Effect of Surface Treatments on Zirconia Roughness and Wear of

Opposing Artificial Enamel

by:

Najm Mohsen Alfrisany

A thesis submitted in conformity with the requirements

for the degree of Master of Science

Faculty of Dentistry

University of Toronto

© Copyright by Najm Alfrisany, 2017

i

Effect of Surface Treatments on Zirconia Roughness and Wear of

Opposing Artificial Enamel

Najm Mohsen Alfrisany

Master of Science

Faculty of Dentistry University of Toronto

2017

Abstract:

Yttria-tetragonal zirconia polycrystal Y-TZP is used in dental restorations. Although occlusal

adjustments are often necessary, their effect on Y-TZP surface roughness and tooth wear is

unknown. This study investigated the effect of surface condition (Control polished–CPZ; ground–

GRZ; repolished–RPZ; Glazed–GZ; Porcelain-veneered–PVZ) on zirconia roughness and wear of

opposing enamel (steatite indenters) after chewing simulation (500,000cycles, 80N, artificial

saliva). A surface profilometer was used to evaluate zirconia roughness (μm) and steatite volume

loss (mm3). Representative specimens from each group were characterized by scanning electron

microscopy (SEM). Three-way Analysis Variance (ANOVA) and Tukey HSD (p≤0.05) showed

that there was a significant interaction effect (p<0.001) between surface condition and CS. CS

significantly increased roughness of GZ specimens. GRZ showed a smoother surface after CS.

Two-way ANOVA and Tukey HSD indicated higher volume loss for indenters abraded directly

against zirconia, as opposed to those abraded against porcelain and glaze material.

ii

TABLES OF CONTENTS

1. Chapter 1: Introduction ---------------------------------------------------------------------------------1

2. Chapter 2: Literature review ---------------------------------------------------------------------------4

2.1 Parameters and methodologies for wear evaluation -------------------------------------------6

2.1.1 The design of wear tests ---------------------------------------------------------------------7

2.1.2 The relevance of wear test parameters--------------------------------------------------8

2.1.2.1 Force -----------------------------------------------------------------------------------8

2.1.2.2 Distance ------------------------------------------------------------------------------10

2.1.2.3 Contact time -------------------------------------------------------------------------10

2.1.2.4 Sliding movements -----------------------------------------------------------------11

2.1.2.5 Antagonist surface ------------------------------------------------------------------11

2.1.3 Two-body and three-body wear test ---------------------------------------------------13

2.1.3.1 Two-body wear----------------------------------------------------------------------14

2.1.3.2 Three-body wear --------------------------------------------------------------------14

2.1.3.3 Three-body wear machines --------------------------------------------------------15

2.1.4 Latest technologies employed in wear measurements ---------------------------17

2.2 Surface properties and wear------------------------------------------------------------------18

2.2.1 The correlation between roughness, hardness and wear--------------------------18

iii

2.2.2 The correlation of aging and wear---------------------------------------------------19

2.3 Study of wear in dentistry--------------------------------------------------------------------21

2.3.1 Systematic reviews on wear evaluation---------------------------------------------21

2.3.2 Effect of occlusal adjustments -------------------------------------------------------21

2.3.3 In vivo zirconia wear ------------------------------------------------------------------22

2.3.4 In vitro zirconia wear------------------------------------------------------------------23

3. Chapter 3: Objectives and hypotheses----------------------------------------------------------------25

3.1 Objectives---------------------------------------------------------------------------------------25

3.2 Hypotheses-------------------------------------------------------------------------------------25

4. Chapter 4: Materials and methods---------------------------------------------------------------------26

4.1 Materials---------------------------------------------------------------------------------------26

4.2 Sample preparation---------------------------------------------------------------------------26

4.3 Experimental groups-------------------------------------------------------------------------28

4.4 Surface treatment-----------------------------------------------------------------------------29

4.5 Roughness measurement --------------------------------------------------------------------34

4.6 Chewing simulation (artificial aging) -----------------------------------------------------36

4.7 Evaluation of antagonist wear --------------------------------------------------------------38

4.8 statistical analysis-----------------------------------------------------------------------------38

iv

4.9 Scanning Electron Microscopy (SEM) --------------------------------------------------- 38

5. Chapter 5: Results ---------------------------------------------------------------------------------------40

5.1. Roughness evaluation ----------------------------------------------------------------------40

5.1.1 Analysis of results------------------------------------------------------------------------40

5.1.2 Effect of aging and surface treatment on the roughness of zirconia surface -----42

5.2 Wear evaluation -------------------------------------------------------------------------------48

5.2.1 Analysis of results ------------------------------------------------------------------------48

5.2.2 Effect of zirconia surface treatments on the wear of opposing artificial enamel -----48

6. Chapter 6: Discussion -----------------------------------------------------------------------------------51

7. Chapter 7: Conclusion ----------------------------------------------------------------------------------58

7.1 Future directions -------------------------------------------------------------------------------58

8. References ------------------------------------------------------------------------------------------------59

1

Chapter 1. Introduction

Yttria-partially stabilized tetragonal zirconia polycrystal (Y-TZP) is frequently referred to

as zirconia. Metal-free dental restorations made of zirconia are one of the most attractive treatment

for teeth requiring indirect restorations, given zirconia’s biocompatibility, white color and high

mechanical properties (Beuer et al., 2012). Zirconia has been the material of choice for frameworks

in indirect restorations as opposed to metal, but because of its whitish and opaque color the coping

needs to be veneered with porcelain, and these restorations are frequently referred to as bi-layer

restorations (Tuncel et al., 2013). One of the most common clinical failures found in veneered

zirconia (bi-layer) restorations is the cohesive fracture within the veneering porcelain (Guess et

al., 2008).

An alternative way to avoid veneer fracture is to exclude the veneering material and to

manufacture monolithic crowns entirely made from zirconia (Long, 2012). This is possible by

using computer-aided-design/computer-aided-manufacture (CAD/CAM) - technology, where

monolithic crowns can be milled in full anatomical contour. The milling of monolithic crowns has

other advantages such as minimized manufacturing time and improved cost-effectiveness. Instead

of building up the porcelain in several layers and firing in multiple firing cycles, the monolithic

restoration can be personalized by using some staining techniques (Johansson et al., 2013).

Monolithic zirconia restorations have been gaining popularity as opposed to the use of bi-

layer restorations (Raigrodski, 2013) because of the improved translucency of the CAD/CAM

zirconia blocks. However, the outstanding mechanical properties of zirconia, such as high hardness

and wear resistance, may have an effect on the opposing natural dentition, especially when the

restoration’s surface is not perfectly smooth (Stawarczyk et al., 2013). Under clinical conditions,

2

monolithic zirconia crowns seem to cause more wear to the opposing enamel than the human

enamel itself (Stober et al., 2014). This is even more problematic when occlusal adjustments of

monolithic zirconia crowns are required after installation of the prosthesis. Grinding with a

diamond bur is known to increase surface roughness of zirconia (Preis et al., 2015). While this

roughness can be reduced depending on the type of material and technique applied for polishing

(Lawson et al., 2014; Rashid, 2014), there is no standard method for polishing monolithic zirconia

crowns yet (Lawson et al., 2014). A glaze layer applied on the surface of zirconia restorations is

easily removed when occlusal adjustments are done or under physiological occlusal loading

(Lameira et al., 2017).

Interestingly, a recent paper showed that the surface wear of artificial enamel was not

affected by the roughness of different brands of zirconia after grinding and polishing (Preis et al.,

2015). However, Amer et al. (2014) observed that material type and surface treatment are

significant factors that affect wear of opposing human enamel. When different materials were

compared, zirconia and feldspathic porcelain resulted in similar wear to the opposing enamel,

which was significantly higher than the wear caused by lithium disilicate ceramic (Amer et al.,

2014). A study performed by Lameira et al. (2017) showed that polished zirconia crowns cause

significantly higher wear of artificial enamel than glazed zirconia crowns after 2.5 million cycles.

If there is no esthetic demand, a highly polished zirconia is recommended over a glazed one

because the glaze layer might wear away during function thus exposing the underlying unpolished

surface to the opposing dentition. Nevertheless, if there is esthetic demand in the area, the zirconia

should be highly polished and then glazed (Janyavula et al., 2013).

The applicability of the data available on recent wear and roughness studies is questionable,

since low number of cycles (Jung et al., 2010) or low loading force (Stawarczyk et al., 2013) are

3

limitations of the methodologies used, compromising the relevance of the roughness data collected

(Luangruangrong et al., 2014). Small sample size is also a compromising factor (Lameira et al.,

2017). Lack of correlation between number of cycles and time of clinical use may also make the

interpretation of the findings difficult. Therefore, the wear of the opposing enamel caused by

monolithic zirconia restorations with or without occlusal adjustments still needs to be better

investigated.

4

Chapter 2. Literature Review

The wear between human enamel and opposing restorative materials involves a very

complex mechanism and the selection of a material for minimal wear of opposing enamel is not

always easy. Wear and loss of enamel are unavoidable and may lead to the need for complex

restorative procedures. In fact, the loss of enamel could cause instability of the occlusal balance as

well as unfavorable effects on both natural and artificial dentition (Ramp et al., 1999). A smooth

restoration surface is needed to avoid dental complications such as wear of the opposing dentition,

plaque accumulation and other oral problems (Ghazal et al., 2009). It is also needed for the patient

comfort (Ghazal et al., 2009). The two factors that play a role in the wear of the opposing dentition

are roughness and hardness, the rougher the surface and the harder the material the greater will be

the wear volume (Anusavice, 2003).

