Abstract .

Introduction: From the discovery of zirconia in 1789 to the biomedical uses as implants or fixed restorative devices, zirconia has been utilised for many reasons, but most of all in dentistry because of its high impact and flexural strength owed to an internal toughening mechanism. Zirconia is now being found indicated for crowns, bridges and more in the dental field, plus with the increasing use of CAD/CAM systems. Veneer porcelains are weaker, and were found to be more likely to crack and de-bond from zirconia substructures than those of the conventional metals.

Methods: Through searching on MMU library which gave results from Science Direct, PUBMED, Quintessence (and any dental journal that MMU had paid the subscription for). Journals were found from specific search categories using the BOOLEAN technique. Then exclusions were made and included data was studied.

Results: A flowchart to show the process of searching methods was created displaying the fields of excluded and included data, with amount of included results.

Discussion: The survival rates that were found, showed that crowns of zirconia-ceramic substructures didn’t last as long in the oral environment than those of metal-ceramics. Generally cracking of the porcelain and de-bonding was found to occur more in zirconia crowns. However zirconia was shown to be a longer surviving substructure material in the use of posterior bridges compared to metals.

Authors Conclusion: Zirconia doesn’t ensure the longevity as other factors have a role in the survival rates of crowns and bridges, and often it was shown that the porcelain would be zirconia’s limiting factor. If high strength porcelains veneered to zirconia substructures or zirconia full contour crowns were utilised, the results might be different and zirconia could therefore be the dominating factor in the survival, without the porcelain fracturing, rendering the restoration a failure.

1

Acknowledgements.

I would like to express sincere gratitude to Dr, S.A. Horne, Dr, R Taylor, Hannah Bates and my parents for the advice and foresight on this dissertation.

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ContentsAbstract ………………………………………………………………………………………………………………………………………….. 1

Acknowledgments …………………………………………………………………………………………………………………………… 2

Contents …………………………………………………………………………………………………………………………………………. 3

1 Introduction...................................................................................................................................5

1.1 Zirconia Background..............................................................................................................5

1.1.1 Discovery leading to use in biomedical science.............................................................5

1.1.2 Chemistry and Structure................................................................................................6

1.1.3 Dental metal alloys:.......................................................................................................7

1.2 Zirconia in Dentistry...............................................................................................................7

1.2.1 Zirconia as a dental ceramic...........................................................................................7

1.2.2 Processing Zirconia into a substructure for use in fixed prosthodontics......................10

1.2.2.1 Slip cast method.......................................................................................................10

1.2.2.2 CAD/CAM.................................................................................................................11

1.2.3 Clinical survival rates....................................................................................................12

1.2.4 Technical Indications for use........................................................................................15

1.2.4.1 Strength...................................................................................................................15

1.2.4.2 Aesthetics.................................................................................................................16

1.2.4.3 Sandblasting and surface treatments.......................................................................17

1.2.5 Clinical Indications for use...........................................................................................17

1.2.5.1 Properties to aid indications....................................................................................18

1.2.5.2 Biocompatibility.......................................................................................................19

1.2.6 Technical Limitations of use.........................................................................................20

1.2.6.1 Ageing......................................................................................................................20

1.2.6.2 Veneer cracking.......................................................................................................21

1.2.7 Clinical Contra-Indications of use.................................................................................22

1.2.7.1 Veneer separation....................................................................................................23

1.2.7.2 Veneer cracking.......................................................................................................23

1.2.7.3 Difficulty in adherence to the porcelain/ceramic veneer.........................................24

1.2.7.4 Workability...............................................................................................................24

1.3 Aim of Investigation.............................................................................................................24

2 Methods......................................................................................................................................26

2.1 Type of study.......................................................................................................................26

2.2 Selection criteria..................................................................................................................26

2.2.1 Keywords:....................................................................................................................26

3

2.2.2 Language......................................................................................................................27

2.2.3 Criteria for success rates evaluations...........................................................................27

2.2.4 Date or time scale of publication.................................................................................27

2.2.5 Mechanical properties selection criteria:.....................................................................28

2.3 Databases............................................................................................................................28

2.3.1 Search terms................................................................................................................28

2.3.2 Online Databases:........................................................................................................28

2.4 Data collection and analysis.................................................................................................28

2.4.1 Study selection:............................................................................................................28

2.4.2 Data extraction:...........................................................................................................29

3 Results.........................................................................................................................................30

4 Discussion....................................................................................................................................32

4.1 Advantages..........................................................................................................................32

4.2 Limitations...........................................................................................................................35

4.3 Future..................................................................................................................................38

5 Authors conclusions.....................................................................................................................39

6 References:..................................................................................................................................40

4

Hypothesis: The selection of Zirconia as a substructure material assures longevity of the final

restoration.

1 Introduction1.1 Zirconia Background 1.1.1 Discovery leading to use in biomedical science.

In 1789 the German chemist, Martin Heinrich Klaproth, found that heating gemstones

produces zirconia as a metal dioxide (Advameg Inc, 2014). In the early 1960’s the use of zirconia was

introduced into biomedical applications, firstly used as a replacement hip joint in orthopaedic

solutions where titanium and alumina had been predominantly used (Madfa et al, 2014). Piconi

(1999) figures through the Research and Development done by Helmur and Driskell in the late

1960’s, furthered the expanding indications for zirconia in biomedical applications.

Increased interest in metal free dentistry, meaning the replacement of metal usage with

materials which have more biocompatible properties i.e. the use of non-toxic zirconia ceramic. This

has become a popular topic in the dental community over the late 20th century and into the present,

and has helped lead to the increased use of zirconia as a biomaterial with specific relation to

dentistry (Manicone, Iommetti and Raffaelli, 2007). In-ceram zirconia became commercially available

in dentistry in 1989, by altering ‘In-ceram alumina’ with the addition of 35% zirconia, to the slip,

producing ‘In-ceram zirconia’ (Vita dental technicians, no date). Then for use in a CAD/CAM

manufacturing technique in 2002, the In-ceram YZ was available (Vita dental technicians, no date).

5

1.1.2 Chemistry and Structure. Zirconia derivations (Knovel, 2008):

Zirconia is chemically the dioxide of the metal Zirconium, which in turn derives from Zircon. Zircon,

which naturally occurs, is often associated with silica (ZrSiO4) which is a gemstone, hence the

original find by Klaproth. The less pure deposits from zircon are used for stabilized zirconia in

ceramics and the higher purity deposits form zirconium. Zirconium is produced by: extraction from

zircon (mainly) and baddeleyite by use of chlorination; subsequently refined to ZrCl4; followed by

solvent extraction to purify the material; finally a reduction process with magnesium, producing the

Zr element.

Zirconia is a polymorphic material which means that at different temperatures its structure will

change. There are 3 different structures and when they are understood they can be utilised for

desired mechanical properties.

