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The Endodontic Restorative Harmonic: Part I Access To Apex, Apex To Access


May 7, 2019
by Kenneth S. Serota, DDS, MMSc

The restoration of endodontically treated teeth (ETT) has been guided historically by anecdotal empiricism rather than biomechanical dynamics. Decisions regarding the configuration of the restoration, the diameter of the post channel and the post and core materials to be used have plagued foundational dentistry for decades.1,2 The loss of coronal tooth structure due to caries,3 excessive access cavity design4,5 and the taper of the root canal preparation are vectors that will create stress at the cervical region during functional loading.6,7 The ongoing confusion regarding design and materials has caused paradoxical statements and illogical contradictions to be factored into the restorative matrix for ETT.

The introduction of fibre posts has altered the root to restoration harmonic. Fibre posts provide a reliable alternative to metal posts (cast or prefabricated) as their modulus of elasticity (20 GPa) is closer to dentin than metal posts (200 GPa). Stiff, hard metal posts transfer forces along their long axis creating a wedging effect on tooth structure. This can lead to catastrophic failure. The use of fibre posts obviates such an event.8,9

Factors in the decision-making process for the use of posts include:10-24

ETT are weakened by the loss of strategic tooth architecture resulting in structural and occlusal compromise. This is dependent on the amount of native tooth structure removed due to previous or existing caries, the state of the current restoration and the volume of tooth structure removed during the endodontic treatment procedure. ETT are more vulnerable to tooth loss than teeth with vital pulps due to the possibility of either recrudescent or persistent post-treatment disease subsequent to root canal treatment.25 Historically, ETT were considered to be brittle, subject to fracture due to the loss of tooth vitality, decreased moisture content, resultant inelasticity and the loss of collagen cross-linking. Contemporary studies comparing ETT to contralateral vital teeth challenge these findings; no decrease in compressive or tensile strength was associated with changes in the water content of dentin.26-30

In the case of ETT, the distribution of stress concentration zones and the magnitude of tensile stresses has been perceived to increase significantly when tooth structure is lost, or occlusal loads are delivered off-angled to the long axis of the tooth. Fracture resistance is a function of resistance to deformation under load. Restorative materials are less likely to endure stress vectors with sustained load further validating the need for bio-minimalism of ETT restorations.31-35 As well, the loss of tooth structure at the floor of the pulp chamber in ETT leads to significant biomechanical changes in as little as three weeks, with ensuing recontamination of the pulp canal space, resulting in a higher incidence of fractures.36,37 The more native tooth structure retained in ETT, the more enhanced the load management during function, the more effective the stress management and the more predictable the long-term prognosis.38,39

Biologic Width
Biologic width (BW) is the natural seal that develops around teeth and that protects the alveolar bone from infection and disease (Fig. 1). The dimension of BW is not a constant; it depends on the location of the tooth in the alveolus, it varies from tooth to tooth and the configuration of the tooth. BW is essential for the preservation of periodontal health. It is sustained by the removal of any irritation that might damage the periodontium (marginal discrepancies). Exposure of sufficient sound tooth structure in the case of a deep subgingival tooth fracture or carious lesions enhances the retention of the restoration, ensures accurate impression taking and enables correct placement of the restoration margins without violating the BW. This is an imperative in the esthetic zone in patients with uneven gingival margins or excessive gingival display.40,41

Fig. 1

A representation of the attachment apparatus as developed by Gargiulo, Wentz and Orban in 1961.

The choice of marginal position is either supragingival, equigingival or subgingival. Restorations placed where the alveolar bone is thin or the gingiva is thin and highly scalloped are prone to recession.42,43 When a restoration invades the biological width, the body’s response is to move the attachment zone apically until a tolerable biological width is re-established.44

Ferrule
The “ferrule effect” is an enduring foundational tenet in restorative dentistry. It is defined as the height of natural tooth structure extending from the crown margin coronally. Numerous studies have reported that 2 mm of ferrule is required to resist displacement45,46 of the crown from the remaining tooth structure. The effectiveness of the ferrule in the restoration of ETT is determined by; 1) the height and width, 2) the number of remaining walls, 3) the location of the ferrule, 4) the condition of the residual tooth structure, 5) the tooth type and 6) the degree of parafunctional loading (Fig. 2).47

Fig. 2

The ferrule is a predictor of long-term treatment success, whereas the same effect cannot be demonstrated for posts. Logically, the number of walls of coronal tooth structure remaining is a predictor of long-term treatment success due to the configuration factor (C factors) (52).

