Planning of total knee replacement: analysis of the critical parameters influencing the implant

This paper describes a planning software developed for computer assisted Total Knee Replacement (TKR) and analyses the influence of surgical parameters on the final prosthesis positioning and planning protocol. The authors´ software allows the surgeon to simulate the implant on a 3D model of the patient computed from CT images. The surgeon identifies anatomical characteristics, and the system computes automatically the position/orientation of the implant in order to restore the mechanical axis, to minimise the bone loss and preserve the articular line; then the surgeon can interactively adjust the position of the prosthesis component on sections or planes. The system was repeatedly used by 3 surgeons on 10 patients. The authors have analysed the repeatability of the surgeons planning, the efficacy of the automatic computations and the influence of the different steps of the planning process on the result. Surgeons were able to identify the anatomical points with 4 mm mean error, however the automatic computations of the mechanical axis and prosthesis implant resulted in a final mean error of 1° and 1.5 mm. The surgeons relied mainly on the automatic restoration of the mechanical axis and on the preservation of the articular line, while the ideal fitting of the prosthesis on the bone shape is often adjusted by expert surgeons on the basis of a careful analysis of sagittal and coronal sections. The use of accurate images and computer assistance for planning TKR has shown great potentiality and satisfactory accuracy to improve the surgeons´ insight and confidence