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Factors determining post selection: a literature review
Factors determining post selection: A literature review Aquaviva S. Fernandes, BDS, MDS,a Sharat Shetty, BDS, MDS,b and Ivy Coutinho, BDS, MDSc Goa Dental College, Bambolim Goa, India; Kargar Dental College, Mumbai, India Most endodontically treated teeth require a post-and-core build-up for restoring the teeth to optimum health and function. Selection of an appropriate post-and-core system from the wide variety of those available may be a clinical dilemma. Therefore, the purpose of this article is to review the literature to identify the various factors that influence the selection of the post-and-core assembly. English-language peer-reviewed articles between 1961 and 2002 were identified using MEDLINE, as well as a hand search. The following key words were used: post, design, retention, fracture resistance, survivability, and esthetics. Selection of a post and core system should satisfy many interrelated biologic, mechanical, and esthetic factors to optimally restore the endodontically treated tooth to adequate form and function. This review may serve as a guide to aid the clinician in the selection of a post-and-core system. (J Prosthet Dent 2003;90:556-62.)
T
he longevity of endodontically involved teeth has been greatly enhanced by continuing developments made in endodontic therapy and restorative procedures. It has been reported that a large number of endodontically treated teeth are restored to their original function with the use of intraradicular devices.1 These devices vary from a conventional custom cast post and core to one-visit techniques, using commercially available prefabricated post systems.2,3 In the last few decades, various prefabricated posts systems have been developed. The selection of post design is important, because it may have an influence on the longevity of the tooth.4 This article reviews the literature to determine the various factors influencing the post selection for restoring an endodontically treated tooth. The English-language literature from 1961 to 2002 was searched using MEDLINE. This was complemented with a manual search. The following key words were used: post, design, retention, fracture resistance, survivability, and esthetics. According to the reviewed articles, the factors influencing post selection were root length, tooth anatomy, root width, canal configuration, amount of coronal tooth structure, torquing force, stresses, development of hydrostatic pressure, post design, post material, material compatibility, bonding capability, core retention, retrievability, esthetics, and crown material.
ROOT LENGTH The length and shape of the remaining root determines the length of the post.5 It has been suggested that root length should be considered for the selection of the ideal post length.6 It has been demonstrated that the greater the post length, the better the retention and stress distribution.5-9 However, it may not always be possible to use a long post, especially when the remaina
Lecturer, Department of Prosthodontics, Goa Dental College. Lecturer, Department of Prosthodontics, Kargar Dental College. c Lecturer, Department of Prosthodontics, Goa Dental College. b
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ing root is short or curved. Several studies suggest that it is important to preserve 3 to 5 mm of apical gutta-percha to maintain the apical seal.10,11 When the root length is short, the clinician must decide whether to use a longer post or to maintain the recommended apical seal and use a parallel-sided threaded post. On the other hand, the result of an in vitro investigation has demonstrated that reinforced composite luting agents may compensate for the reduced post length.12 For molars with short roots, the placement of more than one post will provide additional retention for the core foundation restoration.6,12
TOOTH ANATOMY Each tooth in the arch exhibits anatomic characteristics such as root curvature, mesio-distal width, and labio-lingual dimension. Hence, root anatomy dictates post selection.12 Teeth may have anatomic variations, which may adversely affect the post placement. However, most investigators have disregarded anatomic variations such as invagination, depression, and ribbonshaped roots.8,10,13,14 A consideration of the root size and length is important, because improper post space preparation and use of large-diameter posts present the risk of apical or lateral perforation. Moreover, an active post can initiate cracks in the thin dentinal wall. A thorough knowledge and radiographic evaluation of the root anatomy will aid the clinician in avoiding catastrophic damage to the roots when planning the post space. Radiographs help the restorative dentist to evaluate the root length, width, anatomic variations, canal structure, and the surrounding hard tissue structures. Occasionally, the radiographs can be misleading because of proximal root concavities and the magnification factor. Therefore, use of a grid is recommended while a radiograph is made so that the length, diameter, and design of the post can be correctly determined.15 Gutmann16 reviewed the anatomic considerations in some detail and stated that roots of maxillary centrals VOLUME 90 NUMBER 6
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and laterals, and also mandibular premolars, have sufficient bulk to accommodate most post systems. This knowledge aids the dentist in determining the post suitable for a given root.
POST WIDTH Preserving tooth structure, reducing the chances of perforation, and permitting the restored tooth to resist fractures are criteria in selection of the post width.17,18 Tilk et al19 evaluated maxillary and mandibular teeth, and recommended a range of post widths. Various investigators19-21 have recommended different approaches regarding the selection of the post diameter. These approaches were summarized by Lloyd and Palik22 into 3 categories, conservationist (13 proponents), preservationist (7 proponents), and proportionist (6 proponents) approaches. Stern and Hirshfeld23 suggested that post width should not be greater than 1 third of the root width at its narrowest dimension. This proportionist approach was advocated with the intent of preserving sufficient tooth structure. Other investigators have proposed the preservationist approach where in the post should be surrounded by a minimum of 1mm of sound dentin.24 Others, including Pilo and Tamse,21 advocated minimal canal preparation and maintaining as much residual dentin as possible, thereby suggesting restriction of the post diameter in an effort to conserve the remaining tooth structure (conservationist approach). The influence of post width on its retention and fracture resistance has also been studied. It was shown that an increase in post width has no significant effect on its retention.8 The tooth restored with larger diameter posts is reported to provide the least resistance to fracture with a decrease in the width of the remaining dentin.18
CANAL CONFIGURATION AND POST ADAPTABILITY Canal configuration aids in making a choice between a custom-designed post and a prefabricated post.14,25 If the selected post closely fits or conforms to the canal shape and size, it may be a more conservative option because less dentin removal is required, thus enhancing fracture resistance of the tooth, as well as retention of the post.4,26 Often, a dilemma arises in funnel-shaped canals, whether to use a parallel-sided post and fill the remaining post space with cement or to use a tapered post that closely adapt to the canal wall. A third option is to use large prefabricated parallel-sided posts by removing additional tooth structure, so that an intimate contact between the canal and post is attained. It has been suggested that if a canal requires extensive preparation, a well-adapted cast post and core restoration will be more DECEMBER 2003
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retentive than a prefabricated post that does not match the canal shape.27 In addition, root reinforcement with composite is suggested for wide canals.28 In vitro studies4,29 have demonstrated that a well-adapted tapered post provided increased resistance to fracture. However, on fracture, it resulted in an extensive loss of tooth structure compared with teeth restored with well-adapted, passively cemented parallel-sided posts. Recently, Morgano and Milot30 questioned the results of an in vivo study,31 which concluded that close adaptation of a post causes more catastrophic failure. Morgano and Milot30 stated that 44% of the cast posts were less than one half to one quarter the length of the clinical crown and that the failure rate reported was the result of compromised length rather than post type. A recent investigation regarding custom-cast posts reported a success rate of more than 90% after 5 years in function.30
CORONAL STRUCTURE The amount of remaining coronal tooth structure is also a critical factor in determining the post selection. The bulk of the tooth above the restorative margin should be at least 1.5 to 2 mm to achieve resistance form.32 It was found that teeth restored with carbon fiber posts had inferior strength compared with those restored with metal posts when subjected to forces simulating those in a clinical setting.33,34 The use of cast post and cores in restoring endodontically treated teeth with moderate to severe coronal tooth loss has demonstrated a success rate of 90.6% after 5 years of service.35 The results of in vitro33,34 and in vivo studies35 indicate that nonmetal posts, such as a carbon fiber post system, can be used when ample coronal dentin remains and the crown is well supported by remaining tooth structure35; otherwise, cast post and core may be used when there is moderate to severe tooth loss.