Exposure of sufficient sound tooth structure in the case of deep subgingival fractures and/or carious lesions enhances the retention of the restoration and enables correct placement of the restoration margins without violating the biologic width. This improves aesthetics in patients with uneven gingival margins and excessive gingival display.48,49 In situations where a substantial volume of tooth structure is lost, adhesive materials will not overcome the lack of ferrule and should not be an alternative to sound engineering principles when restoring ETT.

There are two ferrules; the crown ferrule and the core ferrule. The greater the height of residual tooth structure above the margin of the preparation (crown ferrule), the greater the fracture resistance. The same premise applies to the buccal thickness (core ferrule).50 Ideally, 2.0 mm of crown ferrule and 2.4 mm of dentin thickness (core ferrule) minimizes the fracture potential in molars. Full coverage preparation for ETT maxillary and mandibular bicuspids, regardless of the coronal ferrule height, results in diminished buccal thickness and increased fracture potential (Fig. 3).51-53

Fig. 3

The ETT preparation has 2 ferrules; the crown ferrule and core ferrule (49).

There are numerous contraindications to achieving an ideal ferrule; immunological disease, close adjacent roots, tori, the ascending ramus, occlusion, muscle insertions, furcation exposure and lip position.54 It is best to do a risk-benefit analysis by creating a provisional restoration prior to crown lengthening to ensure the restorative treatment plan objectives can be met (Fig. 4).

Fig. 4

A Crown lengthening algorithm.

The Paradigm Shifts
The developments in adhesive restorative technologies and techniques enable functional and aesthetic reconstruction of debilitated tooth structure when adequate coronal tooth structure remains. A more conservative non-invasive rehabilitation is possible for rebuilding the integrity of the residual tooth structure.55 Improvements in direct restorations relating to the enhanced properties of composite resins engender a shift in the traditional default full coverage treatment plan.

In order to compensate for parafunctional occlusal forces, the marginal placement of a protective restoration should provide cuspal coverage.56 Posterior teeth that have undergone ETT have been reported to have greater cuspal flexure then non-ETT. Teeth missing marginal ridges allow greater cuspal flexure than teeth with intact marginal ridges, whether ETT or non-ETT. Direct or indirect onlays can be used when there is residual tooth structure that is not undermined and if the marginal ridges are intact. Onlays with cuspal shoeing and full coverage crowns restrict cuspal displacement and will prevent coronal fracture under loading (Fig. 5).57

Fig. 5

A Critical thinking algorithm.

Clark58 advocates a substantially altered perspective on the reconstruction of ETT; 1) enamel bevels, 2) flared walls, 3) aluminum oxide blasted 4) and etched and uncut enamel. He advocates monolithic composites in contrast to porcelain fused to metal, dilithium silicates, and zirconia. He recommends not to layer, the use of a translucent matrix system and the removal of biofilms with a pressurized air-water-mild abrasive slurry.58 This will be discussed in depth in Part II.

Access Cavity Design Revision
Recently, a trend to patient centric conservative endodontic cavities (CEC) has changed the focus of the restoration of ETT.59 As the adhesion era in restorative dentistry progresses, extra-coronal restorations requiring minimal preparation are replacing full coverage restorations depending upon the volume of tooth structure remaining. The CEC concept recognizes that endodontics is restoratively driven; an access to apex, apex to access paradigm that preserves peri-cervical dentin to diminish the potential for fracture. Traditional endodontic cavities (TEC) are procedure centric and prone to structural compromise. Logically, a balance between the preservation of native tooth structure and dentin removal for access to the canal system is beneficial. However, a minimal access could compromise the efficacy of debridement and disinfection as it limits access to the entirety of the root canal system. The size reduction engenders the possibility that infected tissue could remain and iatrogenics ensue (Figs. 6A-6D).60

Fig. 6A

Prior to the nickel titanium era, the traditional endodontic cavity preparation for incisors included the coronal aspect of the cingulum. The coronal tooth structure was weakened by the removal of excessive lingual tooth structure.

