Although the results of a total knee replacement (TKR) are described as being good, many patients are not satisfied after the operation with certain limitations such as the inability to perform sports or other vigorous activities [10, 15]. This motivates engineers and surgeons in their search for superior implant materials and designs, leading to innovations such as surgical navigation and patient-specific cutting blocks, as well as high flex and anatomically formed knee designs. There is even a revival of interest for the bi-cruciate retaining knee implant. Besides these technical topics, the discussion about some more fundamental issues still continues. Still no consensus whether the posterior cruciate ligament (PCL) should be retained or substituted in TKR has been reached. Advocates for both PCL substitution and PCL retention can point to excellent clinical and radiographic results in the literature. Furthermore, findings in the area of biomechanics, histology, and gait analysis do not show convincing evidence for one technique above the other [11]. However, PCL balancing seems to be a factor of major importance. To function properly, the PCL must be accurately tensioned during TKR. This may seem evident, but clinical papers reporting on cruciate-retaining TKRs scarcely report on the functional status of the PCL or the contact point in the operated patients, even though several authors did discuss their finding that PCL insufficiency might be an explanation for some inferior results found for PCL-retaining implants [4, 14]. Thus far, little scientific data have been published regarding balancing issues related to the PCL, a topic generally considered to be complex. It seems crucial that the PCL is placed under appropriate tension such that the kinetic benefits of its retention can be enhanced while the adverse effects of either an excessively tight or lax PCL can be avoided. It is essential that a balanced situation is (re)created during TKR since a too tight PCL may result in decreased ROM, potentially affect PE wear, and implant fixation due to posterior loading [12, 13], whereas a too slack PCL may lead to AP instability, which could result in pain, effusion, impaired function, and higher contact stresses [1, 13]. But what actually happens during PCL balancing? In two cohort studies, we investigated the relation between flexion gap height and translation of the tibia when the knee was distracted by a tensor. By measuring distances, forces and by performing statistical analyses, we found that balancing the flexion gap is an extremely delicate technique. With a mono-block tensor, the ratio between gap height increase and anterior tibial translation was 1:1.25 [3]. For a bi-compartmental tensor, a higher ratio was found, being 1:1.9. To make it more complex, this ratio was the highest for knees with a steep PCL and with those in which the collateral ligaments had been released during the procedure. In short, the effects of gap distraction were highly variable between patients [7]. Hence, only a 2-mm change in tibia bone cut height, a change of 2–3 degrees in slope of the tibia cut, a change of 2–3 mm in the bone cut on the posterior condyles of the femur or in PE-insert thickness all can make a difference between a perfectly balanced PCL and one that has been too tightly or too loosely balanced. Without computer navigation, these slight variations are easily made. When the tibia moves forward during distraction of the flexion gap (whether from the implant, a spacer or a P. J. C. Heesterbeek (&) Research, Development & Education, Sint Maartenskliniek, Nijmegen, The Netherlands e-mail: p.heesterbeek@maartenskliniek.nl