Restoration of the endodontically treated tooth is a subject of great interest to dentists, as is evidenced by the large number of articles on the subject that have appeared in the dental literature over the past two decades. This is a sign of the spread of endodontic therapy within the profession and the accompanying demand for it in the patient population. Teeth that were once removed are now saved by endodontic treatment. However, the endodontic treatment should not be considered complete until the tooth has been restored to complete function in a cosmetically acceptable way.

There are at least fourteen ways to build up an endodontically treated tooth for subsequent restoration. The number of commercially available systems for accomplishing this is even greater. Through this bewildering maze of techniques and products run some common principles governing their use.

Restoration of the Endodontically Treated Tooth identifies these principles and applies them to each of the many categories for rebuilding teeth.

An effort has been made to classify these systems logically and to give a step-by-step technique for their use. Each chapter presents the entire technique-from start to finish-for each method of restoring endodontically treated teeth. There is also a chapter on the important subject of temporary restorations for teeth with little or no coronal tooth structure.

Of the 739 illustrations, 227 of them are in color. The over 300 references present a rationale for a systematic approach to restoration of endodontically treated teeth, and to provide the reader with the opportunity to pursue topics of interest in even greater depth.

This book will guide the novice through the first few experiences in the restoration of endodontically treated teeth. It should also enable the experienced practitioner to evaluate different solutions to the problem and to feel comfortable in trying techniques or systems not previously used.

Understanding the most recently developed facts of dental science is of utmost important to each practicing dentist

Therefore, a good professional journal is indispensable for continuing up-to-date study. “Quintessence International” provides everything the dentist desires: contemporary reports of new methods and devices, understandable descriptions of research result, criticism of poor quality materials and outdated methods, tips for better practice administration, etc.

Through the concentrated and precise layout of its report and its many illustration, some of which are in color, “Quintessence International”, now united with “Dental Digest”, has already won many friends in its short period of existence. Write to the publisher for details.

Restoration of the Endodontically Treated Tooth

Restoration of the Endodontically Treated Tooth

Herbert T. SNPngburg, Jr., D.D.S.
Professor and Chairman

James C. Kessler, B.S., D.D.S.
Assistant Professor

Department of Fixed Prosthodontics,
University of Oklahoma, College of Dentistry,
Oklahoma City, Oklahoma

Quintessence Publishing Co., Inc. 1982
Chicago, Berlin, Rio de Janeiro, Tokyo

© 1982 by Quintessence Publishing Co., Inc., Chicago, Illinois.
All rights reserved.

This book or any part thereof must not be reproduced by any means or in any form without the written permission of the publisher.

Lithography: Industrie- und Presseklischee, Berlin; time litho, Leinfelden-Echterdingen
Composition and printing: F. W. Wesel, Baden-Baden
Binding: F. W. Wesel, Baden-Baden
Printed in Germany

ISBN 0-86715-108-0

eISBN 978-0-86715-108-4

Dedicated to

Connie

Peggy

Preface

Endodontic therapy in general, and the restoration of nonvital teeth in particular, fell into disrepute in the United States during the 1930’s and 40’s. The widespread disregard for prevention or elimination of infection in the restoration of pulpless teeth became linked with the medical profession’s embrace of the focus of infection theory. Since the early 1950’s, when endodontic therapy was put on a sound scientific footing, the restoration of nonvital teeth has once again increased in the United States.

The use of endodontic therapy has spread within the profession in this country, and its acceptance has grown within the patient population. This has created a new problem for dentistry. Teeth which would have been extracted at one time are now saved. Root canal therapy should not be considered as an end in itself, however: it is only the first phase of treatment. Not until the tooth has been restored to function consistent with the esthetic demands of the patient’s mouth can the endodontic treatment be considered completed.

There are a number of dowel designs and techniques from which to choose in the restoration of the endodontically treated tooth. The last twenty years have seen a profusion of manufactured “systems” utilizing prefabricated dowels, and the literature has been filled with articles describing techniques for restoring the pulpless tooth. However, reports of controlled studies were rare before Colley’s classic article on retention in 1968 and Standlee and Caputo’s equally classic analysis of stress distribution in 1972. There have been many more research reports since that time. The group at U.C.L.A., with Dr. Jon Standlee and Dr. Angelo Caputo as its nucleus, continues to contribute objective data to this important clinical area.

