IMPACT OF BONDING PERICERVICAL DENTIN ON

BIOMECHANICAL RESPONSE IN ROOT FILLED

MAXILLARY SINGLE-CANAL PREMOLARS

by

Nghia Quang Huynh

A thesis submitted in conformity with the requirements for the degree of Master of Science

Graduate Department of Dentistry

University of Toronto

© Copyright by Nghia Quang Huynh 2017

ii

IMPACT OF BONDING PERICERVICAL DENTIN ON

BIOMECHANICAL RESPONSE IN ROOT FILLED MAXILLARY

SINGLE-CANAL PREMOLARS

Nghia Quang Huynh

Master of Science

Graduate Department of Dentistry

University of Toronto

2017

Abstract

Introduction: This study evaluated the impact of bonding pericervical-dentin (PCD) with

composite-resin on microstrain distribution (MD) and fracture strength of root-filled maxillary

premolars.

Methods: Microstrain: Ten single-canal maxillary premolars were decoronated 2mm above the

cemento-enamel junction (CEJ) and instrumented to Protaper Universal F3. Canals were root-

filled with gutta-percha, either to CEJ (Group-1) or to 6mm below CEJ and restored with bonded

composite-resin (Group-2). Digital moiré interferometry was used to evaluate MD in PCD.

Fracture strength: Thirty premolars were prepared and root-filled as above (n=10/group). Cyclic

loading with compressive load-to-failure were used. Mechanical data were analyzed with one-

way ANOVA and post-hoc Tukey test at 5% level of significance.

iii

Results: Coronal and apical dentin showed bending and compressive MD. Group-1 MD was

unaltered. Group-2 MD suggested stiffening at PCD. Load-at-failure values were not statistically

significant (p=0.464).

Conclusion: Bonding of PCD might impact the biomechanical responses in maxillary premolars

at low-level continuous loads.

iv

Acknowledgements

There exists many paths that one can take in life. The journey that has led me to this moment has

been full of interest, perseverance, and tremendous reward. There is a saying that nothing comes

about from nothing and thus my journey has resulted from the many people who have guided and

helped me along the way. I would like to thank Dr. Friedman and Dr. Dao, my committee

members, for their continued support and help in allowing me to achieve my goals. Dr.

Friedman, you have been instrumental in my understanding of clinical endodontics and your

mentorship will never be forgotten. Thank you. Dr. Alice Fang Li; without your expertise and

patience while educating me on the intricacies of interferometric analysis, I would not have

succeeded and therefore owe you a multitude of gratitude for your tremendous help.

Furthermore, I am indebted to my supervisor Dr. Kishen for all his support, encouragement and

enlightenments. You are a phenomenal teacher, and an inspiration to all that has had the

privilege to work and learn from you. Your patience and understanding are truly a rare quality in

today’s society. Thank you for all your support and encouragement.

In addition to my professional acknowledgements, I am fortunate to have a loving and supportive

family. My parents have always encouraged and supported me through all my ventures and I

would not achieve all that I have without their continued love and support.

To my beautiful and loving wife, Gloria, you have always been an inspiration to me in both my

professional and personal life. Your intelligence, smile and comfort have always carried me

through rough sailings. This thesis is a reflection of your encouragement and love and it is my

hope that our children, Sophia, William and Audrey will one day understand the value of hard

work, dedication and sacrifices that we have both made for them. Lastly, a reminder to myself

and my children, life is a journey. Journeys are rarely a straight path. Journeys have significant

v

inflection points, usually at 38.2, 50, and 61.8%. When the paths from these inflection points

converge to a single destination, an important life cycle is complete. What we do at these

completions, determines the next journey that one takes. Life is 38.2% what happens to you,

61.8% how you respond.

Nghia Quang Huynh

vi

Table of Contents

Acknowledgements …………………………………………………………………………….... iv

Table of Contents ………………………………………………………………………………... vi

List of Figures ………………………………………………………………………………….... ix

I. Introduction ………………………………………………………………………………. 1

1. Root fracture in endodontically treated teeth ………………………………… 1

1.1. Prevalence of root fractures in endodontically treated teeth ……………................. 1

1.2. Predisposing factors to root fractures ……………………………………………… 1

2. Minimally invasive healthcare ………………………………………………………. 2

2.1. Paradigm shift ……………………………………………………………………… 2

2.2. Minimally invasive dentistry ………………………………………………………. 2

3. Dental Biomechanics …………………………………………………………………. 3

3.1. Definition …………………………………………………………………………... 3

3.2. Relevance in endodontic research …………………………………………………. 3

4. Pericervical dentin …………………………………………………………………… 3

4.1. Definition …………………………………………………………………………... 3

4.2. Biomechanical role of pericervical dentin …………………………………………. 4

4.3. Strategies to conserve pericervical dentin …………………………………………. 4

5. Root canal instrumentation ………………………………………………………….. 5

5.1. Nickel titanium rotary instruments ……………………………………………….... 5

5.2. Root canal instrumentation and dentin removal …………………………………… 5

6. Strain measurements ………………………………………………………………… 6

6.1. Application of strain measurement in teeth …………………………………….. ….6

7. Digital moiré interferometry ………………………………………………………… 8

7.1. Principles and properties ……………………………………………………………8

vii

7.2. Application in endodontic research ………………………………………………... 9

8. Micro-Computed Tomography ……………………………………………………… 9

8.1. Principles …………………………………………………………………………... 9

8.2. Applications in endodontic research ……………………………………………. 10

9. Load to fracture analysis of teeth ………………………………………………....… 10

9.1. Properties of dentin …………………………………………………...…………. 10

9.2. Hybrid mechanical testing of the fracture strength of teeth ………………..…….. 11

9.2.1. Cyclic fatigue …………………………………………………………………11

9.2.2. Load-at-failure ………………………………………..……………………… 12

10. Restoration of endodontic treated teeth………………………………………….. 12

10.1. Principles ………………………………………….……………………………… 12

10.2. Composition of composite resins …………………………………….....…………13

10.3. Application in endodontic restorations …………………………………………… 14

II. Objectives and Hypothesis ……………………………………………………………….. 15

III. Article Submitted for Publication ……………………………………...………………. 16

The Biomechanical Effects of Bonding Pericervical Dentin in Maxillary Premolars.. 16

Acknowledgements ……………………………………………………………………... 17

Highlights ………………………………….……………………………………………. 17

Clinical Relevance ……………………………………………………………………… 17

Abstract …………………………………………………………………………………. 18

Introduction ……………………………………………………………………………... 19

Materials and Methods ……………………………......………………………………… 20

Results ……………………………...…………………………………………………… 24

Discussion ………………………………….…………………………………………… 25

References …………………………………..…………………………………………... 28

viii

Legends to Figures ……………………………………...………………………………. 31

IV. Discussion …………………………………………………………………………………36

V. Conclusion …………………………………..……………………………………………. 42

VI. Future Direction …..…………………………………...…………………………………..43

VII. References …………………………………......…………………………………………. 44

VIII. Figures ………………………………….………………………………………………… 52

ix

List of Figures

Figure 1: Schematic of grating application prior to DMI analysis. The applied grating material

acts as a deformation- sensing element. .................................................................. 527

Figure 2: Schematic of DMI experimental arrangement. The moiré interferometer consists of

two mutually coherent light beams from a diode laser (wavelength = 670nm), which

are incident on the specimen grating at an oblique angle and generate a virtual

reference grating on 2400 lines/mm. ....................................................................... 538

Figure 3: An example of a virtual reference grating interacting with the deformed specimen

grating to produce a moiré fringe pattern when the specimen is subject to a mechanical

load. ......................................................................................................................... 49

1

I. Introduction

1. Root fracture in endodontically treated teeth

1.1. Prevalence of root fractures in endodontically treated teeth

With approximately 15 million endodontic treatments performed annually in the USA (1), the

reported 5-10% (2-5) prevalence of vertical root-fracture (VRF) leading to tooth loss post-

treatment represents considerable societal burden (2). Teeth that show the highest incidence of

root fractures in endodontically treated teeth are maxillary second premolars and mandibular first

molars (4). Narrow root morphology accompanied by dentinal loss and parafunctional occlusion

have been implicated as playing an important role in initiating and disseminating root fractures in

these teeth.

