Repeatability and Accuracy of Bone Cutting and Ankle Digitization in Computer-Assisted Total Knee Replacements

In conventional total knee replacement (TKR) surgery, a significant fraction of implants have varus/valgus alignment errors large enough to reduce the lifespan of the implant, so we are developing a more accurate computer-assisted procedure aimed at reducing the standard deviation (SD) of the implant’s frontal alignment to under 1? . In this study we measured the contributions to overall alignment error of two steps in our proposed procedure: ankle digitization and manual bone cutting. We introduce a new digitizing probe that quickly and robustly locates the midpoint between the ankle malleoli. Based on repeated measurements on 8 cadavers by 6 operators (318 measurements), we estimate that the new probe introduces only 0.15? (SD) of variability into the definition of the tibial mechanical axis in the frontal plane. We also measured the accuracy and repeatability with which surgeons can implement a bone cut using conventional cutting guides to see if conventional cutting techniques are sufficiently accurate. A total of 53 tibial plateau, distal and anterior/posterior (A/P) femoral cuts approximated primary and revision TKR resections. In 20 test cuts on cadaver bone made by 2 expert TKR surgeons, we found a SD of 0.37? (bias of 0.29?) in the varus/valgus difference between the guide orientation and the implant orientation before cementing, but in 23 additional cuts performed by four less-experienced surgeons, the SD was 0.83? (bias of 0.31?). Ten A/P femoral cuts showed similar trends. We conclude that, in the hands of an experienced surgeon, our current technique (based on our previously-reported non-invasive hip centre locating technique [Inkpen 1999b], robust ankle digitization and manual cutting using computer-guided cutting guides) can approach our target alignment variability goal of a SD less than 1? .