A refining plate segment for a mechanical refiner of lignocellulosic material including: a refining surface on a substrate, wherein the refining surface faces a refining surface of an opposing refiner plate, the refining surface including bars and grooves between the bars, wherein an angle of each bar with respect to a radial line corresponding to the bar increases at least 15 degrees along a radially outward direction, and the angle is a holdback angle in a range of 10 to 45 degrees at the periphery of the refining surface, and wherein the bars each include a leading sidewall having an irregular surface, wherein the irregular surface includes protrusions extending outwardly from the sidewall towards a sidewall on an adjacent bar and the irregular surface extends from at or near the outer periphery of the refining surface extends radially inwardly along the bars without reaching an inlet of the refining surface.