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Measuring joint kinematics is a key requirement for a plethora of biomechanical research and applications. While x-ray based systems avoid the soft-tissue artefacts arising in skinbased measurement systems, extracting the object's pose (translation and rotation) from the x-ray images is a time-consuming and expensive task. Based on about 106'000 annotated images of knee implants, collected over the last decade with our moving fluoroscope during activities of daily living, we trained a deep-learning model to automatically estimate the 6D poses for the femoral and tibial implant components. By pretraining a single stage of our architecture using renderings of the implant geometries, our approach offers personalised predictions of the implant poses, even for unseen subjects. Our approach predicted the pose of both implant components better than about 0.75 mm (in-plane translation), 25 mm (out-of-plane translation), and 2° (all Euler-angle rotations) over 50% of the test samples. When evaluating over 90% of test samples, which included heavy occlusions and low contrast images, translation performance was better than 1.5 mm (in-plane) and 30 mm (out-ofplane), while rotations were predicted better than 3-4°. Importantly, this approach now allows for pose estimation in a fully automated manner., PLoS ONE, 17 (6 6), ISSN:1932-6203

Video-fluoroscopic analysis can provide important insights for the evaluation of outcome and functionality after total knee arthroplasty, allowing the in vivo assessment of tibiofemoral kinematics without soft tissue artefacts. To enable measurement of the knee throughout activities of daily living such as gait, robotic systems like the moving fluoroscope have been developed that follow the knee movement and maintain the joint in front of the image intensifier. Since it is unclear whether walking while being accompanied by moving fluoroscope affects normal gait, the objective of this study was to investigate its influence on gait characteristics in healthy subjects. In addition, the impact of the motors’ noise was analysed. By means of skin markers analysis (VICON MX system, Oxford Metrics Group, UK) and simultaneous measurement of ground reaction forces (Kistler force plates, Kistler, Switzerland), gait characteristics when walking with and without the moving fluoroscope as well as with and without ear protectors in combination with the moving fluoroscope, were obtained in young (n = 10, 24.5y ± 3.0y) and elderly (n = 9, 61.6y ± 5.3y) subjects during level gait and stair descent. Walking with the moving fluoroscope significantly decreased gait velocity in level gait and stair descent over the respective movement without the fluoroscope. Statistical analysis, including gait velocity as a covariate, resulted in no differences on the ground reaction force parameters. However, some kinematic parameters (ankle, knee and hip ranges of motion, minimal knee angle in late stance phase, maximal knee angles in stance and swing phase) seemed to be modified by the presence of the moving fluoroscope, but statistical comparison was limited due to velocity differences between the conditions. Wearing ear protectors to avoid the influence of motor sound during walking with the moving fluoroscope caused no significant difference. Walking with the moving fluoroscope has been shown to decrease gait velocity and small alterations in kinematic parameters were observed. Therefore, gait and movement alterations due to the moving fluoroscope cannot completely be excluded. However, based on the absence of differences in ground reaction force parameters (when adjusted for velocity within ANCOVA), as well as based on the comparable shape of the angular curves to the slow control condition, it can be concluded that changes in gait when walking with the moving fluoroscope are small, especially in comparison to natural slow walking. In order to allow assessment of joint replacement with the moving fluoroscope, including an understanding of the effects of joint pain, clinical analyses can only be compared to gait activities showing similarly reduced velocities. Importantly, the reduced gait speeds observed in this study are similar to those observed after total knee arthroplasty, suggesting that analyses in such subjects are appropriate. However, the moving fluoroscope would likely need to be optimized in order to detect natural gait characteristics at the higher gait velocities of healthy young subjects. The moving fluoroscope can be applied for comparisons between groups measured with the moving fluoroscope, but care should be taken when comparing data to subjects walking at self-selected speed without the moving fluoroscope., PLoS ONE, 13 (7), ISSN:1932-6203

PLoS ONE, 11 (11), ISSN:1932-6203

Designing orthopaedic implants to reconstruct native anatomy and function continues to be a major challenge in the development of new products. This thesis provides insights and suggestions for designing next generation knee arthroplasty with a focus on native joint kinematics. Like in many other specialties, biomimetics is the ultimate objective in joint reconstruction. Knee arthroplasty is the most successful treatment option for severe osteoarthritis, providing patients with a reliable solution to relief pain and regain mobility. However, native joint function is not fully restored and for an aging population with growing interest in demanding activities, several limitations are present. Particularly, existing total knee arthroplasty (TKA) delivers kinematic deficits, associated with abnormal feeling of the knee, following joint surgery. These limitations are related to sacrificed ligaments, and none-anatomic geometries of current implant designs. Mostly, the anterior cruciate ligament (ACL) is sacrificed and its function is lost. Therefore, a biomimetic process was implemented to reverse engineer articular surfaces, by directly incorporating native knee kinematics. For the first aim, we designed a biomimetic implant that would preserve all major ligaments of the native knee joint. Therefore, an anatomically designed femoral component was virtually moved through native knee kinematics, to carve out a tibial articular surface, through the biomimetic process. This anatomical surface was incorporated into a novel implant, specifically designed to preserve the native ACL. For the second aim, we established a dynamic simulation platform to reliably evaluate implant design variations and the effect of ligaments. Knee kinematics, driven by ligament function and contacting implant geometry, allowed for comparison of different knee arthroplasty systems, against in vivo and simulated native knee motion. The first computational study revealed that a biomimetic, bi-cruciate retaining (BCR) implant provides activity dependent kinematics, similar to healthy knees in vivo, during deep knee bend, chair sit and level walking. This was in contrast to symmetric BCR systems, which nonetheless showed kinematic improvements over ACL-sacrificing implants. This indicates that restoring anatomic knee geometry together with ACL preservation is required for optimal implant function. The second computational study showed that a novel ACL-substituting mechanism could improve kinematic deficits of contemporary PCL-retaining (ACL-sacrificing) implants. ACL-substituting and ACL-retaining implants both provided similar improvements over the ACL-sacrificing implant in comparison to a native knee model. This was shown during walking, stair-ascent, chair-sit and deep knee bend simulations, indicating that ACL-substitution may be a valuable treatment option when ACL preservation is not feasible. For the third aim, we evaluated an extended treatment indication for unicompartmental knee arthroplasty (UKA), via in vivo kinematic analysis, using a moving fluoroscope. UKA is known for better functionality over TKA due to ACL retention and partial preservation of the native articulation. An ACL-deficient population, undergoing an altered surgical technique, with appropriate adaptation of implant placement, was compared against conventional UKA patients, with an intact ACL. The first in vivo study showed kinematic similarities between ACL-deficient and conventional UKA patients, in contrast to TKA. A posterior femoral shift was observed with ACL-deficient UKA, while kinematic trends and range of motion were similar during deep knee bend, downhill walking, level walking and stair descent. Based on kinematics, this confirms that the indication for UKA can be extended to include selected ACL-deficient patients. The second kinematic study revealed no differences in ground reaction forces between ACL-deficient and conventional UKA patients. Adequate kinematic and kinetic symmetries among implanted and contralateral leg indicate that UKA may not always be a contraindication due to ACL deficiency. In summary, this thesis outlines the importance of biomimetic implants for the objective of “forgotten knees”, following joint arthroplasty. Native knee function may be restored with anatomic geometries and ligament retention. Preservation or substitution of either the articular geometry or the ACL, results in kinematic improvements over conventional TKA. Long-term clinical outcome studies will be required for the ultimate proof of this concept.