Introduction: The aim of this work was to compare the posterior tibial slope of the tibial component after performing a total knee arthroplasty, as an intramedullary or extramedullary guide was used during the surgical technique, as well as comparing the range of mobility obtained according to the Instrumentation used.Material and methods: We conducted a descriptive, retrospective, observational study of a series of 57 patients operated in our Center during 2012 and 2013, with the same model of total knee arthroplasty (Sigma PS® DePuy), divided into two homogeneous groups in terms of age, sex, degree of osteoarthritis evolution, the first with patients operated by extramedullary guidance and the second by intramedullary guidance. A radiographic study was performed, measuring the posterior slope angle in the sagittal plane. The range of mobility achieved after arthroplasty and implant survival was studied.Results: Together, in both groups, measurements of the posterior slope angle were made, which was 4.35º preoperative average. In groups, the mean postoperative posterior fall angle was 4.04 ° in the patients who underwent an EM guide, while those who underwent an IM guide the mean was 1.76 °; the differences being statistically significant. The range of mobility in the ATRs operated by intramedullary guidance was 102.7º on average (range 80-125º), while in the group where the extramedullary guide was used, it was 104.3º (range 80-130º) no these differences being statistically significant. Regarding the survival of the implant, during the study period two cases of patients undergoing surgery for the replacement of prostheses were found, both of which belonged to the ?Extramedullary guide? group.Conclusion: The posterior drop angle, for the same PS prosthesis model, was statistically different according to the guide used, being within the 3-7º range in the group where the extramedullary guide was used; but despite these differences, no impact on the range of mobility was observed.Currently, it is recommended to restore the patient's posterior tibial slope, and in cases with a slope greater than 10º use a PS ATR.