OBJECTIVE The aim of this biomechanical in vitro study was to assess posterior cruciate ligament (PCL) strain following two different total knee arthroplasty (TKA) designs (fixed versus mobile) with regard to modification of the tibial slope. METHODS We investigated eight Natural Knee I (NK I) prosthesis with fixed bearing and eight Low Contact Stress (LCS) prosthesis with mobile meniscal bearings. TKA was performed using fresh frozen human cadaveric knee joints. PCL strain was measured with implantable force transducers. Knee kinematic assessment was made with a load of 300 Newton and without load using a six-degrees-of-freedom testing device. Modification of the tibial slope was analysed radiographically. Statistical analysis was performed using Student's t test, Wilcoxon rank sum test, and the Spearman coefficient of correlation. RESULTS Assessment of the tibial slope showed a non-significant increase of 2.1 degrees (p = 0.14) following TKA using the NK I, and of 1.1 degrees (p = 0.12) using the LCS, respectively. Analysis of PCL strain following implantation of the NK I prosthesis revealed non-significant alterations both with (p = 0.74) and without load (p = 0.20). Concerning the LCS prosthesis, a significant decrease in PCL strain was seen with load (p = 0.01), whereas non-significant modifications were measured without load (p = 1.0). The modified tibial slope and modified PCL strain following LCS TKA showed no (with load: r (s) = 0.01) and modest correlation (without load: r (s) = - 0.43), respectively, whereas it was substantial following NK I TKA (with load: r (s) = 0.64, without load: r (s) = 0.70). CONCLUSIONS As the NK I prosthesis allows PCL tension to be close to normal as the knee flexes, it can be stated that regular PCL tension after TKA is restorable and, moreover, it can be hypothesised that the effected tension of the PCL mainly depends on the interaction between design of the implant (fixed/mobile) and the functional role of the PCL.