A good preoperative plan of orthopaedic reconstructive interventions is of paramount importance for the outcomes of reconstructive surgeries, increasing the information on the patient, reducing the need for taking intraoperative decisions and allowing thus for a safe intraoperative time. The aim of our work is to introduce a Virtual Reality environment into surgeons' practice to assist them during preoperative planning of Total Knee Arthroplasty. The computer-assisted planning is performed on computer-assisted tomographic images of the patient's limb, digitized and elaborated by an in-house interactive program to reconstruct the 3D geometry of the leg. The program helps the surgeon to choose the correct size, orientation and position of the prosthesis components in order to restore the correct alignment of the mechanical axis of the limb and maintain the previous articular time. Moreover ligaments' release can be checked in extension and simulation of non-standard individual strategies can be checked easily preoperatively in order to optimize the results of the intervention. With respect to the present technique this approach is more accurate, allows an effective 3D planning on the whole limb, provides a fast tool for preoperative simulations and can be easily used also by young surgeons with good results. The application of computerassisted planning in an exact robot-assisted execution of TKA is straightforward and is under development in our laboratory.