Introduction: Clinical follow-up studies of total knee arthroplasty (TKA) reveal good results. However, the range of flexion of the knee after TKA remains limited in most cases. The most important factors impacting this range are the length of the quadriceps, the capsular tightness, the tibial slope, the design of the implant, the use of physiotherapy and the surgical technique. Aims: Our aim was to measure maximal knee flexion as a function of the posterior slope of the tibial component. We tried to quantify the relationship between tibial slope and maximal knee flexion. Methods: Seven fresh-frozen cavader-knees were used, all of them had moderate osteo-arthritis. Mean age of the patients was 67. A posterior-cruciate-ligament retaining TKA was used. In each knee, the tibial component was implanted consecutively with a slope of 0°, 4° and 7°. For each slope, the maximal flexion was measured using fluoroscopy in a test rig. Results: The mean maximal flexion achieved was 104° for an implant with a 0° slope; 112° for 4° slope and 120° for 7° slope. We regressed the maximal flexion on the tibial slope, using a Pearson regression analysis and obtained a R2=0.8356; indicating a strong correlation. Conclusions: In PCL retaining TKA, there is an increase of 2° in flexion for every increase of 1° posterior tibial slope.