The incision made must be of adequate length to allow easy insertion of the device through the skin—a 1-cm long incision is inadequate to allow passage of a 1-cm diameter device. It is important to know that the outside diameter of a cannula is larger than the inside diameter, to allow for the thickness of the material used to create the port. In some instances, this can add two or more millimeters to the device diameter, and, therefore, increase the length of the required incision.
During endoscopy, the image must be transferred through an optical system. Although direct optical viewing is feasible and often used for diagnostic purposes, virtually all operative laparoscopy is performed using video guidance.
Figure 23.10 Laparoscopes. Three 0-degree laparoscopes are shown. From top to bottom, 2-mm, 5-mm and 10-mm diameter.
Laparoscopes are more than simple telescopes; they serve a dual purpose—transmission of light into a dark and closed cavity and provision of an image of the operative field. The light is generally transmitted from a cold light source via a fiberoptic cable to an attachment on the endoscope that passes the light to the distal end of the telescope via a peripherally arranged array of fiberoptic bundles. The image is obtained by a distally positioned lens and transmitted to the eyepiece via a series of rod shaped lenses. The eyepiece can be used to view the peritoneal contents directly or can serve as a point of attachment for a digital video camera. Some endoscopes transmit the image through a collection of densely packed fiberoptic bundles—an approach that diminishes resolution, but allows ﬂexibility of the endoscope, and that is of great value for small-caliber telescopes or when the device is designed to be steerable with an articulated distal end. Another option is to position a digital chip on the end of the system, which then functions as a camera, obviating the need to have any lenses or fibers to transmit the image, a design that is colloquially called “chip-on-a-stick.”
A laparoscope with an integrated straight channel, parallel to the optical axis, is called an operating laparoscope because the channel allows for the introduction of operating instruments. Operative endoscopes are of relatively larger caliber than standard laparoscopes, may have smaller fields of view, and may present increased risks associated with the use of monopolar electrosurgical instruments. Standard, viewing-only laparoscopes permit better visualization at a given diameter.
In general, the wider the diameter of the laparoscope, the brighter the image, resulting from either more light or wider lenses, which improve the viewing experience for the surgeon. Narrow-diameter laparoscopes generally allow reduced transfer of light both into and out of the peritoneal cavity; therefore, they require a more sensitive camera or a more powerful light source for adequate illumination. In the past, ideal illumination was provided by 10-mm diagnostic laparoscopes, but improvements in optics allowed the 5-mm diameter laparoscope to become the standard in many operating rooms (Fig. 23.10).
The viewing angle depicts the relationship of the visual field to the axis of the endoscope and typically ranges from 0 to 45 degrees to the horizontal. The 0-degree scope is the standard for gynecologic surgery. The 30-degree angle scope is invaluable in difficult situations, such as the performance of laparoscopic sacrocolpopexy, some myomectomies, and hysterectomy in the presence of large myomas.