Total Knee Arthroplasty: Biomechanical Reflections and Modelling, Based on Quantitative Movement Analysis

Currently more than 1'000'000 total knee arthroplasties, TKAs, are implanted world wide each year, thereof 40'000 TKAs in Switzerland. Because of higher life expectancies it is expected that the number of total knee implantations will increase in the next few years. The goals of a TKA are longevity of the implant components and high satisfactory patient rates. Today the rate of a good patient outcome is around 85%, which still leaves a significant absolute number of patients needing early revision surgery. The most common causes for revision surgery are polyethylene (PE) wear, loosening, knee instability and infection. PE wear, loosening and instability are factors associated with altered joint biomechanics after total knee replacement. The femoral component rotational alignment profoundly affects the knee joint's mechanics in flexion as well as in extension, in all six degrees of freedom. A malrotated femoral component could lead to ligament unbalancing causing lateral flexion instability and pain while standing up from a chair or walking down stairs. In order to avoid these problems a comprehensive mechanical understanding of the knee joint as regards to the alignment of the implant component in TKA is important. A three dimensional computer based model visualising the joint's kinematics during different motion patterns contributes to this understanding and makes it possible to estimate the load at the knee joint. Such a model requires accurate in vivo kinematic and kinetic data to visualise and calculate the load at the joint of different motion patterns of daily activities. Previous investigations in gait analysis used kinematic and kinetic information from skin mounted markers and force plates during level walking. The problem of this measurement technique is the large error in kinematic data acquisition caused by the movement of the skin and muscles relative to the underlying bone. Video-fluoroscopy enables the measurement of kinematics of implant components more accurately by a three-dimensional numeric reconstruction of the single plane projection view in the fluoroscopic images, thus avoiding skin movement artefacts. However, this technique is limited to the field of view of the fluoroscopic screen. This problem was solved by using a motor driven trolley built in the laboratory to carry the fluoroscopic unit (x-ray source, image intensifier, c-arm). This movable system allows the tracking of the knee joint during level walking, and a sit down task. An intensity based registration algorithm reconstructs the six degrees of freedom of the implant components relative to the focus of the fluoroscope. The three dimensional reconstruction is within a translational accuracy of 3.1 mm and a rotational accuracy of 1.6°. Video-fluoroscopy only acquires kinematic data. This means that the loads in the knee joint can not be estimated. In this study was force plate data coupled with the moving fluoroscopic system enabling inverse dynamic calculation. The unit mover was optically tracked by VICON in order to transform the fluoroscopic coordinate system into the global coordinate system with its origin on the centre of one force plate, thereby coupling the fluoroscopic system with the force plate. This transformation was performed within an accuracy of ± 1 mm. This measuring system results in seven times more accurate inverse dynamic calculation than classic instrumented gait analysis would achieve. The local mechanics of the total knee was visualised in a three dimensional computer model. The model included TKA geometry from CAD software, and bone geometry from CT scans of the individual. It visualised the kinematics of the implant components during the activities mentioned above. Furthermore, femoral component malrotation was simulated to estimate the alternated strain at the posterior cruciate ligament, the medial, and the lateral collateral ligaments. The simulation of five different degrees of femoral component malrotation shows the relationship between the strain and forces produced by the ligaments under these conditions. The simulation shows that an internally rotated femoral component has a more profound effect on the forces in the ligaments than an external rotational malalignment. This might lead to pain on the medial side and condylar lift-off on the lateral side during a sit down task, as observed by clinicians. This work sets a basis for further investigation of TKA discussing the patient's outcome and contributing to a better mechanical understanding of the knee joint. It may help clinicians and implant developers discuss the effects of their implant design or alignment of the implant components under dynamic loading.