAL OF PROSTHETIC DENTISTRY portions where the root is generally thinner. Some prefabricated posts are passive, and others actively engage tooth structure with threads.85,86 Active posts are more retentive, but can generate unfavorable stresses and predispose the root to fracture.87-89 The most retentive passive post is a long, parallel-sided post with a rough- ened surface, but a parallel-sided post will often require removal of substantial radicular dentin to achieve the desired length.39,41,90-92 Carbon-fiber reinforced epoxy resin posts Most prefabricated posts are metallic, but there are several newer nonmetallic systems available. A post fabricated from a carbon-fiber reinforced epoxy resin was developed in France by Duret and Renaud, and became commercially available in Sweden in 1992.93 Carbon- fiber reinforced epoxy resin is a recently introduced dental restorative material composed of unidirectional carbon fibers that are 8 µm in diameter embedded in a resin matrix and supporters claim the physical properties are similar to those of natural dentin.94-99 The material is radiolucent and appears to be biocompatible based on cytotoxicity tests reported by Torbjörner et al.99 Two in vitro studies have indicated that these carbon- fiber posts possessed inferior strength compared to metal posts.100,101 Nevertheless, an in vitro study of carbon- fiber reinforced epoxy resin posts that used bovine teeth Fig. 6. Various configurations of commercially available car- suggested that these posts were less likely than metal bon-fiber reinforced epoxy resin posts. posts to cause fracture of the root at failure.102 A retrospective short-term clinical study of 236 teeth restored with carbon-fiber reinforced epoxy resin posts reported ing material suggest that the flexibility of the post will no failures attributable to the posts after a period of 2 to not match the flexibility of the root. Another in vitro 3 years of service.103 These posts are manufactured in study indicated that the form of the post would influ- several configurations (Fig. 6) and are used with com- ence its rigidity and reported that smooth posts were posite cores and resin luting agents. Nevertheless their less flexible than serrated posts.104 ability to bond to adhesive dental resins appears unre- A flexible post can be detrimental especially when markable, and their bond can be improved with mechan- there is little remaining natural tooth structure between ical retention such as serrations.98,104 the margin of the core and the gingival extension of the At this time the long-term effects of restoring pulp- artificial crown. When the ferrule is absent or extremely less teeth with these posts are unknown. Although the small, occlusal loads may cause the post to flex with stiffness of these posts has been reported to be similar eventual micromovement of the core, and the cement to human dentin, Purton and Payne98 reported a trans- seal at the margin of the crown may fracture in a short verse modulus of elasticity for these posts that exceed- time (Fig. 7). Marginal leakage with recurrent dental ed the values recorded for stainless steel posts. Because caries will ensue, but the deterioration will be unnoticed of the parallel arrangement of the reinforcing carbon until substantial destruction of tooth structure occurs.74 fibers, these posts displayed anisotropic behavior Zirconia posts whereby their physical properties differ depending on the loading angles.98 Furthermore, even if the elastic With recent advances in ceramic technology, the all- modulus of the post were comparable to human ceramic crown has become more popular. However, dentin, this property will not ensure similar clinical restoring a pulpless tooth with a metal post and core in behavior for the post and radicular dentin. The root is combination with an all ceramic crown is a challenge. essentially a hollow tube, and the thin rod-shaped post The underlying metal from the post and core can alter is within this hollow tube surrounded by an interven- the optical effects of a translucent all-ceramic crown ing layer of composite resin luting agent. The radically and compromise the esthetics. different configurations of the root compared with the In response to the need for a post that possesses post combined with the interposed composite resin lut- optical properties compatible with an all-ceramic

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Fig. 8. Commercially available post made from zirconium oxide.