STRESS Post and core–restored endodontically treated teeth are subjected to various types of stresses: compression, tensile, and shear. Of these stresses, shear stress is most detrimental to the restored tooth.36 Holmes et al5 have demonstrated that the variation in post dimension greatly influences shear stresses. An increase in the post length with diameter kept to a minimum will help to reduce shear stresses and preserve tooth structure. Thus, the vulnerability of the endodontically treated tooth to fracture is decreased.5
TORSIONAL FORCE Intraorally, post and core–restored teeth are subjected to various types of forces. Torsional forces on the post-core-crown unit may lead to loosening and displacement of the post from the canal, causing failure of 557
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the system.37-39 Therefore, the ability of the selected post design to resist torsional forces will play a key role in stabilizing and retaining the post-core and crown unit.37-39 Burgess et al37 demonstrated the importance of an antirotational feature in the post design and concluded that resistance to torsional forces is integral to the survival of the post-core-crown unit. An investigation into torsional resistance of several post systems revealed that active post designs provide greater torsional resistance than a passive post.38,39
ROLE OF HYDROSTATIC PRESSURE Cementation plays a significant role in enhancing retention, stress distribution, and sealing irregularities between the tooth and the post.40 During cementation, an increase in stress within the root canal has been reported because of the development of hydrostatic pressure.41 This pressure affects the complete seating of the post and may also cause fracture of the root.41,42 Fortunately, there is evidence that the fitting stresses can be reduced by careful placement of the post and by using a proper post design with a cement vent to permit escape of the luting agent and thus reduce the hydrostatic pressure.36 Tapered posts, however, are self-venting and will permit the cement to flow out along the entire surface. Pressure development is also dependent on the viscosity of the cement. The more viscous the cement, the greater the development of the hydrostatic pressure.43 Zinc phosphate cement has been successfully used for the cementation of posts for many years. Recently, resin luting agents have been used, although they are technique-sensitive and more difficult to manipulate.44,45 Care must be taken when using resin luting agents that polymerize anaerobically because they may polymerize prematurely, thus preventing complete seating of the post. Slow-polymerizing dual polymerizing luting agents are recommended in these situations.12
POST DESIGN The available post designs can be classified according to their shapes and surface characteristics. They may be parallel, tapered, or parallel-and-tapered combination. According to their surface characteristics, the posts are active or passive.46 The active post mechanically engages the dentin with threads, whereas the passive post depends on the cement and its close adaptation to the canal wall for its retention. Several studies47,48 have implicated the active post design as a cause of failure of the post and core–restored teeth. Of the designs studied, the tapered post conforms to the natural root form and canal configuration, thus permitting optimal preservation of tooth structure at the post apex. However, this design is reported to produce a wedging effect, stress concentration at the coronal por558
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tion of root, and lower retentive strength.48-50 Parallelsided post designs have been shown to increase retention and produce uniform stress distribution along the post length.9 Concentration of stress has been reported to occur at the apex of the post, especially in a narrow and tapering root end.9 This stress is considered to be caused by unnecessary removal of tooth structure at the apical end of the root and sharp angles of the post.51,52 In the parallel-tapered design, the post is parallel throughout its length except for the most apical portion, where it is tapered. This design permits preservation of the dentin at the apex and at the same time achieves sufficient retention because of parallel design.51 The surface characteristics of the post also change the retentive and fracture resistance values. The highest retention is observed in the threaded post, followed by the post with a serrated surface. The least retention is seen with smooth surface posts.19,48 Unfortunately, the threaded post engages in dentin and may lead to increased undesirable stresses within the root. To avoid placement stresses caused by threads, the following procedures have been recommended: pretapping post channels, limiting the number of threads, counter-rotating the post by one half turn after full engagement, and incorporating a spilt-shank mechanism.53 However, an analysis of the available clinical studies suggests that the performance of a threaded post is inferior to that of a custom-cast post.54 Of all the designs studied, the threaded tapered screw post exerts a greater amount of stress and was considered the least desirable.48,49 The parallel-sided, serrated, and vented posts were found to exert the least amount of stress.55
POST MATERIAL To achieve optimum results, the material used for the post should have physical properties similar to that of dentin, be bonded to the tooth structure, and be biocompatible in the oral environment.56 It should also act as a shock absorber by transmitting only limited stress to the residual tooth structure.57 Unfortunately, the materials used for post and cores, as well as luting agents, have distinct physical properties different from dentin and exhibit fundamentally different fatigue behavior.36 Traditionally, posts were made of metal alloys. Recently, nonmetallic posts have been introduced. Several studies 58-60 have examined post and cores made of various alloys and other materials with different rigidity and demonstrated that rigid materials resisted greater forces without distortion. Therefore, there may be a potential danger in using highly rigid posts. Recently introduced carbon fiber posts are purported to have mechanical properties that closely match those of the tooth.33,61 The presence of the parallel fibers in the resin of carbon fiber posts enable them to absorb and dissipate stresses.33,61 In vitro studies,33,34 however, VOLUME 90 NUMBER 6
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have demonstrated that carbon fiber posts have inferior strength compared with metal posts when subjected to forces simulating those in the oral cavity. Zirconium ceramic, which is presently used for posts, has a high modulus of elasticity, and therefore the forces are assumed to be transmitted directly from the post to the tooth interface without shock absorption.62 The stiffer ceramic post may cause more root fractures compared with the carbon fiber post. Asmussen et al61 have demonstrated that fracture of endodontically restored teeth is less extensive with carbon fiber posts than with ceramic posts. Although various claims are made with regard to nonmetallic posts, there is a need for long term clinical evaluation of both metallic and nonmetallic post systems to allow a definitive recommendation of either of them. Until such time, metallic posts continue to be the standard for most situations because they have stood the test of time.
