The invention relates to a refiner plate segment in a refiner plate for mechanically refining of lignocellulosic material in a refiner, said refiner plate segment comprising at least a first, generally radially extending bar, a second, generally radially extending bar, a groove arranged and defined between said first, generally radially extending bar and said second, generally radially extending bar, and a main dam, which has a height H and is arranged in the groove, wherein a pre-dam is arranged in front of the main dam, said pre-dam has a height h which is less than the height H of the main dam.