Generally speaking, ceramic materials are known to cause more abrasive wear of human

enamel than other restorative materials, but little information is available concerning the effect of

surface roughness of monolithic zirconia against the natural dentition (Ramp et al., 1999; Heintze

et al., 2008). The use of different finishing techniques such as diamond burs and polishing kits

may remove the glazing material of the restoration surface and lead to even more surface roughness

(Rashid, 2014; Al-Wahadni & Muir Martin, 1998; Al-Hiyasat et al., 1997). It is considered

important to re-establish the smooth or glazed-like zirconia surface using appropriate polishing

instruments to minimize the effect of occlusal adjustments on the wear of the opposing dentition

(Ramp et al., 1999; Heintze et al., 2008).

To quantify wear between ceramic materials and tooth structure, many materials have been

used as test substrates. Human enamel may be considered the ideal antagonist to be used in terms

5

of achieving the real clinical conditions and that is due to its complexity in terms of morphological

and structural properties. However, many other materials with similar composition have been

widely used as experimental antagonists such as steatite styluses (Steatite: synthetic material

mainly composed of Magnesium Silicate), ceramic styluses and bovine teeth, which makes it easier

to standardize the shape of the antagonist and more precisely quantify wear. In regard to the recent

approach of standardizing enamel cusps by grinding, studies do not mention changes in the wear

volume of the standardized enamel as opposed to the unstandardized enamel antagonists (Krejci

et al., 1999; Heintze et al., 2008) Even though steatite may not be considered the ideal alternative

to human enamel in terms of its tribological and mechanical properties, the appropriateness of the

steatite material as an antagonist for in vitro wear studies has been well documented (Wassell et

al., 1994a; Wassell et al., 1994b; Hahnel et al., 2009). However, there is no agreement in the

literature as to which material should be used as the antagonist in wear simulation tests (McCabe

et al., 2002; Heintze et al., 2006)

Two-body and three-body wear tests may be used to investigate the wear characteristics of

a given material. Two-body wear represents the attrition between two opposing surfaces, and the

three-body wear methodology adds the third body, represented by food particles, which changes

the distribution of the occlusal forces (Kwon et al., 2015). When results of two-body and three-

body wear tests are compared, the three-body design results in lower wear in comparison to the

two-body design (Amer et al., 2014). However, the direct comparison of the two methodologies

in one experimental study (McCabe et al., 2002) showed that the wear pattern caused by the two

designs is not comparable, and that restorative materials perform differently depending on the

methodology applied (McCabe et al., 2002).

6

To be realistic, quantification of wear should be performed in vivo, but the lack of control

over important variables such as chewing force, dietary intake, or environmental factors (humidity

and temperature ranges) limit the contribution of in vivo studies to the evaluation of the wear of

different materials (Esquivel-Upshaw et al., 2012; Mundhe et al., 2015). Additionally, in vivo

studies are time-consuming, expensive, and very slow in generating precise results when compared

to in vitro studies (Silva et al., 2011). Therefore, the effect of different parameters and

methodologies on the in vitro wear of ceramic materials will be discussed in this literature review.

2.1 Parameters and methodologies for wear evaluation

Tribology is the science or the study of the mechanisms of friction, lubrication and wear

of interacting surfaces that are in relative motion. By definition, friction is the force resisting the

relative motion between rubbing surfaces. Wear, however, is a process that occurs at any time

when a surface is exposed to another surface or when a chemical reaction exists (Powers & Bayne,

1992; Heintze et al., 2005; Zhou et al., 2008), due to the gradual removal of the material from the

surfaces through mechanical or chemical actions. Biomechanically, the oral function can result in

some tribological movements that involve teeth and restorations (Mair et al., 1992; Mair et al.,

1996; Oh et al., 2002).

A wide range of devices for tribological testing of dental materials have been employed

aiming to generate an automated simulation of the mastication movements in an automated motion

(Heintze et al., 2012). Even more sophisticated simulators have been developed to partially mimic

clinical conditions as a way to predict the in vivo performance of restorative dental materials after

mastication against natural enamel and other restorative materials (Sajewicz et al., 2007; Heintze

et al., 2012).

7

2.1.1 The design of wear tests

The quantification of clinical wear is complicated, expensive, and time-consuming

(Janyavula et al., 2013). Previous studies show relatively high standard deviations due to the

biological spread between the studied individuals in terms of different factors that can affect the

design of the test (Mitov et al., 2012; Janyavula et al., 2013).

Variations in the methodological design have also been observed. Some examples are:

wide range of occlusal forces, from 25 N to 150 N (Heintze, 2006; Jung et al., 2010; Kim et al.,

2012; Janyavula et al., 2013) number of cycles varying from 120,000 to 1,000,000 (Heintze, 2006;

Jung et al., 2010; Kim et al., 2012; Janyavula et al., 2013) frequency of cycles (Heintze, 2006)

low number of specimens (Lameira et al., 2017) and the antagonist materials (Wassell et al., 1994;

Krejci et al., 1999; Heintze et al., 2008;). There is also discrepancy in estimating the correlation

between number of cycles and clinical service. For example, Steiner et al. (2009) describe

1,200,000 cycles as being relative to 5 years of oral function, while some other authors estimate

200,000 cycles as corresponding to one year of service (Wiskott et al., 1995; Kelly, 1999).

It is believed that higher number of cycles can generate more wear (Chou & Lopez, 2006),

but how this can correlate to the clinical service of a given material is not clear yet. Furthermore,

most in vitro wear studies initially show a steep increase in wear prior to reaching a plateau phase

(Heintze, 2006; Zhou et al., 2013; Lawson et al., 2014). Monasky & Taylor, (1971) and Mulhearn

et al. (1962) explained that decrease in the wear by the dulling of the abrasive properties of the

surface evaluated.

In terms of substrate, some studies use modified cusps of molars as antagonists (Clelland

et al., 2001; Clelland et al., 2003; Amer, 2014). The cusps are modified aiming at standardizing

the shape and the size of the antagonist specimens, allowing for similar distribution of forces to

8

the opposing surface. However, the use of un-modified human enamel would be the most realistic

alternative as antagonist against crowns to evaluate the wear of tooth structure and ceramic

materials (Al-Hiyasat et al., 1997), but high standard deviations and different wear patterns are

associated with this type of antagonist (Sabrah, 2011).

2.1.2 The relevance of wear test parameters

Although the outcomes of in vitro wear studies are somewhat not directly correlated to the

findings of the clinical data, well designed and controlled experiments are paramount to

characterize the performance of materials under cyclic mastication forces (Kim et al., 2012). As

human tooth’s and restorative material’s wear occur very slowly, it may need years to become

measurable in vivo. Defining the main components of a wear simulator and the relevant outcome

variables is important to understand the shortcomings of the current wear methodologies.

Literature showed that the wear test set-up has to fulfill various requirements (Heintze et al., 2006),

such as force, vertical displacement, contact time, horizontal sliding movement and antagonist

surface.

2.1.2.1 Force

Wear testing simulators should generate clinically relevant forces, which are in the range

of 20-120 N. Studies on human beings who chewed on different food items revealed that this is

the vertical biting force range in the molar area (Schindler et al., 1998). There is considerable

variation of the mastication load presented in the dental literature, and the difference between the

average masticatory forces and maximum biting forces should be noted. The maximum biting

force, which has been reported to be around 400–890 N in the molar area (Anusavice, 2003;

Broadbent, 1999), is much higher than regular masticatory forces (Table 1), and posterior teeth

9

(molars and premolars) are exposed to higher masticatory forces than anterior teeth (Ehrlich &

Taicher, 1981; Riise & Ericsson, 1983; Korioth et al., 1990; Korioth et al., 1997; Kumagai et al.,

1999; Ciancaglini et al., 2002; Hattori et al., 2009). Ferracane et al. (2003) have shown that higher

forces produce higher wear, but this increase in wear is not linear to the increase of loading

(Ferracane et al., 2003). In order to best represent the worst case scenario for materials under

investigation, wear simulators should reproduce masticatory forces of posterior teeth (Table 1).

Forces have varied from 15 N (De Gee & Pallav, 1994) to 75 N (Leinfelder et al., 1989; Leinfelder

et al., 1999) depending on the wear simulator used, and this can affect the outcomes of the wear

studies. A review carried out by Lawson et al. (2013) analyzing the physiological parameters of

clinical wear showed that forces varying between 20 N and 140 N are valid for the simulation of

the masticatory forces.

Test Method Measured Forces Source

Rubber sheet sensors with varying

hardness (molar area) 100 N (hard), 126 N (harder), 140 N (hardest) [max] 28

Encapsulated load cell placed

between teeth (molar area) 50-100 N (elastic), 20-50 N (plastic) and 30-60 N (brittle) [mean] 29

Strain gauge in denture tooth 3-17.5 N [mean] 49-78 N [max] 32

Strain gauge in bridge pontic area 20 N (potatoes) and 60 N (carrots) [mean] 33

Strain gauge in single prosthetic

molar 92 N (meat), 126 N (carrots), 129 N (biscuit) [max] 34

Strain gauge in frame around natural

molars 21.7 N (cookies) and 59.5 N (carrots)[mean] 36

Table 1. Physiologic forces of mastication. (extracted from Lawson et al., 2013)

10

2.1.2.2 Distance

An in vivo study of the mastication of solid food textures observed that the highest values

of the vertical movements were measured to be between 16 and 20 mm, and these values decreased

continuously in correlation to the number of chewing strokes and remained constant between 12

and 16 mm depending on the type of food (Schindler et al., 1998). It is very unlikely that vertical

dislodgement between upper and lower teeth would affect wear and this variable can indeed

significantly increase the time needed for the application of masticatory cycles. Therefore, vertical

dislodgement is often kept to a minimum.

