It is reported that the length of the posterior cruciate ligament (PCL) fibres increases as the knee flexes, and the magnitude of the length change reaches up to 9.8 mm throughout the range of motion, which exceeds the range of failure strain. Therefore, we postulated that a compensatory mechanism must be recruited to overcome this large strain in order to maintain physiologic function as a key component of joint kinematics. Our main objective was to compare the length change pattern for the linear distance between the femoral and tibial tunnels with the length change patterns derived from a real isometer test of different curvatures. We utilized ten intact cadaveric knees and created a vertical femoral tunnel (5 mm medial to the roof of the intercondylar notch and 5 mm proximal from the articular margin) and lateral tibial tunnels (5 mm proximal to the posterior bony ridge on the lateral side of the PCL fibre) and performed a 3D-CT scan at 0A(0), 30A(0), 60A(0), 90A(0), and 120A(0). The distances between the femoral and tibial tunnels were calculated from the 3D coordinates. Real isometry was checked both (1) over the PCL and (2) under the PCL using an isometer with an accuracy of 0.1 mm. The path over the PCL had the longest intra-articular length, followed by the path under the PCL, and the lengths measured by CT, respectively. The path over the PCL had a more curved path compared with the path under the PCL and the lengths measured by CT. The lengths measured by CT showed significantly larger excursion than the real isometer test. The path over the PCL showed the least excursion through the range of motion, followed by the path under the PCL, and the lengths measured by CT, respectively. Our findings suggested that a more curved PCL path has better isometry because the curvature of the PCL compensates for the length change between 0A(0) and 60A(0) flexion. In remnant preservation PCL reconstruction, the passage of graft over the PCL would have increased intra-articular length and better isometry compared with straight under the PCL path. Basic science study.