الفهرس 
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Abstract
This study aimed at evaluating two polymeric materials as substitute for cast Co-Cr alloy in fabrication of major connectors (palatal straps) of maxillary tooth bounded removable partial dentures, regarding their mechanical and degradation properties, in an attempt to avoid using Co-Cr alloy due to concerns about its biocompatibility regarding metal ions release intraorally, and reports of metal allergy among patients.
The tested polymeric materials were CAD/CAM acetal (TSM) and mold-injected PEEK polymer reinforced by ceramic fillers (granular BioHPP®).
The three tested materials in this study were: 1) cast Co-Cr alloy (Wironit® Bego) 2) CAD/CAM acetal polymer (TSM) 3) thermally injected granular BioHPP®. Specimens of the three tested materials were fabricated in three forms: i) palatal straps without rest or clasps ii) rectangular bars iii) specimens for microstructure study.
Seven groups of specimens were fabricated for the three tested materials: three groups of palatal straps and three groups of rectangular bars specimens divided equally between the three tested materials. Each group included 6 specimens of each tested material, except the seventh group included one specimen for each tested material (3 specimens) for the purpose of microstructure study.
CAD/CAM technology was used for the fabrication of specimens, to achieve standardization of specimens’ dimensions, cast Co-Cr alloy and thermally injected granular BioHPP specimens were fabricated using CAD/CAM milled patterns, while acetal specimens were milled from CAD/CAM acetal blocks. Polymer palatal straps (CAD/CAM pink acetal and BioHPP) were fabricated with thickness twice that of metal palatal straps, while rectangular bars of the three tested materials (cast Co-Cr alloy, white CAD/CAM acetal and granular BioHPP) were fabricated with identical dimensions.
Measurements were taken for specimens’ dimensions (palatal straps and bars) to check the standardization of specimens’ dimensions. Each group of specimens was subdivided equally into 2 subgroups (except the microstructure study group), one subgroup tested in dry conditions
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and the other tested after water storage for about 6 months at temperature of 37.5 ± 2 ºC.
Palatal straps were subjected to fracture resistance test, bars specimens were subjected to flexural strength and vicker’s microhardness test. Co-Cr alloy subgroups were considered as control subgroups in the mechanical testing. Fracture resistance and flexural strength tests were carried out using universal testing machine and flexural strength test was carried out using three-point bending test.
Regarding the biodegradability tests, the storage water of the palatal straps of the three tested materials was subjected to ATR-FTIR spectroscopy, UV-Vis spectrophotometer, high performance liquid chromatography (HPLC) and atomic absorption spectroscopy (AAS).
Field emission microscope scanning (FEM) supported by energy dispersive spectroscopy (EDAX) analysis was used for microstructural analysis of microstructure study group specimens, and for scanning and analysis of fractured or deformed palatal straps of the three tested materials either from non-stored or stored subgroups.
After statistical analysis of mechanical testing results, pink acetal palatal straps showed unfavorable properties in comparison to Co-Cr alloy straps, represented by fracture of acetal straps to separate pieces, while Co-Cr alloy straps fractured without separation of pieces. Also, acetal straps showed significant lower mean values of maximum compression load than that of Co-Cr alloy straps either in stored or non-stored subgroup, and lower than the average maximum biting force at first molar area. Water storage caused significant deterioration of maximum compression load mean value of acetal straps.
CAD/CAM White acetal bars showed significant lower mean flexural strength values than that of Co-Cr alloy bars either in stored or non-stored subgroup. White acetal also showed significant lower mean values of microhardness than Co-Cr alloy, either in stored or non-stored subgroup. Water storage caused significant decrease in microhardness of white acetal bars.
The biodegradability tests using ATR-FTIR, UV-Vis spectrophotometer and HPLC, confirmed presence of formic acid in the storage water of pink acetal, which is considered indicator for the
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degradation of acetal to formaldehyde, which in turn oxidized into formic acid.
Microstructural analysis of pink acetal specimen using EDAX revealed the organic structure of the polymer skeleton depending mainly on oxygen and carbon elements. Fluorine element was detected but in a much lower ratio. FEM scanning of the fractured acetal straps confirmed the semi-crystalline nature of acetal microstructure.
Regarding BioHPP straps in fracture resistance test, they showed higher toughness than acetal straps, as straps suffered permanent deformation after the test, without fracture. Water storage had no significant effect on the maximum compression load (MCL) of BioHPP straps. Mean value of MCL of BioHPP straps however was significantly lower than that of Co-Cr alloy straps either in stored or non-stored subgroup, also lower than the average maximum biting force at first molar area.
Flexural strength mean values of BioHPP bars were significantly lower than that of Co-Cr alloy bars either in stored or non-stored subgroup. Water storage did not cause significant change in the flexural strength mean values of white acetal or BioHPP bars but caused significant decrease in the mean values of microhardness of both of them. BioHPP bars showed significant lower microhardness mean values than that of Co-Cr alloy bars either in stored or non-stored subgroup.
FEM analysis of BioHPP microstructure study specimen showed presence of ceramic fillers agglomerates in PEEK matrix. EDAX analysis showed the presence of Al and Si which are most probably related to ceramic fillers composition. Si could also be related to presence of silane coupling agent. EDAX analysis revealed also presence of Ti which is most probably related to titanium dioxide compound used as white pigmentation of BioHPP.
FEM scanning confirmed the leaching of ceramic fillers agglomerates from BioHPP matrix after water storage, it confirmed also the role of ceramic fillers agglomerates as weak sites for crack propagation in non-stored BioHPP straps. FEM scanning also revealed that some fillers were in the range of nanometers.
HPLC did not detect phenols in storage water of BioHPP straps, but ATR-FTIR, UV-Vis together with AAS confirmed presence of Al
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element in the storage water of BioHPP, which could be related to ceramic fillers leaching or corrosion.
Cast Co-Cr alloy straps in fracture resistance test showed no significant changes in maximum compression load (MCL) mean value due to water storage, the same for alloy bars which showed no significant changes in flexural strength, or in microhardness mean values due to water storage.
Leaching of cobalt element from Co-Cr alloy into storage water was suspected by ATR-FTIR and UV-Vis, then confirmed finally using AAS. By calculating the leaching value for each cm2 of alloy surface it is estimated that leaching rate of cobalt element is increasing by increasing time of storage.
FEM scanning of Co-Cr alloy microstructure study group specimen showed the crystals dendritic structure with inter-dendritic areas of cast Co-Cr alloy similar to that reported in the literature. FEM scanning together with EDAX analysis showed that inter-dendritic areas in non-stored metal straps were occupied by carbides, silicon, and oxide layers more than dendritic areas, and also had lower cobalt content than the alloy matrix.
Corrosion and severe loss of cobalt element from inter-dendritic areas in alloy matrix after water storage was confirmed by FEM and EDAX of stored metal straps.
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Within the limitations of this study it can be concluded that:
1- CAD/CAM pink and white acetal (TSM) materials lack the required mechanical properties for the fabrication of maxillary removable partial dentures major connectors.
2- It is not recommended to use acetal polymer as a removable partial denture material due to biocompatibility concerns, as it is highly suspected that saliva could cause its degradation intraorally into formaldehyde.
3- Thermally injected granular BioHPP does not have the satisfactory mechanical properties to be used as a maxillary removable partial denture framework.
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