The forms are as follows (Piconi and Maccauro 1999):

1. At Tr-1170⁰C = monoclinic form

2. At 1170⁰C-2370⁰C = tetragonal form

3. At ≥2370⁰C = cubic form.

The ability of zirconia to transform into different

grain structures is what allows for strengthening in

its solid state. Piconi and Maccauro (1999) mention

that the ability of the tetragonal grains to

transform can be utilised in a positive manner the

tetragonal grains mentioned in Fig1.1 as

‘untransformed particles’ come under tensile

stresses which result in stress-induced

transformation toughening. Which is where the

6

Fig1.1: Particle interaction around a crack propagation (Piconi and Maccauro 1999).

tetragonal grains transform, martensitic in nature, into monoclinic grains (‘transformed particles’)

infront of the crack, stopping it from further propagation.

Garvie (1972) goes into further detail to state that the compressive stresses which are associated

with closing the crack are sufficient enough due to a volume expansion of ~3-5% when the grains

transform (tetragonal to monoclinic).

1.1.3 Dental metal alloys:Dental Casting Alloy Types:

III IV Noble, gold based alloys

High noble, gold alloys

Use: Crowns, short bridges.

Long-span bridges.

Yield strength: 201-340MPa >340MPa 300-520MPa 240-600MPa

The majority of substructures in fixed restorations have always been made using a metal of some

alloying mixture. Here in Fig 1.2 shows a range of relevant dental alloys that would be used as

substructures to fixed restorations such as crowns and bridges.

1.2 Zirconia in Dentistry1.2.1 Zirconia as a dental ceramic

In dentistry there is many forms in which zirconia is used for fixed restorations:

Glass-infiltrated Zirconium toughened Alumina (Vita, no date).

Which is found as ‘In-Ceram Zirconia’.

This can be an alternative technique used to achieve densification of Zirconia Toughened

Alumina products without the intensive grain growth. Intensive grain growth leads to larger

grains deemed undesirable, due to lower strengths. In dental ceramics the glassy phases can

be significantly improved by about 25 – 50%, by a melt-glass infiltration process where the

ceramic obtained has a homogeneous defect-free microstructure and has favourable

7

Fig1.2 ‘Dental casting alloy types and their strengths’ (Sakaguchi and Powers, 2012).

compressive stresses for increased strength. However, the glass-phase content must

controlled, as too much would weaken the material. This process results flexural strengths of

495.2 – 633.5±41 MPa (Zhang et al, 2012). The glass infiltration process can be applied to

zirconia with other additives depending on the mechanical properties required of the

product. In Tinschert et al’s (2007) work in the lifetimes of zirconia-ceramics for bridge work.

It was found that zirconia with an alumina oxide had the better long-term strength.

Yttrium cation-doped tetragonal Zirconia polycrystals.

3Y-TZP, found as ‘Cercon Zirconia’ (DENTSPLY International, no date) plus many other

producers due to its popularity.

It’s most often soft machined, and has a flexural strength of 1087±173 MPa. It’s highly

crystalline and is also known for having the highest fracture toughness of an all-ceramic

material (Sakaguchi and Powers, 2012).

It is produced using tetragonal zirconia polycrystals, with 3 mol% yttrium oxide as the

stabilizer. The stabilizer is needed to ensure that zirconia remains in its tetragonal grains

when cooled, and doesn’t transform to monoclinic grains until tensile stresses are applied

allowing the transformation toughening process to occur.

Two key properties of 3Y-TZP are: Low porosity and high strength. These properties, and

more, make it the most popular choice for application in dental restorations according to

Zarone, Russo and Sorrentino (2011).

Magnesium partially stabilized Zirconia, Mg-PSZ.

Produced by clusters of tetragonal structured crystals in cubic stabilized matrix with 8-10

mol% stabilizer, magnesium oxide. However the maintained stability isn’t guaranteed long-

term, hence why magnesium oxide isn’t the choice over yttrium. Due to its large grain size

the following properties occur: higher porosity levels; lower density and less strength against

8

slip. Furthermore with low dimensional stability and high framework wear. Generally Mg-

PSZ is not seen being indicated for frequent use (Zarone, Russo and Sorrentino2011).

Shrinkage free ZrSiO4 ceramic.

Seen in the Kavo Everest system ‘Shrinkage free ZrSiO4 ceramic’ (Kavo, no date).

Flexural strength of 328.3MPa.

The ZrSiO4 ceramic tested in Binder et al’s (2005) study shows mechanical properties that

are comparable to other all-ceramic dental materials. Full crowns made from the zirconia

ceramic withstand masticatory forces found in the posterior area. While the material

appears to be reliable for clinical use for posterior full crowns, clinical assessment of these

all-ceramic restorations is required.

In conventional CAD/CAM machining of pre-sintered blocks there is significant shrinkage

occurring, so an enlarged amount of material is needed for the process. Which can be

resolved by using ‘shrinkage free zirconia’ (Heydecke et al, 2007).

Dense, shrinkage-free ZrSiO4-ceramics.

Produced by a reaction-bonding process using; ZrSi2, ZrO2, and a polysiloxane, as starting

materials. Sinter shrinkage is compensated by the volume increase during oxidation of ZrSi2.

In addition, the use of a Si-containing so called low-loss-binder (PMSS) reduces shrinkage

further (Hennige et al, 1999). A denser ceramic is less susceptible to microbial attack

Askeland (2011), but harder to machine (Helvey, 2008).

9

1.2.2 Processing Zirconia into a substructure for use in fixed prosthodontics.

1.2.2.1 Slip cast method.Traditional way of shaping ceramics according to Bauew, Ritzhaupt-Kleissel, and Hausselt

(1998) who also states why this method is by far one of the most popular, due to the allowance for

controlling the resulting mechanical properties.

The designed shape is formed in the following stages according to Sakaguchi and Powers, (2012):

Through condensation and capillary reactions the liquid is removed from the zirconia slurry

(slip), which contains the fine ceramic particles in an aqueous state.

It is incrementally built up, shaped and finally sintered, which is where a material is heated

up to a high enough temperature to coherently bind the mass without being melted.

The produced sintered porous core is then glass-infiltrated, by way of capillary action

drawing molten glass into the pores.

Finally generating two interpenetrating networks, a crystalline infrastructure and a glassy

phase.

The combination of two strengthening mechanisms explains why alumina-zirconia slip-cast

ceramics offer the highest flexural strength and fracture toughness of all slip-cast ceramics

strengthening mechanisms (Denry and Holloway, 2010).

1. The stress-induced transformation in zirconia grains produces compressive stresses within

the transformed grains and surrounding glassy matrix, as well as circumferential tensile

stresses around the grains, accompanied by micro-crack nucleation.

2. Crack deflection is expected from the presence of large alumina grains.

In the processing stages it is vital to control the phase transformations for strengthening and so

there is no cracking when upon cooling.

10

VITA (2005) have issued their latest zirconia slip product called ‘VITA In-Ceram’. They are using an

alumina matrix with ~21% tetragonal zirconia oxide, for strengthening. Advantages of this system:

‘no shrinkage during glass infiltration, the material is easy to process in the intermediate stage and

shows a high degree of marginal accuracy after strengthening by infiltration’.