1.2. Predisposing factors to root fractures

Endodontically treated teeth have been thought of being generally more prone to fracture due to

the loss of dentin from caries removal, access preparation, instrumentation effects and lack of

definitive restoration (2, 6, 7). Of multifactorial causes of VRF (6), loss of dentin is important

because it predisposes teeth to mechanical failure under functional stresses (6, 8-11). Typically, a

cumulative process of crack initiation and propagation occurs with time leading to fatigue failure

(6). While crack initiation and propagation induced by engine-driven canal instrumentation is

currently the focus of research (12), the biomechanical impacts of instrumentation on root dentin

have not been explored. Therefore, treatment strategies that minimize excess removal of dentin

have been investigated in an attempt to extend the survival outcomes of endodontically treated

teeth (13-16).

2

2. Minimally invasive healthcare

2.1. Paradigm shift

There is currently a movement in healthcare towards minimally invasive procedures, from

utilizing endoscopic microsurgery in medicine to microendodontic procedures in endodontics,

which are mostly enabled by concurrent technological advancements (17, 18). These

advancements allow for smaller incisions/preparations and improved healing times compared to

traditionally employed surgical techniques (19). With increased life expectancy in the general

population, an individual’s dentition is expected to be retained longer. Advances in the

minimally invasive dental procedures will aid in retaining tooth structure and thus maintain the

functional integrity of natural teeth for the lifetime of the patient.

2.2. Minimally invasive dentistry

Dentistry is unique in that treating dental diseases often involves removing diseased hard tissue,

while the remaining healthy hard tissues lack the propensity to regenerate. Further, the removed

diseased hard tissue is typically replaced with a synthetic polymer with different biomechanical

or material properties when compared to the native dental hard tissues. This material property

mismatch poses many challenges as teeth are subjected to harsh environmental conditions

involving changes in pH, temperature, and masticatory forces. Therefore, by minimizing the

amount of tooth structure removed during dental procedures, one can minimize any deleterious

biomechanical effects on the remaining healthy dental tissues.

3

3. Dental Biomechanics

3.1. Definition

Biomechanics is the study of the structure and function of biological systems using the methods

of engineering mechanics (20). Biomechanics provides information on how different biological

systems behave during function. Natural mineralized tissues are the product of long-term

optimization controlled by evolutionary processes (21); their biomechanical responses to forces

determine the tissues’ inherent mechanism of fracture resistance (21).

3.2. Relevance in endodontic research

It is recognized that many damaging effects produced during endodontic and restorative

procedures are due to the lack of understanding of biomechanical principles underlying the

treatment. Understanding the nature of stress/strain distribution within tooth structure will aid in

understanding how natural/treated tooth structure responds to mechanical forces (22). It should

be realized that micro-crack events leading to catastrophic fracture are locally strain-controlled

(23); thus, assessing biomechanical impacts such as strain distribution in root dentin may provide

insight into fracture resistance of root-filled teeth. Correspondingly, understanding the

biomechanics of root dentin may explain the biomechanical causes of VRF in root-filled teeth

(24).

4. Pericervical dentin

4.1. Definition

Pericervical dentin (PCD) was defined by Clark et al. as the area of root dentin extending 6 mm

apical and 4 mm coronal to crestal bone (15). This region of dentin is believed to be important in

4

minimizing root fracture seen in endodontically treated teeth, as it is an area responsible for

redistributing occlusal forces through the long axis of the root (15).

4.2. Biomechanical role of pericervical dentin

In vivo biomechanical experiments using strain gauges and in vitro experiments on photoelastic

models demonstrated that the major stress/strain distribution in maxillary anterior and

mandibular anterior teeth during physiological loads occurs at the cervical region of the root

dentin, which dissipates towards the apical root dentin (20). Thus it could be suggested that the

loss of PCD may challenge the functional stress/strain distribution in the root (15, 20).

Consequently, dentin loss in this aspect of the root generates higher functional stress

concentrations, which might be a significant risk factor to VRF (25, 26). Furthermore, Finite

Elemental Modeling (FEM) suggests that stress concentrates towards the cemento-enamel

junction (CEJ), which increases internally towards the canal space after access cavity preparation

(25). All the above biomechanical findings warrant conservation of PCD during endodontic and

restorative procedures.

4.3. Strategies to conserve pericervical dentin

Conservation of PCD through contracted endodontic cavities is suggested to improve fracture

resistance in molars and premolars in vitro (27). Although contracted endodontic cavities have

received considerable attention as a means to conserving dentin (27), the method is not widely

accepted and remains controversial. Additionally, the goal of minimizing pericervical dentin

removal has been accomplished by the use of rotary instruments with regressive tapers (16, 28).

However, research studies are limited to evaluate the effectiveness of retained pericervical dentin

on fracture resistance (27, 28).

5

5. Root canal instrumentation

5.1. Nickel titanium rotary instruments

Nickel titanium (NiTi) is a very flexible alloy consisting of a crystalline lattice structure which

alters between austenitic and martensitic phases (29) and the use of NiTi rotary instrumentation

has been used in endodontics to create root canal shapes to fulfill cleaning and shaping principles

outlined by Schilder (30). The introduction of nickel titanium in endodontics has made

endodontic treatment more efficient while maintaining the original canal geometry (31).

However, engine-driven NiTi instrumentation has been implicated in initiating and/or

propagating internal root fractures, craze lines and cracks, rendering the definitive role of rotary

instrumentation in root cracks controversial (31, 32). A recent experimental investigation

suggested that the biomechanical response of root dentin to root canal instrumentation was

influenced by the degree of dentin hydration (33). In hydrated roots instrumentation with hand,

reciprocating or rotary Ni-Ti instruments did not result in residual micro-strain concentrations.

Corresponding instrumentation of non-hydrated roots caused localized micro-strain

concentration and reduced strain relaxation; this biomechanical impact may contribute to

dentinal micro-defects in dehydrated root dentin tested under ambient room conditions (33).

5.2. Root canal instrumentation and dentin removal

Currently it is believed that different degrees of dentin removal may occur during root canal

instrumentation (6, 9, 10, 34), which may alter the biomechanical response of the remaining root

dentin structure (35) and resistance to VRF (36). An investigation based on FEM suggests that

when canals are instrumented larger sizes, radicular stresses formed during loading increase by

up to 37% (37). These internal stresses can be partially relieved by changing the canal

configuration from oval to round (20, 35).

6

Current rationale of contracted access and the preservation of dentin during endodontic treatment

have led to a shift in endodontic file design and metallurgy that promotes dentin conservation in

the pericervical dentin (27, 28). Historically, rotary endodontic files were designed to facilitate a

constant tapered funnel preparation for cleaning and shaping (38, 39). This method of

preparation usually required straight-line access via a conventional endodontic access, but curved

root canal geometry posed significant challenges. These challenges included ledging, zipping,

elbow formation, perforation and loss of working length due to dentin debris compaction (38).

Instrumentation techniques were developed to address these potential complications including

step back, stepdown and balanced force techniques (38). However, the introduction and

advancement of nickel titanium instrument design, which utilizes temperature treatment, as well

as the evolution of rotary instruments from constant to variable taper geometries, allowed for

improved efficiency and conservation of dentin during root canal instrumentation (12, 38, 40).

