الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Summary
The loss of coronal tooth structure for pathologic reasons can be
related to a multitude of individual and/or combined etiologic factors
related to oral and dietary habits which ultimately cause abrasion and/ or
erosion of enamel and dentin. With the pattern and extent of damage to
severely worn teeth, the restoration of these defects becomes increasingly
demanding and often requires multiple alterations to preparation design.
Therefore, a conservative approach as part of “minimally invasive
dentistry” in both the preparation and restoration design is emerging.
To achieve such a conservative preparation; a restorative material
will be needed to attain enough strength while allowing minimum
thickness. These two properties can be attained with the use of modern
ceramics and computerized restorative dentistry (CAD/CAM).
Henceforth, the present study was aimed at studying posterior
occlusal veneers as a recent treatment option for restoring worn occlusal
surfaces. The use of various preparation designs and modern ceramics
milled with CAD/CAM machinery was investigated. Marginal adaptation
and fracture resistance were investigated in an attempt to give an idea
about the durability of the restoration under laboratory conditions.
The aim of this study was to evaluate marginal adaptation and
Fracture resistance of occlusal veneers with two different preparations: the
standard occlusal preparation and standard occlusal preparation with a
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buccal veneer extension ‘vonlay’; using different ceramic materials:
Lithium disilicate ceramic, hybrid ceramic and Translucent solid zirconia.
Sixty extracted, archived, sound and non-carious human
mandibular molars were selected using the average crown dimensions. The
selected teeth were divided into two main groups according to the
preparation of the occlusal veneer where group (O) received a standard
occlusal reduction in enamel following the anatomical form with an angle
of 150o between the cusps and group (OV) that received a standard
occlusal reduction in enamel following the anatomical form with an angle
of 150o between the cusps with a buccal extension ending with a chamfer
finish line “vonlay”.
Each group of teeth was then subdivided into 3 subgroups
according to the material of the occlusal veneer they received where
Subgroup (a) teeth received lithium disilicate ceramic, Subgroup (b)
received hybrid ceramic and Subgroup (c): teeth received translucent solid
zirconia.
All molars were restored with standardized anatomical occlusal
veneers, of 1.5 mm minimum thickness at the position of the central fossa
and the two occlusal planes, buccal and lingual, with a 150o angle between
them. These restorations were generated with the help of CAD/CAM
system.
Self-adhesive resin cement was applied on the occlusal surface of
the occlusal veneers after each restoration received its proper and strict
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surface treatment according to the manufacturer’s instructions. The
fabricated occlusal veneers were then cemented onto their corresponding
teeth.
All specimens were stored in distilled water in an incubator at 37
o
C for 3 days. Samples were evaluated for marginal adaptation. After
initial marginal adaptation evaluation, the samples were subjected to
thermocycling in a deionized water bath with 10,000 cycles between 5°C
and 55°C, with a dwell time of 15 seconds and a 15 seconds transfer time
between baths to simulate temperature fluctuations in the oral cavity. The
cemented samples were evaluated again for marginal adaptation after
thermocycling using microcomputed tomography.
These samples were then individually mounted on a computercontrolled material testing machine with a loadcell of 5kN and data were
recorded using computer software. The load for fracture was recorded in
Newtons.
All the data collected from marginal adaptation and fracture
resistance results was tabulated and statistical analysis was performed.
For marginal adaptation, all the samples showed a volumetric gap
increase after thermocycling, but all the values were within the clinically
recorded reference values. The vonlay preparation recorded statistically
significant higher marginal gap values than the standard occlusal
preparation. Regarding the material, monolithic solid zirconia gave the
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highest marginal gap values, lithium disilicate followed after and the least
values were recorded with hybrid ceramic.
For fracture resistance, regardless of the material of the occlusal
veneer, there was no statistical significance between the two different
preparations. As for the material; again; statistically significant results
were noted between monolithic solid zirconia and the other two materials,
lithium disilicate followed by hybrid ceramic. However, no statistical
significance was found between lithium disilicate and hybrid ceramic
restorations.
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Conclusion
Within the limitations of the study, the following conclusions could
be drawn:
• Thermocycling resulted in an increase in the marginal gap value
regardless of the preparation design or the ceramic material.
• All samples showed marginal gap values that were clinically
acceptable within the range of 150 µm whether before or after
thermocycling.
• Increasing the area of the preparation results in an increase in
marginal gap, as well as the fracture resistance of the restoration.
Clinical recommendations:
- Hybrid ceramics can be considered a promising alternative to
lithium disilicate for they offer better marginal adaptation values.
In case of high occlusal forces, Bruxzir should be considered.
- Both the vonlay preparation and the standard occlusal veneer
preparation can be considered as treatment options in supporting
the tooth structure against fracture when high forces are expected.
Conclusions
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Suggestions for further investigations:
1. The use of a chewing simulator should be considered since it gives
a better and more accurate representation of the parafunctional