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Effect of Inlays, Onlays and Endocrown Cavity Design Preparation on Fracture
Resistance and Fracture Mode of Endodontically Treated Teeth: An In Vitro
Study
Article  in  Journal of Prosthodontics · November 2020
DOI: 10.1111/jopr.13294
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Effect of Inlays, Onlays and Endocrown Cavity Design
Preparation on Fracture Resistance and Fracture Mode of
Endodontically Treated Teeth: An In Vitro Study
Cynthia Kassis, DDS, MSc, DEA ,1
Pierre Khoury, DDS, DESS ,2
Carina Z Mehanna, DDS, CES,DEA,
PhD ,1
Nadim Z. Baba, DMD, MSD, FACP ,4
Fadi Bou Chebel, DDS, MSc,3
Maha Daou, DDS,
CESA,CESB,DEA, PhD ,5
& Louis Hardan, DDS, CES,DEA, PhD3
1
Department of Esthetic and Restorative Dentistry, Saint-Joseph University, School of Dentistry, Beirut, Lebanon
2
Department of Prosthodontics, Lebanese University, School of Dentistry, Beirut, Lebanon
3
Esthetic and Restorative Dentistry, Saint-Joseph University, School of Dentistry, Beirut, Lebanon
4
Advanced Education Program in Implant Dentistry, Loma Linda University, School of Dentistry, Loma Linda, CA
5
Dental Materials, Saint-Joseph University, School of Dentistry, Beirut, Lebanon
Keywords
Endodontically treated teeth; nanoceramic,
inlays; onlays; CAD/CAM; endocrowns;
resistance to fracture; failure mode.
Correspondence
Cynthia Kassis, Department of Esthetic and
Restorative Dentistry, St- Joseph University,
School of Dentistry, Beirut, Lebanon.
E-mail: cynthia.kassis@usj.edu.lb
Funding: Saint Joseph University FMD137
.
Conflict of interest statement: The authors
deny any conflicts of interest in regards to the
current study.
Accepted November 16, 2020
doi: 10.1111/jopr.13294
Abstract
Purpose: To evaluate the fracture resistance and failure modes of endodontically
treated mandibular molars restored with different designs of inlays, onlays and en-
docrowns.
Materials and Methods: Extracted mandibular third molars (n = 180) were used.
An access cavity was prepared on the occlusal surface of each tooth and the roots
were obturated with gutta percha. All specimens were randomly divided into 6 groups
(n = 30/group) according to the cavity design and the restoration material used. C:
control group without access cavity preparation. IE: MOD inlay preparation with Ev-
erX Posterior (GC Europe) in the pulp chamber. IG: MOD inlay preparation with
G-aenial Universal Flo (GC America) in the pulp chamber. OE: onlay preparation
with EverX Posterior (GC Europe) in the pulp chamber. OG: onlay preparation with
G-aenial Universal Flo (GC America) in the pulp chamber. EC: endocrown with an
empty pulp chamber. All restorations were fabricated with CAD/CAM system using
CERASMART®
(GC Dental products Europe, Belgium) CAD/CAM blocks. Speci-
mens were thermal-cycled and were subjected to a compressive load applied at 30°
angle relative to the long axis of the tooth with a universal testing machine. Results
were statistically analyzed by ANOVA followed by Tukey post hoc tests. Chi-square
test and Fisher Exact tests were used for the comparisons among groups.
Results: The mean fracture strength was significantly different between the groups
(p < 0.001); it was significantly highest for intact teeth, followed by endocrowns (p
= 0.021). The strength was significantly lower for inlays (with G-aenial Universal
Flo and EverX Posterior), intermediate for onlays with EverX Posterior followed by
onlays with G-aenial Universal Flo.
Conclusions: Endocrowns exhibited higher fracture resistance than other tested
composite resin groups. Endocrowns and onlays showed a more favorable failure
mode than inlay restorations.
