Restoration of endodontically treated teeth

Typically, root canal treatment is initiated because of deep caries or trauma, both of which often result in extensive loss of tooth structure. Additional tooth tissue is removed for endodontic access, cleaning and shaping of the root canal, and post space preparation, further reducing the structural integrity of the tooth and decreasing its resistance to fracture. At one time it was believed that endodontically treated teeth are inherently more brittle and susceptible to fracture. Subsequent research has shown that the dentin of endodontically treated teeth exhibits mechanical properties equivalent to that of untreated teeth.

It has also been proposed that a portion of the sensory feedback mechanism is lost when the neurovascular tissue has been removed from the tooth in the course of endodontic therapy. Clinically, this means that the patient can inadvertently bite with more force on an endodontically treated tooth than on a vital tooth because of the impaired sensory feedback mechanism.

While endodontically treated teeth may fail for a variety of reasons—eg, caries, endodontic failure, and periodontal disease, among others—tooth fracture remains a significant factor in the posttreatment extraction of these teeth. The choice of definitive restoration plays an important role in survivability. In selected instances, endodontically treated teeth can be restored with a directly placed restoration, particularly in the anterior region. However, restoring a severely damaged tooth to function and esthetics often requires the use of a full-coverage restoration. Providing adequate resistance and retention form when there is a minimal amount of remaining tooth structure can be problematic. When a moderate amount of tooth structure has been lost, the tooth may be built up with restorative material such as amalgam or composite to provide a core to support and retain the crown. When the damage is severe and there is inadequate tooth structure to retain the core, other methods of retention must be employed. This may consist of adhesive bonding techniques, providing auxiliary mechanical retention, and/or placement of a post into the root canal that extends into the core and retains it.

Risk Analysis

The survivability of the individual tooth and its ability to fulfill its intended function must be carefully considered, and the quality of the existing endodontic treatment should be assessed. The absence of symptoms and of periapical pathology, as well as the presence of a radiographically dense and adequately extended obturation, should be confirmed. Periodontal disease should be controlled, and the crown-root ratio should be assessed in view of the position of the tooth in the arch and its anticipated function. Success is enhanced when a restored endodontically treated tooth is in proximal contact on both the mesial and distal. If the root canal filling has been exposed to the oral environment because of leakage or loss of a restoration, contamination of the periapex with bacteria or toxins may occur within days. Retreatment should be considered in situations where the root canal filling has been exposed to the oral environment. There are no clinical data available to give specific guidance on how long the root canal filling may be exposed before retreatment should be accomplished, but in vitro studies suggest a range of days to several months. Because of the difficulty and expense involved in retreatment after restoration, a conservative approach is recommended. Many clinicians use 2 weeks of exposure as an arbitrary guideline for retreatment. Because of the risk of contamination, coronal sealing of the root canal system should be accomplished as soon as possible after completion of the root canal therapy. A long time span between the endodontic treatment and placement of the definitive restoration leads to a significantly higher failure rate.

Perhaps the most important factor related to the survivability of an endodontically treated tooth is the amount of coronal and radicular tooth structure that remains prior to restoration. The tooth structure that will be available following tooth preparation should be visualized and should be assessed for the provision of adequate retention and resistance form for the subsequent crown. Key to this factor is the presence of an adequate ferrule. The term ferrule describes the band of circumferential material that encircles the cervical area of a tooth restored with a crown. As commonly used in dentistry, ferrule also refers to the dimension of remaining vertical tooth structure from the anticipated margin of the preparation to its coronal extent, which is available for encirclement by the crown. The "ferrule effect" serves to reinforce the remaining tooth structure and helps the restoration withstand lateral forces.

Ferrule length was found to be more important than post length in increasing fracture resistance to cyclic loading of crowned teeth. Indeed, some authors have noted that the effect of the remaining dentin height is so significant as to make the effect of the type of post and core employed irrelevant with respect to tooth survival. Lack of an adequate ferrule increases the risk of failure because of post-crown dislodgment, post fracture, and root fracture. A ferrule of only 1.0 mm in vertical height doubles the resistance to fracture of a tooth without remaining coronal tooth structure. Optimal beneficial effect can be expected from a ferrule of at least 1.5 to 2.0 mm of vertical height.

Adequate sound tooth structure should be present to allow placement of a restoration margin that will not encroach on the biologic width, which will ensure continued periodontal health. Biologic width has been defined as "the dimension of the junctional epithelium and connective tissue attachment to the root above the alveolar crest" and has been estimated to be approximately 2 mm. Violation of this dimension with a restorative margin may result in persistent periodontal inflammation. Given the requirement of a minimal ferrule height of 2 mm, at least 4 mm of sound supra-alveolar tooth structure is necessary for mechanical stability and periodontal health.

