HATZIKYRIAKOS,

5. Baraban DG. Immediate restoration of pulpless teeth. J PROSTHET DENT 1972;28:607-12.

6. Priest G, Goering A. Post and core fabrication beneath an existing crown. J PROSTHET DENT 1979;42:645-8. 7. Ruemping DR, Lund MR, Schnell RJ. Retention of dowels subjected to tensile and torsional forces. J PROSTHET DENT 1979;41:159-62. 8. Caputo AA. The mechanics of load transfer by retentive pins. J PROSTHET DENT 1973;29:442-9.

9. Kantor EM, Pines SM. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;38:405-12. 10. Kurer GH. An evaluation of the retentive properties of various permanent crown posts. J PROSTHET DENT 1983;49:633-5. 11. Linde G. The use of composites as core material in root-filled teeth. II. Clinical investigation. Swed Dent J 1984;8:209-16. 12. Miller A. Post and core systems: Which one is best? J PROSTHET DENT

REISIS,

AND

TSINGOS

13. Zmener 0. Adaptations of threaded dowels to dentine. J PROSTHET DENT 1980;43:530-5. 14. Cooney PJ, Caputo AA, Trabered KC. Retention and stress distribution of tapered-end endodontic posts. J PROSTHET DENT 1986;56:540-6. 15. Conrtade C, Timmermans JS. Pins in restorative dentistry. St. Louis: CV Mosby, 1971:145. Reprint requests to: DR. A. H. HATZIKYRIAKOS ECNATIAS STR. 101 54635 THESSALONIKI GREECE

1982;48:27-38.

The dentin-root considerations

complex: Anatomic and biologic in restoring endodontically treated

teeth

James L. Gutmann, DDS* Baylor College of Dentistry, Dallas, Tex. The restoration of endodontically treated teeth has been the focus of considerable controversy and empiricism. Time-tested methods have been highly successful in some respects, but failures are still apparent. The inherent causes of failure are rarely evaluated and the limitations of specific restorative systems are seldom identified. Regardless of the system, there should be a thorough understanding of the anatomy and biology of the dentin and root supporting the restoration on the part of the practitioner, because both endodontic and restorative procedures alter the hard tissues. (J PROSTHET DENT 1992;67:468-67.)

T

he restoration of endodontically treated teeth has been controversial for years because many empirical statements possess varying degrees of scientific support. A plethora of dental literature is available extolling the virtues of diverse techniques to restore the pulpless tooth. While these publications address a multitude of post and core designs including their pros and cons, few articles discuss the “whys” for developing an innovative approach to coronoradicular restorations. There has been a general consensus that endodontically treated teeth are “more brittle” and more subject to fracture.le4 However, there is more recent scientific information to support the contention that endodontically treated teeth have special needs that exceed the requirements of teeth with viable pulp. These unique aspects include: (1) the role of moisture loss and the nature of the dentin; (2) alterations in strength caused by architectural changes in the morphology of the

Presented at the 1991 Annual Meeting of the American Academy of Fixed Prosthodontics. aProfessor and Chairman, Department of Endodontics. 10/l/36088

teeth; (3) concepts of the biomechanical behavior of tooth structure under stress; (4) the nature of dentin toughness in pulpless teeth; and (5) changes in the nature of the collagen alignment in pulpless teeth.

MOISTURE

LOSS

The physical chemistry of calcified dentin deserves specific attention.5 First, the moisture content of the coronal dentin is approximately 13.2 % , but the coronal dentin has twice the tubules of radicular dentin. Presumably with fewer tubules, greater inorganic substrate and intertubular dentin, the radicular dentin would possess less moisture. During aging, greater amounts of peritubular dentin are deposited, diminishing the amount of organic materials that may contain moisture. There are also two major compartments of water content in calcified tissues, one outside the calcified matrix and the other within the calcified matrix. This latter category can be divided into free water to hydrate inorganic ions thus being involved in their movement, but this water can be removed at between 100“ and llO” C, and water firmly bound that does not participate in the movement of ions. This firmly bound portion is called the water of hydroxyapatite crystal, and is not