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The bonding of a post to the tooth structure should improve the prognosis of the post-core restored tooth by increasing post retention68 and by reinforcing the tooth structure.28 It has been postulated that reinforcement of the tooth is due to the stress distribution characteristics of the bonding materials.28 Recently, the importance of bonding on the retention of posts has been demonstrated.69 It was reported that resin luting agents showed good adhesion to carbon fiber posts and glass fiber posts.69 The adhesion to zirconia posts was found to be unsatisfactory.69 It was also observed that to improve retention, the carbon fiber post did not require any surface treatment as compared with the zirconia post. In spite of the creation of microretention on zirconia posts, the adhesion between the post and resin luting agent was not uniform, thus indicating that the nature of post material was responsible for the bonding of the post to the tooth structure.69
CORE RETENTION MATERIAL COMPATIBILITY Corrosion of the post and fracture of the root has been reported in the dental literature.63 Ideally, post and cores are made of the same alloy. Dissimilar alloys may create galvanic action, which can lead to corrosion of the less noble alloy.64 Corrosion of the post may be initiated because of the access of an electrolyte to the post surface, through cementum and dentin, through microleakage around the coronal restoration, through the accessory canals, which may be opened during post space preparation, or through undiagnosed root fracture.42,65 These corrosion products cause a change in volume that has been postulated to cause the root fracture.63 Of the various alloys used for posts, titanium alloys are the most corrosion resistant.43 Alloys containing brass have lower strength and lower corrosion resistance and, hence, are less desirable.66 Noble metal alloys are corrosion resistant, but their cost is higher. With the availability of nonmetallic post materials, the corrosion factor is eliminated.
The primary reason for using a post is to retain the core that substitutes the missing coronal tooth structure. Therefore, the post head design is an important factor.70 The post head should provide adequate retention and resistance to displacement of the core material. Studies30,71 have reported that prefabricated metal posts with direct cores made of glass ionomer, composite, or amalgam are less reliable than a one-piece cast post and core because of the interface between the post and the core. As the number of interfaces increase, the potential for failure also increases.72 Thayer73 has expressed concern that post and core separation is more likely to occur when composite is used as core material. In an effort to avoid this, posts with various head designs are available. Designs vary and include crenellated, flat, and spherical.74 Bonding techniques are advocated to reinforce the retention of the core.75 Some investigators74,75 have concluded that the post head design is crucial and that the lack of retentive features of the post head may reduce post to core retention.
BONDING ABILITY
RETRIEVABILITY
Cores may be bonded to the post and tooth structure so that the tooth-post-core-crown system may function as a unit which, as previously mentioned, may be difficult because of the difference in the physical properties of the materials and tooth structure. Of the cements available, zinc phosphate cement is the one that has been time tested. Newer adhesive resin luting agents are advocated for the luting of posts because they are reported to bond the post to the tooth structure in comparison to the traditional cements, which produce only frictional resistance.67 Moreover, a composite core material can also be bonded to the tooth structure and post with bonding agents.43
Ideally, the post system selected should be such that if the endodontic treatment fails or the post fractures, it is easy for the clinician to retrieve the post without substantial loss of tooth structure.71 Unfortunately, the retrievability of a metal post, especially the cast post and core system is difficult and involves removal of tooth structure around the post, which could further weaken the tooth. Carbon fiber posts have an advantage over metallic and ceramic posts in that the removal is relatively easy, rapid, and predictable.76 Post removal can be performed by means of conventional rotary instruments and solvents,76 which assist in the preservation of the residual dentin and minimize the chances of perforation.
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Removal of zirconia posts has been found to be more difficult.69 Certain post systems facilitate easy removal of the post by providing a milled head, a wrench, or a consecutive series of retreatment drills.77,78 Other commercially available systems to remove posts include the Messeran Kit (Medidenta International Inc, Woodside, New York, NY), Post Removal System (SybronEndo; Orange, Calif), and Endodontic extractors (Brasseler Inc, Savannah, Ga).44 In addition, an ultrasonic unit (Cavi-Endo, Dentsply Intl, Inc, York, Pa), Roto-Pro bur (Ellman International, Hewlett, NY), and a combination of tube extractors with cyanoacrylate, will aid in post removal by breaking up the cement.44
ESTHETICS The post and core material should be esthetically compatible with the crown and the surrounding tissues.79 Several authors have emphasized the need to have the color of the foundation restoration as close to that of natural dentin.76,80 However, in clinical situations in which the root has extensive damage or exhibits immature development, the use of a custom cast post would compromise esthetics as the gray tint of the metal may show through the thin root wall. The overlying gingival tissue would also appear darker or grayish.28 This esthetic concern has led to the development of esthetic posts made from reinforced resins or ceramics in an effort to eliminate the color deficiency. With prefabricated metal posts, the core material can be composite, which may aid in masking the metallic color of the post. However, masking will depend on the thickness of the composite core. A ceramic crown with an opaque substructure may be necessary in situations where complete masking is difficult.80 These solutions may not have an effect on the soft tissues unless a whitecolored post is also used. Therefore, the type of crown material used will affect the post selection. The metalceramic crown will permit the clinician to use any post and core material.76 All ceramic crowns are translucent and allow metal to show through.80 The influence of nonmetallic carbon fiber and zirconia post systems on all-ceramic crowns depends on the substructure and thickness of crown.80 When crown thickness is reduced, the color of foundation restoration shows through a nonopaque thin crown.80 Another alternative to an esthetic post and core system is the use of opaque porcelain fused to the core portion of cast post and core in order to eliminate the grayish effect of cast metal.81 Also, the use of ceramic core material such as IPS Empress cosmo core (IvoclareVivadent, New York, NY) is advocated . The availability of different cement shades permits minor esthetic corrections under all-ceramic crowns.80 560
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DISCUSSION Post and cores contribute in providing predictable restorative options for endodontically treated teeth. After reviewing the literature, it appears that an ideal post system should have the following features: (a) physical properties similar to dentin, (b) maximum retention with little removal of dentin, (c) distribution of functional stresses evenly along the root surface, (d) esthetic compatibility with the definitive restoration and surrounding tissue, (e) minimal stress during placement and cementation, (f) resistance to displacement, (g) good core retention, (h) easy retrievability, (i) material compatibility with core, (j) ease of use, safety and reliability, and (k) reasonable cost. Therefore, the clinician should be knowledgeable in selecting the right type of post and core systems to meet the biological, mechanical, and esthetic needs for each individual tooth. This review identified factors that influence the selection of the post-and-core system and offers the following clinical recommendations: (1) conserve as much remaining tooth structure as possible during the post space preparation, (2) custom-cast post and cores are recommended for noncircular root canals and when coronal tooth structure loss is moderate to severe, (3) parallel-sided, passive, serrated, self-venting prefabricated posts are recommended for small circular canals, (4) posts with an antirotational feature should be used in situations with circular canals, (5) adequate apical seal must be maintained without compromising the length of the post, (6) more than one post may be used for multirooted short teeth, (7) passive parallel posts are advocated for adequate retention but when the apical thickness of dentin is minimal, a parallel-tapered combination post design may be preferred, (8) retentive qualities of the post head may facilitate firm retention of core material, (9) the post should ensure material compatibility, bonding ability, adequate rigidity, and esthetic compatibility with permanent restoration, (10) retrievability in the event of failure should be considered, and (11) the system should be easy to use and cost effective. Although newer prefabricated post systems have been introduced, published long-term clinical data are not available. In addition, the claim that the nonmetallic post systems provide excellent esthetics has yet to be demonstrated through clinical research. Future research evaluating clinical success of the newer post systems on teeth with varying degree of tooth loss is recommended.