2.1.2.3 Contact time

The contact time between stylus and material should be of a clinically relevant duration

and should be stable during the simulation phase. Studies of human who chewed on different food

showed that the actual contact time ranges between 400-600 ms per chewing cycle (Schindler et

al., 1998; Kohyama et al., 2004; Heintze, 2006), but there has been no study so far indicating the

effect of contact time on wear of restorative materials.

2.1.2.4 Sliding movements

The masticatory motion is a dynamic process that may affect the outcomes of the wear test

and can be explained by two steps: attrition and abrasion. Attrition starts when two teeth are in

contact with each other, and ends when the teeth are separated. Abrasion occurs when the upper

and the lower teeth are in contact with a third body (food particles) and ends when the food

particles are eliminated and the upper and lower teeth become in contact (DeLone, 2006). The

average sliding movement in vivo was measured to be 0.3 mm in the first molar towards the

11

anterior and 0.18 mm towards the medial side (Gibbs et al., 1981; Kohyama et al., 2004). Some

chewing simulators offer a range of sliding distances for in vitro testing, while some devices only

include vertical forces, which are translated into impact, without any sliding movements. Yap et

al (1997) observed that wear tests with vertical movements only cannot be on a par with wear

methods combining vertical and horizontal sliding, mainly because the single vertical

dislodgement of the antagonist does not reproduce either attrition or abrasion.

2.1.2.5 Antagonist surface

Many surfaces can act as an antagonists in the in vivo condition, including metal, ceramic

crowns, and human enamel, but enamel is considered as the ideal standard for both in vivo and in

vitro wear studies (Lawson et al., 2013). Nonetheless, it has been reported that less dental wear is

observed on natural enamel-to-enamel contacts than between enamel and any opposing restorative

material (Etman et al., 2008). However, the use of natural enamel also has some limitations due to

its inhomogeneity and the differences in morphology and size of teeth (Maas, 1991; Shortall et al.,

2002). A pointed stylus, for example, produces more wear than a ball-shaped stylus, as the latter

has a greater contact area between stylus and material than a sharp one (Preis et al., 2012;

D’Arcangelo et al., 2016).

The results of previous in vitro studies in which a specific material as an antagonist of the

human enamel were examined have been questioned, due to the fact that the test parameters

differed widely between different tests (Kim et al., 2012). Most studies use flat polished ceramics

and prepared enamel cusp specimens from extracted molars as their antagonists (Hacker et al.,

1996; Ramp et al., 1997; Ramp et al., 1999). The test chambers are filled with water and sliding

movement is integrated in the wear generating processes (Jagger & Harrison, 1994; Jagger &

Harrison, 1995; Ramp et al., 1997; Ramp et al., 1999).

12

Previous studies showed that feldspathic porcelain (low-fusing and conventional) and

natural enamel cause similar wear to the opposing enamel (Ratledge et al., 1994; Clelland et al.,

2001). But when porcelain type was compared, studies demonstrated that low-fusing porcelain

results in less wear to the opposing enamel than conventional porcelain (Derand & Vereby, 1999;

Metzler et al., 1999). Data variability is similar when antagonists of either ceramic or steatite

(MgOSiO2) are compared (Shortall et al., 2002). Steatite has been frequently used as antagonist

alternative to human enamel since many of its mechanical and physical properties are similar to

those of human enamel (Wassell et al., 1994; Mehl et al., 2007).

Study designs where ceramic-to-ceramic surfaces are evaluated have also been used in in

vitro wear testing of dental ceramics. The use of more standardized samples result in decreased

variability of the data when compared to opposing natural enamel, not only due to a better

standardization of the antagonists’ shapes but also because knowing the mechanical properties of

both indenters and surface allow researchers to better correlate the effect of each material on the

wear of the antagonist (Heintze et al., 2008). However, this strategy does not reproduce the clinical

scenario most frequently observed. While a variety of materials have been used as the antagonist

in wear simulator tests, there is no agreement in the literature about which of these materials should

be considered as the ideal and standard choice for wear simulation tests (Heintze, 2006).

13

2.1.3 Two-body and three-body wear test

The main advantage of an in vitro wear design is to identify the basic mechanisms leading

to the undesirable and uncontrolled wear of either tooth structure or restorative material (Amer et

al., 2014). The in vivo dynamics of wear oscillates between three-body and two-body wear

patterns, due to the breaking and dissolution of food particles with the action of cyclic masticatory

loads and saliva. When analyzing the effect of different food particles on wear features on the

occlusal surface of teeth, Maas (1994) carried out compression tests using abrasive grit particles.

The results of this study showed that larger particles produce more wear than small particles which

may be related to the better distribution of forces when small particles are used (Maas, 1994).

Eisenburger & Addy, (2002) found that the amount of load significantly influenced enamel

wear by attrition both in acidic and neutral conditions, but Zhou et al. (2013) stated that wear

seemed to be independent of the loading forces. Although such studies have taken the effects of

food particles and normal load on tooth wear, their focus was on the volume loss, rather than

investigating the wear mechanism in details (DeGee et al., 1986; Eisenburger & Addy, 2002).

Two-body and three-body wear tests seem to be related solely to different methodologies

to evaluate wear, but in fact there is no clear correlation between the results of three-body and two-

body in vitro studies (Condon & Ferracane, 1997). Eight wear testing methods have been

described by the International Standards Organization (ISO - 14569-2), and among these only three

wear testing methods contain a third body (ISO, SO/TS 14569-2:2001).

14

2.1.3.1 Two-body wear

During masticatory function, abrasion results from the sliding forces of one tooth against

another, and the teeth involved are known as first and second bodies. This tooth-to-tooth contact

without the existence of a third body is localized and restricted to one occlusal contact area between

the two teeth (Pallav, 1996; Lambrechts et al., 2006).Several two-body wear simulators have been

designed and used with various degrees of success and limitations in comparison to clinical trials.

Examples of those machines are: capsule, compule concept, two-body abrasion single-pass sliding,

two-body wear rotating counter sample, pin-on-disk tribometer, abrasive disk, etc. (Zhou et al.,

2013).

A two-body wear study performed by Albashaireh et al. (2010) showed that zirconia

samples worn significantly less than other ceramics used in the study in terms of both vertical and

volumetric loss. Stawarczyk et al. (2013) showed that among the zirconia groups used in the study,

glazed zirconia caused the most wear to the antagonist enamel while polished zirconia caused the

least wear, but interestingly developed higher incidence of enamel cracks. The two-body wear

results from Bai et al. (2016) also demonstrated that highly polished zirconia causes the least wear

of the antagonist enamel when compared to glazed zirconia. The authors also reported that the

glazed zirconia surface presented chipping fracture and cracks, which indicated a combination of

fatigue and/or abrasive wear.

2.1.3.2 Three-body wear

In the three-body wear test, abrasion is caused by forceful sliding of one tooth on another

with the existence of food particles or other medium acting as the third body. When the occlusal

surfaces are separated with a third body, the abrasive particles act as a slurry abrading the whole

contact surface (Krejci et al., 1990; Satou et al., 1992). Three-body wear simulators are designed

15

to get as close as possible to the clinical wear that occurs between surface of materials and teeth,

when bodies slide against each other with food particles between them (Kadokawa et al., 2006;

Kwon et al., 2015).

Three-body wear design has been used to evaluate the effect of ceramic restorations on

natural tooth structure. Sorensen et al. (2011) evaluated different full ceramic crowns against

natural enamel using poppy seeds as the abrasive medium. Feldspathic porcelain caused the most

wear to the opposing enamel, while lithium disilicate caused the least, which was similar to gold.

Amer et al. (2014) investigated the effect of different ceramic surface finishing techniques on the

wear of the opposing standardized enamel using a food slurry in between. Results demonstrated

that polished zirconia caused the least wear to the opposing enamel while glazed zirconia caused

the most.

2.1.3.3 Three-body wear machines

Three-body wear simulators often have a chamber where a third body exists. This third

body act as a layer between the two surfaces in contact. The food particles, for example, are

restrained during the whole masticatory movements. Many three-body wear simulators have been

widely used in previous studies, with some variation in terms of stylus, medium, type of

movements, number of cycles, and set up of the samples. The most common systems with their

available parameters are shown in Table 2 (De Gee et al., 1986; Krejci et al., 1990; Satou et al.,

1992; Pallav, 1996).

16

Wear

machine Stylus Medium Movement Force Frequency Cycles Set-up

ACTA wear

machine

A textured and

hardened steel

counter-wheel

Rice/millet

seed shells

suspension

Sliding

15-20 N

adjustable

load 0-50

N

1 Hz 100,000-200,000

cycles

Sample

chamber

holding up to

12 samples

OHSU: Oregon

Health Sciences

University Oral

Wear Simulator

Enamel-

conical

Poppy

seeds+PMMA

beads

Impact

+

sliding

Abrasion

load 20 N

and attrition

load 70 N

1.2 Hz 50,000–100,000

cycles

Multi-mode

simulator

University of

Alabama Wear

Simulator

Polyacetal-

conical PMMA beads

Impact

+

sliding

75.6 N

vertical

1.2 Hz 100,000–200,000–

400,000 cycles

Four stations

device

Zurich computer-

controlled

masticator

Enamel

water +alcohol

+ tooth

brushing

Impact+sliding

Lateral

movement:

0.2mm

49 N 1.7 Hz

120,000, 240,000,

640,000 and

1,200,000 load

cycles

Masticator

Many substrates are employed to simulate the food bolus or slurry during the reproduction of the

masticatory movement in three-body wear tests. Examples of simulated-food mediums are: slurry

of water and unplasticized polymethylmethacrylate beads, polymethylmethacrylate powder,

hydroxyapatite slurry, green carborundum slurry, soft (CaCO3) abrasive, hard (SiC) silicon carbide

abrasive, millet-seed/PMMA-beads (De Gee et al., 1986; Krejci et al., 1990; Satou et al., 1992;

Pallav, 1996). The large variety among the studies using slurry as a third body in terms of materials,

cycles, and forces used, however, compromise the reproducibility and comparison of the reported

results.

