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Quality Resource Guide –

Porcelain Veneers 3rd Edition

www.metdental.com

veneer. Placing a thin film of a bonding resin after

the silane has been applied and dried significantly

improves the bond strength of new resin to old

composite.

26,27

Bonding to existing composite resins compared to

tooth structure causes no significant microleakage

or fracture load differences.

In a clinical trail of

of 3.5 years, porcelain veneers bonded to existing

composites of good quality untreated and treated

with silane and Cojet (a silica coating) showed no

significant difference in outcomes.

Design to Prevent Chipping & Fractures

How do you minimize chipping and fractures

of porcelain veneers? All veneers depend on

adequate support to minimize chipping. Bonding

to tooth structure provides some of the support.

Making sure there is no more than 2mm of free-

standing porcelain will also minimize fractures,

especially for diastema closures. There should

be adequate anterior guidance in protrusive and

lateral movements in order to prevent excessive

stresses on the veneers. Group function may place

too much stress on veneers and can increase the

incidence of chipping. Inadequate bonding can also

cause chipping of the porcelain.

Make sure each

bonding step is followed carefully and without error

or contamination. Fracture can also occur if the

preparation design includes sharp angles rather

than rounded angles. Research is quite conclusive

in demonstrating the concentration of forces in

areas of the preparation that are sharp.

6,7,8

Interproximal Extension

Where should the interproximal margins end?

Some clinicians will favor not breaking the contact

with the adjacent anterior tooth, thereby placing

interproximal margins slightly labial to - or right

at the proximal contact.

Margins placed labial to

the proximal contact may reveal stain over time.

Proximal margins are extended lingually when

closing diastemas; including old restorations that

extend lingually past the proximal line angle; and

when a tooth is being repositioned by veneering

to alter either the inciso-apical tilt, the incisal edge

placement, or the mesio-distal alignment of the

facial and proximal surfaces. In all cases, clinicians

are urged to prepare the tooth especially near the

gingival margin interproximally, so that the original

tooth structure does not show. (Figure 12; note the

areas at the arrows)

Veneer preparations should have a chamfered

margin gingivally and interproximally.

10,11

This

permits sufficient thickness for the veneer to mask

underlying discoloration. A well-defined finish line

permits accurate positioning and smooth marginal

finishing of the veneers. Unless discoloration would

cause a need to do otherwise, the gingival margin

should be placed at the crest of the gingival margin

of tissue. The most favorable response from the

gingiva is achieved with the margins at the crest of

the gingival tissue.

10,11

If dark discoloration must be

covered, then the margin design may extend about

0.5-1.0 mm subgingival. This will assure a favorable

periodontal response.

For many years after porcelain veneers were first

introduced, it was recommended that all veneer

margins end on enamel, because dentin bonding

at the time was not predictable.

10,11

With dramatic

improvements in dentin bonding over the past

twenty years, there is greater acceptance of

preparation designs that place margins in dentin

or cementum. However, the clinician should realize

that, over time, some margins ending on dentin or

cementum will leak and could affect the longevity

of the restoration, even when the best materials

and techniques are used. The greater the percent of

the preparation is on dentin, the greater the risk for

fracture.

11,12,13,14,15,16

Incisal Design

Suggestions for the design of the incisal margin of

the tooth preparation has varied over time, but today

there is general consensus that preparation of the

incisal margin for porcelain veneers is required, but

it should permit no more than 2mm of free-standing

porcelain. It is generally agreed that extending

porcelain more than 2mm beyond the incisal edge

of the tooth significantly increases the likelihood of

fracture.

Suggestions for the design of the linguo-

incisal margin has varied over time but the best

incisal design identified by research is a 1-2mm

incisal reduction with a butt joint (i,e, no lingual

chamfer). This design will minimize incisal chipping,

and will make the fitting, bonding and fabrication

of the porcelain veneer easier.

Newer research is

beginning to support the idea of two designs to the

incisal. For worn and chipped incisal enamel, a butt

joint is recommended; whereas for healthy normal

teeth an incisal overlap, i.e. a palatal chamfer is

preferred.

18,19,20,21

The butt joint design handles

stress better in worn teeth and concentrates stress

less in unworn teeth and have proven over a 7-year

clinical trail to be reliable.

In a situation where a significant portion of the

incisal length or incisal angle is missing, it is

recommended to build the incisal angle up using

composite resin prior to covering the restored

defect with a porcelain veneer. Research has shown

that replacing missing tooth structure with a bonded

composite will improve the outcome in terms of

fracture resistance; as compared to not building up

the fractured tooth and ending up with too great a

thickness of porcelain.

22,23

Other studies show that

bonding to previously placed composites affect

the bond strength and can increase incidence of

chipping, or fracture.

19, 20

The theory underlining this

finding relates to the water sorption of composite

resins and its effect on free monomer. This water

sorption will reduce the availability of free monomer

to which resins subsequently bond. Generally, the

older the composite, the weaker is the bond to

it. The best practice is to replace any composite

material contained within the porcelain veneer

preparation outline, if the existing composite age

is greater than about 2-3 years. Repair strength

studies show bond strengths after that time begin to

fall below 50% of the tensile strength of the material

being studied.

24,25

Research also directs us to apply

silane to any pre-existing composite material under

a porcelain veneer before luting the porcelain

Figure 10

Metered tooth reduction

Figure 11

Completed porcelain veneer preparation