After reading this chapter, the student should be able to:
Describe the main factors involved in the survival of root-filled teeth.
Summarize factors contributing to loss of tooth strength and describe the structural importance of remaining tooth tissue.
Explain the importance of a coronal seal and how it is achieved.
Describe the requirements of an adequate restoration.
Outline postoperative risks to the unrestored tooth.
Discuss the rationale for immediate restoration.
Identify restorative options before root canal treatment is started.
Discuss the advantages and disadvantages of direct and indirect restorations.
Outline indications for post placement in anterior and posterior teeth.
Describe common post systems and the advantages and disadvantages of each.
Describe core materials and their placement.
Describe techniques for restoring an access opening through an existing restoration.
Endodontic therapy is predictable. However, for success the teeth need to be restored to their previous form and function. Before endodontic therapy, restorability must be determined; this involves careful evaluation of the existing tooth structure, including removal of all caries along with any existing restorations, both to reduce the risk of marginal leakage during treatment and to reveal the amount of sound tooth structure. Specific restorative options must be evaluated based on functional demand and remaining tooth structure. , This chapter will discuss the considerations needed to properly restore endodontically treated teeth.
Criteria for a Restorable Tooth
A tooth must retain sufficient sound tooth structure after root canal therapy to allow predictable restoration. Evaluation of a tooth requires clinical, radiographic, and esthetic evaluation. The first step is to remove any existing caries and restorative materials. This process allows clear visualization of the remaining tooth structure and removes bacteria. A periodontal probe can be used to measure the height of the remaining tooth structure that will provide a ferrule; measure pocket depths, which can reveal periodontal status and possible signs of root fracture; and map the subgingival root morphology. A bitewing radiograph should be used to evaluate the remaining tooth structure, pulp chamber, and bone levels; and periapical radiographs should be used to evaluate tooth length and root morphology.
Amount of Remaining Coronal Tooth Structure
Most teeth requiring root canal treatment have been structurally compromised by caries and subsequent restorative procedures. Additional loss of tooth structure occurs during endodontic access, leading to further weakening of the tooth. When the access cavity is surrounded by walls of dentin, it only has a minor weakening effect. In a tooth already seriously compromised by caries, trauma, or large restorations, access preparation is more significant, particularly if some marginal ridges have been lost ( Fig. 16.1 ). , Excessive coronal flaring of the access preparation also results in greater susceptibility to fracture.
The formation of a ferrule around the remaining coronal tooth structure is important to prevent tooth fracture in endodontically treated teeth. Ferrule refers to the amount of cervical tooth structure, or the height of the remaining tooth structure under a crown, that is available to resist tooth fracture ( Fig. 16.2 ). The more tooth structure present, the more resistant the tooth will be to fracture.
Ferrule Wall Height
The ferrule wall height is simply measured from the tooth preparation finish line to the coronal aspect of the remaining tooth structure.
The types of margin preparation or bevels on the remaining tooth structure are unimportant; the total height of the tooth structure is key. The exact crown ferrule height required for success has been debated for many years; recommendations range from 1.0 mm to 3.0 mm heights. We suggest that at least a 1.5 to 2.0 mm high circumferential wall of tooth structure be present above the crown finish line to ensure adequate fracture resistance and enhance the coronal seal provided by the final restoration ( Fig. 16.3 ).
Ferrule Wall Thickness
Endodontic therapy requires the removal of tooth structure to gain access to the root canal system. The ferrule wall width refers to the amount of tooth structure remaining from the pulp chamber and/or access preparation to the external surface of the tooth. The amount of remaining ferrule wall thickness may be jeopardized by crown preparation. Decay and prior restoration may leave minimal ferrule wall thickness. The ability of a tooth to resist lateral forces is directly proportional to the thickness of remaining dentin. More than 1.0 mm of dentin thickness around the root canal is needed for adequate resistance to fracture.
Re-Establishing Coronal Tooth Structure
In a clinical situation where minimal tooth structure remains, the establishment of an adequate ferrule height can be challenging. In such situations, two factors are important to ensure success. First, it is important to reestablish a 1.5 mm to 2.0 mm ferrule and second to maintain the patient’s biologic width. Biologic width refers to the junctional epithelium and connective tissue attachment present from the alveolar bone crest to the depth of the periodontal sulcus. It is suggested that the average biologic width is around 2 mm; commonly the sulcus depth is also included in this measurement, resulting in a suggested measurement of 3 mm. However, the exact biologic width will vary from patient to patient. This suggests that approximately 5 mm of tooth height should remain coronal to the alveolar bone. When this is not available, height can be reestablished by means of surgical crown lengthening or orthodontic extrusion (see Chapter 21 ).