core was formed over the prefabricated zirconia post to develop a post and core that was entirely ceramic. Clinical trials are lacking with this new all-ceramic post, and the ability of these posts to resist intraoral forces are unknown. Ceramics are tough materials with high compressive strengths, but are brittle when subjected to shearing forces.117,118 An alternative to this all-ceramic post is a cast post and core made from a Fig. 7. Flexible post may allow micromovement of core metal ceramic alloy. Opaque porcelain can be fused to (small arrow) under occlusal load (large arrow) when ferrule is small or absent with resultant fracture of cement seal at the core portion to provide a durable post and core that crown margin (curved arrow). will disguise the graying effect that can occur with conventional cast metal posts and cores when combined with all-ceramic crowns.119 Woven-fiber composite materials crown, an all-ceramic post has been developed The manufacturer of a cold-glass plasma-treated poly- (Fig. 8).24,105-108 This post is composed of zirconium ethylene woven-fiber has suggested this material in a oxide, a material that has been used in medicine for resin composite to provide coronoradicular stabilization orthopedic implants. Animal studies have indicated sta- for pulpless teeth.120,121 The fibers are multidirectional bility after long-term aging of this ceramic material and developers of the material have suggested a number without evidence of degradation.109,110 The post is of uses.122 An in vitro study of this material with extract- made from fine-grain, dense tetragonal zirconium poly- ed human teeth indicated that woven-fiber composite crystals (TZP),111,112 and the zirconia post has been posts and cores were significantly weaker than cast metal reported to possess high flexural strength and fracture posts and cores.123 Nevertheless, when this woven-fiber toughness.113 This radiopaque material is biocompati- composite was reinforced with a smaller-diameter pre- ble with some physical properties similar to steel.113-115 fabricated post, the strength of the system increased sig- The zirconia post was designed for use with an adhesive nificantly.123 These prefabricated posts embedded in the resin cement, and one in vitro study has recorded poor woven-fiber composite were not as strong as cast posts resin-bonding capabilities of this post to radicular and cores, but were less likely to cause fracture of the dentin after dynamic loading and thermocycling.116 roots when subjected to failure loads.123 These posts were also designed for use with a com- CEMENTS AND CEMENTATION OF posite core material, but a large composite core may not POSTS be sufficiently rigid to support a brittle all-ceramic crown.11,20 Sorensen24 described a method of combin- Dental cements ing this post with IPS Empress pressed-glass technology Dental cements lute the post to radicular dentin, to compensate for the disadvantages of a composite core and properties such as compressive strength, tensile for an all-ceramic restoration. A custom glass-ceramic strength, and adhesion of the cement are commonly

650 VOLUME 82 NUMBER 6 MORGANO AND BRACKETT THE JOURNAL OF PROSTHETIC DENTISTRY described as predictors for success of a cemented post. AB C Other factors such as potential for plastic deformation, microleakage, water imbibition, behavior of the cement during the setting process, and handling characteristics can also influence the survival rate of a cemented post. There are several luting agents currently available to the dentist and they include: zinc phosphate, polycarboxylate, glass ionomer, resin-modified glass ionomer, compomer, and resin cements. These different classes Fig. 9. Study of pulpless premolars restored with MOD silver of cements represent a variety of products by a number amalgam restorations (A), resin-bonded MOD inlays (B), and of dental manufacturers. There are distinct advantages MOD onlays cemented with zinc phosphate cement (C) and inherent disadvantages to each product. indicated best fracture resistance with onlays. Zinc phosphate cement is the standard cement used for decades to lute dental restorations, and this cement has been extremely successful. The primary disadvantages of zinc phosphate cement are solubility in oral have reported significantly greater retention for posts fluids and lack of true adhesion. Polycarboxylate and cemented with adhesive resins,142-145 whereas others glass ionomer cements provide a weak chemical bond have reported conflicting results.146-148 One factor that to dentin.124,125 Polycarboxylate cements have been has a detrimental effect on resinous cements is eugenol reported to undergo plastic deformation after cyclic contamination of dentin. The setting process of dental loading and may be less retentive than zinc phosphate resins occurs by free-radical addition polymerization, and glass ionomer cements.126 Glass ionomer cement and this process can be inhibited by phenolic com- has been reported to release fluoride127-129; neverthe- pounds, such as eugenol (2-methoxy-4-allyphenol).146 less the ability of glass ionomer cement to inhibit den- Most endodontic sealers contain eugenol, and the tal caries in dentin has not been clearly demonstrat- obturation of the root canal occurs by