July 1, 2001
by Bruce Glazer, DDS, BScD, FADSA, Dip.Prostho., FADI
While the restoration of the endodontically treated tooth has been practised for more than 100 years, the criteria upon which we base either the restoration or the ultimate removal are not altogether clear. We have all experienced the treatment planning dilemma of whether or not a tooth should or more importantly can be saved. Many of these teeth which we ponder over are post endodontic, some are post periodontic and many of them cannot be restored.
Fracture, caries, periodontal disease, internal and external resorption and pulpal death with or without periapical pathology are some of the reasons for endodontic therapy. As well, elective endodontic treatment is sometimes needed for adequate resistance and retention form in the final restoration. The reasons for endodontic therapy are numerous and with an established failure rate (Fig. 1) for post endodontic restorations of 2-10% per year as well as evidence embracing immediate restoration of all endodontically treated teeth, a workable, reliable formula of the “if” and “how” needs to be developed.
FACTORS AFFECTING RESTORATIVE OPTIONS
Based on many long term retrospective studies, certain positional considerations in the arch, existing occlusal schemes, parafunctional activity, abutment potential for fixed or removable partial dentures, single crowns versus abutment retainers for fixed restorations and relative masticatory muscle size must be carefully weighed when considering the long term potential of restorations (Fig. 2). Restoration of the endodontically treated tooth is the very essence of treatment planning. The patient must be pain free but not necessarily committed to final treatment on the initial visit. Once pain free, (on another day), an informed decision can be reached. Complete examination is the only way to assess the final role of the tooth, the need for endodontics and the restorative options. Do not treat in isolation.
AMOUNT OF REMAINING TOOTH STRUCTURE
If the amount of remaining tooth structure is minimal (less than 50%) due to fracture, caries, endodontic access opening or old existing fillings, a new core will need to be created utilizing a post. Crown lengthening, extrusion or both procedures are sometimes necessary in order to create a ferrule to protect the tooth from fracture. A ferrule of at least 2.0 mm is mandatory in order to create one third of a positive resistance triad. Antirotation, a ferrule and the protection of remaining vertical tooth structure create this positive resistance triad. Maximum destruction of coronal tooth structure demands the use of a post which, if metal, extends into the root by an amount equal to the length of the crown. If the post choice is carbon fibre or quartz, the length need only be 7-8 mm. The space created for the post must leave 4-5 mm of gutta percha as an apical seal. The literature is divided as to whether endodontically treated teeth are more brittle than vital teeth but there is a concensus that the strongest tooth is the one with the most remaining sound dentin used to rebuild the core.
The length of solid tooth needed if a post is required should equal the sum of the biological width (2.5 mm), the ferrule length (2 mm), the apical seal (4 mm) and the minimal retentive post length (7-8.5 mm). Coronally, solid tooth refers to dentin that is a minimum of 1 mm thick after preparation. Thus the minimal tooth length needed is 15.5 mm. If you do not have 15.5-17 mm of crown-root length the long term prognosis of the tooth is guarded.
TIMING OF THE PERMANENT RESTORATION
Due to coronal leakage, which is considered to be a significant etiology in endodontic failure, the temporary sealer must be kept intact during the treatment phase with a permanent restoration completed within 2-3 weeks. TERM, G.I., and IRM are the most leak proof temporary restorations for short periods of time. If the time needed is longer than 3 weeks, then a more permanent restoration is needed.
If a tooth has lost the filling which closed the access opening and it has been exposed to saliva for more than 30 days, it is totally contaminated and may need to be retreated. Retreatment is a judgment call and factors such as a single crown versus a bridge abutment, and the importance of the tooth for the long term survival of the masticatory system are only two of many filters used to make the final decision.
Remember that not all teeth which have received endodontic treatment need crown coverage.
The loss of internal architecture due to the endodontic access opening, removal of filling material, and in particular, loss of marginal ridges makes endodontically treated teeth very vulnerable to fracture. An access opening alone decreases tooth stiffness by 5 %, an occlusal preparation by 20%, loss of 1 marginal ridge by 46% and the loss of 2 marginal ridges by 63%. Endodontically treated teeth have a decrease in proprioceptive response due to the loss of pressoreceptors. The loss of this sensory apparatus may prevent the triggering of a protective response, allowing excessive loads to be applied and resulting in the fracture of non-vital teeth.
Horizontal and oblique fractures usually occur in the upper anterior and vertical fractures in the posterior teeth. Teeth with more than 4-6 mm of crestal bone loss and reconstructed using metal posts are more prone to vertical fracture. Corrosion of posts made from non-precious metals and the mismatch of metals between dowel cores and final crowns can cause vertical fracture.
Root morphology also plays a role in the incidence of fractures. Root fracture increases if posts are used in the mesiobuccal roots of upper molars and the mesial roots of lower molars. Bonded corono-radicular amalgam cores are the gold standard for molars. Posts are discouraged in molar teeth. Retention and resistance form should be developed from the remaining walls of dentin, the judicious use of the top 3 mm of canal space and/or pins and potholes.
