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Computer-navigated total knee arthroplasty is aimed at improving accuracy in the positioning of prosthetic components and realigning the lower limb. The optimal position and orientation of the bone resection planes are targeted by the navigation system, but, after these are obtained, additional manual surgical actions, which may considerably affect final component alignment, are necessary for implantation. The aim of this study was to measure the alignment deviation caused by standard impaction of the tibial and femoral components following bone resections with use of navigation control.Ninety-one primary total knee arthroplasties were performed with an image-free knee navigation system. The alignment of the tibial and femoral bone resections was measured in three planes during surgery by the instrumented probe of the system. The alignment measure was repeated after final tibial and femoral component implantation with cement. The alignment deviations between the two measures were considered the positioning error associated with the final manual implantation of the components.The alignment deviations between the bone resections and the subsequent implant placement were >1° in the frontal plane of the femur and in the frontal and sagittal planes of the tibia in 20%, 11%, and 33% of the patients, respectively. The deviations were >2° in 4%, 3%, and 9% of the patients, respectively. Deviations as large as 3° were found at the tibia in the sagittal plane (the posterior slope).Positioning of the femoral and tibial components in total knee arthroplasty, which mainly involves cementation and impaction of the final components, can introduce a considerable error in alignment, regardless of how accurately the resection planes are made. After computer-navigated total knee arthroplasty, it would be useful therefore to check the alignment of the prosthetic component carefully before the cement hardens.