Shaft: Originally purposed for gastrointestinal procedures, flexibility is desirable for atraumatic endolumenal movement; although this may aide in intra-abdominal navigation it also poses a challenge for maneuvering in open space or when attempting to achieve traction or counter-traction. Conversely, laparoscopic tools are rigid, and the combination of spaced ports of entry with transabdominal fulcrum points allow for a stable platform. Alternative means of target fixation, apart from the physical platform, may aide in stability and organ manipulation and help address current limitations of traction and angles of tissue engagement[11]. Additionally, the increased distance between operator and end-effectors, which is typical with flexible endoscopy, limits haptic feedback.

Working channels: Maximal size on commercially available endoscopes is 3.7 mm, limiting the size of available equipment and contrasts greatly to variable size laparoscopic ports. The proximity and parallel orientation of channels limit triangulation, robust tissue manipulation and traction. Furthermore, current endoscopes are restricted to two working channels that are inadequate for some procedures[4].

Ancillary channels: Channels dedicated for insufflation, irrigation and suction are sub-optimal for routine needs of intra-abdominal surgery and may be inadequate in the event of an emergency.

User interface: Current endoscopes are designed for the endoscopist to control field of view and positioning/orientation. The working channels and navigational/field control elements are in close proximity and lead to complicated team interactions.

Site of access: NOTES entry point (transgastric, transcolonic, transurethral or transvaginal) is often selected for proximity or best en-face view, however, this does not ensure direct or adequate visualization of the desired site. Additionally, trans visceral peritoneoscopy may have limited reach and stability in certain orientations; for example targeting the spleen via the transgastric approach. Considerable maneuvering of the endoscope is often required to achieve acceptable positioning and stability. A “bounce-off” technique, utilizing internal structures to redirect scope vectors and trajectory is often necessary, yet technically challenging. This may be due to several reasons, including inter-patient variability and inherent mobility of internal structures. Conversely, procedure specific transabdominal port placement allows optimal laparoscopic site selection and stability.

Visual orientation: While endoscopists accustomed to the confines of the gastrointestinal tract are typically comfortable with inverted positioning, surgeons prefer a fixed horizon. With the camera married to the endoscope shaft, horizon is at the mercy of the endoscope’s final position. Laparoscopic visualization is divorced from the effectors allowing remote imaging with maintenance of the horizon. This may diminish work load and prove beneficial when dealing with complex surgeries[12]. Vantage point is yet another issue; the close proximity and magnified endoscopic views may prove advantageous for meticulous dissection, while a remote view of the operative field may be essential for other tasks and harder to achieve with current flexible platforms. Ancillary visualization technologies with computer tomography and 3D image registration may help mitigate these limitations in the future[13] (Figure 1).

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Figure 1 Image registration illustrating tracking of the endoscope tip within a 3D reconstruction of the subject’s abdomen.

Imaging: Current endoscopic depth of field, although excellent for near vision, lacks the needed distant visualization that would aid in complex procedures and abdominal exploration. Additionally, there may be insufficient light for certain procedures, including exploration and cancer staging. A 10-mm laparoscope provides 380 lumens while a typical endoscope with a 3-mm light bundle provides only 25 lumens. Conversely, the magnified endoscopic images may be superior to laparoscopic images for certain procedures.

There is a fundamental difference in work load between laparoscopic and endoscopic paradigms. Laparoscopically, the field of view is maintained by an assistant while instruments are maneuvered and executed by the surgeon. Endoscopically, positioning and field of view are maintained by a complex coordination of positioning-wheels, torque, placement, and locking mechanisms. The endoscopic tools are also positioned by the endoscopist (in unison with the endoscope) while actuated by an assistant. Additionally, mental workload will likely be increased by fluctuating visual frames of reference and angles of approach associated with NOTES procedures.

While many of these differences pose disadvantages compared to the current laparoscopic paradigm, flexible endoscopy’s added reach and close visualization of an operative field may confer advantages still unknown, for example, inspection of the lesser sac and the supra-hepatic/infra diaphragmatic space in cancer staging, which are not readily attainable with current minimally invasive techniques.

One of the first platforms to be used in NOTES animal models was the “R-scope” (XGIF-2TQ160R; Olympus, Tokyo, Japan) later modified to the NOTES scope. It has been used to perform several procedures in the pre-clinical setting including cholecystectomy[14,15]. This device falls under the traditional endoscopic paradigm. It is a modified dual channel endoscope (DCE) with additional elevator toggles and a larger wheel further down the handle to control a second bending segment (Figure 2). The primary segment is lockable, allowing for a better angle of approach and more precise tissue manipulation with maneuvering of the second segment. Additionally, the two working channels have lifting gates that are orthogonally positioned allowing for simultaneous lifting (vertical motion) and dissection (horizontal motion) (Figure 3). This configuration allows more accurate tissue manipulation off-axis to the visual plane. See Table 1 for device specifics. This endoscope addresses several of the DCE shortcomings, including positioning and to a small degree triangulation. The second bending segment, although useful, can be technically demanding. These features may lead to increased physical and mental work load. Furthermore, the image is still married to the effectors and, as such, has a limited field of view. Essentially the NOTES scope further refined what the current standard endoscope is capable of while partially tackling some tasks germane to complex surgery.

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Figure 2 Natural orifice translumenal surgery (NOTES)/R scope user interface.

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Figure 3 NOTES/R scope distal tip arrangement.