Many manufacturers currently employ a variable taper file geometry, as well as a maximum file

diameter of less than 1mm in an attempt to preserve radicular dentin (40).

6. Strain measurements

6.1. Application of strain measurement in teeth

Strain is defined as the proportion of deformation in the direction of the applied force divided by

the initial length of the material (41). When chewing forces act on a tooth, it experiences bending

related stress/strain distribution. Bending pattern results in stress/strain distribution, which are

highest at the outer aspect and diminish to zero towards the centre of the cross-section. In a

tooth, under compressive force, the maximum stress resulting from bending is predominantly

observed at the cervical aspect of the root (PCD). This bending related stress/strain patterns

7

reduces notably towards the apical region of root (20). This decreased stress distribution in the

middle and apical third of the root was attributed to the shape/angulation of the tooth and its

interaction with the supporting bone. Thus the cervical root dentin and its relationship with the

supporting alveolar bone is crucial for a stable stress distribution from the root to the supporting

bone (26).

The biomechanical impact of root filled teeth utilizing strain gauges within the endodontic

literature has been well established (42, 43). Strain measurements are typically measured via

strain gauges that evaluate strain at only a point within the whole tooth (44). This can pose

challenges in reproducing the exact location of the strain gauge, if a single sample has to undergo

various experimental procedures that require removal and reapplication of the strain gauge (28).

Another limitation of conventional strain gauge measurements is that the strain values from

strain gauges do not provide any information on the distribution of strain while teeth undergo

physiologic stresses (20). Identifying the changes in strain distribution may allow for a more

accurate description of the biomechanical changes that occur in dentin after dentin removal

and/or the restoration of lost dentin. It is critical to realize that the relevance of quantitative strain

measurements from a functionally adapted biological hard tissue such as dentin has been

questioned, while the significance of principle stress distribution pattern and strain distribution

pattern have been emphasized to understand locations of stress/strain bearing in structural

biomaterials (45).

8

7. Digital moiré interferometry

7.1. Principles and properties

Digital moiré interferometry (MI) utilizes the principles of optical interferometry to determine

real-time microstrains in dento-osseous structures with high resolution (46), offering substantial

advantages over conventional microstrain assessments (44, 46).

Digital moiré interferometry is a non-contact, optical based method that can be used to determine

the microstrains with high sensitivity and high resolution from the root dentin. Moiré

interferometry provides complete-field in-plane strain gradients within dentin by utilizing grating

material as a deformation-sensing element (Fig. 1). The moiré interferometer consists of two

mutually coherent light beams from a diode laser (wavelength = 670nm),which are incident on

the specimen grating at an oblique angle and generate a virtual reference grating on 2400

lines/mm (Fig. 2). This virtual reference grating interacts with the deformed specimen grating to

produce the moiré fringe patterns when the specimen is subjected to a mechanical load (44)

(Fig.3).

While numerical modeling efforts for stress distribution in teeth, assuming that the modulus of

elasticity of dentin is constant, may produce misleading conclusions (44), MI provides complete-

field in-plane strain gradients within dentin (46). Generated moiré fringes, representing contours

of displacement components, are analyzed with image-processing software to determine altered

strain patterns within the tooth segment. This supports comparative analysis of microstrain

patterns generated in real-time under varying physiologic loads on the tooth (46). Quantitative

microstrain analysis is calculated via the following formulae:

9

Strain in U field: ΔU/Δx = εx = P/Δx’

Strain in V field: ΔV/Δy = εy = P/Δy’

P is equivalent to the pitch of the reference grating.

7.2. Application in endodontic research

Digital moiré interferometry (DMI) has been previously used in dental research (44). Previous

applications of DMI include evaluating the role of free water on the mechanical deformation of

structural dentin (47) and the investigation of the effect of dehydration as a predisposing factor in

dentin microcracks related to rotary instrumentation (33). DMI is highly sensitive in detection of

microstrain within dental tissues under varying physiologically relevant static loads (44, 46).

Avoiding unrealistic assumptions of elastic modulus of dentin as in FEM studies, DMI supports

a more accurate interpretation of in-plane stress/strain distribution in dentin. Furthermore, DMI

allows for whole field sampling of microstrain patterns that qualitatively illustrate the shift in

strain under applied mechanical loads. This property of DMI allows for testing dentin under

various physiologic loading forces and visually evaluating how strain patterns shift within

dentin.

8. Micro-Computed Tomography

8.1. Principles

The principle of micro-CT imaging involves subjecting a series of x-rays to create cross-

sectional images that can be reconstructed in 3-dimensional imaging software for modelling (48,

49). Micro-CT has wide applications within endodontic research as it is a non-destructive and

10

detailed means to evaluate the macrostructure of dentin (48).

8.2. Applications in endodontic research

Scanning specimens with defined micrometer sections provides tremendous detail, however the

vast amount of data must be compiled via specialized imaging software (48). With the use of

imaging software such as Amira (FEI Visualization Sciences Group, Bordeaux, France), the

image slices can be compiled into a 3D representation of the imaged specimen. Virtual

manipulation of the micro-CT imaging has been commonly used to measure removed dentin

volume during canal instrumentation, search for internal cracks within dentin, as well as

visualizing the complexities of the root canal system (27, 31, 49, 50). Micro-CT has been used

mainly in preclinical studies, however clinical applications are on the horizon and future

applications will allow for microscopic detail of tissues without the need for disuse dissection

(48).

9. Load to fracture analysis of teeth

9.1. Properties of dentin

The composition of dentin consists of 10% of water by weight and is thought to play a vital role

in maintaining the biomechanical properties of dentin in vital teeth (51). It has been speculated

that the loss of this water content in endodontically treated teeth contributes to the “brittleness”

and predisposes the tooth to root fractures (52).

Earlier studies from Helfer et al., reported that moisture content of dentin from root-filled teeth

was about 9% less than their vital counterpart (53). However, there are other studies that

contradict this view. Papa and Messer reported an insignificant difference in the moisture content

11

between root-filled teeth and vital teeth, and emphasized the importance of conserving the bulk

of dentin in maintaining the structural integrity of root-filled teeth (54). Kruzic et al. conducted a

simulation study and found that dehydrated specimens showed significantly lower crack-

initiation toughness compared to the hydrated specimens (55). Khaler et al. found that the work

of fracture of hydrated specimens was significantly higher than dehydrated specimens (56).

Kishen and Asundi used digital moiré interferometry to study the role of free water on the

mechanical deformation of structural dentin. They tested fully hydrated and dehydrated

specimens, dehydrated at 20ºC for 72 hours. They found a strain response characteristic of a

tough material in fully hydrated dentin, while dehydration resulted in a response characteristic of

a brittle material (47). Studies have also highlighted time-dependent properties or viscoelastic

behaviour in dentin (57).

However, current endodontic research and understanding implicate the greater effect of dentin

loss, either by iatrogenic or non-iatrogenic reasons, as a predisposing factor to increased root

fracture (6, 8, 32, 58). Maintaining the bulk of dentin minimizes the required bulk of restorative

material to restore the tooth back into function. Minimal restorative material allows for a greater

propensity to maintain dentin’s biomechanical properties, which is important in retaining

endodontically treated teeth long term. (15, 27, 40, 59)

9.2. Hybrid mechanical testing of the fracture strength of teeth

9.2.1. Cyclic fatigue

The Instron Universal Testing machine is commonly used to assess resistance to fracture of root-

filled teeth (60) in dental research. Fractures within dentin often require a temporal component

involving long periods of physiologic function (6). Cyclic loading has been used in the dental

literature as a means to represent clinical function (61, 62) as physiologic masticatory forces

12

ranging from 9N to 45N with over 250,000 cycles per year represent the average masticatory

function (59, 63, 64). This clinical function is important to simulate as it has been demonstrated

to affect the properties of dentin, dentin-restorative interfaces and the restorative material

property itself (42). As such, cycle fatiguing allows for a more accurate representation of the

temporal component involved in the fracture of endodontically restored teeth whereby a hybrid

mechanical analysis comprised of cyclic loading to simulate physiologic aging and applying a

continuous applied load until failure is used to determine fracture strength (65).

