Eighty-six patients with osteoarthritis or rheumatoid arthritis of the knee were randomized to receive a total knee arthroplasty (TKA) using customized cutting blocks (CCB) or conventional cutting instruments (CCI) for prosthesis alignment. The outcomes of interest included femoral rotation deviation, tibial rotation deviation and the incidence of rotational outliers. Follow up was performed at 2 years post-operation. The results revealed significantly less tibial rotation deviation in the CCB group compared to the CCI group. No significant difference in femoral rotation deviation was observed between the two groups. In 1 of the 3 reference lines for tibial rotation, a higher incidence of rotational outliers was observed in the CCI group compared to the CCB group.