Refiner plate elements for a conical mechanical refiner include: a rotor plate element including at least one rotor plate segment having a feedstock inlet opening disposed at a first end of the rotor plate segment; and a rotor plate segment refining area disposed between the feedstock inlet opening and a second end of the rotor plate segment. The refiner plate elements may further include: a stator plate element including at least one stator plate segment having a stator plate segment refining area; and first and second attaching rails configured to couple to the stator plate segment to a stator support frame of the conical mechanical refiner. A separation between the first attaching rail and the second attaching rail that is not covered by the stator plate segment is configured to form a feedstock outlet opening.