Table 2. Examples of equipment used for three-body wear simulation, and parameters available.

17

2.1.4 Latest technologies employed in wear measurements

In recent years, the number of studies involving dental wear quantification has increased

and includes optical imaging (Amer et al., 2014), laser scanning (Albashaireh et al., 2010; Ghazal

et al., 2009), surface mapping and profilometry technique.

Profilometry analysis may involve contact and non-contact measurements, where a laser

or mechanical stylus is used to trace the surface and record a three dimensional and/or two

dimensional data (DeLong, 2006). Three-dimensional data leads to quantification of wear by

volume (Magne et al., 1999; Albashaireh et al., 2010), while two-dimensional data collection leads

to assessment of wear by vertical loss (Sabrah, 2011; Stober et al., 2016). Burgess et al. (2014)

used a non-contact light profilometer and observed that the main advantage of using non-contact

profilometry is the shorter time needed for the reading (DeLong, 2006; Heintz, 2016). Surface

scanning with a laser in the non-contact profilometry technique requires an opaque surface that

minimizes reflection of the laser shots. Additionally, some non-contact and laser scanning

techniques require a plastic replica or an impression of the worn surface for the scanner to be able

to assess wear (Heintze et al., 2012; Janyavula et al., 2013). Contact profilometers, on the other

hand, present more accurate results, and there is no need for coating, but damaging of the surface,

especially if it is not hard enough, may occur (DeLong, 2006; Stawarczyk et al., 2013).

A possibility to assess in vivo wear is to use an intra-oral optical scanner and automated

3D computer vision (Meireles et al., 2016). Different times of application of phosphoric acid on

the surface of natural enamel and wear analysis by an intra-oral scanner showed the technique to

be accurate, generating precise data without the need for replication of the samples (teeth) to be

evaluated. Authors noted that the approach may also be used to asses wear in vitro.

18

2.2 Surface properties and wear

2.2.1 The correlation between roughness, hardness and wear

Selective restorative materials are known to induce tooth wear. Moreover, surface

roughness and damage of the surface of restorative materials have been known to play an important

role in the wear process. Rough ceramic surfaces can cause more opposing tooth wear than smooth

ceramics (Ghazal et al., 2009). Therefore, occlusal adjustments significantly affect the wear

behavior of ceramic restorations (Güngör et al., 2015). As a result, abrasion between ceramics and

natural teeth remains a cause of concern and the resultant wear may be influenced by both, surface

roughness and the hardness of the restorative material (Tang et al., 2015). Zirconia is a material

with high hardness values, estimated to be about four times higher than that of human enamel

(Chun & Lee, 2014) and steatite, since steatite has mechanical and physical properties that are

similar to those of human enamel (Wassell et al., 1994; Mehl et al., 2007). Literature has shown

that surface roughness has more effect on opposing tooth wear than superficial hardness (Olivera

et al., 2006). At the same time, the wear process may be more closely related to the ceramic

microstructure than to the surface roughness due to the possible early exposure of the material’s

microstructure after the removal of the glaze layer (Elmaria et al., 2006). However, there are no

systematic studies evaluating the effects of the surface roughness on the wear behavior of the

antagonist materials yet.

In the case of zirconia restorations, the surface of aged or transformed monoclinic zirconia

presents higher roughness that may also affect the wear of the opposing surface (Ghazal et al.,

2009), since higher zirconia surface roughness has been shown to increase wear rates of opposing

enamel (Ghazal et al., 2009). Additionally, monoclinic transformation reduces surface hardness

19

because of microstructural changes and microcracking, leading to material loss on the surface of

zirconia (Aldegheishem et al., 2014). Grain size, for example, is an important microstructural

parameter that may affect the mechanical and tribological properties of zirconia. A decrease in the

ceramic’s grain size may increase its wear resistance (Lucas et al., 2015) and decrease the

monoclinic phase transformation rate (Denry & Kelly, 2008).

2.2.2 The correlation of aging and wear

Many previous studies have investigated the effect of aging on the performance of dental

restorative materials, and the existence of a positive and/or a negative correlation between aging,

wear, roughness, and hardness (Burgess et al., 2014). The main issue with zirconia-based materials

is their hydrothermal instability (Flinn et al., 2012; Tang et al., 2015). At low temperatures

(<500◦C) and in the presence of water, the tetragonal (t) phase of zirconia changes to monoclinic

(m). This transformation is known as low temperature degradation (LTD). The t-m transformation

proceeds gradually from the surface into the bulk of the ceramic, and is associated with surface

roughening, micro-cracking and other issues (Burgess et al., 2014).

Considering that monolithic zirconia restorations have only been available for a short

period of time comparing with other dental materials, the aging of full contoured zirconia crowns

can be a concern due to the possible surface phase transformation when they are in a direct contact

with saliva. Studies show different susceptibility of dental zirconia to hydrothermal degradation

from one brand to another (Kohorst et al., 2012). Yttrium oxide content, grain size, density and

presence of a cubic phase are factors that can affect the ceramics’ resistance towards aging (Lucas

et al., 2015).

The most widely accepted zirconia aging method is cited in the ISO standard, which states

20

that autoclave aging under 2 bar of pressure for five hours producing less than 25% of zirconia

monoclinic phase can be considered suitable for clinical use (Lughi & Sergo, 2010; Lucas et al.,

2015). While autoclaving is considered the standard method, other methods such as chewing

simulation (Stawarczyk et al., 2012) and boiling in water (Papanagiotou et al., 2006) have also

been investigated. One of the ways researchers try to study the long-term effects of zirconia LTD

is by correlating artificial aging with in vivo aging (Lucas, 2015). Correlations between in vivo

LTD and artificial LTD are undergoing scrutiny to find the actual likeness of events in which they

occur. For example, correlation of 3-4 years in clinical performance for every 1 hour of autoclave

aging has been established for an orthopedic zirconia (Chevalier et al., 2007), but it seems like the

aging of translucent dental zirconia happens at a much faster rate (Cattani-Lorente et al., 2016).

Based on the material’s properties, both aging and surface treatments on zirconia can influence

phase stability and the mechanical strength of the material (Kvam & Karlsson, 2013; Tang et al.,

2015).

21

2.3 Study of wear in dentistry

2.3.1 Systematic reviews on wear evaluation

As previously mentioned, the in vivo analysis of dental materials is time-consuming and

complicated, and wear is generally assessed in wear simulators in vitro. Various devices with

different force parameters and wear mechanisms are available. Heintze et al (2008) in a systematic

review have shown that the wear data from different wear simulators is not comparable, because

the methods follow different wear testing concepts. For example, systematic reviews on the in

vitro wear testing of ceramic materials were performed previous to the design of a laboratory study,

so that the outcomes could be compared to the existing literature (Heintze et al., 2008; Muts et al.,

2014; Passos et al., 2014). While one systematic review took a more inclusive approach,

considering all in vitro studies performed up to that date (Heintze et al., 2008), the two others

(Muts et al., 2014; Passos et al., 2014) included only in vitro studies with detailed descriptions of

the test methods which were conducted within a specific time period, and concluded that no efforts

have been made so far to systematically assess the possible influencing factors of a laboratory test

on material and the opposing antagonist wear.

2.3.2 Effect of occlusal adjustments

Chair-side adjustment of ceramic restorations is usually required to achieve ideal occlusal

contacts. Such surface adjustments include grinding with a diamond bur. The surface treatments

that follow the adjustments may alter the wear performance of ceramic restorations (Au &

Klineberg, 1994). Grinding per se results in the loss of the glaze layer and compromises surface

smoothness (Janyavula et al., 2013). Intra-oral polishing is a well-established method to re-

establish the glaze-like surface, but this procedure is not employed by all clinicians (Preis et al.,

2015). Even without the application of any adjustment procedure, the thin glaze layer over a

22

zirconia crown is known to be worn within the first six months after the insertion of the restoration

(Preis et al., 2012).

Few available data in the dental literature indicate that occlusal adjustments may cause

several issues to the restoration and to the opposing dentition (Ramfjord et al., 1983; Kawai et al.,

2000). The adjusted rough surface may lead to more abrasive wear of the opposing dentition and/or

increase the rate of plaque accumulation (Monasky & Taylor, 1971; Anusavice, 1996) and it may

also lead to the irritation of the tissues in contact (Swartz & Phillips, 1957; Sarikaya et al., 2011).

Inappropriate occlusal adjustments may even lead to the fracture and clinical failure of the

restoration as it has been addressed in many studies (Kelly et al., 1989; Anusavice et al., 1992;

Harvey et al., 1996; Rekow et al., 2007; Preis et al., 2012).

2.3.3 In vivo zirconia wear

The oral tooth wear is multifactorial with physical and chemical processes interacting

(Zhou et al., 2013). However, it has been accepted that the main advantage of in vivo studies is to

examine the tribological behavior of teeth and restorative materials in the real oral environment

(Mundhe et al., 2015).