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1. Two maxillary incisors (A and B) with excessive dentin removed in the coronal third of the root, encouraging root fracture.

substantially reduced until temperatures of 600° C are reached. In 1956, Battistone and Burnett6 were unable to completely rehydrate teeth after removing the moisture. If the moisture has been lost from calcified tissues, it is unrecoverable even in a saturated atmosphere at body temperature, so moisture loss from endodontically treated teeth is irreversible. In 1972, Helfer et a1.5 determined the moisture content of vital and pulpless teeth in dogs by dividing the total water content of each tooth into free and bound water. The amount of free water was determined gravimetrically while the bound water was calculated by differential thermal analysis. This study demonstrated that there was 9 % less moisture in the calcified tissues of pulpless dog teeth than in vital teeth. This study was the first investigation lend-

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ing credence to the empirical assumption that pulpless teeth may have increased brittleness from moisture loss, but there still is sparse additional evidence to support this concept.

ARCHITECTURAL

CHANGES

While substantial dentin can be removed during endodontic access preparation or canal cleaning and shaping, these procedures apparently do not significantly weaken the tooth. Reeh et a1.7evaluated the effects of endodontics procedures compared with restorative reduction on tooth stiffness. Endodontic procedures reduced tooth stiffness by a mere 5 % , attributed primarily to the access opening, while restorative procedures resulted in appreciable loss of tooth stiffness. An MOD cavity preparation reduced tooth stiffness by more than 60%) with the loss of the marginal

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Fig. 2. Mandibular molar with excessively wide screw post placed in a mesiodistal narrow root.

the pulpless tooth. Tidmarshs described an intact tooth as a hollow, laminated structure that deforms under load. This laminated structure may shorten, its sides may bulge, and its cusps may be wedged apart by opposing cusps. Although there is complete elastic recovery after physiologic loading, permanent deformation may follow excessive or sustained loads. Therefore the tooth appears to respond like a prestressed laminate. In this state, a structure can resist greater loads in the prestressed rather than in the unstressed state because in the prestressed mode it can flex with the varying degree and angle of load. This phenomenon is crucial if the cuspal inner slopes are removed during endodontic access preparation or cavity preparation, thus destroying the prestressed state. Subsequently, stress is released, accompanied by a slight shift in cuspal structure. However, the tooth can now deform to a greater extent under applied loads, and thus be more susceptable to fracture. Grimaldi17 illustrated that there is a direct relationship between the amount of central tooth structure lost in cavity preparation and the deformations under load. This concept would apply to teeth with endodontic cavity preparations, and would be integrated in the nature of the cuspal anatomy, its facial-lingual width, and the angle of inclination.12 DENTINAL

Fig. 3. Scanning electron microscope cross section of dentin that exhibits irregular fracture lines both perpendicular and parallel to the dentinal tubules. (Original magnification X780.)

ridge contributing the greatest loss of tooth strength. This compromise of architectural integrity has been the object of research with respect to cuspal anatomy, flexure, and strength.7-12 With the reduction of the inner cuspal slopes that unite and support or exposure of acute cuspal angles, a greater chance of fracture exists. Despite documentation that the decrease in the strength of endodontically treated teeth is the result of alteration of coronal tooth structure, the cause of the loss of strength is not necessarily endodontics. Conversely, the excessive removal of radicular dentin during canal cleaning and shaping or post space preparation compromises the root (Fig. 1).13-15The same situation occurs if large posts are inserted in small or irregular roots (Fig. 2).i6 BIOMECHANICAL

BEHAVIOR

The behavior of teeth under load has been investigated and has provided information into the changes occuring in 460

