The use of extramedullary and intramedullary guides to prepare the tibial cut was studied comparatively in 100 consecutive primary total knee arthroplasties Low Contact Stress rotating platform. Each type of guide was used in 50 consecutive cases for a total of 100 cases. An ideal tibial alignment (90°± 2°) was obtained in 42 cases (84%) using the intramedullary system (IM group) and in 36 cases (74%) using an extramedullary alignment system (EM group) (p = 0.14). A posterior slope of 10° (± 2°) was achieved in 45 cases (90%) in IM group and in 40 cases (80%) in EM group 2 (p =0.16). The difference was not statistically significant but a greater accuracy was demonstrated when using an IM tibial alignment system.