Do larger heads wear less than smaller heads?
Theoretically, if the metal couple is dry, larger heads should wear more than smaller heads due to their longer sliding distance per step. However, in the presence of the fluid the opposite is true: larger diameter heads should wear less because of their greater sliding velocity. Calculations show that larger diameter wear couples can form a thicker synovial fluid film between components.12 Hmin = 1.64D(_Ú/ED)0.65(W/ED2)-0.21 Hmin is the minimum film thickness D is the head diameter Ú is the entraining velocity According to the formula above, the larger the articulating diameter, the larger the Hmin value. A thicker fluid film means less contact between hard surfaces during motion and, presumably, less wear. Does this theory prove itself? The study cited above compared 22mm, 26mm and 35mm diameter metal-on-metal articulations and found no difference between the three. Isaac compared 16mm, 22mm, 28mm, 36mm and 54.5mm diameter couples and, for diameters 28mm and larger, it was determined
