The object of this study was to develop a method to assess the accuracy of an image-free total knee replacement navigation system in legs with normal or abnormal mechanical axes. A phantom leg was constructed with simulated hip and knee joints and provided a means to locate the centre of the ankle joint. Additional joints located at the midshaft of the tibia and femur allowed deformation in the flexion/extension, varus/valgus and rotational planes. Using a digital caliper unit to measure the coordinates precisely, a software program was developed to convert these local coordinates into a determination of actual leg alignment. At specific points in the procedure, information was compared between the digital caliper measurements and the image-free navigation system. Repeated serial measurements were undertaken. In the setting of normal alignment the mean error of the system was within 0.5 degrees . In the setting of abnormal plane alignment in both the femur and the tibia, the error was within 1 degrees . This is the first study designed to assess the accuracy of a clinically-validated navigation system. It demonstrates in vitro accuracy of the image-free navigation system in both normal and abnormal leg alignment settings.

The objective of this study was to develop a method to assess the accuracy of an electromagnetic technology image-free navigation system for total knee arthroplasty in a leg with normal or abnormal mechanical alignment. An acrylic phantom leg was constructed to simulate tibia and femur deformation. Determination of actual leg alignment was achieved using a digital caliper unit. In the setting of normal alignment, the mean error of the system characterised as the difference between the measured computer navigation and digital caliper angles ranged between 0.8°(frontal plane) and 1.5°(lateral plane). In the setting of abnormal alignment, the mean error ranged between 0.4°(frontal plane) and 1.6°(lateral plane). Deformity had no demonstrable effect on accuracy. The study demonstrates satisfactory in vitro system accuracy in both normal and abnormal leg mechanical alignment settings.Résumé Le but de cette étude était d'évaluer les techniques de navigation sans image par technologie électro-magnét-ique au cours des prothèses totales du genou. Un fantôme acrylique de membre inférieur a été construit de façon à simuler la déformation du tibia et du fémur. La différence moyenne entre la mesure par computer et la mesure instrumentale est de 0,8°dans le plan frontal et de 1,5°d ans le plan latéral. Dans les cas de mauvais alignements la moyenne a été de 0,4°dans le plan frontal et de 1,6°sur le profil. Cette étude in-vitro montre que ce système est fiable aussi bien pour un membre inférieur normalement axé que désaxé.