In the last years a large number of new surgical devices has been developed so to improve the operation outcomes and reduce the patient's trauma. Nevertheless the dexterity and accuracy required in positioning the surgical devices are often unreachable if the surgeons are not assisted by a suitable system. Since a medical robot works in an operating room, close to the patient and the medical staff, it has to satisfy much stricter requirements with respect to an industrial one. From a kinematic point of view, the robot must reach any target position in the patient's body being less invasive as possible for the surgeon's workspace. In order to meet such requirements a suitable performance index to optimal kinematic design is needed. This paper proposes an optimal index which maximizes the robot workspace and minimizes the invasiveness with respect to the surgeon's workspace. The index is used to evaluate the performances of a commercial medical robot and will be very useful in the future design of a new special robot for neurosurgery.