SUMMARY With such a wide variety of materials and post designs available for the restoration of endodontically treated teeth, the clinician should be selective in choosing the post system that best fits the individual needs of each tooth. The criteria and factors presented in this review VOLUME 90 NUMBER 6
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60. Brandal JL, Nicholls JI, Harrington GW. A comparison of three restorative techniques for endodontically treated anterior teeth. J Prosthet Dent 1987; 58:161-5. 61. Asmussen E, Peutzfeldt A, Heitmann T. Stiffness, elastic limit, and strength of newer types of endodontic posts. J Dent 1999;27:275-8. 62. Ichikawa Y, Akagawa Y, Nikai H, Tsuru H. Tissue compatibility and stability of new zirconia ceramic in vivo. J Prosthet Dent 1992;68:322-6. 63. Petersen KB. Longitudinal root fracture due to corrosion of an endodontic post. J Can Dent Assoc 1971;2:66-8. 64. Angmar-Mansson B, Omnell KA, Rud J. Root fracture due to corrosion.1. Metallurgical aspects. Odontol Revy 1969;20:245-65. 65. Luu KQ, Walker RT. Corrosion of a nonprecious metal post: a case report. Qiuntessence Int 1992;23:389-92. 66. Jacobi R, Shillingburg HT Jr. Pins, dowels and other retentive devices in posterior teeth. Dent Clin North Am 1993;37:367-90. 67. Plasmans PJ, Welle PR, Vrijhoef MM. In vitro resistance of composite resin dowel and cores. J Endod 1988;14:300-4. 68. Standlee JP, Caputo AA. Endodontic dowel retention with resinous cement. J Prosthet Dent 1992;68:913-7. 69. Mannocci E, Ferrari M, Watson TF. Intermittent loading of teeth restored using quartz fiber, carbon-quartz fiber and zirconium dioxide ceramic root canal posts. J Adhes Dent 1999;1:153-8. 70. Cohen BI, Condos S, Deutsch AS, Musikant BL. Fracture strength of three different core materials in combination with three different endodontic posts. Int J Prosthodont 1994;7:178-82. 71. Lewis R, Smith BG. A clinical survey of failed post retained crowns. Br Dent J 1988;165:95-7. 72. Hunter AJ, Flood AM. The restoration of endodontically treated teeth. Part 3. Cores. Aust Dent J 1989;34:115-21. 73. Thayer KE. Fixed prosthodontics. Chicago: Mosby; 1984 p. 284. 74. Chang WC, Millstein PL. Effect of post design of prefabricated post heads on core materials. J Prosthet Dent 1993;69:475-82.
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75. Cohen B1, Pagnillo MK, Newman I, Musikant BL, Deutsch AS. Retention of a core material supported by three post head designs. J Prosthet Dent 2000;83:624-8. 76. Freedman GA. Esthetic post and core treatment. Dent Clin North Am 2001;45:03-16. 77. Wilson NHF, Setcos JC, Dummer PMH, Gorman DG, Hopwood WA, Saunders WP, et al. A split-shank prefabricated post system: A critical multidisciplinary review. Quintessence Int 1997;28:737-43. 78. Cohen BI, Musikant BL, Deutsch AS. Comparison of retention properties of four post systems. J Prosthet Dent 1992;68:264-8. 79. Tamse A. Iatrogenic vertical root fracture in endodontically treated teeth. Endod Dent Traumatol 1988;4:190-6. 80. Vichi A, Ferrari M, Davidson CL. Influence of ceramic and cement thickness on the masking of various types of opaque posts. J Prosthet Dent 2000;83:412-7. 81. Hochstedler J, Huband M, Poillion C. Porcelain-fused-to-metal post and core: an esthetic alternative. J Dent Technol 1996;13:26-9. Reprint requests to: DR AQUAVIVA S. FERNANDES SEBASTIAN VILLA AVEDEM PO QUEPEM GOA 403705 INDIA FAX: ⫹91-832-2459816 E-MAIL: [email protected] Copyright © 2003 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2003/$30.00 ⫹ 0 doi:10.1016/j.prosdent.2003.09.006
Availability of JOURNAL Back Issues As a service to our subscribers, copies of back issues of The Journal of Prosthetic Dentistry for the preceding 5 years are maintained and are available for purchase from the publisher, Mosby, until inventory is depleted. Please write to Mosby, Subscription Customer Service, 6277 Sea Harbor Dr, Orlando, FL 32887, or call 800-654-2452 or 407-345-4000 for information on availability of particular issues and prices.
