Implant placement accuracy in total knee arthroplasty: validation of a CT-based measurement technique

Background: The primary goal of many computer-assisted surgical systems like robotics for total knee arthroplasty (TKA) is to accurately execute a preoperative plan. To assess whether the preoperative plan was executed accurately in 3D, one option is to compare the planned and postoperative implant placement using a preoperative and postoperative CT scan of the patient's limb. This comparison requires a 3D-to-3D surface registration between the preoperative and postoperative 3D bone models and between the planned and postoperative 3D implants. Hence, the present study aimed at validating this measurement technique by determining (I) the anatomical regions that result in the lowest 6-degree of freedom (DoF) errors for 3 D-to-3D surface registration of bone models, (II) the 6-DoF errors for 3D-to-3D surface registration of the implant models, and (III) the 6-DoF of the complete measurement technique. Methods: Four different regions of the femur were tested to determine which one would result in the most accurate 3D-to-3D registration of the bone models using 12 cadaveric lower limb specimens. Next, total knee arthroplasties were performed on six specimens, and the accuracy of the 3D-to-3D implant registration was evaluated against a gold standard registration performed using fiducial markers. Results: The most accurate 3D-to-3D bone registration was obtained when using the largest anatomical regions available after TKA, being the full 3D femur model or the femur model without the distal femur which resulted in root mean square errors within 0.2 mm for translations and 0.2 degrees for rotation. The accuracy of the 3D-to-3D femoral and tibial implant registration was within 0.7 mm for translations and 0.4 degrees-0.6 degrees for rotations, respectively. The accuracy for the overall procedure was within 0.9 mm and 0.6 degrees for both femur and tibia when using femoral regions resulting in accurate 3D-to-3D bone registration. Conclusions: In conclusion, this measurement technique can be used in applications where measurement errors up to 0.9 nun in translations and up to 0.6 degrees in rotations in component placement are acceptable.