1.2.2.2 CAD/CAM. The use of CAD/CAM for zirconia substructures has become a more popular technique as the

Research & Development in CAD/CAM technology has increased in the present time, and

demonstrates ‘limitless improvements functionally regarding form’ according to Kurtzman (2014).

Using this method negates impression materials and instead a digital impression is created with

intra-oral scanning. This modern technique can be seen to be more accurate due to the removal of

shrinkage encountered when taking the impression or using the Plaster of Paris for the casted

models. Research conducted by Peluso et al (2004) explains in their article that, Plaster of Paris

models can be damaged and have rates of shrinkage and expansion of which a CAD model would

negate.

Or the design on the computer can be waxed up by a technician and then scanned into the computer

or using a pantographic device, like in key copying, which is where a resin pattern is made and then

copied onto the computer for machining and/or further designs (Sakaguchi and Powers, 2012).

CAD/CAM indicates two main forms of processing ceramics.

1. Soft-machining with pre-sintered blocks:

Zirconia is in the fashion of a ‘block’ which is only partially sintered by a manufacturer and later

fully sintered by the dental laboratory, which produces a recorded 900-1500 MPa flexural

strength fracture toughness which is greater than all other all-ceramic systems (Sakaguchi and

Powers, 2012). This method helps zirconia to achieve a higher flexural strength according to the

theory by Tinschert et al, (2007) that sintering after milling gives the result of better mechanical

properties compared to the densely sintered core (hard machining). ‘Pre-sintered’ 3Y-TZP blocks

11

are used to make either a single or multiunit restoration. The zirconia substructure is then

veneered, with a porcelain of a similar coefficient of thermal expansion. Renishaw (2008) have a

system called ‘Incise’, the main benefit of the soft-machining they say is that it ‘takes a fraction

of the time to mill’, compared to hard machining.

2. Hard-machining with fully sintered blocks:

Renishaw (2006) have a system which is used for hard machining of dental substructures, called

‘Incise’ (different branch of product to soft machined Incise), which utilises the Y-TZP structure for

transformation toughening effects. The use of fully sintered ‘blocks’ is produced via the ‘hot isostatic

pressing’ method which according to 3M (2008) produces surface defects and results in lower

strength. Plus has been shown to contain a significant amount of monoclinic zirconia, which is

usually associated with surface micro-cracking, higher susceptibility to low temperature degradation

and lower reliability (Denry and Gandhewar, 2007). Additionally they also concluded that due to the

high hardness, therefore low machinability of fully sintered 3Y-TZP, so the milling system has to be

particularly robust with specialised diamond burred systems needed. However it was found by (Kou,

Molin and Sjogren, 2006) that following this process the production gave a very smooth finish to the

surface.

1.2.3 Clinical survival rates. Through reviews of clinical papers on the in vitro study of zirconia-ceramic and metal-

ceramic crowns and bridges for comparison. Fig1.5 shows the collation of success rates for such

restorations. Results would indicate for the crown data that zirconia overall is a less successful

substructure over a longer period of time compared to metal (also shown in fig1.6). Whereas when

zirconia is the substructure for a posterior bridge they outperformed the metal comparisons shown

also in Fig1.7.

12

Device Material Success rate % Length of study Reference

Crown Zirconia-ceramic 94 4 (Roediger et al, 2010)

Crowns Zirconia-ceramic 95 3 (Rinke, 2014)

Crowns Zirconia-ceramic 94 3 (Raigrodski, 2004)

Crowns Metal-ceramic 94 12 (Zarone, Russo and Sorrentino2011)

Crowns Metal-ceramic 99 11 (Zarone, Russo and Sorrentino2011)

Crowns Metal-ceramic 84 11 (Zarone, Russo and Sorrentino2011)

Crowns Metal-ceramic 100 5 (Zarone, Russo and Sorrentino2011)

Crowns Metal – ceramic 98 3 (Rinke, 2014)

Posterior Bridge. 3 unit (End Abutment Design)

Zirconia-ceramic 96 4 (Wolfart et al, 2009)

Posterior Bridge. 3 unit (Cantilever Design)

Zirconia-ceramic 92 4 (Wolfart et al, 2009)

Posterior Bridge. 3 unit

Zirconia-ceramic 91 3 (Cercon, no date)

Posterior Bridge. 3 (and 5 unit).

Zirconia-ceramic 100 3 (Sailer et al, 2009)

Posterior Bridge. 3 Unit.

Metal-ceramic 83 5 (Sorrensen et al, 1998 )

Posterior Bridge. 3 Unit.

Metal-ceramic 74 5 (Kern, 2005)

Posterior Bridge. 3 Unit.

Metal-ceramic 65 3 (Sorenson et al, 1998)

Anterior Bridge. 3 Unit.

Metal-ceramic 100 3 (Sorenson et al, 1998)

13

(Fig 1.5: collated success rates of zirconia and ranging metal-ceramics).

(Fig1.6: comparison of single crown restoration, survival rates).

(Fig1.7: Posterior bridge comparison between zirconia and metal-ceramic for survival rates)

14

Zirconia - ceramic

Zirconia - ceramic

Zirconia - ceramic

Metal - Ceramic

Metal - Ceramic

Metal - Ceramic

Metal - Ceramic

Metal - Ceramic

75

80

85

90

95

100

105

Comparison of Zirconia-ceramic to ranging Metal-ceramic crowns

Crown types

Surv

ival

rate

s %

Zirconia -c

eramic

Zirconia -c

eramic

Zirconia -c

eramic

Zirconia -c

eramic

Metal - Ceramic

Metal - Ceramic

Metal - Ceramic

0

20

40

60

80

100

120

Posterior Bridge comparison

Bridge Types

Surv

ival

rate

s %

1.2.4 Technical Indications for use. 1.2.4.1 Strength.

Fig1.3: Mechanical properties of zirconia and alumina based ceramics (Piconi and Maccauro, 1999).

The flexural strength defined by Meckolsky (1995) as the ‘final force required to cause

fracture and is strongly affected by the size of flaws and defects on the surface of the material

tested.’ The fixed restoration undergoes a complex range of stresses, due to the mastication, tongue

and cheek muscles (Wang et al, 2013), therefore the indication of how much stress a material can

withstand is crucial. The flexural strength of zirconia was found to be ~ 900-1200MPa (Piconi and

Maccauro 1999) compared to ~350MPa for lithium disilicate and only ~100-120MPa for glass +

alumina based dental ceramics (Yoshida, Tsuo and Atsuta 2014). Therefore zirconia can undergo far

more ‘bending’ stresses than other ceramic materials out there, however only zirconia and lithium

disilicate would be indicated for use as a substructure or full-contoured restoration. The flexural

strength must be ≥100MPa as a requirement according to the ISO standard 6872:2008 when using

zirconia in the CAD/CAM processing route (ISO, 2008).

