Minimizing electromagnetic interference from surgical instruments on electromagnetic surgical navigation

Background: Electromagnetic computer-assisted surgery (EM-CAS) can be affected by various metallic or ferromagnetic factors. Questions/purposes: We determined to what extent metals interfere with accuracy and identified measures to prevent interference from occurring. Methods: Using an EM-CAS system, we made six standard measurements of tibiofemoral position and alignment on a surrogate knee. A stainless steel mallet was positioned 10 cm from the stylus, and then 10 cm from the localizer to create errors attributable to electromagnetic interference. The experiment was repeated with bars of different metals placed 10 cm from the stylus. Results: The maximum errors recorded with a mallet were: varus/valgus alignment, -2.7° and 2.4°; flexion/extension, -5.8° and 3.0°; lateral resection level, -3.1 and 7.5 mm; and medial resection level, -4.0 and 2.3 mm, respectively. The smallest errors were recorded with cylinders of titanium, cobalt-chrome alloy, and stainless steels. When moved more than 10 cm away from the stylus, errors became negligible. Conclusions: The accuracy of EM navigation systems is affected substantially by the size, type, proximity, and shape of metal objects. Clinical Relevance: Stainless steel objects, such as cutting blocks and trial prostheses, should be kept more than 10 cm from EM-CAS instruments to minimize error.