Silva et al. (2011) carried out a comparison between zirconia and lithium disilicate glass-

ceramics in vivo and in vitro. The results of this randomized and controlled in vivo trial showed

that the performance of lithium disilicate glass-ceramic crowns after four years is comparable to

the performance of zirconia crowns after seven years in terms of replacement rate due to either

maximized or catastrophic failures. Authors also observed that lithium disilicate glass-ceramic is

wear resistant and wear friendly to the opposing enamel in a manner similar to that of veneering

ceramics.

23

Unfortunately, very little information is available in the literature about the mechanisms

that occur as a consequence of different wear patterns among individuals, since most clinical in

vivo studies analyze survival rates and fracture rates of the restorative material rather than

evaluation of wear (Esquivel-Upshaw et al., 2006; Silva et al., 2011; Esquivel-Upshaw et al.,

2012). Therefore, limited clinical data is available regarding the in vivo wear of zirconia and its

effect on the opposing enamel (Etman et al., 2008; Esquivel-Upshaw et al., 2012).

2.3.4 In vitro zirconia wear

The performance of zirconia has been widely investigated in vitro, and that is due to several

factors including the lower cost, the shorter time needed and the larger flexibility in controlling

and changing the variables in an in vitro study design when compared to the clinical investigation.

In the recent years, most of the studies have concentrated on the effect of surface treatments such

as polishing and glazing of zirconia and/or other materials on the wear of the opposing human

enamel or other materials (Beuer et al., 2012; Kim et al., 2012; Janyavula et al., 2013; Kontos et

al., 2013; Preis et al., 2013; Stawarczyk et al., 2013; Amer et al., 2014; Luangruangrong et al.,

2014).

Kim et al. (2012) examined the wear of human enamel and feldspathic porcelain after

chewing simulation against zirconia materials, heat-pressed ceramic, and feldspathic porcelain.

The results of their study demonstrated that zirconia caused the least wear on the opposing enamel

while no significant difference was observed among the three zirconia substrates used in the study.

The authors also reported that the feldspathic porcelain caused highest level of wear on the

opposing enamel while as antagonists more wear resistance than human enamel.

Another study performed by Beuer et al. (2012) investigated the performance of polished

monolithic zirconia, glazed zirconia, and porcelain veneered zirconia against stainless steel

24

antagonists. This study reported higher wear of the opposing antagonist caused by the polished

monolithic zirconia when compared to the glazed and porcelain veneered zirconia. However, there

was higher wear of the antagonist caused by lithium disilicate when compared to zirconia

substrates in another study of different monolithic dental ceramics (Preis et al., 2013). Polished,

ground, and repolished zirconia samples showed no wear of the zirconia surface, while glazed

zirconia showed removal of the glaze layer by the steatite antagonist. Kontos et al. (2013)

investigated the effect of surface treatment (only fired, sandblasted, ground, ground then polished,

and ground then glazed) on the wear of zirconia and opposing steatite antagonist. The authors

reported similar results of the antagonist wear for sandblasted, ground, and ground then glazed

zirconia. Ground then polished samples showed significantly lower wear value than all of the other

groups.

The in vitro evaluation of wear cannot reproduce all the variables that are involved in the

clinical wear of the restoration. The existing in vitro wear simulator devices duplicate only one or

two of the wear mechanisms that are actually present in the mouth, and most of the wear machines

use test samples with a flat surface, while natural teeth and restorations have more complicated

shapes, resulting in complex stress distribution at various sites on the surface (Zhou et al., 2013).

However, the results of in vitro studies are to be considered because they provide simulation of

the most influential parameters for the dynamics of wear under controlled conditions.

Therefore, this thesis experiment will focus on evaluating wear of artificial enamel

opposing zirconia after simulation of occlusal adjustments and other surface conditions by

replicating in vivo conditions, such as high mastication loads, high number of cycles, mouth

temperature and saliva, aiming to design a clinically-realistic experiment with parameters in an in

vitro design that are as close as possible to the clinical scenario.

25

Chapter 3. Objectives and Hypotheses

3.1 Objectives

The present study aims to investigate:

- The effect of surface condition on zirconia surface roughness.

- The effect of surface condition on the wear of opposing artificial enamel.

3.2 Hypotheses

The null hypotheses are:

- The surface condition has no effect on the roughness of zirconia.

- The surface condition of zirconia does not affect the wear of opposing enamel.

26

Chapter 4. Materials and Methods

4.1 Materials

Yttria-partially stabilized tetragonal zirconia polycrystal (Y-TZP) pre-sintered CAD/CAM

materials from two commercial brands (BruxZir, Lot number B 0633325; Expiry date: 10-2017;

Glidewell Laboratories, Newport Beach, CA, USA; Lava Plus, Lot number 520217; Expiry date:

9-2016; 3M ESPE, Seefeld, Germany) were used. BruxZir Standard (OSZ) Milling Blanks were

provided in discs of 98.5 mm diameter and 15 mm height and manufactured with straight edges.

Lava Plus Frame was provided in blocks of 42 mm length, 24 mm width, and 17 mm height.

4.2 Sample preparation

Pre-sintered zirconia materials (BruxZir discs and Lava blocks) were carefully cut with a

diamond-embedded blade (Buehler-Series 15LC Diamond; Buehler, Lake Bluff, IL, USA) (Figure

1) under water cooling in a high speed saw (600 rpm) (Isomet 1000, Buehler, Lake Bluff, IL, USA)

to obtain 6 x 6 mm slices with 2.0 mm thickness. The specimens were sintered following

manufacturers' instructions (Table 3 and Table 4). The final dimension of the samples was 5 x 5 x

1.8 mm due to 20 to 25% sintering shrinkage. Samples were cleaned ultrasonically in isopropanol

solution for 5 minutes and air-dried (Ebeid et al., 2014).

27

Cycle stage Temperature

Start

Temperature

End Heating rate Time

Drying Room temperature Room temperature - 2 hr.

Heating Room temperature 800°C 20°C 39 min.

Heating: 800°C 1450°C 10°C 65 min.

Dwell time 1450°C 1450°C - 120 min.

Cooling 1450°C 800°C 15°C 43 min.

Cooling 800°C 250°C 20°C 28 min.

Ramp up cycle Ramp down cycle

Start temperature: 25°C Start temperature: 1530°C

Heat rate: 10°C/min Ramp time:180 minutes

Ramp time: 151 minutes Final temperature: 70°C

Final temperature: 1530°C -

Hold time: 120 minutes -

Table 3. Sintering cycle employed for BruxZir (Furnace GL-2239-1110, Glidewell Laboratories).

))Laboratories).

Table 4. Sintering cycle employed for Lava (Lava Furnace, 3M ESPE).

Figure 1. Cutting the samples in the pre-

sintered stage with a diamond embedded

blade in the Isomet 1000 saw.

28

4.3 Experimental groups

The sample size calculation was performed based on data (Standard deviation and mean)

available from previous studies and also based on the ability of these studies to statistically

discriminate between groups with a similar protocol (Janyavula et al., 2013; Burgess et al., 2014).

The power analysis indicated that n=5 in a study design of five experimental groups per material

would be sufficient to identify the effect of treatment. Experimental groups were defined as follows

(Figure 2):

Figure 2. Diagram defining the experimental groups according to their surface treatment.

Y-TZP[Lava Or BruxZir]

POLISHED

CONTROL[CPZ]

GROUND[GRZ]

GROUND + REPOLISHED

[RPZ]

GLAZED[GZ]

PORCELAIN VENEERED

[PVZ]

29

4.4 Surface treatment

Samples from both materials were randomly distributed among the groups and treatments

were applied as detailed below (n=5, per zirconia brand).

1. Control polished zirconia (CPZ):

After sintering, samples were polished using the dental laboratory technique according to

instructions from the zirconia’s manufacturers. Polishing was performed with an extra-oral

polishing kit (K0238 Dialite® ZR Extra-Oral Zirconia Polishing System, Brasseler USA®), which

was composed of epoxy-based diamond impregnated polishers with medium and fine grits for

initial polishing of the samples using the dental laboratory technique (Figure 3). The extra-oral

polishing technique was performed as follows: pre-polishing with a medium grit disc-shaped tip

(1/10 mm 2 mm) (H8MZR.HP) applying regular manual pressure always in a parallel direction until

the entire surface presented similar surface finishing; then, final polishing was performed by using

the fine grit disc-shaped tip (H8FZR.HP) with increased manual pressure in a parallel direction

until a smooth or glossy surface could be observed by visual inspection.

30

This procedure was repeated for all samples from groups CPZ, GRZ, and RPZ. Five

samples from this group were kept as control, and the remaining 10 samples were randomly

distributed in the following groups.

2. Ground zirconia (GRZ):

Monolithic zirconia samples polished with the above dental laboratory technique were

subsequently ground with a diamond bur to simulate occlusal adjustments (n=10), as follows: a

diamond bur (Meisinger, 837LF FG 014, 27–76 μm, Neuss, G) (Figure 4) in a high-speed

handpiece and under water cooling (Preis et al., 2012) was applied to the surface of zirconia in

parallel direction for 10 seconds (two strokes, 5 seconds each) (İşeri et al., 2012). A new diamond

bur was used after the preparation of five samples. This procedure was repeated for all samples

from groups GRZ and RPZ. For GRZ group, no further polishing was done. The remaining five

samples received further surface treatment (RPZ).

Figure 3. K0238 Dialite® ZR Extra-Oral Zirconia Polishing

System (Brasseler USA®) used for polishing the samples in

the dental laboratory technique (CPZ, GRZ, RPZ).

