الفهرس 
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Abstract
Recently, the high performance thermoplastic polymer polyetheretherketone (PEEK) gains increasing attention for dental applications. Moreover, it has high mechanical properties, good biocompatibility, a low density as well as high resistance towards external influences. So far, PEEK has been met the growing patient demand for metal-free dental reconstructions as framework material for fixed dental prostheses. However, it has to be mentioned that the material has an unaesthetic grey colour and appears opaque. Because of its innate opacity, PEEK must be covered with veneering composites when used as a framework for dental prostheses if an acceptable esthetic outcome is to be attained.
Therefore, when screening potential restorative materials, it is important that the materials should have the ability to mimic the natural tooth with regard the overall color together with its high resistance to wear.
Our study was designed to evaluate the masking ability and the wear performance of the veneer material of PEEK restoration.
Twenty-fourdisc-shaped PEEK coping specimens were prepared with 15 mm diameter and 0.8 mm thickness. The samples were divided into three groups with regard the veneer thickness (core/veneer: 0.8/1.0, 0.8/1.5 and 0.8/2.0 mm for groups I, II and III respectively).
The color measurement was done by the clinical spectrophotometer (Easyshade V). The color differences (ΔE) and the HSB (Hue, Saturation or chroma, Brightness or Lightness) parameters have been individually evaluated for each sample.
Then, a two-body wear test was conducted in a custom-made wear device which was specially designed and fabricated for this intended study. The veneered PEEK disc specimens were placed in a specimen holder in a lower water bath compartment at constant temperature (20 ◦C, 37 ◦C). Human enamel antagonist cusps were embedded via acrylic resin in stainless steel cylindrical rods mounted in sliding bars of upper part of the wear device to antagonise the veneered PEEK specimens. The wear cycles were adjusted to be of a total 250,000 cycles at loading force of 20N with continuously supplied artificial saliva.
Statistical analysis was carried out the mean value of ΔE was found to be (8.22±0.41) for group I, and(5.13±0.43) forgroup II which considered clinically unacceptable. While for group III the mean ΔEwas (3.23±0.34) which was below the clinically acceptable threshold and considered clinically acceptable.
Regarding ΔLas the thickness of the veneering material increased significant reductions in ΔL values were recorded. group II and III showed a closer correspondence with the lightness parameter with no significant difference between them. Analysis of ΔC showed that group III with the highest veneer thickness resulted in a closer correspondence with the chroma parameter. There was significant, negative correlation between ΔC and thickness for veneering resin. It was not possible to correlate a linear relationship between ΔH and the veneer thickness.
Regarding wear measurements statistical analysis was carried using the weight loss percent and the mean weight loss of the PEEK and enamel antagonist specimens.
The results showed that there was no statistically significant different between the three tested groups of PEEK specimens. Also, there was no statistically significant difference in the wear pattern of the antagonist enamel cusp in the three groups. Thus, the veneer thickness had no effect on the wear pattern. However, the mean wear loss of PEEK specimens was significantly lower than its antagonistic enamel cusp in all the studied groups.