If there is insufficient remaining tooth structure to adequately retain a core, the suitability of the existing root canals to receive a post should be assessed. A radiographic assessment of the availability of a sufficient length of straight canal of adequate diameter should be made preoperatively and factored into the prognosis. Periapical radiographs taken from multiple angles are often helpful.

There are few teeth that cannot be endodontically treated and restored to some level of function provided the dentist has the skills and the patient is willing and able to commit the time and resources. Current techniques provide a high degree of success for endodontic treatment. Even teeth with no tooth structure remaining above the osseous crest can be restored if techniques such as forced orthodontic eruption or crown lengthening surgery are utilized to create more available vertical tooth structure for crown retention. However, these techniques add expense, patient inconvenience, and discomfort and may compromise esthetics and support of adjacent teeth. In addition, because roots are tapered, these procedures result in a decrease in the cross-sectional dimension of the final preparation and also decrease the crown-root ratio. With the ever-increasing success and availability of implant treatment, the concept of "restorability" has changed. The tooth may be indeed be restored, but what is the anticipated longevity? A realistic assessment must be made of the long-term prognosis of the restored tooth compared with that of an implant-retained restoration.

The use of osseointegrated implants for the replacement of teeth offers an alternative for teeth with a questionable prognosis. 

Treatment Planning

Introduction

The type of definitive restoration required for an endodontically treated tooth is primarily determined by the amount of remaining tooth structure, the position of the tooth in the arch, and the functional forces placed on it. An appropriate restoration may range from a direct resin composite in an anterior tooth to a full crown. The greater the amount of tooth structure lost, the greater will be the need for additional retention for the core, often in the form of a post. The ability of a patient to exert heavy occlusal forces, which might displace a core and the crown it supports, decreases from the posterior regions of the dental arch to the anterior. However, the direction of the functional forces transitions from primarily vertical (compressive) in the posterior region to lateral (tensile or shear) in the anterior. These tensile forces tend to displace the crown/core, and therefore the need for additional resistance form is greater in the anterior region, and a post is often required. In the posterior region, although the forces are greater, they are primarily vertical, and sufficient retention and resistance form for the core is often obtained by extending it into the pulp chamber. For these reasons, a post is rarely required in a molar tooth. However, when a restored tooth is to be used as an abutment for a removable or fixed partial denture, greater lateral forces should be anticipated, and the additional resistance form provided by a post should be considered. Because of the differences in magnitude of functional forces, direction of forces, and anatomical differences, anterior and posterior teeth present different restorative challenges.

Anterior teeth

In the anterior region, there will frequently be sufficient remaining tooth structure to restore the tooth to function and esthetics with a direct restoration. In cases of traumatically induced pulpal necrosis, the only restoration required is to seal the access preparation. In other situations, there may be moderately sized Class 3 cavities in addition to the access. In these cases, neither a post nor a full-coverage restoration is required, and the tooth can be restored with bonded resin composite. When a moderate amount of coronal tooth structure is missing but at least 50% of the coronal enamel remains, the bonded porcelain veneer may be the restoration of choice. A tooth conservatively prepared for a porcelain veneer allows for the creation of excellent esthetics without compromising the strength of the remaining tooth structure. Laboratory studies suggest that endodontically treated incisors restored with porcelain veneers have similar resistance to fracture as an intact tooth. There is no need for post placement in these situations. Although some laboratory studies have indicated that a metallic post strengthens an intact anterior endodontically treated tooth, the majority of studies have suggested that the fracture resistance of these teeth is not affected by placement of a post. In addition, placement of a post in an anterior tooth with an intact incisal edge is often complicated by a lack of draw for the post into the canal through the access opening. Removing additional tooth structure in order to place a post is not indicated.

When the decision has been made to restore the tooth with a crown because of loss of structural integrity or esthetic demand, an assessment must be made as to whether there is sufficient tooth structure remaining to retain the core or whether retention and resistance form must be augmented with a post. Because of the predominantly lateral forces that the anterior teeth are subjected to in function, careful consideration should be given to the placement of a post. In some cases, the dentist may wish to defer this decision until after preparing the tooth. An assessment may be made at that time whether sufficient vertical walls remain to resist the lateral forces that the crown will be subjected to without the placement of a post. Because of the size of the canines and the maxillary central incisors, a post can often be avoided in these teeth, and adequate retention and resistance form can be provided by the placement of a core buildup. With maxillary lateral incisors and mandibular central and lateral incisors, providing adequate tooth reduction for the crown often results in very little remaining tooth structure, and the need for a post is clear.