condensation of ed.130 Resin-modified glass ionomer cements possess the gutta percha filling material under pressure to force similar chemical properties and also can leach fluo- the eugenol-containing liquid sealer into the dentinal ride131,132; however, objective proof of the clinical ben- tubules and lateral canals. After eugenol has penetrated efit of this fluoride release is also lacking.133 Adhesive dentin, it is difficult to remove, and the presence of resins are essentially insoluble and provide better reten- eugenol in the radicular dentin can explain the incon- tion in vitro compared with nonadhesive resins and sistent results reported for posts cemented with adhe- conventional cements.134 sive resins.142,146,148,149 There are peculiarities to the handling characteristics The ability of resins to bond to dentin and restora- and clinical behavior of each class of cement. Glass tive materials can enhance retention, but this increased ionomer cement requires several days135 or even sever- retention may not ensure resistance to dislodgment of al weeks136 to reach its maximal strength, so it is the post with normal clinical conditions. One study has unsuitable as a luting agent for posts. Any recontouring reported extremely high retentive values for an unfilled of the core with a dental handpiece soon after cemen- 4-META resinous cement,142 but this cement is rela- tation of the post will cause vibration of the post that tively weak and has been reported to undergo plastic may weaken the immature cement film and contribute deformation that will likely lead to fatigue failure in to eventual retentive failure of the post. vivo.150 Adhesive resin cements are also technique sen- Resin-modified glass ionomer cement has become sitive, and Mendosa and Eakle147 have reported diffi- popular for cementation of complete crowns, and its culty in manipulating a resinous cement in vitro. For use has been suggested for cementation of posts.137,138 example, some posts did not seat completely in their However, this class of cement imbibes water and post channels because of premature setting of the resin. expands with time,139,140 and there is anecdotal evi- Resin cements have also been suggested as a method dence that volumetric expansion of this cement will to reinforce pulpless teeth. One study evaluated the fracture all-ceramic crowns relatively soon after cemen- ability of inlays cemented with resin bonded procedures tation.141 If this cement can fracture all-ceramic in posterior pulpless teeth to bond together the crowns, its expansion will likely cause vertical fracture remaining tooth structure and eliminate the need to of roots if selected to cement posts. Therefore, it cover and surround the cusps with a cast restoration. appears at this time that resin-modified glass ionomer This study compared the fracture resistance of extract- cements should be avoided for cementation of posts. ed endodontically treated premolars restored with Resinous cements have been studied extensively, and MOD silver amalgam restorations, resin-bonded MOD several investigations have evaluated the ability of adhe- inlays, and MOD onlays cemented with zinc phosphate sive resins to retain intraradicular posts. Some studies cement (Fig. 9). The greatest resistance to fracture was

DECEMBER 1999 651 THE JOURNAL OF PROSTHETIC DENTISTRY MORGANO AND BRACKETT

of the available cements possess physical properties that are capable of compensating for problems commonly associated with a poorly designed post. A short, wide, overly tapered post combined with an artificial crown lacking an adequate ferrule is more likely to fail regard- less of cement. If a post is fabricated consistent with sound biomechanical principles, following guidelines in a standard prosthodontic text, conventional cements such as zinc phosphate cement are satisfactory. DIRECT CORES Core materials The three basic direct core materials are silver amalgam, composite, and glass ionomer–based core materials. There have been numerous in vitro experiments that have investigated the physical properties of these core materials. Some studies have been conducted with crowns cemented over cores to more appropriately mimic clinical conditions, and others have loaded the core materials directly to determine their strengths. Conclusions often differ depending on the design of the study, and factors such as applied loading angle have been shown to substantially alter the results.153 Fig. 10. Cement was placed on post only. As post is seated Properties that are important predictors of the clini- (large arrow) entrapped air within canal (small arrows) cal behavior of a core material include compressive, escapes through liquid cement, creating voids in cement shear and tensile strengths, along with rigidity.154,155 film. Silver amalgam has been reported to perform best as a core material under simulated clinical conditions because of its high compressive strength and rigidi- recorded for MOD onlays cemented with zinc phos- ty.11,156 Conversely, a number of studies have indicated phate cement.151 that materials derived from glass ionomer cement perform poorly as a load-bearing core material.155-160 Cementation of posts Composite has a strength intermediate between silver If cement is placed on the post