TOUGHNESS

The physical properties of dentin were initially reviewed by Black in 1895,1swith the determination of the crushing strength. Later research by Peyton et all9 and by Tyldesley20focused on the physical and mechanical properties of dentin, and the results varied appreciably from tooth to tooth and within the same type of tooth. The significant physical aspects of dentin are: (1) the modulus of elasticity, defined as the slope of the stressstrain curve within the elastic limit; (2) the proportional limit or tensile strength, defined as the stress beyond which stress is no longer proportional to strain or the limit above which deformation becomes nonlinear and nonelastic; and (3) compressive strength, the greatest stress the material will resist. The elastic modulus for dentin is approximately 1.90 x lo6 psi. The average tensile strength (proportional limit) is 7.0 x lo3 psi, while the compressive strength or breaking stress is 43.0 x lo3 psi. During experimentation, Tyldesley20 noted that at the breaking stress, fractures were located along the lines of maximal sheer and principle stress rather than parallel to tubule orientation. Even with specimens with distinctly different tubular orientation and distributions, the dentin recorded similar results for physical property tests. Therefore under dynamic catastrophic testing of bending and torsion, dentin has been identified as an isotropic material. Renson21 and Renson and Bradenz2 confirmed these findings by recording similar values for the elastic and mechanical properties (compressive, tensile, and shear) from specimens at different locations with different tubular orientations. In essence, this indicated that the properties of dentin were independent of tubule orientation. This exAPRIL

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Fig. 4. A, Thin tapering root walls in a maxillary first premolar that predispose to perforations along the palatal surface of the buccal root (arrows). B, Anatomic irregularities and root thinness are evident in cross section.

plained why dentin fractured longitudinally and perpendicularly to the dentinal tubules (Fig. 3). Even with an apparent fracture at the gingival or osseous crest, dentinal tubules are not parallel to these landmarks, but display a coronal-to-apical “S-shaped” curvature. In studies designed to examine fractured dentinal surfaces, tubular direction, tooth type, specimen location, age, and sex of the patient did not influence the nature of the surface, further supporting the isotropic nature of dentin.23 The fracture properties of dentin were further evaluated by Rasmussen et a1.,24and they determined the work required to fracture dentin (work of fracture = Wr) as the ratio of the total work expended during fracture to the projected normal area of the two fractured surfaces. This included the work required to initiate and propagate the fracture. Wider variations were noted for fractures perpendicular to the tubules than for fractures parallel to the tubules. These data supported dentin as a more anisotropit material in terms of strength rather than in terms of fracture because these findings were established under controlled experimental conditions and not with catastrophic fractures21-23that are more appropriately seen in the oral cavity. A critical fact was that the Wf to displace the dentin perpendicularly was half of the Wf required to separate the dentin along the tubules. A logical explanation for this phenomenon is that the crack for perpendicular fractures passes between the collagenous networks that form in the planes nearly perpendicular to the tubules, while parallel fractures break these collagenous networks, requiring greater work. However, the degree of work THE

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required may be reduced because collagen intermolecular cross-links may be weaker in endodontically treated teeth.25 Thus the fracture toughness of dentin may vary with tubule orientation, angle and force of the load applied, the crack velocity, and the nature of the collagen fibrils.26 Toughness is measured by the total energy required to fracture a material, but another technique to determine the toughness of a material is microindentation. Imprints are made in the material with specific loads and the depth of the indentation indicates a measure of the hardness of the material. Lewinstein and Grajower,27 using the Vickers hardness test, did not substantiate an increase in dentinal hardness between vital teeth and root canal-treated teeth after 5 to 10 years following treatment. Nevertheless, they cautioned against assuming that the mechanical properties of root dentin are unaffected by endodontic treatment, because their study did not calculate the total energy to “fracture” the material, that is, dentinal toughness. With respect to tubular sclerosis that often occurs in radicular dentin, there is no apparent impact of this process on the fracture toughness of dentin2s or on an elevation in root dentinal hardness.2g Therefore sclerosis of the dentinal tubules of the root would seem to have limited if any influence on strengthening root structure. Because dentin exhibits considerable plastic deformation beyond the yield point,30 it is therefore a weak, biologic ductile material in which the strength and toughness may vary. Carter et a1.31used a punch shear test to evaluate the shear strength values of planoparallel specimens of human dentin from the cervical dentin of vital and endodontically 461

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Fig. 5. A, Maxillary premolar with proximal invaginations and external root split. Root sectioned at level 1 and 2. B, Radicular cross section immediately coronal to the canal divergence, level 1 (A). C, Root and canal anatomy immediately apical to the split into two canals, level 2 (A) with irregular internal and external root anatomy.

treated teeth. These values correlated positively with the approximate toughness values. The shear strengths and toughness of dentin from endodontically treated teeth were lower and significantly different from the values for dentin of vital teeth. The values did not distinguish between anatomic type except for the mandibular incisors, which had

462

the lowest values. Also, maxillary teeth tended to be stronger than mandibular teeth. This study demonstrated a 14 % reduction in the strength and toughness of the cervical dentin in endodontically treated molar teeth. These findings, based on the physical and mechanical properties of dentin, indicated that dentin is basically a

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Fig.