31

3. Repolished zirconia (RPZ):

Monolithic zirconia samples polished with the above dental laboratory technique and

adjusted with the above diamond bur were repolished using an intra-oral polishing system (eZr™

intra-oral Adjustment Finishing & Polishing System, Garrison Dental solution – Figure 5), which

is a diamond-based polishing system with medium and fine grits used in procedures of repolishing.

The repolishing of the zirconia samples was performed as follows: a medium grit flame-shaped tip

(4 x 10 mm) (FPZM020) in a high-speed handpiece was applied in a parallel direction and under

regular manual pressure to the entire zirconia surface; final high-gloss polishing was performed

by using the fine grit flame-shaped tip with increased manual pressure in a parallel direction until

a smooth surface could be observed by visual inspection.

Figure 4. Photomicrographs of the surface of the diamond bur (Meisinger, 837LF

FG 014, 27–76μm, Neuss, G) used to simulate occlusal adjustments (GRZ, RPZ).

A. overview; B. closer view showing evidence of the diamond-impregnated

surface.

B.

B

A.

B

32

4. Glazed zirconia (GZ):

For this group, glazing was applied with the Zenostar glaze system on top of unpolished

zirconia. A thin layer of the spray glaze was applied on the surface, and a firing cycle was applied

according to the manufacturer’s instructions (Table 5).

Stage Firing parameters

Start temperature 480 °C

Heat rate 80° C/min

High temperature 790° C

Drying time 2 minutes

Table 5. Glazing cycle employed to the glaze group (GZ).

Figure 5. eZr™ Intra-oral Adjustment Finishing &

Polishing System (Garrison Dental Solution), used for

repolishing the samples that were ground to simulate the

intra-oral adjustment (RPZ).

33

5. Porcelain veneered zirconia (PVZ):

In this group, the fully sintered unpolished zirconia slices were veneered by using the

powder build-up technique with IPS e.max Ceram veneer material (Ivovlar Vivadent) following

the manufacturer’s instructions.

For this purpose, zirconia samples were cleaned ultrasonically in deionized water for 10

minutes to remove residues and minimize contamination. Veneering material was mixed with IPS

e.max Ceram build up liquid by using a glass spatula to achieve a creamy consistency (Figure 6).

The mixing proportions were 2.5 g powder to 1g liquid. Incremental layers of IPS e.max Ceram

veneering material were added and fired (Figure 7) until the final thickness of 2.5 mm was

achieved (1.8 mm zirconia and 0.7 mm porcelain veneer) (Lameira et al., 2017). After the last

firing cycle, samples did not receive any surface treatment or glaze application, so that this group

could be different from the glazed group.

Figure 6. Mixing of the IPS e.max Ceram

veneer material (Ivovlar Vivadent).

material.

34

4.5 Roughness measurement

After preparation, all specimens were cleaned ultrasonically in deionized water for 5

minutes and air-dried (Ebeid et al., 2014). Initial zirconia surface roughness assessment was

performed using a surface profilometer (Figure 8) (Alpha-Step D-600, KLA Tencor Corp) with a

3D scan recipe shown in Table 6, and a dedicated software (KLA Tencor Apex 3D Mountains)

(Figure 9) (Burgess et al., 2014). An area of 2.1×2.1 mm corresponding to the center of the

specimen was considered for the baseline roughness values. Mean roughness (Ra) of each zirconia

sample was measured in μm prior to the application of chewing simulation (Subaşı & İnan, 2012).

Samples were stored in deionized water in plastic containers at room temperature until chewing

simulation was applied.

After the application of chewing simulation, samples were cleaned ultrasonically and the

area that presented the most visual signs of abrasion of the indenter was considered for 2.1×2.1

Figure 7. Programat® P500 Ivoclar

Vivadent that was used for firing the

veneering porcelain and the glazed groups

(GZ, PVZ).

35

mm roughness evaluation. The mean roughness (Ra) of each zirconia sample was compared before

and after the application of chewing simulation.

X Scan Size (μm) 2100.000

Y Scan Size (μm) 2100.000

Scan Speed (μm/s) 20

Traces 10

Sampling Rate (Hz) 200

Y Spacing (μm) 24.000

Table 6. 3D scan recipe employed for the use of the surface profilometer (Alpha-Step D-600, KLA

Tencor Corp).

Figure 8. Profilometer (Alpha-Step D-600,

KLA Tencor Corp) used for the quantification

of the roughness of zirconia samples and the

wear of steatite indenters.

36

4.6 Chewing Simulation (artificial aging)

Occlusal wear was induced in a chewing simulator (CS-4.4, SD Mechatronik GMBH,

Feldkirchen-Westerham, Germany). The bottom of the zirconia specimens was embedded in

polymethylmethacrylate (PMMA) and inserted into metallic rings connected to the base of the

equipment. The chewing simulation compartments were filled with artificial saliva (Table 7)

(Ablal et al., 2009) and the temperature was kept constant at 37° Celsius. The volume of saliva

and temperature in the chambers were monitored by the operator twice a day. Spherical steatite

indenters (6 mm diameter – SD Mechatronik GMBH, Feldkirchen-Westerham, Germany) were

used to simulate the antagonist surface and 80 N load was applied at a speed of 60 mm/sec in a 1

mm horizontal motion (Figure 11). Four specimens were cycled simultaneously and 500,000

cycles were performed (Stawarczyk et al., 2013). After 500,000 cycles, the specimens were

evaluated in an optical microscope with 60 X magnification for the presence of any fracture or

cracks. After aging the samples, they were cleaned ultrasonically in alcohol, air-dried and

roughness and wear were analyzed (Stawarczyk et al., 2013).

Figure 9. Screenshot of the profilometer

software (KLA Tencor Apex 3D Mountains)

used to measure wear and roughness.

37

Quantity Chemical component

50 g CMC Carboxymethylcellulose

10 g Methyl-4-hydroxybenzoate

0.3 g MgCl2-6H2O

0.83 g CaCl2-2H2O

0.49 g K2HPO4

3.12 g KCL

0.02 g F

5 L Water

Figure 11. Chewing simulation parameters employed.

B A

Table 7. Composition for preparing 5 Liters of artificial saliva.

38

4.7 Evaluation of antagonist wear

One new steatite indenter was scanned using the surface profilometer (Alpha-Step D-600,

KLA Tencor Corp) (Figure 12) with the same recipe showed in Table 6 and with a dedicated

software (KLA Tencor Apex 3D Mountains) ( Figure 13) to register the baseline dimensions of

the antagonist. Following chewing simulation, the wear of the opposing steatite indenters was then

evaluated by calculating the volume of material loss (mm3) based on the diameter and the height

of the worn surface compared to the baseline (unaged) indenter (Lameira et al., 2017).

4.8 Statistical analysis

Roughness data was analyzed by Three-way Analysis Variance (ANOVA) and Tukey HSD

(p = 0.05) with the independent variables being material brand, surface condition and chewing

simulation. Comparisons among groups were performed with Tukey Honest Significance

Difference (HSD) test at a significance level of 5%. Wear data were analyzed by Two-way

Analysis Variance (ANOVA) and Tukey HSD (p = 0.05) with the independent variables being

zirconia brand and surface condition. Tukey HSD test was also used to compare the volume loss

among surface treatment groups.

4.9 Scanning Electron Microscopy (SEM)

For surface characterization, one zirconia substrate and steatite indenter from each

experimental group were cleaned in acetone in an ultrasound bath, and mounted on stubs with

carbon adhesive tape and colloidal silver paint. The specimens were gold sputtered and observed

under Scanning Electron Microscopy (SEM) (JEOL 6610LV) with high vacuum mode under 40,

500 and 1000 X magnifications.

39

Figure 12. Left: Steatite indenter on the base of the profilometer (Alpha-Step D-600, KLA Tencor Corp).

Right: A. a contact reading needle while reading one of the zirconia’s samples, B. the needle’s shadow.

Figure 13. Profile of a control steatite indenter

(Alpha-Step D-600, KLA Tencor Corp).

A.

B.

40

Chapter 5. Results

5.1. Roughness Evaluation

5.1.1 Analysis of results

Data was analyzed by Three-way Analysis Variance (ANOVA) and Tukey HSD (p = 0.05).

The independent factors were material (two levels), aging (two levels) and surface condition (five

levels). There was no effect of brand on zirconia surface roughness (p = 0.216). However, ANOVA

indicated that there was a significant surface treatment*chewing simulation interaction effect (p <

0.001). Mean and standard deviation for zirconia surface roughness following the five surface

treatments were calculated before and after the application of chewing simulation for each zirconia

brand (Table 8 and Table 9), and the results were then pooled due to the lack of effect of brand the

surface roughness (Table 10 and Figure 14). Comparisons among groups were performed with

Tukey Honest Significance Difference (HSD) test at a significance level of 5%.

Treatment Before CS After CS Porcelain veneered 15.3 (4.2) 16.4 (2.7)

Ground 4.1 (1.3) 4.6 (2.5)

Glazed 4.4 (1.5) 11.96 (1.4)

Repolished 3.9 (1.5) 4.2 (1.9)

Control Polished 1.6 (0.47) 1.6 (0.4)

Treatment Before CS After CS Porcelain veneered 12.6 (2.0) 14.0 (2.2)

Ground 4.0 (1.7) 3.4 (1.3)

Glazed 3.0 (1.5) 15.2 (3.1)

Repolished 2.8 (1.4) 4.1 (1.6)

Control Polished 1.0 (0.3) 1.2 (0.3)

Table 8. Roughness values (µm) - Means (and

standard deviations) for Lava groups before

and after chewing simulation (CS).

