Artificial knees get smarter

A patient has been fitted with an artificial knee replacement with a new generation of sensors, to provide a wealth of information on twisting, bending, compressive and shearing loads across the human knee.


The data is transmitted wirelessly and will help scientists and clinicians analyse the forces at work during activities such as walking or rising from a chair.


The benefits may include design improvements for artificial knees, better surgical instrumentation, and better postoperative physical therapy. Historically, knee implants have been designed using predictions based largely on theoretical data.


A custom titanium alloy total knee replacement is the basis for the device, which has been fitted to a patient at the Scripps Clinic in the US.


The tibial component accepts standard, commercially available high molecular weight polyethylene inserts. The stem portion is hollow – and this hollow space houses wireless strain gauge electronics developed by MicroStrain. A polyethylene cap is threaded onto the distal end of the stem, and protects the hermetically sealed radio antenna. The electronics, including the sensing elements, are fully contained within the implant, which is hermetically sealed using laser-welding techniques. The finished, sealed implant is tested for hermeticity using fine helium leak-detection methods, the same methods that are used to test advanced pacemakers.


There are no batteries in the artificial knee – an integral miniature coil within the implant harvests energy from an externally applied alternating field. The remote powering coil is secured to the outside of the patient’s shin, away from the knee.


Twelve sensitive piezoresistive strain gauges, embedded within the implant’s custom-designed tibial component, send data via wireless to a computer. A stored calibration matrix then converts the raw strain data to show the 3D torques and forces about the knee.


The latest device has much greater capabilities than the first smart knee implant, which was implanted in March 2004 and only reported knee compressive forces.


Similar telemetric systems have been used to measure a wide range of forces in the hip, spine and femur but the small space in knee replacements has greatly restricted the data that could be collected. Advances in miniturisation, sensing and wireless capabilities are now overcoming this barrier.


Steven Arms, president of MicroStrain, said, ‘Our expertise in multi-channel strain sensing, power management, hermetic packaging, and digital telemetry have allowed the realisation of this revolutionary new smart total knee replacement.’


The project is part of an initiative begun by clinicians, scientists and industry in 1993. Scripps Clinic biomechanical laboratory, under the direction of Darryl D’Lima MD and Clifford Colwell MD, has been using the prototype of the replacement knee for evaluation implants for the past 10 years. Scripps Clinic staff worked with MicroStrain and two other implant manufacturing companies to design and pretest these devices and make them ready for implantation in patients. After safety testing, the implant was approved by ScrippsHospital‘s Internal Review Board for research purposes and is not yet available commercially.