In writing this book, we have attempted to focus on the underlying principles and the common aspects of the restoration of endodontically treated teeth, while presenting as many different techniques and systems as possible. Products in some classifications are very similar. There is often little to choose between in terms of real difference. The fact that one system was selected for a detailed technique description, while another was simply identified, does not imply an endorsement of one over the other. We hope that the similarities in product and technique will be evident. It is our hope that readers can use this book to become familiar with different ways of restoring endodontically treated teeth and to learn the actual techniques which seem attractive to them. Because new products and techniques are constantly being introduced, it is also hoped that this volume will help students of dentistry, whether undergraduate or practicing, to assess changes.

Acknowledgements

Our introduction to this subject in a rational, systematic manner can be traced to two fine dentists and teachers: Dr. Robert Dewhirst and Dr. Donald Fisher. Many of the principles articulated in this book were learned from them.

Ms. Julie Hall did her usual excellent job of typing, copying, and hieroglyphic interpretation. It is hard to imagine how the project would have progressed without her speed and accuracy.

Finally, the authors want to thank Dr. Randy Atkinson, Dr. Mark Felton, Dr. Ed Harroz, Dr. Robert Johnson, Dr. Neil White, Dr. Brad Williams, Dr. Ken Wilson, and Dr. Doug Woodson for their assistance with the clinical series.

Contents

Preface

Acknowledgements

Chapter 1 Principles of Restoration of Endodontically Treated Teeth

Chapter 2 Custom Dowel-Core (Direct)

Chapter 3 Custom Dowel-Core (Indirect)

Chapter 4 Custom Dowel-Core (Two-Piece)

Chapter 5 Dowel-Core Under a Crown

Chapter 6 Dowel-Inlay Crown Repair

Chapter 7 Precision Parallel Plastic Dowel

Chapter 8 Precision Tapered Plastic Dowel

Chapter 9 Prefabricated Dowel/Cast Core

Chapter 10 Prefabricated Dowel/Composite Resin Core

Chapter 11 Parallel Threaded Dowel (Pretapped)

Chapter 12 Parallel Self-Threading Dowel

Chapter 13 Tapered Self-Threading Dowel

Chapter 14 Amalgam Pin Core

Chapter 15 Composite Resin Pin Core

Chapter 16 Temporary Restorations for Endodontically Treated Teeth

Authors Index

Subject Index

Chapter 1

Principles of Restoration of Endodontically Treated Teeth

Restoration of the endodontically treated tooth is complicated by the fact that much or all of the coronal tooth structure which normally would be used in the retention of a restoration has been destroyed by caries, previous restorations, trauma, and the endodontic access preparation itself. The dentist must employ the principle of substitution, using a dowel in the root canal itself, or pins in the surrounding tooth structure, to build up a replacement for the missing coronal tooth structure. Only then can the tooth be restored.

Solutions to this problem have challenged the inventiveness and ingenuity of dentists for centuries. in fact, teeth were often intentionally devitalized so that the dentist could take advantage of the retention afforded by a dowel placed into the root canal. During the eighteenth century, Pierre Fauchard used a wooden post jammed into the canal to retain crowns. When the wood became wet, it swelled, making the fit even more snug and secure. It also, on occasion, split the root.

Early “pivot crowns” failed frequently because they were placed into poorly treated or totally untreated canals. The problem was such an obvious concern that one of the retentive devices, developed by Dr. F.H. Clark in 1849, consisted of a metal tube in the canal and a split metal dowel which was inserted into it.1 This “spring-loaded” dowel was so designed to allow for the easy drainage of suppuration from within the canal or apical areas!

Even G.V. Black tackled this problem by developing a porcelain faced crown held in by a screw inserted into a canal filled with gold foil.2 One design whose use persisted for a number of years was the Richmond crown. Introduced in 1880, it first consisted of a threaded tube in the canal which held a screw placed through the crown. This design was later simplified to eliminate the tube and make the dowel, by then unthreaded, an integral part of the final restoration or crown (Fig. 1-1).3–6

With the increased interest in the restoration of pulpless teeth in recent years, there has been a gradual changeover from the crown in which the dowel is an integral part. In its place is a separate, well retained device which replaces the missing coronal tooth structure. The crown is fabricated over it as it would be fabricated over a preparation composed entirely of tooth structure (Fig. 1-2).

