Nowadays, the large majority of the instrumentation for orthopaedic surgery consists of mechanical tools with varying degrees of complexity. To increase the accuracy and the safety of orthopaedic interventions, sensors and computers were recently introduced in the operating room. Computer Assisted Orthopaedic Surgery (CAOS) uses a navigation system that tracks the movements of surgical instruments in real-time and displays their exact location in relation to the operative area. Such technology improved the quality of orthopaedic arthroplasties, but it is still limited to the measurement of kinematic parameters such as axial alignments, position and angle measurements. In Total Knee Arthroplasty (TKA), the ligament balance, which is crucial for the stability and lifetime of implants, is currently only qualitatively assessed. The goal of this thesis was therefore to demonstrate the importance of intraoperative measurement of musculoskeletal forces through the development of a force-sensing device designed to improve the ligament balancing procedure during TKA. Three possible device designs were proposed and evaluated using finite element analysis in an iterative optimization process. The final design consists of two sensitive plates, one for each condyle, a tibial base plate and a set of spacers to adapt the device thickness to the patient-specific tibiofemoral gaps. Each sensitive plate is equipped with three deformable bridges instrumented with thick-film piezoresistive sensors, which allow accurate measurements of the amplitude and location of the tibiofemoral contact forces. The net varus-valgus moment is then computed to characterize the ligament imbalance. Laboratory experiments showed that the device has appropriate accuracy and dynamic range for the intended application. The first experimental trials on a plastic knee joint model and on a cadaver specimen demonstrated the proper in-situ functioning of the device. The performance and surgical advantages of the device were then evaluated in an in-vitro study including four different experiments: 1) Six knee joints were axially loaded. Comparing applied and measured compressive forces demonstrated the accuracy and reliability of in-situ measurements. 2) To estimate the importance of keeping the patella in its anatomical place during imbalance assessment, the effect of patellar eversion on the mediolateral distribution of tibiofemoral contact forces was measured. One fourth of the patellar load was shifted to the lateral compartment. 3) Assessment of knee stability based on condyle contact forces or varus-valgus moments were compared to the current surgical method (difference of varus-valgus loads causing condyle lift-off). The force-based assessment found to be equivalent to the surgical method while the moment-based technique, which is considered optimal, showed a tendency of lateral imbalance. 4) Finally the effect of minor and major medial collateral ligament releases was biomechanically quantified. Large variation among specimens reflected the difficulty of ligament release and the need for intraoperative force monitoring. Two clinical trials were carried out to evaluate the device performance in a surgical environment. After the tibial cut, the medial and lateral tibiofemoral gaps ensuring the knee stability were determined from the device measurements and compared to the femoral cuts performed on the basis of standard instrumentation. The agreement between the two approaches was generally good. The only significant difference was measured on the first patient at 90° flexion. At this point, the surgeon also estimated that the knee was not optimally balanced, thus demonstrating the consistency between his perception and the device measurements. In conclusion, the proposed force-sensing device for assistance in ligament balancing during TKA provides accurate, reliable and useful measurements. In addition to the precise imbalance assessment based on the measurement of forces and moments, important clinical advantages, such as the possibility to keep the patella in its anatomical place during the measurement or the real-time force monitoring during the delicate phase of ligament release, were demonstrated. The encouraging results of the in-vivo trial proved the usability of the device in a surgical environment and opens the way for larger clinical studies. The developed device has thus potential to improve the ligament balancing procedure, the consistency of surgery and the lifetime of TKA, illustrating thereby the clinical benefit of measuring forces during orthopaedic surgeries.
EPFL2006