6. Buccally divergent palatal root. Root thinning and/or post perforations commonly occur along the buccal surface of the root (arrows).

Fig. 7. Radicular invagination (arrows) with buccal divergence predispose to perforation during post space preparation.

brittle structure and that alterations in the nature of this material, whether biochemical, structural, or in magnitude, potentially subject dentin to fracture.

in the strength of the tooth structure in pulpless teeth can be formulated. There are fundamental, irreversible changes in the anatomy, biochemistry, and biomechanic properties of the dentin, which makes up the bulk of the remaining tooth structure after pulpal loss and endodontic treatment. However, this is compounded by the loss of strategic architectural aspects of the tooth that have the greatest impact on the ultimate strength of the pulpless tooth. Using this information, the dentist can then make logical decisions regarding the restorative treatment.

COLLAGEN

ALTERATION

Collagen forms the organic matrix of dentin, with inorganic calcium phosphate salts impregnating the fibers of the matrix. When decalcified sections of dentin are viewed in the transmission electron microscope, dentinal collagen consists of large fibrils each demonstrating the 60 to 70 nm axial periodicity characteristic of type I collagen that is a genetically specific, immunospecific form of collagen.32 This pattern is the normal alignment for the tropocollagen subunits that compose the polymerized collagen fibrils. The cross-banding pattern reflects the overlapping of adjacent tropocollagen moieties by one fourth (70 nm = quarter stagger array) of their total length (280 nm). During the polymerization process and the intermolecular cross-linking, the collagen fibers achieve their characteristic physical properties of rigidity, resistance to stretching, and remarkably high tensile strength evident in most calcifying tissues. Changes in these cross-links may contribute to the so-called “brittleness” of pulpless teeth. Preliminary studies by Rivera et al. 25have verified that there are more immature and fewer mature cross-links in root-filled teeth, so this could account for the decrease in tensile strength claimed in these circumstances. When all five of the biologic aspects of dentin are integrated, a reasonable explanation for the purported changes

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FACTORS

INFLUENCING

RESTORATION

The most common misconception in restoring the endodontically treated tooth is to apply the same treatment goals or preparation designs for the vital tooth as for the endodontically treated tooth.16 However, treatment goals must be based upon a multitude of factors specific for each patient that include occlusion, patient function, tooth position, periodontal status, prosthetic needs, economics, amount of remaining viable tooth structure, and root morphology. The status of the root to be restored is critical. Radicular

considerations

Although extensive anatomic and radiographic studies have provided abundant information about root anatomy and its relationship to bone, there remains a tremendous dependency on the radiograph as the essential diagnostic aid for determining the anatomy of the root to be restored. While routine periradicular radiographs provide only two-

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Fig. 8. Internal invaginations (arrows) roots contraindicate use of intraradicular

of mesiobuccal posts.

dimensional cross-sectional anatomy of the radicular tissues from mesial to distal, supplemental views from proximal or occlusal angulations will supply additional information regarding root curvatures or extra roots. However, since the exact faciolingual dimensions or the mesiodistal shape including the presence of invaginations or laminations of the roots between the faciolingual dimensions cannot be accurately ascertained, it is imperative to have a thorough knowledge of the root anatomy before reconstructing the tooth. Anatomic

concerns

A brief review of the major concerns in radicular anatomy before the restoration of the endodontically treated tooth is indicated if a post is to be used. This is not to imply that each endodontically treated tooth should receive a post as part of the restoration. Sorensen and Martinoff indicated that “indiscriminate placement of a post in every endodontically treated tooth is unrealistic.“15 Therefore in those teeth that need a post to retain a core buildup, careful attention must be directed to the root anatomy for selecting the appropriate post design, including shape, length, and method of placement.33-36 Maxillary central and lateral incisors usually have sufficient bulk of roots to accommodate most post systems. However, care must be exercised in using posts with excessive length if the root tapers rapidly to the apex, because the thinner the root walls at the depth of post placement the greater the chance for root fracture. Maxillary canines have wide faciolingual roots and root canal spaces that commonly necessitate a custom cast post for desired adap464