The canals of maxillary and mandibular canines and maxillary central incisors are usually straight and of adequate size and shape for post placement. On the other hand, the canals of mandibular central and lateral incisors, and often maxillary lateral incisors, are small and ribbon shaped and offer variable opportunity for post placement.

Posterior teeth

Clinical studies have demonstrated that a key element in the successful restoration of endodontically treated posterior teeth is the placement of a full coronal cuspal-coverage restoration. In a retrospective study of 1,273 endodontically treated teeth, Sorensen and Martinoff found that a full-coverage restoration increased the success rate of posterior teeth. In another retrospective study, Aquilino and Caplan showed that endodontically treated molars not crowned after obturation were lost at a rate six times greater than teeth that were crowned. In a systematic review of clinical studies of endodontically treated teeth, Stavropoulou and Koidis determined that the 10-year survival rate of crowned teeth is 81% versus 63% for direct restorations. In a study of molar teeth, Nagasiri and Chitmongkolsuk found that the survival rate of uncrowned teeth was 96% at 1 year, 88% at 2 years, and 36% at 3 years.

Examining the status of the endodontically treated teeth extracted during the period evaluated in two large epidemiologic studies reinforces the importance of cuspal coverage. In the previously cited study by Salehrabi and Rotstein, 85% of extracted teeth had no full coronal coverage. In another study, teeth restored with amalgam or composite showed an incidence of extraction of 6.3%; teeth with full coronal restoration had an incidence of only 2.5%.

Regardless of the seemingly poor performance of direct restoration of endodontically treated posterior teeth, it should be cautiously considered in cases where substantial tooth structure remains. Access preparation has little effect on the stiffness of endodontically treated maxillary premolars, but the removal of the marginal ridges with a larger preparation has a significant effect. A 3-year clinical study concluded that the clinical success rates of endodontically treated premolars restored with Fibre Reinforced Composite posts and direct composite restorations were equivalent to full coverage with metal-ceramic crowns.

In the face of confusing and contradictory data regarding full-coverage versus direct restoration of endodontically treated teeth, the clinician should take a conservative approach. The preponderance of clinical data suggests that the full coronal coverage restoration is the more predictable and long-lasting restoration. A direct resin composite restoration may be considered when there is substantial remaining tooth structure and lower functional loads are present. However, it should be realized that the strengthening effect of the bonded resin composite may be lost over time. It has been shown that the effect diminishes significantly with both thermal cycling and functional loading of the restoration. Degradation of the resin-dentin bond over time has also been demonstrated in vivo. Therefore, the long-term strengthening effect of the intracoronal bonded restoration must be questioned, and the clinician should choose this treatment option with caution.

Restoration of endodontically treated teeth with amalgam without cuspal overlay has a low clinical success rate and cannot be recommended. However, in the hands of a skilled operator, a cuspal-coverage amalgam restoration may provide a low-cost alternative to a crown. In 1980, Nayyar et al described the amalcore, or coronal-radicular, restoration. Rather than placement of a post, the pulp chamber and the coronal 2 to 3 mm of each canal are used for retention of the buildup material. A post is generally not needed with the amalcore. A post is required only when insufficient pulp chamber depth is present to retain the core. In a retrospective clinical study of more than 400 coronal-radicular restorations, no failures were noted that could be attributed to the core buildup. If appropriate thickness of occlusal amalgam can be provided and adequate axial contours, proximal contacts, and occlusion can be developed, the amalcore technique can be used to create a long-lasting definitive restoration.

Maxillary premolars are a unique subset of posterior endodontically treated teeth. Anatomically, the root is often small in diameter relative to the dimensions of the crown to be supported, and there may be little tooth structure to retain the core. The pulp chamber of these teeth is typically small and provides little mechanical retention for the core. A post is indicated in these cases. In addition, these teeth are subjected to a mixture of shear and compressive forces, and the need for a post to provide additional resistance form is often similar to that for an anterior tooth. Similarly, lateral forces exerted on a maxillary premolar functioning as the abutment for a removable partial denture often necessitate a post for additional resistance. Maxillary premolar roots are often fluted and may be curved, presenting a high risk of strip perforation during preparation of the post space. The palatal root has been shown to have a larger and straighter canal than the buccal and should be utilized for post placement.

For the molar tooth, the decisions are more clear-cut. The forces on posterior teeth are predominately vertical. Therefore, development of additional resistance form to withstand lateral forces is not as commonly needed as in anterior teeth. Because of the morbidity associated with post placement, a post is indicated in a posterior tooth only when other more conservative retention and resistance features cannot be used for the core. These features include chamber/canal orifice retention, amalgam pins, and threaded pins, all of which have been shown to be exceedingly effective. When a post is deemed necessary, the palatal canal of maxillary molars and the distal canal of mandibular molars are usually the best choice.