9.2.2. Load-at-failure

Fracture resistance (or strength) is the ability of a material sample to resist fracture when

challenged with compressive or shear stress (24). The Instron Universal Testing Machine

(IUTM) (Instron, Canton, MA) is a mechanical device that is able to generate physiologic

and artificial forces to test a material’s fracture strength. In dental research, studies

commonly utilize a 5mm diameter spherical cross-head when assessing fracture resistance of

teeth (66). Utilizing the IUTM, precise measurements of applied forces can be utilized and

measured. This allows for analysis of initial failures of the sample indicated by a sudden

drop in applied forces and is useful in determining the maximum fracture strength of the

tested sample.

10. Restoration of endodontic treated teeth

10.1. Principles

Fracture in dentin is a result of microcrack initiation and propagation under the influence of

occlusal loads (56). Mastication and other parafunctional activity generate cyclic stress/strain

13

that promote fatigue crack propagation in dentin (6, 67). Dentin hard tissue is known to possess

inherent toughness, which aids in resisting fracture (6). This is attributed to the orientation of the

collagen fibrils to the hydroxyapatite that counter the directional effect of the dentinal tubules

(6).

The principle of restoring endodontically treated teeth serves 2 purposes: creating an impervious

seal to prevent recurrent bacterial ingress and to restore biomechanical function to the tooth (68).

Many studies have demonstrated the coronal seal as contributing an important role in the success

of endodontic treatment (69-73) along with the effects of restorative materials in restoring

fracture strength in endodontically treated teeth (74). Several restorative materials investigated

range from amalgam, glass-ionomer cements, and composite resins. (59, 63, 75). Bonded

composite resins have gained interest as a restorative material in restoring endodontically treated

teeth over the years as advances in bonding technologies have improved (76, 77).

10.2. Composition of composite resins

Generally, the composition of composite resin consists of resin matrix and fillers. Bisphenol-A

diglycidylmetacrylate (bis-GMA) or urethane dimethacrylate (UDMA) are typical resin

monomers used in composite resins (78). Short chain monomers, such as triethyenglycol-

dimenthacrylate (TEGDMA) are usually mixed in with bis-GMA to decrease the viscosity of the

composite resin (79). This increased ratio of TEGDMA increases the polymerization shrinkage,

tensile strength, yet reduces the flexural strength of the composite resin (79-81). Fillers of quartz,

ceramic and or silica have been used in composite resins. Increasing the filler component of

composite resin play an important role in its physical characteristic, as an increase in the content

of filler in composite resin decreases the polymerization shrinkage, linear expansion coefficient

and water absorption (79). Consequently, this improves the compressive strength, modulus of

14

elasticity and wear resistance of the composite resin (79, 82). The polymerization process to

harden composite resins involve a chemical reaction between dimethacylate resin monomers to

create a polymer network that is activated by either a light source, chemically activated, or both

(83).

10.3. Application in endodontic restorations

The physical properties associated with composite resins, such as its ability to bond dentin, resist

physiologic chewing forces and have similar modulus of elasticity similar to dentin provide an

opportunity to restore endodontically treated teeth with cost-effectiveness and predictability (25,

68). Concurrently, bonded restorations have been suggested to improve long-term survivability

of root-filled teeth (25, 73, 75, 84, 85). Since the modulus of elasticity of composite resin is close

to that of dentin (25), restoring endodontic cavities with composite resin showed minimal stress

jump at the resin-dentin interface, reduced cuspal flexure and lower probability of crown

fractures (25, 75, 84). With a shift towards minimally invasive dentistry, and specifically

endodontic access and shaping, composite resins may provide a conservative approach in

restoring these minimal preparations.

15

II. Objectives and Hypothesis

This in vitro study aimed to assess the impacts of composite resin bonding of PCD in root-filled

single-canal maxillary premolars, on:

1. Microstrain distribution patterns in PCD under static loads.

2. Root-fracture resistance under simulated functional loading.

The specific aims were to assess the following parameters in single-rooted maxillary premolars:

Microstrain distribution in PCD under continuous compressive loads within physiological

limits before and after root canal treatment.

Changes in microstrain distribution in PCD bonded with composite resin.

Load-at-failure under hybrid (cyclic followed by static) loading.

The following null hypotheses were tested:

1. There would be no discernible difference in microstrain distribution at the PCD

among specimens in all three groups.

2. There would be no significant difference in load-at-failure of roots among all three

groups (root-filled without PCD bonding, root-filled with bonded PCD, intact).

16

III. Article Submitted for Publication

The Biomechanical Effects of Bonding Pericervical Dentin in

Single-Canal Maxillary Premolars

Nghia Huynh, HBSc, DDSa, Fang-Chi Li, DDS, Cert Endoa, Thuan Dao, DMD, MSc,

DipProstho, PhD, FRCD(C)b, Shimon Friedman, DMDa, Anil Kishen, BDS, MDS, PhDa

a Discipline of Endodontics, University of Toronto, 124 Edward Street, Toronto ON M5G 1G6,

Canada

b Discipline of Prosthodontics, University of Toronto, 124 Edward Street, Toronto ON M5G

1G6, Canada

Corresponding Author:

Dr. Anil Kishen

Associate Professor and Head

Discipline of Endodontics, Faculty of Dentistry, University of Toronto

Email: anil.kishen@utoronto.ca

Contact: 1-416-979-4900 (4468)

Fax: 1-416-979-4936

17

Acknowledgements:

Funding from the American Association of Endodontists Foundation, Research Grant Program,

University of Toronto and Canadian Academy of Endodontics Endowment Fund are duly

acknowledged.

The authors deny any conflict of interest related to this study.

Highlights:

1. This study evaluated the ability of pericervical dentin (PCD) bonding in restoring fracture

resistance in maxillary premolars utilizing digital moiré interferometry and load-at-failure

using cyclic and subsequent continuous compressive loading.

2. Bonding PCD shifted the apical microstrain distribution towards the cervical dentin with

increasing physiologic loads. This finding suggested that bonding of PCD caused

stiffening of cervical root dentin, resulting in redistribution of functional loads away from

the apical region.

3. PCD bonding (Group 2) did not provide a significant increase in fracture resistance

compared to Controls and Group 1 (Unrestored PCD) and highlights the limitations to

pericervical dentin bonding.

Clinical Relevance:

Bonding pericervical dentin with composite resin reduced pericervical dentin bending and apical

microstrain distribution for continuous loads, without providing a significant increase in the

load-to-failure subsequent to cyclic loading. The long-term advantage of bonding pericervical

dentin with composite resin was not established.

18

Abstract

Introduction: Pericervical dentin (PCD) loss may increase root fracture propensity in root-filled

teeth. This study evaluated impacts of bonding PCD with composite resin (CR) on radicular

microstrain distribution and load-at-failure of root-filled maxillary premolars.

