Fixation of ceramic prostheses by means of osteointegration implies use of porous structures in which bone tissues can grow. Such structures require total porosity values between 50 and 75 %, and pore size values between 100 and 500 µm. It is possible to manufacture scaffolds that comply with porosity requirements by means of 3D printing processes like Fused Filament Fabrication (FFF). However, such printing technology does not allow to directly select pore size and porosity value to be obtained. On the contrary, process variables such as layer height, nozzle diameter, infill, speed, etc. need to be selected before printing. Main objective of the present work is to define a model that helps selecting appropriate values for printing variables in order to obtain required porosity and pore size values. Such model will be applied to grid structures. In a first step, relationship was searched between pore size and three process variables: layer height, nozzle diameter and infill. In a further step, curves for pore size as a function of infill were searched for the three usual nozzle diameters employed for printing ceramics, 150, 250 and 410 µm. Finally, pore size and infill were determined for mean pore size of 300 µm. Results showed that the higher nozzle diameter, the lower infill should be.