## 1 Introduction
### Related Work  
Each surgical domain, e.g., neurosurgery or laparoscopic surgery, imposes its own requirements and constraints. Consequently, many robotic systems have been proposed for specific surgical scenarios with different human-robot interface design and assistance approaches. The levels of cooperation in robot-assisted surgery can be categorized as active, semi-active, and synergistic systems depending on their level of autonomy [5]. Active systems provide full autonomy during the execution of a surgical task, while semi-active systems assume control of some of the motion DOF executed by the surgeon. These active and semi-active systems require a pre-operative plan to be followed. Synergistic systems  
Figure 1: (**a**) Endoscopic endonasal surgery. (**b**) Instrument trajectory channel and the stellar target representation. (**c**,**d**) Physical human nasal model for endonasal surgery training. (**e**) 3D model of a human nasal model. provide shared control between the surgeon and the robot in which the position and the movement of the surgical instrument are controlled by a master device commanded by the surgeon. Such synergistic systems can be then sub-classified into master-slave, hand-held, and hands-on devices. Master-slave robotic systems have been developed mostly for laparoscopic applications. The commercially available da Vinci Surgical System [6] is a notable example of such an approach. Master-slave interfaces can provide additional capabilities to surgeons such as teleoperation, ergonomics, enhanced vision (3D stereoscopic visualization), motion scaling, and tremor suppression. Haptics can also be included in the master devices such as the PHANTOM [7] and Sigma7 interfaces [8]. However, for EES applications, mater-slave systems have been used with limited success. Attempts have been conducted with the da Vinci system in a transoral [9] and transantral [10] approach. While the use of highly dexterous instruments can provide operation benefits, both approaches generate excessive trauma and remain highly invasive due to the size of surgical instruments. In [11], a continuum robot teleoperated by two PHANTOM devices was proposed for transnasal surgery. However, it required a large workspace and a complex setup procedure. Additional attempts shared similar limitations with the size of the system and instruments, time-consuming setup procedure, and the considerable cost of these systems. Importantly, master-slave systems limit the surgeon's physical access to the patient. Thus, the help of bedside staff would be needed for the use of auxiliary instruments or the change of tools, which is a frequent task in EES surgeries. Hand-held instruments provide direct control over surgical instruments and natural response and haptics. They are significantly smaller and not fixed on a ground frame, designed to exploit the surgeon's dexterity with the use of ordinary tools. Enhanced features include tremor suppression and active guidance. Robotic hand-held instruments have been used in laparoscopic surgery [12; 13], eye surgery [14], and microsurgery [15]. Optical trackers are typically used to obtain the tool pose in the space, which is one of the major drawbacks. Such optical trackers can be easily occluded during a surgical task and represent a challenge for applications in minimally invasive surgery. Furthermore, the lack of a ground frame can produce undesired reaction forces on the operator's hand [16]. Hands-on systems are designed to be an intermediate solution. The surgical tool is attached to a grounded robot arm, and the motion control is shared between the surgeon and the robot through a user interface placed between the tool and the robot. It can provide flexibility in the application and can also reduce the setup space, the number of assistance staff, and the operation cost. This approach is followed in the Acrobot system [17] and the MAKOplasty system [18] developed for orthopedic applications with active constraints limiting the tool motion inside prescribed anatomic boundaries.