The supragingival portion of such a device, either because it replaces coronal tooth structure, or because it forms the center or core of the new restoration, or perhaps because of both, is referred to as a core. If it is retained by a dowel or post in the root canal, it is called a dowel-core. A third component of this system is an encircling band of metal which will support the tooth externally, bracing it against fracture by the dowel. This has been described as the “ferrule effect” by Eissmann, who suggests that it be 2.0 mm. wide.7 It may be provided by a coping,8 a long contrabevel on the core, or the axial walls of the crown, which extend apically beyond the margin of the dowel-core.

Finally, there must be a counter-rotational device on the dowel to prevent its being twisted or rotated by horizontal forces applied to the core. This can take the form of a keyway (a vertical groove cut in the wall of the canal, extending apically from the orifice),9–16 an ovoid or irregularly shaped canal,17–20 a short segment of a second root on a multi-rooted tooth,21 or a pin hole parallel with and separate from the canal preparation.22, 23 There is the possibility that the use of a keyway could endanger the structural integrity of the root.24 If a deep keyway is placed in a thin-walled tooth, it could act as a wedge and produce a cleavage plane. A keyway of normal depth and length placed in an adequate bulk of tooth structure is not likely to have a deleterious effect, however.

If the core is retained by small pins placed in the dentin around the root canal, it is a pin-retained core. Components may be prefabricated or custom-made, and they can be combined with cast metals or plastic material placed in the mouth. In any event, all devices used for building up teeth are variations of the dowel-core or the pin-retained core.

There are distinct advantages in having the retention device separate from the crown restoring the tooth. The adaptation of the axial walls and margin of the crown are totally unrelated to the fit of the dowel In the canal. Secondly, the crown can be easily remade if that should become necessary because of materials failure, caries, or change in the role of the crown (from simple restoration to bridge retainer) at anytime in the future. Replacing a single piece dowel-crown can be most difficult if the dowel is of adequate length. Although devices have been described for removing crowns, the force required to remove a crown with a long dowel just about dictates the cutting of a crown preparation in solid porcelain and metal (Fig. 1-3a and b). If it can be done, it will be time-consuming and tiring for both dentist and patient. Finally, if the tooth being restored is to be used as a bridge abutment, it is not necessary to attempt to align the canal preparation with the path of insertion of the other abutment preparation(s), or vice versa.

The decision of whether to use some form of dowel-core or a pin-retained core is dependent upon several interrelated factors: the thickness of tooth structure around the canal, the bulk and height of remaining supragingival tooth structure, the diameter of the tooth, root morphology, bone support, and the tooth’s role in the final restoration of the mouth (single restoration or abutment). Not every endodontically treated tooth will require a crown. If an anterior tooth has sustained only minimal damage in the past, with no proximal surface involvement, it may be possible to restore the tooth with a lingual composite. Such a circumstance is the exception, however, and more extensive restoration is generally required. It is recommended by several authors that no less be done for an endodontically treated tooth than the placement of a restoration which will provide occlusal coverage.25–30

For those teeth that will require a crown, a decision must be made regarding the type of core required. Numerous clinicians have held that the tooth should be reinforced with a dowel.7, 16, 20, 31–36 Laboratory studies of the subject have been only partially supportive. Guzy and Nicholls, applying forces at 130° to the long axis, found no difference in the failure loads of canines or incisors which were reinforced with 1.0 mm. diameter posts compared with those which were unprotected.37 Kantor and Pines, however, reported that teeth with 1.25 mm. posts were 56% stronger than control teeth upon which no endodontic procedure had been performed, with loads applied at a 45° angle.38 Trabert and associates, using an impact tester, discovered no difference in resistance to fracture between untreated control teeth and endodontically treated teeth without posts.39 When teeth were restored with a steel post 1.25 mm. in diameter, however, the teeth were less likely to fracture than either the controls or teeth “reinforced” with a larger 1.75 mm. post.