Methods: Ten single-canal maxillary premolars, decoronated 2mm coronal to the cemento-

enamel junction (CEJ), had canals enlarged with Protaper Universal instruments to F3. They

were root-filled with gutta-percha, either to CEJ, restored with Cavit (Group 1, n=5), or to 6mm

apical to CEJ, restored with bonded CR to simulate bonding of PCD (Group 2, n=5). Digital

moiré interferometry was used to evaluate pre- and post-operative microstrain distribution in the

coronal and apical root dentin under physiologically-relevant loads (10N to 50N). Another 30

premolars, similarly treated as Groups 1 and 2 or left untreated as controls (n=10/group), were

subjected to cyclic loads (1.2 million cycles, 45N, 4Hz) followed by uniaxial compressive

loading to failure. Mechanical data were analyzed with one-way ANOVA and post-hoc Tukey

test at 5% level of significance.

Results: Microstrain distribution during loading showed bending and compressive patterns at the

coronal and apical root dentin, respectively. In Group 1, microstrain distribution was unaltered.

In Group 2, different microstrain distribution suggested stiffening at PCD. The load-at-failure

values did not differ significantly for Groups 1, 2 and the untreated controls (p>0.4).

Conclusion: Composite resin bonding of PCD might impact the biomechanical responses in

maxillary premolars at low-level continuous loads. The effect of this impact on fracture load

when subjected to cyclic load warrants further investigation.

19

Introduction

With approximately 15 million endodontic treatments performed annually in the United States

(1), the reported 5-10% (2, 3) prevalence of vertical root-fracture (VRF) leading to tooth loss

post-treatment represents a considerable societal burden (2). Of the multifactorial causes of VRF

(4), iatrogenic and non-iatrogenic loss of dentin predisposes teeth to mechanical failure under

functional stresses (4). Typically, when a root-filled tooth is exposed to chewing forces, under

certain conditions, a cumulative process of crack initiation and propagation may occur with time

leading to fatigue failure (4). While crack initiation and propagation induced by engine-driven

canal instrumentation and root filling is currently the focus of research (5), the biomechanical

impact of bonded restoration on instrumented root canal has not been thoroughly explored.

Biomechanics is the study of structure and function of biological systems using the principles of

engineering mechanics (6). The biomechanical response of bulk dentin tissue to functional forces

determines its mechanical integrity and resistance to fracture (7). Micro-crack events leading to

catastrophic fracture are locally strain-controlled (8); thus, assessing the mechanical stress/strain

distribution in root dentin may explain some of the causes of VRF in root-filled teeth (9). Greater

dentin loss generates higher stress concentrations, which significantly compromised fracture

strength of teeth (10). Different degrees of dentin removal may occur during root canal

instrumentation, which may alter the biomechanical response of the remaining root dentin

structure and resistance to VRF (4). Of particular interest in this regard is the pericervical dentin

(PCD), extending 6 mm apical and 4 mm coronal to the crestal bone (11), that distributes

occlusal forces through the long axis of the root (11). Loss of PCD is implicated in weakening of

root structure and decreased resistance to VRF (11).

20

Digital moiré interferometry (DMI) utilizes the principles of optical interferometry to determine

the microstrain distribution in dento-osseous structures with high resolution (12), offering

substantial advantages over conventional microstrain assessments (13). Contrary to conventional

mechanical testing, DMI provides whole-field strain distribution patterns on specimens for low-

level loads within physiological limits. Our group has recently used DMI to assess microstrain

distribution in root dentin in response to root canal instrumentation (4). The aims of this study

were to assess the impacts of restoring PCD with bonded composite resin in root-filled maxillary

premolars on microstrain distribution in root dentin using DMI, and on load-at-failure using

cyclic and subsequent continuous compressive loading.

Materials and Methods

Specimens

The study protocol was approved by the University of Toronto Research Ethics Board. Sample

size calculation considered previous studies (14, 15) on load-at-failure under compressive loads,

where a differences in the range of 22% reached statistical significance with samples of 10

teeth/group. Accordingly, a sample size of 10 teeth/group was used in present study to analyze

data with 80% power and 5% significance.

Thirty human extracted non-carious maxillary premolars with single canals, closed apices and no

cracks/fractures were selected. Teeth were stored in a phosphate-buffered saline solution at 4°C

before testing. Conventional radiographic images of the teeth were captured from two

perpendicular exposures and the teeth characterized for length, and overall dimensions. Three

sets of teeth matched for length and canal dimensions were assembled to minimize variation

among groups.

21

Teeth were decoronated under the dental operating microscope (OPMI Pico, Zeiss, Oberkochen,

Germany) at 10x magnification with a low-speed saw (Isomet, Buehler, Lake Bluff, IL) under

water-cooling. They were sectioned at 2 mm coronal to the average level of the buccal, lingual,

mesial and distal CEJ levels. In the absence of crestal bone, the level of CEJ, located

approximately 2 mm supracrestal in normal periodontal architecture (16), was used as the

reference for measuring PCD in this in-vitro model.

The single root canal was negotiated with a size 10 K-type file (Dentsply Tulsa Dental

Specialties, Tulsa, OK) to the major apical foramen as observed at 4X magnification, and the

working length (WL) was established 1 mm short of this point. Glide path was established with

three PathFile instruments (Dentsply Tulsa Dental Specialties). The canal was enlarged to WL

with ProTaper Universal instruments (Dentsply Tulsa Dental Specialties) to size F3. During

instrumentation, canals were intermittently irrigated with 10 ml of 2.5% NaOCl using a ProRinse

side-vented 30 G needle (Dentsply Tulsa Dental Specialties).

Microstrain distribution

After canal enlargement, specimens were subjected to microstrain assessment with DMI. The

experimental setup for the high-resolution DMI and the process of grating replication were based

on our previous experiments (12). Mesial and distal surfaces of 10 specimens were ground down

equally on wet emery paper of grit sizes 800 and 1200 under constant running water to prepare 3

mm thick, parallel-sided longitudinal sections. High frequency cross grating (f = 1200 lines/mm,

diffraction efficiency of 10% and intensity variation of <15%) was replicated on the longitudinal

root surface with a thin layer of epoxy based adhesive (J-B Marine-Weld, Sulphur Springs, TX),

and was allowed to set for five hours in ambient conditions (22°C, 55% RH). Specimens were

then stored in 100% humidity for 24 hours prior to DMI analysis.

22

Specimens acted as their own control in this experiment. Mounted on a specially fabricated

loading jig (5), the grating-replicated specimen was compressively loaded from 0 N to 50 N with

10 N increments. At each loading interval, whole-field digitized fringe patterns in root dentin

were acquired by a high-resolution charge coupled device camera (12) and recorded as the

control microstrain distribution (after canals were enlarged to F3). Subsequently, the grating

material was gently removed with wet emery paper as described. Specimens were root filled with

thermoplasticized gutta-percha and Pulp Canal sealer (SybronEndo Endodontics, Orange, CA)

by vertical compaction and stored in 100% humidity for 24 hours to allow the sealer to set. In

Group 1 (unrestored PCD; n = 5) canals were root filled to CEJ level and the remaining coronal 2

mm restored with Cavit (3M Deutschland GmbH, Neuss, Germany). In Group 2 (bonded PCD; n

= 5) canals were root filled to 6 mm below the CEJ level, and the remaining coronal 8 mm

restored with dentin-bonded composite resin as follows: 20 sec application and light-cure of

Clearfill DC bond (Kuraray America, New York, NY), followed by Clearfil DC Core Plus resin

(Kuraray America, New York, NY). To minimize voids in the restoration, the dual cured resin

was syringed in place with 2 mm increments and cured. The root-filled specimens were again

grating replicated and subjected to microstrain distribution as described above, which was

recorded for Groups 1 and 2.

Mechanical testing of fracture strength

The remaining 20 specimens were assigned to Groups 1 and 2 as described above. An additional

10 premolars served as untreated controls, having their pulp chambers restored with Cavit (3M

Deutschland GmbH, Neuss, Germany).

