Fundamentals for the journey towards an enhanced understanding of human patello-femoral joint mechanics

Sauer, Adrian (2023): Fundamentals for the journey towards an enhanced understanding of human patello-femoral joint mechanics. Dissertation, LMU München: Medizinische Fakultät

Abstract

Background Anterior knee pain (AKP) is one of the main reasons for patient dissatisfaction after total knee arthroplasty (TKA). Since AKP is localised to the patello-femoral joint and its direct surroundings, it is likely that the causes for AKP are related to this joint. Review of the existing literature on patello-femoral contact forces and patello-femoral wear testing points towards failure-conducing mechanics. This, however, evidently conflicts with the compa-rably low revision rates reported by several national joint registries for patellar implants. This contradiction highlights the need for an improved understanding of the mechanics of the patello-femoral joint in vivo, as herein may lie the key to successful reduction of AKP after TKA. Objectives The first aim of this dissertation was to improve the understanding of the patello-femoral joint by focusing on a new approach to simulate the knee joint. This approach aimed to reduce the dependency of the model on assumptions which are based on non-accurate estimations. In order to validate this model, patello-femoral kinematics need to be further investigated. Therefore, the second aim of this dissertation was to lay the foundation to work with patello-femoral kinematic data with mathematical diligence. Finally, a deeper investigation of the factors influencing patellar kinematics was undertaken by first focusing on analysing the effect of patellar resurfacing in a human donor study. Materials and Methods In a first step, a finite element model including the patella was developed based on stand-ardized in vivo load data for the isolated tibio-femoral joint. Since these data implicitly in-clude all forces and moments induced by gravity, dynamics and soft tissue crossing be-tween the proximal and distal segments of the joint, explicit modelling of most of these factors can be avoided without weakening the validity of the model. This model was ap-plied to analyse the effects of the initial proximo-distal patellar position and the stiffness of the patellar tendon on patellar kinematics. The second part of this dissertation addressed a more fundamental issue, re-establishing solid basic principles with which to analyse and interpret patellar kinematics accurately. The representation of kinematics data, especially of relative patello-femoral rotations, is highly dependent on the underlying mathematical conventions used and their exact exe-cution. A detailed summary of the most common methods to describe patello-femoral rota-tions was formulated and more importantly, accurate conversions between these conven-tions were derived. These were provided to the biomechanical research community in the shape of a virtual computation tool. To improve our understanding of the impact of the parameters which influence patellar kinematics, the effects of patellar resurfacing were isolated from in vitro testing with eight fresh frozen knee specimens on a well-established knee rig at the Musculoskeletal Uni-versity Center Munich (MUM). To identify the specific effects of patellar resurfacing, knee kinematics were measured both pre- and post-TKA, with and without patellar resurfacing, for two tibio-femoral implant variants (posterior stabilised PS and PS+). Results The finite element model made it possible to run comparative studies with different im-plants or parameter variants. For the tibio-femoral joint, both kinematics and contact loads are in good accordance with the values previously reported in the relevant literature. For the patello-femoral joint, it was shown that a proximal initial patellar position and a weak patellar tendon causes more patello-femoral flexion and spin, while patellar tilt and shift are mainly influenced by the initial patellar position. The analysis of underlying mathematical conventions on the representation of patello-femoral rotations showed that both magnitude and the characteristics of the rotation curves can be modified fundamentally by switching between conventions. The derived transformation methods between the presented conventions were shown to be valid by way of mathematical proofs and comparison to benchmark data. In the experimental part of this dissertation, no significant effects of patellar resurfacing on patello-femoral rotations were found. For the translations, it was shown that patellar resur-facing had significant effects on patellar shift. For cases in which the patellar implant was placed in the medio-lateral centre of the patellar cut, this change in patellar shift proved to be significantly correlated to the lateral facet angle of the native patella. Conclusion and Outlook The developed model provides valid results for the tibio-femoral joint. Furthermore, it ena-bles qualitative and comparative analyses between different implant and model configura-tions. Nevertheless, for accurate quantitative results of patellar kinematics and kinetics, additional data for validation are needed, namely in vivo data of patello-femoral kinematics and loads. To handle such kinematic data from different sources, a solid mathematical foundation was established. The relation shown between the native lateral patellar facet angle and the effect of patellar resurfacing on medio-lateral shift shall be taken into account for clinical applications. In addition, the basic research on patellar kinematics brought forward by this dissertation can be used in future projects for further validation of the newly developed model, especially once a combined dataset of in vivo tibial loads and patello-femoral kinematic data be-comes available.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 600 Technik, Medizin, angewandte Wissenschaften ; 600 Technik, Medizin, angewandte Wissenschaften > 610 Medizin und Gesundheit
• Fakultäten: Medizinische Fakultät
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 4. Oktober 2023
• 1. Bericht­erstatter:in: Grupp, Thomas