Another key strength measured is that of the ability to contain a crack, therefore resisting

fracture, which is a vital quality of any dental material, as cracks can be caused in multiple ways but

if kept contained then the material can remain clinically acceptable. Zirconia tested against lithium

disilicate showed it was up to ‘5x stronger’ in this field (Wang et al, 2013). However lithium

disilicates can be indicated for full-coverage restorations in the posterior, like that of zirconia, and

veneers of which zirconia cannot be indicated for (Ferencz, 2015).

15

The fracture load i.e. ‘indication of the material's ability to resist rapid crack propagation and

catastrophic fracture’ (Scherrer et al, 1998) which for a zirconia-ceramic crown is ~804-1067MPa

(Stawarczyk et al, 2011) compared to metal-ceramic crowns which can only be shown to withstand

~228-349MPa (Kang et al, 2003). Owed to zirconia toughening mechanism closing the opening

cracks.

Due to the strength of zirconia it was found that the coping/substructure can be made

thinner, than with substructure metal materials, with thicknesses such as 0.4mm. This leaves more

space for the aesthetical porcelain which needs a greater thickness due to it being a weaker material

(Sakaguchi and Powers, 2012). Lithium disilicate can be reduced to 0.3mm but only for veneers,

therefore zirconia at 0.4mm is still the thinnest substructure for fixed restorations including

posterior bridges.

Ozcan and Vallittu (2003) has also reported that zirconia had superior strength compared to

other all-ceramic materials. However it was also seen, that zirconia is difficult to etch, due to its

increased hardness, which can cause problems in the bonding process, hard machining or when

grinding.

1.2.4.2 Aesthetics. Zirconia, for use in dental technology, is the natural colour of teeth. Due to properties such

as: ‘the grain size is greater than the length of light, high refractive index, low absorption coefficient

and high opacity in the visible and infrared spectrum’ according to Heffernan et al, (2002). The

colour has sufficient opacity as to mask out dis-coloured oral structures below the restoration

(Sakaguchi and Powers, 2012). Whereas in a metal-ceramic restoration the opaque ceramic

preparation stages are utilised to hide the metal under the veneer porcelain. The veneer porcelain

that is added have to be carefully selected so that they don’t show through to the underlying

preparation. With zirconia which is a ceramic shade already, no masking out of underlying colour is

16

needed, leaving the choices of porcelain veneer, to cover a wider spectrum of shades, including the

more commonly desired high translucency shades. While the translucency of zirconia is not as high

as that for dental porcelains, their aesthetics are improved relative to those of conventional core

materials (Zang et al, 2012). This positive attitude is noted throughout the dental community in

review papers such as (Dangra and Gandhewar, 2014) and company literature (Zirconium Crowns,

no date) and (Ivoclar Vivodent, no date).

1.2.4.3 Sandblasting and surface treatments.

Regarding Fig1.4 the sandblasted materials were more resistant towards

hydrothermal degradation than pristine ceramics of the same chemical

composition and grain size, indicating that tetragonal and monoclinic zirconia grains in the surface of

the sandblasted Y-TZP hinder the propagation of the diffusion-controlled transformation during

subsequent exposure to an aqueous environment, such as is the oral environment. Hence why the

strength in air and artificial saliva was higher than any other treatment processed zirconia.

1.2.5 Clinical Indications for use.Zirconia’s range of use or ‘indication’ has become wider the researching and testing zirconia

has been continued since the 1960’s to find properties (such as those mentioned in 1.2.3) that can

be utilised in dentistry and beyond.

Indications for specific relation to fixed restorations where zirconia is the substructure (all can be

anterior or posteriorly subscribed):

17

Fig1.4: Aging in zirconia after differing processing treatments (Kosmac et al, 2007).

1. Single Crowns.

2. Implant abutments.

3. Bridge 3-6 units.

4. Bridge – curved and long-span up to 48mm in length.

5. Cantilever Bridge.

6. Inlay/Onlay Bridge.

1.2.5.1 Properties to aid indications.Christensen (2007) reports that:

Better aesthetics than typical metal-ceramic restorations.

Won’t discolour any natural tissues, such as silver can have a green effect on local tissues.

The margins of the restorations have a more acceptable appearance than those of metal-

ceramic restorations when gingiva recedes, or in cases of thin gingiva therefore no dark

shadowing is seen, such as would be found in metal substructures.

Gingival sensitivity to metal will be eliminated with use of zirconia-based restorations.

Winter (2012) found that zirconia is better at masking out moderate to severe discolouration of

underlying tooth structure and because of zirconia’s high strength the substructure can be more

conservative as zirconia crowns need less space, especially compared to metal-ceramics, zirconia

crowns compare closer to a single gold crown in this later respect.

Zirkonzahn’s (no date) use of zirconia has found that they prescribe no contra-indications

about preparation types, especially not ruling out the knife-edge preparations. Which allows the

dentist more freedom of choice for design aspects.

Zarone, Russo and Sorrentino (2011) states that ‘Zirconia has a favourable radio-opacity’,

which is useful in case of swallowing and when viewing the clinical placements of restorations in the

mouth via X-ray.

18

1.2.5.2 Biocompatibility. According to Warashina et al (2012) zirconia has a less phlogistic reaction in the oral cavity

compared to titanium and other metals. Scarano (2004) found that the bacterial adhesion

comparison between titanium and zirconia resulted with the latter showing a better result due to 7%

less microbial adhesion, this was backed up by Zarone, Russo and Sorrentino (2011), who stated that

‘zirconia doesn’t enhance bacterial adhesion’. Also noted by Askeland (2011) whom concluded that

it is harder for bacteria to adhere to a denser material. Through testing it was evident that zirconia

had the highest density value (5.8g/cm3 or 99.8%) compared to alumina oxide porcelains

(3.98g/cm3) and alumina – titanium restorations (96%) according to Beresnev, et al (2014).

Piconi and Maccauro (1999) also found that zirconia has no cytotoxic affects. The scientific report

was conducted by use of fibroblasts which were cultured with zirconia and by way of monitoring

them it was noted that no significant effects were caused by the zirconia deeming it to be classed as

‘biocompatible’.

Regarding the gingival cells response Zarone, Russo and Sorrentino (2011) found that zirconia also

has a low corrosion potential, certainly compared to that of gold alloys that might release ions into

the oral environment causing inflammatory reactions.

Furthermore zirconia has the smallest of, all restorative materials, for thermal a conductivity rating

when tests were conducted in comparison to a gold alloy which has a thermal conductivity value of

200 W/ (mK), zirconia is 100x less conductive (2W/ (mK)) according to Ceramtec (no date). This

means when put in-situ the material will not absorb the heat from food or drink shocking the nerves,

as much as gold does, certainly a positive in respect to the patient.

However it was seen in Vagkopoulo et al’s (2009) research that significant amounts of alpha

radiation in zirconia based ceramics (surgical implants) because of high levels of ionization, which

could damage cells of oral tissues. For gamma radiation conversely, the radiation level is not

worrisome.