Endodontically treated teeth (ETT) present higher risk of frac-
ture compared with vital teeth, because of their structural dif-
ferences and loss of tooth structure.1
It is a challenge to regain
the fracture resistance of tooth lost due to cavity preparation.2
The restoration of an endodontically treated tooth must en-
sure the biomechanical performance similarly to an intact
tooth. Structural resistance is related to appropriate retention
and adhesive integration between root dentin, core reconstruc-
tion and final restoration, forming a unique and integrated
complex.3
Post endodontic tooth fractures might occur because of the
loss of the tooth substance during the endodontic access cavity
preparation, root canal instrumentation as well as root canal
filling technique or inadequate post space preparation and
selection.3
In fact, endodontic access cavity preparation was
reported as the second largest cause of loss of tooth structure.4
625
Journal of Prosthodontics 30 (2021) 625–631 © 2020 by the American College of Prosthodontists

Effect Cavity Design Preparation on ETT Kassis et al
The quantity and quality of the remaining tooth structure
should be considered in order to define the best restorative op-
tion for each case, since extensive restorations weaken the re-
maining tooth structure.5
Moreover, tooth weakening is caused
by the loss of strategic internal tooth architecture at the center
of the tooth and the marginal ridges.6
A wide range of cav-
ity designs for mesio-occlusal-distal (MOD) preparations were
proposed for the posterior teeth dependent on the caries extent
and residual tooth intact walls.1,7,8
The depth and design of
cavity preparations are critical factors for fracture resistance.
Preparation of an endodontic access cavity compromises the
strength of a tooth, resulting in an increased susceptibility to
fractures.9
According to the cusp coverage, the types of restorations can
be classified as inlays, which are preparations with no cov-
ered cusps, onlays, where at least one cusp is covered, or over-
lays, where all cusps are covered.2–10
Cuspal coverage results
in increasing the longevity of indirect posterior restorations.
Endocrown assemble the intraradicular post, the core, and the
crown in one component leading to a monoblock restoration.11
It uses the pulp chamber to increase the stability through adhe-
sive cement.12
Minimally invasive cavity preparations for pos-
terior restorations demonstrate the benefit of conservation of
tooth structure and improvement of stress distribution.13
Computer-aided design, computer-aided manufacture
(CAD/CAM) composite resins, and CAD/CAM hybrid ce-
ramics are among materials available for use in the treatment
of missing tooth structure. Hybrid ceramics are polymer
infiltrated ceramic materials that merge characteristics of
ceramic and polymer.14–16
CERASMART®
blocks (GC Dental
products Europe, Belgium) are composite resin nanoceramic
blocks that consist of a polymeric matrix reinforced by ce-
ramic nanohybrid fillers.17
EverX Posterior (GC Europe) is a
composite resin-based material comprised of polyethylene and
glass fibers and indicated for the restoration of endodontically
treated teeth. This composite resin increases tooth strength and
mimic the stress absorbing properties of dentine. It is indicated
to be used as bulk base in high stress bearing areas.18
The aim of this study was to compare the fracture resistance
of different teeth preparation designs on endodontically treated
teeth. The null hypotheses were that the different teeth prepa-
rations will not affect the fracture resistance of endodontically
treated teeth, the filling of the cavity access will not affect the
fracture resistance of endodontically treated teeth, and the dif-
ferent teeth preparations will not affect the localization of tooth
fracture.
Materials and methods
This study was approved by the ethical committee of Saint-
Joseph University (USJ-2017-54). One hundred and eighty
extracted mandibular molars free of cracks and caries with
similar bucco-lingual and mesio-distal dimensions were se-
lected for this study. The teeth were cleaned and stored in
0.2% thymol solution (Merck KGaA, Darmstadt, Germany).
Each molar was embedded in a self-polymerizing resin (Nic
Tone®
, MDC Dental, Jalisco, Mexico), perpendicularly, 2 mm
below the cemento-enamel junction and parallel to the long
axis of the tooth. Following root canal treatment, the access
cavity of each tooth was cleaned using ethylene alcohol to re-
move residual sealer and debris from the walls. All access cav-
ities had approximatively the same sizes.