There would seem to be some basis for placing a dowel in the anterior tooth whose small root cross section does not possess the necessary bulk to provide adequate protection from fracture following the endodontic treatment. The dowel must not be made so large, however, that it destroys valuable tooth structure, and with it the structural integrity and natural strength of the tooth. The large circumference of a posterior tooth eliminates the need for a dowel to reinforce the tooth.7 Dowels may be used in posterior teeth to improve retention and to support the core and crown against laterally directed forces when there is no remaining coronal tooth structure to serve that function.

Dowel Retention

There are four factors which can have an effect on the retention of any dowel: length, taper, diameter, and surface configuration.

Dowel Length. It is not surprising that length plays the same role in the retention of dowel-cores that it does in the retention of crowns:/i.e., as length increases, so does retention. Inadequate length in dowels is probably the leading cause of failure of restorations on endodontically treated teeth (Fig. 1-4).

Colley and associates demonstrated 2.23 times as much retention by increasing dowel length from 5.5 mm. to 8.0 mm. (Fig. 1-5).40 Other studies have reinforced this finding. Krupp et al. found an average increase of 47% in retention for an increase from 5.0 to 8.0 mm.41 An increase in retention of approximately 43% was shown by Johnson and Sakumura when the dowel was lengthened from 7.0 to 11.0 mm.42 Standlee et al. showed that the retention for all types of dowels was about one and one-half times as great when embedment depth increased from 5.0 to 8.0 mm.43 Ruemping and associates also determined that tensile retention increased with length, but they found that it was only 1.23 times as great when the dowel was expanded from 5.0 to 8.0 mm.14

Because of its great importance to the longevity of the restoration, dowel length has attracted the attention of numerous clinicians writing on the subject. Many authors feel that the minimum length of the dowel should be equal to the length of the crown.3, 4, 9, 13, 16, 19–21, 28–30, 44–61 Others have stated that the dowel should be two-thirds the length of the root.7, 10, 11,20,29, 44, 51, 52, 62–71 A dowel length equal to three- quarters of the root length has been projected as the ideal by a few authors.11, 62, 63, 72 Other dowel lengths which have been proposed include half the length of the canal,57, 73, 74 80% of the root,75 133% of the crown,12 150% of the crown,76 and, in techniques in which the dowel doubles as the root canal filling, equal to the length of the root.77, 78

Another dimension relating to dowel-cores which must be considered is the length of root canal filling left at the apex (Fig. 1-6). The minimum length of the remaining apical fill has been set variously at 3.0 mm.,13, 21, 47, 58, 79 3-5 mm.,49, 76 4.0 mm.,80 and 4-5 mm.36 As the apex is approached, the possibility of dislodging the root canal filling increases. There is also the possibility that an unfilled accessory or lateral canal will be uncovered, causing an infection of periapical tissues. The optimum distance between the end of the dowel and the apex is 4.0 mm., with a greater length left when a longer root length will permit it.

Besides providing poor retention, a short dowel can also lead to fracture of the root. 58, 69 If the end of the dowel is at or above the alveolar crest of bone, that part of the root investing the dowel will not be bolstered by bone against forces transmitted from the dowel to the tooth. Occlusal forces can produce stresses in the unsupported root, fracturing it diagonally from the tip of the dowel down to the crest of bone17 (Fig. 1-7). For that reason, some authors have suggested that the dowel be embedded far enough into the root so that it will extend at least one-half the distance from the alveolar crest to the apex of the root (Fig. 1-8).15, 17, 35, 81, 82

Kurer estimates that it is not possible to achieve a dowel equal in length to the crown in 20-25% of the cases.83 A quick comparison of crown and root lengths indicates that this assumption is anatomically correct (Table 1-1), although the clinical crown would be slightly shorter. A dowel equal in length to the crown would encroach on the 4.0 mm. apical safety zone in the average maxillary or mandibular incisor.

What then should be the length of the dowel? Simply stated, it should be as long as possible.51 A dowel length equal to that of the crown, or to two-thirds that of the root, is a good rule of thumb. Comparison of the crown and root lengths becomes especially significant when all coronal tooth structure has been destroyed. If intact coronal tooth structure is preserved, as many clinicians recommend,12, 15, 34, 44, 45, 54, 63, 82, 85–87 the dowel length is measured from the occlusal end of that intact coronal tooth structure. It is not necessary under those conditions to extend as far apically to achieve a given dowel length.