Table 9. Roughness values (µm) - Means (and

standard deviations) for BruxZir groups

before and after chewing simulation (CS).

41

*Dissimilar lowercase letters within the same column and uppercase letters within the same row

indicate significant difference (Tukey HSD, p < 0.05).

0

2

4

6

8

10

12

14

16

18

20

µm

Zirconia Roughness

Before After

Porcelain Veneered Ground Glazed Repolished Control Polished

Treatment Before CS After CS

Porcelain veneered 13.43 (3.64) Aa 15.22 (2.90) Aa

Ground 4.04 (1.64) Bb 4.02 (2.20) Bb

Glazed 3.70 (1.80) Bbc 13.54 (3.11) Aa

Repolished 3.37 (1.63) Bbc 4.14 (1.85) Bb

Control Polished 1.30 (0.51) Bc 1.43 (0.41) Bb

Figure 14. Mean and standard deviation for pooled zirconia surface roughness before and after

chewing simulation.

Table 10. Roughness values (µm) - Means (and standard deviations) for pooled Lava and

BruxZir groups before and after chewing simulation (CS)*.

42

5.1.2 Effect of aging and surface treatment on the roughness of zirconia surface

Before the application of chewing simulation, the highest surface roughness was presented

by the porcelain veneered samples (13.43 μm ± 3.64) which was significantly higher than all of

the other four groups (p < 0.0001). Surface grinding significantly increased roughness (4.04 µm ±

1.64) when compared to the control group (1.30 µm ± 0.51) and surface repolishing after grinding

resulted in intermediate values (3.37 µm ± 1.63), which were similar to the ground group and to

the control group. Intermediate values were also observed when a glaze layer was applied to the

surface of zirconia (3.70 µm ± 1.80).

After chewing simulation, Tukey HSD showed that the surface roughness of the porcelain

veneered zirconia samples was significantly higher (p < 0.0001) (15.22 µm ± 2.90) when compared

to the control (1.43 µm ± 0.41), repolished (4.14 µm ±1.85) and ground samples (4.02 µm ± 2.20).

The glazed zirconia samples presented significantly higher surface roughness values (13.54 µm ±

3.11) after chewing simulation. The application of chewing simulation to the glazed zirconia

resulted in a surface roughness that was similar to the porcelain veneered samples and significantly

higher than that of the control, ground, and repolished samples (p < 0.0001). After chewing

simulation, the roughness of the control, ground and repolished samples did not change

significantly.

Representative SEM images for the five groups before and after chewing simulation are

shown in Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19.

43

Figure 15. SEM at different magnifications for the control zirconia: A. before chewing simulation;

B. after chewing simulation, showing even and parallel grazing marks compatible with the

abrasion against the indenter; and C. overview of the surface after chewing simulation, showing

a smoother surface outside the abrasion area.

A. B.

C.

44

Figure 16. SEM micrographs at different magnifications for the ground zirconia. A. before

chewing simulation; B. after chewing simulation showing a smoother surface due to the partial

removal of the peaks by the abrasion against the indenter; C. overview of the surface after

chewing simulation showing a rougher area outside of the abrasion area.

B.

.

A.

C.

.

45

Figure 17. SEM micrographs at different magnifications for the repolished zirconia. A.

before chewing simulation; B. after chewing simulation showing a smoother surface due to

the partial removal of the peaks by the abrasion against the indenter; C. overview of the

surface after chewing simulation showing a rougher surface outside of the abrasion area.

B.

.

A.

C.

.

46

Figure 18. SEM micrographs at different magnifications for the glazed zirconia: A. before

chewing simulation; B. after chewing simulation indicating removal of the superficial

layer and exposure of the material’s structure; and C. an overview of the surface after

chewing simulation, showing a smoother surface outside of the abrasion area.

A.

C.

B.

.

47

Figure 19. SEM micrographs at different magnifications for the porcelain veneered zirconia.

A. before chewing simulation; B. after chewing simulation showing the removal of the

superficial glass layer with exposure of fluorapatite glass-ceramic material; C. overview of

the surface after chewing simulation showing a smoother surface outside of the abrasion

area.

B.

.

A.

C.

.

48

5.2 Wear Evaluation

5.2.1 Analysis of results

Data were analyzed by Two-way Analysis Variance (ANOVA) and Tukey HSD (p = 0.05).

The independent factors were material (two levels) and surface treatment (five levels). ANOVA

showed no effect of material on the steatite volume loss (p = 0.064). However there was a

significant effect of surface treatments on the wear of the steatite indenters (p = 0.025). Tukey

HSD test was used to compare the volume loss among surface treatment groups. Due to the absence

of effect of zirconia brand on wear, results for samples abraded against Lava and BruxZir were

pooled according to the surface treatment. Mean and standard deviation for steatite volume loss

are presented in Table 11.

5.2.2 Effect of zirconia surface treatments on the wear of opposing artificial enamel

The analysis of volume loss of the steatite indenters showed similar wear values for all the

samples abraded directly against monolithic zirconia, irrespective of the surface finishing (Table

11). The highest wear of the indenters was caused by the control samples (0.09 mm3 ± 0.07), while

the glazed group (0.02 mm3 ± 0.03) and the porcelain veneered group (0.02 mm3 ± 0.02) caused

the least volume loss of the opposing indenter (p = 0.008). Abrasion against ground (0.07 mm3 ±

0.06) and repolished zirconia (0.06 mm3 ± 0.07) resulted in intermediate volume loss, statistically

similar to control, glazed, and porcelain veneered zirconia samples. Representative SEM images

for the steatite indenters at baseline and abraded against the five groups with different surface

treatments after chewing simulation are shown in Figure 20.

49

Group Mean SD

Control 0.09a 0.07

Ground 0.07ab 0.06

Repolished 0.06ab 0.07

Glazed 0.02b 0.03

Porcelain veneered 0.02b 0.02

* Dissimilar letters indicate statistically significant differences (p < 0.05).

Table 11. Mean and standard deviation (SD) for steatite volume loss after

chewing simulation (in mm3)*

50

Figure 20. SEM micrographs for the steatite indenters: A. baseline; B. abraded against the

control zirconia; C. abraded against ground zirconia; D. abraded against repolished zirconia; E.

abraded against glazed zirconia; F. abraded against porcelain veneered zirconia. Larger abrasion

areas are shown in B, C and D.

A.

.

B.

.

C.

.

D.

.

E.

.

F.

.

51

Chapter 6. Discussion

The use of monolithic zirconia has become a topic of interest for the restoration of severely

damaged teeth, mainly because the high incidence of chipping or cracks within the veneer layer

can compromise the longevity of the bi-layer restorations (Saito et al., 2010). The application of

zirconia in monolithic configuration is possible because of the high surface hardness of the

material combined with improved translucency (Tinschert et al., 2000).

However, occlusal adjustment of zirconia crowns is required in most cases after installation

of the prosthesis, which will result in a rougher surface and/or removal of the glaze layer. This

surface roughness may be reduced depending on the type of material and the technique applied for

polishing (Lawson et al., 2014; Rashid, 2014), but there is no standard method for polishing

monolithic zirconia crowns yet (Lawson et al., 2014). In the case of glazed restorations, this layer

is easily removed when occlusal adjustments are necessary and also under physiological occlusal

loading (Lameira et al., 2017). As a result, early researchers advised that re-glazing a restoration

after clinical adjustment may be necessary (Barghi et al., 1976).

The relevance of the information available in the literature on wear and roughness is

questionable because of either low number of cycles (Jung et al., 2010; Kontos et al., 2013) or low

loading (Kontos et al., 2013; Stawarczyk et al., 2013). Lack of correlation between the number of

cycles and time of clinical use may also make the interpretation of the findings difficult

(Luangruangrong et al., 2014). In the same way, very low load forces when compared to the

mastication forces also may be another reason that compromises the significance of the findings

(Kontos et al., 2013).

52

Five hundred thousand cycles and 80 N load were applied in the current study, considering

that 500,000 cycles correspond to between 2 and 5 years of clinical service (Wiskott et al., 1995;

Kelly, 1999; Steiner et al., 2009) which can be considered more realistic than the number of cycles

used in previous studies (Jung et al., 2010; Kontos et al., 2013). Similarly, in terms of load, 80 N

load was applied because, based on previous studies, this is considered a high chewing load that

is still within the daily average for an adult without parafunctional habits (Tortopidis et al., 1998).

We hypothesize that a loading that more closely duplicates the forces applied during oral function

will challenge the material in a different way, having an effect on the readings of wear and

roughness as opposed to the low values presented in previous studies (5 N-49 N) (Jung et al., 2010;

Kim et al., 2012; Stawarczyk et al., 2013; Janyavula et al., 2013).

The results of the present study indicate that the first null hypothesis, which stated that the

surface treatments have no effect on the roughness of zirconia substrates, is rejected. The second

null hypothesis, which stated that the surface treatments applied to zirconia surface do not affect

the wear of opposing enamel, is also rejected.