All 30 specimens were mounted in custom devices and imaged with a micro-CT scanner

(SkyScan 1172, Bruker MicroCT, Toronto, Ontario, Canada) at 8 µm voxel size (pre-treatment

23

scan) to standardize root canal geometry and volume, and to rule out any dentinal defects. After

instrumentation, specimens were imaged again (post-treatment scan) to capture the enlarged

canal geometry and post-treatment defects. Amira 3D software (FEI, Hillsboro, OR) was

employed for micro-CT image analysis including comparison of pre- and post-treatment canal

volumes (15, 17).

Specimens were mounted on brass rings with the roots embedded in self-curing resin (Justis

Quick Resin, Ivoclar Vivadent, Schaan, Lichtenstein) up to a level 2 mm apical to the CEJ. A 0.5

mm-thick silicone rubber barrier (Aquasil LV, Dentsply Detrey GmbH, Konstanz, Germany) was

applied to the root surfaces to simulate the periodontal ligament. The embedded specimens were

stored in deionized water prior to mechanical testing. Specimens were then mounted in the

Instron Universal Testing Machine (Instron, Canton, MA). In a custom-made water bath, a force

of 45 N was cyclically applied at a frequency of 4 Hz, with a 5 mm spherical crosshead at the

center of the occlusal access cavity, aligned with the longitudinal axis of the tooth (18). If the

sample survived 1.2 million loading cycles without fracture, simulating approximately 5 years of

function (19), they were subsequently loaded with the same spherical crosshead applying a

continuous compressive force at 1 mm/min until fracture occurred (25% drop in applied force).

The load-at-failure (N) was recorded as a measure of fracture strength. Upon completion,

radiographic images of all specimens were again captured from two perpendicular exposures and

characterized for location (cervical, middle, apical) and pattern (comminuted, buccopalatal,

mesiodistal) of fracture.

Analysis

The acquired whole-field moiré fringe patterns were used to evaluate qualitatively, the

microstrain distribution pattern at different regions of interest at the cervical and the apical third

24

of the root dentin (Fig.1A). The in-plane microstrain values in regions of interest were calculated

using the following relationship. Strain (εxx) in the long axis of the tooth is given as ΔU/Δx =

P/Δx, where U is the relative displacement in the x direction between two points, P is the pitch of

the reference grating, and Δx is the fringe spacing in the x direction. The microstrain values were

used to examine the nature of increase in microstrain with loads at the regions of interest in the

cervical and the apical regions of the root for the unrestored and bonded PCD specimens.

For the mechanical testing under cyclic and continuous loading, mean load-at-failure values of

the three groups were analyzed with one-way ANOVA and post-hoc Tukey test at 5% level of

significance.

Results

Microstrain distribution

The control microstrain distribution (after canals were enlarged to F3) exhibited patterns

consistent with bending at the cervical third of the root and patterns consistent with compression

at the apical third. Both coronal and apical microstrain distribution patterns increased with higher

applied loads (Fig. 1A). In Group 1 (unrestored PCD), bending microstrain distribution patterns

in the cervical dentin and microstrain distribution patterns at the apical regions were lower

compared to controls (Fig. 1B). In Group 2 (bonded PCD), the degree of bending microstrain

distribution at the cervical region was obviously lower compared in this group when compared to

controls, as well as the microstrain distribution at the apical third was much lower (Fig. 1C). All

groups showed an increase in microstrain with applied loads at the cervical region (Fig. 2 - Left).

Control and Group 1 (unrestored PCD) demonstrated an increase in microstrain with applied

25

loads at the apical region, whereas Group 2 (bonded PCD) samples had very little change in

apical microstrain (Fig. 2 - Right).

Load-at-failure and fracture patterns

The mean load-at-failure values were highest in Group 2 (1703.74 N) and lowest in Group 1

(1467.65 N) (Fig. 3). Differences among the groups were not statistically significant (p = 0.464).

When comparing the fracture patterns, all specimens exhibited fractures contained in the cervical

region (Fig. 4). In Group 1 (10/10) and the control group (10/10), the pattern of fracture was

consistent with comminuted fractures (Fig. 4 – Left, Middle). In Group 2, fractures in the bucco-

palatal direction and interfacial failures between the composite resin and dentin surface were

noted in all the samples (Fig. 4 - Right).

Discussion

Cumulative loss of tooth structure due to caries, trauma and treatment procedures increases the

risk of tooth fracture (4, 20). To restore fracture resistance in teeth, modern dentistry has shifted

towards bonded restorative procedures (21). Bonded restorations have been suggested to improve

long-term survivability of root-filled teeth (10). Since the modulus of elasticity of composite

resin is close to that of dentin (10), restoring endodontic cavities with composite resin showed

minimal stress jump at the resin-dentin interface, reduced cuspal flexure and lowered probability

of coronal fractures (10, 22, 23). While previous studies have examined the effects of bonded

restorations at the coronal tooth level (23, 24), this study focused on restoration of PCD with

dentin-bonded composite resin and its impacts on biomechanical responses of root dentin,

including microstrain distribution and load-to failure. Tooth selection were based on the reported

26

higher incidence of root fractures in endodontically treated maxillary second premolars (3).

Whereas, application of pre- and post-microCT scans allowed a non-destructive means to

evaluate specimens for intraradicular cracks, as well as to match root canal volume and canal

geometry for specimens utilized in this study (25, 26).

DMI has been previously applied in dental biomechanics (13). It enables examination of dental

hard tissues under physiologically relevant loads to evaluate high-resolution microstrain

distributions over the whole-sample in real-time (13). One of the inherent challenges in the

application of DMI to biological structures is the difficulty of comparing high-resolution

microstrains quantitatively between specimens (6, 17). Nevertheless, the whole-field microstrain

distribution patterns generated in real-time allow for qualitative comparison within samples

subjected to physiological range of forces, which cannot be achieved with alternative strain

measurements methods.

The Instron Universal Testing machine is commonly used to assess resistance to fracture of root-

filled teeth (27), with cyclic loading considered to represent clinical function (19). In this study,

root specimens were subjected to a hybrid analysis comprised of cyclic loading, to simulate

physiological aging, followed by compressive loading, to failure to determine fracture strength

(28). Cyclic fatigue is important when assessing bonded composite resin restorations, because it

may undermine their bond strength leading to increased flexural bending of supported tooth

structure (29) and to bond failures between the restoration and the tooth (30). Because

incomplete bonding, interfacial voids and polymerization shrinkage may all undermine bond

strength (31), we utilized a dual-cured resin which was applied incrementally within the canal

space.

27

In vivo and in vitro stress/strain analyses of teeth have shown that stress was concentrated

towards the cervical region when restored access cavities were subjected to a compressive load

(10). Our results indicated that both the untreated controls and unrestored PCD group

experienced increased cervical and apical microstrain with increasing applied loads. Restoring

PCD with bonded composite resin contributed to a shift in microstrain distribution away from the

apical root dentin towards the cervical dentin, resulting in the greatest increase in cervical

microstrain and least apical microstrain increase. This finding suggested that bonding of PCD

reduced flexing, or caused stiffening of cervical root dentin, resulting in redistribution of

functional loads away from the apical region.

The definition of a true VRF according to the American Association of Endodontists is a

complete or incomplete fracture initiated from the root at any level, usually directed

buccolingually (32). The reduction in apical microstrain achieved by bonding of PCD might be

expected to reduce the propensity to VRF. Although bonded PCD withstood physiological level

loads, and distributed radicular microstrain away from the apical region, under cyclic fatigue

conditions the value of bonding PCD appeared to be limited. It did not effect a significant

improvement in fracture strength, but possibly a modified fracture pattern characteristic of

interfacial failure.

