الفهرس 
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Abstract
Our study was conducted to evaluate the efficiency of 4 rotary Ni-Ti retreatment file systems which are protaper universal retreatment file system, Rogin retreatment file system, Soco pro retreatment file system and M3 progold retreatment file system. Remaining filling materials left on walls, amount of extruded debris, number of fractured files and the time required for retreatment process were evaluated.
The total number of samples was eighty root canals in forty mesial roots of lower mandibular first molar with two separate root canals and was randomly divided into four groups of twenty canals each according to retreatment system used. Samples were decoronated, the MB and ML canals were prepared and instrumented using manual k files taper 0.02% up to master apical file #25. The step back technique was performed using K-file sizes #30, #35, #40, then obturated with gutta percha and adseal using cold lateral compaction technique. After obturation specimens were sealed with cavit and stored in 100% humidity and 370c to allow sealer to set.
Obturation material of samples was removed using protaper universal retreatment files (group (I)), Rogin retreatment files (group (II)), Soco pro retreatment files (group (III)) and M3 Pro gold retreatment files (group (IV)). The removal was done without using solvent and irrigation was done by distilled water.
The percentage of remaining obturation material was measured by vertical sectioning of the roots followed by image capturing by stereomicroscope and image analysis using imageJ software. The amount of debris extrusion was evaluated by subtracting the pre-retreatment and post-retreatment weight of the Eppendorf tubes for each group.
The number of broken files were evaluated by its length shorter than normal with loss of canal patency. The time to reach working length was measured from the first retreatment file used until the last file reached the working length.
Regarding remaining root canal filling materials according to effect of file type, group I (Protaper retreatment) left less remaining root canal filling material at coronal third than other 3 groups with significant difference. But there was no significant difference between group III (Soco pro) and group IV (M3 pro gold). group I (Protaper retreatment) left less remaining root filling material at middle third than other 3 groups with significant difference. But there was no significant difference between group II (Rogin) and group IV (M3 pro gold).
Remaining root canal filling material at different root levels, values measured at the middle third were significantly higher than value measured at the coronal third in the four groups.
Regarding amount of apically extruded debris, group (IV) showed the lowest value with no significant difference with group (I), while group (IV) showed the highest amount of debris extrusion with significant difference to the other 3 groups.
Regarding time to reach working length, group (I) Protaper retreatment file system was faster than the other 3 groups, while group (III) was the slowest with significant difference between the 4 groups.
Regarding the number of fractured files according to file system the highest value was found in group (IV), while the lowest value was found in group (I). According to file type, D3 file was the highest file to fracture (n= 14), while D1 file was the least (n= 3).