BACKGROUND:To improve the anatomy-based alignment of prosthetic components in total knee replacement, surgical navigation systems have recently been developed, based on anatomical reference frame definitions through landmark digitations and functional calibration. In this study, femoral and tibial resection plane alignments, obtained by conventional tecnique, were measured intraoperatively during total knee replacements by a navigation system to quantify potential errors in conventional bone preparation techniques. METHODS:Femoral and tibial resection plane alignments, obtained by conventional femoral intramedullary and tibial extramedullary cutting guides, were measured intraoperatively in 25 primary total knee replacements by a navigation system. This system enabled the surgeon to calculate, before definitive bone sawing, the final position and orientation of all resection planes. RESULTS:The measurements revealed unsatisfactory alignments in nearly all anatomical planes. Except for tibial varus/valgus, final plane orientations were considerably different from those targeted by the surgeon via the navigation system, respectively 7 degrees, 8 degrees and 10 degrees apart in varus-valgus and flexion-extension at the femur, and in flexion-extension at the tibia. CONCLUSION:Modern computer-aided surgery in total knee replacement, once relevant precision has been established in all femur and tibia anatomical planes, can in the future limit the current critical component misalignments.