Taken together, the DMI and mechanical testing data suggested that at physiologic level

continuous loads, restoration of PCD with bonded composite resin minimized cervical bending

and apical microstrains, but these effects did not impact on fracture strength of the teeth when

they were subjected to cyclic loads followed by increasing uniaxial compressive loading. This

suggests that bonding to PCD has its limitations when cyclic mechanical loads are applied.

Earlier studies have demonstrated that initial bond strength is greatest at the time of bonding, but

28

durability of the bond is reduced with repeated compressive loads (33, 34). The findings also

corroborated the challenge of strengthening root dentin post instrumentation (35).

In conclusion, this study suggested that, in maxillary premolars, restoration of PCD with dentin-

bonded composite resin impacted a shift in microstrain distribution away from the apical region

towards the pericervical region, when the teeth were subjected to loads in the physiologic range.

This effect, however, did not appear to impact on the load-at-failure of the teeth when subjected

to fatigue cycling followed by compressive loading. The long-term clinical effect of composite

resin bonding of PCD in root-filled teeth requires further investigation.

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15. Krishan R, Paque F, Ossareh A, Kishen A, Dao T, Friedman S. Impacts of conservative

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incisors, premolars, and molars. J Endod 2014;40(8):1160-1166.

16. Albandar JM, Abbas DK. Radiographic quantification of alveolar bone level changes.

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18. De Boever JA, McCall WD, Jr., Holden S, Ash MM, Jr. Functional occlusal forces: an

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29. Janda R, Roulet JF, Latta M, Ruttermann S. The effects of thermocycling on the flexural

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Legends to Figures

Fig. 1A. Typical Unfilled Control image showing U-field moiré fringe patterns in the cervical

and apical regions of interest (denoted by the red boxes) where microstrain values were

calculated. The moiré fringe patterns at the coronal and apical regions are consistent with

bending and compression, respectively.

32

Fig. 1B. A typical specimen from Group 1 (unrestored PCD). Both cervical bending and apical

compression moiré fringe patterns are reduced compared to Fig. 1A.

Fig. 1C. A typical specimen from Group 2 (bonded PCD). Cervical bending moiré fringe

patterns are more uniform along the long axis of the root and decreased compared to Fig. 1A.

Fig. 2. Graphical representation of Cervical (left) and Apical (right) microstrain values in

response to increased applied loads. In Group 2, an increased shift of microstrain away from the

apical region towards the cervical region is evident. In contrast, in Group 1 and the Unfilled

Controls increased microstrain values are evident in both regions.

Fig. 3. Average Load-at-Failure was higher in Group 2>Control>Group 1. P>0.4.

Fig. 4. Typical examples of fracture patterns observed in the three groups in this study, all

contained within the cervical region. Left. Untreated Control. Middle. Group 1 (unrestored

PCD). Right. Group 2 (bonded PCD) showing a pattern consistent with interfacial failure.

33

Fig. 1A.

Fig. 1B.

34

Fig. 1C

Fig. 2.

35

Fig. 3.

Fig. 4.

1652.15

1467.65

1703.74

0

500

1000

1500

2000

2500

CONTROL GROUP 1 GROUP 2

Forc

e (N

)

Average Load at Failure

36

IV. Discussion

The trend towards minimally invasive medicine has had an impact on the direction of dentistry

as well as endodontics. Advances in technology that make less invasive surgical approaches

possible have aided this shift in approach. Such advances as CBCT, the dental operating

microscope and microsurgical devices have the ability to minimize the iatrogenic tooth loss

during endodontic procedures. In conjunction with the current armamentarium, advances in

endodontic instrument design and metallurgy have played a role in reducing the amount of dentin

removed during cleaning and shaping procedures. The consequent dentin conservation is

expected to retain the mechanical integrity of endodontically treated teeth.

Tooth structure loss can be categorized into iatrogenic and non-iatrogenic causes. Non-iatrogenic

causes are usually from the result of the caries disease process that allow for macro loss of tooth

structure. Iatrogenic causes, such as cavity preparation and endodontic procedures involved in

cleaning and shaping have been implicated in the structural weakening of dentin. Ultimately, it is

the cumulative loss of tooth structure associated with caries, trauma and treatment procedures

that increase the propensity for tooth fracture (22). Studies that have investigated root fractures

have mainly focused on the coronal tooth loss as well as strategies to minimize dentin loss from

endodontic access (25, 27). However, no studies have focused on the effects of PCD on the

fracture strength of teeth. Clark et al., have speculated that this region of dentin plays an

important role in redistributing occlusal forces along the long axis of the tooth, and thus through

deductive reasoning, loss of PCD would impact on the fracture strength of endodontically treated

teeth (13-16).

Maxillary premolars were selected for this study as the dental literature shows that these

endodontically treated teeth have a higher propensity for root fracture (4). In this study, the

37

utilization of a decoronated methodology allowed for direct investigation of PCD bonding

without the confounder of coronal enamel bonding. Futhermore, root fractures can be initiated in

the crown due to cuspal flexure from loss of coronal structure, which can confound the role of

bonding PCD on fracture initiation and propagation (25, 43, 84, 86-88).

The application of pre- and post-microCT allowed for a non-destructive means to evaluate

specimens for intraradicular cracks, as well as to match root canal volume and canal geometry

for the selected specimens used in this study (49, 50).

DMI has been previously applied in dental biomechanics (44). DMI offers examination of dental

hard tissues under physiologically relevant loads to evaluate high-resolution microstrain

distributions over the whole sample in real-time (44). The advantage of utilizing DMI is the

direct visualization of whole-field moiré fringe patterns under applied loads. However, one of the

inherent challenges in the application of DMI to biological structures is the difficulty of

comparing high-resolution microstrains quantitatively between specimens (20, 89). Nevertheless,

the whole-field microstrain distribution patterns generated in real-time allow for qualitative

comparison within samples subjected to physiological range of forces, which cannot be achieved

with alternative strain measurements methods.

A dual-cured dental composite resin (Clearfil DC Core Plus – Kuraray Dental, New York, NY)

was used in this study in an attempt to overcome the limitation of light penetration in deeper

cavity preparations, and this attribute provided a means to restore the deeper areas of PCD. The

differences in composition of dental composite resins can alter its physical properties as the

amount and type of fillers can have an impact on the hardness, shrinkage and durability of the

composite resin (83, 90, 91). The manufacture specification sheet for Clearfil DC Core Plus

(Kuraray Dental, New York, NY) indicates that a 74wt% filler content is used to provide

Commented [SF1]: Any reference for this?

Commented [SF2]: Verify that statement is consistent with what you wanted to say

38

improved compressive and flexural strength. Furthermore, the modulus of elasticity of Clearfil

DC Core Plus (Kuraray Dental, New York, NY) is similar to dentin at 6-10 GPa (Kuraray

Medical Inc.). This physical property along with its ideal bonding capabilities theoretically

creates a “monoblock” restoration that ideally restores the physical properties of lost dentin (25).

Restoring endodontic cavities with composite resin showed minimal stress jump at the resin-

dentin interface, reduced cuspal flexure and lowered probability of coronal fractures (25, 75, 84).

The Instron Universal Testing machine is commonly used to assess load-at-failure of root-filled

teeth (60). Cyclic loading has been demonstrated in the dental literature to be considered

important in representing of many years of clinical function (62). In this study, root specimens

were subjected to a hybrid analysis comprised of cyclic loading, to simulate physiological aging,

followed by compressive loading to failure to determine fracture strength (65). A hybrid

mechanical testing method was utilized as literature has demonstrated that composite resin’s

durability is reduced when composite resin is mechanically cycled (92). The simulation of long

term clinical use via mechanical cyclic loading under physiologic loads provides a more accurate

representation on the bond strength of composite over time (42, 91, 93, 94). This is important as

fracture initiation and progression involves a temporal component, and the ability to resist this

progression with a bonded composite resin needed to be simulated with composite fatiguing with

cyclic loading. Therefore, when assessing the bonding durability of composite resin restorations,

cyclic loading is important as it may undermine their bond strength, leading to increased flexure

of supported tooth structure (95), resulting in bond failures between restoration and remaining

dentin (42). Since incomplete bonding, interfacial voids and polymerization shrinkage may all

undermine bond strength (90), the utilization of a dual-cured composite resin was applied

incrementally within the canal space in this study.