9. External radicular invagination along with multiple canals in a mandibular first premolar.

Fig.

tation to the root walls, and there is the possibility of proximal root invaginations. Restoration of maxillary premolars presents a variety of problems when one anticipates a post-retained core.37T3s Root walls are commonly thin and roots taper rapidly to the apex, especially when two distinct roots are present (Fig. 4). During preparation of the canal from coronal to apical root structure, proximal invaginations and canal splitting are common, and they present dangerous anatomic scenarios (Fig. 5). Root curvatures to the distal are also common and preclude using long posts. The curvatures of the palatal root can be facial, resulting in a root perforation during post space preparation or cementation. Because of the thinness of these roots, the removal of dentin for the placement of a post results in a weakened root wall that is subject to fracture, either during cementation or during function. These same observations are true for second premolars, but these teeth generally have greater bulk of tooth structure. While the only suitable root in maxillary molars for post placement is the palatal root, even this root presents restorative problems. Eighty-five percent of palatal roots have been shown to curve facially (Fig. 6),3gand when invaginations are present they are located on the palatal and facial surfaces. This combination of root curvature and radicular invaginations predisposes the root walls to weakening or perforation during the placement of long or thick posts (Fig. 7). However, radiographs do not disclose these situations and they are often unnoticed despite patient symptoms. As a result, palatal roots can be fractured, requiring root resection, tooth extraction, or surgical endodontics to repair the perforation. Generally, the placement APRIL

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Fig. 10. A, Internal proximal invagination along the distal wall of the mesiobuccal root of a mandibular first molar. B, Similar invagination along the mesial surface of the distal root of the same tooth.

of intraradicular posts in the mesiobuccal or distobuccal roots is contraindicated (Fig. 8).40 Mandibular incisors are difficult teeth to restore with a post and core and successrates with these teeth have been shown to be higher without apost.15 Root walls are thin and proximal invaginations are common. The placement of a post is commonly compromised by multiple canals or significant bone loss, precluding the placement of a post in an unsupported root. This problem was clearly identified by Reinhardt et a1.41In teeth restored with a post and core having diminished bone support of 4 to 6 mm, stress concentrations occurred both at the post apex and on the adjacent root periphery in a relatively narrow band of remaining dentin. In these clinical situations, the potential for root fracture was deemed great. Mandibular canines present with similar circumstances as maxillary canines. Mandibular premolars have sufficient bulk of root structure to receive most post systems, but care must be exercised to ensure that the entire root canal has been managed because there is a proclivity for multiple canals (Fig. 9). After a post is placed in the root in which canal space has not been treated, an endodontic failure invariably results that requires surgical intervention. One area of concern with the first premolar is the angle of the crown to the root. Often the root will be lingually inclined and active drilling of a post space perpendicular to the occlusal surface will result in a perforation along the facial wall of the root. The major problems posed by the mandibular molars reside in the mesiodistal thinness of the mesial and distal roots (Fig. 10). Along the root curvatures, there are THE

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commonly invaginations and perforations that are invisible radiographically. In addition, roots may be substantially weakened if they are prepared for prefabricated, circular posts because the roots are extremely wide facial-lingually and narrow mesiodistally. In these cases,fractures may occur during post cementation or patient function. These types of fractures have been termed “odontiatrogenic” in origin, and should be recognized by the dentist.42 Often these patients exhibit furcal bone loss or proximal angular defects that are misinterpreted as periodontal problems.