19

1.2.6 Technical Limitations of use.1.2.6.1 Ageing

Aging or more specifically zirconia’s degradation at ‘low temperature is a progressive and

spontaneous phenomenon that is exacerbated in the presence of water, steam or fluids’ according

to Volpato (2011), which can cause consequences such as surface deterioration, microcracks and

decreased resistance in short and long term. Hence why it is of most important in fixed

prosthodontics, due to the low temperature and wetness of the oral environment. Deville et al

(2003) explains that if the zirconia content in the composite, is kept low enough then the micro-

cracks aren’t formed and the percolation doesn’t occur, which means that no ageing ensues. When

zirconia is at 6.7Vol%, ageing can be avoided in medical appliances such as fixed prosthodontics and

will still be slower if below 16vol%. Hence why concentrations of zirconia must be considered and

tested to ensure that the material doesn’t degrade quickly leading to device failure. Additional

factors to be considered are: microstructure patterns, porosity, residual stresses, and particle size,

can affect the ageing ability (Chevalier, 2006).

In an in-vivo accelerated study over 24 months with a chemically aggressive wet

environment (such is the oral cavity), research by Kosmac, Jevnikar and Kocjan (2011) found that the

‘naked’ surfaces of the as-sintered dental Y-TZP are vulnerable to T-M transformation.

There was seen to be an accelerated ageing process in zirconia compared to metal-ceramics.

(Zarone, Russo and Sorrentino, 2011). However this can be argued by Kosmac and Kocjan (2012)

whom under a 24 study noticed no strength degradation of accelerated in-vitro ageing.

Chevalier (2006) found that additives, such as alumina, helped by slowing the ageing of

zirconia, leaving the desired properties to be perceived. They also found that the ageing process is

accelerated when mechanical stress and wetness exposure is increased.

In relation then to zirconia in company literature such as ‘ICE zirconia’ which was recorded at

1400 MPa, showed an average strength degradation of ~30% (980 MPa) which is a higher flexural

strength than conventional metal even post degradation (Zirkonzahn, no date).

20

Using the colloidal processing route i.e. powder mixing, it is possible to increase the zirconia

concentration which gives good mechanical properties. But not too high, so that it is still possible to

avoid the ageing phenomena of grains bunching into aggregates. Which is advantageous, two fold,

by keeping the nanometre grain size, the important residual strains after cooling will improve the

resistance to crack propagation. Also by avoiding zirconia aggregates, it’s possible to increase the

zirconia volume fraction so that the transformation toughening becomes effective in the material,

and therefore both further increasing the toughness. It was seen in this paper that ZTA ceramics had

a better resistance to ageing than the monolithic 3Y-TZP (Deville et al, 2003).

Kosmac, Dakskobler, Oblak and Jevnikar (2007) investigated the aging of zirconia (Y-TZP)

with specific focus on commonly used luting cements. It was found that they absorbed water via

dentine tubules, thereby exposing the zirconia core to moisture which may lead to aging problems

over a shorter period of time than anticipated.

1.2.6.2 Veneer cracking.When zirconia is used with a veneered porcelain then a strong bond is needed between the

two that won’t undermine the strengths of the two components i.e. no matter how great the

strength of zirconia or the veneering porcelain, neither will matter if the bond strength between the

two is weak, as it will cause them to separate which will see the device fail.

Research and development has created modification systems to make more successful bonding

between zirconia and porcelain, such as:

Primers which aids chemical retention (Zandparsa et al, 2013) and (Griffin et al, 2002) and

(Yoshida Tsuo and Atsuta 2014).

Tribochemical coatings using bonding of silica particles to the surface of zirconia, creating

heightened roughness for increased mechanical retention (Matinlinna, Lassila and Vallittu

2007) and (Kern and Wegnera, 1998) and (Ozcan and Vallittu 2003).

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Silane coupling agents help by increasing the wettability will improve the bonding to polar

surfaces such as conventional porcelain ceramics (Yoshida, Tsuo and Atsuta 2014) and

(Matinlinna et al, 2006).

Acid etching, sandblasting and other surface roughening helps to aid the mechanical

retention: (Zandparsa et al, 2013) and (Cassuci et al, 2009) and (Kosmac et al, 1999) and

(Papanagiotou et al, 2006) and (Phark et al, 2009) and (Kosmac et al, 1999) and (Curtis,

Wright and Fleming, 2006) and (Blatz, Sadan and Kern, 2003).

1.2.7 Clinical Contra-Indications of use.Zirconia is found not to be suitable all the time for every fixed prosthodontic restoration, regardless

of its attributes, here are some examples of that:

Prostheses requiring precision attachments or stress breakers are best made with metal-

ceramic restorations (Christensen, 2007).

Veneers, onlays and inlays (Ivoclar Vivodent, no date) – zirconia as a monolithic structure

could be indicated for these restorations (Glidewell Labs, 2015).

The cost of zirconia-based restorations is often higher than that of metal-ceramic

restorations when seen in dental journals such as Christensen’s (2007) however company

literature such as Zirkonzhan (no date) almost agrees stating that ‘Zirconia restorations are

equal to or marginally more expensive than metal-ceramic restorations.’

Heavy Bruxism was researched by Perry et al (2012) and found that due to the overlay

porcelains weakness, a gold alloy crown would be more clinically successful – however a

metal-ceramic restoration can sustain less stress than the zirconia, so they wouldn’t be

recommended for any bruxist patients either.

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1.2.7.1 Veneer separation. Sakaguchi and Powers (2012) refers to issues post-production, caused by unrecognised CTE

differences in earlier production stages of a zirconia crown. It was shown that the two layers,

zirconia and veneered porcelain, can separate leaving micro-gaps for penetration from bacteria,

where they can multiply in colonies and hide away from the mechanical removal such as brushing.

The separation is also of major concern for the strength which could cause loosening, complete

separation of the two parts leading to failure and patient dissatisfaction. The separation could even

be dangerous if lead to materials being swallowed. This can also be caused by other mechanical and

processing techniques, such as sandblasting and grinding done by technicians or dentists. Causing

surface phase changes to the grain structure because stresses on the surface particles, which are

open to attack unlike the protected under layer of tetragonal grains, are then ‘free’ to transform

(Sakaguchi and Powers, 2012). These structural changes can lead to the internal stresses several

microns below the surface waiting to form cracks and later cause failure of the material (Piconi and

Maccauro 1999).

1.2.7.2 Veneer cracking.Veneer cracking/chipping was investigated by Augustin-Panadero (2014) who found that

zirconia-ceramic restorations failed in this way ‘between 6% - 15% over a 3 – 5 year period, while for

metal-ceramics the fracture rate ranges between 4% - 10% over ten years’. Sailer (2009) found that

minor chipping of the ceramic veneer of zirconia-ceramic restorations was 25% compared to 19.4%

of the metal-ceramic crowns. Both crowns and bridges in Rekow et al’s (2011) study, showed a

degree of chipping, but the size was much greater with zirconia cores showing 25% compared to

metal-ceramic bridges with 19.4% chipping rate. Zarone, Russo and Sorrentino (2011) and Tinschert

et al’s (2007) work mentions that overloading and fatigue in the clinical setting could be another

factor for veneer cracking. In greater detail explained by Lee et al (2000), the accumulating

microcracks, from loading in an aqueous environment (such as oral cavity), causes surface defects

that may enhance tension in areas of localised concentration, initiating fracture under lowly applied

stresses.

