In Total Knee Arthroplasty (TKA) the distal femoral and proximal tibial compartments are resected and replaced with two prosthetic components. Inaccurate surgical planning and/or execution can easily lead to failure. Moreover, prosthesis misalignments of a few degrees/millimetres can seriously affect the post-surgical functionality of the operated limb. Hence the usefulness of a computer and robot assisted approach. Aim of this paper is to illustrate the registration phase of an integrated CAS protocol for TKA. Registration is a critical step, linking the planning and execution phases, in which the geometric relationships between the patient's anatomy, the models used for planning and the robotic or sensorised tools employed intrasurgically are estimated. Several registration techniques have been proposed, often needing implantation of artificial fiducial markers. The registration protocol described in this paper, based on the ICP algorithm, does not need any marker implant, thus reducing surgical invasiveness. Special care was taken in trying to adapt the protocol to the relevant application. Laboratory trials were carried out on a synthetic phantom and an animal specimen trying to reproduce realistic environmental conditions. Results showed that many requirements have to be fulfilled in order to get an adequate standard of accuracy. The protocol proved promising, though, as final inaccuracies below 1° and 1mm were obtained.