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T
he longevity of endodontically involved teeth has been greatly enhanced by continuing developments made in endodontic therapy and restorative procedures. It has been reported that a large number of endodontically treated teeth are restored to their original function with the use of intraradicular devices.1 These devices vary from a conventional custom cast post and core to one-visit techniques, using commercially available prefabricated post systems.2,3 In the last few decades, various prefabricated posts systems have been developed. The selection of post design is important, because it may have an influence on the longevity of the tooth.4 This article reviews the literature to determine the various factors influencing the post selection for restoring an endodontically treated tooth. The English-language literature from 1961 to 2002 was searched using MEDLINE. This was complemented with a manual search. The following key words were used: post, design, retention, fracture resistance, survivability, and esthetics. According to the reviewed articles, the factors influencing post selection were root length, tooth anatomy, root width, canal configuration, amount of coronal tooth structure, torquing force, stresses, development of hydrostatic pressure, post design, post material, material compatibility, bonding capability, core retention, retrievability, esthetics, and crown material.
ROOT LENGTH The length and shape of the remaining root determines the length of the post.5 It has been suggested that root length should be considered for the selection of the ideal post length.6 It has been demonstrated that the greater the post length, the better the retention and stress distribution.5-9 However, it may not always be possible to use a long post, especially when the remaina
Lecturer, Department of Prosthodontics, Goa Dental College. Lecturer, Department of Prosthodontics, Kargar Dental College. c Lecturer, Department of Prosthodontics, Goa Dental College. b
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ing root is short or curved. Several studies suggest that it is important to preserve 3 to 5 mm of apical gutta-percha to maintain the apical seal.10,11 When the root length is short, the clinician must decide whether to use a longer post or to maintain the recommended apical seal and use a parallel-sided threaded post. On the other hand, the result of an in vitro investigation has demonstrated that reinforced composite luting agents may compensate for the reduced post length.12 For molars with short roots, the placement of more than one post will provide additional retention for the core foundation restoration.6,12
TOOTH ANATOMY Each tooth in the arch exhibits anatomic characteristics such as root curvature, mesio-distal width, and labio-lingual dimension. Hence, root anatomy dictates post selection.12 Teeth may have anatomic variations, which may adversely affect the post placement. However, most investigators have disregarded anatomic variations such as invagination, depression, and ribbonshaped roots.8,10,13,14 A consideration of the root size and length is important, because improper post space preparation and use of large-diameter posts present the risk of apical or lateral perforation. Moreover, an active post can initiate cracks in the thin dentinal wall. A thorough knowledge and radiographic evaluation of the root anatomy will aid the clinician in avoiding catastrophic damage to the roots when planning the post space. Radiographs help the restorative dentist to evaluate the root length, width, anatomic variations, canal structure, and the surrounding hard tissue structures. Occasionally, the radiographs can be misleading because of proximal root concavities and the magnification factor. Therefore, use of a grid is recommended while a radiograph is made so that the length, diameter, and design of the post can be correctly determined.15 Gutmann16 reviewed the anatomic considerations in some detail and stated that roots of maxillary centrals VOLUME 90 NUMBER 6
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and laterals, and also mandibular premolars, have sufficient bulk to accommodate most post systems. This knowledge aids the dentist in determining the post suitable for a given root.
POST WIDTH Preserving tooth structure, reducing the chances of perforation, and permitting the restored tooth to resist fractures are criteria in selection of the post width.17,18 Tilk et al19 evaluated maxillary and mandibular teeth, and recommended a range of post widths. Various investigators19-21 have recommended different approaches regarding the selection of the post diameter. These approaches were summarized by Lloyd and Palik22 into 3 categories, conservationist (13 proponents), preservationist (7 proponents), and proportionist (6 proponents) approaches. Stern and Hirshfeld23 suggested that post width should not be greater than 1 third of the root width at its narrowest dimension. This proportionist approach was advocated with the intent of preserving sufficient tooth structure. Other investigators have proposed the preservationist approach where in the post should be surrounded by a minimum of 1mm of sound dentin.24 Others, including Pilo and Tamse,21 advocated minimal canal preparation and maintaining as much residual dentin as possible, thereby suggesting restriction of the post diameter in an effort to conserve the remaining tooth structure (conservationist approach). The influence of post width on its retention and fracture resistance has also been studied. It was shown that an increase in post width has no significant effect on its retention.8 The tooth restored with larger diameter posts is reported to provide the least resistance to fracture with a decrease in the width of the remaining dentin.18
CANAL CONFIGURATION AND POST ADAPTABILITY Canal configuration aids in making a choice between a custom-designed post and a prefabricated post.14,25 If the selected post closely fits or conforms to the canal shape and size, it may be a more conservative option because less dentin removal is required, thus enhancing fracture resistance of the tooth, as well as retention of the post.4,26 Often, a dilemma arises in funnel-shaped canals, whether to use a parallel-sided post and fill the remaining post space with cement or to use a tapered post that closely adapt to the canal wall. A third option is to use large prefabricated parallel-sided posts by removing additional tooth structure, so that an intimate contact between the canal and post is attained. It has been suggested that if a canal requires extensive preparation, a well-adapted cast post and core restoration will be more DECEMBER 2003
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retentive than a prefabricated post that does not match the canal shape.27 In addition, root reinforcement with composite is suggested for wide canals.28 In vitro studies4,29 have demonstrated that a well-adapted tapered post provided increased resistance to fracture. However, on fracture, it resulted in an extensive loss of tooth structure compared with teeth restored with well-adapted, passively cemented parallel-sided posts. Recently, Morgano and Milot30 questioned the results of an in vivo study,31 which concluded that close adaptation of a post causes more catastrophic failure. Morgano and Milot30 stated that 44% of the cast posts were less than one half to one quarter the length of the clinical crown and that the failure rate reported was the result of compromised length rather than post type. A recent investigation regarding custom-cast posts reported a success rate of more than 90% after 5 years in function.30
CORONAL STRUCTURE The amount of remaining coronal tooth structure is also a critical factor in determining the post selection. The bulk of the tooth above the restorative margin should be at least 1.5 to 2 mm to achieve resistance form.32 It was found that teeth restored with carbon fiber posts had inferior strength compared with those restored with metal posts when subjected to forces simulating those in a clinical setting.33,34 The use of cast post and cores in restoring endodontically treated teeth with moderate to severe coronal tooth loss has demonstrated a success rate of 90.6% after 5 years of service.35 The results of in vitro33,34 and in vivo studies35 indicate that nonmetal posts, such as a carbon fiber post system, can be used when ample coronal dentin remains and the crown is well supported by remaining tooth structure35; otherwise, cast post and core may be used when there is moderate to severe tooth loss.