Fig. 6B

Conservative endodontic cavity preparation for incisors minimizes tooth structure removal. The use of CBCT in endodontic therapy targets a direct trajectory to the root canal space.

Fig. 6C

Traditional molar endodontic cavity preparation diminishes the amount of peri-cervical dentin retained and heightens the risk of fracture.

Fig. 6D

Conservative endodontic cavity preparation in molars increases the resistance to fracture by minimizing the removal of coronal tooth structure.

Decision making for restoring ETT:
• Best practices; remove the restoration(s) and carious tooth structure prior to endodontic treatment in order to evaluate the restorative matrix.

Best practices; assess how occlusal forces affect the EET in regard to the angulation and the biomechanics of the residual tooth structure. Determine the algorithm of success with cuspid protected occlusion or group function on a single tooth, a bridge abutment, an abutment for an RPD, a single tooth adjacent to an implant retained prosthesis.

• Best practices; crown lengthening considerations

• Short clinical crowns

• Placement of subgingival restorative margins

• Excessive occlusal or incisal wear

• Inadequate interocclusal space

• Partial restorative ferrule

• Best practices; boxes or grooves for secondary retention

• Best practices; Would cementation of a crown on tooth structure be more effective than on core material

• Best practices; Choice of bonding agent, total-etch or self-etch resin cements

Conservative non-invasive rehabilitation in the adhesion era:
• Best practices; Are posts a prerequisite for all ETT?

• Best practices; Is there an evidence- based data analysis that fibre post systems are preferable to cast or prefabricated metal?

• Best practices; Is there evidence-based data analysis that restorations without posts are reliable and predictable?

• Best practices; Are there alternative means by which to reinforce teeth?

• Best practices; Are there predictable adhesive restorative protocols for ETT?

Fig. 7A1

Fig. 7A2

A conservative endodontic access cavity preparation can be restored with a Class I composite inlay. (Figure 7 images modelled from Dietschi, Krejci and Sadan).

Fig. 7B

Fig 7B: A traditional access cavity preparation and significant parafunctional forces is best restored with an overlay. The mass of the restoration over the cusp minimizes contraction and gapping as per Clark (59).

Fig. 7C1

Fig. 7C2

Class II MO/DO/MOD direct composite restorations may be used to restore an ETT when parafunctional forces are light.

Fig. 7D

Composite or ceramic overlays are necessary when parafunctional forces are significant.

Fig. 8A

Composite or ceramic endocrowns may be used when parafunctional forces are light and greater than half of the residual tooth structure remains.

Fig. 8B

Composite cores and full coverage restorations are indicated when parafunctional forces are significant and greater than half of the residual tooth structure remains.

Fig. 9A

Fibre posts, composite cores and full coverage restorations are indicated when less than half the residual tooth structure remains.

Fig. 9B

The cuspal inclines of restorations of ETT are reduced to focus the occlusal forces centrally which mitigates shrinkage.

Conclusion
The voyage from anecdotal empiricism to scientifically validated protocols used for the restoration of ETT continues. Fundamental best practices are changing. Historically, posts/cores and the circumferential reduction of residual tooth structure were the standard technique for restoration of ETT. These aggressive procedures appeared to enhance failure possibilities. Investigations from the past and those ongoing do not provide evidentiary science that conclusively substantiates post composition, post shape, core material choice, ferrule height and width. Improvements in dentinal adhesives and biosmart materials appear to provide an alternative restorative paradigm with maximum preservation of tooth structure and a balance between esthetics and structural resilience.

Oral Health welcomes this original article.

Disclaimer: Part two of this article will provide a matrix for the choice of posts, the choice of core material and the choice of restoration. The author wishes to thank Clinical Research Dental for their support of this manuscript.

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About The Author
Kenneth S. Serota, DDS, MMSc, graduated from the University of Toronto Faculty of Dentistry in 1973 and received his Certificate in Endodontics and Master of Medical Sciences degree from the Harvard-Forsyth Dental Center in Boston, Massachusetts in 1981. Active in online education since 1998, he is the founder of the Endodontic forum ROOTS and the interdisciplinary Facebook forum NEXUS. Dr. Serota is a clinical instructor in the University of Toronto postdoctoral endodontics department. He is the social media and marketing director for Navident Dynamic Navigation.


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