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1.2.7.3 Difficulty in adherence to the porcelain/ceramic veneer. It has been seen that using conventional luting agents to bond the zirconia substructure to

the ceramic veneer, is short lived. This issue was found in Magnea, Paranhosa and Burnett’s (2010)

work. Which has helped to further show why substantial research and development into luting

agents for a sufficient bond to be made is crucial in developing the working life of a restoration.

1.2.7.4 Workability.Due to zirconia’s high level of hardness, there are later issues in the processing such as at the

chair side in the dental practice, which can mean that dentists may be less keen to want to work

with zirconia due to extra time or the need for extra strengthened tools (like that of the more

expensive diamond burs) to be able to successfully grind (Cehreli, Kokat and Akca 2009). Helvey

(2008) found this to be true in retrospect of the time it takes to grind a conventional metal

substructure.

1.3 Aim of Investigation.In answering the hypothesis through thorough collation of informative data, finally

concluding on whether ‘The selection of Zirconia as a substructure material assures longevity of the

final restoration’ in specific regard to fixed restorations.

Longevity of the device can be seen as performing either to a desired length of time or, in this case

specifically, surpassing the average working life of other restorative materials. This could be seen as

a better marker for whether zirconia is in fact the correct selection or not.

The longevity of a device in the oral environment, is determined by the mechanical and chemical

wear it undergoes on a constant to frequent basis. The device failure can occur in many ways for

instance: in a mechanical sense i.e. breaks or is weakened; the device could become less

aesthetically pleasing to the patient, i.e. discoloured by water sorption or could alter the colour of

the oral tissues.

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To gauge a ‘bench mark’ for Zirconia, it must find what dental devices are indicated with zirconia

substructures, so that the comparison to the conventional/other materials used can be assessed,

plus how long those materials survive before they are deemed inadequate for their job.

To fully analyse why zirconia is outliving or antagonistically not, zirconia as a material needs to be

understood, so that the properties and structure of the material can be related and used to answer

for the found longevity of zirconia in a fixed restoration.

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2 Methods. 2.1 Type of study.

This is a systematic review. Information was searched, collated and reviewed in comparison

to other findings on similar or contrasting work for analysis.

2.2 Selection criteria. The method searching for specific information was that of ‘BOOLEAN’, this simple addition

of a ‘code word’ i.e. ‘AND’, ‘NOT’, ‘OR’, meant that the search could be specified and therefore

would reduce the results found.

2.2.1 Keywords: Zirconia

Substructure

Limitations

Indications

Contra-indications

All-ceramic

Fixed prosthodontics / restorations

Fixed partial denture

Crowns

Bridges

Mechanical properties

Aesthetical properties

When searching for restorations that zirconia could be indicated for, the term ‘fixed partial

denture’ (in American papers) kept being used, which was found to mean a ‘bridge’ in the British

terminology. Therefore it is necessary to include this term in my search, as their findings are just as

valid as British work, but might have been excluded from results if not included.

For this method I used the ‘this OR that’ search method i.e. ‘bridge’ OR ‘fixed partial

denture’.

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2.2.2 Language.Only papers in English, were used, no method of translation was used by me as it may

translate wrong and that would give me invalid results.

I did come across a paper that was published abroad in a non-English speaking country, but

it had been professionally translated for English reading. Therefore this was accepted in my

results.

2.2.3 Criteria for success rates evaluations. There were no biases towards age, gender or race in my review of clinical success rates, and

these papers findings were my only needed scientific public study. The rest were all mechanical

property testing studies of zirconia.

Two specific exclusion criteria I made was:

Animal studies, on the basis of differing properties needed for animal e.g. dog oral

healthcare over human oral healthcare.

Any studies that were a mix of crowns and bridges etc. weren’t used, as their findings

weren’t specific enough to each component separately e.g. ‘9 year study, Crowns + bridges,

52.66% - main issue found was chipping’ , here the word main issue is too vague for a

scientific study.

2.2.4 Date or time scale of publication. Any investigation less than 3 years was excluded due to short time span, deemed insufficient

for a longevity study and therefore for true valid results to be extrapolated.

When searching for definitions of ageing for zirconia, I found some old sources e.g. 1949. This was

deemed too old for zirconia specifically, much more relevant to a simple metal definition, but as

zirconia wasn’t understood nor utilised in that period I excluded that from my results.

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2.2.5 Mechanical properties selection criteria: I began to find studies related to zirconia as a cladding material in fuel cells for radioactive

uses. Although the material is the same I wanted to exclude this from my search, as they will alter

zirconia’s properties for high heat needs over what is expected in the oral environment.

2.3 Databases.

2.3.1 Search terms. When searching for a specific topic in mind, the use of double quotation marks (“…..”) tells

the search engine that you want to find results with those specific terms or phrases. This helps to

make the searching much more efficient.

2.3.2 Online Databases: MMU library database searches through the Dental journals we have access to, and all books

that are available for use to us.

PUBMED was used on occasion to go direct to a source if the MMU library search turned up a paper

but couldn't get through to the full text – passwords were supplied for membership for full access to

journals.

Google scholar was also a useful resource for the same reason as PUBMED, but also widens the

search range compared to MMU library, as long as the full text was viewable either for free or with a

given password.

Google search engine was used to find company literature.

Journals reference list/ books reference list. If more information than that given by the paper being

read was required, then finding the paper in question and making a review was quite a good system.

However this method only works for extra investigation of similar data, but for a deeper analysis of a

subject it would be recommended to use a broader search, i.e. the online databases.

2.4 Data collection and analysis.2.4.1 Study selection:

Once papers were found from a database search, the papers then are opened online. If

there can’t be accessed in full then they are disregarded without view, as information was never

taken merely from an abstract.

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The paper abstracts are reviewed to ensure that sufficient qualified informative data was included in

the paper and that it was relevant to my inclusion of search, and if so the paper was then saved as a

bookmark for thorough reading. Once all papers of that search results/enough papers were found,

then the review and noting process took place.

2.4.2 Data extraction: All publishing paper details i.e. author/s, date, journal title etc. were noted for them to be

written up later in the form of ‘MMU Harvard referencing’.

Then a read over of the journal itself, before going back over for further deeper noting of their and

others referenced work. This is easiest done when a first read is done, to find areas of interest to the

topic in hand.

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3 Results.