The specimens were randomly divided into 6 groups of 30
specimens each. Group C is the control group without ac-
cess cavity preparation. Group IE is the MOD inlay prepara-
tion with EverX Posterior (GC Europe) in the pulp chamber.
Group IG is the MOD inlay preparation with G-aenial Uni-
versal Flo (GC America) in the pulp chamber. Group OE is
the onlay preparation with EverX Posterior (GC Europe) in
the pulp chamber. Group OG is the onlay preparation with G-
aenial Universal Flo (GC America) in the pulp chamber and
Group EC is the endocrown with an empty pulp chamber. (Ta-
ble 1) Access cavities of groups IE, IG, OE, and OG were
etched with 37% phosphoric acid (GC Etching Gel; GC Eu-
rope) and the bonding agent (G-premio BOND; GC Europe),
was applied and light-cured for 20 seconds (Satelec Mini Led
curing light, A-dec Inc.). The access cavities of group IE and
OE were filled with fiber reinforced composite (Ever X Poste-
rior, GC Dental) and IG and OG with G-aenial Universal Flo
(GC Dental) and light-cured for 40 seconds. Resin materials
formed a flat restoration wall at the roof of the pulpal cham-
ber. A single operator (CK) prepared a standardized mesio-
occlusal-distal (MOD) cavity preparation on all teeth using a
high-speed tapered diamond bur (6 ˚taper) (Intensiv, Switzer-
land) with copious irrigation. The prepared cavity occupied
one-third of the bucco-lingual distance in order to guarantee a
minimum of 2 mm of buccal and lingual remaining wall thick-
ness, and a horizontal pulpal wall of 3 mm in depth. Proximal
boxes were prepared to create an isthmus with 2 mm depth and
divergent buccal and lingual axial walls. The gingival margin
was located 1 mm above the cemento-enamel junction.
For the onlay preparation, the functional and non-functional
cusps were reduced by 2 mm with 90˚ butt-joint margins
(Fig 1). All restorations were fabricated with CAD/CAM sys-
tem using resin nanoceramic CAD/CAM blocks (Cerasmart,
GC Corp). Preparations were digitized using a Wieland scan-
ner (IvoclarVivadent, Germany). The design of the restorations
was established with similar anatomy and contour with the use
of the Exocad software and saved as Standard Tessellation Lan-
guage (STL) file. The blocks were milled accordingly using
CERASMART®
Flexible Nano Ceramic CAD/CAM blocks
(Cerasmart, GC Corp), with the use of a 5-axis milling machine
(Cerec Inlab MCX5, Sirona, Germany) following the manufac-
turer’s instructions.
Once all the restorations were milled, the intaglio surface of
each restoration was treated with hydrofluoric acid (9% porce-
lain etch, Ultradent), rinsed with water and air dried for then
coated with a silane agent (G Multi-PRIMER; GC Europe) fol-
lowing manufacturer’s recommendations.
All teeth surfaces were etched with 37% phosphoric acid
(GC Etching Gel, GC Europe) for 15 seconds, rinsed with wa-
ter and gently air dried for 10 seconds. A bonding agent (G-
premio BOND; GC Europe) was applied and polymerized for
20 seconds. All restorations were cemented with a dual-cure
resin composite cement (G-CEM LinkAce, GC Europe) ac-
cording to the manufacturer’s instructions. All specimens were
then stored in distilled water at 37 °C for 24 hours before
fracture testing. All teeth were thermal cycled (Thermocycler
626 Journal of Prosthodontics 30 (2021) 625–631 © 2020 by the American College of Prosthodontists

Kassis et al Effect Cavity Design Preparation on ETT
Table 1 Materials used in this study
Material Composition Manufacturer
G-aenial Universal Flo
Flowable, light-cured, radiopaque
resin composite
Resin (UDMA, bis-MEPP and TEGDMA) 31%wt; fillers (silicon dioxide
[16nm], strontium glass [200nm]) 69 wt% and traces of photoinitiator
GC Corp., Tokyo, Japan
EverX posterior
(Fiber-reinforced composite) Bis-GMA, TEGDMA, glass fiber, barium glass, silicone dioxide, PMMA (poly
methylmetacrylate), photoinitiators
GC Corp., Tokyo, Japan
CERASMART Composite resin material (BisMEPP
, UDMA, DMA) with 71 wt% silica and
barium glass nanoparticles
Ceramic type: hybrid nanoceramic GC dental products Europe, Belgium
G-CEM linkAce
Self-adhesive resin luting cement UDMA, dimethacrylate, surface treated silica, silane, synergist GC Corp., Tokyo, Japan
Bis-GMA = bisphenol A diglycidil methacrylate, UDMA = urethanedimethacrylate, TEGDMA = triethyleneglycolimethacrylate, BisMEPP = 2,2-bis (4 methy-
acryloxypolyethoxyphenyl) propane, and DMA = dodecyl dimethacrylate.