IMPACT OF POSTS ON STRENGTHENING ROOT STRUCTURE Historically, the prime intent of a post was to protect the weakened endodontically treated tooth from root fracture as a result of the concentration of internal stresses.43-45 The capability of the post to accomplish this reinforcement and the realistic goal of the post has been the subject of multiple, diverse evaluative techniques. Mechanical stress analysis has been popular, using extracted, restored, and natural teeth placed under an increasing load until fracture occurred.46-4gWhile the tensile forces applied in these studies allowed assessment of post retention, they did not necessarily duplicate masticatory forces. In addition, loads applied at various angles designed to mimic tooth contact do not provide detailed information describing the stress magnitude or distribution before fracture. However, an interesting finding with this testing is that teeth without posts usually fracture in a reparable manner, while teeth 465

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Fig. 11. Removal of excessive dentin outside the long axis of the root in a mandibular molar (A) and in a maxillary premolar (B) resulting in root perforation and significant osseousdestruction. (Premolar courtesy of Dr. Lew Chambless.)

dentin removed with respect to the remaining root structure (Fig. 11). Since the tensile strength of dentin is comparatively weak, the remaining dentinal thickness would be a critical factor in resisting fracture. While a plethora of studies have focused on restoration of these teeth with “retentive” posts, the emphasis should be on factors affecting the resistance to tooth fracture as opposed to mere post retention. 5sThe prime factor is the preservation of sound tooth structure. Nathanson and Ashayeri5g have stated, and their contention is supported by numerous authors,44p5g*60that: “The indication for use of endodontic posts is based on retention and stabilization of the core rather than ‘reinforcement’ of the fragile root.“5g The reference to posts as “root reinforcement systems” is therefore obsolete and dentists should adopt a contemporary view as to why these pulpless teeth have special restorative needs. Many endodontically treated teeth with conservatively enlarged root canals can be restored without a post,61provided sound treatment goals are followed. When a post is truly indicated in these patients, efforts must be made (1) to ensure stability of the post within the root35*58,62;(2) to avoid post systems that are designed to focus stress within a specific root or anatomic situation41; (3) to enhance optimal cement-to-post contact63; (4) to consider replacing zinc phosphate as a cementing medium with a low viscosity resinous cement 5g,64; (5) to minimize post installation and functional stresses65;and (6) to establish the ferrule effect with retained sound dentin if possible,66,67 with maximized clinical crown length.67 Despite innovative designs for the restoration of the endodontically treated teeth, the amount of remaining dentin and the nature of root morphology may be the ultimate factors in the resistance of the dentin/root restorative complex during function. CONCLUSION

with posts fracture so that repair is difficult or impossible because of extensive radicular damage.46,47 A second, common evaluative technique is the photoelastic stress model that provides graphic stress distributions using a birefringent material through which light refraction is analyzed.50-52This test has been used to analyze installation stresses and functional loads, but Reinhardt et a1.41have declared that it is difficult to prepare complex models with this technique and to match materials with the modulus of elasticity of human oral tissues. Recent studies41,53-55have focused on using finite element stress analysis to determine stress concentrations with an intraradicular post. Complex structures are drawn and are divided into smaller segments with specific properties so that stress distribution can then be plotted and detailed evaluations are possible. The fracture resistance of the endodontically treated tooth diminishes with a decrease in dentin56 and the strength of the tooth is directly related to the remaining bulk of dentin.15, 52,57If post space was routinely prepared in all teeth, invariably certain roots would have excessive 466

The dentin of pulpless teeth undergoes alteration in its inherent structure, reducing its tensile strength and flexibility. Because of the moisture and architectural loss of tooth structure, root-filled teeth require unique restorative procedures related to their radicular anatomy and supporting bone. Finally, treatment goals must be in harmony with these biologic tenets and with patient satisfaction. REFERENCES 1. Healey HJ. Coronal restorations of the treated pulpless tooth. Dent Clin North Am 1957;1:885-96. 2. Rosen H. Operative procedures on multilated endodontically treated teeth. J PROSTHET DENT 1961;11:973-86. 3. Baraban DJ. The restoration of pulpless teeth. Dent Clin North Am 1967;11:633-53. 4. Sokol DJ. Effective use of current core and post concepts. J PROSTHET DENT 1984;52:231-4. 5. Helfer AR, Melnick S, Schilder H. Determination of the moisture content of vital and pulpless teeth. Oral Surg Oral Med Oral Path01 1972;34:661-70. 6. Battistone G, Burnett GW. Studies on the composition of teeth. III. The amino acid composition of human dentinal protein. J Dent Res 1956; 35:255-g.