STRESS Post and core–restored endodontically treated teeth are subjected to various types of stresses: compression, tensile, and shear. Of these stresses, shear stress is most detrimental to the restored tooth.36 Holmes et al5 have demonstrated that the variation in post dimension greatly influences shear stresses. An increase in the post length with diameter kept to a minimum will help to reduce shear stresses and preserve tooth structure. Thus, the vulnerability of the endodontically treated tooth to fracture is decreased.5
TORSIONAL FORCE Intraorally, post and core–restored teeth are subjected to various types of forces. Torsional forces on the post-core-crown unit may lead to loosening and displacement of the post from the canal, causing failure of 557
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the system.37-39 Therefore, the ability of the selected post design to resist torsional forces will play a key role in stabilizing and retaining the post-core and crown unit.37-39 Burgess et al37 demonstrated the importance of an antirotational feature in the post design and concluded that resistance to torsional forces is integral to the survival of the post-core-crown unit. An investigation into torsional resistance of several post systems revealed that active post designs provide greater torsional resistance than a passive post.38,39
ROLE OF HYDROSTATIC PRESSURE Cementation plays a significant role in enhancing retention, stress distribution, and sealing irregularities between the tooth and the post.40 During cementation, an increase in stress within the root canal has been reported because of the development of hydrostatic pressure.41 This pressure affects the complete seating of the post and may also cause fracture of the root.41,42 Fortunately, there is evidence that the fitting stresses can be reduced by careful placement of the post and by using a proper post design with a cement vent to permit escape of the luting agent and thus reduce the hydrostatic pressure.36 Tapered posts, however, are self-venting and will permit the cement to flow out along the entire surface. Pressure development is also dependent on the viscosity of the cement. The more viscous the cement, the greater the development of the hydrostatic pressure.43 Zinc phosphate cement has been successfully used for the cementation of posts for many years. Recently, resin luting agents have been used, although they are technique-sensitive and more difficult to manipulate.44,45 Care must be taken when using resin luting agents that polymerize anaerobically because they may polymerize prematurely, thus preventing complete seating of the post. Slow-polymerizing dual polymerizing luting agents are recommended in these situations.12
POST DESIGN The available post designs can be classified according to their shapes and surface characteristics. They may be parallel, tapered, or parallel-and-tapered combination. According to their surface characteristics, the posts are active or passive.46 The active post mechanically engages the dentin with threads, whereas the passive post depends on the cement and its close adaptation to the canal wall for its retention. Several studies47,48 have implicated the active post design as a cause of failure of the post and core–restored teeth. Of the designs studied, the tapered post conforms to the natural root form and canal configuration, thus permitting optimal preservation of tooth structure at the post apex. However, this design is reported to produce a wedging effect, stress concentration at the coronal por558
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tion of root, and lower retentive strength.48-50 Parallelsided post designs have been shown to increase retention and produce uniform stress distribution along the post length.9 Concentration of stress has been reported to occur at the apex of the post, especially in a narrow and tapering root end.9 This stress is considered to be caused by unnecessary removal of tooth structure at the apical end of the root and sharp angles of the post.51,52 In the parallel-tapered design, the post is parallel throughout its length except for the most apical portion, where it is tapered. This design permits preservation of the dentin at the apex and at the same time achieves sufficient retention because of parallel design.51 The surface characteristics of the post also change the retentive and fracture resistance values. The highest retention is observed in the threaded post, followed by the post with a serrated surface. The least retention is seen with smooth surface posts.19,48 Unfortunately, the threaded post engages in dentin and may lead to increased undesirable stresses within the root. To avoid placement stresses caused by threads, the following procedures have been recommended: pretapping post channels, limiting the number of threads, counter-rotating the post by one half turn after full engagement, and incorporating a spilt-shank mechanism.53 However, an analysis of the available clinical studies suggests that the performance of a threaded post is inferior to that of a custom-cast post.54 Of all the designs studied, the threaded tapered screw post exerts a greater amount of stress and was considered the least desirable.48,49 The parallel-sided, serrated, and vented posts were found to exert the least amount of stress.55
POST MATERIAL To achieve optimum results, the material used for the post should have physical properties similar to that of dentin, be bonded to the tooth structure, and be biocompatible in the oral environment.56 It should also act as a shock absorber by transmitting only limited stress to the residual tooth structure.57 Unfortunately, the materials used for post and cores, as well as luting agents, have distinct physical properties different from dentin and exhibit fundamentally different fatigue behavior.36 Traditionally, posts were made of metal alloys. Recently, nonmetallic posts have been introduced. Several studies 58-60 have examined post and cores made of various alloys and other materials with different rigidity and demonstrated that rigid materials resisted greater forces without distortion. Therefore, there may be a potential danger in using highly rigid posts. Recently introduced carbon fiber posts are purported to have mechanical properties that closely match those of the tooth.33,61 The presence of the parallel fibers in the resin of carbon fiber posts enable them to absorb and dissipate stresses.33,61 In vitro studies,33,34 however, VOLUME 90 NUMBER 6
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have demonstrated that carbon fiber posts have inferior strength compared with metal posts when subjected to forces simulating those in the oral cavity. Zirconium ceramic, which is presently used for posts, has a high modulus of elasticity, and therefore the forces are assumed to be transmitted directly from the post to the tooth interface without shock absorption.62 The stiffer ceramic post may cause more root fractures compared with the carbon fiber post. Asmussen et al61 have demonstrated that fracture of endodontically restored teeth is less extensive with carbon fiber posts than with ceramic posts. Although various claims are made with regard to nonmetallic posts, there is a need for long term clinical evaluation of both metallic and nonmetallic post systems to allow a definitive recommendation of either of them. Until such time, metallic posts continue to be the standard for most situations because they have stood the test of time.
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The bonding of a post to the tooth structure should improve the prognosis of the post-core restored tooth by increasing post retention68 and by reinforcing the tooth structure.28 It has been postulated that reinforcement of the tooth is due to the stress distribution characteristics of the bonding materials.28 Recently, the importance of bonding on the retention of posts has been demonstrated.69 It was reported that resin luting agents showed good adhesion to carbon fiber posts and glass fiber posts.69 The adhesion to zirconia posts was found to be unsatisfactory.69 It was also observed that to improve retention, the carbon fiber post did not require any surface treatment as compared with the zirconia post. In spite of the creation of microretention on zirconia posts, the adhesion between the post and resin luting agent was not uniform, thus indicating that the nature of post material was responsible for the bonding of the post to the tooth structure.69
CORE RETENTION MATERIAL COMPATIBILITY Corrosion of the post and fracture of the root has been reported in the dental literature.63 Ideally, post and cores are made of the same alloy. Dissimilar alloys may create galvanic action, which can lead to corrosion of the less noble alloy.64 Corrosion of the post may be initiated because of the access of an electrolyte to the post surface, through cementum and dentin, through microleakage around the coronal restoration, through the accessory canals, which may be opened during post space preparation, or through undiagnosed root fracture.42,65 These corrosion products cause a change in volume that has been postulated to cause the root fracture.63 Of the various alloys used for posts, titanium alloys are the most corrosion resistant.43 Alloys containing brass have lower strength and lower corrosion resistance and, hence, are less desirable.66 Noble metal alloys are corrosion resistant, but their cost is higher. With the availability of nonmetallic post materials, the corrosion factor is eliminated.