Figure 1 Teeth preparations: A, Conventional
inlay preparation; B, Onlay and endocrown
preparations were prepared by reducing the in-
lay preparation 2 mm occlusally.
THE-1200, SD Mechatronik, Germany) in distilled water for
5,000 cycles at 5 °C and 55 °C, with 50 seconds’ dwell time
and 10 seconds’ transfer time. The fracture resistance of each
specimen was tested with the use of a Universal Testing Ma-
chine (YLE GmbH, Walstrabe, 64732 Bad Konig, Germany).
To apply the load on the teeth, a 5 mm diameter stainless-steel
ball was oriented on the center of the occlusal surface at a 30°
angle relative to the long axis of the tooth at a constant loading
rate of 1.0 mm/min. Failure was defined as the load at maximal
load as reported by the Instron universal testing machine, and
force at failure was recorded in Newtons (N) for each speci-
men. The fracture pattern of each specimen was subjectively
evaluated with the use of two classifications. The first classifi-
cation describes the localization of the fracture (Table 2) and
the second one on the prognosis of the tooth (restorable versus
nonrestorable).
Statistical software (SPSS statistics, v25, IBM) was used.
The level of significance was set at p value <0.05.
Kolmogorov-Smirnov tests were used to assess the normality
distribution of continuous variables. Analysis of variance fol-
lowed by Tukey post hoc tests (HSD) were performed to com-
pare the fracture strength between groups. Chi-square tests and
Fisher Exact tests were used to compare the type of fracture
among groups.
Table 2 Classification of fracture patterns
Types of fracture Fracture patterns
Type 1 No visible fracture
Type 2 Fracture restricted to the tooth
Type 3 Fracture restricted to the restoration
Type 4 Fracture of the restoration and the tooth above CEJ
Type 5 Fracture of the restoration and the tooth below CEJ
Results
The fracture loads and fracture modes are presented in Ta-
ble 3. The mean fracture strength (FS) was significantly differ-
ent between the groups (p < 0.001). FS was significantly the
highest for intact teeth, followed by endocrowns (p = 0.021).
The strength was significantly lower for Inlays (with G-aenial
Universal Flo and EverX Posterior), intermediate for onlays
with EverX Posterior followed by onlays with G-aenial Uni-
versal Flo. The difference was not significant between inlays
with EverX Posterior and inlays with Gaenial (p = 1.000).
However, the mean force was lower with onlays with Ev-
erX Posterior compared to onlays with G-aenial Universal Flo
(p = 0.032).
Journal of Prosthodontics 30 (2021) 625–631 © 2020 by the American College of Prosthodontists 627

Table 3 Fracture strength in different groups
n Mean (N) Std Deviation (N) Minimum (N) Maximum (N)
Intact teeth 30 1498.90
d
358.390 830 2200
Endocrown CERASMART 30 1300.53
C
298.167 875 2047
Onlay CERASMART with
30 1065.03
b
233.119 763 1566
EverX
30 930.70
a,b
188.207 707 1400
Inlay Cerasmart with
30 772.10
a
132.388 530 1040
Inlay Cerasmart with Ever X 30 766.90
a
155.806 516 1187
a
Significant difference when compared with Onlay, Endocrown and Intact teeth groups.
b
Significant difference when compared with Inlay, Endocrown and intact teeth groups.
