Introduction: Computer navigation has been shown to improve rotational alignment, angular alignment and sizing, when compared to a conventional jig based approach. These studies have all looked at post operative radiographic evaluation as the indicator. This study measures the intraoperative difference between the conventional jig based approach and the computer navigated system.
Methods: 59 total knee arthroplasties were performed by a single surgeon between September 2006 and February 2007. The author was trained in this technique during fellowship and has performed over 250 CAOS total knee replacements. All knees were DePuy PFC sigma implanted with the DePuy Ci system using Brain-lab software. The femoral sizing was performed using the jig after the distal femoral cut had been made using the navigation system. The difference between the size recommended by the jig was recorded. The implant was chosen by the computer recommendation, and the jig was used only for data collection. The tibial jig was then placed in the standard fashion using an extramedullary jig. The navigation marker was then placed into the jig slot, and the varus/valgus, posterior slope, and resection height were recorded using the computer modeling as the reference. The jig was then re-aligned if the computer measured angle was greater than 2 degrees in any plane, or the resection height was greater than 2mm. The cut was made using the computer recommended position if the differences exceeded these parameters. Tibial plate size was obtained using the “best fit” technique even if that differed with the computer recommendation. All post operative x-rays were then evaluated with x-ray and obvious outliers in size or angulation were recorded.
Results: One tibia was too short to be measured with a jig, so the N for tibial data is 58.
Average measured difference in varus/valgus was 1.26 degrees with 53 valgus (range 0–3.5) and 5 varus (range 0–3.6). Tibial slope average difference was average 2.31 degrees with 54 posterior (range 0–6.5 degrees) and 4 anterior (range 1–2.5 degrees). Tibial resection height difference was average 3.31mm with 4 measured high (0–3.5mm) and 54 measured low (0–6.9mm). Femoral sizing using the jig correlated with the expected size using CAOS in 28 of 34 (82%) of cases. Tibial size “best fit” correlated with CAOS in 46 of 58 cases (84%). The tibial jig was repositioned in 20 of 58 (35%) cases prior to making the cut. No tibial or femoral re cuts after the original cut were required in any case. Without using specific measurements, all post op x-rays had satisfactory alignment and component sizing, however 2 tibial plates had mild lateral overhang.
Discussion: The data suggest that in most cases, the jig approach is satisfactory, however, the computer prevents outliers. The more preoperative deformity was present, the greater variation between the measurements. The femoral jig in the conventional system we used, does not take into account femoral width, and there is no way to correct for posterior condyle deformity, this is why it is felt that the femoral fit is better with the CAOS system. Femoral rotation would not be able to be measured without using the intramedullary jig, so this step was bypassed, but if femoral rotation followed the other data, the computer would prevent malrotation in some cases. Had the conventional jig been used, the data suggests that at least one patient would have had anterior slope of the tibial tray. One patient had a tibia that was too short to use the conventional extramedullary jig. Since no intra-medullary jig was available on the set, the tibia would have had to be placed freehand if the CAOS system was not available. These data suggest that the CAOS system is preventing erroneous cuts in some cases confirming the data published regarding radiographic evaluations with respect to a decrease in the number of outliers.