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7. Reeh ES, Messer HH, Douglas WH. Reduction in tooth stiffness as a result of endodontic and restorative procedures. J Endodont 1989; 15:512-6. 8. Tidmarsh BG. Restoration of endodontically treated posterior teeth. J Endodont 1976;2:374-5. 9. Sheth JJ, Fuller JL, Jensen ME. Cuspal deformation and fracture resistance with dentin adhesives and composites. J PROSTHET DENT 198&60:560-l. 10. Cave1 WT, Kelsey WP, Blankenau RJ. An in viva study of cuspal fracture. J PROSTHET DENT 1985;53:38-42.

11. Hansen EK. In vivo cusp fracture of endodontically treated premolars restored with MOD amalgam or MOD resin fillings. Dent Mater 1988; 4169-73. 12. Khera SC, Carpenter CW, Vetter JD, Staley RN. Anatomy of cusps of posterior teeth and their fracture potential. J PROSTHET DENT 1990; 64139-47. 13. Trope M, Malts DO, Tronstad L. Resistance to fracture of restored en-

dodontically treated teeth. Endodont Dent Traumatol 1985;1:108-11. 14. Rohbins JW. Guidelines for the restoration of endodontically treated teeth. J Am Dent Assoc 1990;120:558-66. 15. Sorensen JA, Martinoff JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J PROSTHET DENT 19&1;51:780-4. 16. Osetek EM, Jameson LM. Restoration of endodontically treated teeth.

Perspect Dent Sci 1986;2:1-2. 17. Grimaldi J. Measurement of the lateral deformation of the tooth crown under axial compressive cuspal loading. Thesis. University of Otago, 1971. 18. Black GV. Physical properties of human teeth. Dent Cosmos 1895; 37:353-421. 19. Peyton FA, Mahler DB, Hershenov MS. Physical properties of dentin. J Dent Res 1952;31:366-70. 20. Tyldesley WR. The mechanical properties of human enamel and dentine. Br Dent J 1959;106:269-78. 21. Renson CE. Elastic and mechanical properties of dentin. J Dent Res 1968;47:992. 22. Renson CE, Braden M. Experimental determination of the rigidity modulus, Poisson’s ratio and elastic limit in shear of human dentin. Arch Oral Biol 1975;20:43-7. 23. Renson CE, Boyde A, Jones SJ. Scanning electron microscopy of human dentine specimens fractured in bend and torsion tests. Arch Oral Biol 1974;19:447-54. 24. Rasmussen ST, Patchin RE, Scott DB, Heuer AH. Fracture properties of human enamel and dentin. J Dent Res 1976;55:154-64. 25. Rivera E, Yamauchi G, Chandler G, Bergenholtz G. Dentin collagen cross-links of root-filled and normal teeth. J Endodont 1988;14:195. 26. El Mowafy OM, Watts DC. Fracture toughness of human dentin. J Dent Res 1986,65:677-81. 21. Lewinstein I, Grajower R. Root dentin hardness of endodontically treated teeth. J Endodont 1981;7:421-2. 28. Rasmussen ST. Fracture properties of human teeth in proximity to the dentinoenamel junction. J Dent Res 1984;63:1279-83. 29. Renson CE, Braden M. The experimental deformation of human dentine by indenters. Arch Oral Biol 1971;16:563-72. 30. Grajower R, Asaz B, Bran-Levi M. Microhardness of sclerotic dentin. J Dent Res 1977;56:446. 31. Carter JM, Sorensen SE, Johnson RR, Teitelbaum RL, Levine MS. Punch shear testing of extracted vital and endodontically treated teeth. J Biomech 1983;16:841-8. 32. Davis WL. Oral histology. Cell structure and function. Philadelphia: WB Saunders, 1986113-34. 33. Zillich RM, Corcoran JF. Average maximum post lengths in endodontically treated teeth. J PROSTHET DENT 19&4;52:489-91. 34. Goerig AC, Mueninghoff LA. Management of the endodontically treated tooth. Part I. Concept for restorative designs. J PROSTHET DENT 1983;49:340-5. 35. Baraban DJ. The restoration of endodontically treated teeth: an update. J PROSTHET DENT 1988;59:553-8. 36. Tilk MA, Lommel TJ, Gerstein H. A study of mandibular and maxillary root widths to determine dowel size. J Endodont 1979;5:79-82. 37. Zillich R, Yaman P. Effect of root curvature on post length in restoration of endodontically treated premolars. Endodont Dent Traumatol 1985;1:135-7. 38. Yaman P, Zillich R. Restoring the endodontically treated bi-rooted premolar-the effect of endodontic and post preparation on width of root dentin. J Mich Dent Assoc 1986;68:79-81. TFE JOURNAL