C
Significant difference when compared with Inlay, Onlay and Intact teeth groups.
d
Significant difference when compared with Inlay and Onlay groups and Endocrown.
Table 4 Percentage of recoverable teeth
Recoverable tooth Nonrecoverable tooth
Endocrown CERASMART 28(93.3%) 2(6.7%)
28(93.3%) 2(6.7%)
EverX
28(93.3%) 2(6.7%)
Inlay CERASMART with
18(60.0%) 12(40.0%)
EverX
17(56.7%) 13(43.3%)
EverX
17(56.7%) 13(43.3%)
Teeth 11(36.7%) 19(63.3%)
Table 5 Localization of fractures
Fracture pattern
2 3 4 5
Intact Teeth 30(100.0%) 0(0.0%) 0(0.0%) 0(0.0%)
Inlay with EverX 3(10.0 %) 9(30.0 %) 6(20.0 %) 12(40.0 %)
Inlay with G-aenial Universal
Flo
3(10.0 %) 12(40.0 %) 3(10.0 %) 12(40.0 %)
Overlay with EverX 0(0.0 %) 23(76.7 %) 5(16.7 %) 2(6.7 %)
Overlay with G-aenial
Universal Flo
0(0.0 %) 24(80.0 %) 5(16.7 %) 1(3.3 %)
Endocrown 0(0.0 %) 25(83.3 %) 4(13.3 %) 1(3.3 %)
The restorability of the teeth depends significantly on the
type of restoration (p < 0.001) (Table 4). Only 36.7% of in-
tact teeth were restorable. With EverX Posterior the inlays was
56.7% and 60.0% of inlays with G-aenial Universal Flo were
restorable. The vast majority of teeth restored with CERAS-
MART onlays with EverX Posterior (93.3%) and G-aenial Uni-
versal Flo (93.3%) and 93% of CERASMART endocrowns
were restorable.
The level of fracture depended significantly on the type of
restoration (p = 0.001) (Table 5). Fracture type 3 occurred in
the vast majority of overlays with EverX Posterior (76.7%),
G-aenial Universal Flo (80.0%) and endocrowns (83.3%)
(p = 0.971). However, the fracture in inlays (in the presence of
EverX Posterior or G-aenial Universal Flo, in the pulp cham-
ber) was recorded as: type 2 (10.0%), type 3 (EverX Poste-
rior: 30.0%; G-aenial Universal Flo: 40.0%), type 4 (Inlay with

EverX Posterior: 20.0%; Inlay with G-aenial Universal Flo:
10.0%) (p = 0.705), and type 5 (40.0%). Fracture resistance
varied significantly according to the preparation design.
Discussion
Treatment of endodontically treated teeth (ETT) remains a
great challenge.19
In fact, tooth fracture’s etiologies are mul-
tiple and uncontrollable by dentists.20
The search for an ideal
adequate tooth preparation and the ideal material to prevent
fracture after ETT in order to decrease the impact on the tooth
resistance strength have been constant.21,22
Indirect restora-
tive techniques demonstrated an improved fracture resistance
in endodontically treated posterior teeth compared with di-
rect techniques.23
Materials used to restore ETT must sat-
isfy a balance between preserving tooth structure and max-
imizing the strength of the restoration.11
For these reasons,
3 different preparation designs have been selected in this
study: inlay, onlay and endocrown. Concerning the choice of
CAD/CAM nanoceramics resin, CERASMART showed a sig-
nificant higher mean fracture resistance load value compared
to other materials.16,17
The results of this study showed that
the highest fracture resistance was with intact teeth, which is
confirmed by many studies that showed that ETT had a signif-
icantly lower resistance to fracture.24
Moreover, there was a significant difference between teeth
preparation designs. The highest fracture resistance was ob-
tained with endocrowns. The strength was significantly lower
for inlays and intermediate with onlays. Fracture resistance
varied with different tooth design preparations, thus reject-
ing the null hypotheses. Moreover, fracture localizations and
modes have changed with different preparation design cavities.