only when it is amalgam and glass ionomer core material and is more cemented, air will be trapped deeply in the prepared flexible than silver amalgam.11 It appears that compos- canal, and as the post is seated the air will travel ite is an acceptable direct core material when substan- through the liquid cement to create voids that will tial coronal tooth structure remains,155,157-161 but less compromise the physical properties of the cement film desirable when there is limited supporting dentin.11 (Fig. 10). Filling the canal with cement before seating Composite is also difficult to condense adequately in the post will avoid air entrapment and ensure a dense the tooth preparation, and a syringe technique has been uniform cement lute.152 Nevertheless few dental reported to produce a denser core compared with a cements provide adequate working time to introduce bulk-insertion technique.162 cement into the canal before the post is seated, and Several composite core materials contain a fluorosil- resin cements are especially prone to premature setting icate inorganic filler similar to the aluminum fluorosili- if this procedure is attempted. Tjan et al146 have cate glass in glass ionomer cements. Consequently, demonstrated substantial voids with an adhesive resin these composites release trace amounts of fluoride that cement, and suggested that these voids were responsi- may continue for up to 5 years.163,164 Nevertheless, as ble for the unexpected low retentive values for posts with the fluoride leached from glass ionomer cements, luted with the resin cement. Zinc phosphate cement is clinically relevant cariostatic properties have not been especially well suited for placement of the cement in the established with these fluoride-containing composite canal before seating of the post because of its extended core materials. working time.152 Bonded cores To date, there have not been any long-term clinical trials of cemented posts that demonstrate the superior- Contemporary adhesive dentistry allows for the ity of a specific cement, and most dentists will select a bonding of cores to the remaining tooth struc- cement empirically. Studies have confirmed that none ture.16,17,165-167 Bonding techniques will augment the

652 VOLUME 82 NUMBER 6 MORGANO AND BRACKETT THE JOURNAL OF PROSTHETIC DENTISTRY mechanical retention of a core, but should not be used dures provided are commonly biased. For example, it as the sole means of retention.18,165,168 The high frac- can be assumed that the dentists who provided treat- ture strength of silver amalgam can be improved in ment for the patients surveyed in a retrospective study vitro with the use of an amalgam bonding agent, used clinical judgment to select the method of treat- although the clinical relevance of the effect of the ment. Teeth with minimal remaining coronal dentin bonding procedure after prolonged intraoral function may have been restored with custom cast posts and are unknown.17 Amalgam bonding techniques result in cores, and those with substantial residual tooth struc- an adhesive resin at the interface between the tooth and ture may have received prefabricated posts or founda- the silver amalgam, and marginal leakage has been tion restorations without posts. Consequently, with this reported after in vitro aging for 30 days.169 However, hypothetical treatment protocol, the teeth restored leakage at the core margin should not be a problem with cast posts would be at a higher risk of failure not when amalgam bonding agents are used because the because of any inherent problems with the procedure margins of the artificial crown completely cover the but because the dentists selected cast posts and cores core/tooth interface. for teeth with the least supporting dentin and the poor- est prognosis. FOUNDATION RESTORATIONS FOR SEVERELY COMPROMISED TEETH FUTURE NEEDS In the past pulpless molars were resected and With the advent of new materials and techniques, restored with complex restorative techniques as a additional in vitro and in vivo studies are required to method to retain compromised teeth and avoid the fully evaluate the efficacy of these recent developments. need for removable prosthetic restorations.170 These The carbon-fiber reinforced epoxy resin post and zirco- restorative procedures are technically demanding and nia post, as well as recently introduced cements and expensive. A failure rate ranging from 32% to 38% with- bonding techniques, are among these new materials in the first 10 years of service has been reported for and procedures. Currently, there is sparse scientific resected molars,171,172 although teeth that survived 10 knowledge relative to the long-term prognosis of teeth years appeared to have a better long-term prognosis.173 restored with these approaches. Construction of a foundation restoration to retain a Further improvements in direct core materials would complete crown is especially difficult with a resected also be welcome. Silver amalgam is the most mechani- tooth, and a relatively high percentage of failures cally sound core material, but health concerns about its appeared to be the result of restorative failures and mercury content continue.186 A silver amalgam core is faulty resective procedures.174 eventually totally covered with a complete