The primary reason for using a post is to retain the core that substitutes the missing coronal tooth structure. Therefore, the post head design is an important factor.70 The post head should provide adequate retention and resistance to displacement of the core material. Studies30,71 have reported that prefabricated metal posts with direct cores made of glass ionomer, composite, or amalgam are less reliable than a one-piece cast post and core because of the interface between the post and the core. As the number of interfaces increase, the potential for failure also increases.72 Thayer73 has expressed concern that post and core separation is more likely to occur when composite is used as core material. In an effort to avoid this, posts with various head designs are available. Designs vary and include crenellated, flat, and spherical.74 Bonding techniques are advocated to reinforce the retention of the core.75 Some investigators74,75 have concluded that the post head design is crucial and that the lack of retentive features of the post head may reduce post to core retention.
BONDING ABILITY
RETRIEVABILITY
Cores may be bonded to the post and tooth structure so that the tooth-post-core-crown system may function as a unit which, as previously mentioned, may be difficult because of the difference in the physical properties of the materials and tooth structure. Of the cements available, zinc phosphate cement is the one that has been time tested. Newer adhesive resin luting agents are advocated for the luting of posts because they are reported to bond the post to the tooth structure in comparison to the traditional cements, which produce only frictional resistance.67 Moreover, a composite core material can also be bonded to the tooth structure and post with bonding agents.43
Ideally, the post system selected should be such that if the endodontic treatment fails or the post fractures, it is easy for the clinician to retrieve the post without substantial loss of tooth structure.71 Unfortunately, the retrievability of a metal post, especially the cast post and core system is difficult and involves removal of tooth structure around the post, which could further weaken the tooth. Carbon fiber posts have an advantage over metallic and ceramic posts in that the removal is relatively easy, rapid, and predictable.76 Post removal can be performed by means of conventional rotary instruments and solvents,76 which assist in the preservation of the residual dentin and minimize the chances of perforation.
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Removal of zirconia posts has been found to be more difficult.69 Certain post systems facilitate easy removal of the post by providing a milled head, a wrench, or a consecutive series of retreatment drills.77,78 Other commercially available systems to remove posts include the Messeran Kit (Medidenta International Inc, Woodside, New York, NY), Post Removal System (SybronEndo; Orange, Calif), and Endodontic extractors (Brasseler Inc, Savannah, Ga).44 In addition, an ultrasonic unit (Cavi-Endo, Dentsply Intl, Inc, York, Pa), Roto-Pro bur (Ellman International, Hewlett, NY), and a combination of tube extractors with cyanoacrylate, will aid in post removal by breaking up the cement.44
ESTHETICS The post and core material should be esthetically compatible with the crown and the surrounding tissues.79 Several authors have emphasized the need to have the color of the foundation restoration as close to that of natural dentin.76,80 However, in clinical situations in which the root has extensive damage or exhibits immature development, the use of a custom cast post would compromise esthetics as the gray tint of the metal may show through the thin root wall. The overlying gingival tissue would also appear darker or grayish.28 This esthetic concern has led to the development of esthetic posts made from reinforced resins or ceramics in an effort to eliminate the color deficiency. With prefabricated metal posts, the core material can be composite, which may aid in masking the metallic color of the post. However, masking will depend on the thickness of the composite core. A ceramic crown with an opaque substructure may be necessary in situations where complete masking is difficult.80 These solutions may not have an effect on the soft tissues unless a whitecolored post is also used. Therefore, the type of crown material used will affect the post selection. The metalceramic crown will permit the clinician to use any post and core material.76 All ceramic crowns are translucent and allow metal to show through.80 The influence of nonmetallic carbon fiber and zirconia post systems on all-ceramic crowns depends on the substructure and thickness of crown.80 When crown thickness is reduced, the color of foundation restoration shows through a nonopaque thin crown.80 Another alternative to an esthetic post and core system is the use of opaque porcelain fused to the core portion of cast post and core in order to eliminate the grayish effect of cast metal.81 Also, the use of ceramic core material such as IPS Empress cosmo core (IvoclareVivadent, New York, NY) is advocated . The availability of different cement shades permits minor esthetic corrections under all-ceramic crowns.80 560
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DISCUSSION Post and cores contribute in providing predictable restorative options for endodontically treated teeth. After reviewing the literature, it appears that an ideal post system should have the following features: (a) physical properties similar to dentin, (b) maximum retention with little removal of dentin, (c) distribution of functional stresses evenly along the root surface, (d) esthetic compatibility with the definitive restoration and surrounding tissue, (e) minimal stress during placement and cementation, (f) resistance to displacement, (g) good core retention, (h) easy retrievability, (i) material compatibility with core, (j) ease of use, safety and reliability, and (k) reasonable cost. Therefore, the clinician should be knowledgeable in selecting the right type of post and core systems to meet the biological, mechanical, and esthetic needs for each individual tooth. This review identified factors that influence the selection of the post-and-core system and offers the following clinical recommendations: (1) conserve as much remaining tooth structure as possible during the post space preparation, (2) custom-cast post and cores are recommended for noncircular root canals and when coronal tooth structure loss is moderate to severe, (3) parallel-sided, passive, serrated, self-venting prefabricated posts are recommended for small circular canals, (4) posts with an antirotational feature should be used in situations with circular canals, (5) adequate apical seal must be maintained without compromising the length of the post, (6) more than one post may be used for multirooted short teeth, (7) passive parallel posts are advocated for adequate retention but when the apical thickness of dentin is minimal, a parallel-tapered combination post design may be preferred, (8) retentive qualities of the post head may facilitate firm retention of core material, (9) the post should ensure material compatibility, bonding ability, adequate rigidity, and esthetic compatibility with permanent restoration, (10) retrievability in the event of failure should be considered, and (11) the system should be easy to use and cost effective. Although newer prefabricated post systems have been introduced, published long-term clinical data are not available. In addition, the claim that the nonmetallic post systems provide excellent esthetics has yet to be demonstrated through clinical research. Future research evaluating clinical success of the newer post systems on teeth with varying degree of tooth loss is recommended.