Surgical Crown Lengthening
Crown lengthening is a way to reestablish a ferrule by removing supporting periodontal structures to expose additional tooth structure. However, if a tooth has reduced bone support, short roots, a poor crown-to-root ratio, or if it could lead to an unacceptable esthetic result, crown lengthening would be contraindicated. Additionally, the apical relocation of the finish line to a narrower part of the root exposes less cross-sectional area, predisposing to a weaker tooth and potentially to furcation involvement. Therefore the ideal tooth for crown lengthening is a periodontally healthy, long, single rooted tooth, with minimal root taper.
Orthodontic extrusion involves the forced eruption of a tooth. The eruption must be performed using rapid orthodontic extrusion allowing only the movement of the tooth. If slow orthodontic extrusion is performed, coronal migration of the bone and gingiva occurs, thereby not allowing the formation of the desired ferrule. It is also suggested to perform supracrestal fiberotomy and root planning to help minimize the coronal migration of bone and gingival tissues when rapidly extruding a tooth. Rapid orthodontic extrusion involves 1 to 3 weeks of activation followed by 8 to 12 weeks of retention of the tooth before starting restoration. However, when extruding a tooth with tapered roots, not only is the bone support reduced, the cross-sectional area is reduced, making an esthetic and structurally sound restoration more difficult.
Complications Associated with Endodontically Treated Teeth
Although root canal treated teeth are at greater risk of extraction than vital teeth, their long-term survival rate is very high. Numerous studies investigating the survival of endodontically treated teeth have documented that at most 1% to 2% are lost per year, and one very large study of almost 1.5 million cases reported that only 2.9% were lost after 8 years. A recent meta-analysis showed a mean tooth survival of 87% after 8 to 10 years. Factors that affect the survival of root-filled teeth include:
Caries and periodontal disease. Caries and periodontal disease are responsible for up to half of all extractions of root-filled teeth. Emerging evidence suggests that root-filled teeth may be more susceptible to caries than vital teeth, though the reasons are unknown. ,
Lack of definitive restoration. A surprisingly high percentage of teeth are not appropriately restored after root canal treatment. , , In one study of U.S. insurance data, almost 30% of teeth had not been restored 2 years after root canal treatment, and 11% of these teeth were extracted.
Inadequate restoration. Lack of coronal coverage for posterior teeth is a major restorative factor in their loss after root canal treatment. , A lack of cuspal protection, coronal coverage predisposes the tooth to unrestorable crown or root fracture. Direct restorations do not provide adequate protection for posterior teeth unless the access opening is very conservative.
Occlusal stresses. Teeth serving as abutments for fixed or removable prostheses are at significantly increased risk of loss, as are teeth lacking mesial and distal proximal support from adjacent teeth. , ,
Endodontic factors. Typically only about 10% of extractions of root canal treated teeth result from endodontic causes, such as persistent pain. , Endodontic pathology (development or persistence of a periapical lesion) is generally amenable to further management rather than extraction; likewise procedural complications, including perforation, may be managed.
Structural, Esthetic, and Restorative Considerations
Teeth function in a challenging environment, with heavy occlusal forces and repeated loading at a frequency of more than 1 million cycles per year for many decades. Caries, restorative procedures, and occlusal stresses add to the risk of serious damage to teeth during normal function, and root canal treated teeth are at greater risk than intact teeth ( Fig. 16.4 ). As noted previously, unrestorable crown fracture is a common sequel to inadequately protected root canal treated teeth. , It is important to understand the basis for this fracture susceptibility when planning the restoration.
Structural Changes in Dentin
It is now generally recognized that many mechanical properties of the dentin of endodontically treated teeth differ only to a minor extent from those of the dentin of vital teeth (strength, hardness, modulus of elasticity). , Prior studies were generally confounded by drying of studied teeth after extraction.
Normal function generates large stresses that are capable of causing cusp fracture and even vertical root fracture in intact vital teeth. Repeated functional loading and cyclic mechanical fatigue have the potential to weaken teeth over time, particularly after tooth structure has been lost to caries, restoration, and access preparation, which places even more stress upon the remaining diminished tooth structure. The distribution of masticatory stresses in the restored root canal treated tooth is markedly and adversely changed from those in the intact, vital tooth. Hence, the restoration must be designed to minimize and to protect against fracture.
Increasingly, patients wish to enhance the esthetic appearance of restorations; for endodontically treated teeth this often involves the use of crowns. Crown preparation necessitates further tooth reduction to provide adequate thickness of the ceramic material to provide a more natural appearance. The amount of required tooth reduction varies based on the material being used. Dark stained teeth may require additional tooth reduction to mask discoloration when translucent all-ceramic crowns are used, further weakening the tooth.