The carbon fibre (RTD) post systems available on the market today boast little or no fractures and appear to be the system of choice for the restoration of endodontically treated teeth.
Non vital teeth used to support fixed or removable partial dentures including overdentures with attachments are more prone to fracture failure. However a recent prospective study using carbon fibre posts has shown that there may be no greater fracture potential utilizing fixed versus single abutments.
Since the average bite force is 168 pounds per square inch and non vital teeth have less proprioception, the occlusion should be carefully evaluated and balancing interferences should be removed to protect against fracture. Protection against parafunction should be introduced early in treatment by means of a flat plane splint. Deep overbites will need the minimal ferrule (2.0 mm) with attention given to the anterior guidance scheme. If possible, during endodontic treatment the tooth should be taken out of occlusion. The use of cantilever pontics is not recommended during the reconstructive stage of treatment even when the opposing dentition is a full or partial denture (Fig. 3).
Anterior teeth with intact marginal ridges, cingula, and incisal edges need only a dentin bonded restoration. The presence of large or multiple restorations or unacceptable aesthetics may preclude conservative options. Discoloured endodontically treated teeth can be bleached externally to avoid the possibility of resorption. Porcelain veneers are also a viable option depending on the amount of coronal destruction. Upper and lower incisors lend themselves well to all post systems (available on the market today) but only a passive system should be cemented to avoid fracture. Round canals need antirotation which may be developed from the remaining coronal dentin or if missing, in the form of pins, grooves or potholes in order to ensure a secure resin core. Cuspids on the other hand have ribbon canals and would do better with a custom dowel. Light cured quartz fibre post systems look very promising for this type of canal configuration and also for the creation of aesthetically superior restorations. (Fig. 5)
Full cusp coverage on molars and premolars is the treatment of choice. This applies even to teeth opposing full or partial dentures due to further proprioceptive loss and the ability to develop heavy forces when only one denture is involved. A bonded corono-radicular amalgam core with or without pins is the material of choice. These substructures fair well even as abutments for fixed or removable partial dentures. The rule of thumb in determining the need for additional retention resistance form is the presence of 4mm of tooth wall. This height can be gained by utilizing the pulp chamber and if needed the first 2-3 mm of root canal.
A post is only necessary to retain a core and as such, diameter, length and design are of utmost importance. Threaded active posts are more retentive than serrated posts, which are more retentive than smooth sided posts. Parallel-sided posts are preferable to tapered. Custom cast posts are usually tapered and more problematic. Cast custom posts should be cast from the same metal that will be used in the final core and crown. As well, they should have a positive occlusal seat and a vent escape path for cement. Parallel sided posts can be created from plastic patterns but they should taper as they approach the root apex.
The post diameter should not exceed 1/3 of the root diameter due to the possibility of perforation and the increase in fracture potential (Fig. 4). The minimum amount of dentin thickness where the post ends should be 1 mm Post length equal to crown height is necessary not only for retention but to soften the forces acting along the root surface. Clinicians believe that a 2.0 mm ferrule assures function on the tooth and not the post core complex. But in reality, when extensive coronal destruction has taken place, the crown does function on the post core and thus creates a potential for fracture and debonding of the core.
Gutta percha is removed with a hot instrument or with Gates Glidden burs. The removal of the gutta percha for the post should be completed at the end of the obliteration appointment. The decision on whether a post is necessary needs to be made before the endodontics is completed, thus the need for early treatment planning. The final flare and shaping of the root canal space should be completed by hand held peeso reamers. Parapost drill sets should never be used in the root canal due to possible perforation.
If 50% or more sound coronal tooth structure remains, do not enter the radicular anatomy with a post.
The function of the core is to provide retention and resistance to the final restoration. Dentin bonded re-enforced resin composites and dispersed phase alloys (less initial leakage) are the materials of choice. These core materials may or may not be combined with a post. All crown finish lines must be 2 mm apical to the core materials. If this finish line cannot be developed due to deep caries or a severe fracture, then extrusion or crown lengthening must be completed (Fig. 6).
The cementing medium enhances retention, aids in stress distribution, and ideally seals microgaps between the post and the tooth. Recent studies have advocated the use of low viscosity resin cements in combination with the removal of the smear layer from the canal walls. Certain cements form a hybrid collagen layer at the dentin cement interface and in addition, bond to metal.
The routine use of dowels in the restoration of endodontically treated teeth is no longer mandatory.
The key to success is to have a 2 mm ferrule of solid tooth structure above the crown margin.
If the criteria developed in this review cannot be met perhaps the best treatment would be removal and replacement with an implant or a fixed bridge.OH
Bruce Glazer is Oral Health’s Prosthodontic editorial board member. He has recently received a Fellowship in the Academy of Dentistry International.
Oral Health welcomes this original article.