Before chewing simulation, the highest surface roughness was presented by the porcelain

veneered samples (Table 7) which was significantly higher than all of the other four groups (p <

0.0001). This result can be due to several reasons. First, the porcelain veneered surface did not

receive a final coverage with glaze, polishing or any additional surface treatment. This procedure

was carried out to avoid having the same surface material on the porcelain veneered and glazed

zirconia specimens, as well as to investigate the effect of the porcelain on the opposing enamel,

since the removal of the glaze material after 2.5 million chewing cycles has been demonstrated

(Lameira et al., 2017). Therefore, the veneer layer investigated in this study was more porous than

in the clinical scenario, when the porcelain would be covered with the glaze layer. Second, the

53

veneering material is a fluorapatite veneering ceramic that is composed of glass powder, fused

silica dioxide (SiO2) (60%) and alumina trioxide (Al2O3) (40%) crystals (IPS e.max® Ceram,

2005; Roselino et al., 2013). The presence of small particles like nano-fluorapatite (300 to 500

nm) extruding from the glassy matrix (Figure 19A) led to greater roughness in comparison to the

glazed one, since the glaze material is composed of a glass matrix with no filler particles

(Stawarczyk et al., 2016). Finally, the smoothness of the glazed surface was confirmed by the

similar roughness values of glazed zirconia and polished (control) zirconia. This finding is in

agreement with previous studies (Bottino et al., 2006; Tholt et al., 2006; Wang et al., 2009;

Yuzugullu et al., 2009).

After the application of chewing simulation, the surface roughness of the porcelain

veneered zirconia samples was still high (Table 10), but interestingly, glazed zirconia samples

presented roughness values that were similar to the porcelain veneered samples. The surface

roughness of the glazed zirconia was then significantly higher (p < 0.0001) than the control,

repolished and ground samples. This increase in surface roughness was due to the removal of the

most superficial and smooth surface, which exposed the inner structure of the glaze layer, with

voids, bubbles and irregularities (Figures 18B and 18C). The visual inspection of specimens

indicated that this phenomenon started around 250,000 cycles. A clinical study has previously

shown that the glaze layer can be removed around the first six months after the installation of the

restoration (Etman, 2009).

The similar baseline roughness values between ground and repolished groups can be

explained by the simplicity of the polishing procedure in an in vitro condition. Samples were flat,

fully accessible, and the operator had the possibility of controlling the pressure applied. This may

not be the case in an intra-oral occlusal adjustment, due to all the limitations associated with an in

54

vivo procedure on a surface with an intricate anatomy such as the occlusal surface of teeth.

Therefore, one should not assume that the intra-oral polishing would be able to generate a surface

polishing similar to the polishing provided by the laboratory after occlusal adjustments in the

clinical scenario.

The comparison of the roughness values before and after the application of chewing

simulation for the ground (Figure 16) and repolished (Figure 17) groups should be combined with

the SEM images of the surface. Even though the statistical analysis showed similar roughness

before and after the chewing simulation (Figure 14), SEM indicated the smoothening of the

abraded area after chewing simulation for both ground (Figure 16C) and repolished (Figure 17C)

specimens. These conflicting results are a consequence of the limitations of the surface

profilometry technique. While opposing steatite indenter was abraded on the zirconia specimen in

an extension of approximately 1 mm, while the profilometer roughness reading was pre-set to an

area of 2.1×2.1 mm2. Therefore, the roughness reading incorporated both abraded and non-abraded

areas. Only a technique sensitive enough to particularly scan the abraded surface would be able to

characterize changes in surface topography of this magnitude. In the absence of such technique, it

is recommended to combine quantitative analyses with the qualitative assessment of the surface

through higher magnification imaging techniques.

The volumetric loss measurement is considered the most effective way to assess wear of

the opposing surface (DeLong et al., 1986; Magne et al., 1999), but many previous studies have

measured the vertical loss instead of measuring the total volume loss (Sabrah, 2011; Stober et al.,

2016). Volume loss obtained by surface profilometer was used in the present study to quantify the

wear of the opposing artificial enamel (steatite indenters). There was a significant effect of surface

treatment on the wear of the steatite indenters (p = 0.025) and analysis of results show that the

55

wear of the steatite indenters was rather affected by the surface material - zirconia, glaze or

porcelain – than by the surface finishing technique – grinding or polishing. The highest volume

loss was caused by the control (polished) zirconia specimens (Table 11) which was similar to

ground and repolished specimens but significantly higher than the wear caused by glazed and

porcelain veneered specimens (p = 0.008). These results are in agreement with the study of Beuer

et al., (2012) which reported higher wear for polished zirconia when compared to the glazed

zirconia. However, zirconia has been considered “wear-friendly” due to results that showed

evidence of significantly lower wear when human enamel is abraded against zirconia as opposed

to glazed and porcelain-veneer materials (Janayula et al., 2013). These contradictory results may

be explained by the methods employed. While Janayula et al. used only 10 N to simulate

masticatory forces and a mix of glycerin/distilled water for humidifying the surfaces, the present

study used a considerably higher load (80 N) and artificial saliva to simulate the oral environment.

It is possible that the higher loading forces between zirconia and steatite maximized the damaging

effect of the significantly harder zirconia (Candido et al., 2014; Aydin et al., 2015) on the artificial

enamel substrate. This, combined with the fact that the present study did not have the glycerin to

act as a lubricating agent during the chewing cycles may have maximized the wear caused by

zirconia. Therefore, the present study indicates that the use of zirconia as a monolithic material

under clinically-relevant masticatory conditions may maximize the wear of the antagonist tooth.

The effect of the polishing technique applied to the antagonist surface and consequent

surface roughness on enamel wear has been previously demonstrated (Albashaireh et al., 2010;

Preis et al., 2011; Kim et al., 2012; Janyavula et al., 2013; Kontos et al., 2013). Nonetheless, this

study showed absence of effect of zirconia surface finishing technique on the volume loss of

artificial enamel, as demonstrated by Preis et al. (Preis et al., 2012; Preis et al., 2013). The initial

56

zirconia roughness values indicated that only ground zirconia was significantly rougher than the

control polished zirconia (Table 10), but it can be hypothesized that the roughness variation was

not suffice to affect the wear of the opposing surface during the application of the chewing cycles,

as indicated by the visual comparison of Figure 15A (control) and Figure 16A (ground zirconia)

The analysis of results showed that glazed and porcelain veneered groups caused the least

volume loss to the opposing indenters, and these findings were corroborated by the SEM findings,

which show a significantly smaller abrasion area for steatite indenters abraded against glazed

(Figure 20E) and porcelain veneered (Figure 20F) zirconia. The SEM of the opposing surfaces

(Figures 18C and 19C), however, show that the material on the surface of the zirconia substrate

was partially removed by the abrasion against the indenter. This was a consequence of the lower

surface hardness of both, veneering porcelain and glaze, when compared to the zirconia substrate

and to the steatite indenter (Shortall et al., 2002; Ghazal et al., 2008). It is possible that longer

chewing cycles (e.g. 2x106 cycles) would result in the total removal of the surface material, with

exposure of the underneath unpolished zirconia, which might have an impact on the dynamic wear

process of the opposing enamel. But this hypothesis is far beyond the scope of the current study

and should be further investigated in a near future as an attempt to establish a correlation between

longer clinical surface and preservation of the opposing tooth structure. The results of the present

study showed that there was no effect of material (zirconia brand) either on surface roughness (p

= 0.216) or on the steatite volume loss (p = 0.064). This can be explained by the similarities in the

chemical composition between the two brands, which resulted in similar flexural and compressive

strength, for example (Alghazzawi & Janowski, 2015). Additionally, in a study with experimental

zirconia materials, Tong et al. (2016) observed that irrespective of grain size, Y-TZP with similar

chemical composition present similar surface hardness (Tong et al., 2016).

57

As above mentioned, one of the limitations of the current study was the number of cycles

applied (500,000). It is possible that a higher number of chewing cycles would have an impact on

the significance of the findings, especially for the glazed and the porcelain veneered groups, since

the total removal of the surface material is expected and may change the wear pattern of the

opposing surface. However, increasing the number of cycles implies in a longer study, which may

compromise the number of groups to be evaluated. Another factor is the technique used for

roughness evaluation and quantification of wear. Re-setting the profilometer reading area to a

smaller surface that would only include the abraded area on the surface might result in a more

accurate reading for the least abraded specimens. On the other hand, this would implicate that

samples with significantly larger abraded surfaces would have some of the abraded area left out of

the reading surface, making it difficult to find a balance between all the groups’ specificities.

Therefore, the evaluation of other techniques for quantification of wear and roughness is

encouraged. Furthermore, even though precise dimensions are critical for quantification of wear,

this study findings demonstrate the importance of the surface characterization under SEM for

understanding the roughness and wear pattern of a given substrate.

58

Chapter 7. Conclusion

Within the limitations of this in vitro study, the following conclusions can be drawn:

1. There is no significant effect of material (zirconia brand) on the surface roughness of

zirconia or on the wear of opposing artificial enamel.

2. Surface finishing technique has a significant effect on roughness of monolithic zirconia.

3. Surface finishing technique applied to monolithic zirconia surface does not have a

significant effect on the wear of opposing artificial enamel (steatite indenter).

4. The material composition applied on top of zirconia significantly affects the volume loss

of opposing artificial enamel.

5. The interaction of surface condition and chewing simulation significantly affects the

roughness of zirconia.

7.1 Future directions

1. To investigate the effect of other technologies (e.g. 3D Scan) for the quantification of

wear and roughness between monolithic zirconia and other ceramics materials and

dental enamel.

2. To compare the outcomes of two-body and three-body wear tests of monolithic

zirconia opposing natural or artificial enamel.

3. To evaluate the effect of finishing techniques on roughness and wear when other

ceramic materials are used (e.g. lithium disilicate, zirconia-toughened alumina).

4. To evaluate the wear and phase transformation on zirconia-based restorations under

extreme masticatory conditions (e.g. higher loads combined with longer chewing

cycles).

5. Correlate wear and other mechanical properties such as fatigue and survival rates.

59

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