Requirements for an Adequate Restoration
The definitive restoration should (1) preserve as much tooth structure as possible, but not forget the appropriate thickness of the restorative material; (2) protect remaining tooth structure, cuspal coverage protecting posterior teeth; (3) satisfy function and esthetics; (4) provide a coronal seal; and (5) be completed in a timely manner. Care must be taken to ensure that esthetic demands do not lead to the weakening of teeth by excessive removal of remaining tooth structure.
Coronal leakage is a major cause of endodontic failure. , Even a well-obturated canal does not provide an enduring barrier to bacterial penetration ; we rely on the restoration for long-term integrity of the coronal seal. The restoration may provide the coronal seal either as a separate step (e.g., placing a barrier over canal orifices) , or, more commonly, as an integral part of the restoration. For direct restoration of a small access cavity, a bonded restoration provides the most reliable seal. Experimental leakage studies consistently demonstrate that leakage occurs around posts, regardless of the type of post or luting cement. However, a crown with an adequate ferrule and sound core foundation provides an effective barrier against coronal leakage. ,
A frequently asked question with regard to lost or leaking restorations is, “How long can a root filling be exposed to oral fluids before it should be retreated?” The question has no clear answer. Experimental studies suggest that complete leakage along the length of the root filling occurs rapidly, within days or weeks. , A recent review, however, concluded that coronal leakage may be clinically less significant than is suggested by experimental laboratory leakage studies. Clinically, bacterial invasion is often limited to the coronal third of the canal, and periapical lesions may take several years to develop. , One commonly accepted guideline is that the root canal should be retreated if it is exposed to oral fluids for more than 2 to 3 months. Others suggest 2 to 3 weeks. However, if the root filling has been performed to a high technical standard and periapical pathology is absent, it may be sufficient to replace the lost or leaking restoration rather than provide endodontic retreatment. Of course, retreatment can be provided after pathology has eventually become evident, but the earlier bacteria are removed, the better the endodontic prognosis.
Unless there are specific reasons for delay, definitive restoration is completed as soon as practical. , , , The core restoration should be placed at the time of obturation, before the rubber dam is removed.
If placement of the core restoration is to be delayed, orifice barriers can be placed at the time of obturation using composite resin or glass ionomer or an adhesive cement. The gutta-percha is removed from the canal orifice 1 mm below the pulpal floor or 1 mm below the level of the cementoenamel junction (CEJ), creating a small depression. The tooth is etched and primer placed for composite resin or the tooth is conditioned for glass ionomer. One option is to seal the orifice with clear composite resin to allow easy visualization of the canal and gutta-percha should reentry into the canal ever be needed; another is to use an opaque white material so that it can easily be distinguished from natural tooth structure.
Most provisional restorative materials commonly used to seal the endodontic access opening allow substantial occlusal wear and loss of the coronal seal within weeks. The tooth has been weakened by access preparation and remains at risk until definitive restoration has been completed. The provisional restoration does not provide protection against masticatory forces, even when the tooth is out of occlusion. Because nonrestorable fracture during or soon after treatment is all too common ( Fig. 16.5 ), protection can be provided in the form of a well-made provisional crown.
For most teeth, it is both unnecessary and unwise to wait for radiographic evidence of healing before the definitive restoration is placed. Prompt restoration will improve the prognosis because it provides better protection against fracture and loss of the coronal seal.
When definitive restoration of the tooth is delayed, the provisional restoration must be durable and must protect, seal, and meet functional and esthetic demands. Provisional materials such as Cavit are inadequate. For posterior teeth, some form of cuspal protection is desirable, even with provisional restorations. A good long-term posterior provisional restoration will cover weakened cusps, thus providing functional and sealing protection. The definitive crown preparation can be completed later without removing the core ( Fig. 16.6 ). Comparable anterior restorations are more challenging owing to esthetic demands and difficulties with the coronal seal. A one-piece provisional postcrown is at risk of dislodgment, thereby compromising an adequate seal. It is preferable to place a definitive post and core immediately after obturation when a provisional crown is indicated. ,
Conservation of tooth structure. Most anterior root canal treated teeth should be restored simply and conservatively using composite resin rather than with a more radical crown or a crown combined with a post and core. , Some data even indicate that molars that are intact (except for endodontic access openings) can be restored using only composite resin. However, most root canal treated posterior teeth require crown placement so that the cusps and remaining tooth structure can be encompassed, minimizing the potential for tooth fracture. , The antiquated and unsound processes used by some practitioners of routinely creating a very large access preparation or decoronating an endodontically treated tooth and then rebuilding it are neither desirable nor in keeping with contemporary knowledge.
