Navigation-assisted surgery in total knee arthroplasty (TKA) is aimed at improving the accuracy with which prosthesis components are implanted in the bones, according to anatomical plane orientations. Traditional surgical techniques based on the identification of transepicondylar and intramedullary axes are replaced with those based on segmental anatomical frame definitions following anatomical landmark identification. These frames are offered on the screen to the surgeon to target in real time the alignment goal by adjusting position and orientation of the bone saw guides. However, immediately after sawing, final bone, and in case cement, preparation and component implantation is necessarily a series of actions performed manually by the surgeon. In the current study, we wanted to compare intra-operatively the final component alignments with the corresponding at the original resection planes.

In this series, 50 Scorpio PS TKAs were analyzed. The navigation system used was the Stryker Knee Navigation System (Stryker-Navigation, Kalamazoo, USA). An ‘anatomical survey’ defined anatomical frames for the femur and tibia, based on relevant anatomical landmark identification, and provided target orientations for all the relevant bone cuts. These references were targeted in all three anatomical planes, and bone cuts were made accordingly. Corresponding alignments of the bone resection planes in the frontal, sagittal and transverse planes for the femur and in the frontal and sagittal planes for the tibia were recorded, with a 0.5° resolution. Then, component implantation was performed and alignments were measured again by means of an instrumented probe. Because of the shape of the prosthesis components, only the alignments in the frontal plane for the femur and in the frontal and sagittal planes for the tibia were recorded.

The difference between the alignment of the bone cuts and the alignment of the prosthesis components, in the frontal plane of the femur, and in the frontal and sagittal planes of the tibia was larger than 2° respectively in 8%, 6%, 10% of the patients.

The present study offers a figure for the different alignment between resection planes and final implanted components, necessarily the effect of the manual procedures implied in TKA for the final implantation of the components. Considering that 1° is the claimed achievable accuracy of the navigation systems, and that the correct alignment goal was achieved at the resection planes, these figures reveal that in up to 10% of the patients the benefit obtained by navigation can be lost by the manual procedures implied in component implantation. These differences in alignment put also concerns in the postoperative statistical comparison between conventional and navigated TKAs.