30

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4 DiscussionTo critically appraise the selection of zirconia as a substructure in fixed prosthodontics, the

hypothesis must be either proved or disproved. So far the history, background, use and limitations of

zirconia in dentistry have been analysed, but to what extent do they impact the longevity of the final

fixed restoration i.e. crown or bridge? Throughout the literature review it has been expressed that

zirconia has high strength, associated with its own toughening mechanism, therefore suggesting that

it will succeed and last longer than weaker conventional substructures in fixed prosthodontics.

However, the strength of zirconia may not be able to outweigh the veneer cracking and separation,

difficulty of bonding, and workability for the technician and the dentist.

4.1 Advantages One of the great advantages for the use of zirconia in dentistry is because of its natural

tooth- like aesthetics. Plus it can have a sufficient opacity to mask-out underlying structures and with

companies such as Ivoclar Vivodent releasing new opacity and translucency ranges, the aesthetics

are improving constantly (Ivoclar Vivodent, no date).

In addition to aesthetics, the main mechanical property that zirconia possesses is the

‘transformation toughening’ effect which lies dormant until needed. If zirconia was pure and cooled

slowly then it would transform from cubic to tetragonal and finally to monoclinic at room

temperature (Piconi and Maccauro, 1999). However; this is not practically seen, as it cools too

quickly and the most commonly used zirconias are doped with a stabilizer e.g. 3mol% yttrium (3Y-

TZP) to ensure that the tetragonal grains remain. With the tetragonal grains remaining at room

temperature then, when tensile stresses are applied, such as that from a crack, it is closed with the

countering compressive stress (3-5% expansion according to Piconi and Maccauro, 1999) as the

tetragonal grains transform into monoclinic grains. This toughening mechanism can be realised

when sandblasting modifications are done, which remove sufficient layers to expose the underlying

TZP (Kosmac et al 2007), this machining gave the outcome of 100% survival rate in air and artificial

saliva solutions, compared to 50% survival rate in a control sample of as-sintered zirconia structure.

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The strengthening effect can been shown as the fracture toughness which for TZP in comparison to

alumina is 3-6KIC greater (Piconi and Maccauro, 1999), showing that impact from the opposing

dentition is less likely to affect that of the TZP restorations, hence their indication for posterior use

where the mastication is strongest. This can been shown diagrammatically as in Figs 1.4 and 1.6 that

the use of zirconia-ceramic bridges in the posterior constantly out-performed the metal-ceramic

bridges.

Due to this increased strength the substructure itself can be thinner (0.4mm instead of >0.5mm for

conventional metal-ceramics) allowing for the porcelain, which is weaker, to be veneered in larger

quantities (Sakaguchi and Powers, 2012) or kept to the same ratio allowing for the zirconia

restoration to be indicated for small occlusal cases. Plus if the porcelain is allowed more space, this

gives extra room for the aesthetical appearance to be perfected i.e. via the porcelain layering

technique.

Wang et al (2013) also confirmed this high strength with the quantifiable data of fracture resistance

which for zirconia was found to be ‘5 times greater than that of lithium disilicate’. In addition, the

flexural strength values can be seen to be up to and above 1GPa for TZP (Piconi and Maccauro, 1999)

compared to that of lithium disilicate, alumina’s and glass aluminas which all can’t exceed 500MPa.

These comparisons are crucial as the flexural strength is what indicates the level to which the

material in loading conditions can withstand cracks without fracture.

Another property of zirconia which indicates the density, comes from the sintering

processes’, this property can have great biocompatibility effects, relating to zirconia’s increased use

in metal-free-dentistry which is becoming more popular as metals are seen as old fashioned due to

corrosive elements and less aesthetical appeal. Literature that suggests this, such as Scarano (2004)

who proved that zirconia had 7% less microbial adhesion compared to titanium substructures, which

can correlate with Beresnev et al (2014) who showed that the greatest density was zirconia (99.8%)

compared to alumina-titanium (96%). Zirconia also produced the least phlogistic reaction, showing

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less tissue cell interaction, compared to titanium and other conventional metals for substructure use

(Warashina et al, 2012).

Other reasons for zirconia’s increased use could be due to the move into modern processing

techniques, with computerisation, we see the increased development of milled machined zirconia

substructures. Kurtzman (2014) found that with the use of CAD/CAM the forms and functionality of

fixed prosthodontics is limitless. Such as the possibility for negation of likely shrinkage in impression

and model casting stages (Peluso et al, 2004) is leading towards the indications of use. However; this

can be outweighed by the fact that the final production of a substructure, shrinks by 20-25%

(Komine, Blatz and Matsumura, 2010) via soft-machining, therefore needing an over-sized design,

and if this has been produced with wax/hard resin, this can be seen as wasteful. Then again with the

increased R&D into the CAM area, companies such as Kavo (no date) have created a product to

reduce said issue e.g. shrinkage free (Kavo Everest system of ZrSiO4 ceramics) therefore resolving

that issue, and their results can be backed up by research from Heydecke et al (2007) and the use of

‘dense shrinkage free ZrSiO4 ceramics by (Hennige et al, 1999). At the same time as the development

of the CAD area, databases of anatomical morphology become more accurate and are stored

reducing the demand for any preparation work to be done manually suggesting that in fact this

method is more conservative as no products are lost compared to the conventional method of wax-

ups, burnouts and casting of metals with sprues.

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4.2 Limitations. The hardness of zirconia is accentuated when milled in the hard-machining process, due to the fully

sintered state of the zirconia ‘blocks’. Which makes for a tougher surface leading to higher rates of

tool wear, and for the technician this can make for an expensive processing technique, in

comparison to soft-machining which uses pre-sintered ‘blocks’, and later sintering (Kou, Molin and

Sjorgen, 2006). Regarding the type of milling used, the hard machining approach which has a higher

tool wear rate (Denry, 2007) leading to more cost on tools has been shown to be a popular method

which could be due to the use of the system but also the manufacturers sales technique i.e.

Renishaw (who makes hard and soft milling machines) who have ‘spun’ the negative, of higher tool

rate loss into a positive by saying that their machinery will measure the loss of the tool during

processing and compensate for this in the amount that is removed from the machined product so

that it remains accurate to its design (Renishaw, 2006). Yet the consumer, be it the technician,

dentist or patient, will be paying more than in comparison to soft machining. However using this

method there is no shrinkage that the technician will have to compensate for, as the block was fully

sintered beforehand (3M, 2008).

This level of hardness can also have later implications, in which ever method the zirconia

substructure is machined, as the dentist often has to do minor but necessary adjustments which may

now only be possible with the use of high strength expensive diamond burs. Plus this grinding will

take more time than the conventional metal-ceramics (Helvey, 2008).