Retention and resistance. The definitive coronal restoration of a root canal treated tooth may consist only of a restorative filling in the endodontic access opening when it is retained by surrounding tooth structure. When the tooth is structurally compromised a crown is needed. It is retained by remaining dentin and a restorative material core that replaces missing tooth structure. Only if the core cannot be adequately retained by the remaining coronal tooth structure should a post be placed into the root canal to provide retention and resistance for the core. Because posts weaken teeth and may produce root fracture or lead to root perforation during preparation of the root canal, they should be used only when the core cannot be retained by any other means, such as mechanical and chemical bonding of a restorative material. ,
Protection of remaining tooth structure. In posterior teeth, this applies to protecting weakened cusps by minimizing undue flexure and preventing fracture. The restoration is designed to encompass the cusps, thereby splinting the tooth and minimizing the chance of tooth fracture.
Planning the Definitive Restoration
A tooth that is intact except for the access preparation can simply be restored using amalgam or composite. All of the provisional restorative materials must be removed along with any cotton or foam pellet, the pulpal floor cleaned of any sealer or gutta-percha, and the round or oval obturations be clearly visualized, so that there will be no void between restoration and the pulpal floor or obturation.
When the tooth requires a crown, the type of definitive treatment can be determined only after the existing restoration (or restorations) have been removed, to ensure that there is no caries present and to expose the remaining sound tooth structure. Therefore the specific crown material will be determined based on esthetic and functional demands.
Whenever possible, direct restoration of the endodontic access opening (e.g., etched and bonded composite resin) is used. Further esthetic issues can be addressed conservatively through internal bleaching and the use of porcelain veneers. For grossly damaged anterior teeth, complete coronal coverage using a crown, or a crown retained by a post, may be necessary.
Either a prefabricated post with direct core buildup (see Fig. 16.7 ) or a cast post and core (see Fig. 16.8 ) can be used for anterior teeth. In esthetically demanding situations, discoloration of the crown by a metal post and core can be concerning. To prevent discoloration one can use prefabricated metal posts that are coated with a thin layer of opaque resin along with a tooth colored composite resin core. Alternatively, a cast post can be fabricated using a metal ceramic alloy that allows the application of opaque porcelain to mask the metallic color, thereby achieving better esthetic outcomes ( Fig. 16.9 ). Tooth colored fiber or ceramic posts may also be considered.
Anterior root canal treated teeth must withstand tipping and lateral forces from mandibular excursive movements which, if transmitted excessively via a post, can fracture the root. Consideration should be given to the occlusal scheme. Where possible, the excursive load should be limited, with more force being borne by adjacent, more structurally sound teeth.
What is the most important determination before endodontic therapy?
Length of root
Curvature of root
Restorability of tooth
Location of tooth
Which of the following are criteria of a restorable tooth?
Amount of remaining coronal tooth structure
Ferrule wall height
Ferrule wall thickness
All of the above
What is the minimally suggested ferrule wall height?
Which of the following are contraindications to surgical crown lengthening?
Reduced bone support
Poor crown-to-root ratio
Poor esthetic results
All of the above
Which complications associated with endodontically treated teeth is the most comm?
Caries and periodontal disease
Lack of definitive restoration
When restoring an anterior tooth with a conservative endodontic access and no existing restorations, the ideal restoration is a bonded composite
Premolars with substantial loss of coronal structure, missing buccal or palatal cusps, particularly maxillary premolars, often necessitate a cast post and core ( Fig. 16.10 ). Premolar roots present many risks to post placement. Narrow mesiodistal root width and large developmental root concavities, coupled with tapered roots, may result in excessive removal of root structure or perforation when the tooth is prepared for a prefabricated post. Additionally, the mesiodistal thinness of the tooth may not permit adequate core thickness to allow strength when using a prefabricated post. Minimal enlargement during post space preparation is essential to preserve sufficient dentin thickness. In maxillary premolars with two roots, the palatal canal is generally used for the post because the buccal root can frequently have a concavity on its furcal aspect. , A small, short (2 mm to 3 mm) post in the buccal canal can be used to provide some retention, resistance, and antirotation. As a rule of thumb, when a root canal is circular ( Fig. 16.11 ) a prefabricated post may be used. However, if the root canal is ovoid, a prefabricated post would be in close proximity to the mesial and distal walls of the canal whereas the facial and lingual areas would be filled with cement ( Fig. 16.12 ). Therefore if the root canal is ovoid or ribbon shaped a custom cast post is suggested.