Abstract Background To prevent excessive tension on the posterior cruciate ligament, some knee prosthesis-systems offer the option of creating a posterior tibiofemoral slope of the tibial component. The objective of this study was to investigate the effect of the amount of tibiofemoral slope on the posterior cruciate ligament load and tibiofemoral contact stress after total knee arthroplasty under isokinetic in vitro conditions. Methods Twelve fresh frozen knee specimens were tested in a knee simulator. After implantation of the Interax I.S.A. ® knee prosthesis-system with a mobile bearing inlay, a bow shaped load transducer was fixed in the medial fibres of the posterior cruciate ligament. A pressure sensitive film was fixed on the femoral inlay surface. The test cycle simulated an isokinetic extension cycle from 120° of flexion to full extension. First, posterior cruciate ligament load and tibiofemoral peak contact stress were measured with the tibial component implanted with a neutral tibial slope and then with 10° posterior slope. Findings After implantation of the tibial component without tibial slope, posterior cruciate ligament load reached a maximum load of 29.5 N (SD 17.1 N) at 97.8° knee flexion. Tibiofemoral contact stress on the medial compartment reached a maximum of 11.9 MPa (SD 2.4 MPa) on the medial compartment and 15.0 MPa (SD 6.1 MPa) on the lateral compartment. With a tibial slope of 10°, posterior cruciate ligament load reached a maximum of 14.5 N (SD 4.9 N, P = 0.04) at 100.5° knee flexion and tibiofemoral stress increased to a maximum of 13.3 MPa (SD 4.7 MPa, P = 0.38) medial and 17.4 MPa (SD 8.2 MPa, P = 0.22) lateral in knee extension. Interpretation Maximum posterior cruciate ligament load was observed at high knee flexion angles, decreasing to full extension. The implantation of the tibial base plate with 10° dorsal slope reduced posterior cruciate ligament load significantly in knee flexion above 50° and slightly increased tibiofemoral contact stress in knee extension. Therefore a posterior tibial slope prevents an excessive load on the posterior cruciate ligament while having little effect on tibiofemoral stress at high knee flexion angles.