Commented [SF3]: Verify that statement is consistent with what you wanted to say

39

In vivo and in vitro stress/strain analyses of teeth have shown that stress was concentrated

towards the cervical region when restored access cavities were subjected to a compressive load

(25). Our results indicated that both the untreated controls and unrestored PCD group

experienced increased cervical and apical microstrain with increasing applied loads. Restoring

PCD with bonded composite resin contributed to a shift in microstrain distribution away from the

apical root dentin towards the cervical dentin, resulting in the greatest increase in cervical

microstrain and least apical microstrain increase. This suggests that bonding of PCD with a

composite resin has the ability to stiffen the PCD to withstand flexing in this area, resulting in

redistribution of functional loads away from the apical region. Similarly, a shift in strain from the

apical towards the cervical region has been demonstrated in maxillary anterior teeth in vivo (20)

and may provide clues on how the biomechanics of endodontically treated teeth is altered by

differing restorative procedures.

The definition of a true VRF according to the American Association of Endodontists (AAE) is a

complete or incomplete fracture initiated from the root at any level, usually directed

buccolingually (96). In addition, VRF is considered to originate from the apical root which then

propagates coronally (96). Previous experimental investigations have displayed increased root

deformation associated endodontic procedures, proportional to the degree of root canal dentin

removal (97). The reduction in apical microstrain achieved by bonding of PCD appears to follow

that of natural teeth (20), thus might be expected to reduce the propensity to VRF. As seen by the

results of the Control and Group 1 samples, microstrain values increased in value in the apical

dentin as well as cervical dentin. This may potentially contribute to VRFs associated with

endodntically treated teeth restored in the conventional manner, ie. GP to the level of the CEJ.

Assuming that VRFs are generated by crack initiation and propagation that originate apically due

Commented [SF4]: Is there a reference suggesting that reduced apical strain reduces propensity to VRF? If not, this sentence needs some kind of support. In the Introduction you mentioned that loss of tooth structure increases propensity, but did not mention the issue of apical strain…

40

to concentrated stresses afforded by the changes in biomechanics associated with endodontically

treated teeth, bonded PCD would favorably reduce the apical microstrain and shift the

microstrain towards the cervical dentin. Therefore, with the shift in microstrain distribution, this

may play a role in alleviating the stresses associated with VRFs associated with endodontically

treated teeth when PCD is bonded. Although bonded PCD withstood physiological level loads,

and distributed radicular microstrain away from the apical region, under cyclic loading

conditions the value of bonding PCD appeared to be limited. Bonding PCD did not effect a

significant improvement in fracture strength, but possibly a modified fracture pattern

characteristic of interfacial failure.

The goal of utilizing composite restorative material to replace dentin is to restore dentin’s

biomechanical properties; however, there are significant challenges in achieving the ideal bond.

Examining the fracture pattern seen in bonding PCD, all samples (10/10) exhibited interfacial

failure between the composite resin and dentin interface. Several factors contribute to the bond

strength of composite resin (76). Although advances in bonding technology have improved,

bonding to dentin and especially to PCD poses some challenges. Bond strength has been

demonstrated to be significantly better when composite resin is bonded to enamel (91). This

difference in bond strength may be attributed to the presence of a water phase within dentin

matrix/tubules (51). The depth of bonding required to restore PCD may be affected by apical

dentin moisture leading to weaker/incomplete dentinal bonding (98). In addition to an inherently

weaker bond strength when bonding dentin, c-factor and polymerization shrinkage have been

shown to affect bond strength in multi-walled cavity preparations (90, 98, 99). These challenges

of bonding PCD impart limitations in achieving the ideal “monoblock” restorative-tooth

interface to restore the biomechanics seen in non-endodontically treated teeth.

41

Considering the findings from DMI and mechanical testing, it could be suggested that restoring

PCD with a bonded composite resin, while shifting the microstrain distribution away from the

apical region of the tooth, did not impart a significant improvement in load-at-failure under

cyclic loads and uniaxial compressive loading. The combined results as a whole suggest that the

biomechanical effect related to bonding of PCD depends on the interfacial integrity of composite

resin-root dentin under cyclic loads. Previous studies have demonstrated that initial bond

strength is greatest at the time of bonding, but durability of the bond is reduced with repeated

compressive loads (92, 93). This may partially explain the current findings. These findings also

highlighted the importance of conserving natural root dentin and corroborated the challenge of

strengthening root dentin post instrumentation using restorative resins (100).

42

V. Conclusion

The risk for fracture of endodontically treated teeth is multifactorial, consisting primarily of loss

of tooth structure as well as the demands of the restorative material used to restore fracture

resistance of the tooth. This study suggested that, in maxillary premolars, restoring PCD with a

bonded composite resin imparted a shift in microstrain distribution away from the apical region

towards the pericervical region when the teeth were subjected to physiologic loads. However,

this effect did not appear to impact on the load-at-failure of the teeth when subjected to cycle

loading followed by continuous compressive loading. The influence of composite resin bonded

PCD on the long-term fracture strength in root-filled teeth needs further investigation.

43

VI. Future Direction

Future investigation into the role of other restorative materials in altering the microstrain

distribution within the root is needed. Whether there is a similar microstrain distribution

difference in other restorative materials such as various types of posts, or ceramics may provide

insight into which restorative material may offer the best biomechanical properties when

restoring endodontically treated teeth. In addition, investigations into the material-dentin

interface will provide valuable insights into the behaviour of the material against radicular

dentin, and whether this material behaviour can alter the biomechanics of dentin in a favorable

manner. Although bonding PCD alters the microstrain away from the apical region towards the

cervical region, future studies are required to evaluate the relevant clinical implications and

whether this microstrain shift has any significant effect on vertical root fracture. In vivo studies

are required to determine the clinical significance of these findings. The role of the periodontium

may play a clinical role in the redistribution of occluding forces that cannot be simulated with in

vitro experiments. Previous studies have shown an improvement in fracture resistance with CEC

(27), yet this study demonstrated that bonding PCD alone imparts no significant load-at-failure

improvement. Therefore, further studies are needed to examine the interplay between remaining

crown tooth structure and PCD. Understanding the relationship between the crown and radicular

dentin and its role in initiating, contributing, or preventing root fractures would be valuable in

formulating directed strategies to better restore endodontically treated teeth. Consequently, this

will provide a basis for understanding the clinical implications of conservation of tooth structure

as it relates to the biomechanical impact of restoring the endodontically treated tooth.

44

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52

VIII. Figures

Fig. 1: Schematic of grating application prior to DMI analysis. The applied grating material acts

as a deformation- sensing element.

Kishen et al., 2001.

Kishen and Asundi, 2000.

53

Fig. 2: Schematic of DMI experimental arrangement. The moiré interferometer consists of two

mutually coherent light beams from a diode laser (wavelength = 670nm), which are

incident on the specimen grating at an oblique angle and generate a virtual reference

grating on 2400 lines/mm.

Kishen 2005, 2006.

54

Fig. 3: An example of a virtual reference grating interacting with the deformed specimen grating

to produce a moiré fringe pattern when the specimen is subject to a mechanical load.