Dowel Taper. The taper of the walls of a dowel has a direct bearing on the behavior of the dowel in the tooth. Parallel-sided dowels are more retentive than tapered dowels (Fig. 1-9). In various studies, this superiority in retention has been demonstrated to be 1.9 times,40 3.3 times,43 and 4.5 times42 as great as that of the tapered dowels. The tapered dowel also generates greater stress than does the parallel dowel,23, 85 showing a potential for splitting the root88 (Fig. 1-10). The tapered dowel tends to produce greater stress in the shoulder area of the restoration, while the parallel-sided dowel causes more stress in the apical area, especially during cementation.89 In an effort to minimize the splitting potential of a tapered dowel, there should be a flat seat at the occlusal end of the preparation to resist apically directed forces and prevent wedging.13, 17, 35

The proximity of the edge of a parallel dowel to the periphery of a tapered root also could increase the danger of a lateral perforation. This has led some dentists to opt for the less retentive tapered dowel to avoid a possible apical perforation or fracture.40, 51 This is more of a concern in teeth whose roots are thin and fragile.

Dowel Diameter. The diameter of a dowel has an effect on both the retention of the restoration and its strength and ability to resist distortion. The smaller the diameter of a dowel, the more likely that it will be displaced, with or without accompanying distortion or fracture (Fig. 1-11). Increasing the diameter of 5.0 mm. long parallel dowels by 0.25 mm. increased retention by 53% in one study41 and by 47% in another,90 while enlarging the diameter of a 7.0 mm. long parallel dowel by 0.35 mm. produced a 45% increase in a third investigation.42 Although it is unquestionably effective, enlarging the dowel is not the safest way of improving retention because it does destroy and weaken the remaining tooth.23 At best, the enlarged dowel does not contribute to the strength of the tooth.39 At worst, it could cause a fracture and loss of the tooth (Fig. 1-12).

One-third the diameter of the root has been proposed as a clinical guideline for the width of the dowel.34, 82 It has also been suggested that there be 1.0 mm. between the wall of the prepared canal and the outer surface of the root.23 The diameter of the dowel will be determined, to some extent, by the narrowest portion of the root. This will be the mesiodistal width of most roots, measured 4.0 mm. from the apex. The thickness of tooth structure in this area is important as a safeguard against perforation of the root.

Because of the faciolingual direction of much occlusal force, the amount of tooth structure between the dowel and the outer surface of the root on the facial and lingual aspects of the root is quite important in protecting the structural integrity of the root. Most roots exhibit considerably thicker dentin faciolingually than they do mesiodistally.

Therefore, a dowel whose diameter is less than one-third the faciolingual dimension of the root at the midpoint of the dowel may be large enough to exceed one-third of the mesiodistal width of the root. This can still be acceptable if the dowel is small enough not to encroach on the mesial and distal surfaces of the root. To this end, the mesiodistal diameter of a dowel should be 2.0 mm. less than that of the root at the midpoint of the dowel and 1.5 mm. less than that of the root at the dowel’s end. Root dimensions and recommended dowel widths are shown for maxillary teeth in Table 1-2, while those for mandibular teeth can be found in Table 1-3. Because of the factors mentioned above, they are slightly larger than those recommended in a previous study.91

These dimensions should be used only for determining the size of dowel that normally would be used. It is just as important to measure the width of a root as it is to assess its length when judging the size of dowel that will be placed in a tooth.

Dowel Surface. Dowels can be categorized in several ways. Classification can be made by their geometric shape-tapered or parallel-and by their surface configuration-smooth, serrated, or threaded (Fig. 1-13). Surface configuration probably plays the single most important role in retention. Threaded dowels are unquestionably the most retentive. Standlee and associates found the pretapped, parallel-sided, threaded dowel to be twice as retentive as a parallel-sided serrated dowel and approximately 6.6 times as retentive as a smooth-sided tapered dowel43 (Fig. 1-14). Ruemping et al. found it 5 and 7 times as retentive, respectively.14