The incisionless operating platform (IOP; USGI Medical, San Capistrano, CA) was initially designed to function within the traditional endoscopic paradigm, with recent modifications bridging to the flexible-laparoscopic model. This device was developed through collaboration of physicians and industry and addresses several NOTES requirements. It has been successfully used in a variety of procedures, and is the first specialized platform to be used in clinical NOTES cases, including human transgastric cholecystectomy[16]. In appearance, the IOP is similar to, but larger than a standard endoscope, with multiple ports and directional wheels at the user interface (Figure 4). Based on endoscopic ergonomics, this platform consists of a 110 cm × 18 mm overtube-like design with a steerable shaft and several channels (Table 1). In some models, the overtube-shaft is capable of stiffening, providing enhanced stability. Its four channels (7, 6, 4 and 4 mm in size) allow for an N-scope (Olympus, Tokyo, Japan) and specialized equipment[17]. The N-scope is independently rotatable within the channel allowing for an adjustable horizon while maintaining instrument position. The other channels can be used for instruments as well as high flow carbon dioxide insufflation[14]. Several specialized tools have been designed for this system including a 2.5-cm grasping jaw, capable of performing tissue plications with unique anchors and several accessories for tissue manipulation. Compared to the DCE, the IOP has enhanced deflection and improved triangulation due to the large channels and effectors’ ability to enter the operative field. However, the IOP’s in-line channel orientation is still subject to parallelism. This may be overcome with instrument modifications. This device presents a new paradigm for the endoscopist as visualization is divorced from the primary operator. Work load for the IOP is high and requires skilled assistants as the primary operator interchanges responsibilities for instrument exchange, device orientation and scope positioning. This is increasingly challenging when the device is in an unstable position. Newer versions of this device allow for bimanual instrument control and are more consistent with the flexible-laparoscopic model.

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Figure 4 Incisionless operating platform.

The EndoSAMURAI (Olympus Corp., Tokyo, Japan) was designed to operate within the flexible-laparoscopic paradigm. It has been tested in animal models for cholecystectomy[18]. It consists of a specialized endoscope with a remote working station and a locking overtube. The distal end of the scope has two short modified independent arms, which upon entry remain parallel with the scope shaft, however, open in an elbow-like fashion when in position (Figure 5). These serve as conduits for different effectors, including standard endoscopic accessories, and are manipulated from a control unit apart from the traditional endoscopic user interface (EVIS EXERA II Universal Platform-Olympus Corp; Figure 6). With 5 degrees of freedom and triangulation capabilities, the arms can tie sutures as well as provide traction and counter traction. In addition to the two conduits, it has a third working channel that may be used for ancillary equipment or suction/irrigation. Stiffened by a locking overtube, the scope articulates in the same manner as a standard endoscope with identical visualization. Although essentially a modified DCE, the EndoSAMURAI overcomes parallelism by the angle at which its effectors are positioned. Additionally, it has better stability compared to the DCE because of the locking overtube. As the arms are married to the camera/scope, it still bears the same image-perspective limitations as the standard endoscope. Interestingly, this system employs a “drive, park and move” methodology. Where the user navigates to the target with the endoscope, locks the overtube system and scope in position and then proceeds to the user interface. This effectively allows one operator to perform most of the work load as the image is theoretically kept in place with the locking system with subsequent maintenance of the image by the assistant, which is somewhat similar to traditional laparoscopy.

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Figure 5 EndoSAMURAI user interface.

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Figure 6 EndoSAMURAI end effectors.

The direct drive endoscopic system (DDES; Boston Scientific, Natick, MA) is a flexible-laparoscopic multitasking platform that consists of a 55-cm steerable guide sheath that houses 3 lumens extending from a rail-based platform with interchangeable 4 mm instruments (Figure 7, Table 1). The user interface consists of ergonomic rail-guided drive handles situated above the surgeon’s waist level. A unique system in the handle allows for seven degrees of freedom: surge, pitch, yaw, roll, tool action, heave and sway. Equipment currently available consists of graspers, scissors, needle pushers and cautery devices. These effectors can traverse a distance from the sheath tip independent of the image (Figure 8). While specialized tools are passed through channels in the guide tube, an N-scope is used for visualization. The N-Scope is freely rotatable and positioned independent of the DDES end effectors. The sheath serves as a guide that can be “docked” once in position. Maintenance of visualization may require adjustment of the endoscope as well as the sheath while tissue is manipulated. This system accomplishes much of what is desired to mimic a laparoscopic approach, including cutting, grasping, suturing, triangulation and knot tying. Of note, current iterations of the DDES do not have a dedicated channel for irrigation and suction and rely on the endoscope’s capabilities, which may not be adequate for intra-abdominal procedures. This system has been tested ex-vivo and in-vivo with suturing tasks accomplished commensurate with laparoscopy, as well as endoscopic mucosal resection and sub-mucosal dissection[15,19].

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Figure 7 Direct drive endoscopic system (DDES).

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Figure 8 DDES-distal end.

Parallel to these technical developments, the human NOTES experience has continued to broaden. Early human work out of Ohio State University used standard endoscopic equipment for diagnostic human peritoneal exploration[20]. This study confirmed that the initial steps of NOTES procedures were safe and feasible in humans. A variety of other human NOTES procedures have been performed to date including: transvaginal and transgastric cholecystectomy[21]; transgastric appendectomy[22]; sleeve gastrectomy[23]; and several others. Many of these procedures have been hybrid in nature with laparoscopic components. As tools are further enhanced, current hybrid procedures may evolve to pure NOTES. However, the hybrid approach may be the best course for the near term to maximize patient safety. In addition to the above prototypes, many others are under development and are in various stages of testing. Currently, the best training tools to acquire skills necessary for NOTES, and develop a comfort level with novel instrumentation, are a combination of animal models and human cadavers, which can only approximate the human surgical experience. NOTES simulators are currently under development and may in the future offer a better means of training.