الفهرس 
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Abstract
Our study was conducted to evaluate the efficiency of three rotary nickel-titanium retreatment systems in terms of remaining obturation material on canal walls, debris extruded out of the canal, time taken to reach full working length and number of fractured files in each system.
One hundred and twenty canals in sixty mesial roots of lower mandibular first molar teeth were used in this study.
The teeth were divided into three groups (n=40) according to the file system used:
• group I: 40 canals were prepared using Protaper universal retreatment file system.
• group II: 40 canals were prepared using Mpro retreatment file system.
• group III: 40 canals were prepared using RT gold file system.
Access cavities were done, the tooth length and canal curvature of the mesio-buccal and mesio-lingual root canals were measured. Canals were cleaned and shaped using k-files then obturated using lateral condensation.
Collection of the apically extruded debris was done with the method described by Myers and Montgomery (92) using Eppendorf tubes. Afterwards the Eppendorf tubes were placed in an incubator at 370 C for 15 days for desiccation.
A microbalance with an accuracy of 10-4g was used to measure the Eppendorf tubes before and after preparation of the teeth. Then, the extruded debris weight was measured by subtracting the initial weight from the final weight.
Percentage of residual filling material on the canal wall was evaluated by splitting the teeth longitudinally then photographing them using stereomicroscopic camera. Finally, images were analyzed using Image J software.
The time required for retreatment was measured from the first retreatment file used until no obturation material can be distinguished on the last file.
The number of fractured files in each system was recorded.
Under the circumstances of this study, it can be concluded that:
1- All systems failed to clear the canals from remaining obturating material especially in the apical third.
2- Protaper universal retreatment system proved to be more efficient than other systems in intra-canal filling removal.
3- Debris extrusion remains problematic in all systems used.
4- Protaper universal retreatment system is the most speed efficient among systems used.
Recommendations
1. More systems should be evaluated for better clearance of the canals from obturation material especially in the apical third.
2. More systems should be evaluated for less debris extrusion during retreatment.
3. Better performance of files in terms of speed and fracture resistance.