The correct determination of the mechanical leg axis (so called Mikulicz-Line) is cruicial step for the correct alignment of prosthesis components in Total Knee Arthroplasty (TKA). The newly developed “genALIGN”-system uses a force-torque sensor to directly measure the torques induced by a deviation from the mechanical axis. During surgery, the device is fixed to the knee center and a force is applied in the approximate direction of the hip center. An unstable system is created which can only reach equilibrium, when both the device and the mechanical leg axis are coaxial, i.e. the device points exactly in direction of the hip center. The result can be easily fixed and the bone cuts can be performed using an attachable cutting jig. To account for the leg’s weight, the sensor is adjusted to zero before starting the actual measurement.
The system was tested in a cadaver study, in comparison to a navigation system with optical tracking. Using the recent prototype of genALIGN, the axis could be determined with a deviation of 2.9°±1.5° (mean±std.dev.) varus and 0.3°±2.4° anterior slope by different users, compared to the optically tracked axis. However, maximum deviation was 5° varus and 11° anterior slope, respectively. Further developments, in particular an improved weight bearing of the leg, will improve the accuracy and are aspects of ongoing work. Moreover, ligament balancing and rotational adjustment will be addressed.
Based on our first experiences we think, that genALIGN potentially offers an accurate, minimal-invasive, simple and low-cost approach for TKA.