In the same method mentioned before by Kosmac et al (2007) when grinding the surface, they found

that this machining process could introduce and increase the surface flaws. It is suggested that when

surface modifications are being made to the zirconia substructure, the manufacturer’s instructions

are followed to control the effects the technician has on the materials structure

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The T-M transformation however can be detrimental to the substructure if certain factors

are not controlled. Grinding of the substructures surface can increase the rate of ageing of zirconia,

brought about by water diffusion in the created cracks. Kosmac (2007) found that Y-TZP with

commonly used luting agents showed ageing problems over a shorter period of time than in metal-

ceramics bonded similarly. The water was found to be absorbed by the dentine tubules and then

contact with the zirconia substructure. In research by Kosmac, Jevnikar and Kocjan (2011) and

Zarone, Russo and Sorrentino (2011) whom all found that in accelerated in vivo studies (with

chemically aggressive wet environments, such as the oral cavity) there was significant ageing

degradation compared to metal-ceramics. However these studies do not allow for the fact that the

zirconia substructure is to be veneered with porcelain which acts as a barrier between the wet

environment and the zirconia, therefore suggesting that these findings are less likely to occur in

practice. The company literature of Zirkonzahn (no date) who produce ‘ICE zirconia’ state that even

with an average degradation of 30% (420MPa) to flexural strength from ageing, their zirconia will

still be at a higher level (980MPa) than that of conventional metals for metal-ceramic’s. It was stated

by both Chevalier (2006) and Deville et al (2003) that with the addition of alumina to the zirconia

substructure there will be sufficiently less ageing i.e. a ZTA ceramic.

Ageing isn’t the only factor for limiting the longevity of zirconia. In a thorough review of

clinical studies on zirconia-ceramic and metal-ceramic crowns and bridges, it was easy to see that

zirconia is often out-performed by the longevity of metal-ceramic crown restorations (as seen in Fig

1.4). In fact the mean results of those zirconia restorations was 94.5% survival rate over a year range

of 3-4 compared to the metal-ceramic studies which ranged up to 11 years and demonstrated and

average of 95% (however there were more metal-ceramic studies to be used, so a more valid

average was found, that did included one result of much lower value that brought the average down

by 2.5%). The newer zirconia systems will need to be tested using longer studies to be as valid as the

metal-ceramic studies, but for that time is needed.

36

The limited longevity of zirconia restorations has seen to be linked to the failure of the restoration as

a whole unit not necessarily the failure of the zirconia substructure. Therefore the failure reasons for

the veneering ceramics must be reviewed. One of the most common causes is veneer

cracking/chipping which can have multiple reasons such as: incorrect CTE matching of porcelain to

zirconia, porosities, improper framework support, unnecessarily high furnace temperatures which

will increase the tensile stresses residually left in the porcelain veneer after firing. Lastly due to the

complex nature of zirconia’s chemical composition on its surfaces, bonding between the porcelain

and zirconia or at the later stage of cementing the all-ceramic crown to the abutment in the mouth

may not have been sufficient (Zarone, Russo and Sorrentino, 2011) and (Tinschert et al, 2007).

Austin–Panadero (2014) found that this resulted in a ‘6%-15% failure rate over a 3-5 year period

compared to that of metal-ceramics with a failure rate of 4%-10% over 10 years’.

Veneer bonding has received a plethora of attention, as it may be a solution to increasing

zirconia’s longevity as a fixed restoration. Due to this limitation of zirconia, there has been much

recent Research and Development which has been linked with the R&D from the 1980’s into

improving the adherence of zirconia as a biomaterial, but now focusing that attention on dental

appliances to see if they can improve the bonding, which should increase the lifespan of the

restoration as a result. If the veneer bond is stronger, then the strength of zirconia can be realised

otherwise the separation of the two components will certainly lead to fracture of the porcelain,

which left unnoticed will become a bacteria trap according to Sakaguchi and Powers (2012).

Airborne particle abrasion (APA) is one of a highly considered method of raising the wettability and

shear bond strength (SBS) of the zirconia’s surface for cementing (Zandparasa, 2013) and (Cauca,

2009). Kosmac (1999) suggests that it also induces the T-M transformation, increasing flexural

strength and SBS, but if over worked the process will introduce micro-cracks (Curtis, 2006). Research

by Phark (2009) proved to show that APA in fact could end up smoothing the surface of zirconia as it

37

was suggested that zirconia began with moderate roughness, which would seem to counteract the

need for improved SBS in the first place.

Therefore alternative research of chemical surface alterations should be considered. The use of silica

which attacks and displaces the hydroxyl groups over the surface, the tribochemical coating method

which increases the roughness for mechanical attachment (Kern, 2006). Backed up by Matinlinna

(2006) who describes the surface tension as lowered, increasing the surface energy, making the

zirconia (often Y-TZP or Y-PSZ) hydrophilic for attachment to the hydrophobic resin.

4.3 Future.With zirconia being realised for its strength potentials and aesthetics too, the uses of

systems such as ‘Zenostar Full Contour Zirconia’ (Ivoclar Vivodent, 2013) may become more widely

indicated because it removes the need for bonding between two structures i.e. substructure and

veneer. Due to it being shown that veneering porcelains are one of the main issues causing failure of

zirconia all-ceramic restoration. Additionally the strength of zirconia can be utilised for cases where

occlusal space is limited.

As mentioned before any ongoing clinical studies reporting survival rates for zirconia will be

more valid for comparison to other substructure materials used, as they will have been in-situ for

longer periods of time than those used for comparison and analysis now.

High-strength porcelains such as lithium disilicate (IPS e.max CAD), seen reporting around

400MPa flexural strength (Kang, Chang, Song, 2013) demonstrating a high strength even compared

to conventional metal substructure values e.g. dental casting alloys types II and IV (yield strengths of

>201MPa). Therefore there is definite reason for indication for the same use as the metal

substructures and veneering e.g. crowns and long span bridges, additionally the aesthetics of lithium

disilicate are better than that of zirconia due to high transparency. So the use in veneering to the

zirconia substructure may then reduce the veneer cracking problems which have caused such

38

detrimental results for longevity. However lithium disilicate may become more widely used on its

own, as it can be indicated for posterior full-coverage restorations, with a small thickness of 0.3mm

which is great for tight occlusal spaces, whilst maintaining that high strength (Ferencz, 2015) plus

lithium disilicate can be indicated for veneers, something that zirconia cannot.

5 Authors conclusions.Through the literature search it is evident that zirconia is being used more in fixed

prosthodontics, especially as it can be seen to be a more successful indication for posterior bridges,

due to the transformation toughening mechanism possessed, opposed to the use of the

conventional metal-ceramic method. Plus with the increasing utilisation of CAD/CAM systems which

help promote the use of zirconia further. Along with the aesthetical tooth similarity found in these

all-ceramic restorations which are being pushed forward as the metal-free dentistry alternative.

However the cracking and difficulty for bonding of the veneered porcelains to the zirconia

substructure, especially in a crown situation, is causing issues for longevity results. Yet with the

research and development into these fields, increasingly finding new methods for achieving better

‘shear bond strength’ coupled with using a higher strength porcelain such as lithium disilicate, could

help the zirconia-ceramic restorations to improve their survival rates and surpass those of the metal-

ceramics.

At the present time the literature shows that zirconia cannot assure the longevity of fixed

restorations as a whole entity. Until the veneered porcelains and bonding strengths are improved or

negated from use i.e. the indication of full contoured zirconia crowns, instead.

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