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39. Bone J, Moule AJ. The nature of curvature of palatal canals in maxillary molar teeth. Int Endodont J 1986;19:178-86. 40. Perez E, Zillich R, Yaman P. Root curvature localisations as indicators of post length in various tooth groups. Endodont Dent Traumatol 1986;2:58-61. 41. Reinhardt RA, Krejci RF, Pao YC, Stannard JG. Dentin stresses in post-reconstructed teeth with diminishing bone support. J Dent Res 1983;62:1002-8. 42. Schweitzer JL, Gutmann JL, Bliss RQ. Odontiatrogenic tooth fracture. Int Endodont 3 1989;22:64-74. 43. Perel ML, Muroff FI. Clinical criteria for post and cores. J PROSTHET DENT 1972;28:405-11.

44. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effects of cement, dowel length, diameter, and design. J PROSTHET DENT 1978;39:401-5.

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55. de Vree JHP, Peters MCRB, Plasschaert AJM. A comparison of photoelastic and finite element stress analysis in restored tooth structures. J Oral Rehabil 1983;10:505-17. 56. Trabert KC, Caputo AA, Abou-Rass M. Tooth fracture-comparison of endodontic and restorative treatments. J Endodont 1978;4:341-5. 57. Hock D. Impact resistance of posts and cores. Thesis. University of Michigan, 1976. 58. Sorensen JA. Preservation of tooth structure: the key to successful restoration. J Clin Dent 1989;1:39-40. 59. Nathanson D, Ashayeri N. New aspects of restoring the endodontically treated tooth. Alpha Omegan 1990$3:76-89. 60. Hirschfeld A, Stem N. Post and core-the biomechanical aspect. Aust Dent J 1972;17:467-8. 61. Hunter AJ, Feiglin B, Williams JF. Effects of post placement on endodontically treated teeth. J PROSTHET DENT 1989;62:166-72. 62. Nicholls JI. An engineering approach to the rebuilding of endodontitally treated teeth. J Clin Dent 1989,1:41-4. 63. Peters MCRB, Poort HW, Farah JW, Craig RG. Stress analysis of a tooth restored with a post and core. J Dent Res 1983;62:760-3. 64. Nathanson D, Ashayeri N. Effects of a new technique. J Calif Dent Assot 1988;16:27-31. 65. Standlee J, Caputo AA. Biomechanics. J Calif Dent Assoc 1988;16:4958. 66. Barkhordar RA, Radke R, Abbasi J. Effect of metal collars on resistance of endodontically treated teeth to root fracture. J PROSTHET DENT 1989;61:676-8.

67. Sorensen JA, Engelman MJ. Ferrule design and fracture resistance of endodontically treated teeth. J PROSTHET DENT 1990;63:529-36. Reprint requests to: DR. JAMES L. GUTMANN PROFESSOR & CHAIRMAN DEPARTMENT OF ENDODONTICS BAYLOR COLLEGE OF DENTISTRY 3302 GASTON AVE. DALLAS,

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The dentin-root complex: anatomic and biologic considerations in restoring endodontically treated teeth.

The restoration of endodontically treated teeth has been the focus of considerable controversy and empiricism. Time-tested methods have been highly su...
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