## 1 Introduction  
Recently, surgeons have applied a transsphenoidal approach (TSA) to remove pituitary tumors, using an endoscope, as shown in Figure 1. This method of operation involves approaching the pituitary tumor through the shortest path--by incising the nasal septum and removing the rostrum bone. The actual clinical workflow for this procedure is shown in Figure 2. TSA surgery has evolved from surgery using a microscope to surgery using an endoscope. There are several reasons for this change. The main reason is to secure the surgeon's field of view. When a microscope is used, no obstacle can intrude between the surgical space and the microscope. This meant that it was necessary to remove more of the body structure than was necessary. To secure the surgical space view, TSA surgery using an endoscope has been developed [1; 2; 3]. It is also difficult to identify the presence of residual tumor during tumor removal surgery. Minimally invasive surgery has become possible since the introduction of the endoscope, as it enables a minimal incision; however, there are still problems such as the difficulty in identifying residual tumors remaining after surgery and the possibility of occurrence of complications in TSA surgery. For example, a tumor in the blind region, as shown in Figure 3, is difficult to remove by relying on the endoscopic view; moreover, in minimally invasive surgery, there is always a blind region that cannot be examined using a conventional straight endoscope. Removal of tumors in this region requires a surgical navigation system, a flexible (bendable) endoscope and a bendable surgical tool [4; 5; 6; 7; 8; 9; 10; 11]. Figure 1: The concept behind the transsphenoidal approach to pituitary surgery. Figure 2: Procedures in the transsphenoidal approach to pituitary surgery. Previous research on bendable devices has focused on active types--with motors--using robots mounted around the surgical site, rather than the use of hand-held instruments. The issue here has been that it is difficult for a mounted type, bendable robot to respond quickly when the camera lens becomes obscured with blood, which often occurs in an actual surgical environment. Another problem with TSA surgery is the potential for complications during tumor removal, which can include vision loss due to optic nerve damage, meningitis caused by leaking cerebrospinal fluid and stroke caused by damage to internal carotid arteries (ICAs) that are close to the sphenoidal sinus [12]. In order to avoid these complications, navigation systems have recently been introduced into the operating room [13; 14]. In such a system, patient information--obtained using magnetic resonance (MR), computed tomography (CT), computed tomography angiography (CTA) and so forth, in preoperative procedures--can be used to construct three-dimensional (3-D) patient models. The navigation system consists of 3-D models, a registration algorithm and tracking tools. Its aim is to provide information that is difficult to identify on the endoscopic screen, such as the locations of important patient organs and the target tumor. Other advantages of a surgical navigation system are that it provides as much information as possible to the surgeon, to reduce the need for reoperations and to provide patients with the opportunity for good postoperative progress. For example, the navigation system can indicate the location of optic nerves, thus reducing the possibility of complications arising that involve loss of vision caused by invasion of the patient's optic nerve and can also provide internal carotid artery (ICA) location information, preventing operation delays or reoperations caused by bleeding. The last decade has seen the genesis of research into computer-assisted surgery [15; 16]. Recently, future trends in computer-assisted surgery were predicted by [PERSON] et al. [17], while [PERSON] et al. [18] investigated navigation systems for implant placement and a CT-guided, osteotomy navigation system was applied to maxilla surgery, by [PERSON] et al. [19].