Introduction:
Resistance to sliding and tipping of a restoration must be considered at the time of treatment planning. Some factors have influence in resistance such as type of occlusion, size of occlusal table in comparison with cervix, and place and amount of occlusal forces. When the occlusal table of a restoration is wide and the cervical part is narrow, it is very probable that the line of action of an applied force on marginal ridges passes outside the margins of the restoration and produce destructive torque and causes dislodgment of the restoration. Leverage is probably the predominant factor in the dislodgment of cemented superstructure of implant supported prosthesis, that results in disruption of the cement film. The purpose of  this study was to evaluate the effect of finishing line form on cement microleakage in implant supported prosthesis following occlusal forces in cusp to marginal ridge occlusion.
Materials and Methods:
In this experimental study, 90 metal models were made similar to implant abutment with three types of finishing lines (Shoulder, Sloping shoulder, and Knife edge). Ninety crowns were made from base metal alloy with the occlusal table similar to mandibular first molar and cemented on the models. All specimens were subjected to 1000 thermocycles between 5°^c and 55^oc, with a dwell time of 15 seconds. Each group was divided into 2 subgroups. The subgroups were subjected to 50000 masticatory cycles(equal to 3 months of mastication) with 60 and 100 N occlusal forces on the marginal ridge. All samples were then immersed in 5% basic foshin solution for 24 hours and mounted in polyester to be sectioned vertically. Microleakage was measured under a microscope with magnification of 40 times. The results were analyzed using One-Way & Two-Way ANOVA  and t-test.
Results:
There was no difference in microleakage rate between 3 forms of finishing lines. The interaction between finishing lines and occlusal force of 60 N showed significant difference  (P= 0.0182<0.05).
Conclusion:
The results of this study showed that the least microleakage was observed in shoulder finishing line.