These results are in agreement with previous studies where the
highest fracture resistance was recorded with endocrowns.16,25
Fracture resistance of teeth with inlay cavity preparations de-
pends on the remaining wall thickness limits that should be
more than 2 mm.1,26
Moreover, complete occlusal coverage ex-
hibited the most favorable pattern of stress distribution in the
tooth-restoration complex and large preparations require cus-
pal coverage to prevent possible fracture.27,28
In a 3D finite el-
ement analysis study, Yoon et al29
reported that the onlay cav-
ity design protected the tooth structure more effectively than
the inlay design for adhesively bonded leucite-reinforced ce-
ramic restorations. Other studies have shown that endocrowns
are indicated in endodontically treated teeth and that they had
a better fracture strength outcome when compared to conven-
tional restorations.11,25
However, Gré et al30
reported similar
value of fracture resistance with endocrowns and conventional
crowns.
Recently, new types of composite resin materials have been
introduced to replace the dentin and to absorb stress to min-
imize the risk of fracture. It has been reported that materi-
als with low modulus of elasticity tend to absorb stress, con-
centrating it inside the material and not transferring it to the
tooth structure.31
Özkır found that fiber reinforcement im-
proved the fracture resistance of composite resin.32
In another
study, Goracci et al.33
showed that the highest fracture tough-
ness and flexural strength was recorded with EverX Posterior
compared to other bulk fill and conventional resin compos-
ites when their thickness was over 4 mm. However, the re-
sults of this study showed that the fracture resistance of onlays
with chamber cavities restored with G-aenial Universal Flo was
higher than that obtained when chamber cavities were restored
with EverX Posterior. These results rejected the third null hy-
potheses because the choice of the restorative material in the
pulp chamber influenced the fracture resistance. This is also
confirmed by a study that emphasizes the necessity of retentive
slots with EverX Posterior to prevent cuspal fracture.34
Fur-
thermore, the results are in agreement with a study which found
that short fiber-reinforced resin composite material (SFRC) did
not ameliorate the fracture strength compared to conventional
composites.35
Recent studies have reported that the use of SFRC under
onlay restorations is not useful to increase the load resis-
tance capacity. The thickness of this material will affect the
physical properties and the durability.36
The high thickness
of the CAD/CAM restorations and the loading stress gener-
ated during the fracture could have contributed to limit the
behavior of the FRCs substructure.37
Similar to our results,
Keçeci et al24
showed that intact teeth had a high percentage
of “non-restorable” fractures. However, untreatable fractures
were detected at a significantly high level with ceramic MOD
inlays.38,39
This difference would be explained by the presence
in this study of a composite resin filling in the cavity cham-
ber that could have deviated the fracture and ameliorated the
prognosis.40
According to the localization of fracture lines, in-
lay restorations showed mixed type of failure. Consistent with
these results, it was demonstrated that cusps coverage helped
to distribute stress and to localize fracture lines.1
This in vitro study has some limitations in terms of simu-
lating clinical conditions. In fact, clinical fracture results from
fatigue caused by cyclic loading in multiple directions. In this
study, specimens were subjected to a thermal cycling aging
protocol and compressive and axial loads only. Furthermore,
adhesive resin cement is indicated for hybrid ceramics in-
cluding CERASMART®
. But we have used G-CEM LinkAce
which is self-adhesive resin cement. It could be considered
as a limitation by affecting the integrity of tooth-restoration
complex. For that reason, we have pretreated the enamel be-
fore luting to improve the bond strength of self-adhesive resin
cements.41
Conclusion
There were significant differences between the value of frac-
ture resistance of the groups with different restoration. En-
docrowns and onlays showed a more favorable failure mode
than inlay restorations and fracture modes varied with the
different restorations’ designs. With inlay design, the differ-
ence was not significant between EverX Posterior and Gaenial.
However, the mean force was lower with onlays with EverX
Posterior compared to onlays with G-aenial Universal Flo.
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