artificial With the advent of predictable osseointegrated crown and not exposed to the oral environment; thus, implants to support and retain prosthetic restora- a silver amalgam core is unlikely to contribute any tions,175-179 the practice of retaining severely compro- systemic mercury to the patient. Nevertheless, the day mised teeth has diminished substantially. Often it is in may come when silver amalgam is not available in den- the best interest of the patient to extract teeth with a tistry for any purposes, and a suitable substitute for poor prognosis and replace the compromised teeth direct cores will be necessary. Composite core materials with implant-supported artificial crowns or FPDs. can provide favorable mechanical results when there is adequate remaining supportive dentin, but the flexibili- CURRENT PROBLEMS AND ty of current formulations limit their use when extensive LIMITATIONS coronal tooth structure is missing. Reinforcement with Despite the large volume of published research on silanized glass fibers or polymer-impregnated fibers has foundation restorations, major gaps exist in the profes- been suggested as a method of improving the flexural sion’s current knowledge on this topic. There are strength of dental resins. This approach may improve numerous in vitro studies of different approaches to the physical properties of resin-based core materials.187 foundation restorations, primarily involving methods of In addition, health related issues may also arise with restoring pulpless teeth; but data from these in vitro composites. Organic constituents have been reported to investigations are frequently conflicting and not always leach from dental composite resins, and the biologic applicable clinically.31 Several retrospective clinical effects of these eluded organic materials are studies of restored pulpless teeth have been reported in unknown.188-191 the literature.28,37,180-185 Nevertheless, the results of SUGGESTED DIRECTIONS FOR these studies are also conflicting, and it is difficult to FUTURE RESEARCH formulate meaningful clinical guidelines based on dia- metrically opposed results. Randomized controlled clinical trials would provide With any retrospective study, there are problems the most reliable data on the prognosis of teeth with control of the treatment methods, and the proce- restored with foundation restorations, but these studies

DECEMBER 1999 653 THE JOURNAL OF PROSTHETIC DENTISTRY MORGANO AND BRACKETT are prohibitive, and data relative to long-term success 14. Hormati AA, Denehy GE. Microleakage of pin-retained amalgam and composite resin bases. J Prosthet Dent 1980;44:526-30. rates would be unavailable for many years. Retrospec- 15. Kao EC, Hart S, Johnston WM. Fracture resistance of four core materials tive clinical studies are more cost-effective, and if well with incorporated pins. Int J Prosthodont 1989;2:569-78. executed, can serve as reasonable alternatives to ran- 16. Kanca J 3rd. Dental adhesion and the All-Bond system. J Esthet Dent domized controlled clinical trials. The dental profession 1991;3:129-32. 17. Donald HL, Jeansonne BG, Gardiner DM, Sarkar NK. Influence of denti- needs more high-quality retrospective clinical studies nal adhesives and a prefabricated post on fracture resistance of silver on this topic, and investigations of patients treated in a amalgam. J Prosthet Dent 1997;77:17-22. relatively controlled environment such as a dental 18. Lo CS, Millstein PL, Nathanson D. In vitro shear strength of bonded amalgam cores with and without pins. J Prosthet Dent 1995;74:385-91. school would likely provide more valid results than 19. Oliva RA, Lowe JA. Dimensional stability of silver amalgam and com- those reported from the currently available retrospec- posite used as core materials. J Prosthet Dent 1987;57:554-59. tive investigations. For maximal reliability and validity, 20. Braem MJ, Davidson CL, Lambrechts P, Vanherle G. In vitro flexural fatigue limits of dental composites. J Biomed Mat Res 1994;28:1397-402. retrospective data should be collected from large 21. Braem MJ, Lambrechts P, Gladys S, Vanherle G. In vitro fatigue behavior groups of patients, with records chosen randomly by of restorative composites and glass ionomers. Dent Mater 1995;11:137- chance from a pool of patients who received treatment 41. 22. Indrani DJ, Cook WD, Televantos F, Tyas MJ, Harcourt JK. Fracture tough- that is consistent with recommended procedures found ness of water-aged resin composite restorative materials. Dent Mater in standard texts. 1995;11:201-7. 23. Dionysopoulos P, Kotsanos N, Koliniotou-Koubia E, Papagodiannis Y. SUMMARY Secondary caries formation in vitro around fluoride-releasing restorations. Oper Dent 1994;19:183-8. The topic of foundation restorations involves many 24. Sorensen JA, Mito WT. Rationale and clinical technique for esthetic materials and techniques used daily in dental practice. restoration of endodontically treated teeth with the Cosmopost and IPS This comprehensive article reviewed literature from Empress post system. Quintessence Dental Technol 1998;12:81-90. 25. Shillingburg HT, Kessler JC. Restoration of the endodontically treated various in vitro and in vivo investigations in addition to tooth. 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