If hard-on-hard material pairs are used it is essential to minimize asperity interactions and wear. The
components are manufactured with high accuracy and the smallest realistic roughness. For metal-on-metal
combinations, the femoral head diameters range from about (28-62) mm, the composite surface roughness
(Ra) values for both heads and cups are often in the range (5-20) nm, while diametrical clearances range
from about (50-300) µm. When implanted, surface scratches may result in local higher roughness
compared with the starting values. There are conflicting reports on the influence of, “running-in” upon the
surface roughness in MoM hip joints