SUMMARY With such a wide variety of materials and post designs available for the restoration of endodontically treated teeth, the clinician should be selective in choosing the post system that best fits the individual needs of each tooth. The criteria and factors presented in this review VOLUME 90 NUMBER 6
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may help guide the clinician in selecting the most desirable post system for each tooth. REFERENCES 1. Turner CH. The utilization of roots to carry post-retained crowns. J Oral Rehabil 1982;9:193-202. 2. Shillingburg HT, Hobo S, Whitsett L, Brackett S. Fundamentals of fixed prosthodontics. 3rd ed. Chicago: Quintessence; 1997. 3. Baraban DJ. The restoration of endodontically treated teeth: an update. J Prosthet Dent 1988;59:553-8. 4. Sorensen JA, Engelman MJ. Effect of post adaptation on fracture resistance of endodontically treated teeth. J Prosthet Dent 1990;64:419-24. 5. Holmes DC, Diaz-Arnold AM, Leary JM. Influence of post dimension on stress distribution in dentin. J Prosthet Dent 1996;75:140-7. 6. Hirshfeld Z, Stern N. Post and core—the biomechanical aspect. Aust Dent J 1972;17:467-8. 7. Davy DT, Dilley GL, Krejci RF. Determination of stress patterns in rootfilled teeth incorporating various dowel designs. J Dent Res 1981;60:130110. 8. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effect of cement, dowel length, diameter and design. J Prosthet Dent 1978;39:400-5. 9. Standlee JP, Caputo AA, Collard EW, Pollack MH. Analysis of stress distribution by endodontic posts. Oral Surg Oral Med Oral Pathol 1972; 33:952-60. 10. Mattison GD, Delivanis PD, Thacker RW Jr, Hassel KJ. Effect of post preparation on the apical seal. J Prothet Dent 1984;51:785-9. 11. Kvist T, Rydin E, Reit C. The relative frequency of periapical lesions in teeth with root canal -retained posts. J Endod 1989;15:578-80. 12. Nissan J, Dmitry Y, Assif D. The use of reinforced composite resin cement as compensation for reduced post length. J Prosthet Dent 2001;86:304-8. 13. Gluskin AH, Radke RA, Frost SL, Watanabe LG. The mandibular incisor: rethinking guidelines for post and core design. J Endod 1995;21:33-7. 14. Ash M Jr. Wheeler’s dental anatomy, physiology and occlusion. 7th ed. Philadelphia: WB Saunders; 1993 p. 170-217,274-331. 15. Frommer HH. Radiology for dental auxiliaries. 6th ed. St Louis: Mosby; 1996. p. 265-6. 16. Gutmann JL. The dentin- root complex: anatomic and biologic considerations in restoring endodontically treated teeth. J Prosthet Dent 1992;67: 458-67. 17. Akkayan B, Gulmez T. Resistance to fracture of endodontically treated teeth restored with different post systems. J Prosthet Dent 2002;87:431-7. 18. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-a comparison of endodontic and restorative treatments. J Endod 1978;4:341-5. 19. Tilk MA, Lommel TJ, Gerstein H. A study of mandibular and maxillary root widths to determine dowel size. J Endod 1979;5:79-82. 20. Mattison GD. Photoelastic stress analysis of cast gold endodontic posts. J Prosthet Dent 1982;48:407-11. 21. Pilo R, Tamse A. Residual dentin thickness in mandibular pre-molars prepared with gates glidden and ParaPost drills. J Prosthet Dent 2000;83: 617-23. 22. Lloyd PM, Palik JF. The philosophies of dowel diameter preparation: a literature review. J Prosthet Dent 1993;69:32-6. 23. Stern N, Hirshfeld Z. Principles of preparing endodontically treated teeth for dowel and core restorations. J Prosthet Dent 1973;30:162-5. 24. Halle EB, Nicholls JI, Hassel HJ. An in-vitro comparison of retention between a hollow post and core and a custom hollow post and core. J Endod 1984;10:96-100. 25. Smith CT, Schuman N. Restoration of endodontically treated teeth: a guide for the restorative dentist. Quintessence Int 1997;28:457-62. 26. Craig RG, Farah JW. Stress analysis and design of single restoration and fixed bridges. Oral Sci Rev 1977;10:45-74. 27. Cohen BI, Pagnillo MK, Condos S, Deutsch AS. Four different core materials measured for fracture strength in combination with five different designs of endodontic posts. J Prosthet Dent 1996;76:487-95. 28. Saupe WA, Gluskin AH, Radke RA Jr. A comparative study of fracture resistance between morphologic dowel and cores and a resin-reinforced dowel system in the intraradicular restoration of structurally compromised roots. Quintessence Int 1996;27:483-91. 29. Tjan AH, Whang SB. Resistance to root fracture of dowel channels with various thicknesses of buccal dentin walls. J Prosthet Dent 1985;53:496500.
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75. Cohen B1, Pagnillo MK, Newman I, Musikant BL, Deutsch AS. Retention of a core material supported by three post head designs. J Prosthet Dent 2000;83:624-8. 76. Freedman GA. Esthetic post and core treatment. Dent Clin North Am 2001;45:03-16. 77. Wilson NHF, Setcos JC, Dummer PMH, Gorman DG, Hopwood WA, Saunders WP, et al. A split-shank prefabricated post system: A critical multidisciplinary review. Quintessence Int 1997;28:737-43. 78. Cohen BI, Musikant BL, Deutsch AS. Comparison of retention properties of four post systems. J Prosthet Dent 1992;68:264-8. 79. Tamse A. Iatrogenic vertical root fracture in endodontically treated teeth. Endod Dent Traumatol 1988;4:190-6. 80. Vichi A, Ferrari M, Davidson CL. Influence of ceramic and cement thickness on the masking of various types of opaque posts. J Prosthet Dent 2000;83:412-7. 81. Hochstedler J, Huband M, Poillion C. Porcelain-fused-to-metal post and core: an esthetic alternative. J Dent Technol 1996;13:26-9. Reprint requests to: DR AQUAVIVA S. FERNANDES SEBASTIAN VILLA AVEDEM PO QUEPEM GOA 403705 INDIA FAX: ⫹91-832-2459816 E-MAIL: [email protected] Copyright © 2003 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/2003/$30.00 ⫹ 0 doi:10.1016/j.prosdent.2003.09.006
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