A "hands-on" robotic system for total knee replacement (TKR) surgery is presented. A computer tomography-based preoperative planning software is used to accurately plan the procedure. Intraoperatively, the surgeon guides a small special-purpose robot, called Acrobot, which is mounted on a gross positioning device. The Acrobot uses active constraint control, which constrains the motion to a predefined region, and thus allows the surgeon to safely cut the knee bones to fit a TKR prosthesis with high precision. A noninvasive anatomical registration method is described. The system has been successfully used in seven clinical trials with encouraging results.