Measurement of Load-Induced Electrical Potentials at the Surface of the Knee

Osteoarthritis (OA), the most prevalent chronic progressive disease of the elderly, is one of the leading causes of disability in our society with increasing incidence throughout the world. OA results from the deterioration or loss of articular cartilage acting as protective cushion between the bones. As the incidence of OA is on a continuous increase, there is an urgent need for noninvasive assessment techniques that can be effective before the onset of irreversible degradation. Current technologies to assess the joint function include clinical evaluation, arthroscopy and imaging techniques such as X-ray, magnetic resonance imaging (MRI) and computed tomography (CT). Although these techniques are widely used to assist the diagnosis of OA by evaluating osteophyte formation, joint space narrowing and bone damage, none of them can show osteoarthritic changes of the knee until the later OA stages. The need for an early and accurate evaluation of the integrity of cartilage has prompted many investigators to explore a variety of techniques. For example, the electromechanical transduction of cartilage, known as streaming potentials, is a sensitive index of the integrity of articular cartilage that has been validated by many in-vitro experiments. The underlying mechanism is associated with the separation of fixed negative charges and mobile counterions in the liquid phase of the cartilage under compression. A hand-held arthroscopic instrument (Arthro-BST) has thus been designed to measure the electrical potentials directly over compressed articular cartilage to assess its quality. A non-invasive approach, electroarthrography (EAG) was later developed to measure the electrical potentials appearing on the surface of knee during loading and reflecting the underlying streaming potentials. Mechanical loading simply consisted of shifting the body weight of the erect subject to the instrumented leg. Even though the first EAG measurements were repeatable, variability in the EAG signals recorded in the same subject was observed, which can hamper its application as a diagnostic tool for OA. The aim of our study was thus to investigate the nature of the variability of the EAG signals and to contribute to the development of better measurement techniques. The first objective of this study was to investigate how the contraction of certain leg muscles in supine subjects affects the contact force of the knee joint and, in turn, the EAG values. Voluntary isometric muscle contractions were repeatedly conducted to selectively activate four leg muscle groups while six subjects were lying on their back. Two EAG signals were recorded on both