الفهرس 
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Abstract
Fracture of dental amalgam restorations is a common clinical failure that is encountered in dental offices. The present study aimed at studying the electrochemical characteristics of aged high copper amalgam systems, determining bond strength after their repair and investigating the effect of corrosion on the determined bond strength.
A total of 240 samples of two main groups; unicompositional and admixed amalgam alloys, were investigated in this study. 120 samples for each amalgam type in the form of disks 7 mm in diameter and 2 mm in thickness were constructed, weighed and divided randomly into four groups, 30 disks each according to the immersion periods namely: one week, one month, two months and three months. Each amalgam disk was stored in separate glass tube containing 1% NaCl solution and kept at 37o C. At the end of each storage period, each disk was rewired and the weight loss was calculated.
For each amalgam type, a disk representing each storage period was studied for its electrochemical behavior using potentiodynamic polarization test. The results were compared to that obtained for unstored sample.
The storage solution at the end of each period was collected and chemically analyzed using atomic absorption spectrophotometry. Three elements were detected; silver, copper and mercury and the quantities of these released elements were determined.
One surface of each disk was roughened and the samples of each group were then subdivided into three subgroups; 10 disks each. In the first subgroup, fresh amalgam mix was condensed to the roughened disks directly without using any bonding agents. In the second subgroup, fresh amalgam was condensed after application of Panavia F resin cement. In the third subgroup, fresh amalgam was condensed after application of All-Bond 2 adhesive resin. According to the type of freshly condensed amalgam, each subgroup was divided into two divisions; 5 disks each. Five disks were repaired with freshly condensed unicompositional amalgam while the other five disks were repaired with freshly condensed admixed one.
Using a universal testing machine, shear force was applied on the plane of repair at crosshead speed of 0.5 mm / minute and the bond strengths were calculated and statistically analyzed.
The amalgam disks were examined at different magnifications with scanning electron microscope (SEM) to study the surface of the disks before and after storage. After shear force application, the broken samples were also scanned to study the topography of the fractured surfaces. SEM equipped with energy dispersive x – ray spectrometer (EDX) was used to determine the elements and chemical compounds on the surface of amalgam specimens after storage and to quantify their percentages.
The results of weight loss showed that both amalgam types did not loose significant weight along the four storage periods. Furthermore, the weight loss of unicompositional amalgam was insignificantly greater than that of admixed one.
Potentiodynamic polarization test revealed that the unicompositional amalgam had general trend of greater corrosion potential compared to the admixed one. In addition, unicompositional amalgam had worse passivation than those of tested admixed amalgam samples.
Elemental analysis of the storage solutions showed that copper was the most released element from both amalgam types followed by silver while mercury was the least. Tin could not be detected in the solution as it formed insoluble corrosion products on the surface of amalgam samples. Generally, the amounts of elements released from unicompositional amalgam were greater than those from admixed one.
Statistical analysis of bond strength values showed insignificant difference among all investigated groups. However, when the effect of each variable on the bond strength was statistically analyzed, the results revealed that both types of aged amalgam and durations of the storage periods had insignificant effect on the bond strength values. On the other hand, the specimens repaired without any bonding agent as well as those repaired using Panavia F showed significantly higher bond strength values than those repaired using All – Bond 2. In addition, the specimens repaired with unicompositional fresh amalgam had significantly higher bond strength value than that repaired with admixed one.
Upon SEM examination, the corrosion of the unicompositional amalgam appeared as surface roughening, while that of admixed amalgam showed corrosion reaction zone surrounding the Ag – Cu eutectic alloy and extending to the grain boundaries of the adjacent 1 matrix. The micrographs of the fractured surfaces showed an adhesive failure mode between the bonding agent and the roughened aged amalgam surface.
SEM/EDX analyses of the corroded unicompositional and admixed amalgam surfaces revealed that mercury and tin were the most prevalent elements. However, tin oxides and chlorides were present in huge amounts on the surface of both amalgams compared to other oxides and chlorides.