PhD Thesis

Title: “Study, Design and Evaluation of an Actuated Knee Prosthesis for Postoperative Ligament Imbalance Correction”

Laboratories:

• Laboratory of Medical Information Processing (LaTIM), INSERM U1101, Télécom Bretagne – Brest, FRANCE
• Montpellier Laboratory of Informatics, Robotics, and Microelectronics (LIRMM), UMR 5506 CNRS, University of Montpellier 2 – Montpellier, FRANCE

Description:

Total Knee Arthroplasty (TKA) consists in the complete replacement of the knee joint by means of a prosthesis. Nowadays, a set of computer-assisted techniques can be employed during TKA surgery in order to assist the surgeon. The correct positioning of the prosthetic components can be precisely planned in the preoperative period and effectively achieved intraoperatively. However, in some cases the proper alignment of the prosthesis with respect to the mechanical axis of the lower limb may not be sufficient to ensure a long-lasting stability of the installed implant. To this end, a key achievement of TKA surgery consists in setting up a proper tension for the two collateral ligaments of the knee. In order not to give rise to any postoperative joint instability, none of these two ligaments should be looser (or tighter) than the other. Ligament tensions are manually set by the surgeon intraoperatively, according to the outcome of both the bone cuts and the component positioning stages. Different instruments have been proposed in the last decades in order to set up perfect balance conditions during TKA. Unfortunately, despite the assistance provided by such tools, ligament balance still majorly depends on the surgeon’s experience and perception, thus remaining an unsolved problem.

The prosthetic components are passive components which are not able to fit the morpho-functional changes that the patient normally undergoes during and after the rehabilitation period. Even the slightest inaccuracy in the ligament tensioning process risks being amplified throughout the first years after primary TKA. Thus, knee balance conditions could become suboptimal and lead to postoperative complications, such as misalignments, component loosening and polyethylene early wear. Consequently, the implant lifespan (usually 15 to 20 years) might be severely reduced. In the worst-case scenario, revision surgery is necessary to install a new knee prosthesis and restore proper balance conditions. In this context, over the last decade the orthopaedic community has started developing the idea of active knee implants, which could adapt to the physiological evolution of the body and fit the patient’s morphological changes. This project falls within this framework. The goal of this work is to design an instrumented tibial component, part of a fixed-bearing knee prosthesis, able to monitor ligament balance conditions in the postoperative period. An embedded actuation system should allow to correct any detected ligament imbalance right after the rehabilitation period, thus compensating for the unavoidable inaccuracies of TKA surgery.

Our research team developed the first smart knee implant able to postoperatively monitor and assess ligament balance conditions. It consisted of an instrumented tibial component that exploits the presence of four piezoelectric elements embedded in the tibial baseplate. An approach based on the center of pressure of the net tibiofemoral forces acting on the platform allowed to detect any imbalance condition. The piezos were meant to serve also as energy harvesters, thus dispensing with the need for an external power supply. An embedded telemetry system allowed the wireless control of the prosthesis and data communication to the clinician’s computer. Upon achievement of such diagnostic capabilities, the next step consisted in the actuation of the proposed device. The idea was to end up with an active knee implant, able to slightly modify the relative position of its components according to the detected ligament imbalance. Restoring optimal balance conditions right after the rehabilitation period should, in the long term, reduce the need for revision surgery.

Thanks to a compact and robust miniaturised actuation system embedded in the tibial baseplate, the inclination of the tibial tray could be adjusted so as to re-tighten a too lax collateral ligament. Optimal balance conditions can therefore be restored before the occurrence of more severe complications. The proposed system good efficiency, resulting from smart design choices, was assessed by means of static force analysis. Simulations run on the implant 3D model allowed to perform useful design optimisations and to estimate the components robustness properties. Upon fabrication of a full-scale prototype, stability and force sensors tests on a knee simulator were defined to evaluate the actuation system working principle and assess the effectiveness of the proposed design choices under normal working conditions. The very promising results of the experimental validation stage also provided precious indications for future work and improvements.