Purpose To assess, using model‐based dynamic radiostereometric analysis (RSA), the biomechanical behaviour of a new design posterior‐stabilized (PS) fixed‐bearing (FB) total knee arthroplasty (TKA) in vivo while patients performing two common motor tasks. The hypothesis was that model‐based dynamic RSA is able to detect different behaviour of the implant under weight‐bearing and non‐weight‐bearing conditions. Methods A cohort of 15 non‐consecutive patients was evaluated by dynamic RSA 9 months after TKA implantation. The mean age of patients was 73.4 (65–72) years. The kinematic evaluations were performed using an RSA device (BI‐STAND DRX 2) developed in our Institute. The patients were asked to perform two active motor tasks: sit‐to‐stand in weight‐bearing condition; range of motion (ROM) while sitting on the chair. The motion parameters were evaluated using the Grood and Suntay decomposition and the low‐point kinematics methods. Results The dynamic RSA evaluation showed a significant difference (p < 0.05) between the biomechanical behaviour of the prosthesis during the two motor tasks. When subjected to the patient weight (in the sit‐to‐stand) the low point of the medial compartment had a shorter motion (5.7 ± 0.2 mm) than the lateral (11.0 ± 0.2 mm). This realizes a medial pivot motion as in the normal knee. In the ROM task, where the patient had no weight on the prosthesis, this difference was not present: the medial compartment had a displacement of 12.7 ± 0.2 mm, while the lateral had 17.3 ± 0.2 mm. Conclusions Model‐based RSA proved to be an effective tool for the evaluation of TKA biomechanics. In particular, it was able to determine that the fixed‐bearing posterior‐stabilized TKA design evaluated in this study showed a medial pivoting movement under weight‐bearing conditions that was not present when load was not applied. Under loading conditions what drives the pattern of movement is the prosthetic design itself. By the systematic use of this study protocol future comparisons among different implants could be performed, thus contributing significantly to the improvement of TKA design. Level of evidence IV.

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Purpose Which total knee arthroplasty (TKA) design represents the better solution to restore a correct knee biomechanics is still debated. The aim of this study was to compare posterior stabilized (PS) and cruciate retaining (CR) version of the same TKA design (femoral component with an anatomic sagittal radius—J‐curve design) by the use of dynamic Roentgen stereophotogrammetric analysis (RSA). The hypothesis was that the two models influence differently in vivo knee kinematic. Methods A cohort of 16 randomly selected patients was evaluated 9 months after surgery: Zimmer PERSONA® was implanted, eight with CR design and eight with PS design. The kinematic evaluations were performed using a Dynamic RSA (BI‐STAND DRX 2) developed in our Institute, during the execution of the sit‐to‐stand motor task. The motion parameters were obtained using the Grood and Suntay decomposition and the low‐point kinematics methods. Results PS TKA lateral femoral compartment had a wider anterior translation (17 ± 2 mm) than the medial one (11 ± 2 mm), while the two compartments of CR TKA showed a similar anterior translation (medial: 9 ± 2 mm/lateral: 11 ± 2 mm). T test for comparison between CR and PS TKA of antero‐posterior translation showed a statistically significant difference (p < 0.05) in the flexion range between 15° and 40°. The CR prosthesis did not anteriorly translate during flexion. The PS design translated anteriorly showing a roll‐forward mechanism during extension from 80° to 18° of flexion and a posterior translation from 18° to 0°. The same significant differences (p < 0.05) between the PS and CR groups were found comparing the low‐point positions of the femoral condyles in the range of flexion between 25° and 40° for the medial compartment and between 15° and 25° for the lateral compartment. Conclusions Dynamic RSA was able to investigate for the first time in vivo the kinematic behaviour of PS and CR version of the same TKA J‐curve design. PS type showed a medial pivot during sit‐to‐stand motion task, while the CR type showed a cylindrical movement. Further studies are needed to evaluate the impact of different TKA designs on clinical results. Level of evidence IV.

Radio-Sterometric-Analysis (RSA) and x-ray fluoroscopy require dedicated software to reconstruct the radiological scene and the position of the objects in space. It is important to have a reliable validation to correctly use these softwares. The two major regulations that deal with the definition of "accuracy" are the ISO-5725 and the GUM 1995. The aim of this work, is to present a protocol for the evaluation of the accuracy of a radio stereometric software in terms of "trueness" and "precision", according to the standard ISO-5725. The protocol consisted in a series of computer simulations of the radiological setup. Each simulation changed the position and orientation of the x-ray sources, detectors and objects. Then, radiological images were generated. The noise level of the images was also changed in order to evaluate the accuracy with different image qualities. Then, the images can be processed with RSA softwares to evaluate their accuracy. The protocol was tested on a custom RSA software developed at the Istituto Ortopedico Rizzoli and the accuracy of the results was evaluated. The radiological scene reconstruction accuracy was found of the order of (0.092 ± 0.14) mm for tube position and (0.38 ± 0.31) mm/(2.09 ± 1.39) deg for detectors in the direction other than the source-detector direction. In that case, in fact, the accuracy is of the order of (2.68 ± 3.08) mm for the tube position and (0.16 ± 0.27) mm/(0.75 ± 1.16) deg for detectors. This fact is an intrinsic limitation of the scene reconstruction technique and it is widely discussed in the literature. The model positioning and orientation evaluations was also very accurate: (0.22 ± 0.46) mm/(0.26 ± 0.22) deg. No differences were highlighted about the noise level. The accuracy remains the same independently from the the noise of the images. The protocol was also useful to detect and fix hidden bugs in the software and optimize the algorithms.