Accuracy of image-free navigation for severely deformed knees.

This study evaluated the accuracy of an image-free navigated total knee arthroplasty (TKA) system when used to align deformed tibia bone models. The accuracy was assessed in normal, 10 degrees varus, 20 degrees varus, 10 degrees valgus, and 20 degrees valgus tibia bone models (a total of five tibial models) by direct measurement of the navigated cutting guide. The mean angular errors in the tibial mechanical axes of the normal, 10 degrees, and 20 degrees varus models, respectively, were 0.0 degree, 0.7 degree varus, and 2.4 degrees varus. Thus, the errors seen with the two varus models were significantly larger than that associated with the normal model. The mean angular errors were 0.1 degree varus and 0.4 degree valgus in the 10 degrees and 20 degrees valgus models, respectively. These errors were not significantly different from those obtained with the normal model. These results suggest that in varus-deformed knees, image-free navigation has a tendency to cut the tibia in varus. This fact is considered to be one of the reasons for the lack of superiority of TKA alignment in severely deformed knees when using image-free navigation. Therefore, special attention must be paid when using image-free navigation TKA in such cases.